Resources

Articulating Concrete Block for Erosion Control

August 2018

INTRODUCTION

An articulating concrete block (ACB) system is a matrix of individual concrete blocks placed together to form an erosion-resistant overlay with specific hydraulic performance characteristics. The system includes a filter layer underlay that allows infiltration and exfiltration to occur while providing particle retention of the soil subgrade. The filter layer may be comprised of a geotextile or properly graded aggregate or both. The blocks within the matrix must be dense and durable while providing a matrix that is flexible and porous.

Articulating concrete block systems are used to provide protection to underlying soil materials. The term “articulating” implies the ability of individual blocks of the system to conform to changes in subgrade while remaining interlocked or otherwise restrained by virtue of the block geometric interlock and/or additional system components such as cables, ropes, geotextiles, or geogrids. The interlocking property provided by the special shapes of ACBs also allows for expansion and contraction. Long-term durability and sustainability relies on an appropriate engineered design based on site-specific hydrological and geotechnical conditions. They are either hand-placed or installed as pre-assembled mats on top of a filter layer on prepared subgrade, and act as a soil revetment.

Articulating concrete blocks (ACBs) are an effective erosion control system used to solve a wide variety of erosion problems:

drainage channels
wildlife habitat
river fronts
bridge abutments/piers
coastal shorelines
dikes and levees
pipeline protection
spillways
boat ramps
retention basins
lake shorelines
overflow channels
low water crossings
dam overtopping

The systems are easy to install, simple to produce, and environmentally friendly. ACB systems are often used as an alternative to cast-in-place concrete bulkheads and slope paving, gabions, soil cement, roller compacted concrete, or rock riprap. ACBs can also be used as grid pavers. However, grid pavers manufactured according to ASTM C1319, Standard Specification for Concrete Grid Paving Units (ref. 7), are not considered ACBs.

Articulating concrete blocks are produced in accordance with ASTM D6684, Standard Specification for Materials and Manufacture of Articulating Concrete Block (ACB) Revetment Systems (ref. 3). They can be made in a variety of shapes and thicknesses, and may even be colored according to preference. ACBs have excellent resistance to hydraulic shear and overtopping conditions. Design resources for ACBs include the Design Manual for Articulating Concrete Block and ACB Design Spreadsheet (refs. 2, 8).

Stream Channelization, Before and After ACB Installation

ENVIRONMENTAL BENEFITS

One of the environmental benefits of ACB erosion systems is that vertical cores and spaces can be incorporated throughout the system, which allows vegetation growth. Properly selected plant species can almost completely cover the entire hard surface of the ACBs, allowing them to blend in with the natural look of the project as well as water purification by absorbing nutrients and breaking down other pollutants. During peak storm events, the ACB layer beneath the vegetation will protect the soil from erosion. The ability to support the ecosystem’s habitat is a major advantage of ACB systems over other erosion control methods. Additional advantages of ACB systems are:

serviceability
aesthetics
pedestrian safety
sustainability
cost effectiveness
flexibility

The permeability of ACBs allows their use to preserve natural drainage and treatment systems. ACBs installed on filter media are pervious surfaces that reduce water runoff and flooding risks, improve water quality, reduce pollutants, recharge aquifers and prevent erosion. These environmental characteristics permit the use of ACBs on sustainable developments to preserve or improve existing sites and can be applicable to credits in some green building rating systems, such as LEED^TM, which is discussed in more detail below.

ACBs installed over drainage layers can also be used in a Best Management Practices (BMP) plan to preserve or improve existing sites, or in new developments. The system installed over a drainage layer preserves the natural drainage and treatment systems of the soil, reducing water runoff and flooding risks, improving water quality, reducing pollutants, recharging aquifers, and preventing erosion. Research on permeable pavers that are installed in a similar manner to ACBs (block over a gravel drainage layer) has also shown a promotion of aerobic biodegradation of hydrocarbons and reduction of nutrients (refs. 11, 12). When vegetated, they can also generate habitat. These qualities make ACBs a great solution for use in sustainable projects where water quantity and quality control are extremely important.

