International Concrete Abstracts Portal

The civil engineering community is acquainted with the fact that existing reinforced concrete structures require constant maintenance in order to increase their service life. Nevertheless, maintenance might be not sufficient when a change of use, which entails for an increase of the service loads, damage due to aggressive environment, or the need of withstanding natural and man-made hazards were not taken into account when the structure was designed. Fiber-reinforced composites are a suitable tool to strengthen and thus increase the capacity of the structural element without changing the original configuration and adding mass. Steel reinforced grout (SRG) is a relatively new tool in the realm of fiber-reinforced composites that consists of steel fibers embedded in a cementitious mortar. This paper presents a state-of-the art on SRG that includes a literature review and points out the effectiveness of SRG for flexural strengthening and confinement of concrete members. It also proposes a design approach to strengthen reinforced concrete beams based on the available data on flexural tests on beams, bond tests on SRG-concrete joints, and tensile tests on SRG coupons.

U.S. and Japan have had a history of collaboration to mitigate the effects of earthquakes. In 1977, a cooperative agreement set forward a plan to improve the safety of building structures. This plan led to collaborative research over the next several decades that included studies of reinforced concrete, masonry, steel, and precast structural systems. This paper summaries some of the research on structural subassemblages at the University of Minnesota that stemmed from that collaboration. The intent of this paper is to also recognize the contributions provided by Prof. James K. Wight who played a key role in the U.S. Japan collaboration and in the dissemination of the outcomes of research through his leadership at the American Concrete Institute (ACI) including his roles with technical committees including ACI Committees 318 – Structural Concrete Building Code and 352 – Joints and Connections in Monolithic Concrete Structures.

Most maintenance problems associated with industrial concrete floors result from the joints. This paper emphasizes a method to eliminate saw-cut joints in slabs-on-grade by the use of steel fiber reinforced concrete (SFRC) only. The performance of the composite material is directly linked with the choice of a specific concrete mix design and an improved technique to uniformly mix a high dosage of steel fibers. Tests and experience have shown that high level post-cracking ductility of the SFRC can control micro-cracking caused by flexural and shear stresses combined with restrained shrinkage. The proposed design approach, based on the yield-line theory, gives an objective view of the safety factor in relation to the ultimate state. Case studies demonstrate that typical areas of 25,000 ft² (2322 m²), without saw-cut joints, are regularly achieved by experienced contractors with relevant site quality control. Practical site aspects such as armored contraction joints, slab details, aspect ratio, installation techniques etc., are an integral part of the case study as well. The second part of this paper details the use of this technique for structural applications such as suspended slabs on piles and mat foundations. To demonstrate the structural capacity of concrete solely reinforced with a high dosage of steel fibers, real scale tests and practical case studies are presented.

The shrinkage of concrete originates stresses on restrained structural elements, which can cause cracks. In some cases, for instance to reduce the number of structural joints, it is important to decrease drying shrinkage by the use of a shrinkage reduction admixture (SRA). The effectiveness of a SRA depends on porosity and stiffness of concrete. In order to evaluate the performance of two SRA products, shrinkage tests have been done on concretes with and without SRA. Two different concrete mixtures were evaluated: a low strength concrete mixture (for housing purposes) and a medium strength concrete mixture (use in bridges). Results show the effectiveness of the SRA on the two reference concrete mixtures, with different W/C and different cement contents.

Expansion joints in bridge slabs are designed to absorb horizontal displacement due to the temperature fluctuation and moving vehicular load. The expansion joints, however, are very often damaged due to a repeated loading of the moving vehicles. The joints in the existing simple span structures can be eliminated by converting the structural types into continuous spans. When existing simple span bridges are converted into continuous spans, rigid (pin and roller) support conditions have to be changed to elastic supports in order to absorb and distribute the energy of horizontal motion due to an earthquake loading. However, development of additional reaction forces and stresses on the concrete slab due to the unequal displacement of the elastic supports have been overlooked. In this study, two-span continuous specimen, which is converted from two simple span structures is tested and analyzed to investigate the stress distributions of the concrete slab. Durability of the concrete slab under the service load is also discussed. The results of this study show that the change of strain in the longitudinal direction can be reduced by using the elastic supports and the vertical shear stress increases, directly affecting the fatigue life of the concrete slab.