Mix design for concrete
concrete mix design can be defined as the process of selecting type and proportions of raw materials to produce a concrete with the required properties such as strength and durability. The strength of the concrete will decide by designer, concrete delivered at the site should meet the designer requirement because the design calculation will be based on specific concrete properties, also designer will determine fresh concrete properties, fresh concrete properties will be governed by the type of structure and method of placement.
the main purpose of a concrete mix is to produce concrete meeting a predetermined requirement. the requirement as follows
- strength and durability: concrete strength and permeability are related to each other, increasing of W/C will reduce concrete strength and increase concrete permeability which mean decreasing durability, in general concrete mix design focus on concrete strength and workability, concrete durability which affected mainly by permeability should be considered in aggressive environment, the ingress of aggressive materials such as chloride could lead to steel rusting this will be followed by concrete cracking and spalling.
- workability: workability is an important property of fresh concrete because it is reflecting the ease of placing and compacting concrete, tests are used in determining concrete workability are not accurate, in general, slump test used to evaluate concrete workability.
- Economy: produced concrete shall be economic, the most expensive ingredients of concrete is admixture followed by cement, admixture used in small quantity, therefore cements quantity shall be reduced without scarifying of strength and quality of concrete. fly ash and GGBS or silica fume can be used to replace part of cement
Procedure for concrete mix design:
Here I will explain ACI method
- before beginning with concrete mix design, type of cement to be used in concrete mix shall be determined, cement type may vary depends on environment and structure type.
- the first step, choosing of a slump, a suitable value of slump that fit the structure shall be selected from Table 1
- selection of the maximum size of aggregate, well-graded aggregate with large maximum size will produce a mix with less void for the same volume of aggregate with a smaller size, reducing void will reduce mortar in a unit volume of concrete, maximum size aggregate shall be less than 1/6 of the narrowest structure member, also less than 1/3 of structure slab and less than 3/4 of reinforcement bar minimum clear spacing.
- Estimation of mixing water and air content: water content depends on aggregate shapes and grading, particle sizes and slumps if this information not available table 2 can be used to determine water content with or without air entrained
- selection of W/C, from table 3 we can select w/c based on concrete compressive strength for non-air-entrained and air-entrained concrete. w/c shall comply with exposure condition so the selected w/c shall be verified using table 4
- after computing w/c and water content, cement content can be calculated by dividing water content by w/c.
- Estimation of coarse aggregate, coarse aggregate content shall be increased as much as possible, several tests have shown that using a well-graded aggregate the larger size and the finer sand the produced concrete mix will have satisfactory workability and strength. dry bulk volume of coarse aggregate can be determined using table 5, a dry bulk volume of coarse aggregate depends on the maximum size of aggregate and fineness modulus of sand
- Estimation of fine aggregate content. At the completion of step 6, all the ingredients of the concrete have been estimated except the fine aggregate. The amount of fine aggregate can be determined by either the weight method or volume method. According to the weight method, if the unit weight of the wet fresh concrete is known from previous experience, then the required weight of fine aggregate is simply the difference between the unit weight of concrete and the total weight of water, cement, and coarse aggregate. In the absence of a reliable estimate of the unit weight of concrete, Table 6 can be used as a guide to choosing the unit weight of fresh concrete.
In the absolute volume method, the total volume displaced by the known ingredients (i.e., water, air, cement, and coarse aggregate) is subtracted from the unit volume of concrete to obtain the required volume of fine aggregate. This, in turn, is converted to weight units by multiplying by the density of fine aggregates.
- adjustment amount of free water: aggregates assumed to in SSD condition, in general, aggregate stock is an unbalanced condition, aggregates may air-dry or wet condition, so it will absorb or increase the water content, also using the mixture in liquid condition extra water will be supplied, if those conditions and other conditions are not considered w/c will be wrong, therefore adjustment shall be done for water content to consider these conditions.
- Trail mix: after the completion of the design of concrete mix, the trial mix should be done to verify the concrete properties, if the mix failed, concrete shall be re-design and test again until all tests are passed.