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Concrete is a construction material that consists of two basic components, paste that includes water, cement, and chemical admixture and aggregates which consist of both fine and coarse particulates. The maximum size allowed for coarse aggregates is 25 mm. The total volume of concrete is made up of 60-75% aggregates and 25-40 % paste. In addition, water makes up between 28-32 % of the volume of the paste (Steven et al. 2002).
Interestingly, concrete began to be used in construction after the Portland cement was passed the required strength test. According to Orchard (1979), Portland cement made in five types: normal, which is used for general members of structure, moderate sulfate resistance that used in conditions which concrete is exposed soil with low sulfate, high early strength which gains high strength in minimum time, low heat of hydration for large mass of concrete ( i.e., more than 1 m cross section), and high sulfate resistance that used in which concrete is exposed to soil with high sulfate. In fact, the concrete is used now in structural members in construction for all types of buildings and bridges that are built with reinforced concrete.
The use of concrete has many advantages over the other types of building materials such as steel structure. These advantages include the following point: it is the safest material that resist fire, isolated sound, and it is offers high strength under compression loads. Nevertheless, it has also disadvantages such as the following: it has high weight compared to its strength, low strength for tensile loads and cracks occur on the surface.
Concrete is divided into two specific types based on the location of casting: cast on site and precast concrete (i.e., concrete that cast in factories under special conditions helps to control concrete cracks). On other hand, the concrete that cast on site has different characteristics compared to precast concrete. For example, one of the problems with casting concrete on site is the increased of cracks.
Indeed, concrete cracks are the most problem in construction and these cracks allow air and water to access the steel reinforcement which is the most important elements in structural design. In fact, this problem will lead to corrosion of steel. In sequence, if the steel is exposed to corrosion, the construction will collapse. In addition, these cracks make the aggregates and paste separate that leading to concrete weakness.
This essay shows some problems which cause the cracks that include volume changes in concrete ( plastic and chemical shrinkage and swelling), temperature fluctuation, soil settlement, metal corrosion, and external loads. Also, this essay investigates the most effective methods of cracks prevention including design factors (expansion joints, settlement joints, soil improvement, and improvement of structural design for reinforcement), cooling materials, and concrete curing, and repairing cracks by using epoxy injection, cement grouting, and routing and sealing.
2- Causes of Concrete Cracks
The cracks of concrete are the cracks which be on the surface of slabs, beams and foundations after the concrete has been casted (Figure1) and they are a result of one or more factors. The common factors are volume changes in concrete, temperature fluctuation, soil settlement, metal corrosion and external loads.
Fig.1 Concrete cracks (Chrysalis 2007)
2.1- Volume Changes in Concrete
Volume changes in concrete are the most frequent cause of concrete cracks. Volume change cracks appear on the concrete's surface. According to Steven et al.(2002) the concrete is changed in volume (both expanding and contracting) after concrete dries. The changes in concrete volume affect the quality of concrete and cracks appear on the surface because the concrete is weak in the tensile zone ( bottom in the middle of the span, top at the supports). The problems that can happen in volume changes are chemical shrinkage, plastic shrinkage and swelling.
2.1.1- Plastic Shrinkage
Plastic shrinkage is the decrease in concrete volume and occur when the original volume of concrete shrinkage before completely dries (Slag Cement Association 2005). Indeed, of all types of volume changes, plastic shrinkage is the main reason for concrete cracks and appear on the surface of concrete as a result of the evaporation of water from the surface of concrete mixture and the water's rate of evaporation is more than the water demand in the concrete mixture that makes the surface too dry and the cracks be between the aggregates and paste .However ,there are three types of surface drying. Two of them are related with tensile strength which is the maximum strength of concrete (Figure 2). The first type of the surface drying is not problem, it starts after the tensile strength developed and it will not cause the concrete cracks. On other hand, if the drying starts before the tensile strength develope, the cracks will be occurred. The third type is rapid drying. If the concrete has rapidly dry, the cracks will happen on its surface later (National Ready Mixed Concrete 1998). Moreover, plastic shrinkage usually occurs in hot weather areas that have high air temperature which increase the concrete's temperature and therefore decrease its humidity(Steven et al. 2002).
Fig.2 Tensile diagram (Webster's online dictionary 2006)
( concrete has high strength in point 3 and weak between 3 and 1)
2.1.2- Chemical Shrinkage
According to Steven et al. (2002), the chemical shrinkage is the reduction of both solid and liquid materials in concrete mixture during the action between water and cement as a result of heat of hydration. The chemical shrinkage continues to reduce the volume until finish the heat of hydration and the cracks occur in the internal mixture. They do not occur in the external mixture or on the surface.
