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WSP is one of the leading consultancy and management companies that provide services for the local and international community. It’s a company that employees engineers, scientist, and many other people with different degrees and fields of expertise to provide long lasting sustainable solutions for different types of sectors such as transportation and infrastructure. They plan projects for their clients depending on their needs and give them advice to ensure their clients make the correct and most sustainable choices which helps with the success of the project. For example, for the bridge sector they have experts that worked on all types of bridges and mastered all construction method using specialized materials, new technologies and tools (2). Not only that but they also provide maintenance and inspection to old bridges such as General Inspection and Principal Inspection to make sure they don’t fail and last longer than the life expectancy they had when built.
Common types of deterioration in bridges
Deterioration of material is when the material losses quality due to many reason such as the environment, due to the poor quality of the material when made, and due to overloading of the bridge. A very common type of deterioration is corrosion. Corrosion occurs due to electrochemical reaction in steel and without the presence of both water and oxygen corrosion won’t occur. In this report we will discuss these types of deterioration and ways to reduce them and increase the durability of the bridges to make them last longer than expected. Which include the following types of deterioration:
- Rebar corrosion
- Concrete cracking
- Surface defects in concrete
Figure 1: Rebar corrosion
Rebar corrosion as in Figure 1 is one the most common type of corrosion which occurs when the chloride ions penetrate concrete and travel to the steel bar that is embedded in them (3). Steel that is embedded is difficult to corrode as the concrete is very alkaline which provide a bad environment for corrosion to occur. However, corrosion can still occur when oxygen and carbon dioxide travel through the concrete and the carbon dioxide can lower the alkalinity of the concrete making it suitable for the steel to corrode. Once the steel starts to corrode due reaction with different due to reaction with different ions. The product of corrosion occupies a great volume than the steel that produced this corrosion which exerts and outward pressure making the concrete crack as the concrete is not good with tension. When the concrete cracks this allows further access of chloride ions which creates further corrosion until the bar collapses as it cannot withstand the tensile stress hence causing the structure to collapse (4). Corrosion can be controlled by controlling the factors that causes it such as humidity, temperature and chloride ions. However, there are mechanical methods which are physical barrier that delays or prevent the entry of oxygen or chloride to reach the steel beams that are placed inside the concrete. For example, the use of corrosion resistant material such as the fiber reinforced bar polymer rebar instead of the normal steel rebar used. It is expensive, but it will make the bridge last longer saving cost of future repair or maintenance. There are other corrosion control methods but they all vary from structure to structure depending on factors such as the environment it will be built in and the life expectancy of this method of control (5).
Figure 2 : shows the type of cracks in concrete and their causes.
Concrete is a very common material that used to build bridges due to many reasons one of them being that it is very strong in compression but it’s very weak in tensions which causes cracking. There are many reasons why cracking in concrete occurs figure 1 shows some types of cracking and their causes. There are mainly two types of cracking as figure 1 shows one after hardening and one before hardening.
Plastic shrinkage cracking
This type of cracking occurs before hardening this when the moisture on the concrete top surface evaporates while the bottom layer is still hydrated this causes a force to the lower part of the concrete causing cracks. To prevent evaporation from the surface we can use plastic sheets to cover it. If plastic shrinking occurs and the crack size is small, it can repair by self-heal through continual cement hydration. However, if the crack is big and cannot self-heal you need to place a proper coating to prevent further crack and to prevent other chemicals from entering and causing corrosion.
Corrosion of reinforcement
Figure 1 shows that some of the cracking is as a result of corrosion of reinforcement this occurs once the concrete is hardened. This was discussed previously where corrosion causes rust inside the concrete which cause pressure that is exerting outward in the concrete and since concrete is weak in tensile tension this pressure causing cracking. There are ways to avoid corrosion of reinforcement by using corrosion resistant steel such as stainless steel or by using epoxy coated reinforcement that can be used in all material of reinforcement these are all measures to be done before corrosion occurs so when the reinforcement inside the concrete is being made. There are also some measures that can be done after corrosion occurs and they are called electrochemical techniques such as Cathodic protection (6).
Surface defects in concrete
To achieve a high-quality concrete with specific properties great care must be taken during construction process to avoid common defects that can occur as a result poor construction method. Some of defects are very hard to avoid even with proper construction methods. A common defect is efflorescence.
