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Characteristics of Concrete Masonry Units With Integral Water Repellent

  • August 2018

INTRODUCTION

A concrete masonry unit’s characteristics are a function of the properties and proportions of the materials used, as well as the manufacturing processes. The unit characteristics do not singularly define the characteristics and performance attributes of a concrete masonry wall, but they certainly play a significant role in influencing those attributes. When used as part of a breathable exterior wall for an inhabited structure, or as a barrier for any conditioned or protected space, concrete masonry is expected to contribute to the water penetration resistance and moisture control of the wall assembly. Current model building codes include provisions intended to ensure that exterior walls provide adequate weather protection for the building (ref. 1).

Design of concrete masonry walls to mitigate or control moisture migration includes many considerations beyond the characteristics of the concrete masonry unit, such as flashing, weeps, workmanship, mortar or grout characteristics, vents, coatings, vapor barriers, air barriers, temperature differences, and accommodation of differential movement, plumbing and roof leaks, as well as other considerations. The potential for condensation, whether at the wall’s interior surface, weather-exposed surface, and/or interior of the wall, should also be considered. Proper design and construction of concrete masonry, considering all of these elements, is critical to the water resistant performance of the wall system. These topics are addressed in References 2 through 7 and in other literature sources.

Mortar joints are especially critical to a wall’s water penetration resistance. Achieving good bond between the mortar and the unit surfaces is essential and is largely influenced by the mortar material itself, tooling procedures, and joint profile as well as by the configuration of the concrete masonry unit. Ribbed units, for example, make it difficult to adequately tool the mortar joints. Reducing mortar’s absorption characteristic is also important for achieving success in moisture control in a concrete masonry wall. This can be achieved using integral water repellent admixtures in the preparation of the mortar.

While all of the aforementioned aspects significantly affect wall performance, this TEK focuses specifically on evaluating the water penetration resistance characteristics of concrete masonry units and their role in contributing to control of moisture in the wall.

THE ROLE OF CONCRETE MASONRY UNITS

The concrete masonry unit’s role and contribution to the concrete masonry wall assembly’s water penetration resistance depends in part on how the units are used in the design. The unit characteristic requirements for contributing to success of the exterior wall may vary depending on the design of the masonry wall in which it is used. For example, the role of concrete masonry units is more critical relative to moisture control when they are part of a weather-exposed surface or exterior wall assembly for a protected and conditioned building than if they are used as an interior wall.

There are three primary forces influencing moisture control of a concrete masonry wall: positive or negative air pressures created by the weather or building ventilation systems, internal moisture absorption and/or adsorption through the matrix of the concrete material, and condensation/evaporation. For the purposes of this discussion, absorption is considered to relate to the cementitious material’s attraction to or affinity for water at the molecular level. Generally speaking, mortar tends to have a much greater affinity for water than does a concrete masonry unit. Adsorption is the affinity of water at the individual surfaces of the cementitious materials. For instance, capillary pressure creates the tendency for water to migrate into a porous object along the surfaces of the interconnected voids, such as a sponge placed in very shallow water. The same tendency may be observed in a mortar joint or an untreated concrete masonry unit due to interconnected voids.

When units are used on a building exterior, it is desirable to limit moisture migration through the first barrier of defense at the wall surface. Wind driven rain can be a significant cause of water breaching a mortar joint, the front face shell of a single wythe wall, or a veneer unit. These weather-induced positive pressures can create a challenge to barrier defenses. As a driving force, they are highest at the surface of the masonry and rapidly diminish a few inches into the mortar joint, the unit, or into the cavity of a drainage wall.

Water repellency characteristics of concrete masonry units can be defined by their contribution to barrier defenses at the surface of the wall (which will help limit the effect of the positive pressure of wind driven rain), by their ability to limit the potential for absorbing and adsorbing moisture through their matrix, and by their contribution to controlling condensation.

PERTINENT UNIT CHARACTERISTICS

Barrier defenses in concrete masonry units can be provided at the surface as well as within the mass of the concrete layer. Surface protection can be enhanced by post-applied breathable materials, external coatings and wall coverings. When coatings are used, the most important characteristic of the unit may be its compatibility with the type of coating used. Some clear sealers and certain paints may not be suitable for a particular concrete masonry unit since some coatings may not be able to bridge open pores or fill all surface irregularities or textures. For example, the proper performance of stucco relies on a rougher and more open unit surface texture of the concrete masonry unit to ensure adequate mechanical bonding.

