Concrete durability problems in structures can be due to environment to which the concrete structure is exposure or due to internal causes within the concrete.
The following conditions causes the concrete durability problems in structures:
- Physical factors
- Chemical factors
- Biological factors
Durability of concrete in structure occurs due to above factors which cause weathering in concrete, abrasion or chemical reaction with concrete or reinforcement.
Durability problems in concrete structures related to environmental causes include the following: steel corrosion, delamination, cracking, carbonation, sulphate attack, chemical attack, scaling, spalling, abrasion and cavitations.
Durability problems due to Temperature
Concrete contracts and expands due to change in temperature. Concrete expands when temperature increase and contracts when temperature decreases. The effect of these expansion and contraction will not be in unrestrained concrete member. But when a concrete is restrained by connecting members such as columns, beams, slabs, foundations etc, these changes produces significant stresses in concrete which lead to development of cracks.
Warping of Concrete due to Temperature Change
Concrete exposed to temperatures greater than 95°C (203°F) can have significant effects. These effects are caused due to change in volume of cement paste and aggregates. Cement paste shrinks at high temperature due to dehydration while aggregates expands. The net result of high temperature on concrete is expansion. Therefore, exposure to very high temperatures (i.e. fire) will result in concrete spalling, particularly when the concrete is exposed to high temperatures for a long time.
Factors such as moisture condition of concrete, types of aggregates and their stability, cement content, duration of exposure to high temperature, rate of change in temperature, age of concrete and support conditions etc affects the durability of concrete at high temperature.
Concrete Durability Problems due to Moisture:
Concrete expands or swells due to increase in moisture and contracts when moisture reduces. The effects of moisture gains and losses on the volume of concrete are illustrated in Figure 2.
These changes in moisture in concrete causes it to swell and shrink. When concrete starts to dry, shrinkage first occurs at the surface of concrete. This shrinkage of concrete at the surface will develop tensile stresses on concrete surface which leads to cracks.
If a section of the concrete is restrained, and if concrete joints are not provided, major random cracks may develop.
Shrinkage of reinforced concrete is less than the shrinkage of plain concrete. The difference depends on the amount of reinforcing steel used. Steel reinforcement restricts but does not prevent drying shrinkage. The concrete will crack if the shrinkage strain of the concrete exceeds the limiting tensile strain of the concrete.
Problems in Concrete due to Moisture:
The three main problems with moisture and concrete are as follows:
- The moisture cycle
Physical Factors Affecting Concrete Durability:
Many times with the age of concrete, concrete surface is subjected to wear due to sliding, impact, scraping etc. In case of hydraulic structures, the action of the abrasive materials carried by flowing water generally leads to erosion of the concrete. Another cause of damage to concrete in flowing water is cavitation .
Abrasion in concrete is caused by the sliding or scraping of equipment across the concrete. Abrasion damage to concrete may also be caused by subjecting the concrete to abrasive materials (such as sand) that are carried by wind or water.
Tests on concrete results indicate the following facts:
- That abrasion resistance is clearly related to the compressive strength of the concrete.
- Strong concrete has more resistance than weak concrete.
- Since compressive strength depends on the water-cement ratio and adequate curing, a low water-cement ratio and proper curing of the concrete are necessary for abrasion resistance.
- Hard aggregates are more abrasion resistant than soft aggregates.
- Steel-trowelled surfaces resist abrasion more than a surface that is not trowelled.
Cavitations in concrete occurs when a high-velocity, flow of water (or any other fluid) suffers an abrupt change in direction or velocity.
Biological Factors affecting durability:
Concrete may be damaged by live organisms such as plants, sponges, boring shells, or marine borers.
Mosses on concrete
Rotting seaweed has been known to produce sulfur. Sulfur can be easily converted to sulfuric acid. The presence of sulfuric acid on concrete leads to concrete disintegration.
Seaweed on concrete
Chemical Factors AFFECTING CONCRETE DURABILITY
Durability of concrete is affected by chemical reaction due to chemical interactions between aggressive agents present in the external environment and the constituents of the cement paste.
Among the exceptions are alkali-aggregate reactions which occur between the alkalies in cement paste and certain reactive materials when present in aggregate, delayed hydration of crystalline CaO and MgO if present in excessive amounts in Portland cement, and electrochemical corrosion of embedded steel in concrete.
Chemical reactions in concrete results into increase in porosity and permeability, decrease in strength, and cracking and spalling. Sulfate attack, alkali-aggregate attack, and corrosion of embedded steel etc due to chemical reactions in concrete are responsible for deterioration of a large number of concrete structures. Concrete structures in coastal and offshore structures are exposed to chemical and physical processes of deterioration, which aptly demonstrate the complexities of concrete durability problems in practice.
Salt in the surrounding ground, ground water, or air diffuses into the concrete. Steel corrosion results in an increase in the volume of the corroded portion of the reinforcing steel bar. This increase in steel volume causes the concrete to crack and to disintegrate.