Compressive strength of concrete Blocks or concrete masonry units are required to know the suitability of these in construction works for various purposes.
Concrete masonry blocks are generally made of cement, aggregate and water. Which are usually rectangular and are used in construction of masonry structure. They are available in solid and hollow forms. The nominal dimensions of concrete masonry block vary as follows.
Length: 400 or 500 or 600mm
Width: 200 or 100mm
Width: 50, 75, 100, 150, 200, 250 or 300mm.
Tests on Concrete Masonry Block Units
Different tests are conducted on concrete masonry unit to satisfy the all requirements. But now we are discussing about three tests conducted on concrete masonry block which are as follows.
Blocks of same mix shall be taken and divide them as follows to conduct the following tests.
 Dimension measurement (All blocks)
 Density of block (3 blocks)
 Compressive strength of block (8 blocks)
Dimension Measurement
All blocks should be checked in this step. The length, width and height are measured with steel scale. If it is a hollow block, then the web thickness and face shell are measured with caliper ruler. And prepare a report of average length, width and height of block and average minimum face shell and minimum web thickness using recorded dimensions.
Density of Concrete Masonry Block
As said above, 3 blocks shall be taken to conduct this test. To determine the density of block, first heat the block in the oven to 100^{o}c and then cooled it to room temperature. Now take the dimensions of block and from that find out the volume and weigh the block. The density of block is determined from the below relation and the average density of 3 blocks will be the final block density.
Density of block = mass/volume (kg/m^{3})
Density values of different grades of blocks should be as follows.
Type of unit  Grade  Density of block (kg/m^{2}) 
Hollow type unit  A(3.5)  >/= 1500 
A(4.5)  >/=1500  
A(5.5)  >/=1500  
A(7.0)  >/=1500  
A(8.5)  >/=1500  
A(10.0)  >/=1500  
A(12.5)  >/=1500  
A(15.0)  >/=1500  

 
B(3.5)  11001500  
B(5.0)  11001500  

 
Solid type unit  C(5.0)  >/=1800 
C(4.0)  >/=1800 
Compressive Strength Tests on Concrete Masonry Blocks
Eight blocks are taken to determine the average compressive strength of concrete masonry block. The blocks should be tested with in 3days after collected in lab. The age of each block shall be 28 days.
The compressive strength testing machine consist of two steel bearing blocks, one is in rigid position on which the masonry unit is placed and another one is movable which transmit the load to the masonry unit when applied.
If the bearing area of masonry unit is more than the bearing area of steel blocks, then separate steel plates are used. The plates are arranged on steel blocks in such a way that the centroid of masonry unit coincide with the center of thrust of blocks.
Bearing area of concrete masonry units are capped with the Sulphur and granular materials coating or gypsum plaster capping.
After placing the unit in testing machine, onehalf of the expected maximum load is applied at a constant rate, and the remaining load is applied in not less than 2 minutes. Note down the load at which masonry unit fails and the maximum load divided by gross sectional area of unit will give the compressive strength of block.
Similarly, test the remaining 7 blocks and the average of 8 blocks strength is the final compressive strength of concrete masonry unit.
Below table represents the value of minimum average compressive strength of individual units.
Type of unit  Grade  Min average compressive strength of individual units (N/mm^{2}) 
Hollow type concrete masonry unit  A(3.5)  2.8 
A(4.5)  3.6  
A(5.5)  4.4  
A(7.0)  5.6  
A(8.5)  7.0  
A(10.0)  8.0  
A(12.5)  10.0  
A(15.0)  12.0  

 
B(3.5)  2.8  
B(5.0)  4.0  

 
Solid type unit  C(5.0)  4.0 
C(4.0)  3.2 