Concrete Bleeding and its Effects on Crack Resistance
Concrete bleeding can be defined as the process where free water in the mix is pushed upward to the concrete surface as a result of the settlement of heavier solid particles such as water and settlement. The bleeding in concrete can directly affect the crack resistance of concrete structures. The biggest factor that determines the rate of concrete bleeding is the ratio of cement to water since the higher the ration, the excessive the bleeding. Concrete bleeding reduces the ration of water to cement and defines densifies the concrete. The concrete that bleeds too long and too fast can result in numerous problems such as voids under aggregate particles and rebars, weaken horizontal construction joints, sands streaking in walls, and jamming of rock in pump lines.
Even if the concrete bleeding is not excessive, finishing the concrete at the wrong time may result in problems related to concrete bleeding such as dusting, scaling, and blistering surfaces. The prevention of these harmful concrete bleeding effects requires an understanding as to why the concrete is bleeding and how the proportions of the concrete mixture affect the bleeding. This research paper reviews the effects of concrete bleeding on crack resistance of concrete structures. Some of the effects of concrete bleeding on the crack resistance of concrete structure include surface delamination blisters, mortar flaking, scaling, durability, steel-paste bind, aggregate-paste bond, shrinkage of the concrete structure, post-bleeding expansion, and volume change.
Concrete is a mixture of cement blended with numerous sizes of aggregate and water which forms a paste that glues the aggregate together during hardening. When the concrete is still fresh, it is a workable mixture that can be used to make any shape desired. After sometimes, the paste slowly stiffens to gain the required strength. Almost all freshly placed concrete bleeds. This is because the cement and the aggregate settle hence forcing the excess water to go upward due to its low density. This process continues until settlement stops and the rate of bleeding depends on the number of fines such as fine sand, fly ash, and cement, the water content, and properties of the mixture (Beletich, 2013).
There are two different categories of concrete bleeding, these include channel bleeding where water rises through a specific path and the normal bleeding where there is uniform seepage of water in the whole surface of the structure. The accumulation of water in the concrete mixture can take place through uniform seepage over the whole of the structure surface or at the localized channels conveying water to the structure surface. The effects of concrete bleeding can affect the crack resistance of a concrete structure through influencing the concrete structure characteristics by impacting its strain, tensile strength, compression strength, specific gravity, water absorption, and visual appreciation of localized defects (Clifford, 2008).
Categories of Concrete Bleeding
Numerous researchers have assessed the effects of plastic concrete bleeding only on crack resistance of concrete structures, however, the major literature gap in their researches is that they fail to analyze the effects of bleeding on other types of concrete structures. The researchers majorly focus on the plastic concrete bleeding and the plastic concrete structure. Majority of these researchers only focuses on the effects of bleeding on plastic concrete while failing to review the other types of concrete such as precast concrete, high-density concrete, reinforced concrete, plain concrete, lightweight concrete, and normal strength concrete (Committee, 2013). The researchers also fails to mathematically analyze how the concrete bleeding affects the crack resistance of concrete structures through the changes in train, tensile strength, compression strength, specific gravity and water absorption of the concrete structures.
Concrete bleeding is a tendency of water rising to the surface of the structure of freshly placed concrete. It is a form of segregation where soma water quantity comes to the surface of the concrete structure after compacting and placing, before settling. The content of water conveys some particles of cementing materials and sand. As the concrete bleeding continues, the layer of water at the structure surface maintain the initial height of the concrete sample. Concrete bleeding is normally assessed in terms of bleeding capacity and bleeding rate. The bleeding rate is the rate at which the bleeding water moves through the concrete structure and it is controlled by the permeability of the plastic concrete paste. The bleeding capacity is the amount of water bleeding for a specific concrete structure and it is normally denoted by the change in height or settlement of the surface of the concrete structure (Council, 2008).
The duration of concrete bleeding is influenced by the settling properties of the materials used and the depth of the concrete structure. A thin concrete structure will bleed or settle for s shorter duration compared to a deep section of the concrete structure. Majority of the bleeding takes place during the dormant moments when the concrete structure has no reaction (Delatte, 2009). Some of the effects of concrete bleeding on the crack resistance of concrete structures include:
The combination of the aggregates, water, and cement in a mixer creates a suspended and disbursed state of particles in the concrete structure. The state of suspension is not stable due to the heavier particles of aggregates and cement are forced downwards by gravity through the lighter water. The downward motion of the solid particles is replaced with water, and the solid matter decreases resulting in the decrease in the crack resistance of the concrete structure. Settlement can also take place without the occurring of bleed water on the concrete structure. This is due to numerous instances such as windy periods, the evaporation rate is enough to eliminate the bleeding water as it comes off the concrete structure surface (Dhir, 20).
