Environmental News

Preventing ASR in the management of concrete

In Australia we have numerous rock sources, in mineable quantity, covering a broad range of geologies across the nation.

There are some areas where volcanic rock types dominate, some metamorphosed regions and large sedimentary deposits in a number of locations.

Due to this inherent diversity in rock type across a vast geography, technical specification, testing and research in rock and aggregate properties are critical to understanding all rock sources and their potential use.

Knowledge about the suitability of aggregates in application helps guide us in ensuring the ongoing consistent performance of rock for the benefit of the broader community and our economy.

Cement Concrete Aggregates Australia (CCAA), along with others, has for some time researched the properties of aggregates for use in concrete and other applications.

MANUFACTURED SANDS {{image2-A:R-w:220}}As the shortage of construction sands becomes more acute, the use and specification of manufactured sands has become vitally important to cover the increasing volumes of concrete required in new infrastructure projects. 

A CCAA research report – National Test Methods and Specification Values January 2007 – set the scene on how manufactured sand was being used in concrete.

The CCAA guide T60 – Specification and Use of Manufactured Sand in Concrete November 2008 – provided critical detail on the properties of quality manufactured sand for use in concrete. 

Research in 2012 – Surface Properties of Concrete Containing Manufactured Sands – confirmed that concrete containing quality manufactured sands can be successfully used in skid-resistant pavements.

The ultimate test of the value of research is when it has a direct influence on contractual specification.

In the area of manufactured sand durability, the concept of the deleterious fines index (DFI) was developed during this research to assess the quality of an individual manufactured sand sample, by determining the amount of material finer than 75 micron (<75mm) and multiplying it by the methylene blue value (MBV).

This DFI (<75mm x MBV) property has since been adopted in Australian Standard AS 2758.1 Concrete Aggregates, along with a number of other specifications, including RMS B80 on concrete for bridges, where it is used to confirm suitability of manufactured sand for use in concrete. 

RMS R83 focused on the skid resistance of concrete pavements, and recent changes have moved away from the concept of the free silica content of sands to the Micro-Deval loss, which measures the performance of the sand itself in an abrasive test environment.

AS2758.1 has undergone several significant changes, with the introduction of new clauses to reflect advances in understanding of manufactured sand and in addressing the risks associated with alkali aggregate reaction (AAR) in concrete. This document is in the final stages of review and publication.

It is well documented globally that AAR, and in particular the subgroup alkali silica reaction (ASR), is a significant and ongoing issue for concrete infrastructure, and research undertaken by CCAA and the construction industry has continued since 2004.

This work focused on describing what ASR is, the damage it may cause to concrete and how specification and testing can minimise the risk to structures and the community as a whole.

A complete overhaul of the HB79 Guideline on Minimising Damage from AAR reflects the changing global position with respect to AAR mitigation and, in particular, the strategy Australia must adopt to minimise risk to concrete. While some Australian states rely on deemed to comply provisions of adding supplementary cementitious materials to control ASR risk, other areas focus heavily on laboratory testing and specification in an attempt to quantify and mitigate risk.

To better understand the efficacy of laboratory test methods in classifying AAR risk to concrete structures, CCAA supported the long-term research and development of two new Australian Standards: AS1141.60.1 Accelerated Mortar Bar Test and AS1141.60.2 Concrete Prism Test. These two methods are based on the American ASTM C1260 and ASTM C1293 methods respectively, with some modifications to accommodate Australian cements and improve within-laboratory repeatability and between-laboratory reproducibility.

These particular ASTM methods have been used extensively in a number of countries over an extended period and the introduction of the first Australian methods for determination of potential ASR in 2014 will herald a new era in measurement and quantification of risk.

Research is currently under way to test more than 120 commercially supplied aggregate sources, to understand the suitability of these methods to Australian aggregates and better understand the relevance of results derived.

A new and exciting concept of reaction kinetics has recently been developed by the CCAA research team that challenges previous concepts of the way ASR develops in concrete over time. If the rate of reaction can be managed, then damage to a concrete structure can be managed and this may place far more control in the hands of the asset owner and the community that uses it.

Upon completion of this aggregate testing stage of the research, a better understanding of potential future specification approaches may be developed to minimise risk to concrete containing potentially reactive aggregates.

The next logical step will be to investigate the link between laboratory testing and field structure performance. A few researchers in the Australian construction industry have only addressed this, and further work in this area is essential to fully understand ASR risk into the future.

The AS2758 series of Australian Standard specifications underpin the construction materials industry, and the correct interpretation of these documents is critical to ensure quality materials are made and supplied to critical infrastructure projects in Australia.

A series of new technical notes have been developed by CCAA that provide a background to each of the relevant tests and procedures called up in this important series of documents:

AS2758.1 Concrete Aggregates
AS2758.2 Sealing Aggregates (for Sprayed Bituminous Surfacing)
AS2758.4 Gabion and Mattress Aggregates
AS2758.5 Coarse Asphalt Aggregates
AS2758.6 Guide to Specification of Armourstone
AS2758.7 Railway Ballast

All CCAA publications noted here are available on its website, many at no charge.

With continued technical focus on all aspects of the quarry industry, the future for Australian aggregates looks bright. 

Michael van Koeverden is the director of Engineered Material Solutions. Together with Warren South of the CCAA, van Koeverden is presenting at CMIC14 in Brisbane on 4 September, 2014.

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