Revised 2022

Bedding sands are a critical component of all sand-set segmental concrete paving systems. Especially for vehicular applications, specifiers and contractors need to consider bedding sand selection. While gradation is an important consideration, other characteristics should be assessed in order to ensure long- term pavement performance. This technical bulletin examines these characteristics and provides guidance to specifiers and contractors.

BACKGROUND

Bedding sand provides four main functions. It beds the pavers during installation; helps initialize interlock among the pavers; provides a structural component for the system (as described in CMHA Tech Note PAV-TEC-004–Structural

Design of Interlocking Concrete Pavement for Roads and Parking Lots) and facilitates drainage of water that infiltrates through the joints. Typical specifications require bedding sands to conform to ASTM C33 Standard Specification for Concrete Aggregates and CSA A23.1 Concrete materials and methods of concrete construction FA1 gradation for concrete sands with an additional limit of 1% passing the No. 200 (0.075 mm*) sieve (See Table 1). To achieve this low percentage of fines, washing the sand is typically required. A common name for the recommended bedding material is washed concrete sand. In vehicular applications, experience and research have shown that other factors besides gradation contribute to the successful function of the bedding layer in vehicular applications. Knapton (1994) notes that since 1980 the amount of material passing the No. 200 (0.075 mm) sieve has been reduced in the British Standard BS 7533-1 Pavements constructed with clay, concrete or natural stone paving units. Code of practice for the structural design of pavements using modular paving units. He notes that fines have reduced from 10% in 1980, to 3% in 1991, to 1% for heavily trafficked pavements, further reducing to 0.1% for bus stations. North American standards currently limit the amount of allowable material passing these sieves to 1%.

Other studies (Lilley and Dowson 1988) (Beaty 1996) have investigated failures of segmental concrete pavements subjected to channelized vehicular traffic. They have also concluded that more comprehensive specifications are required. Lilley and Dowson (1988) suggested that bedding sands in segmental concrete pavements designed to carry more than 1.5 million equivalent standard axle loads, ESALs (18 kip/80 kN), should be subjected to grading and degradation tests. For the purposes of this Tech Note, vehicular traffic is defined as roads exposed to a minimum of 1.5 million lifetime ESALs and axle loads up to 24,250 lbs (11,000 kg).

FAILURE MECHANISMS

Failure of the bedding sand layer occurs in channelized vehicular loads from two main actions; structural failure through degradation and saturation due to inadequate drainage. Since bedding sands are located high in the pavement structure, they are subjected to repeated applications of high stress from the passage of vehicles over the pavement (Beaty 1996). This repeated action, particularly from higher bus and truck axle loads, will degrade the bedding sand and cause failure. For these applications sand should be selected based on their ability to withstand long-term degradation.

Bedding sand permeability also is a significant factor in the selection process. Wherever difficulties have been experienced with laying course materials in heavily trafficked pavements, water has been a major factor (Knapton 1994). As they approach higher moisture levels in service, bedding sands may become unstable. Smaller particle sizes (fines) become suspended in water, forming slurry that lubricates the entire bedding layer. Choosing bedding sand with a gradation as shown in Table 1 will help to reduce the risk of poor drainage and instability. However, these sands will be susceptible to drainage problems if they do not have the hardness to withstand long term degradation from vehicular wheel loads.

SELECTION AND PERFORMANCE DESIGN PRINCIPLES—GOING BEYOND GRADATION

Selecting Durable Bedding Sands—Durability of aggregates has long been understood to be a major factor in pavement performance. ASTM C88 Soundness of Aggregate by use of Sodium Sulfate or Magnesium Sulfate is an example of a typical test method used by road agencies to assess aggregate durability. The test involves soaking an aggregate in a solution of magnesium or sulfate salts and oven drying. This is repeated for a number of cycles, with each cycle causing salt crystals to grow and degrade the aggregate. The test method takes a minimum of 6 days to complete. The percent loss is then calculated on individual size fractions. This test method, however, is considered highly variable. Jayawickrama, Hossain and Phillips (2006) note that when ASTM initially adopted this test method they recognized the lack of precision, saying, “it may not be suitable for outright rejection of aggregates without confirmation from other tests more closely related to the specific service intended.” CMHA recommends using ASTM C88 as a measure of aggregate durability as long as other material properties described in this bulletin are also considered.

The Micro-Deval test is evolving as the test method of choice for evaluating durability of aggregates in North America. Defined by CSA A23.2-23A, The Resistance of Fine Aggregate to Degradation by Abrasion in the Micro-Deval Apparatus and ASTM D7428 Standard Test Method for Resistance of Fine Aggregate to Degradation by Abrasion in the Micro-Deval Apparatus, the test method involves subjecting aggregates to abrasive action from steel balls in a laboratory rolling jar mill. In the CSA test method a 1.1 lb (500 g) representative sample is obtained after washing to remove the No. 200 (0.080 mm) material. The sample is saturated for 24 hours and placed in the Micro-Deval stainless steel jar with 2.75 lb (1250 g) of steel balls and 750 mL of tap water (See Figure 1). The jar is rotated at 100 rotations per minute for 15 minutes. The sand is separated from the steel balls over a sieve and the sample of sand is then washed over an 80 micron (No. 200) sieve. The material retained on the 80 micron sieve is oven dried. The Micro-Deval loss is then calculated as the total loss of original sample mass expressed as a percentage. ASTM and the American Association of State Highway Transportation Officials have both adopted the coarse aggregate version of the Micro-Deval test, ASTM D6928 and AASHTO TP 58. Both are also considering a version for fine aggregates. Since the test apparatus uses the same size drum and rotates at the same speed, no modifications to the apparatus are required to perform the fine aggregate test in laboratories currently equipped to perform the coarse aggregate test procedure.

A study conducted by the Interlocking Concrete Pavement Institute (ICPI 2004) investigated nine sands from across the United States reported by contractors to have “good to excellent” serviceability in vehicular applications. The results of this study indicated that eight of these sands had Micro-Deval degradation losses less than 8% when measured according to CSA A23.2-23A. The same study subjected these sands to the ASTM C88 soundness loss and found that no sample had greater than 6% loss. The Micro-Deval test is recommended as the primary means to characterize bedding sand durability (See Table 3) and the magnesium or sulfate soundness should be considered when the Micro-Deval test is not locally available. The variability of the soundness test method should always be a consideration unless measured in relation to other material properties.

A test method similar in nature to Micro-Deval is the Lilley and Dowson test (Lilley Dowson 1998). This test method specifically developed for bedding sands is recognized internationally and is referenced in CMHA manuals Port and Industrial Pavement Design with Concrete Pavers and Airfield Pavement Design with Concrete Pavers. This test method is performed on 3 lbs (1.4 kg) randomly selected, oven-dried sand samples with two 1 in. (25 mm) diameter steel balls together weighing 0.3 lb (135 g). Three sub-samples each weighing 0.5 lbs (0.2 kg) are derived from the main sample. Each sub-sample is sieved according to ASTM C136 then re-mixed and placed in a nominal liter capacity porcelain jar with the two steel balls. The three jars are rotated at 50 rpm for six hours and sieved again. Sand durability is assessed from resulting increases in the percent passing the No. 50, 100 and 200 (0.300, 0.150, and 0.075 mm) sieves. Developed in the UK, the test is not readily available at laboratories in North America. The CSA and ASTM Micro-Deval tests may be more available.

Beaty (1996) demonstrated a correlation between the two tests with a correlation coefficient greater than 0.99. The relationship between the two tests is:

L = 1.97 + 1.21 M

Where:

M = CSA Micro-Deval Degradation Loss (%)

L = Lilley and Dowson Degradation Loss (%)

Beaty’s correlation involved a modification to the test procedure by reconstituting the test aggregates into a standard gradation shown in Table 2 and performing the Micro-Deval and Lilley Dowson tests on the re-graded aggregate. In this modified version of the Lilley Dowson test procedure the loss (L) is measured as the total increase in percentage of fines passing the No. 200 (0.075 mm) sieve at the completion of the test. Using the correlation described above, an 8% Micro Deval degradation (See Table 3) would have a corresponding Lilley and Dowson degradation of 12%.

Bedding Layer Drainage—Bedding layer drainage is important for early and long term performance of a pavement. One failure documented by Knapton (1993) describes a segmental pavement that was opened to bus traffic and within hours of construction subjected to continuous heavy rain. The bedding sand in this case had a high percentage of fines. As a result of the continuous rainfall, finer sieve fractions in the sand were transported into the drain holes of the underlying concrete slab. With the drainage compromised the bedding sand liquefied and was pumped through the joints of the pavement, resulting in immediate rutting and failure of the system. The pavement was subsequently reconstructed with bedding sand that had 0% material passing the No. 200 (0.075 mm) sieve and reported excellent performance. Although gradation is an important factor in drainage (since it affects permeability) eliminating all of the fines can sometimes be impractical. Therefore, CMHA recommends up to 1% passing the No. 200 (0.075 mm) sieve.

