Concrete Masonry | Manufacture, Construction, Advantages of Concrete Masonry

Concrete masonry or cement-concrete masonry uses cement concrete blocks, either hollow or solid, for wall construction, with or without stone facing.

A hollow unit, is defined as a unit that has a core-void area greater than 25% of the gross area.

Various types of concrete masonry units, depending upon shape and size, are manufactured, and these can be grouped in two heads:

(i) Regular concrete blocks

(ii) Hollow concrete units

Regular concrete blocks are manufactured from dense aggregate and are used in load-bearing walls. Hollow concrete units are manufactured from lightweight aggregates.

They may be used both for load-bearing as well as non-load-bearing walls. They are light in weight.

There are various forms of concrete masonry units. Concrete Association of India recommends that the face thickness of the hollow blocks should at least be 5 cm, and the net area should at least be 55 to 60% of the gross area.

The cores in the blocks should at least be two in number and should preferably be oval-shaped.

The recommended size of common blocks are 39 cm x 19 cm x 30 cm; 39 cm x 19 cm x 20 cm and 39 cm x 19 cm x 10 cm.

The aggregate used in the block manufacture consists of 60% fine (i.e. sand) and 40% coarse aggregate of 6 to 12 mm size, with a combined fineness modulus of 2.9 to 3.6.

The cement-aggregate mix is in 1:6 proportion. The strength of the blocks should be at least 3 N/mm.

Concrete masonry blocks are manufactured in the following surface finishes:

(i) Common finished surface

(ii) Glazed finish

(iii) Slumped finish

(iv) Specially faced finish

(v) Coloured finish

Common finish surface has fine to coarse texture which can be obtained by varying the mix proportions and by using appropriate aggregates.

If the exposure of the aggregates is required, it can be obtained either by treating the surface with a dilute acid solution or by scrubbing it while the concrete has not fully set.

A glazed finish is used for decorative work. It can be obtained in a manner similar to the glazing of tiles. Glazed-finish concrete blocks are water-resistant.

The slumped finish is the rough finish that is obtained by using the concrete of the desired slump. When the forms are open, the blocks settle slightly, causing a rough surface.

In the specially faced finish, finishing material such as marble, etc. is incorporated on the facing side of the block.

A colored finish can be obtained by mixing various pigments into the concrete mix.

  1. Manufacture of Concrete Masonry Blocks  

The following points should be kept in mind while manufacturing the concrete masonry blocks:

1. The cement-aggregate ratio should not be leaner than 1: 6.

2. The aggregate should have a mixture of fine aggregate 60% and coarse aggregate (6 to 12 mm size) 40%. The fineness modulus as the mixed aggregate should be between 2.9 to 3.6.

3. Blocks should be taken out from the molds only when the concrete has been sufficiently set.

4. Concrete should not have very lean consistency. If hand molding is done, the hollows should be vertical. Proper compactions should be obtained.

5. Machine casting is preferable to hand casting, to obtain a better finish.

6. After taking the blocks out of the mold, they should be kept under shade for at least 24 hours, and then immersed in a water tank for curing for at least 24 hours, and then immersed in a water tank for curing for at least one week.

After that, the blocks may be, stacked with cells horizontally.

7. Blocks should be used only after about 3 to 4 weeks of their taking out of the curing tank.

8. The compressive strength of blocks should not be less than 3 N/mm² after 28 days of curing.

  2. Construction of Walls  

The method of constructing the wall with the concrete block is the same as that used for brick masonry.

First, the corners or ends of the wall are constructed with a few courses of blocks.

Mortar is applied to the bottom of the concrete block at the horizontal face members only. For vertical joints, the mortar is applied to the projections at the sides of the block.

For building the portion in between the corners, the string is spread between the two horizontal end blocks of a course, and the blocks are laid in between.

The final closing block is fitted carefully. The following points should be kept in mind while supervising the construction work:

1. Before use, it should be ensured that the blocks are dry. They should not be drenched in water before use.

2. Blocks of successive courses should be so laid that vertical joints are staggered.

3. The joints should be 5 to 10 mm thick and should be uniform.

4. The mortar used for construction should not be stronger than the concrete mix used for the manufacture of blocks. Generally cement-lime-sand mortar of mix proportion 1: 1: 10 is used.

5. The blocks used for external walls should have absorption of less than 10%. For internal walls, the absorption should be less than 15%.

6. Concrete blocks have a high thermal expansion, due to which walls crack at corners. Long walls may have cracked even at their mid-length.

Hence at the junction of walls, solid concrete blocks or hollow blocks filled with concrete should be used.

  3. Properties of Hollow Block  

1. Moisture Movement: Not more than 0.09%

2. Water Absorption:  Not more than 10%

3. Drying Shrinkage:  Not more than 0.06%

4. Compressive Strength: 

For Grade A: 3.5 to 15.0 N/mm²

For Grade B: 3.5 and 5.0 N/mm²

5. Density:

For Grade A: 1500 kg/m³

For Grade B: 1100 kg/m³ to 1500 kg/m³

  4. Uses of Concrete Masonry  

Hollow concrete blocks are utilized in all types of masonry construction such as

a. It is used in exterior load-bearing walls

b. It is used in interior load-bearing walls

c. It is also used in curtain walls

d. It is used in partition walls and panel walls

e. Backing from brick, stone, and another facing

f. Fireproofing over a structural member

g. Fire-safe walls around stairwells, elevators

h. Piers, column and retaining walls, etc.

i. Boundary fences

  5. Advantages of Hollow Concrete Masonry  

The advantages of hollow concrete masonry are as follows:

1. Concrete blocks are regular in size, requiring no dressing work. Hence construction is very rapid.

2. Blocks are light and therefore easy to handle.

3. Because of their lightness, the loads transferred to foundations is much less than the stone masonry. This is an important consideration in locations where the soil has a low hearing capacity.

4. There is a great saving in the material.

5. Hollow blocks are structurally stronger than bricks.

6. Thinner walls can be easily constructed, resulting in an increase in the floor area.

7. Because of the large size of the blocks, the number of joints in the masonry is less. This results in saving in mortar.

8. Because of hollow space, the resulting wall has better-insulating properties against sound, heat, and dampness.

9. Blocks can withstand atmospheric actions and do not require plaster or any other covering or facia work.

  6. Disadvantages of Hollow Concrete Masonry  

The disadvantages of hollow concrete masonry are as follows:

1. It has a low bearing capacity.

2. Dangerous to hang heavy objects on walls.

3. Maintenance cost is high.

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