Characterization of Effective Built-in Curling and Concrete Pavement Cracking on the Palmdale Test Sections
Differential expansion and contraction between the top and bottom of a concrete slab results in curling. Curling affects stresses and deflections and is an important component of any mechanistic-empirical design procedure. A significant portion of curling can be attributed to the combined effects of nonlinear "built-in" temperature gradients, irreversible shrinkage, moisture gradients, and creep, which can be represented by an effective built-in temperature difference (EBITD).
Several instrumented test sections utilizing several design features were constructed and evaluated using the Heavy Vehicle Simulator (HVS) in Palmdale, California. These instrumented slabs were loaded with a half-axle edge load without wander in order to study the effects of curling and fail the slab sections under accelerated pavement testing. A procedure for estimating EBITD using loaded slab deflections was developed using the HVS results. The advantages of using loaded slab deflections are that they can be used for measuring EBITD of slabs with high negative built-in curl and can also be adapted for a Falling Weight Deflectometer, making the procedure efficient and cost-effective for the back-calculation of EBITD of in-service pavements. Differences in restraints and variability in concrete material properties resulted in EBITDs ranging from –5�C to greater than –30�C.
The HVS field tests were also used to examine Miner's hypothesis along with various fatigue damage models. Results indicate test slabs cracked at cumulative damage levels significantly different from unity. New models that incorporate stress range and loading rate along with peak stresses were developed. The coefficients for these models were developed to incorporate transverse cracking, longitudinal cracking, and corner breaks. The models can also be used for slabs that exhibit high negative EBITD. For slabs susceptible to high shrinkage gradients, microcracking resulting from restraint stresses during early ages can significantly reduce the slab's nominal strength. Early-age restraint can vary considerably from one slab to another, depending on restraint. A procedure to model slab strength reduction and slab size was developed using nonlinear fracture mechanics principles. A parameter called the "effective initial crack depth" is introduced to characterize the early-age surface microcracking.