LEED Certification

ACBs can also make a significant contribution to project certification under Leader in Energy and Environmental Design (LEED) registered projects. Under LEED v3 for New Construction (ref. 9), ACB products can help attain points toward certification in the following categories: Sustainable Sites, Materials & Resources, and potentially Innovation in Design and Regional Priority Credits. These are briefly discussed below, based on LEED v3 for New Construction. Other versions of LEED may have different credit numbers, or slightly different criteria. For more detailed information on ACB’s potential contributions towards LEED credits, see TEK 06-09C, Concrete Masonry and Hardscape Products in LEED 2009 (ref. 10).

Sustainable Sites: ACBs can help capture up to 5 credits of the 26 possible in the following areas:

Prerequisite—The project is required to reduce the pollution generated during construction. ACBs can contribute to preventing erosion, reducing dust when working as drivable surfaces, and constructing sediment basins with the added support for vegetation growth that increases water quality.
Credit 5.1, Protect or Restore Habitat—ACBs can be used to restore erosion-prone areas by retaining soil and promoting plant growth.
Credit 5.2, Maximize Open Spaces—Ponds constructed with ACBs may count as open space as long as they are vegetated and meet the maximum 1:4 (vertical : horizontal) side slope limit.
Credit 6.1, Storm Design: Quantity Control—Using ACBs can reduce a site’s impermeable hardscaping for slope stability and erosion control, allowing stormwater to percolate into the ground and reducing runoff. In addition, receiving channels protected with ACBs can reduce erosion, reduce impervious surfaces and, when planted, contribute to the overall aesthetics and ecology.
Credit 6.2, Storm Design: Quality Control—ACBs can also be used to reduce the amount of stormwater that requires treatment. As discussed above under Environmental Benefits, ACBs on gravel filters have been shown to improve runoff quality.
Credit 7.1, Heat Island Effect: Non Roof—Open grid ACBs (at least 50% pervious) or with a Solar Reflectance Index (SRI) of at least 29 can be used in lieu of traditional concrete or asphalt to reduce the urban heat island effect.

Materials and Resources: ACBs can contribute towards 8 of the 14 possible credits in the following areas:

Credit 2, Construction Waste Management— Damaged concrete products can be redirected to the manufacturing process for recycling and unused products can be reused on another project.
Credit 3, Material Reuse—ACBs can potentially be salvaged from one project and reused in another.
Credit 4, Recycled Content—Concrete products, including ACBs, can be manufactured using recycled materials that have been diverted from the waste stream.
Credit 5, Regional Materials—Concrete hardscape products are usually available close to the point-of-use and are manufactured with local materials.

Innovation in Design: Projects can earn Innovation in Design credits by exceeding the LEED credit requirements, or by addressing innovative environmental strategies not specifically addressed in LEED, such as the life cycle environmental impact of the materials used.

Regional Priority Credits: Up to 4 LEED credits can be earned through Regional Priority Credits, which are designed to encourage designers to focus on specific regional needs. The credits are classified by zip code and designers have the option to excel in aspects of the project that are priorities in their region. ACBs could contribute to attain higher percentages on all the credits mentioned above to qualify for extra points.

ACB UNITS

ACB units complying with ASTM D6684, Standard Specification for Materials and Manufacture of Articulating Concrete Block (ACB) Revetment Systems (ref. 3), are produced as dry-cast (in a block machine) or wet-cast (with concrete and molds). Sampling and testing of dry-cast ACB units are performed in accordance with ASTM C140, Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units (ref. 6), for conformance with the requirements in Table 1. Sampling and testing of wet-cast units is performed in accordance with ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens and ASTM C42, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete (refs. 4, 5).

Several varieties of ACB systems are available: interlocking, cable-tied and non-cable-tied matrices, and open cell and closed cell varieties. Open cell units contain open voids within individual units that facilitate the placement of aggregate and/or vegetated soil. Closed cell units are solid concrete elements that are capable of allowing vegetation growth between adjacent units. Figure 1 shows a variety of ACB units in plan view.

