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Essay: Concrete Cracks

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ABSTRACT

Cracks have a significant effect on the safety of concrete structures but estimating the tribute of crack in a specific measurement is very difficult due to multiple factors applied in a single concrete member. The main emphasis of this seminar is mainly established on the observation results of some completed and under construction randomly visited projects in Addis Ababa. In the first four parts of this paper the review of cause and effect of concrete cracks in the wellbeing of different construction projects has been discussed, in the fifth part existing concrete cracks which leads to failure and structures which are exposed to the major causes of cracking has assessed, lastly the papers concludes that, preventing the causes and effects of concrete cracks through good construction practice is mandatory.

1. INTRODUCTION

Cracks are the failures of construction materials in any part of the structure before and after imposed to service. It is unexpected failure of structures due to unconsidered factors. Cracks can exist in any type of structures. Some cracks are visible and can be controlled before following extra damage in the structure. The others are not precautions until partial failure of the structure. However cracks in concrete are very popular, cracks in existing brick and concrete block walls are also public.

Concrete is a versatile material which is widely used in construction projects all over the world. It is due to its nature to have the property of the ingredients, so that we can use concrete in any situation by changing the property and proportion of the ingredients. Cement concrete has high compressive strength but low capacity in resisting tensile loads. Tensile loads, harmful reactions and environmental loadings in concrete can lead to tensile stress. These will result cracking when the tensile stress becomes greater than the tensile strength of concrete which can reduce performance of concrete. On the other hand asphalt concrete is mainly used in a pavement and water proof part of the structures. It can be a result of different causes including poor quality of asphalt mix, unconsidered load and environmental factors. Even though the failures caused by cracks can be reduced by proper design and construction practice, it can lead to collapse of the structures if not properly repaired (Abebe 2017 and TRB 2006).

Concrete cracks are complete or partial separation of concrete in the surface or deeper in internal part of the concrete produced by fracturing. Usually it is unavoidable complicated phenomena and has a significant effect on the chloride diffusion and deterioration of concrete. All concretes will face cracking to some extent but high amount of cracking leads to failure. Cracks are early sign of distress which is to be maintained properly; otherwise it leads to a repetitive failure and unnecessary costs. The materials and techniques to repair cracks are quite different based on the type and magnitude of cracks. So that examining the causes, effects and ways of controlling cracks are necessary to overcome the dreads and malaises caused by unfortunate failure of structures (Abebe 2017 and Pooja et al 2015). This paper mainly concerns on the cause and effect of cement concrete cracks.

2. DISTINGUISHING CONCRETE CRACKS

Concrete cracks may or may not have a measureable effect on the serviceability of the structure, however there is no a definite baseline to consider cracks as a problem. Even though, many intellectuals try to judge either the crack is creepy or not, by measuring the width of the crack, there is no specified width for separation. The criteria of judgment can vary from project to project based on the individual’s perspective. The other way of measuring the danger of cracks is measuring the width of the cracks. Even an internationally accepted institute like American Concrete Institute (ACI) has not any numerical standard or recommendation on the width of cracks which requires a repair. ACI recommends, estimating maximum crack width for tensile members, by multiplying maximum crack spacing (four times concrete cover) by average strain in the reinforcement. Many authors recommend that cracks in the exterior concrete with a width of more than 0.3mm leads to corrosion of reinforcement and a crack with width of more than 0.45mm needs corrosion resistance reinforcement, Even though cracks with smaller width has its own effect (ACI 1997 and Pooja et al 2015).

As many scholars agree that cracks running through the depth of concrete can be the sign of structural difficulty. So that these cracks should be repaired with no need of measuring the width. In other hand hairline cracks are difficult to clearly justified that they may be a cause for a problem and requires repair. As Pooja et al (2015) states, to know the status of hairline cracks considering the following questions is necessary.

• Is it active? (if dormant it does not need repair)

• Is it in a wide enough horizontal surface?

• Does it allow moisture seepage?

• Is it located in highly observable area?

• Does it hold dirt and leads to maintenance and sanitation issue?

