DEFECTS IN CONCRETE- ALL YOU NEED TO KNOW

There can be different types of defects that can occur in concrete structures it may be crazing, cracking, blistering, dusting, delamination, efflorescence, scaling, spalling, and Curling. These defects can be due to various reasons or causes.

Causes for Defects in Concrete Structures

Causes of defects in concrete structures can be broadly categorized as:

1. Structural deficiency resulting from errors in design, loading criteria, unexpected overloading, etc.
2. Structural deficiency due to construction defects.
3. Damage due to fire, floods, earthquakes, cyclones etc.
4. Damage due to chemical attack.
5. Damage due to marine environments.
6. Damage due to abrasion of granular materials.
7. Movement of concrete due to physical characteristics.

Types of Defects in the Concrete and Prevention:

Different types of defects which can be observed in hardened concrete surface and their prevention methods are explained here:

•  Cracking:

Cracks are formed in concrete because of different reasons but when these cracks are very deep, it is not safe to use that concrete structure. Different reasons for cracking are not proper mix design, insufficient curing, a gap of expansion and contraction joints, use of high slump concrete mix, not suitable sub-grade, etc.

To stop cracking, use a low water-cement ratio, and maximize the coarse aggregate in the concrete mix, admixtures containing CaCl2 must be avoided. The surface should be prevented against the quick evaporation of moisture content. Loads must be applied on the concrete surface only after getting their maximum strength.

• Crazing

Crazing also called as pattern cracking or map cracking, is the formation of closely spaced shallow cracks in an uneven manner. Crazing occurs due to rapid hardening of top surface of concrete due to high temperatures or if the mix contains excess water content or due to insufficient curing. Pattern cracking can be avoided by proper curing, by dampening the sub-grade to resist absorption of water from concrete, by providing protection to the surface from rapid temperature changes.

• Blistering:

Blistering is the development of hollow bumps of various sizes on the concrete surface due to entrapped air under the finished concrete surface. It may cause due to extreme vibration of concrete mix or presence of excess entrapped air in the mix or improper finishing. Extreme evaporation of water on the upper surface of the concrete will also cause blistering

• Delamination

Delamination is also similar to blistering. In this case also, top surface of concrete gets separated from underlying concrete. Hardening of top layer of concrete before the hardening of underlying concrete will lead to delamination. It is because the water and air bleeding from underlying concrete are struck between these two surfaces, hence space will be formed. Like blistering, delamination can also be prevented by using proper finishing techniques. It is better to start the finishing after bleeding process has run its course.

• Dusting

Dusting, also called as chalking is the formation of fine and loose powdered concrete on the hardened concrete by disintegration. This happens due to the presence of excess amount of water in concrete. It causes bleeding of water from concrete, with this fine particles like cement or sand will rise to the top and consequent wear causes dust at the top surface. To avoid dusting, use low slump concrete mix to obtain hard concrete surface with good wear resistance. Use water reducing admixtures to obtain adequate slump. It is also recommended to use better finishing techniques and finishing should be started after removing the bleed water from concrete surface.

• Curling:

When a concrete slab is twisted into a curved shape by upward or downward movement of edges or corners, it is called curling. It happens mainly due to the dissimilarity in moisture content or temperature between the slab surface (top) and slab base (bottom).

The curling of the concrete slab may be downward curling or upward curling. When the upper surface is dried and cooled before the bottom surface, it begins to contract and upward curling takes place. When the bottom surface is dehydrated and cooled due to high temperature and high moisture content, it will shrink before the top surface and downward curling occurs.

To prevent curling, use a small shrink concrete mix, provide control joints, provide heavy reinforcement at edges or provide edges with good thickness.

• Efflorescence:

Efflorescence is the formation of the accumulation of salts on the concrete surface. Formed salts are generally white in color. It is because of the presence of soluble salts in the water which is used in manufacturing concrete mix.

When concrete is hardening, these soluble salts get heave to the top surface by hydrostatic pressure, and after absolute drying, salt deposits are formed on the surface.

It can be stopped by using clean and pure water for mixing, using chemically unproductive aggregates, etc. And confirm that cement should not contain alkalis more than 1% of its weight.