High-strength concrete will perform well when recycled concrete aggregate takes the place of virgin aggregate, but an admixture is required, according to new peer-reviewed research from China and Alaska, presented at the 87th annual meeting of the Transportation Research Board in Washington, D.C., in January.

More than 10,000 researchers, engineers, technicians and stakeholders attended the meeting this year. Following are summaries of some of the papers of interest to the aggregates community. For more information about TRB, visit http://trb.org.

WITH HELP, WASTE CONCRETE IS FINE FOR HIGH-STRENGTH CONCRETE MIXES

So-called waste concrete, when crushed, has the potential to replace virgin aggregate in critical high- strength concrete (HSC) mixes, according to Juanyu Liu, Ph.D., University of Alaska-Fairbanks, and Bing Chen, Ph.D., Shanghai Jiaotong University, in their paper, “Property of High Strength Concrete Made with Field-Demolished Concrete Aggregates.” But it should incorporate silica fume, natural sand, and undergo a modified mixing method.

“Environmental and economic factors are increasingly encouraging higher-value utilization of demolition debris,” they write. “While the use of waste concrete aggregate (WCA) is mainly confined to low-grade application[s], it would be more beneficial if good-quality concrete can be produced with demolished concrete for new concrete construction.”

Urbanization, population growth and reconstruction has led to an abundance of construction waste, the authors write. “The debris from these demolished buildings is thrown away, causing environmental pollution, or is simply used as filling material,” they add. “In the meantime, quality aggregates are in short supply in many places where concrete technology is needed, making it necessary to import quality aggregates from distant locations, which can significantly increase the overall budget of a project. This together with environmental, economic and energy considerations are encouraging the recycling of demolished concrete structures and pavements as aggregates.”

These waste concrete aggregates [the most common industry term is recycled concrete aggregate, RCA] are crushed and ground by means of different methods so that they could be used as concrete aggregates, the researchers say. The use of crushed waste concrete as concrete aggregates began in Europe at the end of World War II, and it has been the subject of investigation for a long time, the authors write, adding the use of WCA is mainly confined to low-grade applications, such as unbound road base and fill. “It would be more beneficial if good-quality concrete can be produced by taking advantage of the debris of the buildings damaged in destructive earthquakes or demolished due to special needs, provided that the debris is used properly,” they say.

It's well accepted that WCAs make proper aggregates, they say, but add that the original demolished concretes are often composed of concretes with different design strengths from different structural parts, and contaminated with other building materials such as pigment, gypsum.

“WCAs are always attached with substantial amount of relatively soft cement mortar paste, making these aggregates more porous and less [resistant] to mechanical attacks,” Liu and Chen write. “Therefore, there are many unsolved problems encountered in controlling the quality of concrete made with WCAs including low compressive strength, wide variability of quality, high drying shrinkage, large creep and low-elastic modulus, which hampers the application of WCA for higher grade applications.”

In previous studies, WCAs were obtained from crushed concrete specimens made in the laboratory, to control the properties of the original concrete, but this may not well represent the behavior of those obtained from field-demolished concrete. Also, they say, most studies recommend only partial replacement of natural aggregates.

Therefore, the researchers undertook an experimental study to explore use of field-demolished concrete as full replacement of natural aggregates to produce high-strength concrete. The workability of fresh concrete and mechanical properties of hardened concrete made with WCAs also was investigated.

The cement used in this study was ordinary portland cement produced by the Shanghai cement factory, with a 28-day compressive strength of 53.5 MPa. To enhance the bonding, silica fume was used, provided by the Elken company, with particle size of about 0.01 — 0.1 um. A sulphonated naphthalene formaldehyde superplasticizer also was used. The waste concrete came from one demolished building in Shanghai, China. Before crushing treatment, concrete cores were retrieved from different locations of the building for strength measurement.

“The preliminary results indicated that HSC with up to 80 MPa of 28-day compressive strength made with field-demolished concrete as coarse aggregates, and natural sand as fine aggregates, can be obtained with the facilitation of a modified mixing method and addition of silica fume,” they write. “By providing a general basis for evaluating demolished concrete as aggregates for good-quality concrete, the investigation should contribute significantly to the understanding of recycled aggregate concrete as a viable option in the HSC construction industry.”