3. TYPE AND CAUSE OF CONCRETE CRACKS

Cracks in concrete are a natural phenomenon that can be exist in different level of impact, the types and cause of cracks are various depend on the environments and function of concrete structure. Some common types of cracks their nature and their cause are listed below.

3.1. Structural cracks

Structural crack results from incorrect design, faulty construction or overloading and may endanger the safety of a building. The cracks in beam, column, slab and footing are considered as structural cracks. As James G. and James K. (2012) states that tensile stresses induced by loads, moments, shears, and torsion cause distinctive crack patterns, as shown in Figs below.

3.1.1. Tension cracks

Members loaded in direct tension crack right through the entire cross section, with a crack spacing ranging from 0.75 to 2 times the minimum thickness of the member. In the case of a very thick tension member with reinforcement in each face, small surface cracks develop in the layer containing the reinforcement.

Figure 1, tension crack (James G. and James K. 2012)

3.1.2. Flexural Cracks

Members subjected to bending moments develop flexural cracks, as shown in Fig.

2

Figure 2, flexural cracks (James G. and James K. 2012)

3.1.3. Shear cracks

Members subjected to shear develop shear cracks. Have a characteristic inclined shape, as shown in fig below. Such cracks extend upward as high as the neutral axis and sometimes into the compression zone.

Figure 3, shear cracks (James G. and James K. 2012)

3.1.4. Torsion cracks

In a normal beam where shear and moment acts, they tend to be pronounced on the face where the flexural shear stresses and the shear stresses due to torsion added, and less pronounced (or even absent) on the opposite face, where the stresses counteract.

Figure 4, Torsion cracks (James G. and James K. 2012)

3.1.5. Bond cracks

Bond stresses lead to splitting along the reinforcement, concentrated loads will sometimes cause splitting crack.

Figure 5, Bond cracks (James G. and James K. 2012)

3.2. Non-structural crack

Non-structural cracks are not making that much damage when compared to structural cracks, but it should have significance effect on the aesthetic va
lue of the structure. This crack will happen depending on the material properties due to moisture variation, temperature variation, crazing, effect of gases and liquids. As Rajat et al (2015) and Abebe (2017) says Non-Structural Cracks can occurred during the concrete is either plastic or hardening state. The following are commonly known cracks in concrete;

 Pre-hardening concrete crack

• Plastic Settlement

• Plastic Shrinkage

 hardening concrete crack

• Crazing

• Drying Shrinkage

• Thermal Expansion and Contraction

• Chemical reaction

o Due to Alkali-Aggregate Reaction

o Sulfate attack

o Due to corrosion of steel

3.2.1. Plastic Settlement concrete crack

Concrete has a capacity of settling after initial placing. The settlement of concrete restrained due to the diameter of the reinforcement bars and congested reinforcement bars, strength of formwork and inadequate vibration. In this case the void space occur and leads to concrete settlement crack (Meraj and Sabih, 2014).

Figure 6, plastic settlement (Meraj and Sabih, 2014)

As Meraj et al (2014) and Prasad et al (2013) clearly explains that plastic settlement cracks of concrete are caused by congestion of rebar, Improper vibration, use mixes with excessive bleeding characteristics, decrease in the ratio of concrete cover to reinforcing bar diameter, retarding admixture, improperly and un-rigidly set all formwork so that it will move during concrete placement, minimum compaction, improper curing, large size aggregate particle, gap graded proportion mix.

3.2.2. Plastic Shrinkage

After the plastic or fresh concrete has been placed, concrete has its own properties which is known as volume reduction or contraction during the plastic state of the concrete. Beside that concrete plastic shrinkage is developed (pre- hardening or pre- setting shrinkage). Due to this analogy plastic shrinkage will happened because of excessive evaporation of bleeding water over the recommended rate of evaporation (Abebe 2017). If the concrete is early dry its surface will result shrinkage due to that the fresh concrete cannot resist any kind of tension. In this case short cracks will be developed. These cracks occur until one to two hours from placing of concrete and these cracks are most of the time parallel one to another and also spaced 0.3m to 1m apart, 5cm to 10cm in depth and up to 3mm in width. On the surface these cracks are wider and spill down rapidly (Meraj and Sabih 2014).

