Crack

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Abstract

There are several types of reinforced concrete which has been wisely used in many functions. It plays an important role in both infrastructure and superstructure and it also contributes to several buildings such as bridges, dams and marine structures. Nevertheless, it still has some problems in construction. This project considers those in reinforced concrete by examining both the causes and the effects of the most dangerous and common failure in concrete structures ' Crack caused by corrosion in reinforcement' as well as focusing on the repairs of this crack along with evaluating the solution of this failure.

It is concluded that other failure such as deflection in buildings can occur as a result of the crack. This serious problem must be taken into account to avoid such dangerous reaction.

Introduction

Reinforced concrete has been widely used since the need to build huge concrete structures. The first sign of using it took place in the end of nineteenth century. Gaventa (2001) points out that by the 1890; concrete was being extensively used for concrete structures, such as bridges, riverbanks and docks. It can be seen that military requirements and even marine buildings require the reinforced concrete rather than other materials. Engineers, therefore, take advantage of using it for these purposes.

Reinforced concrete is a mixture of cement, stone (aggregate), sand and water, this combination must be reinforced ' add a quantity of steel ' to give it an extra strength.

There are many external effects which result in stresses in concrete, those could be external loads, environmental effects such as temperature, and even the load of concrete itself, because concrete has a good durability compared with other materials, so it has a sufficient resistance to compressive stresses, whereas it dose not have an adequate resistance to tensile stresses' loads causing stretch in reinforced concrete ', concrete therefore must be reinforced to resist these strengths. The design of reinforced concrete is not always accurate. Therefore, engineers provide it an extra material to avoid serious troubles.

Nevertheless, there are still some failures in concrete structure which can either cost a lot to repair or cause the end of edge of building.

One of these failures is crack which might happen as a result of poor design. Moreover, there are other types of cracks. For example, share cracks which are caused by structural loads or the movement of foundations of building. These types are not due to the rust in reinforcement. However, Pullar-Strecker (1987) points out that they could result in rusting if they are left untreated. In addition to plastic shrinkage cracks, which occur during the construction on concrete surface such as floors or slabs, are not a result of rusting of reinforcement, but they may cause rust to reinforcement in which they provide contamination close to the reinforcement (Pullar-Strecker, 1987). Nonetheless, the most dangerous type of crack is one which is caused by rust in reinforcement ' corrosion '. This might lead to the end of edge of building, engineers take this condition seriously, and they pay more attention to prevent this type.

The aim of this project is to describe the corrosion of steel in concrete in order to examine both the causes and the effects, with a focus on the solutions of this failure and the assessment of each type of repair.

Section 1: The Causes and the Effects of the Crack

1.1- Definition of corrosion

Generally corrosion is a process which happens to metals as a consequence of external environmental factors. In other words, it is the change in the properties of the metal caused a layer on the surface of metal. According to ISO 8044 standard, corrosion is defined as physicochemical interaction between a metal and its surroundings which leads to changes in the properties of the metal and which might often result in impairment of the function of the metal (Reza, 2008). More importantly, Tatum (2009) explains that corrosion is a process which takes place in a material and deteriorate it, after exposure to environment .In this case, the chemical reactions will set up by an exposure of electrons in the metal to water and oxygen.

1.2- The causes of the crack

As explained above corrosion is the result of environmental factors. However, in reinforcement concrete while steel is surrounded by concrete, the rust may occur as a consequence of reactions between concrete and metal. According to Bohin (2006), corrosion is an electrochemical process for iron in concrete. In this reaction, the metal can be dissolved in phases to result rust and also this process is induced by carbonation of concrete which is a result of chemical reaction of the alkaline components of cement with the atmosphere; moreover, Pullar-Strecker (1987) and Bohni (2006) explain that rusting occur because of carbonation which reduce the alkalinity of the concrete. Steel in concrete is protected from rusting by the alkalinity of the concrete. This protection might be effective for a long period; however, it is possible for this protection to be permanently reduced by either the penetration into the concrete of acid gases, which are in the air, or by the penetration into the concrete of chlorides from seawater. The ability of both gas and salt to penetrate into concrete relies on the permeability of the concrete (Pullar-Strecker, 1987).

