Cement concrete hollow blocks have been in use for several masonry constructions. Several advantages have given room for rapid development and use of the same in place of traditional construction materials like stones and bricks.
Typical concrete masonry units are shown in Fig. 16.9 (Sharma, 1988), in addition to the concrete hollow blocks referred to in Chapter 9.
Figure 16.9 Typical concrete masonry units
Based on the job requirement, the concrete blocks may be made. There is no standard size of concrete blocks. However, Concrete Association of India (CAI) recommends that the face thickness should not be less than 5 cm and the net area should be atleast 55–60% of the gross area.
The common sizes for building blocks are given in Table 16.5.
Table 16.5 Common sizes of blocks
Use of Hollow Concrete Blocks for Masonry
Following are the uses of concrete blocks for masonry construction:
- Large in size but easy to handle
- Uniformity in design
- Easy handling and placing
- Adequate strength
- Attractive appearance
- No need of plastering for outside work
- Less number of masonry joints
- Less cost
- Easy method of manufacturing
16.4.2 Laying of Concrete Hollow Block Masonry
Different types of basic hollow blocks used in construction are discussed in Section 9.6.1. Additional hollow block units are given in Fig. 16.9 Now the construction features are explained.
1. Construction of Walls
A mortar bed is spread on the foundation concrete and levelled to have a uniform thickness everywhere. The corner block is first placed and positioned accurately. Mortar is applied to the other end and one block is positioned to the end and aligned. The level of the course is checked after placing a few blocks. If necessary, the blocks are tapped with additional mortar such that the mortar thickness is 2 cm below and on the ends. The first course is checked to be in plumb before planning second and additional courses.
As done in brick laying, the successive courses are laid in such a manner so as to break the joints vertically. For vertical joints the mortar is applied to the projection at the sides of the block. As followed for the first course, the courses are built starting from the corners only. Every time, the verticality and horizontality are checked. All the four vertical edges of the final block and the edges of the opening are covered with mortar and pushed in position. The face of the masonry may be pointed by running a tool. The type of joint recommended are weathered, V-shaped or concave, such that the joints shed off water easily.
2. Construction of Columns
Columns are used wherever a large pressure to be transferred through large bearing surface. The columns may form an integral part of the wall or it can be a separate unit. The columns are made of standard stretcher and corner blocks or other special shapes are used. For better stability, the hollows within the blocks may be filled with plain or reinforced concrete (Fig. 16.10(a)).
Figure 16.10 Pilasters and piers
3. Construction of Window and Door Openings
Blocks with hole (jamb) near the opening should be filled with concrete and wooden plugs (Fig. 16.10(b)). The door or window frames are screwed to the wooden plugs. It is also fixed in the lintel with small dowels of mild steel. Under the base of the window or door, a course of solid concrete block masonry is laid which is extended into the adjacent walls up to a distance of at least 30 cm on either side. Lintels are also of hollow channel-shaped sections which can be filled with concrete and provided with steel reinforcement at their bottom.
4. Construction of Reinforced Walls
This is made by providing vertical reinforcement in the hollow with concrete (Fig. 16.11). In order to increase the strength of the wall, reinforcement is provided at the horizontal joints. Because of this provision, expansion cracks which may occur due to moisture and change of temperature may be reduced. Further, two horizontal bars of 6 mm diameter are placed on each on the face of the wall. Instead of steel rods, welded steel mesh may be used wherever needed (Fig. 16.11).
Figure 16.11 Jamb details for 200 mm thick hollow block wall
Concrete Blocks with Concrete Filling
Concrete hollow blocks constructed with two through holes to form continuous vertical hole are ideal for concrete filling. These holes can be completely or partly filled with concrete during the process of construction itself. For this purpose, mixes with large size aggregates may be used. This provision will add greater strength to walls. It has been found from the tests that filling of hollow blocks with plain concrete will give basic strength approximately halfway between these for unfilled blocks and the solid blocks with the same quality of concrete.
Reinforced Concrete Hollow Block Masonry
In the conventional hollow block masonry, the vertical holes may be made to accommodate steel reinforcement and can thus be made of reinforced concrete construction. Similarly special concrete blocks can be made to accommodate horizontal reinforcements. Such a horizontally reinforced hollow block can be used as lintels and horizontally strengthened reinforced concrete bonds can be built up with these blocks. This type of construction is known as reinforced concrete hollow block masonry construction.
16.4.5 Special Features of Concrete Block Masonry
The important features to be incorporated, compared to regular brick masonry, are control joints, band beams and joint reinforcements. These factors are to be provided failing which the block masonry is bound to result in random cracking due to shrinkage and moisture movements. These factors are discussed below.
1. Control Joints
Concrete has higher coefficient of expansion than brickwork. In order to control thermal movements, these joints are built as continuous and vertical. Further, to minimise cutting of units, they are to be located at change of sections. These joints may be shrinkage joints and expansion joints. The shrinkage joints are continuous vertical joints approximately 18 mm thick. These joints are later raked and caulked. Expansion joints are located at spacing of 45–60 m in suitable locations. These joints are filled with joint filers, which are compressible materials that close the gap but allow movement.
2. Bond Beams
Bond beams are similar to lintels which are to be provided on top of openings or top of walls such as compound walls. This is constructed using special channel-shaped units which are reinforced and filled with concrete. It serves as a structural element as that of a lintel to control cracks on top of openings.
3. Joint Reinforcement
For crack control, horizontal joint reinforcements in the form of minimum two No. 9 gauge wires at intervals can be used. As they are expensive, they are not generally provided. 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 (Fig. 16.12).
Figure 16.12 Load-bearing wall
Compound Walls in Hollow Block
Compound walls are nowadays preferred to be of unplastered concrete blockwork. The construction is cheap and no maintenance is needed. There are many variations in the construction of these walls. The convenient method is to install under-reamed or cast-in-situ piles at stipulated intervals (of 3–4 m) with grade beam on top of the piles and the blockwork built on top of the grade beams.
Vertical reinforcements are provided in the hollow blocks at regular intervals from the piles. This provision strengthens the wall along its length giving lateral resistance to wing loads or earth pressure. The top of these walls are finished with a coping cast in a channel-shaped block on top (Varghese, 2012).
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