Designing and Proportioning Normal Concrete Mixtures The process of determining required and specifiable char- acteristics of a concrete mixture is called mix design. Characteristics can include: 1 fresh concrete properties; 2 required mechanical properties of hardened concrete such as strength and durability requirements; and 3 the inclusion, exclusion, or limits on specific ingredients. Mix design leads to the development of a concrete specification. Mixture proportioning refers to the process of deter- mining the quantities of concrete ingredients, using local materials, to achieve the specified characteristics of the concrete.
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Tahir Rao 2-ACI In this summary, I will borrow heavily from the language, graphs, and tables of the manual to create a guideline for mix design of normal weight concrete that will be used for educational purposes at Stevens. For example, a bag of cement 94 lbs. If the cement was consolidated and without voids it would approximately occupy only 0. Therefore, the volume the cement will actually occupy in a batch of concrete will be its absolute volume of 0. Design for a strength of psi. The available materials, along with the data that we will need to design the mix, are given in the table below.
Choice of Slump If slump is not specified, use Table 6. In this example, for footings the slump should be between 1 and 3 inches. Use a slump of 3 for a mix that is more workable.
STEP 2. Choice of maximum size aggregate. The nominal maximum size of aggregate should be the largest that is economically available and consistent with the dimensions of the structure.
See full report for more information. In this example, the maximum size is 1 inch. STEP 3. Estimation of mixing water and air content. The quantity of water per unit volume of concrete required to produce a given slump is dependent on: the nominal maximum size, particle shape, and grading of the aggregates; the concrete temperature; the amount of entrained air; and the use of chemical admixures.
Table 6. Depending on aggregate texture and shape, mixing water requirements may vary above or below those shown in the table. Before you can use Tabel 6. Usually, if the concrete is exposed to freeze-thaw, as is the case of concrete placed in foundations, you will need air-entrainment. For our example, for a slump of 3 inches and a maximum aggregate size of 1 inch, use the air-entrained part of the table to find that you will need lbs of water per cubic yard of concrete.
The fine print under the table specifies that the water should be reduced by 25 lbs if you are using well rounded aggregates. Therefore, in this example we will use lbs of water.
STEP 4. Selection of Water-Cement or Water-cementitious materials ratio. In absence of such data, Table 6. The strength shown in the tables is for test specimens cured for 28 days in laboratory conditions. Calculation of cement content. Estimation of coarse aggregate content. The volume of coarse aggregate for one cubic yard of concrete is given in Table 6.
For an aggregate size of 1 inch and Fineness Modulus of Sand of 2. Estimation of fine aggregate content. At this point all the ingredients of the concrete have been estimated except the fine aggregates. The ACI committee report allows either the weight method 6. The second method is the most accurate and will be summarized here. Absolute Volume f3 Water 1 4. Adjustments for aggregate moisture.
The weights that we calculate above are dry weights. Absorbed water does not become part of the mixing water. However, the free water will add to the water content so we must account for it. The surface water contributed by the coarse aggregate is 1. See Table 6. Dry unit wt of coarse aggr. Maximum aggr. Structural considerations require it to have an average day compressive strength of psi. A slump of 3 to 4 inches is required.
aci 211.1 table 6.3.3 shared files: