Concrete mixture design is done with the objective of creating a balance between strength, durability, placeability and aesthetic conditions, and low cost.

Concrete mixture design consists of 2 main stages:

(1) Selection of suitable components (cement, aggregate, water, and additives)

(2) Calculating the ratio of these components in order to obtain the most economic concrete possible with optimal strength, durability, and workability.

Strength is the most important characteristic property of concrete. There is a one-to-one relation between water/cement ratio and strength. As long as concrete components’ properties and environmental conditions remain fixed, strength decreases as water/cement ratio increase, and strength increases are this ratio decreases.

In addition, concrete strength for a fixed water/cement ratio is affected by;

– The largest aggregate size,

– Aggregate grain size distribution (granulometry), shape and surface ruggedness,

– Type of used cement,

– Air amount in the concrete,

– Type and amount of the used additives.

# How to Determine Concrete Mixture Ratios?

Determination of concrete mixture ratios is done based on volume. In 1 m^{3} compressed concrete, the amounts of mixture components are calculated with the below formula.

Where;

c : Mass of cement to be entered in the mixture (kg)

p : Mineral additive amount to be used as an addition to the cement in the mixture (kg)

k : Chemical additive amount to be used in the mixture (kg)

ρ_{c} : Cement density (kg/dm^{3})

ρ_{p} : Mineral additive density (kg/dm^{3})

ρ_{k} : Chemical additive density (kg/dm^{3})

W : Volume of water to be entered in the mixture (dm^{3})

Wa : Amount of aggregate to be entered in the mixture (kg)

ρ_{a} : Aggregate’s average specific mass (kg/dm^{3})

A : Total air amount in the concrete (%)

## How to Select the Water / Cement Ratio (W/C)?

Water/cement ratio is related to strength class of the concrete and the severity of external effects it will be exposed to (environmental effect classes).

As per TS 13515 and/or TS EN 206 Chart F.1, the environmental effect class for the environment the concrete will be located in should be determined and parameters such as least cement dosage conforming to this class, lowest characteristic compressive strength, and highest w/c ratio **(Chart.1)**.

**Chart 1.** Limit values suggested for concrete composition and properties (TS 13515 Chart F.1)

## How to Decide on Target Compressive Strength?

Target compressive strengths to be used in concrete mixture design are given in Chart 2 according to the concrete classes and W/C ratios depending on 28-day compressive strengths are given in Chart 3 and Figure 1.

**Chart 2 **– Target compressive strengths (fcm) to be taken as basis in mixture calculations according to concrete classes and average compressive strengths the test samples must have

**Chart 3 – **Approximate W/C ratios according to 28-day concrete compressive strengths

**Figure 1 **– Graphical display of the approximate relation between W/C ratio given in Chart 3 and compressive strength. This graphic may be used for strength values belonging to different W/C ratios not included in Chart 3.

## How to Decide on the Water Amount in a Concrete Mixture?

The amount of water in the concrete mixture changes depending on viscosity, the largest aggregate grain size and whether the concrete has chemical additives or air entrained.

Plasticizer chemical additives are used to provide plasticity in the concrete and to decrease water amount. Used chemical additive type and effectiveness degree significantly affect mixture water amount in the concrete.

Figure 2 presents the approximate mixture water amounts of concrete which contains crushed stone aggregate, without chemical additive and non-air entrained. Approximate mixture water amounts depending on air entrainer additive use and other aggregate type are given in TS 802. When chemical additives are used in producing concrete, additive-added concrete mixture water amount is determined with a certain amount of water decrease from the mixture water amounts found in the graphics depending on the chemical additive’s type.

**Figure 2 **– Approximate mixture water amounts for non-air entrained and without chemical additive concrete for concrete using crushed stone aggregate with different largest aggregate grain sizes and different settling values

## How to Decide on the Cement Amount?

After water/cement ratio and water amount are determined, the cement amount to enter in the mixture is calculated with the below formula.

Where:

c : Mass of cement to be entered in the mixture (kg)

s : Mass of water to be entered in the mixture (kg)

s/c : Water/cement ratio.

Other than this, cement amount can be selected as an estimated value determined based on experience in the beginning. Cement density value should be taken from cement test report. The concept of k-value given in TS EN 13515 should be used in case fly ash or blast furnace slag are used in concrete mixture.

## How to Decide on the Air Content of Concrete?

The air amount to enter the concrete mixture is determined by the envisaged aggregate largest grain size, grain size distribution and climate conditions (Figure 3).

**Figure 3** – Total air contents to be used in the concrete mixture calculations depending on aggregate largest grain size and climate conditions

## How to Decide on the Aggregate Amount?

The remaining volume from cement, water, chemical and mineral additives, and air in the concrete mixture is filled with aggregate. And aggregate volume is calculated using the following formula.

In order to calculate the mass of total aggregate to be used in 1 m^{3} concrete, specific mass ρa belonging to each grain class should be determined.

Here, Ma gives the total aggregate mass entering into 1 m^{3} concrete mixture and masses of each aggregate grain class (M1, M2, M3 and …….Mn) are determined by multiplying with aggregate mixture ratios (x1, x2, x3 and ………xn).

**How to Select the Aggregate Grain Size Distribution?**

Aggregate grain size distribution to be used in the concrete mixture should be selected to be in the regions number 3 and number 4 (Figure 4, Figure 5). Grain distributions to fall into region number 3 is preferred as it is the suitable region.

**Figure 4 **– Limits of aggregate grain size distribution curve determined for the concrete with aggregate largest grain size of 16.0 mm

**Figure 5 **– Limits of aggregate grain size distribution curve given for the concrete with aggregate largest grain size of 32.0 mm.

## How to Calculate Moisture Correction in Aggregates?

As reference specific mass values used for aggregates are determined as saturated dry surface (SDS), the found aggregate amounts are also SDS values.

Aggregates are not usually in the saturated dry surface (SDS) state when preparing concrete mixtures and their moisture states should be checked continuously and determined in regular intervals. Moisture correction should be made according to the moisture (R) and water absorption (Se) values of the aggregates as given below.

The result of Se – M = …..

The difference between these values is assessed as below;

If ( + ) material is “AIR DRY”

If ( – ) material is “WET”

If ( 0 ) material is in the state of Saturated Dry Surface “SDS”.

Corrected mixture water amount and aggregate moisture correction for each aggregate class is calculated as described in TS 802.

Calculations are made with the formulas.

## Verification of Mixture Calculations Through Tests

The limit values given in TS 802 for grain distribution, w/c ratio and water amount which significantly affects concrete properties, and which are taken as basis for mixture calculations are values obtained from the results of many tests and they are not indefinite values.

Therefore, the concrete samples to be prepared using aggregate, water, cement, air, and additive amounts obtained as the result of mixture calculations should be tested and the results to be obtained should be assessed in terms of whether they have the properties taken as the basis of the calculations. In case a difference is identified between the envisaged properties and properties found at the test, the mixture calculations should be repeated by changing the inputs as required.

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