Because concrete is weak in tension, a little rust could crack it by growing around the reinforcement. In this case, the crack could run parallel along with reinforcement as a result of either poor quality of concrete cover, which covers the reinforcement, or the thickness of this cover ( see 1 below )that might be thin. In this case, the concrete carbonates quickly reach the steel, and the rust becomes apparent for period of time depending on the conditions inside the concrete. Another case is that reinforcement could rust within the thickness of the concrete. This case the rust spread from the bar to the next (Pullar-Strecker, 1987).

In terms of both quality and thickness of concrete, providing a good quality to steel reinforcement is one of high alkalinity, which protect steel from rusting, due to the presence of the hydroxides of sodium produced during the chemical reactions. However, the alkalinity are reduced by atmospheric carbon dioxide and chloride are able to reach the steel through the concrete after that corrosion of the reinforcement occurs which then results in damage to the cover of concrete ' increasing the volume of the crack '. In addition, the cover of concrete covering over reinforcement, its function to prevent the ingress of both carbonation and chloride through the concrete reaching the steel. However, in some cases, the thickness of the concrete can not completely prevent the entry of those substances. This might be related to the poor quality of the concrete or the presence of much permeability in the concrete itself (pullar-Strecker, 1987). Moreover, the bond between both the concrete and the steel is a major factor which measures the extent of the crack in reinforced concrete in addition to the physical properties of the concrete itself (Critchell, 1968).

With regard to the influence of the environment, it can be obviously clear that the local environment is one of the important factors influencing on material, the Interaction of atmosphere with other material such as iron in particular, results in changes in its properties due to the reaction between oxygen and chloride, which are in air or seawater, with the steel. In addition to the climate to which the materials are exposed to the change of the temperature during the day, this change leads to contraction and expansion of the reinforced concrete causing the cracks. Lambert (2002) paints out that the most important aspect of the local environment is the moisture level where the proportion of the water vapor is higher along with the carbonation and the chloride.

It has shown above the causes by the influence of the concrete itself on steel along with the effect of the quality of the reinforced concrete, and the impact of the stresses in presence of little rusting as well as the influence of the local environment, which is the one of the most important factors, on reinforced concrete by temperature and also moisture. All these causes have effects on concrete structural which should be taken into account before the construction.

1.3- The Effects of the crack

Crack in reinforced concrete is a failure in which the resistance of compressive stresses could be less than the effects of loads and reactions besides the increase in possibility of being more dangerous during the time. In other words, cracks might increase in volume as a consequence of both the corrosion of steel and the loads on reinforced concrete, in this case, other elements of building might be deteriorated. For example, Walls, which are used to separate the areas of the building, might deteriorate due to the deflection of the building; this deflection may be caused by the cracks. Critchell (1968) points out that a lot of consideration must be given to the deflection of reinforced concrete structures which is resulted under loads along with the development of cracks. Moreover, as the crack developing and increasing in volume during a time, the corrosion of steel could occur quickly especially in outsides of building which allowed the water to inter (Pullar-Strecker, 19 87).

Having examined that the crack caused by corrosion in reinforced concrete can develop by the time and results in serious problems effecting on buildings, therefore it must be treated since the crack begins to occur.

Section 2: The Repairs of the Crack

2.1- Introduction

One of the most dangerous signs that the reinforcement is rusting is the crack, it is also very important to identify the type of crack during the initial survey. The crack may not be easily seen. However, cracks less than 0.05mm wide could be identified very easily, therefore, more attention should be paid for this case whether the crack is less than about 0.3mm wide or is greater than 0.5mm wide (Pulla-Strecker, 1987). However, Bohni (2006) shows that crack must be at least 0.3mm if it is required to be sealed. There are various options of repairing cracks in reinforced concrete which is damaged by corrosion of steel depending on the condition of the structure.

