Description of Material
Discuss About The Science Construction Material Defect Report.
Defects are very common in construction industry. These defects have numerous effects on the safety and health of end users of the built structures and facilities (Abisuga, et al., 2016). The defects are caused by a wide range of factors including faulty design, poor quality of materials, incompetent workers, inappropriate construction method, defective workmanship, inadequate supervision, etc. (Ahzahar, et al., 2011); (Ali & Wen, 2011). The aim of this report is to examine a construction material defect. The defect selected is spalling concrete on a storey residential building along 327 Mowbray Road, Chatswood NSW. The building is twenty years old. The location of the building is shown in Figure 1 below.
Figure 1: Location of the building
Figures 2, 3 and 4 below shows spalling concrete on different parts of the building.
Figure 2: Spalling concrete above the window
Figure 3: Spalling concrete at the edge of slab
Figure 4: Palling concrete on the stairs
Reinforced concrete is a composite material comprising of concrete and steel reinforcement. These materials are used because of their mechanical properties, which contribute to the structural integrity and soundness of a building. Buildings are exposed to both tensional and compressional forces hence they need to be constructed using materials that can withstand forces in both tension and compression. Concrete has high compressive strength and low tensile strength. Concrete is a composite material made up of cement, aggregates, admixture and water. Cement is the material that binds together all other ingredients of concrete. For the concrete to attain its desired strength, it has to be made up of the correct proportions of good quality ingredients, mixed using the right method, poured and compacted correctly and cured adequately. On the other hand, steel reinforcement has high tensile strength and low compressive strength. When these materials are combined to form reinforced concrete, they complement each other thus making the structure have high tensile and compressive strength. These materials are also used because they have almost the same expansion and contraction rates, which reduces stress fractures. Durability of reinforced concrete is usually increased by use of an appropriate water repellent on the surface (Tkach, et al., 2015); (Zhang, et al., 2017). The water repellent inhibits water penetration through concrete surfaces thus preventing concrete spalling (Meier & Wittman, 2011); (Zhang, et al., 2017). This material is very essential especially for buildings in coastal regions (Dai, et al., 2010).
Description of the Defect
Spalling concrete (also known as concrete cancer) is a failure mode of reinforced concrete. This defect occur when water (moisture) penetrates through concrete cover and reaches internal steel reinforcement causing it to corrode, rust and expand significantly. As a result of this, stresses get created on the surrounding concrete causing it to break away (spall). Once the concrete starts getting displaced and flaking, more water penetrates into the affected area causing the steel reinforcement to rust further. This speeds up the process of concrete spalling and worsens the situation. Spalling concrete is a very major problem in Australia, more so in coastal areas such as Sydney (Howe, 2015). Buildings that were constructed without using an appropriate waterproof membrane or water repellent sealer are vulnerable to spalling concrete.
There are numerous causes of spalling concrete. Some of them include: inadequate concrete cover that allows water to penetrate easily and reach the steel reinforcement; use of poor quality steel reinforcement; use of poor quality concrete; using incompatible metals too close to each other; and defective workmanship; among others (Sydney Strata Specialists, 2016). Concrete has high content of alkali, which is meant to help protect the steel reinforcement against corrosion. But once water penetrates through the concrete cover, it neutralizes the alkalis thus making them ineffective in protecting the steel reinforcement. That is why it is important to ensure that adequate concrete cover provided, i.e. at least 25.4mm (one inch).
Figures 5, 6, 7 and 8 below shows different spalling concrete defects. The defects are very similar to the ones identified in the case study analyzed in this report. This shows that spalling concrete defects are caused by similar factors ad occurs in a similar manner. The defect is characterized by broken or spalled concrete and exposure of reinforcement, as shown by the photos.
Figure 6: Example of spalling concrete defect
Figure 8: Example of spalling concrete defect
Reinforced concrete is a very common and widely used material in construction because of its properties. The main properties of reinforced concrete are: high tensile, high compressive strength, high shear strength, resistant to fire, durable, high ductility, relatively high modulus of elasticity, low coefficient of thermal expansion, moderate thermal conductivity and creep resistance. When plain concrete is combined with steel reinforcement, a thin film develops on the reinforcement’s surface. This is as a result of the alkaline environment from the lime present in cement. This makes the steel reinforcement extra-resistant to corrosion since the lime layer cannot allow easy penetration of moisture (Merlo Construction, 2017). This material is used because it has properties that are desired for most buildings. The concrete provides compressive strength whereas the steel reinforcement provides tensile and flexural strength. This enables the material to resist compressional forces and tensional forces.