The swelling of concrete paste is the swell of its surface as a result of drainage of the external water of concrete and the chemicals replaced the water. The volume of swelling is percentage of external water that has drained. On other hand, external water might comes from curing process of concrete which is a wet sackcloth that used in concrete after casting to keep the temperature constant (Steven et al. 2002).
2.2- Temperature Fluctuation
Temperature fluctuation also affects concrete. If the temperature of concrete mixture increase and decrease, the mixture will expand and shrink creating joints between the paste and the aggregates that cause the cracks (Slag Cement Association 2005). In addition, a mass concrete that has more than 1 m2 section area, temperature fluctuation happened in both external and internal temperatures after removing the forms. After removing the forms, the external temperature will rapidly decrease as a result of outside temperature and the internal temperature will be high temperature as a result of heat of hydration. The high difference between the internal and external temperatures will cause the cracks.
2.3- Soil Settlement
The soil settlement and consolidation are two basic problems in soil and they are both defined as the drop of soil as a result of the weight of the concrete footings above the soil that transit the loads from the structure to the soil. The difference between settlement and consolidation is the settlement has suddenly foundation drop and consolidation is the footing drop during time. However, both the settlement and consolidation increase the stresses on the part of the concrete structure that is designed to resist the loads. In addition, unequal settlement is defined as the drop of only some footings and these footings drop into different levels which lead the beams and slabs to drop in one side and other side does not drop. Moreover, the different elevation of one beam, slab or foundation causes cracks as shown in Figure 3.
Fig.3 Footings cracks by soil settlement
(Engineering Foundation Technologies 2008)
2.4- Metal Corrosion
According to National Ready Mixed Concrete (1995), metal corrosion is defined as the chemical reactions between the metal and environment that includes air and moisture contents and it causes to destroy the metals. The corrosion makes holes in the surface of the metal and decreases its strength and section area. Moreover, the corrosion of steel and other materials inside reinforced concrete leads to decrease the quality of concrete. Steel bars in concrete are used inside concrete to expose the tensile strength because the concrete's tensile strength is about 10 % of its compressive strength. When the steel corrodes, the rust takes higher volume than origin steel volume that causes the concrete to be under tensile stress and causes the steel lose its properties. The loss of properties makes the concrete cracks.
2.5- External Loads
The concrete structures are exposed to external loads that can affect the quality of concrete if the loads are higher than designed load. However, the external loads consist of dead loads(concrete's own weight and irremovable materials) and live loads (people, furnishings and other removable materials) that loads transit to stresses which cause bending, shear and torsion stresses. The bending stress affect the concrete that is exposed to tension and it is on the bottom of the middle span of slabs and beams. Moreover, the concrete's tensile strength is 10 % comparison with its compressive strength and when the loads are over the design, the cracks occur on the tension zone. Another type of stress is shearing that happens in the supports of slabs (beams) and the beams (columns) as a result of axial loads. Therefore, the reinforced concrete resists the axial loads that might be overdesign and the cracks occur as vertical on the member. The third type of stress is torsion which is a rotation load resistance and it occur on beams that support another beam. The beam that is supported causes the supporting beam rotate around its centre which affect its surface and the crack appear there.
3- Prevention of Concrete Cracks
The concrete cracks are the main problem of concrete and the cost of repairing cracks is high and it takes a long time. The prevention of cracks is important to save money and time. However, to prevent cracks, design factors must be improved, materials must be cooled and the concrete must be cured.
3.1 Design Factors
To prevent the cracks in concrete, the design factors must be improved. The factors include expansion joints, settlement joints, soil improvement and improvement in the structural design reinforcement.
3.1.1 Expansion Joints
Concrete like other materials expands and shrinks with temperature and moisture. The decrease and increase in its volume causes the cracks that usually happened in the structures that have long spans. Expansion joint is the joint between concrete that is filled by rubber. It is one factor to control the cracks in concrete and it is important to allow the concrete expand and shrink in the different temperature and keep the structure safe. However, if the designer ignore expansion joints and the concrete expands, it will swell because there are not joints and the cracks must occur because the swelling make tension on surface and concrete is weak for tension. To construct the joints, the maximum space is 2.5 times of thickness and the panels should be square and the expansion joint should be one quarter of the thickness of the slab(National Ready Mixed Concrete Associate 1998). In addition, according to National Ready Mixed Concrete Associate (1998):
"Control joints can be tooled during finishing or sawed with a carborundum blade at an early age. Sawed joints may not be practical if the concrete is made with hard aggregate such as quartz gravel or trap rock. Sawing is easier if coarse aggregates contain materials such as limestone or sandstone. If the joint edges ravel during sawing it must be delayed, but if sawing is delayed too long it may become difficult. With abrasive saw bladesââ‚¬â„¢ sawing is often done at an age of one day or even earlier".