2.2- Removal of the defective concrete

In terms of both the quality and the thickness of concrete, the use of poor quality to cover the reinforcement is likely to lead to corrosion of steel on the external faces of the members of concrete structures such as beams, slabs and columns. Furthermore, the thickness which covers the steel is another cause resulting the rust if it is not thick enough or the carbonation is saturated into it. In this case the concrete could become defective; the steel then could be exposed to aggressive environment. Moreover, Perkins (1976) and EI-Reedy (2008) demonstrate that all the defective concrete should be removed by either using the hammer with the chisel or using other machines such as 'high velocity water jet'. This leaves the concrete and the steel clean and damp. Moreover, another advantage of using this machine is there is no huge noise except that from the motor and also its performance is quicker compared with other tools. After removing the defective concrete, the rust also must be almost completely removed from the reinforcement, then when there is sufficient clean and space, the steel and surrounding concrete should be given a coat of Portland cement grout which is a mixture of an organic polymer and fine aggregate. The function of the grout is to create an alkaline environment around the reinforcement and also to provide bond between the old concrete and the new mortar.

Pullar-Strecker (1987) points out that the concrete cover must be removed when the steel is rusting enough to crack the thickness of concrete, but how deep for the concrete to be cut away? , this usually up to 50mm, this depth could be easy to remove when the concrete is hollow. However, it is often difficult in some types of reinforced concrete to cut that away therefore using high pressure water blasting is the quickest method of removing a large area of high strength concrete and also has an ability to remove the rust from the steel. There is an important point that the removal of the concrete should be within all the affected reinforcement. This depth is measured at least 20mm beyond reinforcement. Some studies showed that the depth of removal should be for all zone of carbonated concrete (see 2 below). After ensuring that all spalled concrete is cut away and there is an adequate depth around the reinforcement. This depth will be poured by the new mortar covering all the steel. This mortar can prevent the access of both chloride and carbonation to the reinforcement. Moreover, there are many ways to remove the part of the defective concrete which has cracks, however, it is important and necessary to remove concrete for a distance deeper than the depth required for removal of spalled concrete, and then the steel can be reached. The removal of concrete is around 25mm behind the reinforcement (see 3 below), and also making sure that there is no traces of chlorides after repair process.

One of the easiest ways of removal the defective concrete is to use the hammer with the chisel for this case. This is considered one of the cheapest methods, after removing that; the high strength mortar would be poured.

In some cases of repair, it might be found that the reinforcement must be cut away as a result of too much rust surrounding it and then the section can not be able to resist tensile stresses. Therefore, some steel must be added to ensure that there is an adequate resistance to carry these loads. In this case, it might seem difficult and inaccurate to provide more steel by using some tools. However, there is a new method to strengthen reinforced concrete structures. Using fiber reinforcement polymer has many advantages over the steel. This substance dose not corrode and therefore can be used in any environment which is exposed to corrosion. Moreover, it can also reduce the deflection of buildings (EI-Reedy, 2008).

There is another case that when the crack occurs at joints of structures which is built to divide large areas of buildings. In this case the joints can be widened and sealed at the external faces of structure ,however, if these joints are contaminated or the steel is already rusting , they must be fixed by cutting the affected concrete away and reconstruct the joints. The removal of the concrete at the joints is often a big problem; therefore, the water blasting is a good way again of doing it. This will give an accuracy to remove as far as 0.5mm on either side of the joint (Pullar-Strecker, 1987).

2.3- Filling the crack

In some conditions of deterioration, there is no need to remove from the concrete. This is in case of the width of the crack is very small, in other words, it dost not seem to widen more during the time. However, the mortar which would be used to fill the cracks should penetrate into the smallest cracks; otherwise this repair would be failed. Bohni (2006) explains that the Epoxy resin ' a mixture used to fill and fix cracks' is suitable for small cracks width down to 0.1mm. Other substances used to seal cracks which move, however, there must be sufficient width at least 0.3mm if the crack requires to be sealed. Before filling, the cracks needs to be clean and the edges of the cracks may need to be dried before filling it. Moreover, these substances should be flexible enough for any expected movement during the time (Holland, 1997).

