Contents 1. Introduction2 2. Concrete attacks2 3. Alkali-Silica Reaction – ASR3 4. Sulphate Attack5 5. Reference List8 1. Introduction 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. . 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 Reaction will form a mapping crack at the surface of the concrete. Fig 1 is and extreme example of macrocracking found at the Hoover Dam, USA. Fig1. Example of cracking due to ASR at the Hoover Dam, USA Image taken from Hobbs, D. W. (1988, pp. 16)
As it can be seen from the image, there are specific signs that this is an Alkali-Silica Reaction such as damp patches at the junction of the cracks and the edges of the cracks often appearing to be light in colour. Cracking like this will often be confused by surveyors as being caused by an expansion or contraction. As it was said before, one major feature of Alkali-Silica Reaction in concrete is cracking. In order to record data for further investigations, the surveyor should sketch or photograph the crack pattern. One other characteristic of Alkali-Silica Reaction is discoloration.
This occurs along the cracks and although similar to rust caused by reinforce bars within the concrete, the surveyor is advised that colour photographs are to be taken for an off-site second investigation. If occurred in reinforced concrete, the cracks caused by Alkali-Silica Reaction will tend to follow the lines of the reinforcing bars. Although often confused with the cracks produced by the corrosion of the reinforcements, in order to provide a definite confirmation of ASR, the surveyor should enforce a microscopic examination of a sample taken from the interior of the concrete.
It is often that the surveyors confuse the cracking pattern of the affected cement. Other characteristics of Alkali-Silica Reaction are discoloration, efflorescence, exudations and pop-outs. 4. Sulphate Attack “Sulphate attack is the term used to describe a series of chemical reactions between sulphate ions and the components of hardened concrete, principally the cement paste, caused by exposure of concrete to sulphate moisture” ( Skalny et al. 2002, p. 3) It is well known that sulphate attack mainly affects the brickwork and concrete by creating a disruption of the mortar. The sulphate attack can create expansion, bowing and/or cracking of affected material. The chemical and mineralogical compositions of Ordinary Portland cement (OPC) are the most vulnerable to sulphate environments (Bonshor 1996, Amin et al. 2007). OPC is one of the most common cement used in construction industry. Its main composition is ground limestone and clay.
When burned, these components form the basis of most concretes. According to Ramson (1993, p. 19) if bauxite is used instead of clay, a high-alumina cement is produced. The main characteristic of this cement is its rapid rate of strengths developed and also if not ‘covered’ the high resistance to sulphate attacks. This can be one of the first evidence for surveyors that the concrete is not affected by sulphate attack. The main idea of sulphate attacks is simple.
Bonshor and Bonshor (1996) describes that the sulphate salts migrating from neighbouring building materials, or sometimes even enclosed in the groundwater react with elements of the OPC to produce ettringite or thaumasite. The most common circumstance of sulphate attack is when the unprotected concrete contains sulphate based materials or is exposed to sulphate groundwater. There are three main requirements necessary for sulphate attack to occur: (i) soluble sulphate salts such as sodium, potassium, calcium and magnesium.
It is important to specify that attacks from different sulphates will have different result. Mortars or concretes attacked by sulphates such as calcium or sodium will have a soft mush; on the other side when attacks form magnesium sulphate occurs, this being considered the most aggressive, the main feature of this attack are the salts that sometimes crystallize out or near the surface of the attacked material; (ii) tricalcium aluminate consisted in ordinary or rapid hardening cement; (iii) a persistent wetness on the material.
To understand the main manifestations of sulphate attacks in building components, the author will describe the visual characteristics that a surveyor will look for, in order to distinguish and recognise when sulphate attack has occurred. * The mortar in the brickwork is considered by Addleson and Rice (1995) to be under sulphate attack from as early as two years after construction. One of the main visual appearances of the attack is the white colour of the cement. The mortar subjected to sulphate attack will become loose at the surface, sometimes presenting cracks along the bed joists.
It is important to mention that surveyors often confuse the horizontal cracking from rendered walls caused by corrosion of strip ties in cavity walls with the sulphate attack. Bonshor and Bonshor (1996) recommend that if not confident with the diagnosis from visual inspection, the surveyor should sample the affected mortar and further examine in a specialist laboratory. University of the West of England (UWE), Bristol (2006) advice that sulphate attacks occurs where saturation is greatest and usually around parapet walls and chimney.
This is due to the large exposer to rainfall. UWE believe that although in some cases repairs are possible, in most instances once started, the sulphate attack is impossible to stop therefore the only option is the re-building. * When the sulphate attack is detected in rendered brickwork there are several visual signs for a surveyor to distinguish the type of attack. Wide horizontal and vertical cracks will appear in the rendering. Outward curling of the rendering in the cracks might appear as a result of sulphate attack. Fig2. Example of Sulphate Attack on chimney brickwork
Image taken from University of the West of England, Bristol, (2006) The adhesion of the rendering on the brickwork may fail; this can result in rendering falling off either from one brick or even a large portion this depending on the seriousness of the attack on brickwork. If untreated, the brickwork may be exposed to efflorescence. * There are several occasions when the sulphate attack occurs on the underside of the ground slabs. If not isolated by a damp proof membrane, the salts in the ground will react with the Portland cement causing a map-pattern of cracking.
