What are the potential problems 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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Do engineers need to cater for corrosion protection of lifting anchors in precast concrete units?

The corrosion of lifting anchors in precast concrete units has to be prevented because the corroded lifting units cause an increase in steel volume leading to the spalling of nearby surface concrete.

Consequently, steel reinforcement of the precast concrete units may be exposed and this in turns results in the corrosion of steel reinforcement and the reduction in the load carrying capacity of the precast units. To combat the potential corrosion problem, the lifting anchors could be covered with a layer of mortar to hide them from the possible external corrosion agents. Alternatively, galvanized or stainless steel lifting anchors can be considered in aggressive environment.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What the preferable size of cover blocks?

The purpose of cover blocks are:

i. Maintain the required cover.

ii. Prevent steel bars from getting exposed to the atmosphere so that steel corrosion may result.

iii. Place and fix reinforcement based on design drawings.

As cover blocks after concreting shall form part of concrete structure, it is preferably that the cover blocks shall possess similar strength to the concrete structure. Moreover, the size of cover block should be minimized so that the chances of water penetration to the periphery would be reduced.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm. Does an increase of maximum aggregate size benefit the structures?

To answer this question, let’s consider an example of a cube. The surface area to volume ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to volume ratio decreases with an increase in volume. Therefore, when the size of maximum aggregate is increased, the surface area to be wetted by water per unit volume is reduced. Consequently, the water requirement of the concrete mixes is reduced accordingly so that the water/cement ratio can be lowered, resulting in a rise in concrete strength.

However, an increase of aggregate size is also accompanied by the effect of reduced contact areas and discontinuities created by these larger sized particles. In general, for maximum aggregate sizes below 40mm, the effect of lower water requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

How does concrete cover enhance fire resistance?

In the event of exposing the concrete structures to a fire, a temperature gradient is established across the cross section of concrete structures. For shallow covers, the steel reinforcement inside the structures rises in temperature. Generally speaking, steel loses about half of its strength when temperature rises to about 550oC. Gradually, the steel loses strength and this leads to considerable deflections and even structural failure in the worst scenario. Hence, adequate cover should be provided for reinforced concrete structure as a means to delay the rise in temperature in steel reinforcement.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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