Alkali-Silica Reaction – ASR
This report was issued in order to provide an in depth understanding of how a surveyor can differentiate between Alkali-Silica Reaction and Sulphate attacks in concrete when inspecting a building. In order to satisfy the requirements for this report, the author will give a detailed description of both kind of attacks, different study areas, experiments, diagnosis and forms of repair. To be able to define and analyse this topic, the author of this report has used different sources of reference such as books, academic journals, World Wide Web and several British Standards. By the end of this report, the writer will be able to demonstrate that the questioned concrete attacks can be differentiated by any professional surveyor when inspecting the concrete in a building. 2. Concrete attacks
Chemical attacks usually occurs when using poor quality cement although good concrete has been known to be subjected to conditions that can lead to its deterioration. The environment “supplies” several physical and chemical forces which can contribute to concrete deterioration. BRE (2005) delivered a full list of chemical attacks that can arise both land contaminated by human and natural ground. There are several rarely occurred chemical attacks that are mainly caused by contaminated land; these are chemical species such as ammonium or chromium, but also organic such as phenols. The higher the quantity of these chemicals is, the higher the concrete attack. The most known forms of concrete attacks are: * Chloride penetration leading to corrosion of steel and spilling of the concrete cover; * Inadequate cover of reinforcing steel.
Less common causes of concrete deterioration caused by chemicals or chemical reaction are: * Cycles of freezing and thawing;
* Carbonation resulting in an increase of steel corrosion; * Sulphate attack;
* Shrinking aggregates;
* Alkali-aggregate reactions.
3. Alkali-Silica Reaction – ASR
It is believed that there are three types of alkali-aggregate reactions that will affect the condition of concrete: alkali-silicate reaction, alkali-carbonate reaction and alkali-silica reaction. It is believed that the alkali-silica reaction “may be found in the concrete because microcrystalline quartz or stained quartz is often present in aggregates contacting phyllosilicates” (Hobbs D.W., 1988). The Institution of Structural Engineers (1988) described Alkali-Silica Reaction as being a chemical process in which the alkalis, found mostly in cement, when combined with specific types of silica found in aggregates, particularly in moist condition, will produce an alkali-silica gel that eventually will absorb the moisture from concrete, causing cracking and disruptions of concrete. British Cement Association (1993) advised that in order to determine that the inspected concrete cracking is a result of Alkali-Silica Reaction, the surveyor should sample and test the area “before any definite interpretation can be attempted”. Because of poor workmanship, shrinkage, weathering or differential stresses, the concrete can produce characteristics that will often be confused with Alkali-Silica Reaction. BCA (1993) are aware that “it is not always easy to distinguish these features from those indicative of ASR”. Their recommendation is that if suspected the sample should be taken into laboratory and further investigated. Because of the damping characteristics, the surveyor should allow dry weather when inspecting a suspected Alkali-Silica Reaction area. The degree of wetting should be recorded by the surveyor as this might be due to rain, condensation, leaking pipes, water run-off or poor detailing of construction. A second inspection is recommended if damp patches at the junction of the cracks are observed. It is known that Alkali-Silica...
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