QAQC in Construction

1. Drying Shrinkage Concrete by nature is a porous material. This means that when it is curing, the water in the concrete takes up space. As concrete hardens, it loses its moisture through evaporation resulting in empty capillary pores and a slightly smaller shape. This process is known as drying shrinkage. Since concrete is very rigid, this shrinkage causes an increase in tensile stress, which may lead to, cracking, internal warping, or external deflection. Concrete cracks when the drying shrinkage stresses exceed the tensile strength of the concrete. The spacing and size of cracks depend on the amount of shrinkage, degree of restraint, and amount and type of reinforcement. Drying shrinkage is the number one reason for cracks in concrete. 2. Lack of / improper use of control joints Control joints are most commonly installed in slab-on-grade concrete that is used in places such as driveways, pavements, house slabs, basement floors, and industrial spaces.  Control joints are general...

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and portland cement, are an essential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories--fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inches but generally range between 3/8 and 1.5 inches in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder. Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. Crushed aggregate is produced by crushing quarry r...

1. Natural Sources of Sand: The sand particles consist of small grains of silica (SiO 2 ). It is formed by the decomposition of sand stones due to various effects of weather. According to the natural sources from which the sand is obtained, it is of the following three types: (1) Pit sand (2) River sand (3) Sea sand. (1) Pit Sand: This sand is found as deposits in soil and it is obtained by forming pits into soils. It is excavated from a depth of about 1 m to 2 m from ground level. The pit sand consists of sharp angular grains which are free from salts and it proves to be excellent material for mortar or concrete work. For making mortar, the clean pit sand free from organic matter and clay should only be used. When rubbed between the fingers, the fine pit sand should not leave any stain on the fingers. If there is any stain, it indicates the coating of oxide of iron over the sand grains. (2) River Sand: This sand is obtained from banks or beds of rivers. The river...

By Eng. Muhammad Arif  [ B.Sc. Civil Engineering ( UET, Lahore, Pakistan ),  Member (PEC), (SCA) and (ACI)] Portland Cement Association Aggregates are generally thought of as inert filler within a concrete mix. But a closer look reveals the major role and influence aggregate plays in the properties of both fresh and hardened concrete. Changes in gradation, maximum size, unit weight, and moisture content can all alter the character and performance of your concrete mix. Economy is another reason for thoughtful aggregate selection. You can often save money by selecting the maximum allowable aggregate size. Using larger coarse aggregate typically lowers the cost of a concrete mix by reducing cement requirements, the most costly ingredient. Less cement (within reasonable limits for durability) will mean less water if the water-cement (w/c) ratio is kept constant. A lower water content will reduce the potential for shrinkage and for cracking associated with restrained vo...