Different types of shrinkage in concrete can be seen during its life. Most of the time shrinkage has a detrimental effect on the durability and strength of concrete.
Definition of Shrinkage in Concrete
Shrinkage is the inherent property of concrete which results in volume change in concrete.
Volume change induces stresses in the concrete, which leads to cracks. These cracks are called shrinkage cracks.
Volume change in concrete also occurs owing to external factors like alkali-aggregate reaction, thermal properties of aggregates, suphate attack, etc.
Why does Shrinkage in Concrete occur?
Shrinkage occurs mainly due to the loss of water. Now, loss of water may take place due to various reasons:
- Evaporation
- Hydration of cement
- Carbonation
Types of Shrinkage in Concrete
- Plastic shrinkage
- Drying shrinkage
- Autogenous shrinkage
- Carbonation shrinkage
- Chemical shrinkage
1. Plastic Shrinkage in Concrete
Loss of water after placing of concrete, when it is in the plastic state, is the primary cause of plastic shrinkage.
Plastic shrinkage is the volume reduction in the concrete in a plastic state after the placing of concrete when there is water loss. Volume reduction leads to cracks in concrete, which are termed plastic shrinkage cracks.
2. Drying Shrinkage in Concrete
Drying shrinkage is the inherent property of concrete. Drying shrinkage can be reduced but not eliminated.
The reinforcement and aggregates pose restraint on the drying shrinkage resulting in high internal stresses. The stresses induce microcracks in concrete. Drying cracks affect the strength as well as the durability of the concrete.
Moisture loss taking place at the surface of the concrete may not be compensated by the moisture movement from the concrete interior. This also induces stresses resulting in cracks.
3. Autogenous Shrinkage in Concrete
Self-dryness occurring in concrete due to cement hydration is one of the major causes of autogenous shrinkage in concrete.
Autogenous shrinkage occurs throughout the depth of the concrete member when the capillaries are formed inducing stress in the concrete.
In the past, autogenous shrinkage did not hold much importance as it was found to happen only in the concrete mix, which has a very low w/c ratio.
However, now concreting at very low w/c ratios has become possible owing to superplasticizers and other admixtures. So, autogenous shrinkage is now an important factor in concrete shrinkage.
4. Carbonation Shrinkage in Concrete
The presence of carbon dioxide and moisture in the atmosphere results in carbonation shrinkage. Know the entire mechanism of carbonation shrinkage to know how these two factors cause carbonation shrinkage even in the normal atmosphere.
5. Chemical Shrinkage in Concrete
Volume reduction owing to the hydration reaction of cement is the cause of chemical shrinkage in concrete.
Key TakeAway
Shrinkage is the inherent property of concrete. It is not possible to make concrete, which does not exhibit shrinkage.
Mechanism and factors affecting different shrinkage types have to be understood to control the shrinkage in concrete.
| Shrinkage | Mechanism |
|---|---|
| Plastic Shrinkage | After placing of concrete, it is exposed to atmosphere. Water will be lost during evaporation. Some of the water will be absorbed by dry concrete below and aggregates. Loss in water reduces volume. Reinforcement and aggregates pose restraint, thus inducing tensile stresses. |
| Drying Shrinkage | Withdrawal of gel water from air voids in hardened concrete. |
| Autogenous Shrinkage (occurs at w/c ratio < 0.4) | CO2 from the atmosphere forms carbonic acid upon exposure to moisture. This carbonic acid reacts with Ca(OH)2 of hydrated cement to form calcium carbonate and water. Volume reduction takes place as products have less volume than reactants. This leads to shrinkage. |
| Carbonation Shrinkage | CO2 from atmosphere forms carbonic acid upon exposure to moisture. This carbonic acid reacts with Ca(OH)2 of hydrated cement to form calcium carbonate and water. Volume reduction takes place as products have less volume than reactants. This leads to shrinkage. |
| Chemical Shrinkage | Owing to the hydration reaction of cement, volume reduction takes place as products formed have lesser volume than reactants. This leads to shrinkage in concrete. |
| Shrinkage | Occurrence Time | Factors Affecting Shrinkage | Control Measures | Remark |
|---|---|---|---|---|
| Plastic Shrinkage | Soon after concrete placing (when concrete is in plastic state) | – Rapid drying – w/c ratio – Bleeding in concrete – Atmospheric temperature – Relative humidity in the atmosphere – Unintended vibration – Yielding of formwork – Slump of concrete – Entrained air | – Reducing rate of evaporation – Laying polyethylene sheet cover – Applying monomolecular coatings – Conducting concreting operation at night – Using shrinkage compensating cement or expansive cement – Using low-slump concrete mix | Cracks appear near aggregates/reinforcement or concrete surface as concrete is weak in plastic state. |
| Drying Shrinkage | After concrete is set and hardened Major drying shrinkage occurs in first few months (Ever-ongoing process) | – Drying condition – Time period – w/c ratio – Aggregate cement ratio – Aggregate type – Cement type – Size of concrete member – Fineness of cement gel – Carbonation in concrete | – Using low heat cement or PPC – Adding lime to OPC – Using lime plaster instead of cement plaster – Curing by high pressure steam curing method | Cracks appear in thin slabs and walls over several weeks/months. |
| Autogenous Shrinkage (occurs at w/c ratio < 0.4) | After mixing of concrete. | – W/C ratio – Cement type – Usage of pozzolanic material n cement – Curing method – Usage of admixtures – Rate of hydration – Degree of hydration | – Wet curing – Using a higher w/c ratio – Using low heat cement – Using shrinkage reducing admixtures | Cracks are induced. Autogeneous shrinkage occurs throughout the depth of the concrete member. |
| Carbonation Shrinkage | Time period of occurrence is similar to drying shrinkage | – Concrete Permeability – Atmospheric Moisture content – CO2 content in the atmosphere – Relative humidity – W/C ratio – Curing technique – Presence of gel pores – Cement content | – Providing suitable contraction joints in concrete – Reducing permeability of concrete – Reducing w/c ratio – Ensuring proper curing of concrete – Reducing cement content – Prevention of carbonation | Carbonation starts from the concrete surface. Its rate of penetration through the surface is quite slow. Carbonation shrinkage reduces alkalinity of concrete, which may make the reinforcement prone to corrosion. |
| Chemical Shrinkage | As soon as water is added to cement, chemical shrinkage starts | – Addition of superplasticizers – Blast furnace slag cement usage – Addition of silica fume – Degree of hydration – Cement composition | – Using low C3A cement – Using cement having higher C2S content – Addition of silica fume |
FAQ
Which is the expected type of shrinkage in a high strength concrete with low w/c?
Autogenous shrinkage.
As high performance concrete generally deals with w/c ratio lesser than 0.4, autogenous shrinkage is bound to happen.
