a study into perfecting of concrete construction and the avoidance of cracks

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Introduction

Concrete is the most versatile and robust construction material available, and it composed of mostly cement, and it is defined as a quasi-brittle material with a low capacity for deformation under tensile stress. Within the composition of concrete there is also water, aggregate (gravel, sand) and other chemical compounds like admixture. Concrete has many important advantages, which make it a universal building material. It is durable, which can last for years, even in extreme temperatures, and it is crush-resistant and fire-resistant and it provides insulation against sound and heat.

Concrete has been used in ancient times, starting with lime mortars from 12.000 to 6000 B.C. in Crete, Cyprus, Greece, and the Middle East. Modern concrete came into play in 1756, when British engineer John Smeaton added pebbles as a coarse aggregate, mixing powdered brick into the cement. English inventor Joseph Aspdin invented Portland cement in 1824, and this became the dominant cement used in concrete production.

At present concrete is used for many purposes, for instance it can be used in roads, parking garages, bridges, buildings, dams, pavements, and even artwork, and other structures that will last for decades. Apart from natural disasters that might affect the concrete, there are other problems that can affect the concrete, and the most common, form of concrete deterioration is cracking. Cracking occurs over time in virtually all concrete. Not all concrete cracking are the same; as they can takes many forms such as: Plastic shrinkage cracks, Cracks due to construction jointing, Cracks due to continuous external restraint, Cracks due to lack of isolation joints, Cracks from freezing and thawing, Craze cracks, Settlement cracks. Cracks can vary in depth, width, direction, pattern, location, and cause, and they can be either active or dormant (inactive). Active cracks widen, deepen, or migrate through the concrete, while dormant cracks remain unchanged. Cracking can be the result of the drying surface layer, tensile stresses develop in the hardening concrete, resulting in shallow cracks of varying depth.

During the past few decades methods have been designed to fix the cracks in concrete, but no methods have been designed to produce concrete free from crack . Considering the fact that this material is the most used material in the whole world and that it is used in all countries, under various climates, atmospheres and for different purposes we note that the construction material concrete reacts differently and also deteriorates differently.

Through this paper, we will try to review the various causes of crack formation in concrete structures, and the relevance of such cracks to concrete performance, and we will describe the types of cracking, and discus the various solutions methods that can be used to assess how we can minimize, and reduce concrete cracking, and we will end this essay with a brief discussion and conclusion.

Components of concrete

Cement which is the basic component of concrete is a man-made material, which consists of oxides of calcium, silicon and aluminium. Cement is manufactured by heating limestone which is a source of calcium with clay, then grinding this product with a source of sulphate. It will be the cement that will bond the other components together which makes it the most important ingredient in concrete.

Water is another important ingredient in creating concrete. Water will allow the cement to mix and react with the other aggregates. The less water in the mixture will give the concrete a stronger resistance and durability and more water will provide an easy flowing concrete but less strong. The purity of water when mixed to the other ingredients not be impure as the will not allow the compounds to mix properly thus resulting to an early weakening or failure of the structure.

Other aggregates such as gravel, crushed stones and sand are used in the concrete mixture. More recently recycled materials from demolished structures have been used as part of the aggregates. The quality and characteristics of aggregate play a significant role on the properties of concrete and the quality of being is about (70-75%) of the total volume of mass of the concrete.

Another important ingredient in the composition of concrete is the chemical admixtures. Admixtures are added during mixing to improve the functionality or more of the properties of the concrete mixture. These chemical admixtures are not essential for small structures but make a huge difference for huge structures. Accelerators such as CaCI2 is used to provoke the concrete so that it dries quicker thus accelerating the hydration process. Another admixture is C12H22O11 which is a retarder. This compound will slow down the hydration process. This is a desirable and required step when building huge structures as it will prevent the concrete to harden quickly while constructors are still pouring concrete. Another important admixture is corrosion inhibitors. The corrosion inhibitor will prevent the steels inside the concrete to rust and corrode. This is a crucial admixture as the corrosion of steel with the concrete proved out to be a major issue as it provokes cracks in the concrete. The special property of concrete is that it solidifies after placing; this takes place due to a process call hydration. During this chemical process water will react with the cement which will create a bond with the other compounds.

Causes of concrete cracking

A crack in concrete is a phenomenon occurs in different part of structure and in different forms based on time of occurrence, before solidification or after solidification. Cracks might happen in concrete structure due to several reasons such as settlement, structure movements, Mechanical loading, shrinkage, drying , Environment condition and inappropriate ingrident. Cracks in a structure will weaken the structure and consequently allowing other problems to occur. The concrete itself can crack for different reasons.