3.1.2 Settlement Joints
A settlement joint is a joint between concrete that filled by rubber like the expansion joint, these joint is vertical on building and starts from the foundation to the end of roof. All of the structural members on a building that include beams, slabs, and walls should use settlement joint to protect the building from unequal soil settlement which cause a foundation drop that leads the cracks occur. Settlement joints are located in areas between different soil types to allow the foundation which is located in frail soil to drop and the cracks do not occur. In addition, if a beam drops on one side, the area that is exposed to cracks is settlement joint and the stresses that are on the beam will not exceed the design estimation. Moreover, the joints should be taken between areas of unequal weight which the area with high weight is more drop on the foundation than low weight (Kuznestov & Pechenov 1972). In addition, the joints are used if the foundation of a column and a wall's footing are adjoining and they have different sizes. Also, they should be used in a building that has different levels of footings (Kuznestov & Pechenov 1972).
3.1.3 Soil Improvement
Soil is important in structural engineering which supports the structure. If the soil collapses or settles, cracks can occur in concrete, also this can happen if the soil swells. To control settlement and swelling in soil, soil must be improved. To improve soil that has a tenancy to settlement, mechanical characteristics must be changed by adding another material mixture such as cement or another soil, preloading, or compact the soil. Soil improvement by adding another materials can resist high loads. Using a large diameter drill to cut through the earth and grouting the soil with other materials can lead to make strong soil. However, despite the effectiveness of this method in technical terms, it is not economical because its cost is high due to heavy materials and amounts of human labour needed. Other type is preloading. it requires loading the soil before the structure is built. Interestingly, preloading is used to allow the soil to settle before the structure will be built which protects the concrete from cracks because the settlement occurs before the concrete is cast. In addition, the soil can be compacted which is similar to the preloading idea and saves time by using equipment to compact the earth and the voids escape from soil. This crates soil resists high weight. On other hand, swelling soil can be prevented by using water for few days.
3.1.4 Reinforcement Prevision
When a beam is exposed to overdesign load, a deflection will happen, thereby causing cracks to occur at the bottom of the beam (Figure4). According to, MacGreger (1998) , the thickness of the beam should equal (L/24)where, L is the length of beam because the deflection is associated with the depth of beam according to the following equation:
âË†â€ /L= (C * L) / d
where, d is the depth of the beam. Based on the above equation, if the depth of the beam increase, the deflection decreases, thereby protecting it from cracks. In addition, cracks can be caused by shear, but they can be prevented by using shear reinforcement called stirrups and located around the cross section of the beam as shown in Figure 5.
Figure 4. Deflection
(CENCO physics 2003)
Figure 5. Shear reinforcement
(Euro Code2 1992)
3.2 Cooling Materials
Cooling concrete thought cooling materials is the most effective method for keeping concrete temperatures low, specially in hot weather that lead to protect the concrete from plastic shrinkage which causes the cracks. The materials that must be cooled are water, cement, and aggregates. Specifically, water and aggregate temperatures are the most important. For every 2 oC decrease in the water temperature, the mixture is lowed by 0.5 oC . In addition, aggregates make up about 70 % of the concrete mixture and every 0.8 oC temperature decrease in the aggregate temperature causes a decrease in the concrete temperature by 0.5 oC . in contrast, when the cement temperature is decreased by 5Co the temperature of the mixture is decreased by 0.5 oC (Steven et al. 2002). However, water must be protected in isolated tanks to keep it cold and the aggregates must be sprayed with water and the cement must be isolated from the sunshine. In addition, ice can be used to replace some of the water in the concrete mixture during the casting to cool the mixture.
3.3 Concrete Curing
In concrete curing, the concrete is treated to maintain the moisture content in the concrete as it hardened after casting. According to Orchard (1979), three methods of curing are available: wet curing which wet coverings are placed on the concrete as it hardened (i.e. wet sackcloth is used to cool the mixture), steam curing which steam is used cover the surface area of the concrete which replaces the water that evaporates from the mixture, and compound curing which a chemical component is spread on the concrete surface after casting. The last method is painted with curing compound that seals the concrete. in addition, concrete curing can control the temperature in concrete mixture.
