Shrinkage and Thermal Cracking of Fast Setting Hydraulic Cement Concrete Pavements in Palmdale,California

Jointed Plain Concrete Pavement (JPCP) test sections were constructed using Fast Setting Hydraulic Cement Concrete (FSHCC) as part of the California accelerated pavement testing program (CAL/APT). Many of the longer slabs cracked under environmental influences before any traffic load was applied to them. Cores drilled through the cracks indicated that cracking initiated at the top of the slabs and propagated downwards. Concrete shrinkage and thermal strain data from field instrumentation was recorded and analyzed along with laboratory test data to determine the cause of the cracking. Finite element analysis using the measured strains and temperatures predicted high tensile stresses at the top of the test section slabs as a result of the differential drying shrinkage between the top and base of the slab and the non-linear nature of the negative temperature gradients through the slab. Laboratory free shrinkage tests on the test section cement indicated significantly higher shrinkage than ordinary Type II Portland cement. Based on the analysis it is recommended that the use of high shrinkage hydraulic cements in rigid pavement construction should be discouraged as these can result in high differential shrinkage gradients and premature cracking. Laboratory tests indicated fast setting hydraulic cements do not necessarily have high shrinkage and some can have significantly lower shrinkage than typical Type II cements. Shorter slab lengths (4.5 m) will reduce tensile stresses and thereby reduce the chance of premature failure in the event high shrinkage cement is used. Stiff bases such as lean concrete bases, will increase the stresses in pavements because of friction between the base and slab. Bases which are flexible under long-term loading and stiff under short-term traffic loading (for example asphalt concrete bases) are preferred.