Background

The realistic assessment of alkali-silica reactivity (ASR) and identification of preventive measures remain major challenges to the concrete and aggregate industry. The recently developed AASHTO R 80 Practice (previously AASHTO PP 65) and the American Society for Testing and Materials ASTM C 1778 Guide have significantly improved the way the concrete industry assesses aggregates for potential ASR and selects appropriate mitigation means to allow the use of potentially reactive aggregates in new concrete construction. These publications were a result of a number of FHWA- and state departments of transportation (DOTs)-sponsored studies on evaluating the potential for, and the prevention of, ASR as well as a consideration of the current approach by the Canadian Standards Association. 

The above-mentioned studies used long-term outdoor exposure sites to benchmark accelerated laboratory tests to concrete exposed to actual environmental fluctuations. These studies investigated concrete mixtures primarily that followed mixture proportions specified in ASTM C 1293, including cement and alkali contents, which is often regarded as the most reliable test method for assessing aggregate reactivity. As a result, the current guidance documents (AASHTO R 80 and ASTM C 1778) are based on mixtures that have high cement [708 lbs./yd³ (420 kg/m³)] and alkali (0.95% or 1.25% Na₂O_eq) contents. However, the issue has been raised that they do not properly capture concrete mixtures with lower cement contents [for example, <708 lbs./yd³ (<420 kg/m³)] and/or lower alkali loadings. 

Also, recent results from long-term exposure sites indicate that the amount of supplementary cementitious materials (SCMs) needed to control ASR expansion in the concrete prism test (ASTM C 1293) or that required according to the AASHTO R 80 Practice or ASTM C 1778 Guide may not be adequate to control expansion in outdoor exposure blocks with high contents of high-alkali cements. Furthermore, testing SCM mixtures in exposure blocks with more moderate (and realistic) alkali levels is needed to ascertain whether or not this is merely an artifact of the severe alkali loadings used in previous exposure-block studies.

 The objectives of this research are to improve guidance in AASHTO R 80 and ASTM C 1778 through:

1. Constructing and evaluating field exposure blocks with varying concrete materials placed in diverse environmental conditions to supplement the existing information.

2.  Enabling better benchmarking of current performance and job-mixture tests that have been or are being developed currently.

To achieve the objectives of this research study, concrete blocks will be cast and exposed to a range of real highway environmental conditions, alkali loadings, and cement contents to provide the crucial long-term benchmarking for the developments of ASR test methods for job mixtures and to validate and/or calibrate the prescriptive measures in AASHTO R 80 and ASTM C 1778. The results from the exposure blocks cast in this research study are expected to improve current guidelines for assessing ASR in field concrete in real environmental conditions. 

A sufficient number of concrete mixtures should be considered to include the following variables:  

• A range of fine and coarse aggregates (from non-reactive to very highly reactive) with an emphasis on moderately reactive aggregates;
•  Various alkali loading levels, as achieved through a range of cementitious contents (300 – 445 kg/m³; 500-750 lbs./yd³) and use of cements of various alkali levels (0.4%-1.0% Na₂O eq.) in the range used in pavements and highway structures with an emphasis on lower alkali loadings;
•  A range of SCM types and contents (0 to 50% SCM); and
• Variation in environmental exposure; blocks from each mixture will be placed in at least three different climate locations to represent various temperature/humidity ranges across the United States as reflected in AASHTO R 80/ASTM C 1778.

In addition to concrete blocks, samples will also be cast from each mixture for laboratory testing using the current and most promising performance tests for assessing aggregate reactivity and mitigation of ASR.

The performance of the concrete exposure blocks will need to be monitored indefinitely beyond the proposed study period. This may include further data collection and evaluation and any refinement of AASHTO R 80/ASTM C 1778 in a future project.

Deliverables shall include, at a minimum:

Phase I

 An interim report that provides the following:

 1. A critical review of the literature discussing issues relating to laboratory tests for the ASR potential of concrete and the field performance. The review will also address the aggressive nature of tests, the alkali content of cements, the cement content and the adequacy of the SCMs in controlling the ASR. Gaps and disconnects in the current state of evaluation of field concrete mixtures for ASR potential will be identified to help enhance the reliability of current and emerging methods for determining the susceptibility of concrete to ASR and improve the AASHTO R 80/ASTM C 1778 guidance documents.

2. A detailed task-by-task work plan, including the test matrix, to meet the objectives of this research, based on an assessment and analysis of the current state of knowledge and practice and the findings of the critical literature review.

Phase II

Final deliverables include the following:

1. A final report that documents results, summarizes findings, and draws conclusions; 

2. An extensive database fully documenting information from field-exposure blocks and laboratory testing delivered in an easily usable format and made available on the project webpage;

3. Proposed revisions to AASHTO R 80 and ASTM C 1778 and its referenced material standards; 

4. Recommendations for improving current and emerging test methods; and

5. A draft TRNews article highlighting the products of this research and their implementation.

Status: The projrect has been completed. The final report has been printed and is available.