Chapter 1. Introduction

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Introduction

Many distresses occurring in concrete structures are attributed to the corrosion of steel reinforcing rebars, a condition to which steel-reinforced continuously reinforced concrete pavement (CRCP) is typically subjected. According to a CRCP performance report, corrosion has been a major deteriorative factor for CRCPs in Wisconsin, causing delamination, spalling, and steel rupture.⁽¹⁾ Therefore, glass fiber reinforced polymer (GFRP) rebars-which are increasingly gaining attention for structural application because of their noncorrosiveness and high longitudinal strength, light self-weight, and nonmagnetic quality-can also be viable alternatives to steel reinforcing rebars for CRCP.

Conventional steel-reinforced CRCP has been built across the country since 1921, and its design can be found in a 1993 guide by the American Association of State Highway and Transportation Officials (AASHTO).⁽²⁾ The behavior of conventional CRCP in response to concrete volume change has been well understood using mechanistic and numerical analysis methods, and its field performance has been monitored and analyzed as well; this has provided valuable information regarding some factors affecting the behavior and distresses deteriorating performance over time. (See references 1, 3, 4, and 5.) However, since little research concerning the replacement of steel reinforcing rebars with GFRP rebars in CRCP has been conducted, initial studies of mechanical behavior and design considerations for GFRP-reinforced CRCP need to be completed prior to any field application.

At the onset of this study, the effects of using GFRP reinforcing rebars on shrinkage and thermal stress development in concrete were investigated. Shrinkage and thermal stresses in concrete have been known to cause incipient cracking in CRCPs or bridge decks. In order to optimally control concrete cracking (which has a strong influence on the performance and longevity of such structures), it is important to understand the development of these stresses in the structures. In the case of a freely supported reinforced concrete slab subjected to shrinkage or temperature variation, concrete stresses result from the restraint provided by the reinforcement. While concrete shrinkage causes tensile stresses in concrete, temperature variation can cause either tensile or compressive stresses in concrete, depending on whether the temperatures drop or rise and on the way of combining the coefficients of thermal expansion (CTEs) of the concrete and the reinforcement used.^(6,7) The CTE of concrete varies primarily with the type of coarse aggregate used, and the CTE of the GFRP depends on the types of fibers and resins and the volume fraction of fiber used. In the case of slab on ground or bridge decks, however, other restraining forces acting on the concrete slab-such as friction from the subbase under concrete pavements, restraints from the girders underneath bridge decks, or restraints from reinforcement ties to neighboring slabs-need to be considered. When these restraints are considered, the overall resulting stress in the concrete will differ from that for a freely supported concrete slab (mostly by having a higher tensile stress level).

In this report, analytical studies for a freely supported reinforced concrete slab are first presented to describe the effect of GFRP reinforcing rebars on shrinkage and thermal stresses in concrete, and the results are compared with the experimental measurements. A finite element (FE) model for the freely supported reinforced concrete slab was developed, and the results were verified by the analytical results. FE analyses were also conducted for the GFRP-reinforced CRCPs; transverse cracks were spaced at 1.524 meters (m) (5 feet (ft)), which are subjected to both concrete shrinkage and temperature change. In the FE analyses, various CRCP design considerations (such as the CTE of concrete, the friction from the pavement's subbase, and the bond-slip between concrete and reinforcement) were studied to understand their effects on stress development and crack width in the CRCP; additional effects of crack spacing were studied using CRCP slab segments of lengths 1.067 meters (m) (3.5 feet (ft)), 1.524 m (5 ft), and 2.438 m (8 ft).^(8,9)

The FE analysis has also been employed to create a feasible longitudinal reinforcement design for a 25.4-centimeter-(cm-) (10-inch-) thick GFRP-reinforced CRCP as an example. Using number 6 GFRP rebars at 15-cm (6-inch) spacing at the middepth of the slab is shown to be an economically feasible design for GFRP-CRCP on a flexible subbase (or lime-treated clay subbase). Although a larger amount of GFRP reinforcement is sometimes required to satisfy the allowable crack spacing (1.067 to 2.438 m (3.5 to 8 ft)), crack width (≤1 millimeter (mm) (0.04 inch)), and reinforcement stress level (20 percent of GFRP ultimate tensile strength) comparable to those from number 6 steel rebars at a 15.24-cm (6-inch) spacing at the middepth, the proposed design for GFRP-CRCP has been further examined using a mechanistic analysis program, CRCP8, and proven to perform satisfactorily for given material properties and design conditions provided by the Virginia Department of Transportation (VDOT). Other feasible designs for the given condition were also investigated, considering the effects of concrete coarse aggregate and subbase types on the development of crack by the mechanistic analysis.

Objective and Scope

The objective of this study is to investigate the effects on stress development in pavement and on critical design factors from substituting GFRP reinforcement for conventional steel reinforcement in CRCPs to determine the performance characteristics of the GFRP-reinforced concrete pavements. The results of this study target the design of CRCP with GFRP rebars as an applicable reinforcement and propose feasible GFRP-CRCP designs to be constructed.

The scope of this report includes studying the effect of GFRP reinforcing rebars on shrinkage and thermal stresses in concrete by analytical and numerical methods as well as by experimental measurements, and proposing a series of designs for the GFRP-reinforced CRCP based on the numerical and mechanistic results. The study also reveals some areas where further studies are recommended.