Beyond the unit’s exterior surface compatibility with the type of breathable post-applied material, coating or wall covering used, if any, an important consideration is the characteristics of the concrete used to produce the unit. The water penetration resistance of concrete is determined by the characteristics of the matrix and its resistance to absorbing moisture. The properties and proportions of the raw materials used to produce the units and the manufacturing procedures employed influence the water penetration resistance of those units. For example, a greater volume of interconnected voids within the unit may provide an easier path for moisture migration. Alternatively, reducing the volume of voids, such as by increasing the unit compaction, may limit moisture movement through the unit. Aggregate type and gradation, cement to aggregate ratio, mix water content, alkalinity, machine compaction, curing processes, and plasticizing and integral water repellent admixtures are some of the parameters that can have an influence on water repellency characteristics.

INTEGRAL WATER REPELLENTS

Integral water repellent admixtures can be used in the mix design of the concrete masonry unit during production to limit a unit’s tendency to absorb moisture through its matrix. Integral water repellent admixtures are usually polymeric products that utilize hydrophobic materials to significantly reduce the absorption characteristics of the concrete. Without these admixtures, even those units with excellent compaction will absorb some moisture through the concrete matrix. Integral water repellents significantly limit absorption by changing the chemistry of the matrix, which may include coating the pores in the concrete with a hydrophobic material that reduces the chemical affinity for water. Thus, concrete masonry units with integral water repellents are positioned to repel water rather than automatically allowing it to migrate through the unit. However, use of integral water repellent admixtures alone does not assure a water-resistant unit. Care must still be taken in production as discussed above to reduce the volume of interconnected voids that will permit moisture migration via other forces, such as wind or gravity.

An advantage of integral water repellent admixtures is that they remain a permanent part of the concrete matrix. Unlike post-applied products, integral water repellent treatments require less maintenance since they are more durable, and they are active throughout the whole concrete matrix and not just at the surface. In addition, integral water repellents can reduce efflorescence by reducing water migration through the concrete masonry (including latent water introduced to the system from grout or mortar).

When integral water repellents are used in concrete masonry units, it is important that the same or a compatible admixture be used in the mortar as well in accordance with manufacturer’s recommendations. Failure to use an integral water repellent admixture in the mortar may compromise the water repellency characteristics of the wall.

EVALUATING UNIT WATER REPELLENCY

The water repellency characteristics of a concrete masonry unit can be evaluated using simplistic field methods or more involved laboratory test methods. Three methods are described briefly below, and in more detail in the referenced published industry test methods (refs. 8, 9, 10).

All of these tests are suitable for evaluating units to be used in wall construction. It is important that field testing, if considered necessary, be conducted prior to wall construction since most of these tests can not be accurately performed on a constructed wall surface. For instance, small amounts of mortar left on the surface of a unit even after cleaning, as well as the cleaning techniques themselves, may alter the surface characteristics of the unit relative to its as-delivered condition. Similarly, water introduced into the system from grout or mortar (water of latency) and in turn absorbed into the unit may change the unit’s characteristics. Before, after, or during construction, accumulated dust or pollution may also alter the surface characteristics. When water repellency characteristics are evaluated prior to unit placement, any unexpected results from field testing can be addressed in a timely manner using the default laboratory test methods described below.

Water Bottle and Water Droplet Tests

The water bottle and water droplet test methods (ref. 8) can be effective as a first pass evaluations of water repellency. The water droplet method is typically conducted on individual units in a horizontal position as shown in Figure 1 (90 degrees to the “as laid” or construction orientation), but as a variation the water bottle test can also be conducted on units placed in a vertical (“as laid”) orientation. Typically, a concrete masonry unit manufactured with an integral water-repellent admixture will be able to support at least three out of the five water droplets for a period of five minutes or more.

At the immediate surface of the concrete masonry unit, the effectiveness of an integral water repellent may diminish over time due to exposure to elements such as dirt, contaminants and UV light. The water repellency characteristics of the concrete just below the surface, however, remain unchanged and provide continuing protection. Therefore, while the water droplet test is rather reliable for identifying a sufficient level of water repellency, it may not be a good indicator of poor water repellency. In other words, if a unit fails to support a droplet of water, the unit should not be considered inadequate, but rather should be taken to a laboratory for further testing using the spray bar and water uptake methods.