Another important material property is permeability. Even specifications that allow up to 3% of fines can result in a five fold decrease in permeability from the lowest to highest percentage passing (Bullen 1998). In research conducted by the Interlocking Concrete Pavement Institute (ICPI 2004) the permeability of “very good to excellent” bedding sands was measured. Using the test method described by ASTM D2434 Standard Test Method for Permeability of Granular Soils (Constant Head) the permeabilities ranged from 2.8 in./hr (2.1 x 10-3 cm/second) to 15.6 in./hr (1.1 x 10-2 cm/second). These values correspond to fines that range from 2.5% to 0% passing the No. 200 (0.075 mm) sieve but, more importantly they also are associated with Micro-Deval maximum degradation values of 8%. Table 3 indicates a minimum permeability of 2.8 in./hr (2.1 x 10-3 cm/second) that should also be considered at the same time as the other primary properties listed.

Other Material Properties—Studies have indicated that bedding sand shape plays a role in bedding sand performance. (Knapton 1993) notes that rounded or cubic grains lead to stable sands, whereas more angular grains are frequently associated with sands that fail. The sands tested by ICPI (ICPI 2004) showed that eight of the nine “good to excellent” performing sands were characterized by having a predominance of sub- angular to sub-rounded particle shapes when tested according to ASTM D2488 Description and Identification of Soils (Visual- Manual Procedure). Specifiers and contractors should consider bedding sand angularity using Figure 2 as a guide. Figure 3 shows a photograph of one of the ICPI test sands at high magnification. Table 3 suggests that a combined percentage of sub-angular to sub-rounded particles should be a minimum of 60%.

Geology—Geology of bedding sands has been noted by a number of studies to play an important role in their performance. For example, bedding sand with quartz mineralogy is preferred over crushed sandstones (Knapton 1993). In the study by the Interlocking Concrete Pavement Institute (ICPI 2004), eight of the nine “good to excellent” performing sands were noted to consist predominately of silica minerals with over 80% of the material either quartz or quartzite. Table 3 recommends a minimum 80/20 ratio of silica/ carbonate mineralogy. A tenth sample, included in the study (and noted as poor performing in the field) was characterized as having up to 50% carbonate content. Petrographic analysis was conducted according to the Ministry of Transportation of Ontario laboratory method MTO LS-616 Procedure for the Petrographic Analysis of Fine Aggregate (MTO 1996). ASTM C295 Standard Guide for Petrographic Examination of Aggregates for Concrete offers an alternative test method.

Limestone screenings and stone dust are not recommended for bedding sand. In addition to being unevenly graded and having excessive material passing the No. 200 (0.075 mm) sieve, screenings and stone dust will break down over time from wetting and abrasion due to vehicular loads. Unlike soft limestone screenings and stone dust, hard, durable concrete sand meeting the requirements in Table 3 will not break down easily. Limestone screenings also tend to break down during pavement construction under initial paver compaction. Depressions will eventually appear in the pavement surface with limestone screenings or stone dust.

Recommended Material Properties—Table 3 lists the primary and secondary material properties that should be considered when selecting bedding sands for vehicular applications. Bedding sands may exceed the gradation requirement for the maximum amount passing the No. 200 (0.075 mm) sieve as long as the sand meets degradation and permeability recommendations in Table 3. Micro-Deval degradation testing can be replaced with sodium sulfate or magnesium soundness testing as long as this test is accompanied by the other primary material property tests listed in Table 3. Other material properties listed, such as petrography and angularity testing content range of 6% to 8% has been shown to be optimal for most sands (Beaty 1992). Contractors can assess moisture content by squeezing a handful of sand in their hand. Sand at optimal moisture content will hold together when the hand is re- opened without shedding excess water. Although it can be difficult to control the exact moisture content on the job site, uniformity of moisture content can be maintained by covering stock piles with tarps. Digging into sand piles at mid- height to avoid saturated material that may be at the bottom of the pile is also recommended.

While on the job site, a contractor should check the hardness of the bedding sand particles. Particles of sufficient hardness will not break under the pressure of a Swiss Army pocket knife. This field test, although not recommended for pre-selection of bedding sands, helps assess a material at the time of delivery. Table 4 lists the recommended bedding sand properties that need to be considered by a contractor during installation.

Interlocking concrete pavements should also be designed and constructed such that the bedding sand should not be able to migrate into the base, or laterally through the edge restraints. Dense- graded base aggregates with 5% to 12% fines (the amount passing the No. 200 or 0.075 mm sieve), will ensure that the bedding sand does not migrate down into the base surface. For pavements built over asphalt or concrete bases, it is necessary to provide adequate drainage by providing 2 in. (50 mm) diameter weep holes around the perimeter on 10 ft (3 m) centers and at the low points in the concrete base to drain excess water from the bedding layer. Holes should be filled with washed angular gravel and covered with geotextile to prevent the loss of bedding sand. Figure 6 on the next page shows a detail. Specifiers can visit the CMHA website to download similar details for use in specifications from www.masonryandhardscpes.org. To control lateral loss of bedding sand, Figure 7 shows geotextile installed at the interface of a concrete curb. To ensure that the sand cannot migrate through the joints in the curb woven geotextile is placed on top of the aggregated base, extending approximately 1 ft. (300 mm) into the pavement and wrapped up the sides of the curb to fully contain the bedding sand.

ROLE OF JOINTING SAND

Jointing  sand  provides two primary functions in a segmental concrete pavement; it creates interlock and helps seal the pavement. CMHA recommends that the same material properties listed in Table 3 also apply to jointing sand. Panda and Ghosh (2002) describe laboratory research on pavements using fine and coarse joint sands. Simulated loading consisted of 11-kip (51 kN) over 80 mm pavers with varying joint widths and joint sand gradations. Deflection of the pavement was then measured with coarser sand exhibiting lower deflections. The study concluded that “the coarser the sand, the better the performance.” The coarser sands used in the study correspond to the gradations for Joint Sand listed in Table 1 and the study recommended joint widths up to 3/16 in. (5mm). CMHA recommends joint widths of 1/16 to 3/16 (2 mm to 5 mm).

Contractors can benefit from using one sand source. There are advantages to using the bedding material for the jointing sand during construction. Using one material allows the contractor to monitor and control one sand product on the job site. Over time the joints become filled with detritus, providing some degree of sealing. Regardless of the sand used, segmental concrete pavements will always allow some water penetration through the joints.

Coarse bedding sand may require additional effort to place it in the joints. In some cases, smaller joint widths may require the use of finer graded sand. In this case, the use of mortar sand is recommended. Mortar sand should conform to the gradations of either ASTM C144 or CSA A179 but should also meet the material property requirements of Table 3.

Although joint sand selection is an important factor, design and construction play a more important role. Considerations such as joint width, ensuring that the sand is swept in dry, degree of compaction, and ensuring the joints are completely filled, are just as critical to the long term success of pavement performance. Information on joint sand installation can be found in CMHA Tech Note PAV-TEC-002—Construction of Interlocking Concrete Pavements).

MAINTENANCE OF INTERLOCKING CONCRETE PAVEMENT

Occasionally interlocking concrete pavements will require maintenance for them to deliver peak performance. Refer to Tech Note PAV-TEC-006–Operation and Maintenance Guide for Interlocking Concrete Pavement for information on preventative maintenance, identifying and remedying aesthetic and structural distresses and best practices for the disassembly and reinstatement of interlocking concrete pavement.

REFERENCES

Refer to the latest published ASTM and CSA standards and CMHA Tech Notes and manuals

1. ASTM–American Society for Testing and Materials International, Conshocken, PA. www.astm.org
2. CSA–Canadian Standards Association, Rexdale, ON. www.csagroup.org
3. CMHA-Concrete Masonry and Hardscapes Association, Herndon, VA. www.MasronyandHardscapes.org
4. Knapton J., “Paver Laying Course Materials – State of the Art” in Proceedings of the 2nd International Workshop on Concrete Block Paving, Oslo, Norway, pp. 246 – 264, 1994.
5. BS 7533-101: Pavements constructed with clay, concrete or natural stone paving units. Code of practice for the structural design of pavements using modular paving units. British Standards Institution, London, UK, 2021
6. Lilley A.A. and Dowson A.J., “Laying Course Sand for Concrete Block Paving” in Proceedings of the 3rd International Conference on Concrete Block Paving, Rome, Italy, pp. 457-462, 1988.
7. Beaty A.N.S. “Laying Course Materials: Specification and Performance,” in Proceedings of the 5th International Conference on Concrete Block Paving, Tel Aviv, Israel, pp. 129-139, 1996.
8. Jayawickrama, Hossain and Phillips, Evaluation of Aggregate Durability Using Micro-Deval Test, Transportation Research Board 2006 Annual Meeting, 2006.
9. ICPI Bedding Sand Laboratory Testing, File no. L04-0127AT, DAVROC Testing Laboratories, ICPI Herndon, VA, USA 2004.
10. Knapton J., The Nature and Classification of Bedding Sand, Proc. BIBM 1993, pp 135-141, Washington, DC, 1993.
11. Bullen F. and Knapton J., “The Role of Bedding Sands in Segmental Paving Instability,” in Proceedings of the 3rd International Workshop on Concrete Block Paving,
12. Cartagena, Colombia, May 10-13, 1998 pp. 36-1 to 36-4,1998.
13. MTO LS – 616 Procedure for the Petrographic Analysis of Fine Aggregate, Ministry of Transportation Ontario, Downsview Ontario, Canada, 2005.
14. Beaty, A., “Predicting the Performance of Bedding Sands,” in Proceedings of the 4th International Conference on Concrete Block Paving, Auckland, New Zealand, pp. 273- 284, 1992.
15. Panda and Ghosh, “Structural Behavior of Concrete Block Paving—Sand in Bed and Joints,” Journal of Transportation Engineering, March 2002.