NOTE – For units produced by the wet-cast method, tests shall be conducted in accordance with Test Methods ASTM C39 and C42. For units produced by a dry-cast method, tests shall be conducted in accordance with Test Method ASTM C140.

Figure 1—Examples of Proprietary ACB Systems Shown in Plan View

(Note that this is not all inclusive of available configurations. No endorsement or recommendation is intended.)

ACB INSTALLATION

Articulating concrete blocks can be installed by small construction crews with a modest amount of equipment. Installations are fast and are easy to inspect due to the visibility of all components.

ACBs are installed on top of a filter layer over a prepared subgrade. The filter acts to protect and hold the subgrade in place while allowing bidirectional water penetration. Final subgrade elevation should be 0 to + ½ in. (12.7 mm) under a 10 ft. (3.05 m) straight edge. After the ACB installation is complete, the open cell voids or joints between the ACB units are filled with granular material or soil. Unit to unit vertical offset should be limited to the value utilized in the design. If vegetation is required, hydraulic seeding or mulching provides a low cost and highly effective method of establishing commonly used grasses and plants. In applications subject to continually flowing water, solid units should be used below the normal waterline or the voids of hollow units should be filled with gravel. Figures 2 through 4 show typical ACB cross-sections.

Installation methods depend on whether the ACB product being used is classified as cabled or as non-cabled.

Figure 2—ACB Trapezoidal Channel Lining, Typical Cross-Section

Figure 3—ACB Embankment, Typical Cross-Section

Figure 4—ACB Boat Ramp or Creek Crossing, Typical Cross-Section

Cabled Articulating Concrete Mats

Cabled interlocking blocks have preformed horizontal holes cast in them so that high-strength cables or ropes (synthetic or steel) can be installed through the matrix of blocks binding them into a monolithic mattress. The cables or ropes are used to facilitate placement of the mat, normally by a spreader bar and crane, as shown in Figures 5 and 6. For design purposes, cables offer no hydraulic stability or structural value to the ACB mat or block system.

The blocks are pre-assembled into cabled mats in a controlled environment on or off the job site. The mats are lifted by the cables’ end loops, placed on the back of a flatbed truck, and shipped to the job site where they are again lifted and placed on the prepared slope. The mats can also be fabricated on site near the prescribed area, eliminating the need for additional trucking. Assembly of the blocks into mats makes it possible to install these systems underwater and on steep slopes.

Several commercially available cable systems can be manually placed on a shoreline or channel, and the cables then hand-inserted through the holes. An advantage of this process is that revetments with fewer seams can be constructed. Also, it is easier to hand-place the blocks around a radius or projection on a sloped bank, such as a culvert outlet. To accomplish this with pre-assembled ACB mats, custom-shaped mats must be fabricated or field piece work insertions are required. Both require careful planning, detailing, and execution in the field.

Figure 5—Placement of Cable ACB System With Crane

Figure 6—Cabled ACB Installation Behind a Sound Barrier

Non-Cabled Articulating Concrete Mats

Non-cabled blocks are cast in interlocking shapes to provide a positively connected matrix that is individually hand-placed by semi-skilled labor. The blocks are individually installed according to the geometry of the product. The typical block layout is oriented in the application’s centerline position. These blocks make it possible to install ACBs in areas that are restrictive to large construction equipment.

ACBs without cables can be suited to projects where complex site geometries and limited access are present. These non-cabled ACBs offer comparable shear resistance and some systems offer a higher percentage of open surface area when desired. These blocks are typically delivered in palletized cubes and are hand-placed at the job site. Since there are no cables involved, the material costs can be considerably less. ACBs without cables can be constructed in virtually seamless fields.

DESIGN CONSIDERATIONS

Existing and proposed project characteristics combined with the hydraulic design objectives will determine, through a factor of safety analysis, the ACB product that will best serve the project’s erosion control needs. A 1.5 factor of safety is common.

