Journal of Materials in Civil Engineering

The use of fiber reinforced polymer (FRP) composites for the seismic retrofit of columns and piers, flexural strengthening of beams and girders, and strengthening of slabs has been fairly well established on a structural basis. However, aspects related to materials selection and use, design detailing, fracture and failure mechanisms, and durability are still not well understood. The functional efficiency of the rehabilitated structure is largely dependent on materials-related issues, which are highlighted in this paper, emphasizing effects pertaining to constituent materials, interfaces, and overall systems response. Major implications of processing and fabrication schemes are highlighted focusing on materials use efficiency, potential of defect formation, and effects on durability. Examples of the importance of detailing are shown, and design guidelines pertaining to materials are discussed.

Bond quality is an important factor that affects the performance of repaired members. In patching repair, interfacial cracking and delamination due to the shrinkage of repair materials are often observed. In this study, flexural bond strength tests using jointed specimens were carried out, and the tension softening diagrams were determined by using experimental load-CMOD curves. These tension softening diagrams were adopted as the bond properties in numerical analyses to predict shrinkage and cracking behavior. For crack injection repair, bond strength tests based on the Japanese Industrial Standard (JIS) were carried out under conditions that are different from those normally encountered in the field (i.e. the tests involved smaller specimens, smoother surfaces, and finer grained materials). The bond properties of crack injection materials were evaluated using tension-softening diagrams. Size effect on flexural bond strength of repair materials was observed and clarified by numerical analysis with these tension softening diagrams. The analyses indicate size effect should be considered when evaluating the strength of larger structures in the field.

Concrete beams are often retrofitted with bonded steel or composite plates. The failure of the retrofitted member is usually due to delamination of the plate from the beam. Delamination can occur either at the end of the reinforcing plate or at the location of a crack in the concrete beam. This paper will focus on the analysis of delamination initiated at the mouth of a flexural crack. When loading is applied, the flexural crack tends to open and hence induces high interfacial shear stress that may result in delamination. The relation between interfacial shear stress and applied moment is affected by many material and geometric parameters. In this study, the bridging stress in the bonded plate and the maximum interfacial shear stress τ_max are first related to the crack mouth opening. A fracture mechanics based analysis is then carried out to obtain the crack mouth opening and τ_max for a given applied moment. Through a systematic parametric analysis, the effect of various parameters on τ_max is deduced. A combination of parameters that will increase τ_max, and hence increase the likelihood of delamination, can hence be identified and avoided in design.

Thin bonded cement-based overlays include surface repairs, toppings, and linings, especially on slabs, slabs on soil, concrete pavements, and tunnel walls. The aim of this paper is to show that, contrary to a widely held belief, debonding of cement-based overlays is governed not by the shear stress along the overlay-base interface but by the built-in tensile stress perpendicular to this interface. After a brief comparison with laminated composite materials and with beams reinforced by bonded strips (made of metal or of composite), the specific case of thin cement-based overlays is presented. Different conditions of debonding of an overlay are investigated. In all cases, the tensile stress perpendicular to the base-overlay interface is confirmed as the designing parameter. In terms of fracture mechanics, debonding of cement-based overlays is governed by a mixed mode in which mode I is widely dominant. There is a lack of appropriate tests to accurately characterize the mechanical behavior of the interface. Nevertheless, finite-element calculation relying on the mean values of the shear and tensile strength of the interfacial bond (calculation was performed with CESAR code) provides a good approximation of debonding initiation and propagation.

New cement-based repair overlays are subject to restraint and eigenstresses that can lead to crack formation and delamination. The origin of this complex state of stresses is found in the dimensional instability of the new coating. Drying shrinkage, autogeneous shrinkage, and temperature gradients are the most important loads acting on the system. In most cases the design of such coating systems is carried out in an empirical way. In this paper a new approach based on a numerical model is proposed to determine the relevant key parameters for the design of repair systems. By means of this model time-dependent moisture distributions and crack evolution in a drying concrete repair system can be simulated in a realistic way. Crack formation and delamination is described, with the smeared and discrete crack models, applying nonlinear fracture mechanics. The relevant material parameters influencing shrinkage cracking and delamination are identified by means of a sensitivity analysis. Finally, this approach is applied to analyze in a rational way the repair measures on a concrete bridge. This study primarily aimed to decide which repair material is more suitable with respect to shrinkage-induced damage and consequently which one provides the highest durability for the rehabilitated structure.

Reinforced concrete (RC) slabs on the steel girders of old highway bridges occasionally get damaged. Various repair methods for damaged deck slabs have been developed and adopted on site. A new method for repairing damaged RC slabs, deck restoration by adhesive panels (D-RAP), was developed, in which double layered prefabricated panels are glued on the top surface of the concrete by means of epoxy resin mortar. It is not clear how the system behaves under static and dynamic load. Various tests were performed on the mechanical behavior of beams and slabs reinforced by this method. Fatigue tests of full-scale model slabs were also carried out on two alternative points. It was found that the D-RAP method increased the load-carrying capacity and fatigue strength up to the level of newly constructed slabs. The test results show that the D-RAP method is an effective reinforcing method for deteriorated slabs.

The electrodeposition method for repair of cracked reinforced concrete is presented here. Electrodeposition means precipitation of material on the surface of an electrode by electrolysis. The ultimate purpose of this study is to apply this technique to land-based reinforced concrete structures. Investigations on mortar specimens with embedded steel were conducted. After cracking the specimens with a specified load, they were immersed in several types of solutions. Then a constant current between the embedded steel in mortar and an electrode in the solution was applied. The deposition pattern on the surface, the crack closure development, and surface coating due to the precipitation of electrodeposits were checked and evaluated. In order to assess the effectiveness of this method to improve the mortar surface, water permeability, pore size distribution, and adhesion strength were measured. The results indicate that electrodeposits formed on the mortar surface are able to close the crack and coat the mortar surface. In addition, the investigations show that the deposition product can improve the microstructure of the mortar surface.

This paper presents a continuation of the findings of a long-time study of the performance of cements varying in their chemical and physical characteristics. Most of the cements were provided by the Portland Cement Association, and others were obtained form a local manufacturer. The study was initiated in 1940 by the Portland Cement Association in collaboration with the U.S. Bureau of Reclamation. The 28 cement types studied varied in chemical and physical characteristics and were used in the construction of the parapet wall located at Green Mountain Dam. Each of the 28 cements was used in 3 or 4 of the 104 panels of the parapet wall. The Green Mountain Dam is located on the Blue River, a tributary of the Colorado River, 21 km southeast of Kremmling, Colo. The dam itself is an earth-fill structure, 94 m high, with a crest length of 350 m. Some of the cements used were high-alkali cements, and the aggregates contained reactive silica. This paper investigates the development of alkali-aggregate reaction during the past 53 years on the parapet wall and its impact on strength and durability of concrete. Specifically, the results of a visual inspection and compressive strength are presented as a function of the amount of equivalent alkalis present in these different cements for the years 1943, 1946, and 1997.

The lack of construction materials in the coastal areas and the high cost of importing good-quality soil creates the need for developing new technologies to manufacture engineered fills using low-quality materials obtained from a seabed. This study presents results of a series of tests conducted to obtain field data related to basic engineering properties of a geomaterial comprised of the mud dredged from Tokyo Bay, mixed with lightweight additives, such a foam or expanded polystyrol beads, and portland cement as a binder. Due to discrepancies that may exist in laboratory-prepared samples, a field study was launched to replicate the actual construction conditions. Relationships are developed to correlate various engineering properties of the geomaterial in terms of strength, modulus of elasticity, and deformation characteristics.