Revised 2019

Introduction

An increasing amount of new and rehabilitated roof decks use segmental concrete paving units to support pedestrian and vehicular applications. The units provide an attractive, durable walking surface for pedestrian plaza decks. They can be used to create outdoor space, usable exterior living environments at commercial and residential buildings e.g. next to offices, hotels, hospitals, universities, observation areas on commercial buildings and at cultural centers. See Figure 1. Parking structures and the roof decks of underground buildings use concrete pavers to support vehicular traffic as shown in Figure 2.

Segmental concrete paving units protect roofing materials from damage due to foot traffic, equipment, hail and vehicles. Concrete provides a heat sink that reduces the thermal stress and deterioration of waterproofing materials. The units flex with the movement of the structure as well as with vehicular and seismic loads. Additionally, the units provide a slip-resistant surface and are especially attractive when viewed from adjacent buildings. They can exhibit high durability under freeze-thaw and deicing salt conditions.

A primary role of segmental concrete units is ballast for roofing materials to prevent uplift from high winds. When caught by high winds, gravel ballast on roofs can shift and distribute unevenly. This leaves roof materials exposed to winds, thereby increasing the risk of their uplift. In some cases the gravel can be blown from roofs creating a hazard for glass, pedestrians and vehicles. Concrete units are preferred over gravel ballast because they provide a consistent, evenly distributed weight for protection from wind uplift and damage. Furthermore, concrete unit paving is required by many building codes as roof ballast for high-rise buildings.

This Tech Note provides guidance on the design and construction of roof assemblies using precast concrete pavers or concrete paving slabs using with various setting methods for pedestrian and vehicular applications. There are many kinds of roof assemblies placed under these types of paving units. The compatibility of paving units and setting methods with the components of roofing assemblies such as waterproof membrane, protection board and insulation should always be verified with the manufacturers of such components.

Vegetated, low-slope roof surfaces or “green roofs” are receiving increased attention from designers and clients interested in reducing building energy costs and the urban heat island. This trend is changing the aerial view of our cities. Furthermore, sustainable building rating systems such as LEED® recognize green roof technology as well as highly reflective roof surfaces. Concrete unit paving offers designers a reflective surface that can be easily integrated into green roof projects while earning LEED® credits. CMHA Tech Note PAV-TEC-016-Acheiving LEED® Credits with Segmental Concrete Pavement provides additional information on how to integrate green roofs with concrete unit paving.

Plaza Deck Components

Concrete pavers and slabs—There are two categories of segmental concrete deck materials for roofs, concrete pavers and slabs. See Figure 3. Concrete pavers are units that are a minimum thickness of 23/8 in. (60 mm) and whose length to thickness (aspect ratio) does not exceed 4 to 1. They conform to the requirements of ASTM C936 (1) in the U.S. or CSA A231.2 (2) in Canada. These units can be used in pedestrian and vehicular applications. Concrete pavers 23/8 in. (60 mm) thick are commonly used in pedestrian plaza or terrace applications. When the capacity of the structure is limited to additional weight, units as thin as 11/2 in. (40 mm) have been used in pedestrian applications. For vehicular uses, the recommended minimum thickness of units is 31/8 in. (80 mm).

Precast concrete paving slabs range in nominal size from 10 x 10 in. (250 x 250 mm) to 48 x 48 in. (1200 x 1200 mm). These products should conform to CSA A231.1 (3) in Canada ASTM C1782 (4) in the US. Like pavers, concrete paving slabs can be manufactured with a variety of colors, special aggregates and architectural finishes to enhance their appearance. Surface finishes include shot-blasted, hammered and ground or polished. They differ from pavers in that slabs typically require at least two hands to lift and place them, and the length to thickness (aspect ratio) is 4 to 1 or greater. Paving slabs generally range in thickness from 11/2 in. to 2 in. (40 to 50 mm) and thicker units are also applied to roofs. Slabs are only for pedestrian plaza applications and are not recommended for vehicular use. Slabs risk tipping, cracking from bending forces, and shifting under repeated forces from turning and braking tires.

In ASTM and CSA paving slab product standards, flexural (rather than compressive strength) is used to assess unit strength since the larger slabs are exposed to bending and cracking. Compressive strength is excluded from these standards because it is not a true measure of the performance of the concrete. It can increase as the thickness of the tested unit decreases. Therefore, a high compressive strength test result required from a thin slab gives a false indication of a slab’s resistance to bending since thinner slabs will break in bending more readily than thicker ones.

Unit dimensions are measured on samples and compared to the dimensions of the manufacturer’s product drawings. Allowable tolerances for length and width in ASTM C1782 and CSA A231.1 (3) are –1.0 to +2.0 mm from the manufacturer’s product drawings. Height should not vary ±3.0 mm. Units should not warp more than 2 mm on those up to 450 mm in length and/or width. For units over 450 mm, warping should not exceed 3 mm. Tighter dimensional tolerances may be required for pedestalset, bitumen-set and some sand-set applications. Recommended tolerances are typically ±1.5 mm for length, width, height and no greater than 1.5 mm for warpage. These tolerances are needed for precision required in these construction assemblies and management of tripping hazards. These tolerances are often met through grinding the units, also called gauging.

There are some lightweight, low-flexural strength ballast slabs (mistakenly named roof pavers) manufactured with a tongueand-groove or bevels along their sides to increase their interlock. Other designs include plastic fasteners to connect one unit to the next. These methods of joining the sides to one another provide greater resistance to uplift from wind. Figure 4 illustrates one type of unit with tongue and grooved sides (not visible) and connecting tabs between each unit. Some of these types of units are made with lightweight concrete, or are thinner in order to reduce the dead load on the roof structure. Some designs have grooves on their bottom surface. When installed, these follow the roof slope to help remove water. These types of units offer limited architectural enhancement from patterns, colors, or surface finishes.

ASTM has issued C1491, Standard Specification for Concrete Roof Pavers (5). This product specification is appropriate for ballast-only type paving units (pavers or slabs) used only in direct contact with roof materials and only for limited pedestrian use such as walkways for maintenance personnel. Products that meet this standard should not be subject to constant pedestrian use, not placed on pedestals and never be subject to vehicles. Specifiers and contractors are advised to use roof paving products for vehicular and pedestrian applications that meet the previously mentioned ASTM or CSA standards. CMHA takes a conservative approach by not recognizing differences among shapes with respect to structural and functional performance. Certain manufacturers may have materials and data that discuss the potential benefits of shapes that impact functional and structural performance.

Setting Materials

Pedestals—Paving slabs for plaza decks are often placed on plastic or fiberglass pedestals. The result is a level deck, concealment of slope and drains and water storage space under the units during very heavy rainfalls. Pedestal-set paving units install quickly and enable fast removal for repair of waterproofing materials and for maintenance of deck drains. The units can be reinstated after repair with no visible evidence of movement. Damaged paving units can also be easily removed and replaced. Figure 5 shows a diagram of a pedestal system with paving slabs.

In most pedestal-set applications, units are 18 x 18 in. (450 x 450 mm) or larger but can be configured to support nearly any unit dimensions. The corners of paving units rest on plastic pedestals. These units usually require shimming after placement. Shims are inserted under the corners of a nonaligned paving unit until its surface is even with adjacent units. Some plastic pedestals have a built-in leveling device to reduce the amount of labor involved with shimming. Some are telescoping cylinders whose length can be changed by rotating an adjustable sleeve within another. Other designs have a base that tilts slightly to compensate for the slope of the roof.

Vertical spacers are often molded in the plastic pedestals to ensure uniform joint widths among the paving units. The open joints allow runoff to pass through them onto the waterproof membrane and into roof drains. The joint created by the spacer should not exceed 3/16 in. (5 mm) and this will minimize the likelihood of tripping.

With any segmental paving system, the final, installed result should provide a smooth, stable, and even surface. For pedestrian plaza deck applications, lipping tolerances among adjacent paving units should be no greater than 1/8 in. (3 mm). Surface tolerances of the finished elevations should be no greater than ±1/8 in. (±3 mm).