Guidelines for the selection and design of the appropriate ACB product are provided by the Design Manual for Articulating Concrete Block (ref. 2) and TEK 11-12A, Articulating Concrete Block Revetment Design—Factor of Safety Method (ref. 1).

The hydraulic forces which are considered in the design of erosion control projects using this technology are hydraulic lift, drag, and impact. ACBs have demonstrated the ability to resist high velocity flows in excess of 25 ft/s (7.6 m/s), usually associated with drainage channels, water control structures, dam spillways, and fast flowing rivers. While many of the forces created by water can be readily calculated, particularly uplift and drag, each project application has separate design considerations.

ACBs are not designed to add structural strength to the slopes. The protected slope must be geotechnically stable prior to placement of surface protection. Known as “flexible revetments,” ACB installations should not be placed on slopes which are steeper than the natural angle of repose of the soil. This is a different technology than retaining walls which resist lateral earth pressures.

Protruding height variances between adjacent blocks should be minimized and must be in accordance with the design value utilized. Grading beneath the block and fabric is critical to establishing an acceptable finished profile of the ACBs.

Filter layers are always placed under the ACBs. The function of the filter is critical, as it must retain the soil in place while letting water pass through without clogging. The filter layer must remain in intimate contact with the block and the soil to preclude soil particles from being transported down the slope beneath the geotextile. The key design points to consider in the filter selection are:

Only woven monofilament or nonwoven needle-punched geotextiles should be considered for filter applications.
The filter layer must have a long-term permeability capable of handling the required volume of water through a restricted surface area equal to the joint area of the articulating concrete block.
The permeability of the filter layer must always be equal to or greater than the permeability of the protected soil unless a special bedding layer is provided.
The filter layer needs only to retain the majority of particles beneath it, thus creating a filter bridge.

Care should be taken in the specification of the underlying geotextile filter layer. Appropriate ASTM test methods are available to characterize soil properties and should be done to develop the retention criteria and proper permeability. The geotextile must possess adequate strength and endurance properties to survive the process of installation and any long-term forces applied to it. ASTM D6684, Standard Specification for Materials and Manufacture of Articulating Concrete Block (ACB) Revetment Systems (ref. 3), provides strength requirements for geotextiles.

In most of these designs, the owner or architect should rely upon the engineering expertise of a qualified engineer to select the appropriate block type, filter layer, soil compaction, and design parameters. Proper material selection and construction practices are required and should be checked during installation.

ACBs for Shoreline Protection Against 2- to 3-Foot (0.61 – 0.91 m) Wave Attack

REFERENCES

Articulating Concrete Block Revetment Design—Factor of Safety Method, ACB-TEC-002-11, Concrete Masonry & Hardscapes Association, 2002
Design Manual for Articulating Concrete Block, ACB- MAN-001-20, Concrete Masonry & Hardscapes Association, 2020.
Standard Specification for Materials and Manufacture of Articulating Concrete Block (ACB) Revetment Systems, ASTM D6684. ASTM International, 2018.
Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, ASTM C39/C39M. ASTM International, 2021.
Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, ASTM C42/42M. ASTM International, 2020.
Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140. ASTM International, 2023A.
Standard Specification for Concrete Grid Paving Units, ASTM C1319. ASTM International, 2021.
ACB Design Spreadsheet
Leadership in Energy and Environmental Design (LEED) Green Building Rating System, v3. United States Green Building Council, 2009.
Concrete Masonry and Hardscape Products in LEED 2009, TEK 06-09C, Concrete Masonry & Hardscapes Association, 2009.
Hunt, W.F., Collins, K.A., Hathaway, J.M. Hydrologic and Water Quality Evaluation of Four Permeable Pavements in North Carolina, USA. Proceedings of the 9th International Conference on Concrete Block Paving. Buenos Aires, Argentina, 2009.
Kirkpatrick, R., Campbell, R, Smyth, J., Murtagh, J., Knapton, J. Improvement Of Water Quality By Coarse Graded Aggregates In Permeable Pavements. Proceedings of the 9th International Conference on Concrete Block Paving. Buenos Aires, Argentina, 2009.

