How to Estimate and Order Ready Mix Concrete (RMC)?

Before ready mix concrete is ordered for your site, it is essential that you order it knowing the basic behind it. This won't let you order the wrong mix design and prevent any loss of money and material. Three important things behind this procedure is to:

    1. Estimation of Quantity
    2. Understand the strength, slump and type of mix required
    3. Ordering ready mix concrete
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    1. Estimation of Quantity of ready mix concrete

    In U.S, the concrete volume is ordered in cubic yards. Calculate the volume of the area of concreting. Calculate the volume by multiplying it with depth of structure. If there are several individual elements with different dimensions, determine their individual volumes and sum it up. 

    If, we need to concrete a floor of depth 0.33 feet, length 60 ft and width 4 feet, then volume of concrete = 0.33 x 60 x 4 = 79.2 cubic feet.
    1 cubic yards = 27 cubic feet
    Hence, the volume obtained in cubic feet is divided by 27 to get the volume in cubic yards i.e.
     79.2/27 = 2.93 cu. yards.

    Provide an extra of 5% to compromise for any spillage and uneven base construction.

    2. Understanding the strength, slump and type of mix of ready mix concrete

    Once the quantity of concrete is calculated, next step is know know the required strength and characteristics of the concrete to be delivered.

    a. Strength of Ready Mix Concrete

    Before ordering RMC, you must let the manufacture know the compressive strength of concrete. In general,
    For slabs, a compressive strength between 3500 to 4000 psi, for slabs on grade 3500 to 5000 psi and a strength of 3000 to 5000 psi is used for beams, slabs, girders, columns and walls. For pavement strength between 4000 to 5000 psi and for footings and walls, a strength of 2500 psi is enough. For outdoor home improvement a strength between 3500 to 4500 psi is enough.

    b. Aggregates for Ready Mix Concrete

    The size of the aggregates must be specified before ordering the ready-mix concrete. If we use large aggregates where the steel ratio is high, there are chances for honey combing. The maximum size of the coarse aggregates must be as specified by the American Concrete Standard (ACI) and it is dependent on the spacing of the reinforcing steel that are placed inside the formwork.
    Generally, the size of the aggregates must not exceed one fifth of the narrowest inside form dimension and must not be greater than three forth the spacing between the rebars. The structural engineer provides the guidance and the direction regarding the size of aggregates.

    c. Slump of Ready Mix Concrete

    Slump refers to the consistency of concrete that is to be placed. It is specified based on the spacing of the reinforcement, the type of formwork and the slope of the place. ACI standards specifies the following values of slump:
    1. A slump between 2 and 4 inches for R.C.C Foundation
    2. A slump of 1 to 4 inches for footings and caissons
    3. For beams, column, slabs and R.C. walls, a slump between 3 and 6 inch is employed.
    4. For pavement, between 2 and 3 inches
    5. For heavy mass concrete, slump between 1 and 3 inch.

    3. Ordering Ready Mix Concrete

    In US, generally ready mix concrete is ordered in cubic meters or cubic yards. A normal truck type would deliver 9 to 11 cubic yards maximum capacity concrete. When we do not order full capacity of concrete, we have to pay an amount for volume of truck not used. 
    The amount of concrete ordered is dependent on the place of construction and the forms used. 

    1. If the site is every distant apart, check whether you need to specify the addition of admixture to compensate the setting time of concrete
    2. The arrival time of concrete
    3. The mode of pumping concrete, pump or tailgate or wheelbarrow
    4. Air or non-air mix
    5. Contact persona and mix id number
    6. The purpose of the concrete.

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