Concrete is the most important building material and the worldwide consumption of concrete today is 8.8 billion tons per year. Concrete, although a very durable material, can show loss of strength and deterioration in course of time because of various reasons.
25.1.1 Design deficiency
Faulty design is one of the major causes of structural failure. This may include underestimation of loads, faulty mix design, poor detailing, faulty analysis, poor foundation, etc.
25.1.2 Material deficiency
Use of poor quality materials, physical and chemical properties of concrete, increased water–cement ratio, etc. are also some of the major reasons.
25.1.3 Construction deficiency
Poor workmanship is one of the most important causes of distress in buildings. Inadequate control in batching, mixing, transporting, placing, compacting, finishing and curing of concrete leads to various problems including honeycombing, segregations, poor strength, etc. Very often sufficient cover is not provided for the reinforcement. Bad-quality formwork and poor preparation of joints also lead to distress.
25.1.4 Environmental deficiency
Attack of chemicals and acids, proximity to marine environment, thermal variations, entrapped air, leaching because of the presence of hydrated compounds, etc. can also end up in problems to the structure.
25.1.5 Man-made deficiencies
Reckless modifications and alterations, installation of additional load, poor or no maintenance, etc. make the performance of the building poor.
Deterioration of concrete basically starts from leaks and cracks. Because of any of the reasons said above, concrete can become porous and leakage can happen. Because of poor workmanship, improper mixing, inadequate compaction or poor quality formwork, etc. segregation of material can occur and honeycombs can be formed. The quality of the joints is also a major cause of leakage.
25.1.6 Cracks in concrete
The cracks in concrete vary in width from 0.1 to 1.0 mm. Concrete starts cracking, even at the plastic state and when hardened concrete is exposed to weather, because of thermal and shrinkage strains. The cracks in concrete are caused primarily because of the following reasons.
- Temperature gradient including frost action
- Humidity gradient (drying shrinkage)
- Structural overloading, cyclic or impact loading
- Rapid drying conditions (plastic shrinkage)
- Inadequate structural design and detailing
- Chemical causes including corrosion of reinforcement
The shrinkage strain causes tensile stress in concrete, and when this exceeds the tensile strength of concrete, cracks begin to happen. However, due to the viscoelastic behaviour (creep) of concrete, some stress is relieved and it is the residual stress, after the relaxation due to creep, that is responsible for cracking.
Cracks on concrete surfaces seriously affect the durability of concrete and 0.15 mm is often recommended as the maximum allowable crack width.