Thursday, May 1, 2014

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Poor construction methods and workmanship is responsible for the failure of buildings and structure. The poor construction methods and workmanship is caused due to negligence and inadequate quality control at construction site. The effects of some of the poor construction methods are discussed below:

(a) Incorrect placement of steel
Incorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in  the wrong position, collapse can occur when the element is fully loaded.
(b) Inadequate cover to reinforcement
Inadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.
(c) Incorrectly made construction joints
The main faults in construction joints are lack of preparation and  poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor joints allow ingress of moisture and staining of the concrete face.
(d) Grout leakage
Grout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.
(e) Poor compaction
If concrete is not properly compacted by ramming or vibration the result is a  portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction is essential to give a dense, impermeable concrete.
(f) Segregation
Segregation occurs when the mix ingredients become separated. It is the result of
1. dropping the mix through too great a height in placing (chutes or pipes should be used in such cases)
2. using a harsh mix with high coarse aggregate content
3. large aggregate sinking due to over-vibration or use of too much plasticizer

Fig: Seggregation of concrete
Segregation results in uneven concrete texture, or porous concrete in some cases.
(g) Poor curing
A poor curing procedure can result in loss of water through evaporation. This can cause a reduction in strength if there is not sufficient water for complete hydration of the cement. Loss of water can cause shrinkage cracking. During curing the  concrete should be kept damp and covered.
(h) Too high a water content
Excess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete,which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.


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