Investigation of Alkali Threshold Limits and Blended Aggregate in ASR Risk-Assessed Concretes

Concrete structures are designed for a specific design life to tolerate deterioration caused from various aggressive environmental loads such as carbon dioxide, chloride and aggressive soil conditions. The approach to prevent deterioration in concrete due to alkali-silica reaction (ASR) is by the avoidance of any such dissolution reaction taking place in concrete. ASR can in part be prevented by limiting the alkali content and restricting the use of potentially reactive aggregates. In this paper, the alkali threshold of several aggregates originating from New Zealand were determined using a modified version of RILEM AAR-3.2 and AAR-7.1. The AAR-2 accelerated mortar bar test (AMBT at 80°C) and AAR-3.2 concrete prism test (CPT at 38°C) were replaced with Australian Standard AS 1141.60.1 and 60.2 test methods, respectively, to evaluate expansion. Additional accelerated CPT in accordance with AAR-4.1 (ACPT at 60°C) was also conducted to examine the adequacy of shortening the test period. Petrographic examination taken before and after expansion testing was also carried out to qualify the presence of reactive silica and ASR gel contributing to expansion. The findings of this study suggest the potential for specifying the alkali threshold in concrete based on the reactivity classification of aggregates allowing a relaxation of the CCANZ Technical Report TR 3 alkali limit of 2.5 kg/m3 that is currently in place in New Zealand. This approach allows greater flexibility in the use of potentially reactive aggregates as sustainable concreting making materials

Mitigating Alkali Silica reactions in the absence of SCMs: A review of empirical studies

© 2019 by the fib. All rights reserved. The mechanism and severity of alkali-silica reaction (ASR) is subjective to the conditions of the availability of moisture and sufficient alkali content, and the presence of reactive aggregates. Since the 1940s, key focus has been placed on the reduction of alkali content by way of addition of supplementary cementitious materials (SCMs). However, the cost of SCMs and the realization that the availability of these materials could become limited in the untold future has influenced some researchers to investigate the development of protocols for the use of aggregates minimizing the likelihood of potential severe ASR. This paper presents a summary and review of the various strategies that have been adopted in recent years for the mitigation of ASR without utilising the addition of SCMs

Interfacial Reaction During High Energy Ball Milling Dispersion of Carbon Nanotubes into Ti6Al4V

The unique thermal and mechanical properties of carbon nanotubes (CNTs) have made them choice reinforcements for metal matrix composites (MMCs). However, there still remains a critical challenge in achieving homogeneous dispersion of CNTs in metallic matrices. Although high energy ball milling (HEBM) has been reported as an effective method of dispersing CNTs into metal matrices, a careful selection of the milling parameters is important not to compromise the structural integrity of CNTs which may cause interfacial reactions with the matrix. In this study, multi-walled carbon nanotubes (MWCNTs) were purified by annealing in argon and vacuum atmospheres at 1000 and 1800 °C, respectively, for 5 h to remove possible metallic catalyst impurities. Subsequently, 1, 2 and 3 wt.% MWCNTs were dispersed by adapted HEBM into Ti6Al4V alloy metal matrix. Raman spectroscopy (RS), x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectrometry and transmission electron microscopy techniques were used to characterize the as-received and annealed MWCNTs, as well as the admixed MWCNT/Ti6Al4V nanocomposite powders. The experimental results showed that vacuum annealing successfully eliminated retained nickel (Ni) catalysts from MWCNTs, while the adapted HEBM method achieved a relative homogeneous dispersion of MWCNTs into the Ti6Al4V matrix and helped to control interfacial reactions between defective MWCNTs and the metal matrix

Protocols for investigating the reactivity of aggregates and alkali thresholds for ASR prevention

Alkali in concrete pore solution and reactive silica in aggregate are integral features required for alkalisilica reaction (ASR). When high amounts of alkali are present, expansive ASR gel forms that cause cracking of concrete. Thus, limits have been imposed, restricting allowable alkali contents for use in concrete. However, these limits are known to be generalised with a single limit specified for all aggregate types. This study investigates the reactivity potential and critical alkali threshold for individual aggregates and aggregate combinations, by increasing alkali content (0.60-1.25% Na2Oe) in concrete, varying exposure temperature (38-80°C) and extending test duration. A combination of RILEM recommended methods and modified versions of the standard accelerated mortar bar test (AMBT) and concrete prism test (CPT) expansion test methods have been used. The key findings of this study suggest that the potential exists for specifying a determined alkali threshold in concrete based on the reactivity classification of aggregates used, thus, allowing a relaxation of the current alkali limit for concrete. This approach permits greater flexibility in the potential safe use of reactive aggregates in concrete. Furthermore, this study shows that the determination of an aggregate’s reactivity and potential to ASR is not only highly dependent on its chemical composition and the alkali content in the concrete but also the test method, exposure temperature and test storage age used to assess changes in expansion

Mitigation of ASR using aggregate fines as an alternative for SCMs

The use of supplementary cementitious materials (SCMs) for mitigating alkali-silica reaction (ASR) is the most common and practical approach adopted by concrete producers since the early 1950s [1]. However, with the future supply of commonly available SCMs such as fly ash and ground granulated blast furnace slag set to decline, alternative materials for mitigating ASR needs to be considered. The objective of this experimental work is to investigate the potential of using ground reactive aggregate fines as SCM substitutes to mitigate ASR. The mechanism of mitigation has been investigated using characterization and expansion tests assessed under AMBT conditions. Mortar bars containing 0%, 10%, 25% and 40% ground reactive aggregate fines by mass of cement replacement were prepared for modified accelerated mortar bar testing. The results obtained indicated that a reduction in ASR expansion was achieved with increasing ground reactive aggregate fines content. Further characterization including XRF and ICP-OES analyses were carried out on ground reactive aggregate fines to understand the efficacy of these materials as potential additives for ASR mitigation