The objective of this study was to quantify the early-age Portland Cement Concrete (PCC) slab deformations due to pure environmental loading at critical periods immediately following construction. A newly constructed Jointed Plain Concrete Pavement (JPCP) section on highway US-30 near Marshalltown, Iowa, USA was instrumented to monitor the pavement response to variations in temperature and moisture during the first seven days after construction in the summer of 2005. Temperature and humidity sensors installed within the test sections monitored the variations in temperature and moisture. The PCC slab deformations were measured using the Linear Variable Differential Transducers (LVDTs) installed at corner, edge, and center of the PCC slab. The surface profiles were measured along the diagonal and transverse directions of the slab using an inclinometer-profiler. The measurements revealed the zero-gradient temperature deformation in the form of slab curling due to built-in temperature difference. Two-dimensional (2-D) and three-dimensional (3-D) Finite Element (FE) models were employed to characterize the zero-gradient temperature deformation. Comparisons between the field-measured and the FE-computed slab deformations are presented in this paper.