If the unit is already installed in the wall, the water bottle test can be used to evaluate the unit. If water applied to the face of the unit is not absorbed immediately, but rather freely runs down the surface of the unit, it likely has sufficient water repellency. Again, if the water is absorbed at the surface, it can not be assumed that the unit does not have sufficient water resistance. Water can be sprayed on a larger wall surface area to determine if isolated units appear to have significantly higher absorption characteristics, since these may appear to have a darker surface color as a result of absorbed water. However, remember that conclusions based upon any field testing, especially on units installed in construction, are not definitive relative to water repellency determinations.

Figure 1—Specimen Undergoing the Water Droplet Test Method

Spray Bar Test

A spray bar test (ref. 9) is a good method to evaluate a unit’s ability to limit absorption as well as verify its effectiveness as a barrier against free moisture migrating through pinholes in the unit face. This laboratory test requires relatively inexpensive equipment and can be conducted in a single day. A spray bar is attached to the unit such that it applies a steady stream of water onto its face (see Figure 2). The inside of a hollow unit is visually inspected to assess if and how moisture has migrated through the front face shell.

Moisture may be present on the interior as dampness that can be seen on the inside surface of the front face shell, on the center or end webs, or even on the interior or exterior surfaces of the back face shell. Moisture may also be observed on the inside of the front face shell from “pinholes.” Pinholes are locations where water has found a path through the face shell to the interior of the unit. Free water will appear as a droplet and may eventually trickle down the inside of the front face shell. A good water repellent unit will limit moisture migration in both forms: dampness and pinholes. If a unit allows an excessive amount of water to migrate through the unit, the type of failure can give an indication of the corrective action that should be taken by the producer. Excessive dampness, for example, may indicate that additional integral water repellent admixture or process adjustment is needed. Excessive pinholes may indicate that an adjustment to the aggregate blend and/or increased compaction may be necessary to reduce the volume of interconnected voids in the unit.

Figure 2—Specimen Undergoing the Spray Bar Test Method

Water Uptake Test

Another good method for evaluating a unit’s resistance to moisture migration is the water uptake test (ref. 10). The test involves placing an oven-dried unit face down (non-split side) in ⅛ in. (3 mm) of water and measuring the water absorption by means of its weight gain over time.

While the water uptake test may be very good at distinguishing between the levels of resistance to absorption uptake, it will not indicate compaction or other flaws that might result in pinholes. Therefore, it is recommended that the results of this test be used to complement the results of the spray bar test and not used exclusively as a means of evaluation.

Figure 3—Specimen Undergoing the Water Uptake Test Method
SPECIFYING WATER REPELLENCY

REFERENCES

  1. International Building Code, 2003 and 2006 editions. International Code Council, 2003, 2006.
  2. Water Repellents for Concrete Masonry Walls, TEK 19-01, Concrete Masonry & Hardscapes Association, 2006.
  3. Design for Dry Single-Wythe Concrete Masonry Walls, TEK 19-02B, Concrete Masonry & Hardscapes Association, 2012.
  4. Preventing Water Penetration in Below-Grade CM Walls, TEK 19-03B, Concrete Masonry & Hardscapes Association, 2012.
  5. Flashing Strategies for Concrete Masonry Walls, TEK 19-04A, Concrete Masonry & Hardscapes Association, 2008.
  6. Flashing Details for Concrete Masonry Walls, TEK 19-05A, Concrete Masonry & Hardscapes Association, 2008.
  7. Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
  8. Water Droplet Test Method for Concrete Masonry Units, CMHA Method CMU-WR1-07, Concrete Masonry & Hardscapes Association, 2007.
  9. Spray Bar Test Method for Concrete Masonry Units, CMHA Method CMU-WR2-07, Concrete Masonry & Hardscapes Association, 2007.
  10. Water Uptake Test Method for Concrete Masonry Units, CMHA Method CMU-WR3-07, Concrete Masonry & Hardscapes Association, 2007.
  11. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C 90-06. ASTM International, 2006.
  12. Standard Specification for Concrete Facing Brick, ASTM C 1634-06. ASTM International, 2006.

NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication.

Sampling and Testing Segmental Retaining Wall Units

  • August 2018

INTRODUCTION

Segmental retaining wall (SRW) units are subject to the minimum requirements of Standard Specification for Dry-Cast Segmental Retaining Wall Units, ASTM C1372 (ref. 1). This standard includes criteria for minimum compressive strength, maximum water absorption, maximum permissible variations in dimensions, and, when required, freeze-thaw durability. Test methods used to demonstrate compliance with these requirements are outlined in this Tech Note.