Bonshor and Bonshor (1996) recommend that BRE Digest 363 will provide guidance in the case of a sulphate attack on concrete. Generally sulphate attack in ground-bearing slabs will form cracks in a solid ground floor mainly if the recycled colliery shale has been used as capping layer for the ground underneath the slab. Because the sulphate attack in ground bearing slabs, the surveyor will have to investigate further whether the slab has a damp proof membrane and if possible what sort of material has been used as colliery shale fill.
WRAP Organisation (2011) recommends colliery shale should be tested for sulphates especially if it is to be used in proximity to concrete. As building professional, a surveyor will be able to differentiate between Alkali-Silica Reaction and Sulphate attacks in concrete. There are several visual differences between these two chemical attacks. One of the major confusion made by surveyors is when inspecting a cracking pattern in a building. It is highly recommended that if suspected, the surveyor should take samples for laboratory examination. There are numerous chemical reactions that are likely to produce disruptive cracking in buildings.
This is the reason why a professional surveyor should not rush and give diagnosis unless entirely sure about the cause. Word count: 1759 5. Reference List Addleson, L. and Rice, C. (1995) Performance of materials in buildings. Oxford: Butterworth-Heinemann. Alan Wood & Partners (2012) Sulphate attack . Available at: http://www. alanwood. co. uk/pdf/Sulphate-Attack. pdf (Accessed on 5th October 2012). Amin, M. M. , Jamaludin, S. B. , Pa, F. C. & Chuen, K. K. (2008) ‘Effects of magnesium sulphate attack on Ordinary Portland Cement (OPC) mortars’, Portugaliae Electrochimica Acta, (26), pp. 235-242. Bonshor, R. B. and Bonshor, L.
L. (1996) Cracking in buildings. London: Construction Research Communication. British Cement Association (1993) The diagnosis of alkali-silica reaction. Available at: http://homepage. tudelft. nl/n89v3/LinkedDocuments/1992-DiagnosisOfASR. pdf (Accessed on 5th October 2012). Cook, G. K. and Hinks, A. J. (1992) Appraising building defects: perspectives on stability and hygrothermal performance. Essex: Longman Scientific & Technical. El-hachem, R. , Roziere, E. , Grondin, F. & Loukili, A. (2012) ‘New procedure to investigate external sulphate attack on cementitious materials’, Cement & Concrete Composites, (34), pp. 57-364. Farny, J. A. & Kosmatka, S. H. (1997) Diagnosis and control of Alkali-aggregate reactions in concrete. Available at: http://www. nebrconcagg. com/assets/PromotionPages/Mix%20Design/ASR1. PDF (Accessed on 6th October 2012). Giaccio, G. , Zerbino, R. , Ponce, J. M. & Batic, O. R. (2008) ‘Mechanical behaviour of concretes damaged by alkali-silica reaction’, Cement and Concrete Research, (38), pp. 993-1004. Hobbs, D. W. (1988) Alkali-silica reaction in concrete. London: Thomas Telford. Mittermayr, F. , Bauer, C. , Klammer, D. , Bottcher, M. E. , Leis, A. Escher, P. & Deitzel, M. (2012) ‘Concrete under sulphate attack: an isotope study on sulphur sources’, Isotopes in Environmental and Health Studies, 48 (1), pp. 105-117. Ransom, W. H. , (1993) Building failures: diagnosis and avoidance. 2nd edn. London: E & FN Spon. Sachlova, S. , Prikryl, R. & Pertold, Z. (2010) ‘Alkali-silica reaction products: Comparison between samples from concrete structures and laboratory test specimens’, Materials Characterization, (61), pp. 1379-1393. Sarkan, S. , Mahadevan, S. , Meeussen, J. C. L. , van der Sloot, H. & Kosson, D. S. 2010) ‘Numerical simulation of cementitious materials degradation under external sulphate attack’, Cement & Concrete Composites, (32), pp. 241-252. Skalny, J. , Marchand, J. & Odler, I. (2002) Sulphate attack on concrete. London: Spon Press. The Concrete Society (1985) Alkali-silica reaction: new structures-specifying the answer existing structures-diagnosis and assessment. London: Concrete Society. The Institution of Structural Engineers (1988) Structural effects of alkali-silica reaction: interim technical guidance on appraisal of existing structures. London: the Institution of Structural Engineers.
University of the West of England, Bristol, (2006) Durability of clay bricks. Available at: https://environment7. uwe. ac. uk/resources/constructionsample/Conweb/walls/bricks/section6. htm (Accessed on 20th October 2012). WRAP, (2011) Burnt and unburnt colliery spoil, Available at: http://aggregain. wrap. org. uk/applications/wrap_pdf/aggregain/pdf_material. cfm? id=2910 (Accessed on 22th October 2012). Zerbino, R. , Giaccio, G. , Batic, O. R. & Isaia, G. C. (2012) ‘Alkali-silica reaction in mortars and concretes incorporating natural rice husk ash’, Construction and Building Materials, (36), pp. 796-806.
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