Efflorescence is common defect that occurs with all types of concrete especially colored concrete. In most cases efflorescence is not harmful however if it keeps increasing it can damage the surface and its aesthetically unpleasing. They occur when soluble salts from the material moves to the surface and evaporates leaving salts on the surface. It is very common in the winter due to slow rate of evaporation which allows more salt to reach the surface. When Effloresce first occurs it can be removed easily by scrubbing it. If efflorescence combines with Carbon dioxide its more difficult to remove and would require using acidic wash and after that wash the slab with baking soda to neutralize it. (7)
Methods of detecting deterioration in bridges
- Ultrasonic pulse velocity
Ultrasonic Pulse Velocity is a non-destructive testing used to check the quality of the concrete and to detect any damage caused to the material such as cracks. The disadvantage of this method is small defects such as small crack is sometimes undetected.
- Ground penetrating radar
Ground penetrating radar is also a type of non-destructive method where it can be used show objects that are within the structure such as position of reinforcement bars in the structure and the thickness of concrete. It’s also a fast method of scanning large areas in short time. However, GPR cannot detects corrosion in the steel reinforcement but can show a possible sign of corrosion. (8)
- Visual inspection
This type of inspection is called general inspection which occurs every two years and this inspection of all visible elements occur without the need of road closure or the requirement of specialist equipment. This is done by recording significant defects and non-significant defects. If the defect is serious then Urgent Defect Report is used to report this defect, so it can be treated. (9)
To create a successful bridge project there should be a plan for the future to avoid possible deterioration such as corrosion by using methods that avoids these types of deterioration for example using corrosion resistant material as reinforcement this might be expensive but in the long term it cheaper. There should also be strategies and measures if certain unexpected defect occurs. Furthermore, bridges in good conditions should also be inspected to prevent in it from becoming worse as this will reduce future cost in maintenance and increase the life time of the bridge. (10)
(3) Corrosionpedia. (2018). What is Rebar Corrosion? – Definition from Corrosionpedia. [online] Available at: https://www.corrosionpedia.com/definition/1380/rebar-corrosion [Accessed 13 Dec. 2018].
(4) Stainlesssteelrebar.org. (2018). Why does rebar corrode? – Stainless Steel Rebar – ISSF. [online] Available at: http://www.stainlesssteelrebar.org/why-does-rebar-corrode/ [Accessed 13 Dec. 2018].
(5) Material and Methods for Corrosion Control of Reinforced and Prestressed Concrete structures in New Construction. (2000). [ebook] Virginia: J.L. Smith and Y.P. Virmani, pp.14-16. Available at: https://www.fhwa.dot.gov/publications/research/infrastructure/structures/00081.pdf [Accessed 13 Dec. 2018].
(6) CORROSION CONTROL PLAN FOR BRIDGES. (2012). [ebook] Houston: NACE international’s White paper, pp.25-26. Available at: https://www.nace.org/uploadedfiles/corrosion_central/corrosion_101/white_papers/corrosioncontrolplanforbridges.pdf [Accessed 11 Dec. 2018].
(7) Nasvik, J. (2005). WHAT CAUSES EFFLORESCENCE AND HOW DO YOU REMOVE IT ?. [online] Concreteconstruction.net. Available at: https://www.concreteconstruction.net/products/decorative-concrete-surfaces/what-causes-efflorescence-and-how-do-you-remove-it_o [Accessed 12 Dec. 2018].
(8) FPrimeC Solutions. (2017). 4 Methods of Condition Survey for Bridge Decks | FPrimeC Solutions. [online] Available at: https://www.fprimec.com/4-methods-of-condition-survey-for-bridge-decks/ [Accessed 13 Dec. 2018].
(9) C1 topic structural inspection & Assessment by Alex Stevinson slides 18-25
(10 ) CORROSION CONTROL PLAN FOR BRIDGES. (2012). [ebook] Houston: NACE international’s White paper, pp.5-6. Available at: https://www.nace.org/uploadedfiles/corrosion_central/corrosion_101/white_papers/corrosioncontrolplanforbridges.pdf [Accessed 11 Dec. 2018].
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