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Evaluating the Compressive Strength of Concrete Masonry – 2015 IBC/2013 MSJC

April 2019

INTRODUCTION

Structural performance of concrete masonry is largely dependent upon three key criteria:

the engineering rationale forming the basis of the structure’s design;
the physical characteristics of the materials used in the construction (i.e., the masonry units, grout, mortar, and reinforcement); and
the quality of the construction used in assembling these components.

The first step in the design of any engineered masonry structure is determining anticipated service loads. Once these loads are established, the required strength of the masonry can be determined. The designation f’_m, indicates the specified compressive strength of masonry. It is used throughout the design and, in accordance with the appropriate code, to predict the strength and behavior of the masonry assembly and thus to size masonry elements. It should be stressed that the specified compressive strength of the masonry is related to but not always equal to the tested compressive strength of the masonry.

To ensure that a safe and functional structure is being constructed that will meet or exceed the intended service life, measures must be taken to verify that the compressive strength of the assembled materials (including masonry units, mortar and grout if used) meet or exceed the specified compressive strength of the masonry.

Compliance with the specified compressive strength is verified by one of three methods: the unit strength method, the prism test method, or by removing units from existing construction. These methods are referenced in masonry design codes (refs. 1, 3, 4, 6), specifications (ref. 2,4), and standards (ref. 5) as rational procedures for verifying masonry compressive strength.

UNIT STRENGTH METHOD

The unit strength method is often considered the least expensive and most convenient of the three methods. However, the unit strength method also tends to yield more conservative masonry strengths when compared to the prism test method.

Compliance with f’_m by the unit strength method is based on the net area compressive strength of the units and the type of mortar used. The compressive strength of the concrete masonry assemblage is then established in accordance with Table 1 for concrete masonry designed in accordance with the 2013 Specifications for masonry Structures (MSJC) (ref. 2), and Table 2 for concrete masonry designed in accordance with the 2016 Specifications of Masonry Structures (TMS 402) (ref. 4).

Use of the unit strength method requires the following:

Concrete masonry units must be sampled and tested in accordance with ASTM C140, Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units (ref. 7) and meet the requirements of ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units (ref. 8). (Note that ASTM C140 allows the test of one set of units to be applied to any number of concrete masonry units or related units of any configuration or dimension manufactured by the producer using the same materials, concrete mix design, manufacturing process, and curing method.)
Mortar bed joints used in construction must not exceed ⅝ in. thickness (15.9 mm).
If grouted masonry is used in construction, the grout must meet either the proportion or the property specification of ASTM C476, Standard Specification for Grout for Masonry (ref. 9), and the 28-day compressive strength of the grout must equal or exceed f’_m but not be less than 2,000 psi (14 MPa). When property specifications are used, the compressive strength of the grout is determined in accordance with ASTM C1019, Standard Test Method for Sampling and Testing Grout (ref. 10).
Mortar must comply with requirements of ASTM C270, Standard Specification for Mortar for Unit Masonry (ref. 11).

In addition to this TEK, CMHA has generated a spreadsheet that calculates the specified compressive strength (f’m ) for masonry based on the compressive strength of the concrete masonry unit and the type of mortar used. Additionally, the spreadsheet also calculates the required strength of a unit to obtain a specific value of f’m . See CMU-XLS-004-19, CMU Unit Strength Calculator (ref. 17).

Using either Table 1 or Table 2 for example, for concrete masonry units with a compressive strength of 2,600 (17.93 MPa), the maximum f’m used in design would be 2,250 (15.51 MPa) if Type M or S mortar were used. Note that per footnote A of Table 1 and Table 2, compressive strength of masonry values must be multiplied by 85% when the unit strength is established on units less than 4 in. (102 mm) in nominal height.

When higher strength masonry materials are specified, it may be more cost effective to utilize the prism test method to demonstrate compliance with f’_m due to the level of conservatism inherent in the unit strength method; i.e., the cost of prism testing are well offset by the construction savings resulting from a more economical design that takes advantage of using a higher compressive strength for the same specified materials.

Note that the unit strength values in the 2013 and 2016 Specification for Masonry Structures (i.e., those in Table 1 and Table 2) are less conservative than values in previous editions. Note that in Table 2 the minimum compressive strength allowed for Type M or S mortar and Type N mortar is set to 2,000 psi (13.79 MPa) versus the 1900 psi (13.10 MPa) listed in Table 1. This change is as a result of changes made in ASTM C90 which sets the minimum compressive strengths of both Type M or S mortar, and Type N mortar, to 2000 psi (13.79). The historical conservatism was due to two primary reasons:

The original database of tested compressive strengths was based on the testing procedures and equipment that were considerably less refined than they are today. Current ASTM C1314, Standard Test Method for Compressive Strength of Masonry Prisms (ref. 3), requirements produce more consistent and repeatable compressive strengths, particularly the requirements for more stable bearing platens on the compression testing equipment.
Historical testing procedures did not strictly control the construction, curing, and testing of the masonry prisms. As a result, a single set of materials could produce various prism test results depending the construction, curing and testing procedures used.

The database of compressive strength values used to generate the values in Table 1 and Table 2 was compiled using modern concrete masonry materials, modern test equipment, and current ASTM test procedures, providing a more realistic estimate of masonry compressive strength.

Table 1—Compressive Strength of Masonry Based on the Compressive Strength of Concrete Masonry Units and Type of Mortar Used in Construction (ref. 2)

Table 2—Compressive Strength of Masonry Based on the Compressive Strength of Concrete Masonry Units and Type of Mortar Used in Construction (ref. 4)

PRISM TEST METHOD

ASTM C1314 contains provisions for determining the compressive strength of a masonry prism: an assemblage made of representative units, mortar and grout (for grouted masonry construction). Although constructed using materials used in the project, the prism is not intended to be a reduced-scale version of the wall, but rather a quality assurance instrument to demonstrate how the masonry components work together. For this reason, prisms are typically constructed in stack bond with a full mortar bed joint, regardless of the wall construction. The tested compressive strength of the prism is corrected to account for different permissible height to thickness ratios of the prisms. This corrected strength must equal or exceed f’_m. Understandably, prism testing should be undertaken before construction begins to verify that the compressive strength of the assembled materials is not less than the specified compressive strength used in the design.

Prisms should be tested at an age not greater than 28 days old to document compliance with f’_m, When prisms are tested as part of an inspection program periodically during the course of construction, an earlier age, such as 3 or 7 days, is often preferred. To confidently interpret the results of these earlier age prism tests, the relationship between prism age and strength development should be determined using the materials, construction methods and testing procedures to be used throughout the job. Only when this strength/time curve is generated can early age test results be extrapolated to predict the 28-day strength.

Prism Construction

Masonry prisms are constructed using units representative of those being used in the construction. One set of prisms (containing three individual prisms) is constructed for each combination of materials and each testing age for which the compressive strength is to be determined. Note that for concrete masonry units of different configuration but from the same production lot, separate prisms are not required for each configuration. For example, if a project uses 8-in. (203-mm) and 12-in. (305-mm) units from the same lot, prisms need only be tested using either the 8-in. (203-mm) or the 12-in. (305-mm) units, but not both. ASTM C140 (ref. 7) defines a ‘lot’ as any number of concrete masonry units of any configuration or dimension manufactured by the producer using the same materials, concrete mix design, manufacturing process, and curing method. For multi-wythe masonry construction, with different units or mortar in each wythe, separate prisms should be built representative of each wythe, and tested separately. Prisms should be constructed on a flat and level location where they can remain undisturbed until they are transported for testing, at least 48 hours.

All units used to construct the prisms must be of the same configuration and oriented in the same way so that webs and face shells are aligned one on top of the other. Units are laid in stack bond on a full mortar bed using mortar representative of that used in the corresponding construction. Mortar joints are cut flush regardless of the type of mortar joint tooling used in the construction. Prisms composed of units that contain closed cells must have at least one complete cell with one full-width cross web on either end. Various prism configurations are shown in Figure 1.

Since masonry prisms can be heavy, especially grouted prisms, it often proves effective to construct prisms using half-length units. The criteria for constructing prisms of reduced-sized units are (also see Figure 2):

that hollow units contain fully closed cells,
that the cross section is as symmetrical as possible, and
that the length is not less than 4 in. (102 mm).

As a result, handling, transporting, capping, and testing the reduced sized prisms is easier, resulting in less potential for damage to the prisms. Using reduced length prisms also reduces the required plate thicknesses for compression machines and typically result in higher and more accurate assessments of masonry strengths.