One reason for concrete cracking is settlement; not all the types of settlement, but the most important type of settlement which causes cracks in building is the differential settlement. In this situation one portion or many portions of building move upward or downward with unsimilar value of the rate of settlement and the quantity of it with other parts so it will cause vertical cracks. Whereas, when the building settle at the same rate and amount that will not cause a huge problem as the differential settlement will do. For an extremely example OF that the Empress Hotel in Victoria BC, was built on pilings ,has uniformly settled down over 65 years causing the building to sink by one floor so that now the second floor is being used as a hotel's "Lobby"

(http://article.pubs.nrc-cnrc.gc.ca/RPAS/rpv?hm=HInit&afpf=t71-051.pdf&journal=cgj&volume=8)

Another reason for cracking is construction movement; the earth planet is under movement occasionally due to earthquakes or storms. These could move building from one side to another or up and down direction, which it will generate vertical cracks in building structure; for example The Northridge Earthquake in Los Angeles, California in January 1994 caused extensive damage and the building has cracked in some parts.

(http://www.inspect-ny.com/structure/Earthquake_Damaged_Foundations.htm).

Another reason for a crack is when a Mechanical loading initiates strain can exceed strength capacity of concrete. Concrete with a low tensile capacity has particularly subjected to crack at the early-age (Kasai,1972).When the loads are applied recurrent or over long time period, creep and fatigue can affect the strength development which can guide to crack (Bazant and Celodin,1991).

Shrinkage might causes cracks during of the rapid evaporation of water from the surface of concrete. This rapid evaporation depends on many factors; the most important factors are temperature and speed of direct sun to make the evaporation rate higher than the rate of water floating on the surface of concrete. The shrinkage cracks are usually short and superficial and they appear in two opposite directions at the same time. In the case of pre-cast components of facilities that manufacture in the closed area and have curried very well do not problems with the risk of shrinkage cracks because it will be too small. An excess of water in the mixture will reduce the concrete strength and cause the concrete to shrink. This shrinking is caused by the excess of water being evaporated. The less water in the concrete will provide a stronger concrete and lesser risks or cracking. Another reason for concrete to crack is the quick drying of the concrete. During this process the water evaporates quicker than it should take which increases the possibility of cracks occurring in the structure. Concrete will also crack due to exposure to extreme climates. In freezing temperatures the concrete structure will deteriorate within 3 to 5 years. This occurs due to the accumulation of water around the structure and eventually the saturation of the aggregates. Cracks can also occur if inappropriate concrete is used for the job. Different concrete are used for different structures and different climates. For instance Pozzolanic cements are more suitable when mixing concrete to be used where there will be exposure to sea water. The pozzolanic cement has a 60% extra resistance to sea water than pure Portland cement.

Types of concrete cracking

There are two types of concrete cracking; the first one is the crack that happens before solidification (referred to as plastic cracks), and the other one occurs after the concrete has solidified. Each type of crack has its individual reasons for occurrence and its own ways for avoidance. Plastic cracks that take place before solidification are due to excessive evaporation of water , construction movements, and settlement. Plastic cracking can be mostly eliminated through close attention to the mixture design, curing and material placement. Cracks that arise after the concrete has solidified might be due to a wide range of reasons. These cracks may be due to moisture, mechanical loading, chemical reactions of incompatible materials (e.g., alkali-aggregate reactions) and environmental condition (e.g. freezing of water in paste or unsound aggregate).

Table 1 shows a brief summary of the type of concrete cracks ,form of crack,cause and appearance time (Transportation Research Board 2006).

Problems resulting of concrete cracking

During the chemical process of hydration the water added to the concrete will evaporate, leaving the other materials to bond together. This is an important part in the solidification of concrete. A problem may arise if the process of hydration does not take place effectively. If the amount of water added to the concrete composition was not enough or more than it should be, this will cause the concrete to crack, thus leaving the structure vulnerable, and when this happens this takes away the most important characteristics of this construction material. Once concrete is cracked it becomes vulnerable to the penetration of damaging fluids and is more prone to spalling, wear and abrasive damage. Cracks in concrete are a huge problem, and they may occur unexpectedly and in unanticipated locations, and they affect the buildings structure, and the safety of users, as a crack will only get bigger which will result in the slow but continued weakening of the structure.

Solutions to prevent or minimize concrete cracking Before occurring:

All concrete has a tendency to crack, and it is not possible to produce completely crack- free concrete. However, cracking can be reduced and controlled if the following basic concerting practices are observed:

Prevention of plastic shrinkage cracks:

Plastic shrinkage cracks are the type of cracks that occur before the concrete hardens, because the concrete is still in a pre-hardened or plastic state. The rapid loss of water while the concrete is still unhardened or settlement over embedded item will usually result of shrinkage of the surface. Plastic shrinkage cracking occurs most often in summer with conditions of low humidity, wind and or high temperature.

We can prevent this type of crack by using a process such:

Fibers: the use of fibers in the mix is one of the best ways to prevent plastic shrinkage cracks. The fibers allow the bleed water to come to the surface uniformly, relieving internal stresses that can cause plastic shrinkage cracking. Both microfibers and fibrillated fibers are effective at reducing plastic shrinkage cracks.

Subgrade: Another good way to minimize these cracks is by making sure the subgrade is damp, so moisture will not be sucked out of the concrete from below.

