Question

Why concrete mix proportions need to be designed? After completing the design mix process, explain what are the next steps to be done? If the same concrete mix proportions calculated in (a) is to be used for application near marine environment or in heavy industrial area, suggest with justification the possible modification that can be done.         

Answer 1

A good concrete mix design creates the foundation of a sound infrastructure.

Concrete mix design involves a process of preparation in which a mix of ingredients creates the required strength and durability for the concrete structure. Because every ingredient in the mix consists of different properties, it’s not an easy task to create a great concrete mix. It is imperative that all ingredients be tested to determine their physical properties and the bearing capacity of the project location.

The ingredients to be tested: water, fine aggregate (sand), coarse aggregate, cement, chemicals, reinforcement, and soil.

The values of the physical properties obtained after testing will be used as the basis for all concrete mix design considerations. This will ensure the structure will be sound and prevent failure of the mix. It is important to note that the ingredients for the mix might vary from one project location to another, so the physical properties must be tested for the requirements specified for each location.

Types of concrete mix

The two types of concrete mix are normal performance concrete and high-performance concrete, and they’re characterized by their compressive strength:

Normal Performance Concrete

A normal performance concrete mix has a performance strength ranging between 20 and 40 MPa. It is the more used mix of the two. Normal performance concrete has good workability if all of the mix ingredients are in accurate proportions. When freshly mixed, the concrete must be plastic or semi-fluid so that it can be molded.

High-Performance Concrete

A high-performance concrete mix has a performance strength above 40 MPa. The main purpose of using high-performance concrete is to reduce the weight, creep or permeability issues, and to improve the durability of the structure. Like the normal performance concrete mix, this mix must be plastic or semi-fluid when freshly mixed so that it can be molded.

Because high-performance concrete has a high cement content, it is often sticky and hard to be handled and placed. However, this does not cause the cement to bleed, an issue that normal performance concrete might run into.

After mix desin we check the properties of concrete by some test

1. Slump cone test to determine the workability of concrete

2. Compaction factor test is the workability test for concrete conducted in laboratory

3. Compressive strength of concrete.

Modifications of mix design when site near to the marine environment :

Marine environment are basically have cool climate , the evaporation of water is low compare to hot conditions. So water to cement ratio of concrete is decreases. And we use different types of admixtures

1. Water reducing admixture :

Water reducing admixtures, the name itself defining that they are used to minimize the water demand in a concrete mix. Workability is the important property of concrete which is improved with the addition of water but if water is added more than required the strength and durability properties of concrete gets affected.In addition to increase in workability it also improves the strength of concrete, good bond between concrete and steel, prevents cracking, segregation, honeycombing, bleeding etc.Water reducing admixtures are also called as plasticizers and these are classified into three types namely plasticizers, mid-range plasticizers and super plasticizers. Normal plasticizer reduces the water demand up to 10%, mid-range plasticizers reduce the water demand up to 15% while super plasticizers reduce the water demand up to 30%.Calcium, sodium and ammonium lignosulphonates are commonly used plasticizers. Some of the new generation super plasticizers are acrylic polymer based, poly carboxylate, multicarbovylatethers etc.

2.Accelerating Admixtures

Accelerating admixtures are used to reduce the initial setting time of concrete. They speed up the process of initial stage of hardening of concrete hence they are also called as accelerators. These accelerators also improves the strength of concrete in it early stage by increasing the rate of hydration.Earlier hardening of concrete is useful in several situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low temperature regions etc.Some of the accelerating admixtures are triethenolamine, calcium formate, silica fume, calcium chloride, finely divided silica gel etc. Calcium chloride is the cheap and commonly used accelerating admixture.

3. Air Entraining Concrete Admixture

Air entraining admixtures are one of the most important inventions in concrete technology. Their primary function is to increase the durability of concrete under freezing and thawing conditions. When added to concrete mix, these admixtures will form millions of non-coalescing air bubbles throughout the mix and improves the properties of concrete.Air entrainment in concrete will also improve the workability of concrete, prevents segregation and bleeding, lower the unit weight and modulus of elasticity of concrete, improves the chemical resistance of concrete and reduction of cement or sand or water content in concrete etc.Most used air entrainment admixtures are vinsol resin, darex, Teepol, Cheecol etc. These admixtures are actually made of Natural wood resins, alkali salts, animal and vegetable fats and oils etc.

By following this we modify the mix design of concrete.

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