Effects of Bleeding on Different Types of Concrete Structures
The minute quantity of volume or settlement reduction is not of concern for numerous normal construction applications or practices. However, the applications in which concrete structure is being situated under an object that must be supported, there should be no concrete bleeding to prevent the development of spaces or cracks between the concrete structure surface and the supported object. The concrete bleeding also promoted the risk of settlement cracks over the objects embedded such as reinforcing steel (Garber, 2011).
After the process of concrete bleeding, expansion takes place within the paste. The expansion during post-bleeding is as a result of chemical and physical reactions happening during the initial phase of setting. This expansion may lead to pressure exertion within the concrete structure hence reducing the crack resistance of the structure and the structure may collapse. Majority of the expansion takes place during the initial days of construction and the successive days have less than half expansion range (Klieger, 2011). The table below shows the expansion during post-bleeding of a concrete structure:
Figure 1: Expansion during post0bleeding of a concrete structure (Lamond, 2009)
Shrinkage of the concrete structure is sometimes referred to as setting shrinkage and this process takes place before the hardening of the concrete. The shrinking of the concrete structures is caused by the combination of the loss of free water from the concrete structure as a result of structure surface evaporation and bleeding and also the intake of water by the cement during the process of hydration. The rate of surface evaporation and bleeding determines the quantity of concrete shrinkage. The shrinkage of the concrete during the settling reduces the crack resistance of the concrete structures (Leung, 2009).
The differential settlement takes place between the past and aggregate and once the aggregate can no longer settle, the past continues settling hence allowing bleeding water to collect and rise under the aggregate. This reduction of the bond between aggregate and paste bind minimizes the strength of the concrete in the concrete structure hence exposing the structure to the risk of cracking by reducing the crack resistance. There are instances when the mortar of the concrete settles faster compared to the rest of the concrete because of the hot conditions of the weather, there will be a settlement of particles of coarse aggregates leaving minute voids of air on top of the particles of aggregates which further reduce the crack resistance of the concrete structure (Limbachiya, 2008).
Bleeding Capacity vs Bleeding Rate
The concrete bleeding water is expected to be collected under other embedded items and reinforced steel. This is due to the bleeding water having the tendency collecting under huge items in the concrete, however, during the settlement of the concrete, the concrete pulls away from the steel leaving voids of air and water can easily collect in these voids hence reducing the crack resistance of such concrete structure. The small collection of a settlement or bleeding water under reinforced concrete is not detrimental to the development of strength in the bar since the majority of stresses are applied to the deformation of the bar. However, extreme collection o water and development of void can promote corrosion of the steel at the location of voids, minimize past-steel bond, and embedded strength especially in the presence of chlorides, carbonation, and moisture (Maganlal, 2008).
The mixture of concrete is normally meant to be durable despite daily exposure to advanced weather conditions. Low quantity of concrete bleeding is most cases does not minimize durability, however, extreme concrete bleeding may have advanced impacts in case necessary preventive measures are not taken. The resistance of the concrete to sulfates, acids, chlorides, and aggressive chemicals and durability are related directly to the cement water ratio and permeability of the concrete used in the structure. The concrete bleeding weakens the concrete structure surface and reduces the durability of the structure resulting in a structure with lower crack resistance (Mailvaganam, 2010).
The relationship between scaling and bleeding is influenced by the curing practices, finishing, and placing. Increased concrete bleeding can promote resistance to scale contrary to the common belief. The concrete resistance to scaling can endanger whenever the concrete structure has varying water content (Nilson, 2015). The figure below shows the relationship between the subsidence and scaling of the concrete structure:
Figure 2: Relationship between subsidence and scaling (Paillere, 2009)
There is need of increasing the content of cement or reducing the ration or cement to water of the concrete structure surface through sprinkling dry cement on the slab of the structure so as to absorb the extra water or finishing the slab before the accumulation of bleeding water. The accumulation of water may result in the development of voids below the surface or in some cases weakened regions of paste hence resulting in the decrease in the crack resistance of the concrete structure (Perkins, 2012).