SAMPLING SEGMENTAL RETAINING WALL UNITS

Segmental retaining wall units are sampled using the same procedures as used for other concrete masonry units. The purpose of selecting multiple test specimens for unit testing is to ensure that the range of results is representative of the entire lot of units from which the specimens were taken. Selecting units from only one portion of a pallet, or choosing only the most or least desirable units may misrepresent the properties of the lot.

Selected specimens should be randomly chosen from each lot, and should all have similar unit configurations and dimensions. A minimum of three units are required to be sampled for compression, absorption and dimensional evaluation in accordance with ASTM C140/C140M, Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units (ref. 2). When freeze-thaw durability testing is also performed, a total of five units should be selected. Since testing for compressive strength, absorption, and freeze-thaw are performed on coupon specimens, all tests can be performed on each sampled unit. Each test specimen is marked with a unique identification, which makes the test specimen immediately identifiable at any point during the testing. Immediately after marking, each unit is weighed to determine the received weight. Note that any loose material should be removed prior to weighing.

MEASUREMENT OF DIMENSIONS

Unit dimensions are measured to verify that the overall length, width and height are within the allowable ±⅛ in. (3.2 mm) tolerances permitted by ASTM C1372. This tolerance does not apply to architectural surfaces, such as split faces.

For each unit, the overall width is measured at the mid-length of the unit across the top and bottom bearing surfaces of the unit using a steel scale marked with ⅒-in. (2.5-mm) divisions (or smaller). Similarly, the overall length is measured at the mid-height at the front and back of each specimen. For height, six total measurements are taken. Four of these measurements are at each corner of the specimen, and the remaining two are taken at mid-length of the front and back of the unit (See Figure 1). The reported overall dimensions are determined by averaging the respective measurements for width and height, and reporting the front and back length of the unit separately.

Additional dimensional and testing information can be found in Segmental Retaining Wall Units, SRW-TEC-001-15 (ref. 3).

Figure 1—Height Measurements for SRW Units (ref. 2)

ABSORPTION TESTING

Absorption describes the amount of water a unit can hold when saturated. Absorption can be an indicator of the level of compaction of the concrete mix, the aggregate gradation, and the volume of voids within a unit. Data collected during absorption testing is used to calculate absorption and density. During absorption testing, the weight of each specimen is determined in the following order and condition: received weight; immersed weight; saturated weight; and oven-dry weight. The immersed and saturated weights are determined following 24 to 28 hours of immersion in water and prior to oven drying the specimens.

ASTM C140/C140M allows for absorption testing of either full units or coupons. Because of the size and weight of SRW units, coupon specimens are typically tested in lieu of full size units. When reduced-size unit 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.

The absorption specimens are immersed in water with a temperature between 60 and 80°F (15.6 to 26.7°C) for 24 to 28 hours, and each specimen is weighed while suspended and completely submerged in water to determine the immersed weight. After determining the immersed weight, the units are removed from the tank and allowed to drain for 60 ± 5 seconds by placing them on a ⅜-in. (9.5-mm) or coarser wire mesh. A damp cloth is used to remove surface water, since a dry cloth may absorb water from the masonry unit. Each unit is weighed again to determine the saturated weight.

Testing larger specimens for absorption requires particular attention to drying times, because it takes a greater length of time to remove all of the moisture from larger masses. To reach an oven-dry condition, the units must be dried for at least 24 hours in a ventilated oven at a temperature of 221 to 239°F (105 to 115°C). For most laboratories, this means a drying time of more than 24 hours, since several hours are typically required to raise the oven temperature to the specified range after the room-temperature SRW units are inserted.

After at least 24 hours, unit weights are recorded in two-hour intervals to ensure the units are no longer losing weight due to moisture loss. The unit is considered oven dry when two successive weighings differ by 0.2% or less. Note that when weighing the units using an electronic scale, insulating materials for the scale may be necessary, because heat radiating from a unit just removed from the oven may cause the scale to return inaccurate results.

ASTM C1372 (ref. 1) includes the maximum water absorption requirements shown in Table 1.

Table 1—Maximum Water Absorption for SRW Units (ref. 1)

a Based on oven-dry density of concrete.

COMPRESSIVE STRENGTH TESTING

Compressive strength tests are used to ensure that the SRW units meet the minimum strength requirements of ASTM C1372: minimum net average compressive strength of 3,000 psi (20.7 MPa) for an average of three units with no individual unit less than 2,500 psi (17.2 MPa).

