Miniature concrete prism test - A new test method for evaluating the ASR potential of aggregates, the effectiveness of ASR mitigation and the job mixture.

摘要

Existing test methods to characterize ASR potential of aggregates and mitigation ability of supplementary cementing materials such as the Accelerated Mortar Bar Test (AMBT) and the Concrete Prism Test (CPT) are widely accepted in the industry. Although, the AMBT is a rapid test, the results from this test can be unreliable, particularly with certain types of aggregates. The CPT is considered as a more reliable test; however, the duration of this test method renders it impractical for routine usage in the industry to screen deleterious materials or inefficient ASR mitigation measures. This research presents a new test method-Miniature Concrete Prism Test (MCPT) that overcomes the deficiencies of the AMBT and the CPT. In this test, 2 in. x 2 in. x 11.25 in. concrete prisms are used and both coarse and fine aggregate can be evaluated in their native states without the need of excessive crushing as seen in the AMBT. Concrete prisms prepared with aggregates in question and high-alkali cement with boosted alkali content to 1.25% Na2Oeq. by weight of cement are conditioned in a bath of 1N NaOH at 60°C for a period of 56 days or 84 days depending on the 56-day expansion behavior of the specimens. This test method can also be employed to assess the mitigation effectiveness of supplementary cementing materials. For the majority of aggregates, concrete prism expansion at 56 days determines the reactive nature of the aggregate. For a small minority of aggregates that tend to be slow/low reactivity, expansion measurements up to 84 days are needed to characterize their behavior. Different parametric investigations were carried out to determine the influence of specific test variables on the results and calibration of the test method using a wide range of reactive and non-reactive aggregates (33 in total, 19 CA and 14 FA). The aggregates studied in this project were selected based on their established field performance. Results from the MCPT of 12 reactive and non-reactive aggregates with known field performance were compared with the results from the AMBT and the CPT methods. MCPT 56-day data was best correlated with CPT 1-year data (R2= 0.9945). Once the protocol was established, the effectiveness of different ASR mitigation measures such as -Fly ash (9 different types at different dosages, 15%, 25%, 35%), Slag (40% dosage), Silica fume (10% dosage), Meta-kaolin (10% dosage) and LiNO3 (50%, 100% and 150% dosages) were investigated. MCPT 56-day results of mitigation measures were compared with the CPT 2-year and ASTM C 1567 14-day expansion results. MCPT 56-day data (normalized as % of limiting value of 0.020%) matched well with the CPT 2-year data (normalized as % of limiting value of 0.040%). The Factorial design (a statistical model) concept was employed to investigate the basic parameters' effects on the ASR related expansion of job concrete mix design (such as w/c, cement content, alkali content of cement, etc.). Finally, an approach to evaluate ASR potential of job concrete mixture was proposed and two airfield taxiway-job mixtures with known field history were tested using the MCPT in this regard.