Chapter 7. Summary, Conclusions, and Recommendations

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Summary and Conclusions

In this report, the concrete stresses induced in a GFRP-reinforced concrete slab due to concrete shrinkage and temperature variations have been analytically calculated in comparison to those induced in a steel-reinforced concrete slab, and the validity of the analytical model has been verified numerically and experimentally. The analytical solution indicates that a low Young's modulus of GFRP rebar results in a stress reduction in concrete slabs. The thermal stress in concrete can be either tensile or compressive, depending on temperature variation and the CTEs of the concrete and the GFRP reinforcement used.

From comparison with the finite element calculation, the analytical solution of average axial concrete stresses is shown to be valid throughout the longitudinal (x-) direction, especially in the vicinity of the slab's middle section, where the maximum average axial stresses in concrete appeared. Meanwhile, the comparison also reveals the applicability of the FE method to the CRCP analysis. A 1.524-m (5-ft) CRCP segment FE model was built to simulate the behavior of the CRCP and to study the effects of CRCP design considerations (such as the CTE of concrete, the friction from the pavement's subbase, and the bond-slip between concrete and reinforcement) on stress development and crack width in the CRCP. It is shown that using concrete with a lower CTE reduces the concrete's tensile stress level when exposed to temperature drop, and a weaker bond between the concrete and reinforcement also decreases the concrete tensile stress level in the CRCP and the tensile reinforcement stress level at its cracks. With a higher subbase friction, the CRCP is more likely to crack from its bottom area since the concrete stress level at the bottom will be higher than that at the top. The results give ways to favorably control the larger crack spacing and wider cracks of the GFRP-reinforced CRCP caused by the GFRP's low elastic modulus; in order to satisfy the limiting criteria, with a given CTE of concrete, one can choose to (1) increase the amount of reinforcement, (2)increase the bond between concrete and reinforcement, and/or (3) increase the bond between concrete slab and subbase.

From the FE study, a feasible longitudinal reinforcement design of 25.4-cm- (10-inch-) thick GFRP-reinforced CRCP has been proposed for a given condition by evaluating concrete stress developments in the 1.067-m (3.5-ft), 1.524-m (5-ft), and 2.438-m- (8-ft-) long CRCP slab segments with limestone concrete. Using number 6 longitudinal GFRP rebars at 15.24-cm (6-in.) spacing at the middepth of the slab is shown to be an economically feasible design for GFRP-CRCP on the flexible subbase (or lime-treated clay subbase). In addition, number 5 GFRP rebars spaced at 1.219 m (48 inches (4 ft)) will be adequate as transverse reinforcement.

The mechanistic analysis program has been used to analyze the GFRP-CRCPs with different types of concrete coarse aggregate and subbase. Higher CTE, higher Young's modulus, lower tensile strength, and larger drying shrinkage of concrete appear to reduce the crack spacing in the CRCP. However, even though the crack width is generally narrower as the crack spacing is smaller, it can open wider after the crack formation settles down. This occurs when ambient temperature and air humidity levels drop enough to cause large concrete volume change so that the crack width of those smaller crack spacing sections will become larger. Higher subbase friction always causes smaller crack spacing followed by narrower crack width. From the mechanistic analysis results, the GFRP-CRCPs with the limestone, sandstone, or siliceous river gravel concrete on asphalt-stabilized subbase seems to perform satisfactorily without raising the GFRP-reinforcement ratio under a given design condition provided by VDOT. In addition, the subbase that can provide a bond-slip of about 1.357 GPa/m (5,000 lbf/in²/inch) is able to provide a satisfactory performance of the GFRP-CRCP with granite concrete.

Currently, the designs of a 27.94-cm- (11-inch-) thick GFRP-reinforced CRCP and a steel-reinforced CRCP section have been prepared and are waiting to be constructed in West Virginia in 2006.

Recommendations

• Further studies of the effect of a high transverse CTE of the GFRP rebar on its bond with concrete at various temperatures and on cracking in the GFRP-reinforced CRCP are necessary.
• Investigations into the effect due to the low transverse (shear) strength of the GFRP rebar on the load transfer at the cracks generated in the GFRP-reinforced CRCP should be conducted. Investigation of the relationship between the load transfer efficiency and the crack width is required.
• Developing the previously mentioned alternative reinforcement position for minimizing the amount of GFRP reinforcement needed while still favorably controlling the cracking in the GFRP-reinforced CRCP would be economically beneficial.
• Development of standard design guidelines to clearly specify the limiting criteria (such as crack spacing, crack width, and reinforcement stress level for the GFRP-reinforced CRCP) is needed. Currently, the AASHTO guideline for the steel-reinforced CRCP design can only serve as a reference.