Some critical areas of compression testing that are necessary to ensure accurate testing include:

  • appropriate capping stations with stiff, planar plates with smooth surfaces,
  • compression machines with spherically seated heads and bearing plates meeting the requirements of ASTM C140/C140M (ref. 2), and
  • proper specimen alignment within the testing machine (specimen’s center of mass aligned with machine’s center of thrust).

ASTM C140/C140M testing procedures for compressive strength of SRW units are the same as those for conventional concrete masonry units (see TEK 18-7, ref. 4), with the exception that coupons are tested in lieu of full-size units.

The tested compressive strength can be influenced by the size and shape of the specimen tested and the location where the coupon was taken. For these reasons, it is important that all retaining wall units be tested using a similar size and shape specimen. In addition, the SRW unit supplier should be contacted for the recommended coupon sample location. 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 large enough to make each required cut in a single pass.

ASTM C140/C140M requires coupons to have a height to thickness ratio of 2:1 before capping and a length to thickness ratio of 4:1 (see Figure 2). The coupon width must be as close to 2 in. (51 mm) as possible based on the configuration of the unit 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). Coupon dimensions must be within ⅛ in. (3 mm) of the targeted dimensions. The coupon height is taken to be in the same direction as the unit height dimension. If these procedures are followed, the compressive strength of the coupon is considered to be the same as the strength of the whole unit.

Figure 2—SRW Coupon for Compressive Strength Testing

FREEZE-THAW DURABILITY

In areas where the segmental retaining wall is likely to be exposed to repeated freezing and thawing under saturated conditions, ASTM C1372 requires that freeze-thaw durability be demonstrated in one of the following ways:

  1. proven field performance,
  2. each of five specimens must have less than 1% weight loss after 100 cycles, or
  3. four of five specimens must each have less than 1.5% weight loss after 150 cycles.

When required, testing is 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. 5), an accelerated laboratory test that provides an indication of relative performance when the units are placed in service. Testing in accordance with ASTM C1262 can be conducted using water or saline (3% sodium chloride by weight) as the test solution. ASTM C1372, however, does not require freeze-thaw evaluation in saline, recognizing that for most applications tests in water are considered sufficient. If the units are to be exposed to deicing salts on a regular basis, local project specifications should be consulted to determine if testing in saline is required.

Freeze-thaw durability test methods are prescribed because freeze-thaw durability cannot be reliably predicted based on factors such as compressive strength, absorption or concrete density. A unit’s freeze-thaw durability can be influenced by manufacturing variables such as:

  • aggregate type,
  • production methods,
  • cement content and
  • presence of admixtures; as well as field variables, including:
  • exposure to moisture (source, volume, frequency)
  • environment (drainage, exposure to shade or sunlight, exposure to salt and chemicals) and
  • temperature (rate of change, peak values, number of cycles, cycle lengths).

C1262 testing is carried out on five specimens representative of the entire lot. These units should be marked for identification, as for C140/C140M testing. Specimens are not permitted to be oven-dried prior to starting freeze-thaw testing.

One coupon is saw-cut from each SRW unit. The side of the coupon has a surface area 25 to 35 in.² (161 to 225 cm²) and a thickness of 1¼ in. ± 1/16 in. (32 ± 2 mm) (see Figure 3). The coupon should be cut from the exposed face of the unit (as it will be placed in service), unless that face is split, fluted, ribbed or otherwise nonplanar. In these cases, the coupon should be cut from another flat molded surface. Saw-cut coupons are then rinsed in water (not submerged), brushed with a soft bristle brush to remove residue and any loose particles, then allowed to air dry on edge for at least 48 hours.

Each specimen is placed in a container, as shown in Figure 4, with the appropriate test solution. After one hour, more liquid is added as necessary to maintain the prescribed level. After 24 hours in the container, the specimen is removed and allowed to drain for one minute on a ⅜-in. (9.5-mm) or coarser wire mesh, removing surface water with a damp cloth. The specimen is immediately weighed to determine the reference weight Wp, after which the specimen is returned to the container and additional water or saline is added if necessary prior to the cyclic freeze-thaw testing.

Specimens are then subjected to freezing and thawing cycles, as follows (see Figure 5):

Freeze cycle: 4 to 5 hr, or longer to ensure that all water is frozen, at 0 ± 10°F (-17 to -5°C) air temperature


Thaw cycle: 2.5 to 96 hr, to ensure that all ice has thawed, at 75 ± 10°F (24 ± 5°C) air temperature.