Figure 7, plastic shrinkage crack (Prasad et al 2013)

As Prasad et al (2013) clearly describes that the causes of plastic shrinkage concrete cracks are happened due to; concrete placed on dry subgrade condition, the fresh concrete exposed to strong wind, use of warm water and hot aggregate to speed up the temperature of fresh concrete, wrongly dampen the subgrade and formwork because of excessive water is removed during placing of concrete, wrongly preserve the aggregate under shadow, not place and finish the concrete fast, excessive evaporation of bleed water, not start curing as soon as possible after placing of concrete.

3.2.3. Thermal Expansion and Contraction

In most cases materials expand when they are heated, and contract when they are cooled. With related to this thermal stresses are produced when there is normal expansion and contraction of concrete due to surrounding change in air temperature. The strain produced by the temperature usually associated with the magnitude in ranges of temperature change and coefficient of thermal expansion of the material.

In the exception of extreme climate condition, most concrete structures has an ignorable stress from change in environment. Though in huge structures the amount of heat produced by the cement-water reaction is high and the system of removing this heat is very low, an internal temperature of concrete becomes high. In other case as the external temperature is very low the thermal shrinkage of concrete and produce tensile stress in concrete resisting cracks (Abebe 2017 and Prasad et al 2013).

As Meraj et al (2014) and Rajat et al (2015) has clearly shows that when there is no provision of thermal expansion, concrete will crack. This cracks will be a source of seepage in water retaining structures, the rate is dependable on the consumption of Portland cement. This phenomena is not dependable on the cross section and other dimensional characteristics of the members.

3.2.4. Drying Shrinkage

It is generally accepted that hydration process is very high in some hours after the ingredient is mixed. Even though the initial setting time of the concrete is mostly the starting point of hydration, the major concern to a drying of concrete is occupied after ten hours of mix that is final setting time. At this stage concrete become stiff, it produces ability of resisting external forces and gradually increases through time.

Concrete has specified dimensional related volume at the initial stage but after drying it becomes decrease in volume because of that the volume of ingredient is not proportionally equal to the volume of the dried concrete. In the time of volume change, stresses usually developed within structure due to combination of shrinkage and restraint provided by another part of the structure. Cracks are developed when the tensile resistant of concrete is exceeded by the restraints. This type of cracks exist in the surface of the concrete and in later stage they goes to the deepest part. As Meraj et al (2014) explains that the major factors which affect drying shrinkages are the type of aggregates and water to cement ratio. In most cases the concrete with smaller size aggregate shrinks more than that of concretes with coarser size. In accordance with this, the higher water to cement ratio of concrete it will have unreacted water in initial stage of hardening. Over a long period of time the excess water will be removed progressively and the spaces that was provided by the water molecule will become weak. In the presence of restraints as shown in figure 8, this contraction leads to crack and called drying shrinkage (Prasad et al 2013). Consider this type of failure as they mostly takes place in the first few months. As he clarifies briefly these cracks are time dependent approximately 15-30%, 40-70% and 66-80% of total shrinkage occurs in two weak, three month and one year respectively. From the produced drying shrinkage very small present can be recovered by immersing the concrete member in to water.

Figure 8, draying shrinkage with & without restraint (Doug 2016)

3.2.5. Craze cracks

These type of cracks are fine random cracks in the surface of caused by shrinkage of the layer day after placement. Since the only external layer is pretentious they can’t affect structural stability of the concretes but usually they are the main causes for deterioration and ugliness of concrete. In most concrete surfaces these type of cracks exists in an irregular hexagon shape with less than 50-100mm width which can be seen when the surface is wet, and no more than a few millimeter in depth (Abebe 2017 and Prasad et al 2013). As He describes the general reasons for appearance of Crazing cracks are;

• Poor or inadequate curing

• Too wet mix, excessive floating, depression of the coarse aggregate which leads to an excessive concentration of cement paste and fines at the surface

• Finishing while there is bleed water present on the surface or the use of steel trowel at a time when the smooth surface brings up too much water and cement fines. The use of a derby or bull float while bleed water is present will produce a high water-cement ratio at the surface

• Sprinkling cement on the su
rface to dry up bleeds water. This concentrates fines on the surface.

