Are hollow blocks a viable alternative?

Simplicity of design, environmental characteristics, speed and ease of construction and reduced costs have made hollow blocks highly desirable.



STRENGTHENING THE STRUCTURE: Hollow blocks protect the building against heat and cold and damage due to fire. Photo: K. Ramesh Babu

STRENGTHENING THE STRUCTURE: Hollow blocks protect the building against heat and cold and damage due to fire. Photo: K. Ramesh Babu  

Masonry is one of mankind's oldest building materials and also one that is least understood. Misconceptions regarding its behaviour have led, over the years, to serious misuse of the material through inadequate or even non-existent design procedures. Added to this, the poor construction practices have worsened the situation. However, perhaps because of considerable amount of information and data available today andsound design techniques and improved manufacturing process, vastly improved construction practices have evolvedin the recent years.

Over the years, new forms of hollow structural masonry have evolved from the traditional bricks. A wall does more than enclosing a building in an attractive form; it must have strength to support floors and roofs and should resist wind and earthquake effects. It must give adequate protection against noise, heat/cold and damage due to fire. Brickwork is inherently brittle and hence needs to be reinforced if it has to carry tensile stresses.

The present concept of reinforced masonry utilises the floors and roofs as diaphragms to distribute lateral loads to walls, which in turn provide horizontal shear resistance needed in addition to carrying the normal vertical live and dead loads. These walls if constructed of plain masonry would be incapable of resisting the magnitude of horizontal shear and bending forces imposed on them. For this reason, modern reinforced masonry contains reinforcing steel to resist the shear and tensile stresses so developed.

Load bearing masonry supports its own weight as well as the dead and live loads of the structure and all lateral wind and seismic forces. Non-load bearing masonry (including in fills) also resist lateral loads and may support its own weight for the full height of the building or be wholly supported by the structure at each floor level. Solid masonry is built of solid units and hence is inconvenient to reinforce. Walls of hollow units have open cores in units, which can be judiciously reinforced and grouted at required locations or joints. Reinforced masonry bearing walls are ideally suited for multi-storey flat construction. The development of high strength masonry block along with improvements in grouting and reinforcing techniques which are now available, have made masonry bearing walls practical for such multi-storey construction especially when combined with roof slabs.

High-rise masonry construction has several desirable features. The important of these are

Simplicity of design

Excellent environmental characteristics

Speed and ease of construction

Reduced building costs

Buildings may be rectangular square or circular in plan, however, it is highly desirable to maintain as much symmetry as possible to preclude difficulties when providing seismic resistance.

It would be appropriate at this point to mention briefly some of the qualities of the concrete block which are superior to conventional brick.

The sound-absorbing qualities of concrete masonry surfaces are rated well above the effective levels recommended by sound engineers. Sound transmitted from room to room through walls, ceiling and floors, can be minimised through the use of hollow masonry wall construction. The reinforcements in the hollow blocks are automatically protected from corrosion because of embedding them in the grout inside the hollow block. Reinforced masonry possesses those inherent fire resistance characteristics that make the fire ratings for multi-storey buildings easily achievable.

In the design of air-conditioning systems for a building, the thermal inertia of the building material is important. This means that the material possess the ability to reduce the effect of maximum heat gain or loss in cyclic changes. The greater the heat storage capacity (thermal inertia), the smaller the instantaneous rate of flow into the interior (conductivity), making structural hollow block masonry very advantageous.

Construction time is minimised, primarily because of typical repetitive nature of layout. When the walls are placed, floors can be built offering continuity of work. The floors next provide work area reducing the cost of exterior scaffolding.

There are no columns or projecting beams to form. The elimination of beam reduces the floor-to-floor height by at least 30 cm. This could add up to 3 m height for a 10-storey building. The structural masonry walls have a surface that can be painted, stained or left natural. No plastering is really required. The structural concrete block masonry has innumerable applications. Its potential has been exploited only to a limited extent in India. In the years to come, more applications are bound to come to the fore. The applications are limited only by designer's imagination and use of this versatile material.

(The author is Emeritus Professor, Dept. of Civil Engg. IIT-Madras and former Dean, Anna University)

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