After every 20 cycles when using water (or 10 cycles using saline) any residue is collected, dried and weighed to determine the percentage weight loss, as follows:

  • determine weight of residue from each evaluation period, Wr, from (weight of the dried residue and filter paper) – (initial weight of the filter paper)
  • add Wr from each evaluation period to determine total accumulated residue weight, Wresidue
  • after the freeze-thaw testing is complete, dry each specimen and weigh to determine Wfinal
  • calculate the initial weight of the specimen from: Winitial = Wfinal + Wresidue
  • determine the cumulative weight loss of each residue collection interval both in grams and as a percentage of Winitial as shown in Table 2.
Figure 3—Coupon for Freeze-Thaw Durability Testing
Figure 4—Freeze-Thaw Immersion
Figure 5—Freeze-Thaw Cycle Requirements
Table 2—Procedure for Calculating Weight Loss Due to Freeze-Thaw Testing (ref. 3)

REFERENCES

  1. Standard Specification for Dry-Cast Segmental Retaining Wall Units, C1372. ASTM International, 2017.
  2. Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M14a. ASTM International, 2022b.
  3. Segmental Retaining Wall Units, SRW-TEC-001-15, Concrete Masonry & Hardscapes Association, 2014.
  4. Compressive Strength Testing Variables for Concrete Masonry Units, TEK 18-07, Concrete Masonry & Hardscapes Association, 2004.
  5. 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 Concrete Masonry Units

  • August 2018

INTRODUCTION

Standards for sampling and testing concrete masonry units are developed by the technical committees of ASTM International in accordance with consensus procedures. These standards reflect the expert opinion of researchers, concrete masonry manufacturers, designers, contractors and others with an interest in quality standards for concrete masonry.

The most commonly used ASTM standards for concrete masonry unit testing include: Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140 (ref. 1), and Standard Test Method for Linear Drying Shrinkage of Concrete Masonry Units, ASTM C426 (ref. 2).

SAMPLING & TESTING CONCRETE MASONRY UNITS, ASTM C140

Unit Sampling

The purpose of selecting multiple samples for unit testing is to ensure that the range of results is representative of the entire lot of units from which the specimens were taken. Consequently, concrete masonry units chosen for testing should be randomly sampled. Choosing units from one portion of a pallet, or choosing the most or least desirable units may misrepresent the properties of the lot.

Although a shipment may consist of several different unit configurations, samples for testing should all have the same configuration and dimensions. In some cases, such as shrinkage results under ASTM C426 (ref. 2), it is generally acceptable to consider the test results of one unit configuration to be representative of units with different configurations provided they were made using the same mix design, manufacturing and curing procedures.

Units that are representative of the entire lot of units are sampled from the job site or may be sampled from the manufacturer’s storage inventory. Sampled units are marked with a unique identification and weighed.

Measurement of Dimensions

Unit dimensions are used: to verify that the overall length, width and height are within allowable tolerances; to calculate normalized web area and equivalent thickness; and to verify that face shell and cross web thicknesses meet the requirements of the appropriate unit specification (see Figure 1). Minimum face shell thickness is prescribed to address concerns such as ease of mortar placement, sufficient mortar coverage over joint reinforcement and resistance to lateral pressure from grouting. Minimum web thickness and area considerations include transfer of shear, flexural strength in the horizontal span, and resistance to tensile splitting of walls under compression.

Included in ASTM C140 since 2012 is testing to determine minimum normalized web area. Its purpose is to ensure that the unit has sufficient web material connecting the face shells. It replaces the equivalent web thickness criteria in previous versions of the standard. To determine the normalized web area, the minimum thickness and height of each web is measured and used to calculate the total web area of the unit. This total web area is divided by the nominal unit face area to determine normalized web area in in.²/ft² (mm²/m²).

Although not specified in ASTM C140 (ref. 1), the units set aside for absorption testing are typically used for measurement of unit dimensions, before the units are immersed in water. This way, the gross volume (determined from overall unit dimensions) and the net volume (determined from water displacement) for the units are both determined from the same set of test specimens.

Figure 1—measurement of Web & Face Shell Thickness

Absorption

Absorption describes the amount of water a unit can hold when saturated. Absorption can be an indicator of the level of compaction of the concrete mix or of the volume of voids within a block. For a given mix design and manufacturing and curing process, variations in absorption can be an indication of deleterious materials in the mix, mixing quality, and/or compaction of the concrete mix, which also can indicate variations in compressive strength, tensile strength, durability, laboratory procedural problems, or other causes. Data collected during absorption testing is used to calculate absorption, density, net area, net volume and equivalent thickness.

