Concrete, An In-depth Study » Concrete Mixes Explained

What are concrete mixes?

Concrete mixes are composed of five major components in various proportions: cement, water, coarse aggregates, fine aggregates (i.e. sand), and air.

Chemical admixtures and pozzolanic materials can also be incorporated into the mix to give it certain desired properties.

A concrete mix design, on the other hand, is the process of choosing elements for a concrete mixture and determining their proportions. Always keep the intended strength, durability, and workability of the concrete for the project in mind while designing a mix.

Needless to say, all ready-mix concrete companies attempt to find the ideal amounts of these materials in order to improve their concrete mixes and offer them strength, durability, workability, and other desirable characteristics. To assure the lowest cost while retaining the best strength of your mixture, it’s critical to optimize concrete.

This is far from simple, as every addition or subtraction to the concrete mix necessitates component modifications, making the process inefficient and difficult.

Design of Concrete Mix

The term “cement mix design” is frequently used mistakenly with “concrete mix design.” Cement, on the other hand, is just one of the elements in concrete. It’s a binding agent that helps concrete set, harden, and stick to other surfaces. As a result, it cannot and should not be confused with concrete mix design.

Concrete Mixes Calculation

All guidelines and conducts of calculations need to follow the ACI standard for concrete mixes.

How to Design a Concrete Mixture

In general, concrete mixtures must conform to the guidelines (ACI Committee, 2009). The tables and calculations supplied in the standard can be used to create a concrete mix.

Because each concrete mix has its own set of characteristics, the design procedure can be time-consuming and difficult. 

On the other hand, there are various concrete apps that overcome the difficulties of producing a custom concrete mix. Nowadays, there are various commercial and free apps that are useful in the determination of concrete mix designs.

Step 1. Slump Flow

The maximum and minimum slump for the fresh mix qualities must be defined in the initial step of the application.

The flowability/workability of a concrete mix is represented by the concrete slump. A higher slump, for example, enables better positioning in congested reinforced elements. The guideline is based on the ACI standard’s Table 6.3.1 as shown below.

Step 2: Size of the Aggregate

You’ll also need to figure out what size aggregate you’ll need for the mix design.

In general, the maximum dimension of coarse aggregate is limited by the cross-sectional and reinforcement design parameters of the structure.

The increased aggregate size is normally more cost-effective since it reduces the amount of cement used per unit of volume, but it may affect the mix’s workability. Reducing the maximum size of the coarse aggregate, on the other hand, permits your concrete mix to achieve better strength at the same water-cement ratio.

Based on the limitations of Table 6.3.3, suggests alternate aggregate sizes as shown below.

When a particular amount of coarse aggregate is used per unit volume of concrete on an oven-dry rodded basis, concrete with nearly the same nominal aggregate maximum size and grading will achieve sufficient workability.

Table 6.3.6 provides appropriate values for this aggregate volume. It can be shown that the volume of coarse aggregate in a unit volume of concrete is only determined by its nominal maximum size and the fineness modulus of the fine aggregate for equivalent workability. Due to changes in particle shape and grading, variances in the amount of mortar required for workability with different aggregates are automatically compensated for by differences in oven-dry-rodded void content.

On an oven-dry-rodded basis, the volume of aggregate in ft³ for a yd³ of concrete is equal to the value from Table 6.3.6 multiplied by 27. By multiplying this volume by the oven-dry-rodded weight per ft³ of coarse aggregate, it is converted to the dry weight of coarse aggregate required in a yd³ of concrete.

Step 3: Mixing Water and Air Content

Based on the slump flow and aggregate size, you now have a first estimate of how much water you’ll need to get the right workability for your mix.

For air-entrained concrete, the amount of entrapped air required must be calculated.

When the concrete structure is exposed to freezing or de-icing salts, the entrapped air is a critical metric. Increased air content increases concrete durability in these conditions because it permits water to expand in the entrapped air when it freezes. The internal pressure induced by the formation of ice is reduced as a result.

Based on the ACI committee’s recommended values, the calculation of the water weight and amount of entrapped air required. shall be based upon Table 6.3.3 as a reference showing from step 3.

Step 4: Water/Cement Ratio and Concrete Strength

The water/cement ratio is the most significant factor in concrete mix design; it determines the concrete mix’s strength, durability, and workability. You’ll need to fill in the appropriate compressive strength and the water/cement ratio here.

Reducing the water/cement ratio, for example, will improve the strength and durability of the concrete. However, lowering the water/cement ratio can considerably diminish the concrete’s workability. In these conditions, adding a water reducer to the mix could be a viable option (see Step 7).

You can set the desired compressive strength and retrieve the associated water/cement ratio using Table 6.3.4(a) ACI 211.1-91. In addition, based on the structural exposition, you will be given parameters for the maximum allowable water/cement ratio (Table 6.3.4(b)).

Step 4.1 Pozzolanic Materials

This phase also allows you to incorporate pozzolanic elements into the mix, such as fly ash, silica fumes, or slag.
It is more environmentally friendly and cost-effective to use pozzolanic material to replace a portion of the cement. It usually slows down the curing process and gives the concrete better properties.
You have the option of selecting your favorite method of calculation. A new adjusted water/cementitious material ratio, amount of pozzolanic material, and adjusted cement weight will be computed based on the specific gravity of the pozzolanic material.

Step 5 Coarse Aggregate

You must now define the coarse aggregate unit weight, finesse modulus, and coarse aggregate volume per volume of concrete.

You will get the amount of coarse material you’ll need.

You can also choose the coarse aggregate size and the modulus of fine aggregate finesse, and then calculate the volume of oven-dry-rodded coarse aggregate based upon the Table 6.3.6 data.

This table is based on the concrete’s workability.

Step 6 Fine Aggregate

Depending on the method of computation (per weight or per volume), the amount of fine aggregate is calculated differently.

The volume procedure determines the amount of fine aggregate using 1 yd³ (1m³) of concrete, whereas the weight method uses an estimate of the concrete weight.

The first estimate of concrete weight can be determined using ACI Table 6.3.7.1 depending on the type of concrete (non-air-entrained or air-entrained).

For the final calculations, you now have the determined quantity of fine aggregate required for the planned concrete mix.

Step 7: Moisture in Aggregates Adjustment

Based on the moisture content and degree of moisture absorption of coarse and fine aggregates, the final step in the calculations modifies the amount of water in the mix design.

Because the quantity of water the aggregates supply to the mix and take away from it creates variance in the water/cement ratio, it’s critical to look at it.

Chemical admixtures such as water reducers can further lower the amount of water.

Step 7.1 Chemical Admixtures

You can add a water reducer, air-entrained admixtures, or other chemical admixtures to the mix composition during this phase.

Using a water reducer allows you to maintain a steady water/cement ratio while reducing the cement ratio without sacrificing strength or enhancing workability.

When aiming to improve the durability and workability of a concrete mix, air-entrained admixtures can be quite effective.

Step 8: Design Summary

Obtaining your summary report of your findings is the final step in the process. This is composed of the details of the concrete mix, as well as the quantities of each item needed for the desired volume of concrete.