There are various types of defects in concrete:
1. Cracks
Cracks are inherent in concrete and cannot be completely prevented but can be minimized.
The reasons for cracks in concrete are:
(a) excess water
(b) early loss of water
(c) Alkali aggregate reaction
(d) Freeze and thawing
2. Efflorescence
It is the appearance of fluffy white patches on the surface of concrete members.
It is caused by the poorly washed aggregate, salty water used in making concrete the salts being leached out to the surface by rainwater afterward on evaporation while patches appear on the surface.
3. Segregation
It is a separation of (a) coarse aggregate from fine aggregate, (b) paste from coarse aggregate or water from the mix, and the ingredients of the fresh concrete no longer remain uniformly distributed.
It can be reduced by increasing the small size of coarse aggregate, air entrainment, using dispersing agents, and pozzolana.
Causes of segregation are excess water, dropping concrete from heights, badly designed mixes, concrete carried over long distances excess water-pumping, belt conveyor system, etc. over vibrations, and during concrete finishing, extra floating and tamping.
4. Bleeding
It is a flow of mixing water within or emergence to the surface from freshly placed concrete is usually due to excessive vibrations imparted to concrete to achieve full compaction.
Bleeding can be reduced by the use of uniformly graded aggregates, pozzolana by breaking the continuous water channel, or by using – entraining agents, finer cement, and a rich mix.