Immediately following construction of the prisms, each prism is sealed in a moisture-tight bag, as shown in Figure 3. The prism test method requires prisms to be cured in sealed plastic bags to ensure uniform hydration of the mortar and the grout if used. Under actual field conditions, it may require longer periods for hydration and the corresponding strengths to be achieved. Curing prisms in sealed plastic bags results in measured strengths which are representative of those exhibited by the masonry throughout the life of the structure. Bag curing also provides a uniform and repeatable testing procedure.

Where the corresponding construction is to be grouted solid, each prism is grouted solid using grout representative of that being used in the corresponding construction. When prisms are used for field quality control or assurance, prisms must be constructed at the same time as the corresponding construction and grouted when the construction is being grouted. When prisms are used for other purposes, such as preconstruction or for research, prism grouting must occur between 4 hours and 48 hours following the construction of the prisms.

After grouting, the grout in each prism is consolidated and reconsolidated using procedures representative of those used in the corresponding construction. After each consolidation, the grout in the prism will likely settle due to water absorption from the grout into the masonry units. Therefore, after each consolidation, additional grout should be added as necessary and be screeded level with the top of the prism to facilitate capping. Reinforcement is not included in prisms. Immediately following prism grouting, the moisture-tight bag is resealed around each prism.

If the corresponding construction will be partially grouted, two sets of prisms are constructed—one set grouted and one set ungrouted.

Figure 2—Saw-Cut Locations for Reduced-Size Prisms

Figure 3—Constructing a Half-Length Prism in a Plastic Bag

Transporting Prisms

Since mishandling prisms during transportation from the job site to the testing facility can have significant detrimental effects on the tested compressive strength of prisms, extreme care should be taken to protect against damage during transport.

Prior to transporting, the prisms should be strapped or clamped as shown in Figure 4 to prevent damage. Tightly clamping or strapping plywood to the top and bottom of a prism prevents the mortar joint from being subjected to tensile stresses during handling. The prisms should also be secured during transport to prevent jarring, bouncing or tipping.

Curing Prisms

As previously stated, each prism is constructed in a moisture-tight bag (Figure 3) large enough to enclose and seal the completed prism. The bags should have adequate thickness to prevent tearing; a thickness of 2 mils (0.0051 mm) or greater has been found to work well. After the initial 48 hours of job site curing in the moisture-tight bag, each prism is carefully moved to a location where the temperature is maintained at 75 ± 15° F (24 ± 8° C) for full curing prior to testing.

Prism Net Cross-Sectional Area

To provide accurate an accurate strength calculation, the laboratory needs to determine the net area of the prisms. Ungrouted masonry prisms should be delivered to the testing agency with three additional units, identical to those used to construct the prism. If reduced-length prisms are used, additional reduced-length units should accompany the prisms to the laboratory for this purpose.

The net cross-sectional area used to calculate compressive strength of a prism depends on whether the prisms are grouted or ungrouted. For ungrouted full-size prisms, the cross-sectional area is the net cross-sectional area of the concrete masonry units determined in accordance with ASTM C140 on concrete masonry units identical to those used to construct the prisms. When reduced sized units are used to construct ungrouted prisms, the net cross-sectional area is based on the reduced sized units.

When testing fully grouted prisms, net cross-sectional area is determined by multiplying the actual length and width of the prism per ASTM C1314. These areas are illustrated in Figure 5.

Figure 5—Net Cross-Sectional Areas of Grouted and Ungrouted Prisms

Testing Prisms

Two days prior to the 28 day time interval or the designated testing time, typically 28 days, each prism is removed from the moisture tight bag. Prism age is determined from the time of laying units for ungrouted prisms, and from the time of grouting for grouted prisms.

To provide a smooth bearing surface, prisms are capped with either a sulfur capping material or high-strength gypsum compound in accordance with ASTM C1552, Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing (ref. 12). No other capping materials are permitted, nor are unbonded caps.

Capping provides level and uniform bearing surfaces for testing, thereby eliminating point loads due to surface irregularities. The result is more uniform and reliable compressive strength values. Patching of caps is not permitted because it is difficult to maintain a planar surface within the tolerances of ASTM C1552.

Capping materials must have a compressive strength of at least 3,500 psi (24.13 MPa) at an age of 2 hours when cubes of the material are tested in accordance with ASTM C617, Standard Practice for Capping Cylindrical Concrete Specimens (ref. 13).

The average thickness of the cap must not exceed ⅛ in. (3.2 mm). Caps are to be aged for at least 2 hours before testing the specimens, regardless of the type of capping material. Capping plates of adequate stiffness and smoothness are critical to achieving accurate results. Machined steel plates of 1 in. (25.4 mm) minimum thickness are required as a base. Glass plates not less than ½ in. (12.7 mm) in thickness may be used as a wearing surface to protect the plates. The capping wear plate must be plane within 0.003 in. in 16 in. (0.075 mm in 400 mm) and free of gouges, grooves and indentations greater than 0.010 in. (0.25 mm) deep or greater than 0.05 in.² (32 mm²).

One of the most common oversights in testing masonry prisms is compliance with the established requirements for the testing machine itself. The testing machine is required to have a spherically seated head with a minimum 6 in. (150 mm) diameter and capable of rotating in any direction. The spherically seated head is then attached to a single thickness steel bearing plate having a width and length at least ¼ in. (6.4 mm) greater than the length and width of the prism being tested. The required thickness of the steel bearing plate depends on the diameter of the spherically seated head and the width and length of the prism being tested. The thickness of the steel bearing plate must equal or exceed the maximum distance from the outside of the spherically seated head to the outmost corner of the prism—designated d in Figure 6. Failure to provide the required minimum bearing plate thickness decreases the measured compressive strength of the prism due to the bearing plate bending during testing. It is also required that the bearing faces of the plates have a Rockwell hardness of at least HRC 60 (BHN 620).

The last step prior to testing a prism in compression is determining the prisms center of mass. The center of mass of a prism can be thought of as the point on the cross-section of a prism where it could physically balance on a point. The prism is then centered within the test machine such that the center of mass coincides with the center of thrust (which coincides with the center of the spherically seated head).

Failure to align the center of mass with the center of thrust results in a nonuniform application of load and therefore lower measured compressive strengths. For prisms having symmetric cross-sections, the mass centroid coincides with the geometric centroid—or the center of the prism as measured with a ruler. For prisms that are non-symmetrical about an axis, the location of that axis can be determined by balancing the masonry unit on a knife edge or a metal rod placed parallel to that axis. If a metal rod is used, the rod must be straight, cylindrical (able to roll freely on a flat surface), have a diameter between ¼ in. and ¾ in. (6.4 and 19.1 mm), and it must be longer than the specimen. Once determined, the centroidal axis can be marked on the end of the prism.

To test the prism, it is placed in the compression machine with both centroidal axes of the specimen aligned with the machine’s center of thrust. The maximum load and type of fracture is recorded. Prism strength is calculated from the maximum load divided by the prism net area. This prism strength is then corrected as described below.

Figure 6—Determination of Bearing Plate Thickness

Corrections for Prism Aspect Ratio

Since the ratio of height, h_p, to least lateral dimension, t_p,—designated the aspect ratio or h_p/t_p—of the prism can significantly affect the load carrying capacity of the masonry prism, ASTM C1314 contains correction factors for prisms having different aspect ratios, as outlined in Table 2.

To use the values in Table 2, simply multiply the measured compressive strength of the prism by the correction factor corresponding to the aspect ratio for that prism. Correction factors shown in Table 3 can be linearly interpolated between values, but cannot be extrapolated for aspect ratios less than 1.3 or greater than 5.0.

PRISMS FROM EXISTING CONSTRUCTION

The majority of quality assurance testing of concrete masonry materials is conducted on samples representative of those used in the construction. In some cases, however, it may be necessary or desirable to evaluate the properties of existing masonry construction using the actual construction materials instead of representative samples. Examples where the in-place (in-situ) masonry properties might need to be considered include old or damaged construction, or during the construction process, when: a testing variable or construction practice fails to meet specifications; a test specimen is damaged prior to testing; test records are lost; or representative samples are not otherwise available.

The procedures covered in ASTM C1532, Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units and Specimens from Existing Construction, (ref. 14), are useful when physical examination of an assembly’s compressive strength, stiffness, flexural strength or bond strength is needed on a representative sample of the actual construction. These specimens are a portion of the existing masonry, and may include units, mortar, grout, reinforcing steel, collar joint and masonry accessories. The specimens can be taken from single or multiwythe construction. The procedures outlined in C1532 focus on documenting the condition of the masonry and protecting the specimens from damage during removal and transportation to the testing laboratory.

