Shrinkage is the reduction in the volume of concrete, which may result from factors such as water absorption in the plastic state, evaporation of moisture, and heat dissipation during hydration. The effects of shrinkage are considered in the verification of serviceability limit states when necessary.
Why do Concrete Shrink?
Concrete is a mixture of cement, fine aggregate, coarse aggregate, and water. During the early stage of mixing, water in the concrete reacts with cement in a process called hydration. The hydration reaction generates lot of heat which makes the concrete to shrink as the heat escapes. Also, the excess moisture not used in hydration gradually escapes through pores as the concrete continues to dry, leading to a further reduction in volume. This migration of excess moisture through the pores would continue throughout the service life of the structure.
Factors affecting Shrinkage
There are several factors that affect shrinkage. Some of these factors are mentioned below:
Cement content: High cement content leads to increase hydration which is a chemical reaction that generates heat. This loss of heat in the cement matrix results to increase in (autogenous) shrinkage
Cement types: The type and nature of cement also affect shrinkage. For example, concrete that contains rapid-hardening cement experiences faster hydration and shrink more than those that contain ordinary Portland cement.
Aggregates: Large sizes of aggregates restrain the concrete from shrinking freely. The stiffer the Aggregates the slower the rate of shrinkage and vice versa.
Water-cement ratio: High water content causes drying shrinkage to occur for a long period of time
Dimension of elements: Element with large dimension and surface area exposed to the atmosphere experiences more shrinkage.
Curing: Better curing reduces pores and slow down evaporation
Relative Humidity: Relative humidity also affects the rate of shrinkage. High relative humidity slows down shrinkage as it reduces rate of evaporation from the concrete.
Ways of Minimizing Shrinkage
Shrinkage cannot be totally eradicated but can be minimized through
- Partial Substitution of cement with alternative materials like fly ash
- Ensure quality concrete with optimal water-cement ratio
- Improving workability of concrete by adding admixtures rather than water
- Pouring concrete under low temperature
- Casting of concrete in successive bays rather than alternate bays
- Proper curing for days after initial setting of the concrete
Estimation of Shrinkage Strain
Shrinkage can be categorized into different forms such as plastic, chemical, thermal etc., however according to EN 1992-1-1, the total shrinkage of a member can be simplified to be addition of autogenous shrinkage strain (∈ca) and drying shrinkage strain (∈cd)
Autogenous shrinkage develops during hardening of concrete due to hydration of cement, in the early days. This makes it a vital component of shrinkage to be considered, especially when new concrete is cast against hardened one. The drying shrinkage strain develops slowly, since it is due to loss of water through the hardened concrete. Because of the graduality of drying shrinkage, its effect is more critical when estimating long-term deformation in structures.
The total shrinkage strain is given by the below expression:
∈cs = ∈ca + ∈cd
where:
∈cs is the total shrinkage strain
∈cd is the drying shrinkage strain
∈ca is the autogenous shrinkage strain
The total value of drying shrinkage (∈cd∞) is Kh∈cd,0 while that of autogenous shrinkage is 2.5 (fck
– 10) x 10-6
∈cs = Kh∈cd,0 + 2.5 (fck – 10) x 10-6
∈cd,0 can be taken from table 3.2 of EN 1992-1-1
Kh is a coefficient that depends on the notional size h0 (its value can be obtained from table 3.3 of EN 1992-1-1)
h0 is the notional size (mm) of the cross-section 2A0/u
For simplicity the value of ∈cd,0 can be taken from table 3.2 of EN 1992-1-1. However, a detailed formular is given in Annex B of the standard which is:
∈cd,0 = 0.85 [(220 + 110 h0αcds1). exp(-αcds2 fcm/fcm0)] x 10-6
βRH = 1.55 [1 – (RH/RH0)3]
Where:
fcm the mean compressive strength (MPa)
fcm0 = 10Mpa
αcds1 is a coefficient which depends on the type of cement
= 3 for cement Class S
= 4 for cement Class N
= 6 for cement Class R
αcds2 is a coefficient which depends on the type of cement
= 0,13 for cement Class S
= 0,12 for cement Class N
= 0, 11 for cement Class R
RH is the ambient relative humidity (%)
RHo = 100%
Shrinkage Stress
When concrete is allowed to shrink freely, there would not be stress induced in the concrete due to volumetric change (Shrinkage). However, when the free shrinkage is restricted by restraints, then tensile stresses is induced in the concrete which can lead to cracking if the tensile strength of the concrete is exceeded. Restraint is almost always present in a reinforced or prestressed concrete. This restraint could be due to internal reinforcement, friction of the earth surface for members cast against the earth, fixity with adjoining members etc. Therefore, the effect of shrinkage stress should be catered for to minimize serviceability issues such as cracking.
Having evaluated the shrinkage strain, the shrinkage stress on a concrete restricted from shrinking freely would be obtained by multiplying the elastic modulus and shrinkage strain.
σ = εshrinkage x Ecm
Effects of Shrinkage in Structures
Effect of Shrinkage maybe critical in structures where serviceability and performance criteria govern their design. Two examples of such scenario’s structures are discussed
Differential Shrinkage in Composite Bridges
One of the major causes of shrinkage stress in concrete members is differential shrinkage. Differential shrinkage occurs between parts of the cross-section cast at different ages and this is always the case in composite bridges.
In composite concrete bridges—where reinforced concrete decks are integrated with steel girders or precast concrete girders —shrinkage can induce significant internal stresses. As the concrete deck contracts over time, it tends to pull on the stiffer girders, leading to differential strain between the materials. This can cause additional bending moments, redistribution of internal forces, and in some cases, cracking of the deck slab. The effect is more pronounced in long-span bridges or when early-age shrinkage is restrained by the girder. Design codes typically account for shrinkage in serviceability limit state checks, and proper detailing, material selection, and construction sequencing are used to mitigate adverse effects.
Differential axial deformation in Tall Buildings
Shrinkage in vertical members such as columns, shear walls, and core walls in tall buildings also contributes to overall axial shortening of those buildings. This would not in itself be a challenge but for differential axial shortening. Differential axial shortening occurs when vertical members shorten at different rate thereby impeding the serviceability of the tall building as the horizontal members spanning between vertical members tend to tilt. Shrinkage is from the major contributing factor to differential shortening alongside creep.