Another type of pedestal system consists of 8 in. (200 mm) square extruded polystyrene blocks (typically 2 in. or 50 mm thick) glued together, spaced on a grid across the deck and adhered to a polystyrene insulation board that rests on the waterproof membrane. Many contractors use 60 psi (0.4 MPa) polystyrene blocks to support the paving units. To support heavier loads, 100 psi (0.7 MPa) extruded or expanded polystyrene foam can be used. The bottom block of foam may have grooves in contact with roofing materials to facilitate drainage. The grooves should point toward drains.

A patented leveling system trims the tops of the polystyrene blocks to the required height. Shimming is not necessary except for the occasional paving unit that might be slightly out of dimension. Spacing is typically maintained with neoprene rubber spacer tabs adhered to the corners of the paving units, although plastic pedestals can be used. This pedestal system supports units up to 36 x 36 in. (910 x 910 mm). The foam pedestals can extend as high as 5 ft (1.5 m). Figure 6 shows the foam pedestals in place and receiving the paving slabs (6).

Another approach to creating roof decks is placing a plastic or fiberglass grid system, geotextile, pavers and jointing sand. Figure 8 shows this application as overlay onto an existing concrete roof deck. This assembly is for pedestrian applications only.

Bedding and Joint Sand for Pedestrian Applications—Sand-set pavers and slabs (up to 12 x 12 in. or 300 x 300 mm) are common options for pedestrian applications. The typical sand thickness is nominal one inch (25 mm). Figure 8 illustrates a sand-set application for pedestrians.

A key design consideration is not allowing the bedding sand to become saturated. Continually saturated sand and joints can support moss or vegetation that eventually clogs roof drains. Saturated sand can increase the potential for efflorescence that might exist in some concrete paving units. While not attractive, efflorescence will eventually disappear and it is not detrimental to structural performance.

The risk of saturated bedding sand is reduced by adequate slope of the roof structure and correct sand gradation. Sand requires at least a minimum deck slope of 2% to drain. Gradation of the bedding sand for pedestrian applications should conform to ASTM C33 (7) or CSA A23.1 “FA 1” (8). It is important that no material (fines) pass the No. 200 (0.075 mm) sieve as the presence of this size of material will greatly slow the movement of water through the bedding sand.

Recommended gradations for pedestrian applications are provided in Table 1. Limestone screenings or stone dust should not be used since they typically have fines passing the No. 200 (0.075 mm) sieve. It is accepted construction practice to use bedding sand for joint sand. Additional effort in sweeping and compacting joint sand may be required to work the larger particles down the joints. The sand should be dry when applied so that it flows freely into the joints.

Bedding and Joint Materials for Vehicular Applications—As with pedestrian plaza or terrace applications, bedding materials for vehicular applications need to freely drain water so that they do not become saturated. Again, an essential roof structure requirement is a 2% minimum slope. Parking decks with saturated bedding sand subjected to constant wheel loads will pump sand laterally or upward and out of the paving assembly. Joint sand is carried out as well, and loss of interlock follows. An unstable surface results where loose pavers receive damage (chipping and cracking) from continued wheel loads. Loss and lateral movement of bedding sand can result in damage to and leaks in the waterproof membrane from loose paving units.

In a few older, vehicular roof deck applications, there have been instances of bedding sand becoming clogged with fines over several years. The source of fines is likely from a combination of a lack of adequate slope, dirt deposited from vehicles and sometimes from degradation and wearing of the sand into finer material under constant traffic. The fines eventually accumulate in the bedding sand and slow drainage.

To help prevent the bedding layer from becoming saturated or becoming clogged, bedding material with a coarser gradation than that shown in Table 1 may be advantageous for vehicular or pedestrian applications. An example is material conforming to the gradation of ASTM No. 9 or No. 89 aggregate (9). See Table 2. The void space in this aggregate can allow for movement and removal of fines.

Joint sand should have sufficient coarseness such that it does not vacate the joints by working its way down and into the bedding material. The bedding material gradation should overlap with that of ASTM C33 or CSA A23.1 joint sand to help prevent it from working into the bedding sand.

Joint Sand Stabilization—Joint stabilization materials are recommended in sand-set roof applications for pedestrian and vehicular use. They are applied as a liquid or mixed dry with the joint sand and activated by moistening the joints with water. These materials reduce infiltration of water and ingress of fines brought to the surface by vehicles, and they achieve early stabilization of joint sand. Stabilization can help prevent the joint sand from being washed out by rainfall or blown out by winds. CMHA Tech Note PAV-TEC-005-– Cleaning, Sealing and Joint Sand Stabilization of Interlocking Concrete Pavements offers further guidance on the types of joint stabilizers and their applications.

Neoprene adhesive with bitumen-sand bed—This setting method typically involves applying an asphalt primer to the substrate and then placing a 3/4 in. (20 mm) (1 in. or 25 mm maximum) thick asphalt-stabilized sand layer over it, followed by a neoprene adhesive. The sand asphalt mix is applied hot and compacted. The units are set into the adhesive after a dry skin forms and the joints are then filled with sand. Figure 9 provides a schematic cross-section. Due to the high temperature of the bitumen-sand during installation, it may not be compatible with certain waterproof membranes. The waterproof membrane manufacturer should confirm compatibility of the primer, asphalt setting bed and adhesive with the membrane. Joint sand stabilizer can provide early stabilization of the joint sand. Cement mixed with sand to stabilize it in joints is not recommended since the cement can stain the surface of the paving units.

Drainage for roof applications should use bi-level drains which provide surface and bedding layer drainage. Bi-level drains include holes in the sides of roof drains to remove water that collects below the paving units. Details on drains are discussed later.

Mortar—While it is not a common setting material, a mortar bed (approximately 3:1 sand to cement) may be used to level and secure pavers or slabs. This setting method is not used over drainage mats. See Figure 10. Like a bitumen setting bed, mortar is costly to remove and replace should there be a need for roof maintenance. In addition, mortar deteriorates in freeze-thaw climates, and especially when exposed to deicing salts. In ASTM C270, Standard Specification for Mortar for Unit Masonry, the Appendices include a table on the Guide for the Selection of Masonry Mortars. While Type S is recommended, the guide states caution in selecting mortar for horizontal applications. While they are not foolproof, latex or epoxy modified mortars can reduce the onset of deterioration from freeze-thaw and salts making them acceptable for some pedestrian applications. However, loading and environmental factors preclude the use of mortar-set paving units for vehicular applications, and this setting method is better suited for non-freezing areas.

Geotextiles, Protection Board, Insulation and Drainage Mats

Geotextiles—With sand or aggregate bedding materials, geotextile will be needed to contain them and keep them from migrating into deck drains or through wall drains such as scuppers. In addition, sand or aggregate requires geotextile under it to prevent loss into the protection board and insulation (if used). Geotextile manufacturers should be consulted on geotextile selection. The fabric should be turned up against drains, vents and other protrusions in the roof and along parapets and walls.

To contain sand and aggregate bedding materials, the geotextile should extend up the side. Figure 11 shows this detail which will help prevent loss of bedding materials from a deluge of rainfall that causes temporary ponding around the drains. A separate piece of geotextile is wrapped around the roof drain to prevent loss of bedding sand or aggregate.

Protection board—Most waterproofing systems require a protection board over them to prevent damage to the waterproofing from paving units and to reduce thermal stresses from temperature changes. This can be an asphaltic protection board or other materials. The manufacturers of waterproofing systems can provide guidance on the use of protection layers and they can recommend specific materials when this option is required. Protection board is generally not used in vehicular applications.

Insulation—If a pedestrian plaza deck covers an inhabited space, insulation may be required. Insulation typically consists of foam or fiber boards placed over the waterproofing. Sometimes they are adhered directly to the waterproofing. Insulation may be tapered to roof drains to facilitate movement of water into the drains. Insulation board in contact with the waterproof membrane should have drainage channels to facilitate drainage of water under it. Insulation under pavers in vehicular applications requires careful design and execution. As with other engineered pavements, consult an experienced designer familiar with these applications. A secure location for insulation is sandwiched in place inside the concrete deck.

Drainage mats—Drainage mats in pedestrian applications are generally placed under bedding sand and over waterproof mem- branes to accelerate drainage of water from the sand. Drainage mats are typically 1/4 to 3/8 in. (6 to 10 mm) thick. They consist of a plastic core, called a geonet, covered by geotextile. Some mats consist of a “dimpled” plastic sheet, which will have limited ability to support loads. Geonets have a lattice like structure capable of carrying greater loads. The geonet and geotextile support and contain the bedding sand under the paving units while allowing water to move into it and laterally to roof drains. They are only recommended in pedestrian applications under a sand setting bed. They should be pulled taught and secured, placed at a minimum of 2% slope.

Installation of drainage mats for pedestrian applications should start at the lowest slope on the roof with the work proceeding upslope. Flaps on each should go under the next (in a manner similar to placing roof shingles) so that the water drains from one section to the next. This helps prevent water from leaking under the mats. While mats reduce the amount of water reaching the waterproof membrane, they are not a substitute for deck waterproofing. The paving installation contractor should install mats.