Segmental Retaining Wall Units

August 2018

INTRODUCTION

Mortarless segmental retaining walls are a natural enhancement to a variety of landscape projects. Applications range from 8 in. (204 mm) high terraces for erosion control to retaining walls 20 ft (6.1 m) or more in height. The individual concrete units can be installed to virtually any straight or curved plan imaginable.

Segmental retaining walls are used to stabilize cuts and fills adjacent to highways, driveways, buildings, patios and parking lots, and numerous other applications. Segmental retaining walls replace treated wood, cast-in-place concrete, steel, and other retaining wall systems because they are durable, easier and quicker to install, and blend naturally with the surrounding environment. Concrete units resist deterioration when exposed to the elements without the addition of toxic additives which can threaten the environment.

A variety of surface textures and features are available, including split faced, stone faced, and molded face units, any one of which may be scored, ribbed, or colored to fit any project application. Construction of segmental retaining walls does not require heavy equipment access, nor does the system require special construction skills to erect. Manufactured concrete retaining wall units generally weigh 30 to 100 lb (14 to 45 kg) each and are placed by hand on a level or sloped gravel bed.

Successive courses are stacked dry on the course below in the architectural pattern desired. Mechanical interlocking and/or frictional shear strength between courses resists lateral soil pressure. In low-height walls, overturning forces due to soil pressure are resisted by the weight of the units, sometimes aided by an incline toward the retained soil. Higher walls resist lateral soil pressures by inclining the wall toward the retained earth, or by other methods such as anchoring to geosynthetic reinforcement embedded in the soil. Further information on the design of segmental retaining walls can be found in Design Manual for Segmental Retaining Walls (ref. 1).

Segmental retaining wall units are factory-manufactured to quality standards in accordance with ASTM C1372, Standard Specification for Segmental Retaining Wall Units (ref. 2). These requirements are intended to assure lasting performance, little or no maintenance, structural integrity, and continued aesthetic value.

Segmental retaining wall units complying with the requirements of ASTM C1372 have been successfully used and have demonstrated good field performance. Segmental retaining wall units currently being supplied to the market should be produced in accordance with this standard so that both the purchaser and the supplier have the assurance and understanding of the expected level of performance of the product.

ASTM C1372 covers both solid and hollow units which are to be installed without mortar (dry-stacked). Units are designed to interlock between courses or to use mechanical devices to resist sliding due to lateral soil pressure. If particular features are desired, such as a specific weight classification, higher compressive strength, surface texture, finish, color, or other special features, they should be specified separately by the purchaser. However, local suppliers should be consulted as to the availability of units with such features before specifying them.

Figure 1—Examples of Segmental Retaining Wall Installations

Materials

ASTM C1372 includes requirements that define acceptable cementitious materials, aggregates, and other constituents used in the manufacture of concrete segmental retaining wall units. These requirements are similar to those included in ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units (ref. 3).

Compressive Strength

Minimum compressive strength requirements for segmental retaining wall units are included in Table 1. Units meeting or exceeding these strengths have demonstrated the integrity needed to resist the structural demands placed on them in normal usage. These demands include impact and vibration during transportation, the weight of the units above them in the wall, nonuniform distribution of loads between units, and the tensile stresses imposed as a result of typical wall settlement.

Segmental retaining wall units will not fail in service due to compressive forces since axial loads are only a result of self-weight. Due to the direct relationship between compressive strength and tensile strength, this minimum requirement is used to ensure overall performance.

Compressive strength testing of full size units is impractical due to the large size and/or unusual shape of some segmental retaining wall units. Therefore, compressive strength of these units is determined from testing coupons cut from the units. The results of tests on these smaller coupons will typically yield lower strengths than if the larger, full-size specimen were tested. The reason for the difference is size and aspect ratio. However, it is important to keep in mind that the compression test is not intended to determine the load-carrying capacity of the unit, since segmental retaining walls are not designed to carry vertical structural loads. Compressive strength is used solely to assess the quality of the concrete.