Section 3: The Evaluation of the Solutions

3.1- Introduction

The most important goal of repairing cracks is to ensure that the crack is maintained in a good method; otherwise, the crack can be more dangerous on structure. In other words, the crack can continue causing failure of buildings.

As explained in the repair of the crack, there are different ways of maintenance depending on the condition of the failure.

3.2- Evaluation of the removal of the Defective Concrete

Starting with the removal of the defective concrete in which the spalled concrete is removed. This seems to be more effective in terms of using the mortar, moreover, as explained by Perkins (1976) and EI-Reedy (2008) that using machines such as ' high velocity water jet ' is convenient for bigger areas to cut the concrete away and also clean the steel from the rust. The failure sometimes occurs in some elements of structure where such big machines cannot be used, therefore, the hammer with the chisel are more suitable tools for this case, more importantly, those tools can also be used for places where the crack is difficult to be reached by machine. However, the performance of these tools is too slow and is inaccurate sufficiently for both the removal of the defective concrete and making enough depth for mortar. Moreover, it requires human power along with paying more attention. Perkins (1976) mentioned that when such expensive machines are used as a results, contractors tend to reduce the required quantity to a minimum and this can result in insufficient removal of the old spalled concrete, in addition to that resin mortar is very much stronger than the normal concrete and this strength of the mortar should be compatible with the strength the old concrete. If there is various between strength of the new mortar and the old defective concrete, the mortar will therefore tend to pull itself away from the base concrete. Moreover, Broomfield (1996) points out that it is very difficult to match the new concrete used for repair to the original work and it is almost impossible to obtain the mortar in the same way for the old concrete.

Generally it seems that resin mortar has a good substance for this purpose and is almost compatible with the concrete. However, it should have an adequate bond with the old work and also is impermeable enough.

3.3- Evaluation of Adding the Steel to the Concrete

With regard to adding steel to the concrete, in some cases there is need to add some steel to areas where the reinforcement has too much rust and is required to remove away ,however, providing new steel seems to be difficult, inadequate and very slow work , therefore, it is much better for fiber reinforcement polymer to be used instead. EI-Reedy (2008) points out that this alternative is a quick solution to strengthen the concrete and also to reduce the deflection which occurs when the concrete member is overload. On the other hand, this production seems to be very expensive compared with the steel.

Conclusion

To sum up, this project has firstly introduced some failures in concrete structure. Among these problems is the most dangerous type of crack caused by corrosion in reinforcement. However, it has also shown that other failures can result in corrosion of steel as long as the crack is not cured. It has also defined the corrosion generally along with examining both the significant causes of the crack and the effect of this failure. Secondly, it has focused on different ways of repair of the crack which are almost the same depending on the condition of the problem as well as evaluating these solutions in terms of the quality and the accuracy. Therefore, in my view, the corrosion in reinforced concrete is the principle reason for causing cracks, this can also be the cause of the deflection in buildings and the work during the construction should be on the high level of accuracy to avoid such corrosion in reinforcement.

References

Bohin, H. (2006) Corrosion in Reinforced Concrete Structures. Cornwall, England

Broomfield, D. (1996) The Repair of Reinforced Concrete [Online]. Available from > http://www.buildingconservation.com/article/concrete/concrete.htm< [10 May 2009]

Critchell, P. (1968) Joints and Cracks in Concrete. London, England EI-Reedy, M. (2008) Steel-Reinforced Concrete Structure: Assessment and Repair Of Corrosion

Gaventa, S. (2001) Concrete Design. China Holland, R. (1997) Reinforced Concrete. United States of America

Javaherdashti, R. (2008) Microbiologically Influenced Corrosion: An Engineering Insight. London, England

Perkins, P. (1997) Repair, Protection and Waterproofing of Concrete Structures. Essex, England

Perkins, P. (1976) Concrete Structures: Repair, Waterproofing and Protection. Essex, England

Pullar-Strecker, P. (1987) Corrosion Damaged Concrete: An assessment and Repair. Essex, England

Tatum, M. (2009) What is Corrosion? [Online]. Available from > http://www.wisegeek.com/what-is-corrosion.htm< [10 May 2009]

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