Details about nominal mix and design mix

Definition for Nominal Mix: The concrete is extensively applied as construction materials and several mixes of fixed ratio are now being used with concrete to retain adequate strength. These mixes are known as nominal mixes.

Under normal situation, it contains margin of strength over that specified.

Nominal mix concrete is applied for concrete of grades M5, M7.5, M10, M15 and M20.

Ratios of ingredients in nominal mixes

The ratios of materials for nominal mix should be as follow:-

Grade : Ratios (C, FA, CA)
M5 : 1:5:10
M7.5 : 1:4:8
M10 : 1:3:6
M15 : 1:2:4
M20 1:1.5:3

With 1:1.5:3 ratio, it signifies that these should be measured as 1 kg of cement, 1.5 kg of fine aggregate and 3 kg of coarse aggregate.

But, usually they are selected on the volume basis and multiplied with 1.55 constant (in Nominal Mix Concrete Calculation) to obtain the bulk volume of the material.

Design Mix Concrete: The concrete mix created under quality control on the basis of the strength, durability and workability is known as the design mix.

Prior to arrive at the mix ratio, several other factors should be taken into consideration which range from accessibility of equipment for compaction, curing process selected, type of cement, quality of fine and course aggregate etc.

The design mix or controlled mix is extensively utilized for different types of vital structures due to superior strength; curtailed inconstancy; leaner mixed with consequent economy and improved quality.

In design mix, the concrete is formed in weight basis especially for greater volume where the load is vital. It is required to examine each and every property of the ingredients mentioned below.

Weight of each ingredient
• Brands
• Mix Proportions
• Type of exposure
• Properties of Cement: Initial & Final Setting Time, Specific gravity, Cement Grades
• Properties of Fine & Coarse Aggregates - Particle Size, Silt Content, Unit Weight, Fineness Modulus, Crushing Value

On the basis of the above things, the mix design will be set up as per requirements and then examined with trail mix. As soon as the trial mix is examined and passed on 7th & 28th Days for compressive strength, then it will be ready for batching and execution at the site level.



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Published By
Rajib Dey
www.constructioncost.co
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Properties and benefits of rebar cages

In drilled shaft construction, rebar cages are generally utilized to reinforce the shaft throughout excavation. To retain the strength of this cage as well as complete the entire construction project successfully, the design of the cage should be perfect.

Normally, a rebar cage for a drilled shaft comprises of longitudinal bars which are allocated with uniform spacing along the boundary of a cylinder.

In order to reinforce these bars, steel is arranged transverse to the bars and connected with ties, clamps or welds. Other elements of rebar cages may contain hoops for sizes, guides for centering the cages in the borehole and the premie inside of the cage, and stiffeners and pickup devices applied to facilitate lifting the cages.

Larger cages should contain temporary or permanent strengthening components to get rid of permanent distortion due to the stresses of lifting and placing.

As rebar cages are vital drilled shaft construction, it is essential that these cages should be properly constructed on the basis of a calculation of the stresses that it will endure.

The amount of reinforcing steel in a rebar cage should adhere to the structural requirements, taking into account combined stresses of axial load, lateral load and moment.

Properties of Steel Used for Rebar Cages

The American Society for Testing and Materials (ASTM) defines various steels which are employed for reinforcing drilling shafts.

The American Association of State Highway and Transportation Officials (AASHTO) approves most of these ASTM steels for being applied in building rebar cages for drilled shaft construction.

Normally, the steel accessible for these cages is AASHTO M 31 (ASTM A 615) in Grade 40 or Grade 60. If welding is required, then weldable steel, like ASTM A 706, should be utilized.

In case, there is risk for corrosion, galvanized or epoxy-coated steel are useful for longitudinal and transverse reinforcement. This is often stated for marine environments where the chloride content of ground or surface water is extreme.

As nicks and blemishes in the coating may happen at the time of the lifting and disposition of the rebar cases, rapid corrosion may occur. Under this situation, rebar without epoxy should be utilized and the drilled shaft should be filled with a low-permeability concrete to enhance the resistance strength against corrosion.

In unusual situations, high strength reinforcement is suitable. It may comprise of threaded couplers for splice connections and higher-strength rebar.

Contractors should compute the structural requirements of a drilled shaft cautiously while deciding the requirements of a rebar cage.

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Published By
Rajib Dey
www.constructioncost.co
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