Drainage mats are typically supplied in rolls making them difficult to flatten, and they often don’t remain flat during installation. An adhesive between the mat and waterproof membrane will likely be required to maintain flat drainage mats during their installation. Before using an adhesive, confirm that it will not harm the other materials and in particular the waterproof membrane.

Drainage mats can be used under foam or plastic pedestal systems. While drainage mats may be tested according to the compressive strength test method in ASTM D1621 (10), they may require additional testing by pre-loading to ensure that they will not crush under loads from the pedestals.

Drainage mats should not be used directly under bedding sand in vehicular applications. In such cases, mats can deflect under wheel loads, eventually fatiguing, compressing and deforming. Repeated deflection tends to shift pavers, bedding and joint sand, making interlock difficult to maintain. Deflection causes the joint sand to enter the bedding, thereby loosing interlock. Joint sand loss with possible eventual crushing of the mat can lead to saturated bedding sand. These distresses can avoided in vehicular applications by placing the mat under a minimum 4 in. (100 mm) thick concrete base that supports anticipated traffic loads under the bedding sand and concrete pavers.

Waterproof Memberanes

The choice of waterproofing is influenced by the application, the project budget, the deck materials under it and the type of structure supporting the roof. There are three broad types of waterproofing materials used under concrete paving units. They are single-ply, liquid membranes, built-up or modified bitumen roofing. A brief description of these materials follows with their compatibility to segmental paving (11,12).

Single-ply roofing is strictly for pedestrian applications and it is the most widely used waterproofing. It is typically made from vulcanized (cured) elastomers such as ethylene propylene diene monomer (EPDM), neoprene, or butyl. These flexible sheets have excellent weathering properties, high elongation and puncture resistance. When assembled on a roof, the sheets are spliced together at the job site with an adhesive. The entire assembly of sheets can be loose-laid and ballast provided by paving units. They also can be partially or fully adhered, or mechanically fastened to the roof deck.

Another type of single-ply membranes includes non-vulcanized elastomers such as polyisobutylene (PIB), chlorinated polyethylene (CPE), chlorosulfanated polyethylene (CSPE). These materials are usually reinforced with a polyester mat laminated between two plies. Thermoplastics such as polyvinyl chloride (PVC) sheets are heat welded in the field. Like the elastomers, PVC is loose-laid with ballast paving units, partially or fully adhered, or mechanically fastened to the deck material.

Rubberized asphalt membranes and polyethylene laminates have been used extensively to waterproof pedestrian plaza decks. Prefabricated sheets are made in small sheets and are spliced together in field. They generally are fully adhered to the concrete deck, so their longevity is highly dependent on the quality of the workmanship in splicing and on the smoothness and quality of the concrete.

Manufacturers of single-ply membranes should be contacted about the extent of warranties on the field splices under paving units. Additional measures may be necessary to protect the splices from the paving. This can include installation of a second, sacrificial membrane layer directly under the paving units.

Liquid applied membranes are installed either hot or cold depending on the materials. Rubberized asphalt membranes are hot applied to the concrete deck to form a continuous coating with no seams. These are for pedestrian plaza decks only. Cold- applied liquid resins and elastomers such as polyurethane are generally suitable as waterproofing on concrete decks subject to vehicular use. Sprayed-in-place polyurethane foam acts as an insulator and as waterproofing. The material is soft and is not recommended for use with concrete paving units.

Built-up roofing is made from paper, woven fabric or glass fiber mats, polyester mats or fabrics adhered together in alternating layers with bitumen or coal tar. The exterior surface of the layers is covered with bitumen or coal tar. Built-up roofs use concrete pavers or slabs as a walking surface to prevent wear and puncture of the membrane, especially around mechanical equipment. The use of pedestal systems should be avoided in built-up roofing due to the likelihood of indentations in the layered waterproofing materials.

Modified bitumen consists of plastic or rubber additives pressed into asphalt sheets. They are installed by heating the sheets with a torch and applying them to the deck substrate, or by mopping bitumen and securing them to the substrate with it. Some systems use cold cement or mastics to adhere the sheets to the substrate. Some modified bitumen waterproofings create overlap “bumps” every yard (meter) or so. There can be an additional construction cost to avoid these when using a pedestal system.

These systems do not require segmental paving ballast unless insulation needs to be secured in place. While these systems are generally compatible with concrete paving units in pedestrian applications, manufacturers should be contacted for verification of use with paving units under vehicular traffic.

Each of these waterproofing systems has advantages and disadvantages on speed of installation, costs, durability and manufacturer warranties. Many waterproof membrane manufacturers require the use of roofing contractors that have been certified to install a particular manufacturer’s roofing system. The subject of roof waterproofing is large and outside the scope of this publication. There are many references on roofing and waterproofing systems. An overview is provided in Roofing—Design Criteria, Options, Selection (12). Other resources are publications by the National Roofing Contractors Association at http://www.nrca.net and the Roof Consultants Institute at http://www.rci-online.org.

Deck Structure Systems

Concrete—There are four types of concrete deck structural systems (11). They are reinforced concrete slabs, post-tensioned slabs, pre-stressed precast elements such as “T” beams with a concrete topping, and concrete poured onto and formed by steel decks. Each type responds to waterproofing differently. For example, volumetric changes in reinforced concrete slabs can cause reflective cracking in liquid-applied membranes and some fully adhered bituminous systems. Posttensioned slabs are generally suited for liquid applied membranes because the slabs have a low amount of deflection and cracking. Loose-laid waterproofing systems are suited for over precast elements because they can accommodate the many joints in the deck, whereas liquid-applied and fully adhered membranes are prone to reflective cracking and splitting at joints.

In lighter, less expensive roofs, the concrete deck is poured onto and formed by a corrugated steel deck. In some cases the concrete is lightweight, i.e., weighing less per cubic foot or cubic meter than ordinary ready-mixed concrete. The weight of lightweight concrete is reduced by using lighter aggregates and by air-entraining the concrete mix. Lightweight concrete reduces loads on the columns and beams, thereby reducing their size and expense. (See Reference 13 for further information on lightweight concrete.) Steel decks topped with concrete should be vented so that moisture can escape if waterproofed with liquid-applied or fully adhered materials. Some waterproofing manufacturers do not recommend use of their materials over lightweight concrete.

Steel—Corrugated steel decks are generally covered with insulation and loose-laid single-ply membranes. This inexpensive assembly often uses ballast made with lightweight concrete paving units. These assemblies typically do not use heavier precast concrete pavers or paving slabs.

Design Considerations

Detailing for movement for pedestrian applications—Roof joints should be located when there is a change in roof direction, dimension, height, material, or when there are extreme differences in humidity or temperature within a building. Most roof structures have joints that allow each part of the structure to move independently due to settlement, seismic activity and thermal expansion/contraction. There is usually a flexible sealant in the joint to prevent water from entering and leaking into the space below. The sealant can be a compression seal squeezed into the joint, or a more expensive and durable strip seal. A strip seal is a length of flexible material fastened to metal clips secured to the concrete deck. The strip seal flexes with the movement of the adjacent structures.

Figure 12 illustrates a joint in a concrete structure and with sand-set paving units over it. Expansion joints should be treated as pavement edges. As with all segmental pavement construction, an edge restraint is required to hold the units together. Figure 12 shows steel angle restraint on both sides of the joint and secured to the concrete deck. There should be a compression seal at the top against the steel edge restraints, as well as one between the concrete decks. This detail is recommended at roof expansion joints for pedestrian applications.

This detail shows the paving pattern stopping at a joint in the deck and resuming on the opposite side. The sealant is joined to the edge restraint and not to the sides of the paving units. The use of a sailor or soldier course of pavers on both sides of the joint will present a clean visual break in the pattern. Figure 13 shows the consequences of not stopping the pattern with an edge restraint at an expansion joint. The pavers separated and exposed the bedding sand and waterproof membrane.

Parapets or building walls can typically serve as edge restraints. For sand-set paving assemblies, expansion material should be placed between the outside edge of the pavers and vertical walls of buildings when functioning as separate structures from the deck on which the paving units rest. Figure 14 shows this detail with expansion material. It should not adhere to the paving units or the wall, but should independently expand and contract with their movement. Expansion materials at the perimeter of the pavers are not necessary to place against walls or parapets when the pavers are resting on the same structure as the walls. Figure 11 illustrates this condition.

Detailing for movement for vehicular applications— Deck structures are designed to consider the maximum deflection at their mid-span. Unfortunately under vehicular loading some designs can allow a vertical movement in excess of 1 in. (25 mm). Repetitive vertical deflection can lead to cracking and chipping of the segmental paving units, and the crushing and eventual loss of joint sand leading to further forms of distress. Deck structures should be designed to have limited vertical deflection. Figure 15 details an expansion joint in a roof application subject to vehicles such as a parking structure. Although compression seals can be used, this assembly uses a strip seal for bridging the joint. The ends of the concrete deck are formed as edge restraints to hold the concrete pavers in place.