Literature Review
Preventive measures for spalling concrete are: providing adequate concrete cover (based on AS 3600-2009 standard recommendation); using good quality concrete materials (cement, coarse and fine aggregates, admixtures and water); use of good quality steel reinforcement; use of suitable construction method to ensure good quality workmanship (to ensure that the steel reinforcement is properly installed and the concrete is correctly mixed, poured, vibrated or compacted and cured); employing competent workers to do the concreting job; ensuring adequate supervision of concrete works; and applying a suitable water repellent on finished concrete surfaces. These measures will help in preventing voids in the concrete and penetration of moisture into the steel reinforcement.
But if this defect occurs, it can be remedied. First is to remove the cracked, broken or spalled concrete from the affected area, using small chipping guns, until reaching the reinforcement. This is so as to allow inspecting the level to which the steel reinforcement has corroded, to enable easy cleaning of the reinforcement and to ensure a good bond between the new patching material and the steel reinforcement. Care must be taken to avoid cutting the existing steel reinforcement. Second is to sandblast and clean the steel reinforcement using a hard brush and applying a suitable priming material (such as epoxy coating) on it. This will help to protect the steel reinforcement against further corrosion. Third is to replace the removed concrete with good quality concrete. Fourth, is to apply a cement mortar on the surface so as to achieve the desired finish. Last but not least is to apply a suitable water repellent on the finished surface (Pestine, 2014). The remediation work should be done by a qualified and certified contractor so as to ensure good workmanship. The reinstated area should also be cured for at least seven consecutive days.
Conclusions
Reinforced concrete is a common and widely used construction material. This material has desirable properties that makes structure strong and durable. However, reinforced concrete is exposed to numerous defects due to faulty design, use of poor quality materials, poor workmanship, incorrect construction method, environmental factors, etc. This report analyzed spalling concrete defect on a residential building in Chatswood, one of the Sydney suburbs. This defect can be prevented by considering the factors or conditions that cause them when designing or constructing a reinforced concrete building. In the event that the defect occurs, it can be remedied by following a procedure that has been discussed in this report. Therefore spalling concrete is a defect that can be prevented, but if it occurs it can still be remedied.
References
Abisuga, A., Famakin, I. & Oshodi, O., 2016. Educational building conditions and the health of users. Construction Economics and Building, 16(4), pp. 19-34.
Ahzahar, N., Karim, N., Hassan, S. & Eman, J., 2011. A Study of Contribution Factors to Building Failures and Defects in Construction Industry. Procedia Engineering, Volume 20, pp. 249-255.
Ali, A. & Wen, K., 2011. Building Defects: Possible Solution for Poor Construction Workmanship. Journal of Building Performance, 2(1), pp. 59-69.
Dai, J. et al., 2010. Water repellent surface impregnation for extension of service life of reinforced concrete structures in marine environments: The role of cracks. Cement and Concrete Composites, 32(2), pp. 101-109.
Howe, M., 2015. Peak Strata Body Warns of Concrete Cancer. [Online]
Available at: https://sourceable.net/peak-strata-body-warns-of-concrete-cancer/
[Accessed 23 May 2018].
Meier, S. & Wittman, ,. F., 2011. Recommendations for water repellent surface impregnation of concrete. Restoration of Buildings and Monuments, 17(6), pp. 347-358.
Merlo Construction, 2017. Properties of Reinforced Concrete. [Online]
Available at: https://www.merloconstructionmi.com/properties-reinforced-concrete/
[Accessed 23 May 2018].
Pestine, G., 2014. Concrete Spalling – Expert Article. [Online]
Available at: https://www.robsonforensic.com/articles/concrete-spalling-expert-article
[Accessed 23 May 2018].
Sydney Strata Specialists, 2016. Spalling in Strata Buildings. [Online]
Available at: https://sydneystrataspecialists.com.au/tips/spalling-in-strata-buildings/
[Accessed 23 May 2018].
Tkach, E., Semenov, V., Tkach, S. & Rozovskaya, T., 2015. High Effective Water-Repellent Concrete with Improved Physical and Technical Properties. Procedia Engineering, Volume 111, pp. 763-769.
Zhang, P., Liu, S. & Han, S., 2017. Visualization of rapid penetration of water into cracked cement mortar using neutron radiography. Materials Letters, Volume 195, pp. 1-4.
Zhang, P. et al., 2017. The Effect of Water Repellent Surface Impregnation on Durability of Cement-Based Materials. Advances in Materials Science and Engineering, Volume 2017, pp. 1-9.
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