• Occasionally carbonation of the surface can cause crazing.

Figure 9, Crazing crack (Prasad 2013)

3.2.6. Chemical reaction

Chemical reactions has a two way relation with cracks; chemical reaction inside concrete creates suitable condition for the existence of cracks, in other side cracks are also the main causes of undergone chemical reaction in concrete. The commonly known reactions that has a significant relation with cracks are alkali silica reaction and corrosion of steel reinforcement (it is briefly described in section 4 of this paper). Reactive aggregate in concrete reacts with the alkali hydroxide in cement paste to produce an expansive gel causing map cracking or directional cracking in the loaded structure (Fred 2010).

4. EFFECT OF CONCRETE CRACKS

Meraj and Sabih (2014) says “Cracks can be treated as cancer in R.C.C structure, as cancer which in its primary stage is curable to a certain extent but becomes danger to life in later stage; same happens with cracks”. He expresses that naturally cracks can exist in any structure, because of unpredictable environment they can’t be prevented by best design and construction practice only. It needs periodical checkup with in the serviceability of the structure. The best way to minimize the damages of cracks is ensuring methods of keeping concrete cracks inactive and repair before it reaches at an extreme level. Some major effects of concrete cracks are;

4.1. Fluid transport mechanism

Concrete structures are designed for a limited serviceability time in which they can give an acceptable function without a significant extra maintenance cost. In most concrete structures degradation problem can exists in different level of effect depend on the quality of concrete and circumstance environment. Even though there are many factors that affect the degradability of concrete structures, the main factor that play a significant role is the presence of fluids herein.

The accepted and useful reaction in concrete structures is hydration of cement, but in the life of the structure it is impossible to keep the structure out of other reaction. Depending on the availability of the moisture and other chemicals inside, concrete becomes deteriorated from time to time. An aggressive chemical from the environment can facilitate chemical reactions in concrete and affect the durability. Since oxygen, dissolved salts (chlorides) and other poisonous chemicals can easily transported from environment to the internal parts of concrete, deterioration is very alarming in the presence of concrete cracks.

The main way of preventing concrete from degradation is preventing the entrance of moisture and chemicals in to concrete. Though it is very difficult to perfectly make concrete non accessible for moisture, minimizing the volume of moisture which can be entered to the concrete through cracks is very crucial to decrease the rate of degradation. It can be done by design and producing of non-permeable and crack resistance concrete structure (HDCS, 2011).

4.2. Carbonation of concrete

Carbonation of concrete is a reaction of calcium in cement paste with spontaneously introduced carbon dioxide to produce calcium carbonate. As cement past has some amount of calcium hydroxide it has basic property, so that it can react with carbon dioxides which entered to the concrete from air or water and form a neutral compound (with PH=7). Even after removal of calcium hydroxide, calcium oxide which is produced from calcium silicate hydrate will carbonate.

Cracks in reinforced concrete increase the speed of carbonation by allowing an entrance of extra aggressive gas and air inside concrete from the environment. In addition carbonation process requires water to solve carbon dioxide, which mostly introduced through cracks and can transmit in porous structures. So that the main factors that affect the rate of carbonation are porosity and moisture content in the concrete. In a dry concrete carbon dioxide cannot be dissolved, in opposite of this, when concrete is to wet carbon dioxide cannot entered to the concrete so carbonation will not occur, it needs an optimum condition with relative humidity commonly range from 40% to 90%.

In addition to carbonation, bi-carbonation will also occur if concrete was produced with high water to cement ratio that forms hydrogen carbonate ion with less than 10 PH value. This process mostly has not an advantage because of that, it makes a concrete soft and friable. In contrary to bi-carbonation, carbonation has an advantage of decreasing porosity by making strong carbonated cement paste mostly in concretes without reinforcement (Timur et al 2017).