23/8 in. (60 mm) vs. 31/8 in. (80 mm) thick pavers for vehicular applications—Most vehicular applications with pavers are supported by a concrete structure. The support from such a structure is often used as rationale for using pavers that are less than 3 1/2 in. (80 mm) thick. Thicker units render greater vertical and rotational interlock. Using concrete pavers less than 3 1/2 in. (80 mm) thick in vehicular of applications increases the risk of reduced surface stability by reducing horizontal and rotational interlock under turning and braking vehicles.

Weight—Concrete pavers, slabs and bedding materials exert substantial weight on roof structures. The structure supporting these materials should withstand dead and live loads. The advice of a structural engineer should be sought to assess the capacity of the roof and tolerable deflections from paving-related loads especially when units are added to an existing roof deck structure. The weight of paving units can be obtained from manufacturers for the purposes of calculating loads. Bedding sand (1 in. or 25 mm thick) weighs approximately 10 lbs. per sf (49 kg/m2).

Resistance to wind uplift—The designer should consult Loss Prevention Data for Roofing Contractors Data Sheets published by Factory Mutual (FM) Engineering Corporation (15). Data Sheets 1-28 and 1-29 provide design data including the minimum pounds per square foot (or kg/m2) of paving unit weight required for resistance to wind uplift. The FM charts consider wind velocity pressure on roofs at various heights in different geographic locations. Design pressures are then compared to the type of roof construction, parapet height and the whether the paving units have tongue-and-groove, beveled joints, or are strapped together. Some high wind regions may have local building codes with additional weight requirements for paving units, especially on high-rise buildings.

Slope for drainage—A flat or “dead level” roof, i.e., one with no pitch, should never be designed. A dead level roof does not drain, creating a high risk of leaks in the waterproofing, as well a potential saturation of bedding sand (when used). The membrane will be exposed to continual standing water and ice that accelerates its deterioration and increases the potential for leaks. Likewise, paving units and bedding materials in constantly standing water subject to many freeze and thaw cycles will experience a decrease in their useful life.

Regardless of the deck substrate, it should be built with a minimum 2% slope to drain. This may be difficult to achieve with certain decks sloping toward area drains and some decks are built flat and then a topping applied to achieve slopes. The designers should take every opportunity to use deck systems that enable construction of a minimum 2% slope as some toppings are not waterproof and flat roofs will eventually leak.

Slopes for pedestrians and vehicles —The maximum slope is constrained by the need for a comfortable walking surface and the maximum percentage is typically 8% (4.5°). For driving surfaces, the maximum recommended slope should not exceed 20% (11°) and ideally should not exceed 8% as such surfaces will often see pedestrian use. For slopes exceeding 4% with exposure to vehicles, consideration should be given to using bituminous-set rather than sand set systems.

Roof drains—Depending on the design, roofs are drained at their edges and/or from the interior with roof drains. When roofs decks are loaded with dead and live loads, they will deflect. Continual deflection over time results in deformation of the roof. This movement can make drain inlets or scuppers adjacent to columns or on frame lines at the perimeter of the highest points of the roof. Therefore, sufficient pitch to the roof that accounts for such deflections is essential to continual drainage. In addition, the surface of the paving should be a minimum of 3/16 in. (5 mm) above the inlet of roof drains. When sand or aggregate is used for bedding or fill, it is essential that holes be in the sides of drains to allow water to escape the bedding sand. The bottom of the holes should be at the same elevation as the top of the waterproof membrane. As previously noted, drains should be wrapped in geotextile or fiberglass screen to prevent loss of bedding material through the drain holes.

Figure 16 illustrates ponding around a parking deck roof drain that didn’t have drain holes in its sides to drain subsurface water. Figures 17 and 18 illustrate a possible drain solution with holes for a pedestrian roof and parking deck. For paving slabs with pedestals, the slabs generally are located over roof drains, or are cut to fit around drains (see Figure 5). Bitumen-set assemblies require holes in the sides of the drains to remove water that may collect below the paving units. Bitumen and neoprene must not be allowed to clog roof drains or holes on their sides during installation.

Raising elevations—New and rehabilitated roofs may require fill material for raising the paved surface so it conforms to adjacent elevations. The deck surface receiving the fill material should slope a minimum of 2%. Fill materials are typically concrete, asphalt, or open-graded base. The structure should be evaluated first by a structural engineer for its capability in taking the additional load. Lightweight concrete may be considered if there are load limitations. These fill materials are often placed over a water-proof membrane. Consideration should be given to using insulation and protection board over the waterproof membrane. Attention in detailing and during construction should be given to how the fill materials will meet vents, skylights and other protrusions in the roof without damage to them, their flashing, or to their waterproofing. Opengraded bases will require geotextile under them to contain them. The fabric should cover all sides of the base.

Dense-graded aggregate base fill materials without drains are not recommended since water can collect at the bottom of the base and soften it. Over time, this condition can increase the potential for deformation of the base under repeated vehicular wheel loads. In addition, aggregate base materials can shed fine particles that, over time, can clog geotextiles and drains.

Concrete, asphalt, or open-graded bases are preferred as fill materials since they do not deform when continually exposed to water. In addition, they seldom shed particles into the roof drains so they present a much lower risk of clogged geotextile and drains.

Due to its high temperature at application, asphalt may not be compatible with some waterproof membranes, insulation, or protection board. All fill materials should be reviewed with the manufacturer for compatability with these components. Other important considerations are the minimum thickness to which the fill materials can be applied without cracking and deterioration from freeze-thaw cycles and salts. The design and selection of fill materials should address movement from temperature changes, vibration (if exposed to vehicles) and seismic activity.

Construction Considerations

Low slope roofs and waterproofing systems are generally installed by a specialty roofing subcontractor. A second subcontractor specializing in the installation of segmental paving supplies and installs bedding materials, pedestals, pavers or slabs after the waterproofing is placed by the roofing contractor. Installation of protection board and/or drainage mat may be by the paving contractor or roofing contractor depending on the project specifications. Testing of the waterproofing for leaks and any repairs should be completed prior to starting the paving.

Job Planning—Roof jobs are typically built in a very limited space. There will be an additional expense of moving the paving units from the ground to the roof. Most roofs may not have space to store cubes of pavers and stockpiled sand, and if they did, they most likely do not have the structural capacity to withstand their concentrated weight. The advice of a structural engineer should be should be sought on assessing the maximum load capacity of the roof to safely support the weight, packaging and distribution of all materials delivered to the roof, or a crane used to lift them from the exterior.

Forecasting delivery time for moving pavers to the roof, as well as sand, pedestals, saw(s), tools, geotextile and crew to the roof is critical to accurately estimating roof projects. Labor functions and costs must be tracked on each project for use in future bids. For example, additional time and expense may arise from the need for the paving contractor to place temporary protection on the waterproof membrane to prevent damage during construction. A one-story parking garage may allow all materials to be driven onto and delivered quickly to the roof. A multi-story parking garage with pavers on the top floor may have a 6 ft – 6 in. (2 m) ceiling height that will not allow delivery of pavers and sand in large trucks. Trucks with a low clearance will be needed to move materials through the structure and to the roof, or craned to the roof.

The packaging of most concrete pavers and slabs allows their transport to the roof via elevator or crane during construction. Roof access, construction scheduling, the capacity of the roof to withstand loads from packaged materials, and reduction of labor costs will dictate the economics of using a crane to transport materials to the roof. The roofing contractor often handles this.

In some cases, an elevator may be the only means of transport. An example of using only an elevator to move crews, tools and materials was to the observation deck on the 86th floor of the Empire State Building in New York City (Figure 1) where the deck was rehabilitated with concrete pavers.

The layout of paving slabs can be more demanding than the layout of interlocking concrete pavers. Some designers prefer joint lines to be located in particular places such as centered at columns or staircases. Careful planning of the layout will spare wasted cuts and adjusting the pattern on site to conform to the drawings and design intent.

Sometimes railing posts along the perimeter of a roof may require coring holes in paving slabs to fit around them. In addition, paving units may need to be cut to fit against moldings and other protrusions from parapets. The location of the pattern and cutting should be anticipated in advance of the construction.

Installation of bedding sand—After placing the geotextile, the bedding sand is screeded using screed bars and a strike board to 1 in. (25 mm) thickness. Mechanical screeders may be used on large deck jobs as shown in Figure 19. This shows 40,000 sf (3,715 m2) of pavers on a concrete parking deck next to a condominium housing project. Once the bedding sand is screeded, the pavers are compacted into the bedding sand. Sand is spread, swept and vibrated into the joints with at least two passes of a plate compactor. Excess sand is removed upon completion of compacting.

For larger than 12 in. x 12 in. (300 mm x 300 mm) slabs, bitumen or pedestals are recommended as the preferred setting methods rather than a sand bed. If placed on bedding sand, larger slabs tend to tip and tilt when loads are placed on their corners. Pedestals and bitumen are more stable assemblies for pedestrian applications. When compacting paving slabs with a plate compactor, using “add-on” rollers on this equipment should be considered to help eliminate risk of damage.

Some jobs may require slabs to completely cover the roof right up to the parapets and protruding vents. If full slabs do not fit next to vents and parapets, the slabs are saw cut and placed on pedestals next to them.

