In the early ages, the strength of concrete constructions was queered by cracking. This situation was prevalent in cases where structures of massive sizes were involved. The probability of a concrete structure to crack is determined by a variety of factors including stress and hydration. Cracks characteristics are highly composite, and there are no universal methods for determining their likelihood of occurrence (Attanayaka & Aktan, 2004). The fundamental principle surrounding the occurrence of cracks is the low tensile concrete strength. In the early age concrete, it has been determined that most cracks results when tensile stresses outmatch tensile strength of the concrete material.
Visible cracks raised a lot of concern for the constructors (Lea, 1971). This is because they provided room for the percolation of aggressive solvents into the concrete, a situation that strained the reinforcing material of the construction. The strain resulted into deterioration of the structure. Regardless of the concrete durability issues, demand for durable, and monolithic concrete has been on the rise in modern cities. Shrinkage and creeping of concrete under limited condition on first few days after hurling is described by experimentations that render data on tensile creeping strains and shrinkage (Cannon, 1993). Comprehensive methodologies for concrete cracking control, therefore, necessity to ensure sustainability of complex, concrete structures. This paper discusses various mechanism employed to control early age concrete cracking.
Objective of the Study
Concrete mechanical content increases with age, and at an early age, low strength of the concrete can cause it to crack under lower stress. Concrete can be exposed to mechanical actions inferred from shrinkage which result to concrete dimensional alterations and can stress the concrete. Early age concrete cracking arises from rapid changes such as thermal deformation autogenously shrinkage and drying shrinkage (Emmons, 1993). These volume alterations cause tensile stresses in the material when strength is comparatively scummy.
Competition inside the material between the development of tensile stress and the development of strength evolves with time and these results to premature concrete deterioration. The stimulated stresses may cause immediate cracking as residuary stresses that suffice to limit capability of the concrete material. Premature deterioration affects durability, integrity, and long-term service life of concrete construction (Grzybowski & Shah, 1990). Various methods are employed to curb concrete cracking. The objective of this study is to evaluate ways and means of controlling cracking in the early age concrete.
Scope of the Study
This study evaluates those activities that were involved in the preparation of concrete during the early ages. These activities include the alternative drying and wetting, selection and proportioning of materials, the curing of creeps and shrinkages (Jacobs & Whitcomb, 1997), and conformity to the construction practices. The scope aims at assessing how these activities influenced cracking.
Alternate Drying and Wetting
This method subject concrete to drying conditions in three days, and then covered with wet material for one day and the process repeats until the concrete become entirely stiff. The stress evolve during drying conditions, and on wetting, the stress relaxes. Alternate drying and wetting balances the built up stress and the tensile strength and this successfully achieves the goal of curbing cracks (Jensen & Hansen, 1995). Wetting drastically reduces the shrinkage strain that build up during drying conditions, and this significantly recovers early age concrete cracking. The speed of shrinkage in the first days drying is high, but a high rate of shrinkage. Wetting also affects the aggregate tensile creep, and this result from the decrease in tensile stress, and the related recuperation of tensile creep. The alternate drying and wetting influences both the creep behavior and restrained shrinkage of concrete at an early age (Nilsson & Winter, 1985).
Proper materials selection and proportioning
Proper material selection and proportions significantly reduce early age concrete cracking. Concrete mixtures with a high water to cement ratio have a comparatively high piousness, and the mixture demonstrate considerable drying shrinkage and lowers protection of the rewarding steel from chemical compound collusion (Krauss & Rogalla, 1996). Tendency for cracking increases as the strength of concrete increments and particularly if concrete is insufficiently treated. Low water to concrete is sensitive to shrinkage cracking at concrete early age.
Extended moist curing raises the coefficient of elasticity and reduces the creep making the concrete more prostrate to cracking. Concrete mixtures built using mellower cement substances are extremely contributing to cracking by developing higher heat of hydration (Cady, 1995), higher coefficient of elasticity, lower creep, and greater shrinkage. Use of high strength concrete encourages increased cement contents increasing cracking and with a proper planning during materials selection and mixture proportioning, a crack resistant concrete which meets durability, and functioning specifications is produced (Attiogbe & Darwin 1987). Proper selection of reinforcing materials helps in reducing grade the strength gain, reduce stiffness development, and, therefore, reduces potential for concrete cracking. These materials increase the rate of strength development, and create conditions that are favorable for cracking.
Initial Curing on Creep and Shrinkage
This method involves sealing the concrete for few days before subjecting it to drying to increase hydration. During sealing, the shrinkage stress is low, and it has not eliminated because sealing does not suppress the internal drying. The stress increases on unsealing because sealing increases the elastic modulus of concrete by raising the hydration of cement and later shrinkage strains results to a higher stress (Balaguru & Shah, 1985). Also, the exposure shocks speed the shrinkage regardless of the concrete age. Sealing improves the stiffness and strength of the material. This improvement reduces early age shrinkage, a situation which enables the structures to remain firm.
Proper following of construction Practices
Following proper construction technique reduces water evaporation from fresh concrete and as a result, reduces the potential for shrinkage cracking. This practice ensures proper concrete curing after concrete placement that reduces the speed of evaporation. Use of wind breakers, and controlling high temperature by protecting fresh concrete from the sun reduces evaporation loss (Nawy, 2001). This technique also ensures proper joint spacing in ordinary concrete and the proper amount of steel in the continuously reinforced concrete essential. This ensures sufficient thickness and proper drainage that are indispensable in controlling early age cracking on concrete.
Significance of the Study
Shrinkage of concrete results from a complex combination of both external and internal drying. These types of drying result into concrete stress for first few days upon the casting of the concrete. As the performance of the concrete is improved, the probability of cracking reduces drastically. The rate and history of stress evolution are significant factors that affect the failure stress, and time of cracking and these two parameters must be looked at in the analysis for exact anticipation of shrinkage cracking. Tensile creep at an early age makes a substantial part of the time dependent distortion (Neville, 1996). Its function in lowering the shrinkage strain and loosening the shrinkage stresses can be extracted as the ratio of aggregate creep to free shrinkage. Tensile creeps relax shrinkage stresses by at least half for normal and high performance concrete. It also stretches the time to break by two to three meters than that would be anticipated based on free shrinkage entirely.
Fiber reinforcement causes a little increase in the aggregate tensile creep for a normal concrete. The large holdup in fracture seems discrepant with the minor advance in tensile creep entirely, and other factors must exist which should be related to drying creep mechanisms associated with micro cracking.
Sealing the concrete after drying does not eradicate early age shrinkage nor enhances the purpose of fibers on altering the limited shrinkage and creep conduct. The potentiality for cracking of the initially covered concrete when subjected to drying increases as the initial curing betters stiffness of the concrete (Bentur et al, 1997). The remainder amongst the improved force and hardness of concrete and the changed magnitude risk of cracking must be, therefore, looked at. The shoplifting stress loosens rapidly in the first passing water and formulates again on re-drying. Shrinkage and creep recovery of restrained concrete is determined by both wetting and drying application.