4.3. Corrosion of reinforcements

As concrete is pretty best in compressive strength and week in tensile, it is commonly accepted that using reinforcement bars to produce strong reinforced concrete member both in tensile and compressive strength. Reinforcement bars need to be neutral for the strength of reinforced concrete structure, but it becomes reactive as air and moisture which are the preconditions for existing of corrosion introduced to the concrete member. Corrosion is a natural process of iron reacts with water in the presence of air on which oxides, chlorides and sulfates should be produced.

The corrosion of reinforcement bars in reinforced concrete mostly leads to failure of structure by reducing the bond strength of bars with concrete cement paste. Even though there are various sources for the existence of moisture and air inside concrete members, the main way for introduction of those fluids is through the cracks of concrete. In a normal condition the reinforcement bar creates a passive layer to resist corrosion but as the moisture action increases this layer reduces its strength slowly.

As William et al (2009) clearly describes that, reinforcement corrosion occurs at concrete cracks involves a macro-cell where steel exposed at the crack tip and immediately thereto becomes an anode and actively corrodes once the critical chloride concentration is exceeded here with adjacent passive rebar serving as a cathode for oxygen reduction. This macro cell activity reduced with time and they become independent of time mostly after two years. In order to maintain charge conservation, the anodic corrosion reaction can occur only as rapidly as oxygen is reduced at cathode sites. This phenomenon can be reduced by enough concrete clear cover and quality of concrete mix (William H, 2009).

Figure 10, macro-cell corrosion activity at a concrete crack (William et al 2009)

Corrosion of reinforcement is very crucial in concrete structures which are imbedded in a saturated ground. As (William et al 2009) argues that, in these type of structures the action with related to cracks should be considered in different zones of corrosion which has different level of effect. These zones are;

• Zone 1: When concrete members are fully embedded in saturated ground, ground water with sulfate and chloride can easily introduced to the concrete cracks with no dependency on the crack width. But no significant corrosion exists because of unavailability of dissolved oxygen in groundwater.

• Zone 2: In this zone both chloride and dissolved oxygen are available at the time of high ground water table and relatively low ground water table respectively. Corrosion will occurs even at dry condition because of a macro cell between bars at the crack tip (anode) and oxygenated concrete (cathode). But the magnitude is dependable on the range of water table variability.

• Zone 3: In this zone the source of chloride and sulfate is diffusion fr
om soil through crack face. So that the risk of corrosion is less than zone 2.

• Zone 4: In this zone potential corrosion comes from the diffusion of chloride and sulfate present in the surface of concrete and in smaller amount with in cracks (Pooja et al 2015).

Figure 11, reinforced concrete element which transitions traversing different inland corrosion zones (William et al 2009)

4.4. Concrete Strength

However the strength of concrete hasn’t a single basic measurement, the effect of concrete crack can be seen based on difference mechanical responses of concrete structures. Many scholars agrees that, drying shrinkage is one of major factors affecting crack width, deflections, losses in pre stress, as well as bending moment and forces in structures. Though the cracks produced in concrete surface, it allows the entrance of chemicals and affect the bond between past and reinforcement bars.

Hikotsuku et al, Assess the effect of drying shrinkage on tensile strength of reinforced concrete in the laboratory and concludes that; the shear tension strength of reinforced concrete beams exposed to drying condition decreased by 17% compared with that of companion sealed beams and shows significant size effect. In addition to this, He proposed that equivalent tension reinforcement ratio, for estimating the effect of autogeneous shrinkage which is applicable to evaluate shear tension strength of RC beams before yielding of reinforcement (Chien 2016).

Reinforced concrete structures are the composite of two materials which are concrete for compressive strength and reinforcement bars to overcome weakness of concrete in tensile resistance. Even though the entrance of fluids through cracks become a cause for the existence of corrosion in reinforcement bars, the concrete part also injured in compression softening effect that is related to degree of transverse and straining present. It influences the compressive strength, ductility and load deformation response of concrete element. When compared to that of relatively non cracked concrete, the compressive strength of cracked concrete is very smaller (Vecchio et al 1995).

In the presence of crack, the internal void of concrete structure increases gradually through permeability and diffusion mechanism. Since fluid fraction of the members is not able to resist a load, it leads to decrease the shear, tensile and compressive strength of reinforced concrete members. As most of concrete crack experts recommends that, the residual maximum shear crack which exceed 0.4 mm width and peak maximum shear crack exceed 1mm is not advisable in long term loading. This is the maximum tolerable value of cracks even in the moderate condition.

4.5. Aesthetic value

Any concrete structure in the world which is constructed for difference purposes has a common consideration that is stability of the structures. Concretes with internal deterioration has their own effect on external character of the structures. Structural stability alone is not enough for the appropriate functionality of structures, considering the internal stability is basic but must be together with the external visual character.

Most cracked and degraded concrete structure has an external grey surface which is not color of concrete in normal condition. For the fulfillment in needs of the occupant, the aesthetics value of the structure must be considered. In case cracks exist in the surface of the concrete structures it is not possible to maintain by plastering of the surface, So that it has a vigorous effect on the pleasure of users and including the community.

4.6. Durability of the structure

The durability of concrete structures is the collective result of all implications, which indicates the serviceability of the existing structure for the designed function within designed period of time. Even though the main factors leading most structures to be out of service is exposing the structure to an extra load more than that of considered in the design, the cracking of concrete structure by environmental factors also has a significant effect on the durability of existing structures. In any case the affected structure will develop internal and external cracks, therefore it cannot be functional for the design period of time.

The effect is more complex in case of the structures which are constructed in water bodies, since the entrance of moisture allows chemicals to access and move in the structures. Generally all effects caused by cracks are the sign of minimizing in the durability of the structures.

5. CONCRETE CRACKS IN SOME ADDIS ABABA CITY PROJECTS

5.1. Introduction

Addis Ababa city is the capital of Ethiopia, which has many construction projects in the outskirt. Our assessment focuses on some construction projects which has existing cracks or exposed concrete members which probably leads to crack. Even though it is very difficult to distinguish cause and effects of cracks because of effects can be a cause for extra cracks and vice versa, we try to look on both causes and affected concrete structures separately.

5.2. Existing Cracks

We randomly visits some areas and we found different existing cracks. These happened in the serviceable structures or in concrete structures which are under implementation. Some of the existing cracks that we get in visiting some areas are listed below.

A. Plastic Settlement

In most visited areas, concrete moves away from reinforcement bars after some days. In these concrete structures reinforcement bars are clearly visible with void spaces below them. As Mr. Yonanes Zewuge who is site engineer of the projects says that, these cracks exist in our site because of high water cement ratio produced by non-skilled labors.

Figure 12, Plastic settlement around Bole Arabsa, Katla

B. Plastic Shrinkage

As we have visit in the sites, fresh concrete which is casted in the ground produces a hairline cracks in the surface.

Figure 13, Plastic shrinkage of concrete around Sidist Kilo

C. Thermal Cracks

These type of cracks in concrete has shown in existing structures after starting of giving a service. Some long spanned concrete retaining walls in the visited areas have expansion joints in a specified length but it is mostly up to limited depth from top to bottom ward. As shown in figure, concretes retaining walls for the asphalt road in Megenagna area produce a vertical thermal cracks below the end point of the expansion joints.

Figure 14, Thermal cracks in retaining wall around Megenagna

D. Craze Cracks

We have visit a scratched concrete surfaces in existing ground floor slab, which is near the kitchen and exposed to dampness. As the users of the building says that they do not care for this cracks because of no effect in them.

Figure 15, Craze crack in ground slab in front of Anbesa Gibi

E. Drying Shrinkage

As shown in figure below these type of cracks has been popular in existing building slabs because of volume change of ingredients after hardening of concrete due to different factors.

Figure 16, Drying shrinkage in front of Anbesa Gibi

F. Tension Cracks

As shown in figure below, beams which are directly fixed between two columns exposed to tensile loads because of volume change.