Standard Practice for Preparation of Field Removed Manufactured Masonry Units and Masonry Specimens for Compressive Strength Testing, ASTM C1587 (ref. 15), provides procedures for preparing field-removed specimens for compressive strength testing, and covers procedures such as removing hardened mortar and cleaning.

Compressive strength test results of field-removed masonry units and assemblies are expected to vary from, and will likely be less than, compressive strength test results of new masonry units and newly assembled prisms. Therefore, drawing relationships between the results of tests conducted on field-removed specimens to those of masonry units prior to use or of constructed prisms is difficult.

Prior to removal of specimens from existing construction, a repair plan should be developed. This plan should include replacement of units removed and repair of any disturbed or cut reinforcement, including those unintentionally damaged during the removal process.

Selecting Specimens

Specimens should be representative of the masonry construction as a whole, considering variations within the construction such as: parapets, corbels, areas where different masonry units are combined for architectural effects, as well as variations in the condition or exposure of the masonry. C1532 includes guidance on random sampling, location-specific sampling, and on condition-specific sampling. When testing to help quantify the effects of various exposures or conditions, the sampling should represent each exposure condition.

Thorough documentation of the specimen’s condition prior to removal is necessary to assess whether the specimen was subsequently damaged during removal and transport, and for comparative purposes with the other specimens.

Removing Specimens

Carefully remove each specimen at its perimeter, ensuring the specimen is the appropriate size for the intended testing. Note that hydraulic or electric impact equipment should not be used, due to the potential for damaging the specimens. Saw-cutting or hand chiseling is preferred.

The following procedure is recommended. Make the first cut along the bottom of the specimen (on both sides of the wall if necessary) and insert shims. Make the two vertical cuts at the sides of the specimen, then make the top cut. Provide any necessary shoring, bracing and weather protection for the remaining construction. Similar to the pre-removal documentation, assess and document the specimen’s condition to determine if the specimen was damaged during removal.

Transporting Specimens

The specimens should be confined as described in Transporting Prisms, page 4. In addition, each specimen should be protected on all sides with material such as 1 in. (25 mm) thick packaging foam or bubble wrap, placed in sturdy crates, and the crates completely filled with packing material to ensure the specimens cannot move within the crate during transport.

Testing Specimens

It is not permitted to test grouted or partially grouted specimens that contain vertical reinforcement. Specimens cut from existing construction containing horizontal reinforcement can be tested, but the presence and location of reinforcement should be noted and reported.

Prisms must: include at least one mortar bed joint; have an aspect ratio (h_p/t_p) between 1.3 and 5; have a height of at least two units (each of which is at least one-half the height of a typical unit); have a length one-half the unit length and two unit lengths; not include vertical reinforcement. In addition, when prisms contain units of different sizes and/or shapes, the unit height and length are considered to be that of the largest unit height or largest unit length within the prism.

The specimens should be prepared for capping by smoothing and removing loose or otherwise unsound material from the bearing surfaces, to produce a plumb and level surface.

Note that grouted or partially grouted specimens cannot contain vertical reinforcement. The specimens are photographed to document specimen condition prior to capping. Capping and testing procedures are identical to those for constructed prisms except that a slower loading rate is used for field-removed prisms to account for uncertainty in expected loads for these prisms. For field-removed prisms, the first one-quarter of the expected load can be applied at any convenient rate, and the remaining load should be applied within 2 to 4 minutes.

Field-removed prisms may have non-uniform dimensions that should be considered when determining net cross-sectional area for calculating compressive strength. Professional judgement should be used to determine the minimum bearing area of a non-uniform prism. One effective method for face-shell bedded specimens is to multiply the length of the specimen at the bed joint by the sum of the face shell thicknesses to determine minimum bearing area. A more detailed discussion of making this determination is available in CMU-FAQ-012-14, How can the Bearing Area of a Concrete Masonry Prism Removed from Existing Construction be Determined? (ref. 16).

REFERENCES

Building Code Requirements for Masonry Structures, TMS 402-13/ACI 530-13/ASCE 5-13, Reported by the Masonry Standards Joint Committee, 2013.
Specifications for Masonry Structures, TMS 602-13/ACI 530.1-13/ASCE 6-13, Reported by the Masonry Standards Joint Committee, 2013.
Building Code Requirements for Masonry Structures, TMS 402-16, The Masonry Society, 2016.
Specification for Masonry Structures, TMS 602-16, The Masonry Society, 2016.
Standard Test Method for Compressive Strength of Masonry Prisms, ASTM C1314-16. ASTM International, Inc., 2016.
International Building Code, International Code Council, 2012/2015.
Standard Test Methods of Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140/C140M-16, ASTM International, Inc., 2016.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-16a. ASTM International, Inc., 2016.
Standard Specification for Grout for Masonry, ASTM C476-16, ASTM International, Inc., 2016.
Standard Test Method for Sampling and Testing Grout, ASTM C1019-16, ASTM International, Inc., 2016.
Standard Specification for Mortar for Unit Masonry, ASTM C270-14a. ASTM International, Inc., 2014.
Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing, ASTM C1552-16, ASTM International, Inc., 2016.
Standard Practice for Capping Cylindrical Concrete Specimens, ASTM C617M-15. ASTM International, Inc., 2015.
Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units and Specimens from Existing Construction, ASTM C1532/C1532M-12, ASTM International, Inc., 2012.
Standard Practice for Preparation of Field Removed Manufactured Masonry Units and Masonry Specimens for Testing, ASTM C1587/C1587M-15, ASTM International, Inc., 2015.
How Can the Bearing Area of a Concrete Masonry Prism Removed from Existing Construction be Determined? CMU-FAQ-014-14, Concrete Masonry & Hardscapes Association, 2014..
CMU Unit Strength Calculator, CMU-XLS-004-19, Concrete Masonry & Hardscapes Association, 2019.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.

Evaluating Existing Concrete Masonry Construction

August 2018

INTRODUCTION

The majority of quality control testing of concrete masonry materials is conducted on samples representative of those used in actual construction (ref. 1, 2, 3, and 4). In some cases, however, it may be necessary or desirable to evaluate the properties of existing masonry construction using the actual construction materials instead of representative samples. Examples where the in-place (in situ) masonry properties might need to be considered include old construction, damaged construction or during the construction process when:

a testing variable or construction practice fails to meet specifications;
a test specimen is damaged prior to testing;
test records are lost; or
representative samples are not otherwise available.

This TEK outlines guides and practices for the physical evaluation of masonry units, grout, mortar, and assemblies that form a part of an existing structure. Because no single procedure can be considered universally applicable for the evaluation and assessment of all conditions, proper tests or inspections must be selected with care as they form only a part of a broader evaluation, which may also include structural considerations, performance attributes, acceptance criteria, and goals (see Figure 1).

In some cases the physical characteristics of the materials or construction may not be in question, but instead concerns are focused on one or more performance attributes. While possibly stemming from any one of a number of sources, including poor construction, detailing, or materials; common performance related assessments include sources and causes of cracking, mitigating water penetration, and strength evaluation. Options for the evaluation and remediation of masonry structures are virtually endless. A thorough review of this subject can be found in reference 17.

MASONRY UNITS

When it is deemed necessary to remove units from a wall to evaluate their physical properties, the selection and removal of specimens should follow ASTM C 1420 Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units Placed in Usage (ref. 5) to minimize potential damage to the units during their removal and transport and to obtain a representative sampling of specimens from which generalized conclusions can be drawn. Once removed, units can be sent to a laboratory for further assessment using visual techniques, petrographic techniques, or more common tests such those used in determining the compressive strength or equivalent thickness for fire resistant construction. Although comprehensive in its scope, ASTM C 1420 does not contain acceptance criteria or guidance for the interpretation of the results, as the application of such information is nearly always project specific.

While often definitive in their results when properly implemented and interpreted, the option of removing units from existing construction can have its limitations, especially when the existing construction is grouted or contains reinforcement. While it is still physically possible to remove a hollow unit that has been grouted and reinforced from a masonry wall, it becomes difficult (if not impossible) to determine the compressive strength of such units due to the presence of the grout and reinforcement. Hence, for construction that contains grout and/or reinforcement, it may be more appropriate to remove prisms or cores from the assembly, particularly when structural stability is the primary reason for the evaluation.

