10.6 practical steps to ensure shrinkage effects are mitigated - mix design

It is generally assumed that shrinkage and creep are inter-related properties and usually steps taken to control shrinkage will also result in control of creep. Procedures which can be taken to reduce shrinkage include:-
(i) Reduction of water volume in concrete. The most important single factor affecting shrinkage is the amount of water per unit volume of concrete. Concrete with a wetter consistency will shrink more than one with a dry consistency because the wetter consistency is obtained by the use of a higher water/cement ratio, by a greater quantity of paste or a combination of the two.

Consequently, using mix designs which have a reduced water content will result in reduced shrinkage. Measures which minimize the water content of the concrete include:

            -            Minimizing sand content.
            -            Using larger aggregate sizes.
            -            Using lower concrete slumps.

Obviously, these steps also limit workability and ultimately a compromise is required to be reached by the concrete manufacturer.

(ii) Aggregate Selection.Aggregates play an important role in providing volume stability to the concrete matrix. Aggregates should be selected which are well graded for good workability at minimum water contents and contain a minimum of extreme fines. They should be generally rounded or cubical in shape, while flat or elongated particles should be avoided. Further sand should be reduced to the lowest level consistent with adequate workability at the mixing water requirement.

An aspect which becomes extremely important is the use of pumped concrete. Depending on the size of the pump used, the maximum aggregate size can be restricted to 10 mm max size. As a result, shrinkage of pumped concrete is generally much greater than that of standard concrete. If it is imperative that pumped concrete is used, then pipe sizes should be investigated which allow the use of  larger maximum aggregate sizes.

(iii)  Avoidance of Admixtures. The designer should clearly specify that admixtures should not be used by the  concrete supplier except with the specific approval of the designer. This will give an opportunity to the designer to satisfy himself that the admixture proposed by the supplier will not increase the shrinkage in the concrete.

(iv)  Concrete Uniformity. It is important that concrete is uniform and tests should be included, which test the variation of supplied concrete. For example, on a project with a general strength specification of 25 MPa, consecutive trucks may possess 28 day strengths of 27 mPa and 35 mPa, depending on batching or re-routing of trucks, etc. As such, they posses different shrinkage properties and differing tensile strengths. The higher strength concrete, possessing greater cement paste, will display greater shrinkage than the lower grade concrete to which the shrinkage will be applied to. However, the lower strength concrete has lower tensile strength and is less able to resist the added shrinkage and will crack as a result.

(v)  Proper Curing.Ensuring that proper curing is carried out to specification for a minimum of period of 7 days is imperative. In general , curing becomes important since the greater the amount of water retained in the concrete during the hydration process, the better will be the strength and durability of the concrete. This will ensure that tensile strength is maximized to resist applied tensile stresses. Cracking can occur early in a slab due to its attained strength being unable to resist the shrinkage strains applied, at very low strength maturity (1-7 days).