UNIVERSITY OF DUHOK
COLLEGE OF ENGINEERING
CIVIL DEPARTMENT
BUILDING MATERIALS
By:
Youkhanna Z. Dinkha
M.Sc. Construction Materials
youkhanna.zayia@uod.ac
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CHAPTER # TWO
CONCRETE BLOCKS
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Concrete Blocks
Nowadays, Concrete blocks are replacing bricks in masonry construction. They are
extensively used for construction of load bearing wall structures and in partition
walls in reinforced concrete frame works. Concrete blocks are available in three
types :
•Solid
•Hollow.
•Cellular (lightweight). They are called hollow blocks if the percentage of voids is
more than 25%, blocks with voids less than 25% are only perforated blocks.
Concrete blocks are usually manufactured large in size so that the block work is
faster and consume less cement in joints than brick work.
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Advantage of Concrete block compared to brick:
1.One concrete block 390x190x190 mm can replace 8 bricks. Thus
resulting in reduction in cement mortar and speed. Actual size
390x190x190 ; nominal size 400x200x200 mm The dimension of
mortar thickness of about 10 mm.
2. Using hollow concrete blocks instead of solid clay bricks in wall
construction, leads to reduction in dead load on beams, and foundation,
etc,… that have to be considered in structural design.
3. Cheaper than bricks in overall cost (Material cost + Labor cost).
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Disadvantage of concrete Blocks:
1.Shrinkage due to movement of moisture which is absent in bricks.
2.As blocks are much larger in size than bricks, and foundation movement will
cause blocks 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. When the mortar has high
strength, the cracks will be few and very large, but with weak mortar, they will be
small and distributed.
3.In construction of blocks, 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 bricks) when we place them for masonry construction.
4.Ordinary unreinforced block work in walls is very weak for resisting lateral loads
that are caused due to expansion of roof or due to earthquake. Otherwise we have to
introduce reinforcements to take care of these lateral loads.
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Manufacturing of Concrete Blocks:
The concrete mix for concrete blocks shall not be richer than one part of
cement to six parts of volume of combined aggregate. Lean mixes up to
(1:8-1:12) also commonly used.
Coarse aggregate of size 6 to 12 mm is generally used. 60 % fine
aggregate, 40% coarse aggregate is the recommended mix .
Hand-made or Machine made, Cured for 14 days and dried for a period of 4
weeks before being used in the work. Freshly made and uncured concrete
blocks should not be used.
Autoclaved Aerated Concrete Blocks: Are known as light-weight concrete
blocks. They are prepared as solid blocks from cement, water and materials
like ground-sand, pulverized fly-ash together with additives to aerated and
stabilize the air bubbles. Is used for partitions and sometimes in light load
bearing walls.
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Testing of Blocks:
The usual tests prescribed for concrete blocks are as follows :
1.Appearance : must be free from cracks and other defects.
2.Dimensions : the dimensions should conform to the tolerance of ±3
mm or ± 5 mm in length and ±3mm in height and width.
3.Block density.
4.Compressive strength : a batch of 3 blocks is chosen at random.
5.Water absorption.
6.Dry shrinkage.
7.Moisture content.
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