MORTAR

In many cases, the importance placed on the compressive strength of masonry mortars is overemphasized. Because the compressive strength of masonry mortars is not of principal concern in the overall performance of masonry structures there are no test methods that directly measure the compressive strength of mortar taken from an assembly. Yet, there may be circumstances when the removal and evaluation of mortar from existing masonry construction may be deemed necessary. ASTM C 1324 Standard Test Method for Examination and Analysis of Hardened Masonry Mortar (ref. 6) reviews procedures primarily related to the petrographic examination and chemical analysis of samples of masonry mortar removed from masonry construction. Based upon such examination and analysis, proportions of components in masonry mortars can be determined, which can then easily be compared to the volume proportions of ASTM C 270 (ref. 7) to classify a particular mortar or to document the actual proportions of materials used in the mortar.

While ASTM C 1324 can be an invaluable tool for measuring the relative amounts of constituent materials used in a mortar or in mapping the chemical makeup of a mortar, it does have its limitations. For example, even if a mortar is shown to have proportions that do not comply with the requirements of ASTM C 270, the mortar may still comply with the property requirements of C 270, which cannot be reasonably measured through examination of field mortars. Further, the information provided by C 1324 is anecdotal and highly subject to user error. Like all emerging technologies, results stemming from petrographic analyses should be subjected to critical review and careful interpretation.

GROUT

Unlike mortar and units, grout is often hidden from view once placed. Hence, evaluation methods that are focused on grout include both physical tests, such as measuring the compressive strength or grout/unit bond strength, as well as documenting proper placement and consolidation, to ensure as few voids as possible in the resulting construction.

While following the grout lift height and pour height of Specification for Masonry Structures (ref. 8) is a prescriptive means of ensuring high quality grout placement, alternative grouting procedures, such as those permitted by Specification for Masonry Structures through the construction of a grout demonstration panel (refs. 8 and 9), may require supplementary means of documenting proper grout placement and consolidation. Obtaining physical specimens, such as grout cores (see Figure 2) or saw-cut samples (ref. 10), is one means of documenting proper grout placement when non-standardized grouting procedures are used, less destructive (and often less expensive) tests such as ultrasound, impact-echo and infrared photography can be highly efficient tools for measuring the subsurface characteristics of a masonry wall.

ASSEMBLIES

As with individual units, ASTM has published a guide for the selection and removal of masonry assemblies from existing construction, ASTM C 1532 (ref. 11). The procedures outlined in ASTM C 1532 are useful when physical examination of an assembly’s compressive strength, stiffness, flexural strength, or bond strength is needed on a representative sample of the actual construction (ref. 12). When conditions permit, or when less destructive means of evaluation are warranted, several testing alternatives are available.

Modulus of Elasticity

ASTM C 1197, Standard Test Method for In Situ Measurement of Masonry Deformability Properties Using the Flatjack Method, (ref. 13) can be used to evaluate the modulus of elasticity (stiffness) of a single wythe of unreinforced masonry constructed with solid units. To perform the test, two slots are cut into the mortar joints at the top and bottom of the section of masonry to be evaluated. Thin, bladder-like flatjack devices are inserted into these open mortar joints and then pressurized, inducing a controlled compressive stress on the masonry between them. Pressure in the flatjacks is gradually increased and the resulting masonry deformations are measured. The modulus of elasticity is calculated based on the resulting stress-strain relationship. Note that experimental and analytical investigations have indicated that this test typically overestimates the compressive modulus of masonry by up to 15 percent.

Mortar Joint Shear Strength

Guidelines for the Rehabilitation of Existing Buildings (ref. 14) contains a relationship between masonry bed joint shear strength measured in situ to the overall strength of a masonry shear wall. This relationship assumes the wall shear strength is limited by shear through the mortar joints rather than shear through the units. To measure the in situ mortar joint shear strength, ASTM C 1531, Standard Test Method for In Situ Measurement of Masonry Mortar Joint Shear Strength Index (ref. 15), is used. Included in ASTM C 1531 are three test methods for determining an index of the horizontal shear resistance of mortar bed joints in existing unreinforced solid-unit or ungrouted hollow-unit masonry.

In accordance with ASTM C 1531, the mortar bed joint shear strength index is determined by horizontally displacing a test unit relative to the surrounding masonry using a hydraulic jack or specialized flatjacks. The horizontal force required to displace the test unit provides a measured index of the mortar joint shear strength. Some studies have indicated that the in situ mortar joint shear strength may overestimate the actual shear strength index of a masonry wall. While a relationship has been established between the mortar joint shear strength and the shear strength of a masonry wall, there is currently insufficient data to define a similar correlation between the in situ measurement of bed joint shear strength and the actual bed joint shear strength.

Compressive Stress and Strength

For some engineering evaluations of existing masonry it may be necessary to estimate the compressive stress present in the wall. ASTM C 1196, Standard Test Method for In Situ Compressive Stress Within Solid Unit Masonry Estimated Using Flatjack Measurements (ref. 16), provides one such method to determine the average compressive stress in an unreinforced solid unit masonry wythe. The method uses flatjacks above and below the test region similar to ASTM C 1197 previously discussed. When the mortar joints above and below the test area are removed from the masonry to accommodate the flatjacks, the masonry deforms. The flatjack pressure required to move the masonry back to its original position is approximately equal to the compressive stress in the masonry.

The compressive strength of masonry can be evaluated by testing masonry prisms removed from the wall or by using cores cut from a grouted portion of the wall. If vertical reinforcement is present in the wall, testing a prism can be difficult because the vertical reinforcing steel carries load, hence the test is not a true evaluation of the masonry properties. In this case, cored samples may provide a better estimate, because the cores are tested in an orientation 90 degrees from the in situ position, so the reinforcing steel does not interfere with the test.

Limited research (ref. 10) on 6 inch (152-mm) diameter cores cut from grouted masonry compared the compressive strength of the core sample to that of masonry prisms constructed using the same materials. In these investigations, the average ratio of core to prism compressive strength was 1.04 for cores with an aspect ratio (height to diameter) of 1.27. Research on in situ masonry prism removal and testing (ref. 12) found a similar correlation factor when comparing both masonry prisms removed from existing construction to laboratory prepared prisms using similar materials.

NONDESTRUCTIVE EVALUATION

Obviously, the removal of units, prisms, cores, or other materials from a masonry structure is aesthetically detrimental and potentially structurally damaging. When possible, the physical evaluation of existing concrete masonry structures should provide the necessary information that results in the least cost and damage to the structure. A number of nondestructive evaluation procedures are applicable to masonry construction, which are often used in concert with the previously described test methods. The benefit of these techniques is the ability to evaluate portions of a structure with little or no damage.

Ultrasound and Impact-Echo

Ultrasound evaluations (pulse-velocity and pulse-echo) use a transmitter and receiver to pass ultrasonic energy through a wall. The density of the wall is estimated based on the velocity of the waves passing through the wall. Unlike the other methods discussed here, ultrasound requires access to both sides of the wall being evaluated.

Impact-echo differs in two ways from ultrasound: lower frequencies are used, which helps overcome the high signal attenuation and noise often encountered with ultrasound; and access to both sides of the wall is not required. Impact-echo uses elastic stress waves generated by a surface impact. These stress waves are reflected back to the receiver as they encounter internal anomalies or an exterior surface of the wall. Analysis of the reflected signal strength and shape allows evaluation of wall thickness and location of voids and grout areas.

Infrared

Infrared, or heat imaging, technologies measure thermal radiation from a wall surface, and record these emissions as different colors, corresponding to different surface temperatures (see Figure 3). Variations in temperature can be associated with factors such as wall solidity, moisture content, or a change in construction materials or insulation. Infrared cameras allow the user to survey an entire wall relatively quickly.

In order to provide a representative image of the wall, infrared measuring devices require heat to be transmitting through the wall (i.e., a warm interior and a relatively cool exterior ambient temperature). Generally, the larger the temperature flux, the better the resolution of subsurface anomalies.

Fiber Optics (Borescope and Fiberscope)

Borescopes (rigid optical scope) and fiberscopes (flexible optical scope) are useful for viewing interior void areas in a masonry wall. The scope is inserted into a small hole drilled into the wall, and can be attached to a camera or video recorder to document the observations. Borescopes and fiberscopes are often used to visually confirm anomalies detected using ultrasound, impact-echo or infrared methods, or to assess the condition of interior objects or cavities such as wall ties and collar joints.

Electromagnetic Devices (Rebar Locators)

Electromagnetic devices are commonly used to locate metal in masonry walls. Rebar locators generate a magnetic field, which is disturbed when a metallic object is encountered. The magnitude of the disturbance is related to the size of the object and its distance from the probe. Rebar locators can be used to: detect the location and orientation of reinforcing bars, prestress cables and other embedded metal items; measure the depth of embedded metal; and estimate the size of the metal items.

