With the increasing cost of the burning of clay bricks, blocks made out of soil cement, cement mortar, concrete, aerated concrete, and precast stone concrete (large stone chips embedded in concrete) are becoming more and more popular. These blocks can be solid or hollow. However, the proper use of these materials, especially concrete hollow blocks, is still not understood by most users. Strict technical supervision is required in the construction of buildings with blocks, as otherwise, it will lead to heavy cracking of the walls.
Concrete
block as a material of construction is dealt with in the book on Building
Materials. We will now consider its use in masonry. IS 2185 (Parts 1 and 2)
gives the construction details and the standard sizes recommended for blocks to
be used in India.
The
standard sizes prescribed for blocks are the length of 400, 500 or 600 mm, the height
of 100 or 200 mm and width of 50, 75, 100, 150, 200, 250 or 300 mm.
We
use the larger widths for the main walls of the building and the smaller widths
for compound walls and partition walls. The commonly made sizes are 400 mm x
200 mm x 200 mm and 400 mm x 200 mm x 100 mm for main and partition walls
respectively. These are nominal dimensions including 10 mm mortar joints so
that the actual sizes can be 10 mm less. A variation of ±5 mm is allowed as
tolerance.
The
main types of cement blocks available are the following. They may be solid or
hollow.
1. Concrete blocks
2. Soil cement blocks
3. Light-weight aerated
(cellular) concrete block
4. Fly ash cement block
Notes:
1. In addition to these, we
have also hollow blocks made out of ceramics which are generally not used for
walls, but for roofs.
2. Blocks made of aerated
concrete (which are usually made solid) are called cellular blocks.
We
will restrict our study to the more commonly used concrete blocks in this article. Similar rules are applied far other blocks also. Concrete blocks are
made of concrete, not mortar. They come in many types, namely solid, hollow, or
core types. They are called "hollow" when the percentage of the void is
more than 25 per cent (clay bricks in which holes are made for less than 25 per
cent of its face area to increase the burning efficiency of the fuel are called
perforated bricks).
Hollow
blocks are those with one or more formed holes or cavities which run through
the block. Blocks with one end of these holes closed (so that it can be laid
uppermost to provide a continuous surface to spread the mortar for the next course)
are called core blocks or closed cavity blocks. Stabilized soil blocks and
cement mortar blocks are generally used for low-cost housing and
non-load-bearing walls. They have very little strength, only of the order of
1.5 N/mm2.
Hollow
concrete block construction without additional steel reinforcements performed
so poorly and led to so many fatalities during the cyclones in Sydney
(Australia) that its use without additional steel reinforcements is now
prohibited in Australia in regions prone to cyclones. Only reinforced blockwork
(for strength) with core filling (for stability) can withstand such high
cyclonic winds. This will also be true for regions of high earthquake
intensity.
Similarly, the cracking performance of soil-cement block walls in the tropics with
concrete roofs on top of the walls due to movement of the top slab has been very
poor. It is better to roof over these soil cement block walls, usually used for
low-cost construction, with AC sheets, tiles, etc., which are isolated from the
walls and produce no thermal movements on the walls.
Concrete
hollow blocks, as well as solid soil-cement and aerated concrete blocks, are
frequently used in place of brickwork. Concrete hollow blocks have the
advantages that they can be made strong by introducing steel reinforcement or
core filling.
Reinforced
blockwork is quick in construction, and its performance under all conditions
such as cyclones and earthquake loads has been good. Aerated blocks are light
and perform very well as filler blocks for multistorey framed buildings. They
can also be used to carry light loads.
To
comply with 9-inch brick sizes, hollow blocks of normal size — 450 mm x 225 mm
x 225 mm (18" x 9" x 9") — can also be produced. To build a
masonry wall of one square metre, only 10 such blocks are required against
115 clay bricks.
In
addition to the above work, it requires only 13 litres of cement mortar against over 45 litres for brickwork. It can also be built by an average bricklayer four times faster. However, experience shows that the selection of
blockwork should be more for its architectural effect than for its lower cost.
In
actual practice, plastered blockwork tends to be as expensive as brickwork but
can be faster in execution. However, the most popular size of block for
buildings is the 400 mm x 200 mm x 200 mm (nominal size) block, which can
replace 8 standard metric bricks 200 mm x 100 mm x 100 mm (nominal size). For
partition walls, we use blocks of the same length and height as used for
load-bearing walls but only 90 mm or 100 mm thick.
MUST-READ: ALL
YOU WANT TO KNOW ABOUT CONCRETE BLOCKS
What are the considerations to use hollow concrete blocks?
