Bridges are a major element in ground transportation networks. Of the present 70 billion dollar backlog of bridge rehabilitation needs, 28 billion dollars is attributed to the corrosion of steel in concrete. The chloride corrosion of steel in concrete structures is not confined to bridges; but also reduces the service life of parking garages and sea and coastal structures. The service life model for reinforced concrete structures in chloride laden environments consists of the following serial phases: diffusion to the depth of the steel that would precipitate first maintenance actions; corrosion of the steel at the first maintenance depth until cracking and spalling occurs; continuation of spalling until a damage level is reached that is defined as the end-of-functional service life for an element or structure. The diffusion phase is described by Fick's Law and the boundary conditions define the solution form. The time-to-cracking model is dependent upon factors as concrete strength properties, cover depth, and corrosion rate. Corrosion rate has a significant influence on the time-to-cracking that typically occurs in three to seven years after initiation.

This paper presents the responses of specimens representing typical slab-column connections in parking garage structures. Three slab-column specimens were constructed with normal-strength concrete and three with high- performance concrete. Uncoated reinforcing bars and two different epoxy coating thicknesses were used in the test specimens. Each slab-column specimen was supported on a single column and loaded at the four corners to simulate the moments and shears in the slab. The high-performance concrete specimens exhibited smaller crack widths than the normal-strength concrete specimens for both dead and live loads. Specimens reinforced with epoxy-coated bars showed larger crack widths than companion specimens reinforced with uncoated bars. An increased coating thickness resulted in larger crack widths. Crack widths predicted were compared to test results.

Shrinkage cracking is a major concern for concrete, especially for flat structures such as highway pavement slabs for parking garages and walls. One of the methods to reduce the adverse effects of shrinkage cracking is to reinforce concrete with short, randomly distributed fibers. The efficiency of cellulose fiber in arresting cracks in cementitious composites was studied. A ring-type specimen was developed for a restrained shrinkage cracking test. Concretes reinforced with six different types of cellulose fibers, with a fiber content of 0.5 percent by volume (approximately 1 percent by weight of cement) were tested. Cellulose fiber reinforcement showed an ability to reduce the crack width significantly (as compared to unreinforced concrete). For comparison, concrete reinforced with 0.5 percent cellulose fibers showed excellent performance equal to 0.5 polypropylene fibers (maximum crack width about 1/3 that of plain concrete). The long-term mechanical performance of cellulose and polypropylene fiber reinforced concrete was also evaluated. Two types of accelerated aging were used: hot-water soak and repeated wet/dry cycling, respectively. The toughness of unaged cellulose and polypropylene fiber reinforced concrete was approximately 40 and 90 percent higher, respectively, than that of plain concrete specimens. After 60 days of hot water treatment, the toughness performance of the composite was unchanged. However, after being subjected to repeated wet/dry cycling, a 40 percent drop in toughness of both cellulose and polypropylene fiber composite was observed. The influence of the cellulose fiber reinforcement (0.5 percent addition) on other concrete properties, such as free shrinkage, compressive strength, flexural strength, and Young's modulus, were also investigated.

Discusses advancements in impact-echo instrumentation and signal processing that have led to the development of an automated field system. The basic principles involved in impact-echo testing and signal analysis are reviewed. An artificial intelligence technique called a neural network, which has been used to automate signal interpretation from platelike concrete structures, is explained, and examples of its use on concrete slabs containing voids and cracks are shown. Impact-echo instrumentation is discussed, and a new, rapid impact-echo field system is presented. This field system can be used independently when testing concrete structures, such as beams or columns, or in conjunction with the automated signal interpretation software when testing plate-like structures, such as bridge decks, parking garage slabs, and walls. The automated field system makes it possible for the impact-echo test method to become a practical nondestructive tool for condition assessment of concrete structures.

The durability of a prestressed concrete deck in a parking garage is largely dependent on the corrosions of the prestressed strands. The time when the chloride-concentration level exceeds the threshold limit to initiate corrosion on the strands was calculated using Fick's diffusion law. Migration of chlorides was studied within the context of previous research. To develop a prediction method for the movement of the chloride concentration profile with respect to time, a computerized finite difference method was deployed. The method presented in this paper provides a scientific way for maintenance engineers to evaluate concrete structures exposed to salt environments.