Figure 3—Infrared Photograph Used to Verify Proper Grout Placement

REFERENCES

Evaluating the Compressive Strength of CM based on 2012IBC/2011 MSJC, TEK 18-01B. Concrete Masonry & Hardscapes Association, 2011.
Sampling and Testing Concrete Masonry Units, TEK 1802C. Concrete Masonry & Hardscapes Association, 2014.
Concrete Masonry Inspection, TEK 18-03B, Concrete Masonry & Hardscapes Association, 2014.
Masonry Mortar Testing, TEK 18-05B, Concrete Masonry & Hardscapes Association, 2014.
Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units Placed in Usage, ASTM C 1420-99, ASTM International, 1999.
Standard Test Method for Examination and Analysis of Hardened Masonry Mortar, ASTM C 1324-02a, ASTM International, 2002.
Standard Specification for Mortar for Unit Masonry, C 270-02. ASTM International, 2002.
Specification for Masonry Structures, ACI 530.1-02/ASCE 6-02/TMS 602-02. Reported by the Masonry Standards Joint Committee, 2002.
Grouting Concrete Masonry Walls, TEK 03-02A, Concrete Masonry & Hardscapes Association, 2005.
Research Evaluation of Various Grout Consolidation Techniques in Concrete Masonry, MR-13, Concrete Masonry & Hardscapes Association, 1999.
Standard Guide for Selection, Removal, and Shipment of Masonry Assemblage Specimens from Existing Construction, ASTM C 1532-02, ASTM International, 2002.
Research Evaluation of the Compressive Strength of In Situ Masonry, MR-8, Concrete Masonry & Hardscapes Association, 1993.
Standard Test Method for In Situ Measurement of Masonry Deformability Properties Using the Flatjack Method, ASTM C 1197-03, ASTM International, 2003.
Guidelines for the Rehabilitation of Existing Buildings, International Code Council, 2000.
Standard Test Method for In Situ Measurement of Masonry Mortar Joint Shear Strength Index, ASTM C 1531-03, ASTM International, 2002.
Standard Test Method for In Situ Compressive Stress Within Solid Unit Masonry Estimated Using Flatjack Measurements, ASTM C 1196-03, ASTM International, 2003.
Nondestructive Evaluation and Testing of Masonry Structures, Suprenant, B.A., Schuller, M.P., Hanley-Wood, 1994.

Structural Testing of Concrete Masonry Assemblages

August 2018

STRUCTURAL TESTING OF CONCRETE MASONRY ASSEMBLAGES

INTRODUCTION

A considerable amount of research has been conducted on concrete masonry units and assemblages in order to develop design stresses for building codes and to evaluate existing building systems. The properties of concrete masonry which are considered most important and which have been the subject of research on assemblages of concrete masonry units include: structural, fi re resistance, thermal insulation, noise insulation, and resistance to moisture penetration. This TEK is concerned with testing structural properties and reviews the kinds of loads and stresses which concrete masonry walls may be subjected to in service and the principal details of ASTM Test Methods used to investigate the structural behavior of masonry walls.

TYPES OF LOADS

Loads acting on masonry walls may be classified as vertical dead and live loads, sometimes called gravity loads, and lateral loads, these being due to wind, earthquake, earth or water pressure, etc. Vertical loads may be more or less uniformly distributed along the length of the wall or may include one or several concentrated loads which are transmitted to small areas of the wall section. Vertical loads may be centered in the same plane as the centroidal axis of the wall (concentric or axial loading) or at some distance away from this axis (eccentric loading). Eccentric loads produce bending as well as direct compressive stresses and consequently are more severe than concentric loading. Lateral loads may be uniformly distributed on the vertical surface as in the case of wind, or nonuniform according to some function of other factors, as in the case of earthquake or fluid pressure loads. Lateral loads may be concentrated and their direction may be normal or parallel to or at any intermediate angle with the wall surface. Loads may be gradually or quickly applied (impact), permanent (dead loads) or transient (wind load). Vertical and lateral loads may act simultaneously to produce a combination of axial compression and flexural stress in the masonry.

TYPES OF LOADING USED IN TESTS

Despite the variety of load types and stress conditions, masonry walls can be safety designed provided their ultimate strength in direct compression, flexure and shear is known. In developing this information the test procedures employed in the past often have varied as to specimen size, loading method and other details, but when properly presented and interpreted the results have proved to be applicable and useful. Beginning in the 1930’s, the National Bureau of Standards adopted standard methods for use in their own investigations of building constructions, these methods later forming the basis for ASTM Standard E 72, Standard Test Method for Conducting Strength Tests of Panels for Building Construction (ref. 2).

Figures 1 through 6 show schematically the general procedures used in conducting compression, flexural, and racking.

COMPRESSION

Test Methods

According to ASTM E 72, compressive strength tests are made on specimens having a height equal to the height of the wall in actual use and having a nominal width of 4 ft (1.2 m). Generally, story height walls (nominal height = 8 ft) are typical of those tested but compressive strength tests have been conducted on masonry walls over 20 ft high.

Referring to Figure 1, note that compression tests are made with the load line located a distance one-third the wall thickness from the inside face of the wall (eccentric loading) or at the central plane of the wall (axial loading). Eccentric loading is prescribed in ASTM E 72 and for many years has been preferred over axial loading by many investigators since it approximates more closely the loading condition of walls in buildings.

Loading at the central plane or centerline of the wall is true axial loading only when the wall section is geometrically and elastically symmetrical with respect to the center line.

With the advent of engineered loadbearing masonry design, simpler and less expensive test methods for determining compressive strength properties of masonry have come into wide usage: ASTM E 447, Standard Test Method for Compressive Strength of Masonry Prisms (ref. 1), and ASTM C 1314, Standard Test Method for Constructing and Testing Masonry Prisms Used to Determine Compliance with Specified Compressive Strength of Masonry (ref. 3). These tests prescribe methods for testing short compression prisms made of the same masonry units, mortar, and workmanship to be used in the construction. Although the test methods are similar, ASTM E 447 is intended for research purposes only (not for construction quality assurance purposes as is C 1314), and requires collection of additional detailed information associated with research tests.

Stresses Due to Applied Loads

The type of loading largely determines the general shape of the stress distribution diagram for the wall section. For solid walls, Figure 2, axial loading results in rectangular stress diagram, the fiber stresses being uniform over the entire cross section and equal to P/A. If the vertical load is applied eccentrically or off-center by a distance of one-sixth the wall thickness (t/6), the unit stress varies from a maximum of 2P/A at the wall face nearest the load line, to zero stress at the opposite face, Figure 2b. Eccentricity greater than t/6 would produce tensile stresses at the opposite face and a stress diagram which would show zero stress at some point between the wall faces. Eccentricity less than t/6 would result in some compression at the opposite face and a stress diagram of trapezoid shape.

In Figures 2a and 2b, the average compressive stress is the same in each case, assuming the same vertical load, but as noted, the maximum fiber stress for the eccentrically loaded wall is twice that for the axially loaded wall. A logical deduction is that a given wall will support a greater axial than eccentric load. This is borne out by tests which indicate that bending stress due to eccentric location of vertical load or other causes, reduces the ultimate vertical load capacity of masonry below its axial load strength.

FLEXURAL

The different types of loading used in testing masonry walls for flexural or transverse strength are shown in Figure 3, together with the moment and shear diagrams and formulas for calculating maximum shear, moment and deflection assuming simply supported spans. ASTM E 72 specifies a specimen width of 4 ft and either line loading applied at the outer 1/4 points of the span as shown in sketch (b) of the figure, or uniformly distributed loading shown in sketch (c). Uniform transverse loading of upright specimens such as masonry walls has become practical and more commonly used with the development of the “bag method” in which a plastic or rubberized fabric bag, interposed between the wall face and a backboard, is inflated with air to give increments of pressure against the wall until failure. Comparing the deflection formulas for the three loading methods, it will be noted that 1/4-point loading causes the greatest deflection, assuming a given moment and wall section, and from this standpoint is more severe than the other two methods. Also, any line load method produces combinations of maximum shear and moment in the same region which generally results in a lower indicated strength than would be obtained with uniform loading where the regions of maximum moment and of maximum shear are widely separated. It also appears that a test loading which does not include concentrated loads more nearly simulates the more common loads considered in design, such as wind and fluid or earth pressure.