Figure 17, Tension cracks in a fence beam around Ferensay Legation

5.3. Exposed concrete structures

In some areas there are existing structures which are not affected by cracks, but they are exposed to the factors which facilitate cracking. As shown in figure 18, some structures has been exposed to water that may be contai
n sulfate and other chemicals which are the cause of cracks. In addition to chemical attack, reinforcement bars will be affected by corrosion and leads to cracks.

Figure 18, structures exposed to water

As shown in figure 19, in most construction sites there is poor construction practice in concrete casting. Reinforcement bars in a concrete structure has exposed to weathering due to absence of concrete cover. As Mr. Zewudu Tamiru who is the office engineer of these blocks says that concrete covers can be scratched at the time of removing formwork.

Figure 19, Bars without concrete cover in Bole Arabsa 40/60 saving house 03 projects

As shown in figure 20 scratched concrete slabs may be leads to cracking by the effect of weather from the surrounding environment as moisture and other chemicals can be entered to the concrete by means of different transport mechanism.

Figure 20, Scratched slab surface in 5 kilo (AAU, pharmacy campus)

In addition to becoming the main factors leading to cracks, scratched and deteriorated concrete surfaces are the key impacts for loosing aesthetic value of the structures.

6. CONCLUSION

In this report, causes and effects of concrete crack which are caused by different factors has been discussed. Not all type of cracks require the same attention; some cracks has a great effect on the safety of structures and some hasn’t a significant effect with the exception of reducing the aesthetic values. Even though, it is difficult to be assured on preventing the development of different cracks, these potentially dangerous cracks need a series control mechanism before affecting concrete structures. In most under-construction projects a lot of defects which are the main motivators for the development of cracks has been existed.

7. RECOMMENDATION

As it can be understood from the visited areas most types of cracks in Addis Ababa city has been caused by different defects due to poor construction practice with the exception of environmental effect. We highly recommends that to prevent the development and effect of cracks, construction firms shall minimize construction defects through good construction practice in the site.

8. REFFERENCE

Abebe, D. (2017). Construction materials [lecture notes]. Addis Ababa University, Addis Ababa, Ethiopia.

ACI 224.2R-92. (1997). “Cracking of concrete member in direct tension.” ACI Committee report.

Chien, K. (2016). “Shear crack control for high strength reinforcement concrete beams considering the effect of shear-span to depth ratio of member” international journal of concrete structure and materials.

Doug, H. (2016). “Types of cracking and influencing causes.” University of Toronto, Toronto, Canada.

Fred, A. (2010). “Cracks in concrete.” Technical report, Vicroads.

Hikotsuku, H. et al. “Effect of drying shrinkage on shear tension strength of reinforced concrete beams.” VIII International conference of on fracture mechanics of concrete and concrete structures, Hiroshima, Japan.

Hycrete delivery concrete solution (HDCS). (2011). “Concrete durability understanding water transport mechanism in concrete.” Carlstadt, USA.

James, G. and James, K. (2012). “Reinforced concrete structures mechanics and design.” University of Alberta, Alberta, Canada.

Meraj, A. and Sabih, A. (2014). “Cause and evaluation of cracks in concrete structures.” international journal of Technical research and applications, Integral University, Lucknow, India.

Pooja, N. et al (2015). “Study on causes of cracks & its preventive measures in concrete structures.” International journal of engineering research and applications, Career Point University, Kota.

Prasad, YTVV et al (2013). “Shrinkage cracks- causes, preventative measures and repair methods.” Tech mailer Team, India.

Rajat, S. et al (2015). “Study on causes of crack and its preventative measures in concrete structures.” international journal of engineering research and applications, career point university, Kota.

Timur, Z. et al (2017). “Carbonation of concrete taking in to account the cracks in the protective concrete layer.” Asian research publishing network (ARPN), Ufa, Russia.

Transportation research board of national academies (TRB). (2006). “Control of cracking in concrete.” Transportation research circular, Washington DC, USA.

Vecchio, F. et al. (1995). “Compression response of cracked reinforced concrete” Member of American society of civil engineering, University of Toronto, Toronto, Canada.

William, H. et al (2009). “Effect of concrete crack width on corrosion of embedded reinforcement.” Florida, USA.

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