Each unit is weighed a minimum of five times in this order: received weight; immersed weight; saturated surface dry weight; and oven-dry weight (at least twice). The saturated and immersed weights should always be determined following 24 to 28 hours of immersion and prior to oven drying the units.

Because the units are immersed in water and subsequently oven-dried during absorption testing, the units used for this determination should not be used for compression testing, the results of which are influenced by unit moisture content. Six units of identical size and configuration are therefore required for ASTM C140 testing—three for compression testing and three for absorption.

Compressive Strength

Compressive strength tests are used to ensure that concrete masonry units meet the minimum strength requirements of the applicable unit specification (see ref. 11). The unit compressive strength results may also be used to verify compliance with the specified compressive strength of masonry, f’m, when using the unit strength method (ref. 4, Article 1.4 B.2.b). Unit compression tests are easier and less expensive to perform than similar tests on masonry prisms, making the unit strength method the more popular.

Some of the critical areas of compression testing that are necessary to insure accurate testing include:

  • Appropriate capping stations with stiff, planar plates with smooth surfaces.
  • Compression machines with spherically seated heads and bearing plates of adequate planeness and thickness for the size of the specimen being tested. See TEK 18-01B (ref. 8) for details and an example.
  • Proper specimen alignment within the testing machine (center of mass aligned with center of thrust).

For compressive strength determination, three specimens are tested. Wherever possible, full-sized units are used. However, certain modifications are permitted or required as follows:

  • Unsupported projections with a length exceeding the projection thickness must be removed by saw-cutting (see Figure 2). For units with recessed webs, the face shell projecting above the web is removed by saw-cutting to provide a full bearing surface over the net cross-section of the unit, as shown in Figure 3.
  • When the size and/or strength of the unit exceeds the testing machine capacity, a specimen may be cut to conform to the testing machine capabilities. The resulting specimen, however, must contain an enclosed four-sided cell or cells without irregular face shells or webs.
  • If saw-cutting does not produce a test specimen complying with the above provisions, coupons may be saw-cut from the face shells (see Figure 4).
  • For concrete roof paver units, cut three test specimens from three whole paver units to produce a strip of paver with the specimen height equal to its width. Where the paver has supporting ribs, cut the coupon perpendicular to the direction of the ribs, such that any bevelled or recessed surfaces are not included in the top or bottom edges of the specimen.
  • For concrete brick, specimens are required to have an aspect ratio (height divided by least lateral dimension) of 0.6 ± 0.1 (see Figure 5).

The prepared specimens are then capped in accordance with ASTM C1552 (ref. 9) to provide a uniform and level bearing surface. After the specimen center of mass is located, the specimen is positioned in the testing machine such that the specimen’s center of mass is aligned with the machine’s center of thrust. All hollow units are tested with their cores in a vertical direction, except for special units intended for use with their cores horizontal. These special units and units that are 100% solid are tested in the same direction as intended for service. Further information on compressive strength testing is available in references 8 and 12.

Figure 2—Units With Unsupported projections
Figure 3—Units With Reduced Webs
Figure 4—Coupon Requirements
Figure 5—Compression Testing of Concrete Brick

Calculations

Using the data gathered in the preceding test methods, the following characteristics are determined: absorption, density, average net area, gross area, net and gross area compressive strengths, normalized web area and equivalent thickness.

Density, or unit weight, is described in terms of dry weight per cubic foot. It is determined from the saturated weight, immersed weight and oven-dry weight. Using these weights, the volume of concrete in a unit is readily determined and its density is the oven-dry weight divided by its net volume. Among the properties affected by density of concrete in a block are wall weight, building weight, thermal conductivity, heat capacity and acoustical properties.

Cross-sectional area is the basis for expressing compressive strength of concrete masonry units. Unit specifications require that block comply with a minimum net area compressive strength. Net area is described in terms of the percentage of solid material in the cross section, and is measured by the ratio of net volume of the unit to gross volume of the unit. Because water displacement is used to determine net volume, the net cross-sectional area represents the average net area of the unit.

Equivalent thickness is used to determine the fire resistance rating. It represents the average thickness of a hollow unit if the volume is configured into a solid unit of the same face dimension. It is determined by dividing the net unit volume by the unit face area.

