Testing compressive strength of masonry before construction and for every 464.5 m2 of masonry work during construction is required as per Building Code Requirements and Specification for Masonry Structure (ACI 530.1- 11) and under its quality assurance program, certification of compliance.
There are two ways of specifying compressive strength of clay and concrete masonry which are unit strength methods and masonry prism method. These testing methods are provided by ACI 530.1-11.
Either engineers or architects specify one of the two methods. In the case when the method of testing is not determined, the contractor is permitted to select method of determining compressive strength of masonry.
Due to costs related to the making prisms and laboratory tests, the prism testing method is more costly compare with the unit strength method. Nevertheless, results of unit strength method are more conservative than prism strength method.
Methods of Testing Compressive Strength of MasonryThe testing methods for compressive strength of masonry are:
- Unit strength method
- Prism test method
Unit Strength Method of Testing Compressive Strength of MasonryIn unit strength method, masonry units are needed to be tested before and during construction to guarantee their sufficient strength. The values of specified compressive strength of masonry (f’m) depend on not only the compressive strength of masonry units but also on the mortar. Both clay and concrete masonry units should conform to their related ASTM specifications. Clay masonry units need to conform applicable ASTM specifications namely ASTM C 62-05: standard specification for building bricks (solid masonry units made from clay or shale), ASTM C 216-05a: standard specification for facing brick (solid masonry units made from clay or shale), and ASTM C 652-05: standard specification for hollow brick (hollow masonry units made from clay or shale). Clay masonry samples and testing must be as per Test Methods for Sampling and Testing Brick and Structural clay tile. Similarly, concrete masonry unit should conform to the applicable ASTM specifications which are ASTM C55-03: Specification for Concrete Brick and ASTM C90-06: Specification for Load Bearing Concrete Masonry Units in addition to follow either ASTM C 55-03a or ASTM C 90-06 for sampling and testing. In case of grouted masonry, the grout employed for concrete and clay masonry units should conform to ASTM C 476 and grout compressive strength should equal or greater than the compressive strength of masonry and no smaller than 13.79 MPa. Additionally, bed joint thickness for both concrete and clay masonry must be equal or smaller than 15.87 mm.
Prism Method of Testing Compressive Strength of MasonryMasonry prisms as shown in Figure-1 and Figure-2 are an assemblage of masonry units, mortar, and grout if applicable which are constructed and tested for their compressive strength as per the ASTM C 1314-03b: Standard Test Method for Constructing and Testing Masonry Prisms which is employed for determine compliance with specified compressive strength of masonry. The ASTM C 1314-03b specification deals with masonry prism construction procedures, testing, and procedures for determining masonry compressive strength (f’m). This test method can check whether masonry materials used produce masonry which meet the specified compressive strength or not. The masonry units utilized for building masonry prisms are an exemplary of those units that will be used construction. Moreover, in the prism test method, minimum three prisms are required to be constructed and the same material should be used. At least two masonry units should be employed to build prisms and they should be tested at the same age based on determined procedure. The aspect ratio or in other word the height to smallest lateral dimension of the prisms (hp/(tp) need to be between 1.3 and 5. All masonry units must be laid in stack bond in stretcher position and oriented as in corresponding construction and prepared with adequate mortar. The length and width of masonry prism is equal to masonry unit length and width. Not only does the mortar and thickness of joint but also unit positioning and aligning method while prisms are prepared mortar need to be representative of the construction; the prisms might be constructed as solid or hollow ungrouted or solid or hollow grouted depend on the real situation of the structure. The procedure of grouting, its consolidation, and reconsolidation are required to be similar to the corresponding construction. Two series of prisms are required to be grouted; one group is grouted and another group is ungrouted. In the case of grouting prisms solidly, the timing of grouting prisms needs to be no less than twenty four hours and no more than forty eight hours. Added to that, they should be saved in air proof bags and not be disturbed before forty eight hours after that, the stored prisms have to be kept at 24 oC. Tests of prisms are carried out after 28 day or any other specified period but the prism must have taken out of the airtight backs two days before the test is began. .
Fig.1: Masonry Prisms: (A) Hollow Ungrouted Unit, (B) Hollow Grouted Unit
Fig.2: Solid Unit PrismThe compressive strength of masonry is computed in by three steps from prism tests. Firstly, estimate masonry prism strength which is equal to sustained compressive load of prisms by net cross sectional area of that prism. Secondly, calculate masonry compressive strength depend on the aspect ratio (the height to smallest lateral dimension of the prisms) of masonry prisms tests. Thirdly, the compressive strength of masonry is considered to be equal to that of masonry prism provided that the masonry prisms have aspect ratio of 2. When the aspect ratio masonry prism is ranging from 1.3 to 5, the compressive strength of masonry is achieved by correction factor which provided in Table-1 multiply by masonry prism strength. The compressive strength of masonry is computed by averaging the achieved values.
Table-1: Aspect Ratio Correction Factors Compressive Strength of Masonry Prism