The Building Code Requirements and Specification for Masonry Structures (ACI 530.1-11) mandates that the compressive strength of masonry should be tested before construction and after every 464.5 square meters of masonry work during construction. This requirement is part of the quality assurance program, and compliance certification is necessary.
The ACI 530.1-11 provides two ways of specifying the compressive strength of clay and concrete masonry: unit strength method and masonry prism method. Architects or engineers can specify either method, and if they do not, the contractor can choose the method of testing. However, the prism testing method is more expensive because of the costs associated with making prisms and laboratory tests. On the other hand, the unit strength method is more conservative than the prism strength method.
Methods of Testing Compressive Strength of Masonry
There are two commonly used methods for testing the compressive strength of masonry: the unit strength method and the prism test method.
The unit strength method involves testing individual units of masonry, such as bricks or concrete blocks, to determine their compressive strength. This method takes into account the properties of the individual units and the mortar used to bond them together, as well as the geometry of the wall being constructed.
On the other hand, the prism test method involves testing the compressive strength of actual masonry walls or sections, known as prisms, that are constructed using the same materials and techniques as the wall being evaluated. This method provides a more accurate representation of the compressive strength of the actual wall, as it takes into account the effects of the mortar and the overall geometry of the wall.
Both of these methods are important for evaluating the strength of masonry structures and ensuring that they are capable of withstanding the loads and stresses that they will encounter during their intended use.
Unit Strength Method of Testing Compressive Strength of Masonry
The unit strength method requires testing of masonry units both before and during construction to ensure they have sufficient strength. The specified compressive strength of masonry (f’m) is determined not only by the compressive strength of the units, but also by the mortar used. To meet requirements, both clay and concrete masonry units must adhere to their respective ASTM specifications.
For clay masonry units, applicable ASTM specifications include ASTM C 62-05 for building bricks made from clay or shale, ASTM C 216-05a for facing brick made from clay or shale, and ASTM C 652-05 for hollow brick made from clay or shale. Testing for clay masonry units must follow Test Methods for Sampling and Testing Brick and Structural Clay Tile.
Similarly, for concrete masonry units, ASTM specifications must be followed, including ASTM C55-03 for concrete brick and ASTM C90-06 for load-bearing concrete masonry units. Sampling and testing for concrete masonry units must adhere to either ASTM C55-03a or ASTM C90-06.
In the case of grouted masonry, the grout used for both concrete and clay masonry units must conform to ASTM C476. The grout’s compressive strength must be equal to or greater than the compressive strength of the masonry, and not less than 13.79 MPa. Furthermore, the bed joint thickness for both concrete and clay masonry units must be equal to or less than 15.87 mm.
Prism Method of Testing Compressive Strength of Masonry
The ASTM C 1314-03b standard outlines the procedure for constructing and testing masonry prisms, which are assemblages of masonry units, mortar, and grout used to determine the compressive strength of masonry. This test method ensures that the masonry materials used meet the specified compressive strength. The prisms are constructed using at least two masonry units in stretcher position, and the aspect ratio of the prisms must be between 1.3 and 5. The prisms are tested after a specified period and must be taken out of the airtight bags two days before testing.
To compute the compressive strength of masonry, the sustained compressive load of the prism is divided by its net cross-sectional area. The masonry compressive strength is then calculated based on the aspect ratio of the masonry prisms, with a correction factor provided in Table-1 for aspect ratios ranging from 1.3 to 5. When the aspect ratio of the prism is 2, the compressive strength of the masonry is considered equal to that of the prism. The achieved values are averaged to determine the compressive strength of the masonry.
The prisms used for testing must be prepared in a manner representative of the construction, with adequate mortar, unit positioning, and aligning. The prisms may be constructed as solid or hollow, ungrouted or grouted, and grouting procedures must be similar to the corresponding construction. Two groups of prisms are required to be tested, one grouted and one ungrouted. If the prisms are grouted, the timing of grouting must be no less than 24 hours and no more than 48 hours, and the stored prisms must be kept at 24 oC.
Table-1: Aspect Ratio Correction Factors Compressive Strength of Masonry Prism
| tp | Correction factor |
| 1.3 | 0.75 |
| 1.5 | 0.86 |
| 2 | 1 |
| 2.5 | 1.04 |
| 3 | 1.07 |
| 4 | 1.15 |
| 5 | 1.22 |
The process of interpolation can be utilized to derive a correction factor for aspect ratios that are not listed in a table. In other words, if there is missing data in a table for certain aspect ratios, interpolation can be used to estimate the values for those missing entries. Interpolation involves using known data points to estimate values for unknown points that fall within the same range as the known data. This technique is commonly used in mathematics, science, and engineering to estimate missing data or values that fall between two known data points. By employing interpolation, accurate estimates can be made for aspect ratios that are not explicitly listed in a given table.