Mechanical installation—Roof decks can be built by mechanically placing the paving units. Figure 20 shows a parking deck being installed with mechanical equipment. Slabs can be installed with vacuum equipment that relies on suction to grab and place each unit. See Figure 21. For most jobs, these kinds of equipment can not run directly on the waterproofing. They must run over installed concrete pavers. Therefore, a starting area of pavers may need to be placed by hand and the equipment placed on it to continue the paving. Further information on mechanical installation is found in CMHA Tech Note PAV- TEC-011—Mechanical Installation of Interlocking Concrete Pavements. Regardless of the installation method, all federal, provincial, state and local worker safety rules should be followed for fall protection of crews working on roofs.

References

1. ASTM C936, Standard Specification for Solid Concrete Interlocking Paving Units, American Society for Testing and Materials, Vol. 04.05, Conshohocken, Pennsylvania, 2007.
2. CSA A231.2, Precast Concrete Pavers, Canadian Standards Association, Rexdale, Ontario, Canada, 2006.
3. CSA A231.1, Precast Concrete Paving Slabs, Canadian Standards Association, Rexdale, Ontario, Canada, 2006.
4. ASTM C1782, Standard Specification for Utility Segmental Concrete Paving Slabs, American Society for Testing and Materials, Vol. 04.05, Conshohocken, Pennsylvania, 2016.
5. ASTM C1491, Standard Specification for Concrete Roof Pavers, Vol. 04.05, American Society for Testing and Materials, Conshohocken, Pennsylvania, 2003.
6. MacElroy, William P. and Winterbottom, Daniel, “Up on a Pedestal,” Landscape Architecture magazine, American Society of Landscape Architects, Washington, D.C., January, 2000, pp. 66 – 80.
7. ASTM C33, Standard Specification for Concrete Aggregates, American Society for Testing and Materials, Vol. 04.02, Conshohocken, Pennsylvania, 2007.
8. CSA A23.1-2000, Concrete Materials and Methods of Concrete Construction, Canadian Standards Association, Rexdale, Ontario, Canada, 2004.
9. ASTM D448, Standard Classification of Sizes for Aggregates for Road and Bridge Construction, American Society for Testing and Materials, Vol. 04.03, Conshohocken, Pennsylvania, 2003.
10. ASTM D1621, Standard Test Method for Compressive Properties Of Rigid Cellular Plastics, American Society for Testing and Materials, Vol. 08.01, Conshohocken, Pennsylvania, 2004.
11. Gish, Laura E., Editor, Building Deck Waterproofing, STP 1084, American Society for Testing and Materials, Conshohocken, Pennsylvania, 1990.
12. Herbert, R. D., Roofing – Design Criteria, Options, Selection, R. S. Means Company, Inc. Kingston, Massachusetts, 1989.
13. ACI Manual of Concrete Practice, American Concrete Institute, Farmington, Michigan, 2008.
14. Cairns, John E., “Paving of Concrete Roof Decks,” in Proceedings of the Sixth International Conference on Concrete Block Paving, Japan Interlocking Concrete Block Pavement Engineering Association, Tokyo, Japan, 2000, pp. 419-426.
15. Loss Prevention Data for Roofing Contractors, Factory Mutual Engineering Corporation, 1151 Boston-Providence Turnpike, Norwood, Massachusetts 02062-9012, 2000.

Revised 2022

A mobile and ambulatory population requires reduction of pedestrian-related accidents. Snow melting systems for pavements can reduce accidents as well as liability exposure from injuries due to slipping on ice and snow. Moreover, snow melting systems reduce the fatigue and expense related to removing snow. In addition, they can reduce the damaging effects of freeze-thaw cycles, and of de-icing salts experienced by most pavements in cold climates. The inconvenience of spreading de-icing salts is eliminated and interior floor materials are kept cleaner and last longer.

Snow melting systems for interlocking concrete pavements can be used on patios, walkways, residential driveways, building entrances, sidewalks, crosswalks, and streets. A successful project in downtown Holland, Michigan includes a snow melting system in three blocks of concrete paver sidewalks and in the asphalt street (see Figure 1). Holland receives about 75 to 100 inches (190 to 250 cm) of snow each year. By melting the snow, the 167,000 ft2 (15,500 m2) heated pavements reduce pedestrian and vehicular accidents. They also reduce wear on the pavements because practically no de-icing salts are needed. Neither the merchants nor the city crews remove snow in this area of the business district, and the floors inside the stores are kept cleaner.

In addition to exterior applications, heating systems under concrete pavers have been used in interior areas such as around swimming pools, hot tubs, and saunas. The heat creates a comfortable, low-slip walking surface for bare feet and it also warms the room.

Types of Systems

Two kinds of systems are used to convey heat to the pavement surface: electric or liquid. Electric systems use wires to radiate heat. Generally, electric systems have a lower initial cost, but a substantial operating cost. They involve a series of control switches, thermostats, and snow-sensing apparatus. One electric system consists of heat tapes (flat wires) that automatically stop heating when sufficient energy is released. When they cool, the wires automatically allow more heat through them.

Liquid systems (also known as hydronic systems) use a mix of hot water and ethylene or propylene glycol mix in flexible pipes. They have a higher initial cost but a lower operating cost. Hot water systems consist of flexible pipes, pipe manifolds, pumps, switches, a water heater, thermostats, and snow sensors. They typically rely on a boiler that heats a building. Figure 2 illustrates the typical components of an interlocking concrete pavement with a snow melting system.

Snow melting systems generally do not completely dry the pavement surface. Rather, they melt the snow to water which drains away. Completely evaporating the water on the pavement surface is not economically practical since it requires more energy than for melting snow to water. Occasionally, snowfall or drifting may exceed the heat output of the snow melting system. While some snow remains, it will be easier to remove due to the warm pavement surface.

Snow melting systems can be part of new construction or added later. For driveways, pipes or wires can be placed in the wheel tracks to reduce installation costs. However, the remaining snow may require removal if it blocks the movement of vehicles.

The performance of a snow melting system is measured in inches (cm) of snow melted per hour. Its performance is based on heat output measured in BTUs (British Thermal Units) or watts per square meter (m2) of pavement. Performance depends on consideration of three overall design factors. First is the rate of snowfall. Second is the temperature of the snow influenced by the air temperature. About 90% of all snow falls between 35° F (2 C° ) and 10° F (-12° C). On average, snow falls at about 26° F (-3° C). The lower the air temperature, the less dense the snow. For warmer, wetter, and more dense snow, more energy per area of pavement is required to melt it. Third, wind conditions greatly influence performance of a snow melting system. Strong winds remove heat from a pavement faster than calm air. Location of buildings, walls, landscaping, and fences will influence the amount of wind across a pavement, heat loss, and ultimately the design and performance of snow melting systems.

Rate of snow melting will vary with the application. For example, “Melting 1 in. (25 mm) of snow per hour is usually acceptable for a residence but may be unacceptable for a sidewalk in front of a store. Hospital entrances and parking ramp inclines need to be free of snow and ice at all times”(1). Most manufacturers of liquid and electric snow melting systems also provide design guidelines and/or software to calculate the BTUs per square foot (watts/m2) required to melt a range of snow storms for a given region. The guidelines work through a series of calculations that consider the snow temperature (density), ambient temperature, exposure of the pavement to wind, and unusual site conditions. They provide recommendations on the size and spacing of pipes or wires required, as well as the temperature of the fluid, its rate of flow, or the electricity required. The Radiant Panel Association (radiantpanelassociation.org) provides design guidelines for liquid snow melt systems.

Controls for activating the snow melting system can include a thermostat in the bedding sand to maintain its temperature above freezing. Another kind of control is located near the pavement and activates the heating system when snow or ice falls. Sometimes a low level of heat is maintained in the pipes or wires and is increased by the sensor when snow falls.

Construction Guidelines

Snow melting systems with concrete pavers can be built with three types of bases: concrete, asphalt, or crushed stone aggregate. Concrete and asphalt bases are recommended for roads and crosswalks. While these bases may be used for driveways and pedestrian applications, a crushed stone aggregate base may be more cost-effective.

Aggregate Bases for Pedestrian and Driveway Applications

Subgrade Preparation—CMHA Tech Note PAV-TEC-002 Construction of Interlocking Concrete Pavements should be reviewed with this technical bulletin, as it offers guidelines for subgrade preparation, base materials, and installation of bedding sand and concrete pavers. Preparation and monitored compaction of the soil subgrade and the aggregate base are essential to long-term performance. The soil subgrade and base aggregate should be compacted to a minimum of 98% standard Proctor density, per ASTM D 698 (2). Geotextile is recommended over compacted clay soils and silty soils. The geotextile separates the aggregate from the soil, keeping the base consolidated through long-term changes in moisture and temperature, as well as freezing and thawing. Drain pipes may be required in slow draining soils, especially under vehicular applications.

