Methods of Testing Compressive Strength of Masonry
Testing the compressive strength of masonry is a crucial step in construction to ensure the structural integrity and safety of buildings. The Building Code Requirements and Specification for Masonry Structures (ACI 530.1-11) mandates the testing of compressive strength before construction and periodically during construction.
This article explores two widely used methods for testing the compressive strength of masonry: the unit strength method and the prism test method.
A. Methods of Testing Compressive Strength of Masonry
When it comes to testing the compressive strength of masonry, two main methods are commonly employed:
- Unit Strength Method
- Prism Test Method
B. Unit Strength Method of Testing Compressive Strength of Masonry
The unit strength method requires the testing of masonry units before and during construction to ensure their strength meets the required standards. The specified compressive strength of masonry (f’m) depends not only on the strength of the masonry units but also on the mortar used. For both clay and concrete masonry units, adherence to the relevant ASTM specifications is essential.
Clay masonry units should conform to ASTM specifications such as ASTM C 62-05 for building bricks, ASTM C 216-05a for facing bricks, and ASTM C 652-05 for hollow bricks. Testing and sampling of clay masonry should follow the Test Methods for Sampling and Testing Brick and Structural Clay Tile.
Concrete masonry units should comply with ASTM specifications like ASTM C55-03 for concrete bricks and ASTM C90-06 for load-bearing concrete masonry units. Sampling and testing procedures should align with either ASTM C55-03a or ASTM C90-06.
For grouted masonry, the grout used with both concrete and clay masonry units should adhere to ASTM C476, and its compressive strength should be equal to or greater than that of the masonry, not less than 13.79 MPa. The bed joint thickness for both concrete and clay masonry should be equal to or less than 15.87 mm.
C. Prism Method of Testing Compressive Strength of Masonry
The prism test method involves the construction and testing of masonry prisms, which consist of masonry units, mortar, and grout (if applicable). This method, outlined in ASTM C 1314-03b, is used to determine compliance with the specified compressive strength of masonry.
To construct masonry prisms, exemplary units that will be used in the actual construction are employed. At least three prisms, using the same materials, must be constructed, and the masonry units should be tested at the same age according to the determined procedure.
The aspect ratio (height to smallest lateral dimension) of the prisms (hp/tp) should fall within the range of 1.3 to 5. The masonry units must be laid in a stack bond in stretcher position, replicating the intended construction. The mortar, joint thickness, and unit positioning during prism construction should be representative of the actual construction. The prisms can be constructed as solid or hollow, grouted or ungrouted, depending on the specific requirements of the structure.
The grouting procedure, consolidation, and reconsolidation of prisms must mimic the corresponding construction process. Two series of prisms are typically tested: one group is grouted, while the other remains ungrouted. When solidly grouting prisms, the grouting should be carried out between 24 and 48 hours. The prisms should be stored in airtight bags and left undisturbed for 48 hours, after which they can be kept at a temperature of 24°C. The tests on the prisms are conducted after 28 days or any other specified period. The prisms must be removed from the airtight bags two days before testing begins.
The compressive strength of masonry is determined through a three-step process based on prism tests. Firstly, the masonry prism strength is estimated by dividing the sustained compressive load by the net cross-sectional area of the prism. Secondly, the masonry compressive strength is calculated based on the aspect ratio of the masonry prisms.
Testing the compressive strength of masonry is essential to ensure the structural integrity of buildings. The unit strength method and prism test method are two widely used techniques for determining the compressive strength of masonry.
While the prism test method may be more costly due to the construction and laboratory tests involved, it provides more accurate results. Engineers, architects, and contractors play vital roles in selecting and implementing the appropriate testing method to ensure compliance with the required compressive strength standards. By following the specified testing methods, construction professionals can ensure the durability and safety of masonry structures.