Surface Retarders: these cracks are very common when placing concrete in windy condition, when the surface dries out faster than the rest of the slab. The use of windbreaks or surface retarders can keep the surface from setting too quickly, reducing these tensions that cause the cracks.

Prevention of cracks in hardened concrete:

Cracks that occur after hardening can occur for a number of reasons including the sub grade, the concrete mix, jointing or the lack thereof, and loading.

We can prevent this type of crack by using a process such:

Sub grade Preparation: a good sub grade is indeed to support the slab. While concrete is very strong in compressive strength, it is not all that strong under flexural loads, and needs proper support from below. The sub grade need to prepared properly, and be sure it is granular material well compacted and moistened prior to placing concrete.

Concrete Mix: most concrete cracking problems are caused by shrinkage. Concrete tends to shrink from its original volume about ½ inches per 100 feet. The basic volume change mechanism is temperature changes and drying shrinkage. The water to cement ratio is the number one issue effecting concrete quality and shrinkage. Maintain a reasonable water to cement ratio. If you want to place the concrete at a higher slump, order a mid-range or high-rang water reducer. This will increase slump and workability without increasing the water to cement ratio

Joint and joint types: joints are simply pre-planned cracks. By installing joints, if and when concrete does crack. It is important to be active in deciding where joints should be placed, and what types of joint should be used.

Loading: cracks can be included by loading the slab before the concrete has had a chance to develop sufficient tensile strength.

After occurring:

Methods to cure concrete from cracks.

One way to create a nearly perfect concrete is by using the concept of a self- healing concrete. At Delft University of Technology in the Netherlands a group of students and scientists have been working on a self-healing concrete. By using both chemical and biological approaches to tackle this problem, concrete will be able to heal itself in case of cracks.

In a similar approach scientists have used bacteria to allow the concrete to self heal itself. The idea is to place the bacteria in the concrete mixture so then when the concrete cracks and this opens a ways for oxygen and water to enter the crack. This will eventually reactivate the bacteria which will heal the concrete by producing calcium carbonate (limestone). This process of curing concrete is still being perfected and experimented. The costs of research and production are making it difficult to be manufactured and produced for residential uses. This method of curing concrete will be efficient and reasonable in time. (http://www.bu.edu/sjmag/scimag2008/Story%20pages/Self- Healing%20materials.html)

Evalution

mean the possibility to do these solutions or to eliminate the cracks from concrete

Discussion and Conclusion

While cracking is commonly observed in concrete structures, it is important to understand that all cracks may have different causes and different effects on long-term performance due to the confounding effects of design, traffic loads, and climatic conditions relevant to the structure. Cracking need not be alarming and can be addressed appropriately so that the life of the structure is not compromised.

All concrete will crack eventually, but problems arise only when it cracks unexpectedly and in unanticipated locations. Cracking can be the result of the drying surface layer, tensile stresses develop in the hardening concrete, resulting in shallow cracks of varying depth. These cracks often are fairly wide at the surface. This type of cracking usually appears within two days of placement. While cracks may develop in concrete for a variety of causes, the underlying principle is the relatively low tensile strength of concrete. Visible cracking occurs when the tensile stresses exceed the tensile strength of the material. Visible cracking is frequently a concern since these cracks provide easy access for the infiltration of aggressive solutions into the concrete and reach the reinforcing steel or, other components of the structure leading to deterioration.

Since each concrete placement is unique condition that can cause cracking at each job. While a well- designed system of joint can help prevent the random development of structure cracks, we must also protect the concrete from plastic shrinkage cracks and drying shrinkage cracks. Proper concrete practice and awareness of causes and prevention can minimize the occurrence of all types of cracks.

In summery the major steps which can be taken to minimize concrete cracking are:

1- Design the members to handle all anticipated loads

2- Provide proper construction and isolation joints.

3- In slab on grad work, prepare a stable subgrade.

4- Place and finish according to recommended and established practices.

5- Protect and cure the concrete properly.

Concrete failure can be kept at bay by taking proper precautions during the construction phase. If the concrete is correctly placed, consolidated, finished and cured, it has a good start to a prolonged life and delayed deterioration.

References

Nilson, A., Darwin, D. & Dolan, C (2004), Design of Concrete Structures, 13th Edition, pp.5

Susca, S (2006) Concrete Solutions to Concrete Rehabilitation, Journal of architectural Technology, Hoffmann Architects, Vol. 23, Iss. 2, No. 2, pp. 1-2

Pedersen, N. et al (2006), Control of Cracking in Concrete State of the Art, Transportation Research Board, Basic Research and Emerging Technologies Related to Concrete Committee, pp.9

Historic Lighthouse Preservation: CONCRETE, available online: http://www.nps.gov/history/Maritime/handbook/concrete.pdf

Rostam, S (2005) SERVICE LIFE DESIGN OF CONCRETE STRUCTURES - A CHALLENGE TO DESIGNERS AS WELL AS TO OWNERS, ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING), Vol. 6, No. 5, pp. 423-445

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