Concrete Works

Disadvantages of excessive concrete covers

In reinforced concrete structures cover is normally provided to protect steel reinforcement from corrosion and to provide fire resistance. However, the use of cover more than required is undesirable in the following ways:

(i) The size of crack is controlled by the distance of longitudinal bars to the point of section under consideration. The closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Consequently, with an increase in concrete cover the crack width will increase.

(ii) The weight of the concrete structure is increased by an increase in concrete cover. This effect is a critical factor in the design of floating ships and platforms where self-weight is an important design criterion.

(iii)For the same depth of concrete section, the increase of concrete cover results in the reduction of the lever arm of internal resisting force.

Effect of concrete placing temperature on early thermal movement

The rate of hydration of cement paste is related to the placing temperature of concrete. The rate of heat production is given by the empirical Rastrup function:

An 12 degree C increase in placing temperature doubles the rate of reaction of hydration. Hence, concrete placed at a higher temperature experiences a higher rise in temperature. For instance, concrete placed at 32 degree C produces heat of hydration twice as fast when compared with concrete placing at 20 degree C. Hence, high concrete placing temperature has significant effect to the problem of early thermal movement.

Effect of rusting on steel reinforcement

The corrosion of steel reinforcement inside a concrete structure is undesirable in the following ways:

(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results in the decline in bond strength with concrete.

(ii) The presence of corrosion reduces the effective cross sectional area of the steel reinforcement. Hence, the available tensile capacity of steel reinforcement is reduced by a considerable reduction in the cross sectional area.

(iii)The corrosion products occupy about 3 times the original volume of steel from which it is formed. Such drastic increase in volume generates significant bursting forces in the vicinity of steel reinforcement. Consequently, cracks are formed along the steel reinforcement when the tensile strength of concrete is exceeded.

Formation of pedestrian level winds around buildings

When a building blocks the wind blowing across it, part of the wind will escape over the top of the building. Some will pass around the edges of the building while a majority of the wind will get down to the ground. The channeling effect of wind for an escaping path, together with the high wind speeds associated with higher elevations, generates high wind speeds in the region at the base of the building. At the base level of the building, there are three locations of strong pedestrian level winds:

(i) Arcade passages – wind flow is generated by the pressure difference between the front and the back of the building.

(ii) At the front of the building – high wind is produced by standing vortex.

(iii)At the corners of the building – high wind is induced by corner flow.