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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Green construction has been a very important aspect in the concrete production field in the last decade. One of the most problematic waste materials is scrap tires. The use of scrap tires in civil engineering is increasing. This article investigates the dynamic properties of concrete with replacement of fine aggregate with scrap tire. Two different rubberized concrete mixtures were designed. The first set; variable slump (VS) was designed to keep the mix proportions constant with rubber replacement as the only variable. The other set; constant slump (CS) was designed to keep the workability the same using superplasticizer. The compressive strength of the concrete was reduced by the use of rubber. The viscous damping ratio was investigated using free vibration tests with impact hammer on simply supported beams and drop weight tests. The replacement of up to 20% of sand with rubber resulted in an increase in damping with the increase being more in the CS beams as well. Beyond 20%, the effect on damping was insignificant. The average hysteresis damping was found to increase with the increase of rubber content. The fracture energy was found to increase with the increase of rubber content up to 20%. Microstructure investigation was also performed on the two mixes. It is concluded that the choice of the rubber content and the mixing process can have a significant effect on the dynamic properties of rubberized concrete. Recommendations for these two aspects were provided.

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.

Concrete is the world’s most used construction material, and although it offers many advantages over other building materials from an environmental perspective (e.g., durability, thermal properties), the negative environmental impact of traditional concrete is of growing concern as its use increases. This paper highlights significant findings from a recent study focused on identifying alternate materials to be used in concrete to mitigate its negative environmental impacts. This study specifically researched structural-grade concrete in which 100 percent of the portland cement was replaced with self-cementitous hydraulic fly ash and the aggregates were replaced with pulverized post-consumer glass from the container industry. In particular, this paper presents the results of mechanical (compressive and tensile strength, elastic modulus), and durability (ASR, absorption, abrasion, chloride permeability) tests performed on two such concretes made with fly ashes from power plants in Wyoming and Kansas. Overall, the fly ash/glass concretes tested in this research program showed promise for use in the construction industry. They exhibited 28-day unconfined compression strengths in excess of 4,000 psi (28 MPa); although their corresponding tensile strengths were somewhat lower than would be expected based on the behavior of conventional concretes. Relative to durability, the results of the ASR tests were mixed, depending on the manner in which the ASR testing was conducted. The absorption results and abrasion resistance of the two concretes were found to be similar to conventional concretes, and the permeability test results indicate a total charge passed of less than 1,000 coulombs, which correlates with “very low” likelihood of chloride ion penetration being an issue.

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%.