ESTABLISHING PERMANENT CURL/WARP TEMPERATURE GRADIENT IN JOINTED PLAIN CONCRETE PAVEMENTS

The concrete slab in a pavement structure curls due to a temperature gradient and warps in the presence of a drying shrinkage gradient. The curling is upward (downward) when the slab is cooler (warmer) at the top than the bottom. Warping is consistently upward because the slab is more susceptive to drying at the top. Since the slab is not free to curl, tensile stresses form in the slab. These stresses when combined with traffic loadings can result in cracking of the slab. Slabs do not remain flat in the absence of daily gradients. This is because of the temperature/moisture gradient that exists in the slab at zero-stress time. Zero-stress time occurs after the placement of the slab, during curing and following the final set time. These gradients, known as built-in or permanent curl temperature gradients, lock into the slab and either decrease or increase the curling due to the transient gradients. One more factor that influences the future shape of the slab is the permanent warp temperature gradient. A portion of the drying shrinkage in drier seasons can reverse in wet seasons, known as reversible drying shrinkage. Permanent warp is due to the irreversible portion of the drying shrinkage, which progressively increases as the concrete ages and eventually reaches a plateau. This study puts forward a procedure, including three tasks, to establish realistic values for permanent curl/warp in the slab. Task 1 includes identifying the zero-stress time in the slab. This is performed by using the data from four different instrumented pavement structures in Western Pennsylvania. Task 2 focuses on establishing the built-in temperature gradient based on the measured temperature. As part of this task, a computer temperature model is developed to predict the temperature within the pavement based on the ambient conditions and the heat of hydration. Task 3 focuses on estimating the permanent warp in the slab. This is achieved by using long-term strain measurements in two different test sections in Pennsylvania and Minnesota. The drying shrinkage development is also predicted by using a computer relative humidity model. The difference between the predicted and measured drying shrinkage is attributed to the effects of creep and base restraints.