Rapid strength gain concrete repair mixtures that cure within 4 to 12 hr were used to carry out full-depth slab repairs on a section of Interstate highway. The mixtures included a calcium-chloride accelerated mix, a very early strength mix developed by the Strategic Highway Research Program, and a" fast-track" mix previously used for early opening of concrete intersections. Before carrying out the repairs, job materials and mix designs were obtained and used to develop correlations between pulse velocity, maturity, and·'early strength of concrete. Maturity functions based both on Nurse-Saul and Arrhenius (equivalent age) approaches were developed. To account for heat rise of the concretes in actual sections, laboratory correlations were carried out by curing test cylinders in a specially insulated curing box. Pulse velocity and maturity functions were used to predict in situ strength gain of concrete in instrumented test repair sections. Temperatures were monitored through the depths of the test slabs during the initial 8 hr of curing. Temperatures at mid-depth ranged from 60 C to 70 C (140 F to 160 F) for these mixes. After 4 hr of curing, the very early strength mix exceeded 14 MPa (2,000 psi) compressive strength, as determined by in situ methods. The other two mixes gained strength at a slower rate.

In recent years, early opening of concrete pavements to traffic has been given much emphasis. As documented in a synthesis (J) published by the Strategic Highway Research Program (SHRP), many recent developments in materials and processes for concrete paving focus on early opening." Fast track" technology (2) allows concrete pavements to be constructed or repaired and opened to traffic within 4 to 24 hr. That is especially useful when full depth pavement repairs are being carried out along critical segments of highway where traffic disruption must be kept to a minimum. In recognition of the importance of this technology, FHWA initiated a demonstration project on accelerated rigid paving techniques in 1988 to promote the development of fast track techniques. Several pilot projects have been completed with the assistance of the FHW A under this program. As a result of these advances, many states now allow early opening when fast track mixes are used (3). The high-early strength gain typically is obtained by using high cement content, a low water to cement ratio, and accelerating admixtures. A rich, low-water-content Il} ix containing 1 to 2 percent calcium chloride produces adequate strength and abrasion resistance for opening to traffic in 4 to 5 hr at temperatures above 10 C (50 F)(4). Accelerators are added at either the ready-mix plant or at the jobsite, depending on the temperature and distance to the