As suitable sand for concrete becomes harder to find, a research programme to review the alkali limits in concrete may also lead to better tests for industry.
AGGREGATE IN concrete is not chemically inert. It reacts to provide better bonding between the cement matrix and aggregate, which enhances the strength and stiffness of concrete.
However, some reactions between cement and certain reactive aggregates can cause expansion and cracking of concrete structures, which can compromise the engineering properties of concrete and lead to a loss of strength and stiffness. These reactions between alkalis, mainly from the cement, and aggregate are generally referred to as alkali-silica reactions (ASR).
Three conditions must be present to cause ASR-induced cracking of concrete structures – reactive aggregates, alkalis and sufficient moisture. Figure 1 illustrates this process.
Guidance on risk and reactivity limits
New Zealand proactively controls ASR in concrete by limiting the alkali levels in Portland cement and identifying sources of potentially reactive aggregate. In 1991, Concrete NZ (then CCANZ) published TR 3 Alkali silica reaction – Minimising the risk of damage to concrete – Guidance notes and recommended practice. This was the first New Zealand guide to recommend limiting concrete alkali content, rather than cement alkali content, when using reactive aggregate.
Since local volcanic aggregates were found to be highly reactive, the guidance adopted a conservative concrete alkali limit of 2.5 kg/m³, lower than most international standards for similar reactive aggregates.
Pressure from shortages led to request to increase concrete alkali limit
However, shortage of suitable sand for concrete in the Auckland-Waikato region put concrete producers under pressure to use highly reactive sand from the Waikato River.
In 2016, producers also reported increasing demand for high-early-strength structural concrete. This placed additional strain on the supply of suitable sand, making it increasingly difficult to manufacture normal concrete below the recommended alkali limit.
As a consequence, producers asked CCANZ to increase the concrete alkali limit to 2.7 kg/m³ to facilitate the use of Waikato River sand in normal concrete. The increase would raise the maximum cement content from 415 kg/m³ to 465 kg/m³ (Figure 2). It’s a change that would cover almost all cement contents used in construction and precast concrete.
Currently testing concrete’s limits
In 2016, backed by funding from the Ministry of Business, Innovation and Employment, BRANZ and industry, CCANZ convened a working party to address these issues and update the guidance.
The working group began a test programme to find out how concrete alkali contents higher than 2.5 kg/m³ affect the risk of alkali-aggregate reaction damage.
The programme follows internationally accepted testing protocols and focuses on a blend of Waikato River sand and Hunua greywacke mixed with type GP cement. Aggregate of known reactivity from the Auckland market as well as samples of reactive and non-reactive aggregates from around New Zealand and Australia provided a comparison.
Our current understanding of the causes and extent of ASR in New Zealand is mostly based on laboratory investigation. This involves a combination of petrographic examinations and quick chemical and accelerated mortar bar expansion tests together with studies of in-service structures with ASR damage.
Internationally, knowledge and experience with ASR in concrete has undergone significant development in recent years, and much of that understanding was adopted for the research programme.
A typical test protocol starts with quick screening methods and progresses to more long-term techniques (see Figure 3).
Some initial findings
The working party will undertake a full review of findings when the research concludes in late 2018, but results to date suggest the following:
- Petrographic analysis remains the easiest method to assess the potential reactivity of aggregate.
- Chemical analysis methods, such as ASTM C289, are reliable for most New Zealand aggregates but must be carried out by experienced laboratories.
- Rapid mortar bar tests may overestimate the reactivity of some aggregates but are useful for screening new aggregate sources.
- Long-term concrete prism tests can reliably assess ASR but must consider trends rather than compare results with published criteria.
- Research can produce consistent trends for material combinations, despite comparing laboratory techniques and varying characterisation of Portland cement.
The review is also likely to consider recommendations for further action, including updating the TR 3 guidance. Any increase in concrete alkali limits will be based on credible scientific evidence.
The recommendations are also likely to include methods to enable quarries and concrete suppliers to use the research programme’s test framework to assess potentially reactive aggregates.
Articles are correct at the time of publication but may have since become outdated.