The delamination of the concrete structure surface denotes the separation of a huge region of mortar at the surface from the base concrete. The remaining surface exposed resembles a surface scaled with exposed coarse aggregate. The delamination of the surface is caused by the accumulation of bleeding water under the surface producing weaken zone of void (Powers, 2011). The figure below illustrates the bleeding water void or weakened zone under a finished concrete structure surface which may result in the reduction of the crack resistance of the structure:
Factors Influencing the Duration of Concrete Bleeding
Figure 3: Bleeding water void or weakened region under the concrete structure surface (Siddique, 2011)
During the freezing of water in the weakened region or void, the surface of the concrete structure delaminates in a form that resembles a sheet.
The major effect of concrete bleeding on the concrete structures is that the bleeding water lowers the crack resistance of the concrete structure by the bleeding water collecting under huge items in the concrete during the settlement of the concrete, the concrete pulls away from the steel leaving voids of air and water can easily collect in these voids hence reducing the crack resistance of such concrete structure Some of the ways in which the concrete bleeding can be prevented is through the use of finer cement or cement with low alkali content, using air-entraining agent, using finely divided pozzolanic materials which reduces concrete bleeding by developing a longer path of water to traverse, and also proper proportioning and complete and uniform mixing of the concrete materials (Smoak, 2012).
Concrete bleeding can be defined as the tendency of water rising to the surface of the structure of freshly placed concrete. The expansion during post-bleeding is as a result of chemical and physical reactions happening during the initial phase of setting. This expansion may lead to pressure exertion within the concrete structure hence reducing the crack resistance of the structure and the structure may collapse (Society, 2009). Apart from the formation of cracks on the surface of the concrete structure, concrete bleeding also result into negative effects such as reduction in pumping ability of concrete, weakening of bond between two layers of concrete, delays in surface finishing during the construction of pavement, causes permeability of the concrete, and also makes the concrete structure loses its homogeneity (Tazawa, 2009).
Some of the aims and objectives of this research paper include:
- To assess what concrete bleeding is
- To evaluate the general effects of concrete bleeding in concrete structures
- To assess the effects of concrete bleeding on crack resistance of concrete structures
- To evaluate ways of reducing the concrete bleeding in concrete structures
This research paper reviews the effects of concrete bleeding on crack resistance of concrete structure by evaluating the concept of concrete bleeding, its impacts on the concrete structures, and how the bleeding process reduces the crack resistance of the structure. Concrete bleeding can be defined as the process where free water in the in the concrete mixture is pushed upwards to the surface of the concrete structure as a result of the settlement of heavier solid particles such as water and cement. Some of the effects of concrete bleeding on the crack resistance of concrete structure include surface delamination blisters, mortar flaking, scaling effect, the effect on durability, steel-paste bind deterioration, aggregate-paste bond deterioration, shrinkage of the concrete structure, post-bleeding expansion, and volume change (Thomas, 2013).
Immediate and Long-term Effects of Concrete Bleeding
The downward motion of the solid particles is replaced with water, and the solid matter decreases leading to a decrease in the volume of the concrete structure hence reducing the crack resistance of the concrete structure. After the process of concrete bleeding, expansion takes place within the paste. The expansion during post-bleeding is as a result of chemical and physical reactions happening during the initial phase of setting which result into pressure exertion within the concrete structure hence reducing the crack resistance of the structure and the structure may collapse (Woodson, 2011). The shrinking of the concrete structures is caused by the combination of the loss of free water from the concrete structure as a result of structure surface evaporation and bleeding and also the intake of water by the cement during the process of hydration and this process also result in cracking of the concrete structures (Clifford, 2008).
Concrete bleeding also weakens the bond between the aggregates and past in the concrete. This reduction of the bond between aggregate and paste bind minimizes the strength of the concrete in the concrete structure hence exposing the structure to the risk of cracking by reducing the crack resistance. The effects of concrete bleeding on crack resistance of the concrete structure can be reduced by using air-entraining admixtures, using supplementary cementitious materials, increasing the quantity of fine sand, using finer types of cement, and reducing the water content in the cement (Maganlal, 2008). Cracking of the concrete structure can also be caused by the formation of blisters which are as a result of concrete bleeding and these blisters normally occur after troweling of steel in the concrete making water to squirt out. The most predominant admixture used in the reduction of concrete bleeding in concrete structures are water reducers and air-entraining admixtures (Powers, 2011).
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