| Forming during the hardening period of cement concrete, built-in curling effects on the stress and curling behavior of the concrete slab and generates profound influence on the longterm behavior of concrete pavement. The c urrent reseach approaches may not give convicible explanation about the formation of built-in curling. In the current design guide of our country, the effects of built-in curling on structure mechanical behavior haven’t been incoperated. Also no specific techniques has been proposed to control the amount and distribution of built-in curling in slabs. Therefore, based on the previous researches, various method such as field investigation, labrotory test and theoretical analysis were adopted to discuss the formation mechnism of built-in curling in concrete slab. The following work have been complished in this paper.As an intial work, six-slab conctete pavement with semi-regid base layer was implented and instrumented to measure the through-depth temperature field and vertical deflections at different horizontal positons(slab corner and mid-edge) during early-ages. Results showes built-in curling are position-dependent and age-dependent. A single slab over graded stone base was further constructed to investigate the temperature history and strain history along slab depth during early age and hardened age. The residual strain are also found depth-depended and the residual strain gradient shows fluctuation phenomenon during early ages.The finite elment model to simulate the transit temperature field and moisture field in concrete pavement from construction has been established and the user subroutines were developed as well. The adibatic temperature rise tests were conducted on various type of cement and the concrete used in Beijing test slab. Consequently the collected data were applied to reverse the thermal parameters such as hydration heat fuction, absorption coeeficient of solar radiation, heat transfer coeeficient through the differential evolution and finite elment united method. In addition, the parameters in moisture model such as moisture diffusion coefficient and surface factor were reversed. Further, the uniformity index to evalute the the distribution of temperature field and moisture field along slab section was proposed. Effects of curing method and cast time on the uniformity, the spacial and temporal distribution were analyzied consequenly.The literatures on the tension and compression creep feature shows that the creep behavior of early age concrete is hihgly age dependent and the ratio of tensile specific creep to compressive specific creep scatter greatly. Considering that the specific creep of early age concrete is sensitive to the the loading age and load holding time, the holding time-loaindg age equivalence model was presented, the formulus to calcualate shift factor of specific creep and relaxation moudulus were derived and fitted accordingly.The constitutive model which can simultaneously describe the basic creep behavior, the drying creep behavior, the thermal dilation behavior, the different tension specific creep and compressive specific creep, tension and compression strain softening behavior was established. The corresponding user subroutine to accomplish it was also developed and validated. Subsequently, the affections of basic creep, drying creep, tension strain softening and compression strain softening on residual strain in concrete were investigated one by one to verify that all these factors are the possible reason of built-in curling in slabs. Finally, the forward calculation procedure to determine built-in curling was represented and the technique to terminate the calculation and convert the residual deflection into residual stress were both explained.On account of the previous established model which can be called the thermal-moisture-mechanical sequence coupled model and the following steps, effects of the creep or relaxation ability, cast time and slab geometry on the final distribution of residual stress were discussed. The conclusion can be drawled that the uniformity of temperature, the initial drying age, the creep or relaxation ability, the stress history in slab and the constraint degree of the slab would comprehensively impact the built-in curling magnitudes and distribution pattern. The introducing of heavy traffic load into slabs with built-in curling would increase the risk of corner break and transverse crack. In the end, the techniques to control built-in curling in design and construction of concrete pavement were illustrated. |
