Pavement Concepts > Shrinkage

Drying shrinkage and thus crack width is caused primarily by a loss of moisture from the concrete. A direct correlation exists between drying shrinkage and the following factors (4):

•Curing method of concrete, and the environment in which it is applied.

•Initial water content of the concrete. The higher the initial water content of concrete the greater the likely shrinkage (Fig 8.29 in Fulton)(4).

•Age of the concrete, humidity and the environment.

•Aggregate and mix proportions. A higher concentration of aggregate reduces relative shrinkage (Fig 8.25 and 8.28 in Fulton)(4).

•Cement content, composition (web site), performance, fineness and the use of cement extenders.

•Admixtures (web site). The effect of admixtures on shrinkage is variable and depends on the specific admixture and cement and exposure conditions.

•Type of aggregate and aggregate content. For concrete of similar 28-day strength and similar slump, concrete shrinkage can vary.

•The difference in temperature of the concrete at the time of construction and that of the environment.

The following guidelines are useful to lower the risk of high shrinkage:

•The temperature of the concrete at discharge should not be allowed to go too high.

•The slump of the concrete should be low.

•Use larger rather than smaller size of aggregate.

•Excessive haul or a long waiting period at the construction site before placing should be avoided.

•Also, avoid dirt and/or clay contamination during handling.

Considering all the above factors, the estimating of concrete shrinkage in the road becomes difficult. Nevertheless, the program does it for you. From a practical point of view, a shrinkage strain of between 0.04 % and 0.07 % (i.e. between 1.8 mm and 3.2 mm over a length of 4.5 m) can be accepted for design purposes. Designers are encouraged to reduce the risk of shrinkage by taking into account the above factors that influence shrinkage, and by addressing them in specifications and/or quality control.