Base materials and preparation—Recommended gradations for aggregate base materials are those typically used under asphalt pavements that meet standards published by the local, state, or provincial departments of transportation. If no standards exist, the gradation shown per ASTM D 2490, Standard Specification for Graded Aggregate Material for Bases or Subbases for Highways or Airports in Table 1 is recommended (3).

The minimum thickness of the base should be at least 6 in. (150 mm) for pedestrian areas and 10 in. (250 mm) for driveways. Thicker bases, or those stabilized with cement or asphalt, may be required in areas of weak soils subgrades (California Bearing Ratio < 4), in low-lying areas where the soil drains slowly, or in areas of extreme cold and frost penetration. The minimum surface tolerance of the compacted base should be ± 3/8 in. over a 10 ft (±10 mm over a 3 m) straightedge. Density and surface tolerances should be checked before proceeding with installation of the snow melting wires or pipes.

In some instances, rigid foam insulation may be required over the base. The insulation is placed under the bedding sand with wires or pipes in pedestrian applications only. Insulation is not recommended in vehicular applications due to a high risk of breaking as well as trapping moisture above it.

Insulation may be required on heated pavements over a high water table, when the heating system is operated manually, or when the perimeter of the heated area is large in relation to the total area, as with a long sidewalk. The manufacturer of the heating system should be consulted for specific guidance on insulation thickness, as well as when and where to use it.

Some contractors install the wires or pipe into the top of the base without wire mesh. This is accomplished by installing the pipes or wires in the last inch (25 mm) or so of compacted base surface. Base material is added and compacted to bring the level of the base to its final grade. The pipe or wire is exposed and flush with the compacted surface of the base. The absence of wire mesh will facilitate screeding of the bedding sand.

Asphalt and Concrete Bases for Vehicular Applications

For areas subject to constant vehicular traffic such as crosswalks or roads, wires or pipes should be placed in a concrete slab or in asphalt (rather than on top of these materials). This will protect the pipes or wires from damage due to wheel loads. Bedding sand and pavers are placed over them. Figure 5 illustrates a typical construction assembly.

Asphalt or concrete pavement materials and thicknesses should be designed to local standards. The manufacturer of the snow melting system can provide additional guidance on the location and detailing of wires or pipes in asphalt or concrete. Generally, they are placed within the concrete slab with a minimum 2 in. (50 mm) clearance from the top and bottom. For asphalt, the pipes are located at least 11/2 (40 mm) below the bottom of the asphalt layer. Asphalt has a lower heat transfer rate than concrete so asphalt may require more costly, closer spacing of the pipes or wires.

When using an asphalt or concrete base, drainage of excess water in the bedding sand is recommended. Drainage can be achieved by weep holes through the pavement and base at the lowest points. These holes should be 2 in. (50 mm) in diameter and covered with geotextile to prevent loss of bedding sand into them.

Layout of the Heating System

After receiving consultation and design recommendations from the manufacturer, the installation of wiring or pipe should be done by an electrical and/or plumbing contractor experienced with installing these systems (Figure 6). The installed system should be tested for leaks before placing sand or pavers over it. Liquid systems should have their pipes filled and placed under pressure prior to placing asphalt or concrete over them.

The wires are generally no thicker than 3/4 in. (19 mm). Pipes can vary in diameter from 1/2 in. (13 mm) to 1 in. (25 mm) depending on the area to be heated and system flow requirements. Reference 7 provides design and installation guidelines for hydronic pipe and electric wire systems.

Bedding sand—Normally, a consistently thick layer between 1 to 11/2 in. (25 to 40 mm) is recommended under concrete pavers. With snow melt systems, up to 2 in. (50 mm) (before compaction) of bedding sand may be required to cover and protect the wires or pipes. The gradation of the bedding sand should conform to ASTM C 33 (3) or CSA A23.1 (5) as shown in Table 2. Limestone screenings or stone dust should not be used as they often have material passing smaller than the No. 200 (0.075 mm) sieve. This fine material slows the drainage of the bedding sand layer. The bedding should be moist when screeded but not saturated. Screed bars (for screeding bedding sand) will need to be carefully placed so as to not disturb or damage the pipe or wires during screeding of the bedding sand. (See Figure 7.)

All pavers should be compacted, their joints filled with sand and compacted again at the end of each day. If the paver installation takes more than one day, the screeded bedding sand should not extend more than a few feet (1 m) beyond the edge of the open pattern at the end of each day. If there is a chance of rain, this area should be covered with a waterproof covering to prevent the bedding sand from becoming saturated. If the bedding sand is exposed to rain, it will become saturated and will have to be replaced or left to dry for many days. Saturated bedding sand is impossible to compact effectively and often requires removal. This will be very difficult and time-consuming since the pipes or wires will slow bedding sand removal considerably.

Concrete pavers—Concrete pavers should meet the requirements for strength and durability in ASTM C 936 (6) or CSA A231.2 (8). For pedestrian and residential driveway applications, 23/8 in. (60 mm) thick pavers are recommended, and 31/8 in. (80 mm) thick for vehicular uses. Once the bedding sand is screeded smooth, place the pavers in the prescribed laying pattern. All pavers should be constrained by edge restraints. CMHA Tech Note PAV-TEC-003–Edge Restraints for Interlocking Concrete Pavements offers guidance on the selection and application of edge restraints for all applications.

The concrete pavers should be compacted into the bedding sand with a 75–100 Hz plate compactor having a minimum centrifugal compactive force of 5,000 lbf (22 kN). Bedding sand is then spread across the surface of the pavers. A finer sand may be used to fill the joints that conforms to the grading requirements of ASTM C 144 (9) or CSA A179 (10). In either case, the joint sand should be dry so that it easily enters the joints between the pavers.

The concrete pavers are then compacted again and sand swept into the joints between them until they are completely full. Figures 8 and 9 show spreading the joint sand and the final compaction of the pavers. Excess sand is removed. Check with the manufacturers of snow melting systems to see if cleaners and sealers can be applied with no adverse effects to the pipes or wires. For additional guidance on the selection of cleaners and sealers, see CMHA Tech Note PAV-TEC-005–Cleaning and Sealing Interlocking Concrete Pavements—A Maintenance Guide. The minimum recommended slope of the finished pavement surface should be 2%. Water should not drain onto other pavements where it might collect and freeze.

Maintenance of Interlocking Concrete Pavement

Occasionally interlocking concrete pavements will require maintenance for them to deliver peak performance. Refer to CMHA Tech Note PAV-TEC-006–Operation and Maintenance Guide for Interlocking Concrete Pavement for information on preventative maintenance, identifying and remedying aesthetic and structural distresses and best practices for the disassembly and reinstatement of interlocking concrete pavement.

Snowmelt Systems With Permeable Interlocking Concrete Pavements

Hydronic snowmelt systems have been installed within the permeable aggregate bedding layer within permeable interlocking concrete pavements (PICP) for pedestrian applications. Compared to pipes in sand bedding, the pipe spacing will be reduced to account for heat loss to the air voids within the permeable aggregate bedding. For residential driveway applications, the pipe manufacturer should be consulted on durability of pipe material against the bedding aggregate while subject to vehicular tire loads. Figures 10 and 11 illustrate an electric snow melt system installed in the opengraded stone bedding layer of a PICP driveway.

References

1. Snow Melting Calculation and Installation Guide, Hyrdronic Institute of the Gas Appliance Manufacturers Association, Publication S-40, Radiant Panel Association, Hyrum, Utah.
2. Annual Book of ASTM Standards, ASTM D 698 – Standard Test Method for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft3 (600 kN-m/ m3)), Vol. 04.08, American Society for Testing and Materials, Conshohocken, Pennsylvania, 2013.
3. Annual Book of ASTM Standards, ASTM 2940 – Standard Specification for Graded Aggregate Material for Bases or Subbases for Highways and Airports, Vol. 04.03, American Society for Testing and Materials, Conshohocken, Pennsylvania, 1995.
4. Annual Book of ASTM Standards, ASTM C 33 – Standard Specification for Concrete Aggregates, Vol. 04.02, American Society for Testing and Materials, Conshohocken, Pennsylvania, 1996.
5. CSA-A23.1, Concrete Materials and Methods of Concrete Construction, Canadian Standards Association, Rexdale, Ontario, 1994, Clause 5.3.2.1.
6. Annual Book of ASTM Standards, ASTM C 936 – Standard Specification for Solid Concrete Interlocking Paving Units, Vol. 04.05, American Society for Testing and Materials, Conshohocken, Pennsylvania, 2013.
7. Guidelines for the Design and Installation of Radiant Panel Heating and Snow/Ice Melting Systems, Radiant Panel Association, Loveland, Colorado, 2007 edition.
8. CSA-A231.2, Precast Concrete Pavers, Canadian Standards Association, Rexdale, Ontario, 2013.
9. Annual Book of ASTM Standards, ASTM C 144 – Standard Specification for Aggregate for Masonry Mortar, Vol. 04.05, American Society for Testing and Materials, Conshohocken, Pennsylvania, 2013.
10. CSA A179, Mortar and Grout for Unit Masonry, Canadian Standards Association, Rexdale, Ontario, 2009.