Cement and concrete blocks are also used for masonry construction. They are available in three types namely solid, hollow and cellular. They are called hollow blocks if the percentage of voids is more than 25% Blocks with voids less than 25% are only perforated blocks. The term ‘cellular concrete blocks’ especially refer to light-weight aerated concrete blocks as described in Section 3.3. Concrete blocks are usually made large in size so that the block work is faster and consume less cement in joints than the brickwork. These blocks are extensively used for compound walls and non-load bearing walls. Specially made hollow blocks are also used for load bearing walls. Such works are useful in reducing the dead load of masonry in buildings. Blocks can be also with cement and sand called cement-sand blocks or with soil and cement called soil cement blocks.
These are of low strength and used for low-cost construction. The main disadvantage of concrete blocks is shrinkage due to movement of moisture which is absent in bricks. In addition, as these blocks are much larger in size than bricks, any foundation movement will cause block work to crack more than the brickwork. Hence, the first important point to remember in block work is that the mortar strength should not be high.
It should not be more than the strength of the blocks. With high mortar strength, the cracks will be few and very large, but with weak mortar, they will be small and distributed.The second point to remember is that in construction, we should use only blocks which have been cured properly for at least 14 days and dried for 4 weeks. All the initial shrinkages should have taken place by then. For the same reason, we do not wet the blocks (as in case of clay bricks) when we place them for masonry construction.
A third point to remember is that ordinary reinforced block work in walls is very weak for resisting lateral loads that are caused due to expansion of roof or due to earthquake and cyclones (see Section 3.7). Hence, block work, especially ordinary cement sand blocks and soil cement blocks, should not be used as load bearing walls for concrete slab roof which tends to expand and contract with temperature.
Such block work should be roofed over by truss work or other means in which the loads do not come directly on the walls. Otherwise, we should either select specially-designed blocks or we have to introduce steel reinforcements to take care of these lateral effects. Soil blocks (made from stabilized soil) and mortar blocks (made from mixture of soil and sand) are commonly used only for low-cost housing schemes and compound walls.
They have very little strength. It is of the order of 1.5 N/mm2 only, so that they cannot be made to bear any substantial load. They should not be used with solid R.C. roofs but only under light roofs (like A.C.sheet on wooden rafters) which does not produce any thermal effect on the walls. In this chapter, we will mainly deal with concrete blocks and also light-weight cellular blocks. We will examine only the IS specification and the list of tests to be made to determine the quality of concrete blocks. IS 2185 “Specifications for Concrete Masonry Units — Part 1Hollow and Solid Concrete Block” should be referred to for details on the subject.
TYPES OF CONCRETE BLOCKS
- Bottom concrete blocks (Open and closed cavity types). These blocks are classifiedby I.S. into the following three grades.
- GradeA. Theseblocks are used for load-bearing walls. They should have a minimum density of 1500 kg/m3. They should be manufactured for minimum specified compressive strength of 3.5, 4.5, 5.5 and 7.0 N/mm2 in 28 days.
- Grade B. These are also used for load-bearing walls. They may have a density below1500 kg/m’ but not less than 1000 kg/m’. They are made for specified compressivestrength of 2.0, 3.0 and 5.0 N/mm2 in 28 days.
- Grade C. These are used for no-load, bearing walls, and its density is not less than 1000kg/m’.They are made for specified strengths of 1.5 N/mm2 in 28 days. These blocks can also be made with decorative facings like fluted facing to give aesthetic effects.
- Solid concrete blocks.These blocks are used as load-bearing walls. They should havea density not less than 1800 kg/m’ and should be manufactured for specified concrete strength of 4.0 and 5.0 N/mm2 in 28 days.
- Paver blocks.These blocks are solid concrete blocks of various shapes specially madefor exterior ground paving on side walls, drive ways, parking lots, industrial floors, petrol pumps, etc.
DIMENSIONS AND TOLERANCES
As in case of bricks, a block is referred by its nominal dimensions. The term “nominal dimension” means that the dimension includes mortar thickness of 10 mm also. In special cases, where finer joints are specified, the mortar thickness is limited to 6 mm.
Nominal dimension of concrete block according to per BIS are as follow:
Length — 400, 450, 500 or 600 mm
Height — 200 or 100 mm
Width — 50, 75, 100, 150, 200, 250 or 300 mm
(Actual sizes will be less by 10 mm the mortar thickness) In addition, blocks are also made in half lengths. The maximum variation allowed is +5mm in length and +3mm in height and width. These dimensions can easily be achieved in machine made blocks. The width of blocks meant for load-bearing walls will be 200 mm and for parapet or filler walls, it is usually 100 mm.
MANUFACTURING OF CONCRETE BLOCKS
The concrete mix for concrete blocks shall not be richer than one part of cement to six parts of volume combined aggregate. Lean mixes up to 1:8 are also commonly used. Bureau of Indian Standards recommends a fineness modulus (see Section 13.5) of the combined aggregate between 3.6 to 4. Coarse aggregate of size 6 to 12 mm is generally used. Sixty percent fine and forty percent coarse aggregates is the mix recommended. The choice of aggregates for block is of utmost importance as cost of aggregates account for a large part of the total cost. Hence, “baby jelly “aggregates that are not generally used for conventional concrete work are found of much use in making these concrete blocks.
They can be hand-made and also preferably machine-made. A simple machine can make up 1600 blocks in an eight-hour shift. cast block is then cured in a water tank or yard for at least 14 days.
When immersed in the tank, the water shall be changed at least every 4 days. After curing, the blocks are dried for a period of 4 weeks before being used on the work. They should be stacked with voids in the horizontal direction to facilitate easy drying. Otherwise, they should be steam cured and dried. There are only a few modern factories in India that use steam cured in manufacturing of concrete blocks. This process allows the complete shrinkage of the block to take place before they are laid on the wall. This is very important. As already indicated, freshly-made and uncured concrete blocks should never be allowed on the work.