CONSIDERATIONS FOR THE USE OF HOLLOW CONCRETE BLOCKS
We have already seen that
we choose hollow block construction more for architectural appearance and
quickness of construction than for low cost.
The second factor that
needs consideration nowadays is the non-availability of good quality bricks and
the large improvement that has taken place in the quality of concrete blocks
which are being produced in India.
However, we should remember
that if blockwork is to be efficient for construction, the dimensions of rooms,
walls, etc. should fit the block size modules so that there will be a minimum
number of blocks cut and wasted.
We should also note that
full advantage blockwork will not be realized unless the complete range of
block shapes necessary for the construction is available. In addition to
regular blocks, blocks for bond beams for lintels, and blocks for pilasters
(rectangular columns in walls) should be available.
In the construction of
buildings for concealed electric wiring or for fixing of washbasins, etc.,
special arrangements should be made. Otherwise for easiness of construction
brickwork must be prescribed in such places.
Masonry construction with
solid and cellular blocks is very similar to brick masonry.
Hollow concrete block walls are constructed
in the following three different ways:
1.
Simple masonry (similar to brick) with the hollows untreated—the blocks are
laid one over the other breaking bonds in stretcher courses.
2.
Infilled hollow block masonry with plain concrete infilling.
3.
Reinforced hollow block masonry with steel reinforcement and concrete infilling
for higher strength (used for lintels, retaining walls and walls for multi-storeyed buildings).
The two important structural features to remember when
using concrete blocks are as follows:
(i)
There is an inherent weakness in these blocks for shrinkage and moisture
movement. (Hence blocks should be well cured and absorption should be less than
100%.)
(ii)
Blocks being larger in size and more brittle than bricks, the cracks due to settlements
in blockwork will be larger in width than in brickwork. Hence allowable
differential settlements should be small.
Closed
cavity blocks, in which the holes are closed at one end, ensure better load
distribution, better insulation properties in cold places and minimum wastage
of mortar in the blockwork. With open cavities in hot humid regions, the air
current set up in open holes can be considered an advantage.
BASIC STEPS IN LAYING OF CONCRETE BLOCK | LAYING
OF BLOCKS
Blocks
are laid as stretchers breaking bonds as shown in Fig below (p. 64). As already
stated, the most important thing is that concrete blocks should dry out
thoroughly before use. It requires at least 28 days under normal conditions of
normal curing. Solid and closed cavity blocks with one end closed are laid in
walls with a full mortar bed as in brickwork.
If
the walls are constructed of hollow blocks with through holes, the economy in
mortar can be made by "shell bedding" where the mortar is laid only
on a 5 mm (2 in) strip around the outer edge of the hollow block. Blocks should
not be wetted before use (as we specify for brickwork in cement mortar) as the
concrete can take up water and then shrink. Excess mortar is gently removed to
give a good appearance. The bottom layer of blocks on the foundation is always
laid on a full bed of mortar.
The
third important point to remember in hollow block construction is that the
mortar used for blockwork should not be too strong. With strong mortars, when
the wall shrinks, it tends to crack up with a few large cracks. With moderately
strong mortars, the movement results only in a number of small cracks
distributed on the whole length of the wall which will not be noticeable. If
cement mortar is to be used, a mix 1:5 with a plasticizer is found to be
sufficient. 1:1:6 or 1:2:9 cement lime plaster is preferable to pure
cement plaster. The ideal mortar should have the same density, strength and
drying shrinkage as the block.
Pointing
is usually done as in exposed brickwork. The longer the pointing tool, the
smoother and better the joint results. The fresh joint is left for an hour to
set, and then it is worked on for pointing.
Should hollow blocks be filled with
concrete?
HOLLOW CONCRETE BLOCKS WITH CONCRETE INFILLING
Hollow
concrete blocks (especially those with two through holes) form continuous
vertical holes when placed with proper bonding one over the other. They are
ideal for concrete filling, as these holes can be completely or partly filled
with in-situ concrete as the blockwork proceeds. Mixes with large size
aggregates can be used for this purpose. This will add greater strength to the
walls. Tests show that filling hollow blocks with plain concrete will give
basic strength approximately halfway between these unfilled blocks and
solid blocks with the same quality of concrete.
WHAT IS REINFORCED CONCRETE HOLLOW BLOCK
MASONRY?
The
vertical holes in conventional block construction can accommodate steel
reinforcement and thus be made of reinforced concrete construction when infilled.