4- Cracks Repair
Concrete cracks are common problems in concrete. They allow air and moisture to access the reinforcement steel bars which are the most important in structural elements. However, if cracks occur, the concrete must be repaired by using one of the several different methods and the most effective are epoxy injection, cement grouting, and routing and sealing.
4.1 Epoxy Injection
The epoxy injection method is effective for repairing cracks in all structures and it is used for the cracks that do not exceed 0.05 mm (ACI committee 224 1998) and the epoxy is injected into the cracks. To repair the cracks by using this type, the cracks must be cleaned from oil and dust by vacuuming and flushing. After that, surface sealing done to cover the cracks and the holes should be sealed to allow the injection to fix to adhere to the cracks. Then, a mixture of epoxy is injected through the holes. The final step in this method is sealing the epoxy and holes.
4.2 Cement Grouting
This method is used to repair wide cracks and it is similar to the epoxy injection method procedure by using different material. According to ACI committee 224 (1998) The first step to repair the cracks is clean the cracks and covering the cracks with sealant and fixing grouting nipples. Then, the grout which consists of water, cement, and chemical components is used to fill the voids in the concrete. The final step is to sealing the grout and paint it.
4.3 Routing and Sealing
Routing and sealing is the most common method used to repair concrete cracks because it can be used for both narrow and wide cracks and it can fixed for the lowest cost and require the shortest time(ACI committee 224 1998). The routing and sealing method starts with making a groove along the crack to expand it to 1/4 inch to 1 inch in depth. Then, the groove must be cleaned by air. After that, a joint sealant is spread in the groove (Figure 6) and the sealing can be put along the sealant joint. On other hand, this method does not good appearance but the accessories can be used to improve the appearance and make the repair more acceptable in accordance with architecture style.
Figure 6. Grouting and sealing method (ACI committee 224 1998)
5 Discussion and Conclusion
Concrete cracks are common problems in concrete. Indeed, cracking is the most common in structural work. Unfortunately, cracks can lead to disaster if they are wide or if they are in the area that has high moisture content. However, the causes of cracks must be identified to prevent them. Moreover, if cracks occur, they must be repaired.
There are many causes of cracks and the most common are the following: volume change in concrete due to plastic shrinkage, chemical shrinkage, and swelling as a result of the heat of hydration that cause water evaporation from the concrete mixture during the setting time (i.e. the time from casting to hardening of the concrete), temperature fluctuation which is a result of the change in temperature between day and night or the difference between the internal and external temperature in mass concrete, soil settlement, i.e. a drop of soil as a result of added weight and leading to a drop in a beam on one side, metal corrosion that leads to holes in the concrete and decrease its strength, and external loads that cause bending, shearing and torsion stress in overload cases.
Curing concrete which consists of wet coverings, steam and curing compounds is the best method to prevent cracks that happen as a result of volume changes in the concrete. In addition, to prevent the cracks from volume changes, expansion joints can be added to the structure to allow the concrete to expand without creating cracks. Moreover, cooling material is used to control cracks that are caused by temperature fluctuation and concrete curing can be used to reduce the temperature rise that occur during the setting time. The best way to prevent the cracks that occur due to soil settlement is soil improvement and the settlement expansion can be used if the cost of soil improvement is too high.
To control the metal corrosion of the steel reinforcement, the steel of concrete can be coated. The last type of causes is cracks due to external loads that can be controlled by using a reinforcement provision that protects beams, slabs, and other structural members from bending, shear, and torsion stresses that cause the cracks.
However, if cracks occur in the concrete, they must be repaired by using one of the various methods and the choice of method is dependent on the type of cracks. The most common methods are: epoxy injection, cement grout, and routing and sealing. Epoxy injection method is used if the cracks are smaller than 0.05 mm. In this method cracks are repaired by injecting epoxy to seal the crack after it has been cleaned. Another common type of repair for concrete cracks is cement grout and it can be used to repair wide cracks by grouting the cracks after they have been cleaned. The last common type of repair is routing and sealing which can be used for both narrow and wide cracks. In this method, the cracks is routed to produce a groove and sealing is placed in the groove. Routing and sealing is most economical method, but accessories must be used to improve the appearance of structure when this method is used.