Concrete Cracks

Sun (Auckland), Volume 1, Issue 53, 25 May 1927, Page 12

Concrete Cracks

I Temperature Changes A CONSTRUCTION PROBLEM EXPANSION IS IRRESISTIBLE In this article specially written for THE SUN , an expert declares that cracks in ferro-concrete building must occur owing to the expansion of the structure. An article which appeared in a paper a few days ago concerning the Grafton Bridge mentioned that special provision had been made when designing the bridge to allow of the expansion and contraction of the structure due to the difference in temperature by nite and varies according to the difIt is not necessary here to give any figures of what the movement-amounts to except to say that it is quite definite nd varies according to the difference in temperature between day and night or summer and winter. It may not be generally known that in the Grafton Bridge the large centre arched span is actually hinged at its lowest points and that there is no actual joint at the crown of the arch. Each half span is complete in itself and the two parts simply rest one against the other. It is obvious therefore that any expansion of the ferro-concrete is overcome by the two half spans moving independently one from the other. This joint is formed not only in the actual floor or roadway of the bridge, but is continued right through the balustrading. In roadwork expansion joints are left at very frequent intervals, and after the concrete has hardened these joints are filled with bitumen or some other material of a plastic nature which allows the concrete to come and go slightly without detriment. HARD PROPOSITION Ferro-concrete buildings offer a very much harder proposition to the designer. These structures are monolithic, and while it is quite possible to design a building with joints to , allow of expansion, it is impracticable because of the difficulty in making it watertight. The flat concrete roof of a ferroconcrete structure, no matter how it is constructed, invariably expands when the heat of the sun warms it. This movement is irresistible and cracks must appear in certain parts of the building, generally in lintels over windows and through the sills. The outside walls of the building are actually pushed outwards at the top as the roof lengthens. In a building of this nature with a flat concrete roof the movement is very noticeable where brick walls are erected (on a top floor) abutting on to the outer walls and ceiling. The plaster fractures in the angle formed by the two walls, and should the internal brick wall be built hard up against the underside of the roof and plaster, the plaster invariably cracks in the angle. BRICK WALLS CRACK

There are many instances of a brick internal wall, which has been built to the ceiling and tight round a roof beam, having cracked diagonally from beam to floor due to the movement of the beam with the roof. Fractures also occur in many instances where a brick parapet wall joins the concrete

roof. This is due to the difference in movement between the concrete and the brickwork. In other words the expansion of the brickwork due to heat is different to the expansion of the concrete, and the parapet wall being constructed of innumerable small units is able to more or less absorb the movement, thereby allowing the concrete roof to move independently of the parapet. From the point of view of architectural design it is not always expedient to construct a pitched roof of tiles or like material, but where this is done it overcomes the difficulty to a very large extent as the temperature of the different floors being fairly equable there is little movement in the concrete.