Flexural strength of unreinforced masonry assemblages can also be measured by other ASTM methods. Method E 518, Standard Test Method for Flexural Bond Strength of Masonry (ref. 5), is intended to provide simplified and economical means for gathering comparative research data on the flexural bond strength developed with different types of masonry units and mortar or for the purpose of checking job quality control as regards materials and workmanship. Unlike ASTM E 72, Method E 518 is typically not intended for use in establishing design stresses. Specimens are small prisms laid up in stack bond and tested in a horizontal position by applying load at the third-points or by applying a uniform load by means of an inflated air bag.

Method C 1072, Standard Test Method for Measurement of Masonry Flexural Bond Strength (ref. 6) covers physical testing of each joint of masonry prisms using a bond wrench test apparatus. This method permits the measurement of multiple joints in a prism rather than the single joint tests of E 82 and E 518, making statistical evaluations easier and more cost effective. The results are used to: determine compatibility of mortars and masonry units; determine the effect on flexural bond strength of factors such as mortar properties and workmanship; and predict the flexural strength of a wall. The flexural bond strength determined using C 1072 is not typically used to predict the flexural bond strength of a wall constructed of the same material unless testing is performed to document the difference between the two; nor to determine extent of bond for a water permeance evaluation.

RACKING

Racking strength tests are performed according to the general schemes shown in Figure 4 and give an indication of the resistance of the construction to the horizontal component of shearing forces acting parallel to the wall. In the method prescribed in ASTM E 72, shown in the upper sketch, the horizontal component is equal to the load P, and the principal stress, compression acting in a diagonal line between the load point and lower right reaction, is the resultant of load P and vertical reaction Rv.

In the scheme shown in the lower sketch, the load is applied diagonally downward and the horizontal component or longitudinal shear is equal to P cosφ , approximately 0.7P since φ is usually about 45o. This alternative method is not addressed in ASTM E 72, but has often been used because it eliminates the need for tie-down rods.

Results of racking tests of masonry walls generally are given in terms of the maximum horizontal component, pounds per linear foot of wall, although the total load P may also be reported. While failure is considered to be in shear, it actually is caused by a combination of shear and secondary tensile stresses, the latter acting normal to the compressive stress. Although exceeding both the shearing and secondary tensile stresses in intensity, compressive stress is not sufficient to cause a compression-type failure.

As with compression and flexural testing, an ASTM method exists for testing shear on specimens smaller than required in ASTM E 72. In ASTM E 519, Standard Test Method for Diagonal Tension (Shear in Masonry Assemblages) (ref. 4), 4 ft by 4 ft masonry assemblages are positioned in a compression testing machine so that a compressive load is applied along one diagonal, causing a diagonal tension failure with the specimen splitting apart normal to the direction of load (see Figure 5). This method also avoids the need for a hold down force to prevent rotation of the specimen as used in the E 72 method, simplifying the analysis of the state of stress in the specimen.

IMPACT

The impact test of ASTM E 695 (ref. 7) affords a qualitative measure of the capacity of wall, floor and roof panels to resist impact loading. The impact force is obtained from the free-fall of a bag of lead shot through a path which causes it to strike the center of the panel at an angle normal to the surface. The essential details of the method as adapted to the testing of upright masonry wall panels are shown in Figure 6. Panels are typically 4 ft long and are simply supported on a span six inches less than the height of the specimen. The height of drop is increased in increments until failure occurs, but not exceeding 10 ft, and the maximum drop is the value reported.

The structural testing of masonry walls and assemblages encompasses much more than merely determining the ultimate load at failure. At each load increment, strains and deflections are carefully measured with precision instruments. at various locations on the specimen. In some procedures a load increment is applied and measurements are taken after which the load is released and measurements again taken to determine the residual strain or deflection. The specimen is examined and notes made of any cracking, crushing or other visible distress. This process is repeated at each increase in load so that when the test has been concluded the research engineer has accumulated the data needed to give a clear picture of the structural behavior of the specimen through all stages of loading.

REFERENCES

Standard Test Method for Compressive Strength for Laboratory Constructed Masonry Prisms, ASTM E 447-97. American Society for Testing and Materials, 1997.
Standard Test Method for Conducting Strength Tests of Panels for Building Construction, ASTM E 72-95. American Society for Testing and Materials, 1995.
Standard Test Method for Constructing and Testing Masonry Prisms Used to Determine Compliance with Specified Compressive Strength of Masonry, ASTM C 1314-97. American Society for Testing and Materials, 1997.
Standard Test Method for Diagonal Tension (Shear) in Masonry Assemblages, ASTM E 519-81(1993)e1. American Society for Testing and Materials, 1993.
Standard Test Method for Flexural Bond Strength of Masonry, ASTM E 518 -80(1993)e1. American Society for Testing and Materials, 1993.
Standard Test Method for Measurement of Masonry Flexural Bond Strength, ASTM C 1072-94. American Society for Testing and Materials, 1994.
Standard Test Method for Measuring Relative Resistance of Wall, Floor, and Roof Construction to Impact Loading, ASTM E 695-79(1997)e1. American Society for Testing and Materials, 1997.

Evaluating the Compressive Strength of Concrete Masonry Based on 2012IBC/2011 MSJC

August 2018

INTRODUCTION

Structural performance of concrete masonry is largely dependent upon three key criteria:

the engineering rationale incorporated into the design of the structure;
the physical characteristics of the materials used in the construction of the structure (i.e., the masonry units, grout, mortar, and reinforcement); and
the quality of the construction used in assembling these components.

The first step in the design of any engineered masonry structure is determining anticipated service loads. Once these loads are established, the required strength of the masonry can be determined. The designation f’_m, indicates the specified compressive strength of masonry. It is used throughout the design and, in accordance with the appropriate code, to predict the strength and behavior of the masonry assembly and thus to size masonry elements. It should be stressed that the specified compressive strength of the masonry is related to but not equal to the tested compressive strength of the masonry.

To ensure that a safe and functional structure is being constructed that will meet or exceed the intended service life, measures must be taken to verify that the compressive strength of the assembled materials, including masonry units, mortar and grout if used, meet or exceed the specified compressive strength of the masonry.

Compliance with the specified compressive strength is verified by one of two methods: the unit strength method or the prism test method. These two methods are referenced in masonry design codes (refs. 1, 4), specifications (ref. 2), and standards (ref. 3) as rational procedures for verifying masonry compressive strength.

UNIT STRENGTH METHOD

The unit strength method is often considered the least expensive and most convenient of the two methods. However, the unit strength method also yields more conservative masonry strengths when compared to the prism test method especially at the higher range of masonry unit strengths.

Compliance with f’_m by the unit strength method is based on the net area compressive strength of the units and the type of mortar used. The compressive strength of the masonry assemblage is then established in accordance with Table 1. Table 1 is based on criteria from Specification for Masonry Structures (ref. 2) and the International Building Code (ref. 4).

According to both of these documents, use of the unit strength method requires the following:

Masonry units must be sampled and tested in accordance with ASTM C140, Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units (ref. 5) and meet the requirements of either ASTM C55, Standard Specification for Concrete Building Brick (ref. 6) or ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units (ref. 7).
Thickness of bed joints used in construction must not exceed ⅝ in. (15.9 mm).
If grouted masonry is used in construction, the grout must meet either the proportion or the property specification of ASTM C476, Standard Specification for Grout for Masonry (ref. 8), and the 28-day compressive strength of the grout must equal or exceed f’_m but not be less than 2,000 psi (14 MPa). When property specifications are used, the compressive strength of the grout is determined in accordance with ASTM C1019, Standard Test Method for Sampling and Testing Grout (ref. 9).
Mortar must comply with requirements of ASTM C270, Standard Specification for Mortar for Unit Masonry (ref. 10).

Since all concrete masonry units complying with ASTM C90 (ref. 7) have compressive strengths exceeding 1,900 psi (13.1 MPa), by the unit strength method any C90 unit used with Type M or S mortar can be used for projects that have f’_m values up to 1,500 psi (10.3 MPa). If used with Type N mortar, any C90 unit can be used for projects having f’_m values up to 1,350 psi (9.3 MPa). Conversely, if the concrete masonry units have compressive strengths of 2,800 psi (19.3 MPa), then the maximum f’_m used in design would be 2,000 psi (13.8 MPa) if Type M or S mortar were used. Similarly, if 3,050 psi (21.0 MPa) concrete masonry were used in conjunction with Type N mortar, the maximum f’_m that could be used in design would also be 2,000 psi (13.8 MPa). Note that per footnote A of Table 1, compressive strength of masonry values must be multiplied by 85% when the unit strength is established on units less than 4 in. (102 mm) in height.