Because tested strengths are affected by size and shape of the specimen tested, it is important that all retaining wall units be tested using a similar size and shape. ASTM C140/ C140M, Standard Method for Sampling and Testing Concrete Masonry Units and Related Units (ref. 4) requires that specimens cut from full-size units for compression testing must be a coupon with a height to thickness ratio of 2 to 1 before capping and a length to thickness ratio of 4 to 1. The coupon width is to be as close to 2 in. (51 mm) as possible based on the configuration of the unit and the capacity of the testing machine, but not less than 1.5 in. (38 mm). The preferred size is 2 x 4 x 8 in. (51 x 102 x 203 mm) (width x height x length). The coupon height is to be in the same direction as the unit height dimension. If these procedures are followed, the compressive strength of the coupon is considered the strength of the whole unit.

Alignment of the specimen in the compression machine is critical. Care should be taken in capping the test specimen to assure that capping surfaces are perpendicular to the vertical axis of the specimen. Capping needs to be performed in accordance with ASTM C1552, Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing (ref. 5).

Saw-cutting is the required method of extracting a test specimen from a full-size unit. Proper equipment and procedures are essential to prevent damaging the test specimen as a result of saw-cutting. Water-cooled, diamond-tipped blades on a masonry table saw are recommended. The blade should ideally have a diameter sufficient enough to make all cuts in a single pass. Manufacturers of the unit (or licensors of proprietary shapes) should be consulted about recommended locations for obtaining the compression specimen.

^a Based on oven-dry density of concrete.

Weight Classification

Weight classifications for segmental retaining wall units are defined in Table 1. The three classifications, lightweight, medium weight, and normal weight, are a function of the oven dry density of the concrete. Most segmental retaining wall units fall into the normal weight category.

Absorption

Absorption requirements are also included in Table 1. This value is used to represent the volume of voids in a concrete masonry unit, including voids inside the aggregate itself. The void space is measured by determining the volume of water that can be forced into the unit under the nominal head pressure that results from immersion in a tank of water.

Lightweight aggregates used in the production of lightweight and medium weight units contain voids within the aggregate itself that also fill with water during the immersion test. While reduced voids indicate a desired tightly compacted unit, tightly compacted lightweight and medium weight units will still have higher absorption due to the voids in the aggregates. For this reason the maximum allowable absorption requirements vary according to weight classification.

Similar to compression testing, it generally is not practical to test full-size retaining wall units in absorption tests due to their size and weight. Therefore, ASTM C140/C140M permits the testing of segments saw-cut from full-size units to determine absorption and density. When reduced-size units are used for absorption testing, the reduced-size specimen must have an initial weight of at least 20% of the full-size unit weight. This is intended to ensure that a sufficiently sized specimen is tested in order for the results to be representative of the entire unit.

Absorption limits are typically expressed as mass (weight) of water absorbed per concrete unit volume. This is preferred to expressing by percentage which permits a denser unit to absorb more water than a lighter weight unit.

Testing larger specimens requires particular attention to drying times, because it takes a greater length of time to remove all moisture from larger masses. ASTM C140/C140M requires that specimens be dried for a period of not less than 24 hours at a temperature of at least 221°F (105°C). The 24-hour time period does not start until the oven reaches the specified temperature. When placing larger specimens in an oven, it may take several hours for the oven to reach the prescribed temperature. ASTM C140/C140M then requires that specimen weights be determined every two hours to make sure that the unit is not still losing water weight (maximum weight loss in two hours must be less than 0.2% of the previous specimen weight). This will require 48 hours or more for some specimens. If not adequately dried, reported absorptions will be lower than the actual value.

Permissible Variations in Dimensions

Mortarless systems require consistent unit heights to maintain vertical alignment and level of the wall. For this reason, permissible variation in dimensions is limited to ±⅛ in. (3.2 mm) from the specified standard dimensions. Regarding dimensions, “width” refers to the horizontal dimension of the unit measured perpendicular to the face of the wall. “Height” refers to the vertical dimension of the unit as placed in the wall. “Length” refers to the horizontal dimension of the unit measured parallel to the running length of the wall.

