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 Design mix and Nominal mix of concrete A  concrete mix  is a combination of five major elements in various proportions: cement, water, coarse aggregates, fine aggregates (i.e. sand), and air. Nominal Mix These mixes are fixed cement aggregate ratio which ensure adequate strength to the concrete. In the fig. M5, M7.5, M10, M15, M20 and M25 are nominal mixes. Due to variability in mix, they differs in workability and compressive strength. The ratio of mix is required when we estimation of concrete. Design Mix In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. In the fig. M30 and above are design mixes.

Compressive Strength of Concrete Cylinders The most common sample type for tests of concrete compressive strength is the concrete cylinder. There are other ways to determine concrete strength, and some methods may be more cost-effective and arguably better, but concrete cylinder testing remains the standard for acceptance. Well-made concrete cylinders are easy and inexpensive to produce. Typically, one technician posted at the point of discharge or point of placement is sufficient to sample and test concrete batches for the slump, air content, unit weight, and cylinders. Technicians must follow standard practices and be certified in most regions, but training is neither extensive nor complex. To determine the compressive strength of concrete cylinders. Apparatus  1) Compressive strength machine Procedure  1) Take out the cylindrical specimen from the curing tank.  2)Wipe out the excess water from the surface of the specimen. 3)Place the specimen vertically on the platform...

  ADMIXTURES Admixtures are the special ingredients added during concrete mixing to enhance the properties and performance of fresh concrete. Various types of admixtures are available in the market which is used in construction work. FUNCTION OF ADMIXTURES : 1. To accelerate or retard the setting time of fresh concrete. 2. To improve the workability or flowability of concrete. 3. To increase the strength and durability of concrete. 4. To reduce the heat of hydration. 5. To reduce the segregation and bleeding. 6. To decrease the permeability. 7. To achieve other desired properties. TYPES OF ADMIXTURES: 1. ACCELERATING ADMIXTURES : This admixture is added in concrete or mortar for increasing the rate of hydration of hydraulic cement and for shortening the setting time. Calcium chloride (CaCl2) is the most widely used accelerating admixture. 2. RETARDING ADMIXTURES: Retarding admixtures delay the initial rate of hydration of cement and extend the setting time of cement paste. This ad...