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Concrete is the most applied building material in modern times. It is present in all kinds of structures and no other material secms to rcplace concrete as the most selected building material in the near future. The prime advantages are the high performance, the easy production and a facile processability. The sustainability and durability are important requirements to concrete. Even if concrete is significantly less susceptible against factors promoting a damage, compared to other materials, there are still mechanisms affecting it. In particular, high-ways and hydraulic structures made of concrete worldwide show cases of alkali-silica reaction (ASR) damages. This article refers to experience at BAM Federal InstituteforMaterials Research and Testing for several decades to ASR research and darnage assessment. ASR is a severe darnage mechanism that can occur in concrete, when certain conditions related to composition and cxposition arise in a critical combination. The chemical reaction is based on the alkalis and the silica in the concrete and was first identified by Stanton in highway structures in California (Stanton, 1940). Actually three components are needed to initiate ASR: alkali-sensitive siliceous aggregates, alkalis coming from internal (cement or other components) or external sources (de-icing salt or sea water) and water. If all of these components are sufficiently present an alkali-silica gel could be formed (Hobbs, 1988). This gel itself needs additional space when it is formed. Furthermore it is able to swell in the presence of humidity, both processes generating an expansion pressure inside the concrete. As a consequence of intensive expansion, cracks can occur, which significantly lowers thc strength of the material. Parameters like amount and chemical composition of the gel as weil as the pattern and width of cracks can help to classify the ASR darnage (Swamy, 1992). The objective of this article is to demoostrate the microscopic work on ASR affected concrete samples, carried out to assess the damaging process or to evaluate various alkali-sensitive aggregates. It will explain the approach of ASR investigation from the macroscale to microscale, starting briefly with the condition assessment, over treatment and investigations of samples in the laboratory, to the use of different microscopical and analytical techniques. In the main part the characteristic features for identifying ASR in concrete are presented by using microscopic images. Especially the emphasis lies on the role of the aggregate.
Alkali silica reaction (ASR) is a major concrete durability problem resulting in significant maintenance and reconstruction costs for concrete infrastructures all over the world. To determine whether an aggregate is potentially reactive, accelerated concrete tests are used. Aim of this study is the chemical and microstructural characterization of ASR-products formed under the performance conditions of motorway pavements and during subsequent tests on the remaining ASR susceptibility. Samples were taken from concrete motorway pavements. Some of the samples already showed first indications for beginning damages whereas others showed deteriorations only after applying a performance testing procedure for the estimation of future risk of ASR damage. In a first step ASR-products were identified by polarized light microscopy in thin sections. The reaction products where then analyzed by Raman Spectroscopy and finally the chemical composition of the ASR-products was identified by SEM with EDX.
The results indicate that most reaction products show an increasing Ca/Si-ratio with progressive crack length. If an ASR-product develops inside an aggregate and moves through the cement paste, it has a high (Na+K)/Si-ratio at the beginning which decreases with increasing distance from its place of origin. The Raman spectra of the ASR-products are characterized by two broad bands indicating specific Q-species of different amorphous gel networks. Within the scope of the analysis distinctions in the microstructure and the chemical composition of the ASR-gels were detected relating to various conditions of accelerated testing.
The accelerated concrete prism test (ACPT-60 °C) facilitates testing of alkali-silica reactivity of aggregates within five months. Manual measurement is usually conducted to determine expansion of test specimen every 28 days. This conventional procedure gives only limited insight to expansion behaviour of ASR-affected prisms. Due to the large time intervals, expansion exceeding the critical value of 0.3 mm/m is likely to be noticed late. Continuous recording resolves this problem and delivers beneficial information on the shape of the expansion curve.
Displacement transducers integrated into a special testing equipment developed at BAM put continuous measurement into practice. Additionally, ultrasonic velocity and acoustic emissions are measured in-situ and provide deeper insights into hydration and crack formation processes during the test.
Aiming for an automated testing method close to the German guideline, the following technical and methodical challenges arise. Changing of the vertical orientation of the prisms with every manual measurement as standardised cannot be implemented. Furthermore, preliminary results already proved true that cooling and heating cycles, as necessarily associated with each manual measurement, can evoke additional expansion. Therefore, interrupted and continuously measured expansions reveal systematic differences, indicating the necessity to adjust the threshold expansion value for automated testing.
Results presented in the paper: Different types of reactive aggregate have been tested applying continuous and manual expansion measurement to the concrete specimens. Development of continuously measured ultrasonic velocity and acoustic emissions of the aggregates tested diverge significantly while the expansion curves vary in shape and maximum strain. Prisms produced from a slowly reacting aggregate do not exhibit noticeable crack formation. It shows that comprehensive measuring helps describing and distinguishing ASR-damage mechanisms.
Models of ASR-induced expansion, available in the open literature, so far are validated with discontinuous data hardly tracing the modelled curve. Continuous measurement improves validation and further model development.
In concrete elements, simultaneously subjected to cyclic loadings and external alkalis, the risk for damage caused by or under participation of an alkali-silica reaction (ASR) is particularly high. This is of particular concern for concrete pavements due to the increasing heavy vehicle traffic and the application of sodium chloride (NaCl) de-icer during winter. Since 2004, the climate simulation concrete prism test (CS-CPT) has been used successfully to evaluate job mixtures for pavements by considering the impact of alkali-containing de-icers. However, the role of mechanical predamage on ASR is largely unclear. In a joint research project, the CS-CPT has been used to investigate the influence of preexisting microcracks on ASR. It was evident that an ASR initiated earlier in the predamaged concrete prisms due to the more rapid ingress of NaCl solution through the microcracks.
The primary aim of this paper is to analyze the impact of mechanical pre-damage and alkali–silica reaction (ASR) on the fracture mechanical properties of pavement concrete. For this purpose, a four point bending test was applied to large format beams to produce a defined level of cyclic pre-damage. The fatigue-induced concrete degradation process was simultaneously recorded using a testing procedure specifically developed for the purpose. In addition, fatigue-induced cracks on extracted drilling cores were spatially visualized and quantified using micro X-ray 3D-computed tomography (3D-CT). The storage of the small-format test specimens, with and without cyclic pre-damage, in an ASR-conducive environment showed that pre-damage leads to an increase in ASR damage processes. Subsequent structural mechanical investigations on small format specimens with and without pre-damage show that fatigue loading and ASR significantly influence fracture mechanical parameters of the concrete.