International Concrete Abstracts Portal

Editor: Moncef L. Nehdi

With increasing world population and urbanization, the depletion of natural resources and generation of waste materials is becoming a considerable challenge. As the number of humans has exceeded 7 billion people, there are about 1.1 billion vehicles on the road, with 1.7 billion new tires produced and over 1 billion waste tires generated each year. In the USA, it was estimated in 2011 that 10% of scrap tires was being recycled into new products, and over 50% is being used for energy recovery, while the rest is being discarded into landfills or disposed. The proportion of tires disposed worldwide into landfills was estimated at 25% of the total number of waste tires. Likewise, in 2013, Americans generated about 254 million tons of trash. They only recycled and composted about 87 million tons (34.3%) of this material. On average, Americans recycled and composted 1.51 pounds of individual waste generation of around 4.4 pounds per person per day. In 2011, glass accounted for 5.1 percent of total discarded municipal solid waste in the USA. Moreover, energy production and other sectors are generating substantial amounts of sludge, plastics and other post-consumer and industrial by-products. In the pursuit of its sustainability goals, the construction industry has a potential of beneficiating many such byproducts in applications that could, in some cases, outperform the conventional materials using virgin ingredients. This Special Publication led by the American Concrete Institute’s Committee 555 on recycling is a contribution towards greening concrete through increased use of recycled materials, such as scrap tire rubber, post-consumer glass, reclaimed asphalt pavements, incinerated sludge ash, and recycled concrete aggregate. Advancing knowledge in this area should introduce the use of recycled materials in concrete for applications never considered before, while achieving desirable performance criteria economically, without compromising the long-term behavior of concrete civil infrastructure.

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In the past, many investigations have studied the effect of replacing coarse natural aggregate (CNA) with coarse recycled concrete aggregates (CRCA) on “material” properties of concrete, particularly compressive strength. This article reports on a research program in which (1) commonly used and practical methods were used for mixture design, proportioning, and production of CRCA and CNA concrete batches, (2) reinforced concrete beam specimens were produced from both types of concrete and tested in a bending configuration for measuring load-deflection response, moment capacity, and failure mode, and (3) a theoretical investigation was performed to predict the effect of concrete strength on the moment capacity of the beams. The test results showed, as predicted by the theoretical study, that the reduction in moment capacity caused by the strength loss due to the replacement of natural aggregate with CRCA, was negligible. It was also observed that the scatter of load carrying capacities of CRCA and CNA concretes were both very low and had coefficient of variation values of 0.048 and 0.064 respectively.

There are significant environmental benefits of recycling and reusing waste concrete as aggregate for structural concrete. The use of recycled aggregate concrete (RAC), however, is currently limited to non-structural applications such as road base and erosion control. Widespread application of RAC, such as seismic applications, therefore requires an improved knowledge of the behavior under multi-axial state of stresses and development of behavioral models to describe the behavior under compression is essential. This paper presents part of the results of an extensive experimental investigation on mechanical properties of unreinforced RAC where the behavior under quasi-static axial loading was investigated and a one-of-a-kind stress-strain model was developed. It was observed that, quite similar to normal concrete, the stress-strain can be defined by a hyperbolic ascending-descending curve that is primarily a function of compressive strength, a straight descending branch which slope is a linear function of compressive strength followed by a sustaining branch.

Consideration of using high volume of Recycled Concrete Aggregate (RCA) in conventional concrete applications is rare due to the physical properties of RCA and its corresponding drawbacks. In the rare instance where RCA is utilized, replacement levels typically do not exceed 15-20% in order to minimize on the drawbacks. To take another approach, this paper presents results from a study aimed at maximizing the RCA replacement levels, while making only minor adjustments in mix design, to achieve both equivalent strength and durability performance of RCA concrete to their virgin aggregate counter parts. In order to investigate higher replacement levels, 15 MPa concrete criteria were followed to produce a high-volume, low-risk concrete readily produced in the ready-mix industry. Concrete specimens were tested for compressive strength, drying shrinkage, and effects of released alkalis from RCA on triggering disruptive expansion, if used with sand that marginally meets the alkali-silica expansion limit. Through modifications in mix design, the drawbacks of RCA (reduced strength, increased drying shrinkage, and promoting ASR potential) were successfully mitigated at coarse RCA replacement levels up to 100%.

This paper studies the fresh, hardened, and durability characteristics of Self-Consolidating Concrete incorporating Recycled Concrete Aggregate (SCCRCA). Twenty concrete mixtures were divided into five different groups, with constant water to cementitious materials ratio of 0.38, based on the Recycled Concrete Aggregate (RCA) content: 0, 25, 50, 75, and 100% of natural coarse aggregate (NCA) replaced by RCA. All mixtures were designed to have slump greater than 500 mm (19.7 in). The Portland cement was substituted by different percentages of Fly Ash (FA) and Ground Granulated Blast Furnace Slag (S), while the control mixtures were prepared using 100% Portland cement. The fresh concrete properties of all mixtures were investigated, such as: flowability, deformability; filling capacity, and the ability to resist segregation. Moreover, the compressive strength at 3, 14, and 28 days, the tensile strength, and the free shrinkage up to 80 days were studied. Partial replacement of Supplementary Cementitious Materials (SCMs) for cement reduced the 28-days-compressive strength of SCCRCA mixtures compared to those of the control mixtures. Based on the outcomes of this research, replacing the natural coarse aggregate (NCA) in SCC by more than 75% RCA is not recommended. However, this percentage might change depending on a variety of factors.