Aggregate concrete is made of inert granular materials, which include gravel, sand, crushed stone, cement and water. However, aggregates vary in qualities. A good quality should be strong, free from other compounds, strong and hard. Quality of aggregate is what determines whether it is a good surface preparation tools. Aggregate makes up 60 to 75 percent of concrete in volume. They are categorized into coarse and fine concretes. Fine concretes are products of crushed stones or sand. Either sand or crushed stones are sieved through 38 inch sieve to meet the requirements of fine concrete. On the other hand, course aggregate are those particles considered greater than 0.19 inch. Coarse aggregate is mainly composed of gravel and with smaller quantity of crushed stones.
There are different kinds of gravels used in making aggregate concrete. Natural gravel is dredged or dug from riverbanks, sea floor and along the lakes. Crushed gravel is a product of crushed cobbles, quarry rocks and gravels. Usually, only hard stones are crushed to make gravel. Concrete can also be recycled from used mixtures as long as they are clean and have the required granules. The type of concrete is determined by its use. Places where heavy commercial vehicles or cars use, have different concrete aggregate to those only used by human being.
After harvest, concrete is sorted, processed and then screened. The remaining material is washed and graded. For commercial purpose, the mixture is separated through gravitational floatation and jigging to enhance quality. Processed aggregates are stored in minimal segregation and degradation to secure them from any possible contamination. It is important to note that concrete grades determine their use and commercial pricing. Apart from grading based on granular properties, characteristic of aggregates such as hardened properties, proportion of the mixture and economy influence quality of concrete aggregate. Other factors considers in aggregate selection include, grading, durability, texture and shape of particles, moisture and absorption capacity.
Concrete grading depends on the distribution of aggregates and determination of particle size. It is vital in aggregates since it determines the use of aggregate, workability and durability of concrete. Despite the difference in grades, a good ratio of other concrete components, usually, result to a quality concrete and perfect surface preparation tools. However, when gap graded aggregate are stated, some particular size of aggregates are omitted. Gap grading is used to come up with uniform aggregate texture for some specific applications. However, close monitoring of mix proportions is vital to avoid particle segregation. The size of aggregate particles have a lot of impact on fresh concrete than hardened concrete.
Angular, rough textured and elongated particles consume more water to produce than rounded, smooth and compact textured aggregate. Water cement ration must as well be checked to come up with proper mixing ratio. In most cases, flat and elongated particles' ratio in concrete are maintained below 15 percent of the total aggregate. Concrete unit-eight is a measure of the volume of graded concrete and that occupied by the voids. When selecting aggregate, the rate of their water absorption is taken into consideration as this is determined by particle weight and the weight of voids.