When higher strength masonry materials are specified, it usually is more cost effective to utilize the prism test method to demonstrate compliance with f’_m due to the level of conservatism inherent in the unit strength method; i.e., the costs of testing are well offset by the construction savings resulting from a more economical design that takes advantage of using a higher compressive strength for the same specified materials.

^A For units less than 4 in. (102 mm) in height, 85% of the values listed.

PRISM TEST METHOD

ASTM C1314, Standard Test Method for Compressive Strength of Masonry Prisms (ref. 3), contains provisions for determining the compressive strength of a masonry prism: an assemblage made of representative units, mortar and grout (for grouted masonry construction). Although constructed using materials used in the project, the prism is not intended to be a reduced-scale version of the wall, but rather a quality assurance instrument to demonstrate how the masonry components work together. For this reason, prisms are typically constructed in stack bond with a full mortar joint, regardless of the wall construction. The tested compressive strength of the prism is corrected to account for different permissible height to thickness ratios of the prisms. This corrected strength must equal or exceed f’_m. Understandably, prism testing should be undertaken before construction begins to verify that the compressive strength of the assembled materials is not less than the specified compressive strength used in the design.

Prisms should be 28 days old to document compliance with f’_m, When prisms are tested as part of an inspection program periodically during the course of construction, an earlier age, such as 3 or 7 days, is often preferred. To confidently interpret the results of these earlier age prism tests, the relationship between prism age and strength development should be determined using the materials, construction methods and testing procedures to be used throughout the job. Only when this strength/time curve is generated can early age test results be extrapolated to predict the 28-day strength.

Prism Construction

Masonry prisms are constructed using units representative of those being used in the construction. One set of prisms (containing three individual prisms) is constructed for each combination of materials and each testing age for which the compressive strength is to be determined. For multi-wythe masonry construction, with different units or mortar in each wythe, separate prisms should be built representative of each wythe, and tested separately. Prisms should be constructed on a flat and level location where they can remain undisturbed until they are transported for testing, at least 48 hours.

All units used to construct the prisms must be of the same configuration and oriented in the same way so that webs and face shells are aligned one on top of the other. Units are laid in stack bond on a full mortar bed using mortar representative of that used in the corresponding construction. Mortar joints are cut flush regardless of the type of mortar joint tooling used in the construction. Prisms composed of units that contain closed cells must have at least one complete cell with one full-width cross web on either end. Various prism configurations are shown in Figure 1.

Since masonry prisms can be heavy, especially grouted prisms, it often proves effective to construct prisms using half-length units. The criteria for constructing prisms of reduced-sized units are (also see Figure 2):

that hollow units contain fully closed cells,
that the cross section is as symmetrical as possible, and
that the length is not less than 4 in. (102 mm).

As a result, handling, transporting, capping, and testing the reduced sized prisms is easier, resulting in less potential for damage to the prisms. Using reduced-length prisms also reduces the required plate thicknesses for compression machines and typically result in higher and more accurate assessments of masonry strengths.

Immediately following construction of the prisms, each prism is sealed in a moisture-tight bag, as shown in Figure 3. The prism test method requires prisms to be cured in sealed plastic bags to ensure uniform hydration of the mortar and the grout if used. Under actual field conditions, it may require longer periods for hydration and the corresponding strengths to be achieved. Curing prisms in sealed plastic bags results in measured strengths which are representative of those exhibited by the masonry throughout the life of the structure. Bag curing also provides a uniform and repeatable testing procedure.

Where the corresponding construction is to be grouted solid, each prism is grouted solid using grout representative of that being used in the corresponding construction. When prisms are used for field quality control or assurance, prisms must be constructed at the same time as the corresponding construction and grouted when the construction is being grouted. When prisms are used for other purposes, such as preconstruction or for research, prism grouting must occur between 4 hours and 48 hours following the construction of the prisms.

After grouting, the grout in each prism is consolidated and reconsolidated using procedures representative of those used in the corresponding construction. After each consolidation, the grout in the prism will likely settle due to water absorption from the grout into the masonry units. Therefore, after each consolidation, additional grout should be added as necessary and be screeded level with the top of the prism to facilitate capping. Reinforcement is not included in prisms. Immediately following prism grouting, the moisture-tight bag is resealed around each prism.

If the corresponding construction will be partially grouted, two sets of prisms are constructed—one set grouted and one set ungrouted.

Transporting Prisms

Since mishandling prisms during transportation from the job site to the testing facility can have significant detrimental effects on the tested compressive strength of prisms, extreme care should be taken to protect against damage during transport. Prior to transporting, the prisms should be strapped or clamped as shown in Figure 4 to prevent damage. Tightly clamping or strapping plywood to the top and bottom of a prism prevents the mortar joint from being subjected to tensile stresses during handling. The prisms should also be secured during transport to prevent jarring, bouncing or tipping.

Curing Prisms

As previously stated, each prism is constructed in a moisture-tight bag (Figure 3) large enough to enclose and seal the completed prism. The bags should have adequate thickness to prevent tearing; a thickness of 2 mils (0.0051 mm) or greater has been found to work well. After the initial 48 hours of job site curing in the moisture-tight bag, each prism is carefully moved to a location where the temperature is maintained at 75 ± 15° F (24 ± 8° C) for full curing prior to testing.

Prism Net Cross-Sectional Area

To provide accurate an accurate strength calculation, the laboratory needs to determine the net area of the prisms. Ungrouted masonry prisms should be delivered to the testing agency with three additional units, identical to those used to construct the prism. If reduced-length prisms are used, additional reduced-length units should accompany the prisms to the laboratory for this purpose.

The net cross-sectional area used to calculate compressive strength of a prism depends on whether the prisms are grouted or ungrouted. For ungrouted full-size prisms, the cross-sectional area is the net cross-sectional area of the masonry units determined in accordance with ASTM C140 on concrete masonry units identical to those used to construct the prisms. When reduced sized units are used to construct ungrouted prisms, the net cross-sectional area is based on the reduced sized units.

When testing fully grouted prisms, net cross-sectional area is determined by multiplying the actual length and width of the prism per ASTM C1314. These areas are illustrated in Figure 5.

Testing Prisms

Two days prior to the 28 day time interval or the designated testing time, each prism is removed from the moisture tight bag. Prism age is determined from the time of laying units for ungrouted prisms, and from the time of grouting for grouted prisms.

To provide a smooth bearing surface, prisms are capped with either a sulfur or high-strength gypsum compound in accordance with ASTM C1552, Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing (ref. 12). No other capping materials are permitted, nor are unbonded caps.

Capping provides level and uniform bearing surfaces for testing, thereby eliminating point loads due to surface irregularities. The result is more uniform and reliable compressive strength values. Patching of caps is not permitted because it is difficult to maintain a planar surface within the tolerances of ASTM C1552.

Capping materials must have a compressive strength of at least 3,500 psi (24.13 MPa) at an age of 2 hours when cubes of the material are tested in accordance with ASTM C617, Standard Practice for Capping Cylindrical Concrete Specimens (ref. 13).

The average thickness of the cap must not exceed ⅛ in. (3.2 mm). Caps are to be aged for at least 2 hours before testing the specimens, regardless of the type of capping material. Capping plates of adequate stiffness and smoothness are critical to achieving accurate results. Machined steel plates of 1 in. (25.4 mm) minimum thickness are required as a base. Glass plates not less than ½ in. (12.7 mm) in thickness may be used as a wearing surface to protect the plates. The capping wear plate must be plane within 0.003 in. in 16 in. (0.075 mm in 400 mm) and free of gouges, grooves and indentations greater than 0.010 in. (0.25 mm) deep or greater than 0.05 in.² (32 mm²).

One of the most common oversights in testing masonry prisms is compliance with the established requirements for the testing machine itself. The testing machine is required to have a spherically seated head with a minimum 6 in. (150 mm) diameter and capable of rotating in any direction. The spherically seated head is then attached to a single thickness steel bearing plate having a width and length at least ¼ in. (6.4 mm) greater than the length and width of the prism being tested. The required thickness of the steel bearing plate depends on the diameter of the spherically seated head and the width and length of the prism being tested. The thickness of the steel bearing plate must equal or exceed the maximum distance from the outside of the spherically seated head to the outmost corner of the prism—designated d in Figure 6. Failure to provide the required minimum bearing plate thickness decreases the measured compressive strength of the prism due to the bearing plate bending during testing. It is also required that the bearing faces of the plates have a Rockwell hardness of at least HRC 60 (BHN 620).

