ABSTRACTS

Use of High-Strength Concrete and High Strength Rebar in Low-Rise RC Shear Walls

This presentation will discuss the use of high-strength concrete in combination with high-strength reinforcing bars (rebar) in low-rise reinforced-concrete (RC) shear walls. Recently, there has been a concerted effort in the U.S. (and overseas) to allow high-strength materials, especially higher grade rebar, in RC structures. The benefits of reduced steel areas from high-strength rebar can be especially realized with high-strength concrete, to result in a higher-performing composite. For example, high-strength concrete provides better confinement around rebar anchorages, thus reducing development/splice lengths. Further, the increased tension strength of concrete can lead to reduced cracking and deflections under service and thermal loads, which is an important factor when using reduced rebar areas.
Importantly, a limiting factor for high-strength materials in seismic regions is the reduced deformation capacity of high-strength concrete and steel. Therefore, these materials can be particularly suitable for strength-controlled (rather than ductility-controlled) members, such as shear walls with low height-to-length aspect ratios. Results from an analytical parametric investigation of low-rise RC shear walls will be presented to demonstrate the limits and benefits of using high-strength materials in these structures. This investigation includes a cost-benefit analysis to translate the potential economic advantages of adopting these high-strength materials to industry. Experimental results from the initial testing of low-rise walls with high-strength materials will be presented as well. The ultimate goal is to develop validated design/analysis methodologies for low-rise shear walls with rebar strengths significantly greater than allowed in current ACI codes, combined with high-strength concrete, thereby leading to major savings in cost and construction time. This study is funded by the U.S. Department of Energy and is conducted in collaboration with Sandia National Laboratories and AECOM.

Use of Recycled Concrete Aggregates in Prestressed Concrete

This presentation will focus on the use of recycled concrete aggregates (RCA) in reinforced concrete structural applications. The mining and production of natural coarse aggregates (e.g., crushed rock, gravel) contribute significantly to the environmental impacts of concrete. This effect can be mitigated by replacing natural aggregates with RCA from demolished concrete structures. First, a previous project at Notre Dame involving 16 RCA sources and quantitative statistical models for the RCA concrete slump, compression strength, stiffness, creep, and shrinkage will be summarized. This will also include tests and numerical modeling to investigate the effect of RCA on the long-term sustained service-load behavior as well as ultimate load behavior (under flexure-critical, shear-critical, and splice-critical conditions) of non-prestressed reinforced concrete beams. The presentation will then move on to the current work at Notre Dame involving prestressed RCA concrete applications. This will include the effect of RCA on the rate of strength and stiffness gain of concrete, which is important for prestress transfer. Additionally, an empirical quantitative model for the effect of RCA on the bond strength of seven-wire prestressing steel strand will be discussed.
RCA from discarded precast concrete, returned ready-mix concrete, and construction recycling yards can be used as replacement for natural coarse aggregates. The use of RCA from rejected precast concrete members especially creates unique opportunities because the quality of the material is higher and more consistent than RCA from recycling yards. Further, precast plants can recycle their own discarded products, eliminating transportation costs for the aggregate. This will be discussed based on sustained service-load tests of prestressed concrete beams utilizing precast RCA. This study is funded by the U.S. National Science Foundation and is conducted in collaboration with the University of Texas at Tyler and New Mexico State University.