Similarly, special blocks can accommodate horizontal reinforcements so that
horizontal beams such as lintels and horizontal strengthened reinforced
concrete bands can be built up with these blocks (see bond beams described
below). Such construction is known as reinforced concrete hollow block masonry
construction. ACI standard 531-581 and its commentary give valuable information
for the design and construction of such works.
What are the Benefits of
using hollow blocks in construction?
SPECIAL FEATURES OF CONCRETE BLOCK MASONRY
In
addition to using fully-cured blocks and lean mortar joints, the fourth most
important feature to be incorporated in blockwork, for buildings as different
from brickwork, is the need to provide the three features, namely control
joints, band beams and joint reinforcements. Unless these are provided, block
masonry is bound to result in random cracking due to shrinkage and moisture
movements. These are briefly described further.
Control
joints. Concrete has a higher coefficient of expansion than brickwork. Control
joints are continuous joints, usually vertical, built into concrete block
masonry walls for controlling thermal movements. They should be located at the change of sections in the form of vertical mortar joints to minimize the cutting of
units. They can be shrinkage joints and expansion joints. The shrinkage joints
are continuous vertical joints of approximately 18 mm (3/4 inch). These joints are
later raked and caulked (filled in). Expansion joints are located at a spacing of
45 to 60 m in suitable locations. These are filled with joint fillers which are
compressible materials that close the gap but allow movement.
Bond
beams. Bond beams are beams such as lintels that are to be provided on top of
openings or on top of walls without top loads such as compound walls. It is
usually a reinforced masonry course, which is constructed out of specially
channel-shaped units which are filled with concrete and reinforced Fif below. It
usually serves as a structural element and as a means of crack control on top
of openings.
For
lintels, the reinforcement usually consists of at least two 12 mm bars placed
at the top and bottom. For crack control, they are made discontinuous at
control joints. Where continuity is required for structural reasons, a dummy
joint should be given to control the location of the probable crack.
Joint
reinforcement. Horizontal joint reinforcements in the form of a minimum of two No. 9
gauge wires at intervals can be used as in reinforced brickwork for crack
control. They are expensive and are not generally provided. However, when used
as load-bearing walls with RCC slabs resting on the walls, the top three
courses at least should be provided with joint reinforcement as shown in Fig
below.
Is blockwork suggested for a compound
wall?
Compound Walls In Blockwork
Unplastered concrete blockwork which is cheaper than plastered brickwork is used nowadays extensively for constructing compound walls as shown in Fig. below. There are many variations in the construction of these walls. One of the popular methods of construction is to use under-reamed piles at convenient intervals of 3 to 4 m with grade beams on top of the piles and the blockwork built on top of the grade beams.
Vertical
reinforcements can be embedded in the hollow of the block at regular intervals
from the under-reamed piles, thus strengthening the wall along its length
giving lateral resistance to wing loads or earth pressure. The top of these
walls should be finished with a coping cast in a channel-shaped block laid on
tops such as 225 mm thick lightly reinforced precast or cast in place slab over
a closed cavity block laid as the top layer or a bond beam.
CONCLUSION
Concrete
blocks are commonly used in place of clay bricks resulting in a reduction in
cement mortar and also increasing the speed of construction. As manufacturing
clay bricks require good soil and high energy, it is also more environmentally
friendly. However, there are many special points to be taken care of in good
blockwork masonry as compared to brick masonry. They have been briefly
described in this article.
Can I use concrete blocks instead of engineering bricks?
YES
Which is a better hollow block or solid block?
Hollow blocks are lighter in weight, thus reducing the dead load on the structure, so steel consumption per square foot is lower when solid blocks are applied to masonry walls. The production cost of a hollow block per unit is lower because the raw materials consumed per volume of the same volume are less compared to a solid block.
Are hollow blocks good for building?
It offers high dimensional accuracy and stable finish, making it an excellent building material for creating load-bearing structures and partition walls. e. These hollow concrete blocks are capable of withstanding high pressures and are therefore widely used in building construction practices.
Are blocks stronger than bricks?
Concrete blocks should have a minimum compressive strength of 1,900 pounds per square inch, but the strength of many blocks exceeds this limit. If we measure the strength in psi, the concrete blocks will come out on top of the bricks. The former can withstand 3,500 PSI, while the range of bricks is found at 3,000 PSI.
Is it cheaper to build with bricks or blocks?
Yes
As the cost of clay bricks rises, alternatives like soil cement, concrete, and aerated blocks gain traction. However, using concrete hollow blocks requires careful technical supervision to prevent cracking. Standard sizes are outlined in IS 2185. Proper planning for architectural impact and structural needs is vital for successful blockwork construction.
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