The last step prior to testing a prism in compression is determining the prisms center of mass. The center of mass of a prism can be thought of as the point on the cross-section of a prism where it could physically balance on a point. The prism is then centered within the test machine such that the center of mass coincides with the center of thrust (which coincides with the center of the spherically seated head).

Failure to align the center of mass with the center of thrust results in a nonuniform application of load and therefore lower measured compressive strengths. For prisms having symmetric cross-sections, the mass centroid coincides with the geometric centroid—or the center of the prism as measured with a ruler. For prisms that are non-symmetrical about an axis, the location of that axis can be determined by balancing the masonry unit on a knife edge or a metal rod placed parallel to that axis. If a metal rod is used, the rod must be straight, cylindrical (able to roll freely on a flat surface), have a diameter between ¼ in. and ¾ in. (6.4 and 19.1 mm), and it must be longer than the specimen. Once determined, the centroidal axis can be marked on the end of the prism.

To test the prism, it is placed in the compression machine with both centroidal axes of the specimen aligned with the machine’s center of thrust. The maximum load and type of fracture is recorded. Prism strength is calculated from the maximum load divided by the prism net area. This prism strength is then corrected as described below.

Corrections for Prism Aspect Ratio

Since the ratio of height, h_p, to least lateral dimension, t_p,—designated the aspect ratio or h_p/t_p—of the prism can significantly affect the load carrying capacity of the masonry prism, ASTM C1314 contains correction factors for prisms having different aspect ratios, as outlined in Table 2.

To use the values in Table 2, simply multiply the measured compressive strength of the prism by the correction factor corresponding to the aspect ratio for that prism. Correction factors shown in Table 3 can be linearly interpolated between values, but cannot be extrapolated for aspect ratios less than 1.3 or greater than 5.0.

Table 2—Prism Aspect Ratio Correction Factors (ref. 3)

PRISMS FROM EXISTING CONSTRUCTION

The majority of quality assurance testing of concrete masonry materials is conducted on samples representative of those used in the construction. In some cases, however, it may be necessary or desirable to evaluate the properties of existing masonry construction using the actual construction materials instead of representative samples. Examples where the in-place (in-situ) masonry properties might need to be considered include old or damaged construction, or during the construction process, when: a testing variable or construction practice fails to meet specifications; a test specimen is damaged prior to testing; test records are lost; or representative samples are not otherwise available.

The procedures covered in ASTM C1532, Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units and Specimens from Existing Construction, (ref. 14), are useful when physical examination of an assembly’s compressive strength, stiffness, flexural strength or bond strength is needed on a representative sample of the actual construction. These specimens are a portion of the existing masonry, and may include units, mortar, grout, reinforcing steel, collar joint and masonry accessories. The specimens can be taken from single or multiwythe construction. The procedures outlined in C1532 focus on documenting the condition of the masonry and protecting the specimens from damage during removal and transportation to the testing laboratory.

C1532 is very similar to ASTM C1420, Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units Placed in Usage (ref. 15).

Standard Practice for Preparation of Field Removed Manufactured Masonry Units and Masonry Specimens for Compressive Strength Testing, ASTM C1587 (ref. 16), provides procedures for preparing field-removed specimens for compressive strength testing, and covers procedures such as removing hardened mortar and cleaning.

Compressive strength test results of field-removed masonry units and assemblies are expected to vary from, and will likely be less than, compressive strength test results of new masonry units and newly assembled prisms. Therefore, drawing relationships between the results of tests conducted on field-removed specimens to those of masonry units prior to use or of constructed prisms is difficult.

Prior to removal of specimens from existing construction, a repair plan should be developed. This plan should include replacement of units removed and repair of any disturbed or cut reinforcement, including those unintentionally damaged during the removal process.

Selecting Specimens

Specimens should be representative of the masonry construction as a whole, considering variations within the construction such as: parapets, corbels, areas where different masonry units are combined for architectural effects, as well as variations in the condition or exposure of the masonry. C1532 includes guidance on random sampling, location-specific sampling, and on condition-specific sampling. When testing to help quantify the effects of various exposures or conditions, the sampling should represent each exposure condition.

Thorough documentation of the specimen’s condition prior to removal is necessary to assess whether the specimen was subsequently damaged during removal and transport, and for comparative purposes with the other specimens.

Removing Specimens

Carefully remove each specimen at its perimeter, ensuring the specimen is the appropriate size for the intended testing. Note that hydraulic or electric impact equipment should not be used, due to the potential for damaging the specimens. Saw-cutting or hand chiseling is preferred.

The following procedure is recommended. Make the first cut along the bottom of the specimen (on both sides of the wall if necessary) and insert shims. Make the two vertical cuts at the sides of the specimen, then make the top cut. Provide any necessary shoring, bracing and weather protection for the remaining construction. Similar to the pre-removal documentation, assess and document the specimen’s condition to determine if the specimen was damaged during removal.

Transporting Specimens

The specimens should be confined as described in Transporting Prisms, page 4. In addition, each specimen should be protected on all sides with material such as 1 in. (25 mm) thick packaging foam or bubble wrap, placed in sturdy crates, and the crates completely filled with packing material to ensure the specimens cannot move within the crate during transport.

Testing Specimens

It is not permitted to test grouted or partially grouted specimens that contain vertical reinforcement. Specimens cut from existing construction containing horizontal reinforcement can be tested, but the presence and location of reinforcement should be noted and reported.

Prisms must: include at least one mortar bed joint; have an aspect ratio (h_p/t_p) between 1.3 and 5; have a height of at least two units (each of which is at least one-half the height of a typical unit); have a length one-half the unit length and two unit lengths; not include vertical reinforcement. In addition, when prisms contain units of different sizes and/or shapes, the unit height and length are considered to be that of the largest unit height or largest unit length within the prism.

The specimens should be prepared for capping by smoothing and removing loose or otherwise unsound material from the bearing surfaces, to produce a plumb and level surface.

Note that grouted or partially grouted specimens cannot contain vertical reinforcement. The specimens are photographed to document specimen condition prior to capping. Capping and testing procedures are identical to those for constructed prisms.

Field-removed prisms may have non-uniform dimensions that should be considered when determining net cross-sectional area for calculating compressive strength. Professional judgement should be used to determine the minimum bearing area of a non-uniform prism. One effective method for face-shell bedded specimens is to multiply the length of the specimen at the bed joint by the sum of the face shell thicknesses to determine minimum bearing area.

REFERENCES

Building Code Requirements for Masonry Structures, ACI 530-11/ASCE 5-11/TMS 402-11. Reported by the Masonry Standards Joint Committee, 2011.
Specifications for Masonry Structures, ACI 530.1-11/ASCE 6-11/TMS 602-11. Reported by the Masonry Standards Joint Committee, 2011.
Standard Test Method for Compressive Strength of Masonry Prisms, ASTM C1314-10. ASTM International, Inc., 2010.
International Building Code, International Code Council, 2012.
Standard Test Methods of Sampling and Testing Concrete Masonry Units and Related Units, ASTM C140-11. ASTM International, Inc., 2011.
Standard Specification for Concrete Building Brick, ASTM C55-09. ASTM International, Inc., 2009.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-11. ASTM International, Inc., 2011.
Standard Specification for Grout for Masonry,ASTM C476-10. ASTM International, Inc., 2010.
Standard Test Method for Sampling and Testing Grout, ASTM C1019-11. ASTM International, Inc., 2011.
Standard Specification for Mortar for Unit Masonry, ASTM C270-10. ASTM International, Inc., 2010.
Standard Specification for Mortar Cement, ASTM C1329-05. ASTM International, Inc., 2005.
Standard Practice for Capping Concrete Masonry Units, Related Units and Masonry Prisms for Compression Testing, ASTM C1552-09a. ASTM International, Inc., 2009.
Standard Practice for Capping Cylindrical Concrete Specimens, ASTM C617-10. ASTM International, Inc., 2010.
Standard Guide for Seletion, Removal, and Shipment of Manufactured Masonry Units and Specimens from Existing Construction, ASTM C1532-06. ASTM International, Inc., 2006.
Standard Guide for Selection, Removal, and Shipment of Manufactured Masonry Units Placed in Usage, ASTM C1420-03. ASTM International, Inc., 2003.
Standard Practice for Preparation of Field Removed Manufactured Masonry Units and Masonry Specimens for Compressive Strength Testing, ASTM C1587-09. ASTM International, Inc., 2009.