It is very essential to keep the concrete moist and warm enough. An efficient and un-interrupted curing is a must. In the summer, due to combined effect of sun and drying wind concrete surfaces are likely to get dry thereby results in to inferior quality of concrete. Hence, wet covering and proper water curing must be ensured to get the quality
(2007) reported three causes for reflective cracking, the major being horizontal movements from the expansion and contraction of the base pavement that is caused by temperature changes; the differential vertical deflections across transverse cracks, which create shear stresses; and the curling of cement concrete slabs during colder temperature, when the HMA overlay is stiff and brittle. Palacios et al. (2008) reported that cracks propagate to new overlays due to vertical movement of the underlying pavement layer which may be due to traffic loading, frost heave and consolidation of the subgrade soils and/or the horizontal movement of the pavement upper layers due to temperature changes. But, in case of flexible pavements with expansive subgrade, the shrinkage cracks are initiated in the subgrade layer, when the shrinkage induced tensile stress is more than the tensile strength of the material. These cracks are propagated due to the
Shrinkage crack is caused by contraction of the total body due to loss of water content or moisture. Plastic shrinkage cracks are also formed by loss of water content in the concrete structure by evaporation. When the pace at which the water is evaporating exceeds the rate at which moisture is being supplied to it (via bleeding from the concrete), plastic shrinkage cracks are formed. It generally occurs in the plastic state (partially hardened state). Plastic shrinkage crack may not affect the strength of structure but will ruin the appearance of the structure and the crack to full depth may allow water to penetrate it.
2.3 Hydration of cement Chemical reactions that takes place while mixing cement with water is known as hydration of cement When cement is mixed with water, a certain amount of heat is liberated which is known as heat of hydration. This reaction is exothermic reaction. On account of hydration various products are formed. Different compounds hydrates at a different rate and liberate different quantity of heat • Tricalcium silicate, C3S- early heat of hydration is mainly due to the formation of this compound. • Dicalcium silicate, C2S-responsible for later strength in concrete • Tricalcium aluminates, C3A • Tetra calcium aluminoferrite, C4AF • Gypsum, C-S-H Chemical reactions during hydration On mixing water to cement,the following reaction
In other words, the reduction in strain of reinforcement due to tensile stresses in concrete between cracks is termed as “Tension Stiffening”. Before cracking, the concrete tensile stress increases with load. When the stress in the concrete first reaches the tensile strength at a particular section, cracking occurs, Figure 1(a). When cracking occurs, the stress in the concrete at the crack, drops to zero. The concrete stress increases with distance from the crack, due to the steel-concrete bond, until at some distance from the crack, the concrete stress is no longer affected by the crack, as shown in Figure 1(b).
Concrete materials are still a dominant material for construction due to its advantages suchas workability, low cost and fire resistance as well as its low maintenance cost. It is formed from a hardened mixture of cement, fine aggregate, coarse aggregate, water and some admixture. Massive exploration of the natural resources for producing concrete affect to the environment condition and global warning. We have responsibility to reduce the effect of the application of concrete materials to environmental impact. The concrete should be used as efficient as possible.
Precipitation hardening Done after solution treatment and quenching hardening is done either at natural aging or with artificial aging. It relies on changes in solid solubility where it is being heated below the solvus temperature to produce a fine dispersed precipitate since dislocations are often the dominant carriers of plasticity, this serves to harden the material. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging. Experimental
By the reaction of carbon dioxide and calcium hydroxide, impurities are both absorbed by the particles of calcium carbonate precipitated. • PH control and filtration are important factor in the process of carbonation. PH control is important for the complete precipitation of the lime, if PH is not control, then there is destruction of sugar takes place,filtration is important because, in a pressure filtration stage CaCO3 precipitate
Concrete is the composite mix of cement, aggregates, sand and water. Concrete gets hardened like stone on mixing water with cement and aggregates. Concrete have two type ingredients namely active and inactive. The active group consists of water and cement. The inactive part consists of sand and coarse aggregates.
CHAPTER 1 INTRODUCTION 1.1 GENERAL Concrete is an extraordinary and key structural material in human history. Concrete holds the credit of being the most widely utilized tailored or man- made material in the construction industry and will hold good for the years to come. This credit goes to the properties of concrete like excellent strength, durability, less maintenance costs and use in many structural applications. But in the recent years the concrete industry is facing a big challenge mainly due the cement which is a vital component. International Energy Agency states that cement production accounts for 7% of total global CO2 emissions.