The reasons why concrete deteriorates can be attributed to either original construction errors and/or environmental effects.
Construction Defects include structural design errors, incorrect mix ratios, poor workmanship, insufficient curing and inadequate reinforcement cover.
- Design Failure. Inadequate consideration at the design stage such as poor structural design, inadequate provision of expansion joints, incorrect load calculations, excessively slender designs, poor mix design (wrong grading and selection of aggregates, incorrect cement/water ratio etc) can all lead to failure of the concrete.
- Physical Damage. Impact damage from collision, explosion, fire damage, general wear from abrasion, overloading, damage from plants such as ivy and micro organism like algae can leave concrete surfaces damaged and open to attack from other aggressive forces.
- Poor Workmanship. Failure to observe design detail and poor quality control, incorrect placement of the concrete, inconsistent mixes, inadequate compaction or vibration, cement/water ratio not being adhered to, poor curing of the concrete, inadequate cover to steel reinforcement, allowing corrosion promoting chemicals to be included during mixing and placement are all factors which affect the durability of concrete.
- Structural Movement. Structural movement to the whole or elements of the structure by subsidence, shrinkage, incorrect or insufficient expansion/contraction tolerances or joints can cause undue strain and cracking to occur.
Environmental Effects are a major contributor to the Concrete deterioration of structures some of the most common are :
- Frost Attack. Concrete (unless protected) is porous and absorbs moisture. As water freezes it expands and can cause the surface of the concrete to spall and breakaway. This leaves the surface further exposed and reduces the cover to any steel reinforcement.
- Chemical Attack. Aggressive chemicals, particularly acids will affect the concrete causing friable/crumbly surfaces. Chemical spillages and exposure to certain gases, acid rain and some industrial emissions can all have a detrimental effect.
- Carbonation. Carbon dioxide and other gases combine with moisture and penetrate the concrete through micro pores, and react with the calcium hydroxide which gives the concrete its natural alkalinity. This alkalinity, formed during the hydration process, gives the steel reinforcement a passive surface. Once the alkalinity is reduced the steel starts to corrode.
- Alkali-Silica Reaction (ASR). Certain types of aggregate with poor alkali resistance interact with alkaline fluids in the pores of the concrete to form a silica gel around the surface. This gel absorbs moisture, causing it to expand, and ultimately leads to cracking and further deterioration of the concrete.
- Chloride Attack. Dissolved chlorides in the atmosphere, for example de-icing salts, combined with moisture, will penetrate concrete and attack the steel reinforcement, causing corrosion and eventual cracking and spalling of the concrete.
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