DRYING SHRINKAGE, ASTM C426

ASTM C426, Standard Test Method for Drying Shrinkage of Concrete Masonry Units (ref. 2) is intended to evaluate the potential shrinkage characteristics of concrete masonry units due to moisture loss only. Note that concrete masonry may also shrink due to factors such as carbonation and temperature changes, which are not addressed by this test method (although temperature is standardized and corrected so as not to influence the results). This test measures unit length change from a totally saturated condition to an “equilibrium” condition at 17% relative humidity. This represents the potential shrinkage because the masonry is unlikely to encounter these extreme conditions under normal circumstances. The test results are used to determine concrete masonry crack control provisions.

Typically, it is not necessary to run shrinkage tests on units made with the same mix design but having different unit configurations. As long as there are no changes in materials, mix design, production methods or curing, ASTM C426 tests are required to be performed only once every two years, per ASTM C90 (ref. 13).

Test specimens are usually whole units with measurements taken on both faces. Alternatively, coupons may be cut from face shells, as illustrated in Figure 6. Gage plugs are mounted on the test specimens to facilitate length measurements.

This method requires the test specimens to be saturated for 48 hours, at which time the length is precisely measured and recorded. Specimens are then dried in an oven for 5 days. After drying, specimens are cooled and measured. Test specimens are then returned to the drying oven for periods of 48 hours until the length change is negligible.

Figure 6—linear Drying Shrinkage Specimens

PREFACED UNITS

For concrete masonry units with a smooth, resinous tile-like facing adhered to the unit, Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744 (ref. 3) includes requirements and applicable test methods for the facing. The concrete masonry unit to which the facing is applied must comply with the applicable unit specification. Facing requirements include:

Resistance to crazing—Units are subjected to wetting and drying to demonstrate that the facing does not craze, crack or spall.
Resistance to chemicals—The facing must remain unchanged when subjected to the specified list of chemicals and exposure durations.
Adhesion—The facing must remain adhered to the unit when the unit is loaded to failure by an applied compression load.
Abrasion—The wear index of the facing must exceed 130 when the facing is subjected to a standard abrasion test (ASTM C501, ref. 5).
Surface burning—The flame spread and smoke density rating of the facing must not exceed 25 and 50, respectively, when tested in accordance with ASTM E84 (ref. 6).
Color tint & texture—The facing texture must remain unchanged and facing color difference must not exceed 5 Delta units (ref. 7) when subjected to an accelerated weathering test.
Soiling and cleansability—No more than a trace of stain may remain on the facing after cleaning when subjected to a specified list of marking substances.

REFERENCES

  1. Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M-14. ASTM International, 2014.
  2. Standard Test Method for Linear Drying Shrinkage of Concrete Masonry Units, ASTM C426-10. ASTM International, 2010.
  3. Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units, ASTM C744-14. ASTM International, 2014.
  4. Specification for Masonry Structures, TMS 602-13/ACI 530.1-13/ASCE 6-13. Reported by the Masonry Standards Joint Committee, 2013.
  5. Standard Test Method for Relative Resistance to Wear of Unglazed Ceramic Tile by the Taber Abraser, ASTM C501-84(2009). ASTM International, 2009.
  6. Standard Test Method for Surface Burning Characteristics of Building Materials, ASTM E84-14. ASTM International, 2014.
  7. Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates, ASTM D2244-14. ASTM International, 2014.
  8. Evaluating the Compressive Strength of CM based on 2012IBC/2011 MSJC, TEK 18-01B. Concrete Masonry & Hardscapes Association, 2011.
  9. Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing, ASTM C1552-14. ASTM International, 2014.
  10. Standard Specification for Concrete Building Brick, ASTM C55-14. ASTM International, 2014.
  11. Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
  12. Compressive Strength Testing Variables for CM Units, TEK 18-07, Concrete Masonry & Hardscapes Association, 2004..
  13. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-14. ASTM International, 2014.
 
 

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:

  1. Proven field performance,
  2. Five specimens shall have less than 1% weight loss after 100 cycles in water using ASTM C1262 (ref. 6), or
  3. 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

  1. Design Manual for Segmental Retaining Walls, 3rd edition, SRW-MAN-001-10, Concrete Masonry & Hardscapes Association, 2010.
  2. Standard Specification for Dry Cast Segmental Retaining Wall Units, ASTM C1372-14. ASTM International, 2014.
  3. Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-14. ASTM International, 2014.
  4. Standard Methods for Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M-14a. ASTM International, 2014.
  5. Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing, ASTM C1552-14. ASTM International, 2014.
  6. 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.
  7. Sampling and Testing Segmental Retaining Wall Units, SRW-TEC-007-15, Concrete Masonry & Hardscapes Association, 2015.