Concrete block is nowadays replacing bricks in masonry construction especially in many multi stored buildings. They are extensively used for construction of boundary walls and as filler walls in R.C. framework. Un reinforced block masonry construction should not be adopted in earthquake and cyclonic areas.
The usual tests prescribed for concrete blocks are the following:
Appearance: For testing the appearance, 20 blocks are taken for every 5000 blocks for tests. In general, the block units should be free from cracks and other defects. Faces which are to be exposed should be free of chips, cracks or other defects except that not more than 5 per cent of the consignment may contain slight cracks or small chippings not larger than 25 mm.
Dimensions: Twenty full size units are taken and their length, width, height and web thickness are measured. A steel scale graduated to 1 mm is used for measuring dimensions and a caliper rule graduated to 0.5 mm divisions and jaws not less than 15 mm but not more than 25 mm in length is to be used for measuring the web thickness. The dimensions should conform to the tolerances of +5 mm in length and +3 mm in height and width.
Block density: Three blocks are chosen at random and dried at 100°C in an oven. After cooling, its weight is measured correctly to 10 g and its dimensions correctly to one mm. The density should correspond to the class of the block as given in Section 3.4
Compressive strength: A batch of eight blocks is chosen at random. They are tested within 72 hours after delivery and after storage in normal room air before testing. The blocks then are capped with sulphur and granular material or plaster of Paris paste (strength about 25N/mm2). The strength is determined by the load taken to are divided by the gross arra of thr unir which is the area of section perpendicular to the load. Blocks intended for use with their hollow cores in a horizontal direction are tested in the same direction in which they are laid in the field. Individual values should not be less than 20% of the average value specified in Section 3.4 1. according to the grade.
Water absorption: Three specimens are tested separately to find the average value. Firstly, the specimens are completely immersed in water for 24 hours. They are again weighed when completely immersed in water to find its volume. The block is taken out, allowed to be drained for one minute and weighed to find the wet weight of the specimen. It is then dried in an oven at 100–115° C for not less than 24 1. hours, and the dry weight is determined. The following quantities are calculated:
Absorption in Kg = (A-B) / (A-C) x 1000 / M3
Absorption Percentage = ( A-B ) / B x 100 ( by Mas )
Where,
A — wet mass of the block in kg
B — dry mass of the block in kg and
C = suspended immersed mass of block in kg (volume)
The water absorption should not be more than 10% of the mass. (It should be remembered that concrete blocks are never wetted like bricks for using in masonry construction.)
Drying shrinkage: Three specimens, are tested for average value. Generally, three or more specimens are stored in airtight containers for duplicate testing, if required. From these blocks, specimens are cut for testing. Specimens of cross section 7.5 cm x 7.5 cm in solid blocks and 7.5 cm thickness for hollow blocks are taken for testing. They are to be not less than 15 cm in length. Two reference points consisting of 5 mm diameter steel balls cemented with rapid hardening Portland cement are installed at each end. The specimen is completely immersed in water of temperature 27 + 2°C for 4 days. The test is to be conducted by using a special measuring frame with a micrometre and Invar steel rod of suitable length (See IS 2185 (Part I),1979 Appendix E for details).
The test is first made on the wet specimen and then also on the specimen after it is completely dried in an oven for 44 hours and cooled to the room temperature. The difference in lengths is determined (The specimen should be heated, cooled and measured for a number of times till a constant value of its length in the dry state is obtained). The drying shrinkage is the shrinkage as a percentage of its dry length. It should not exceed 0.1 per cent.
Moisture movement: The specimen used for drying shrinkage after completion of the test is immersed in water for 4 days and its elongation is measured by the same apparatus used for drying shrinkage test. The difference between dry and wet lengths is expressed as a percentage of the dry length in the moisture movement. It should not exceed 0.09 percent.