Dimensional tolerance requirements for width are waived for split faced and other architectural surfaces. The surface is intended to be rough to satisfy the architectural features desired and cannot be held to a specific tolerance.

Finish and Appearance

Minor cracks incidental to the usual method of manufacture or minor chipping resulting from customary methods of handling in shipment and delivery are not grounds for rejection. Units used in exposed wall construction are not to show chips or cracks or other imperfections in the exposed face when viewed from a distance of not less that 20 ft (6.1 m) under diffused lighting. In addition, up to five percent of a shipment are permitted to: contain chips on the finished face not larger than 1 in. (25.4 mm) in any dimension; contain cracks on the finished face wider than 0.02 in. (0.5 mm) and longer than 25% of the nominal height of the unit; have dimensions outside the permissible dimensional variations; or be broken.

Freeze-Thaw Durability

Segmental retaining wall units may be used in aggressive freezing and thawing environments. Freeze-thaw damage can occur when units are saturated with water and then undergo temperature cycles that range from above to below the freezing point of water. Freezing and thawing cycles and a constant source of moisture must both be present for potential damage to occur.

Many variations can exist in exposure conditions, any of which may affect the freeze-thaw durability performance of the units. Such variations include: maximum and minimum temperatures, rate of temperature change, duration of temperatures, sunlight exposure, directional facing, source and amount of moisture, chemical exposure, deicing material exposure, and others.

When units are used in applications where freezing and thawing under saturated conditions can occur, ASTM C1372 includes three different methods of satisfying freeze-thaw durability requirements:

Proven field performance,
Five specimens shall have less than 1% weight loss after 100 cycles in water using ASTM C1262 (ref. 6), or
Four of five specimens shall have less than 1.5% weight loss after 150 cycles in water using ASTM C1262.

Segmental retaining wall units in many areas of the country are not exposed to severe exposures. Therefore, the requirements above apply only to “areas where repeated freezing and thawing under saturated conditions occur.”

Freeze-thaw durability tests are conducted in accordance with ASTM C1262, Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry-Cast Segmental Retaining Wall Units and Related Concrete Units, (ref. 6) using water or saline as the test solution. For most applications, tests in water are considered sufficient. If the units will be exposed to deicing salts on a regular basis, consideration should be given to performing the tests in saline. However, no pass/fail criteria has been adopted by ASTM for saline testing.

Compliance

ASTM C1372 also provides guidance regarding compliance. If a sample fails, the manufacturer can remove or cull units from the shipment. Then a new sample is selected by the purchaser from the remaining units of the shipment and tested, which is typically paid for by the manufacturer. If the second sample passes, then the remaining units of the lot being sampled are accepted for use in the project. If the second sample fails; however, the entire lot represented by the sample is rejected.

The specification also provides guidance on responsibility for paying for the tests. Unless otherwise provided for in the contract, the purchaser typically pays for the testing if the units pass the test. However, if the units fail the test, the seller bears the cost of the testing. See SRW-TEC-007-15 Sampling and Testing Segmental Retaining Wall Units (ref. 7) for more detailed information on SRW unit sampling, testing, and acceptance.

REFERENCES

Design Manual for Segmental Retaining Walls, 3rd edition, SRW-MAN-001-10, Concrete Masonry & Hardscapes Association, 2010.
Standard Specification for Dry Cast Segmental Retaining Wall Units, ASTM C1372-14. ASTM International, 2014.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-14. ASTM International, 2014.
Standard Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M-14a. ASTM International, 2014.
Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing, ASTM C1552-14. ASTM International, 2014.
Standard Test Method for Evaluating the Freeze-Thaw Durability of Dry-Cast Segmental Retaining Wall Units and Related Concrete Units, ASTM C1262-10. ASTM International, 2010.
Sampling and Testing Segmental Retaining Wall Units, SRW-TEC-007-15, Concrete Masonry & Hardscapes Association, 2015.