chapter vi mix design of concrete - mert yücel yardimci · civ415 concrete technology chapter vi...

CIV415 CONCRETE TECHNOLOGY

Chapter VI

Mix Design of Concrete

Assist.Prof.Dr. Mert Yücel YARDIMCI

Advanced Concrete Technology - Zongjun Li 1

Mix Design


Mix design can be defined as the processes of selecting suitable ingredients and determining their relative quantities, with the purpose of producing an economical concrete that has certain minimum properties, notably workability, strength, and durability.

Mix design of concrete is frequently done by trial and error. Hence, mix design of concrete is an art, not a science.

Principal requirements for concrete


• Quality (strength and durability) • Workability • Economy

Weight method &

absolute volume method


In the weight method, the unit weight of fresh concrete is known from previous experience for the commonly used raw materials and is used to calculate the weight of the last unknown component of concrete, usually the sand.

If the weights of cement, water, coarse aggregate, and admixtures have been determined, then the weight of sand can be obtained from above equation. The unit weight of wet concrete usually ranges from 2300 to 2400 kg/m3.

Absolute volume method


when the weights of cement, water, coarse aggregate, and admixture have been determined, their corresponding volumes can be calculated, with their densities known.

Since the weight of each ingredient is easier to measure than the volume, the design proportion of concrete is usually expressed as a weight ratio. Hence, the proportion obtained in the volume method has to be converted to weight units by multiplying the volume with the density of the material.

Factors to be considered


• Maximum water/cement ratio • Minimum cement content • Projected strength • Projected workability • Maximum size of aggregate • Air content.

The projected strength is usually specified by the structural designer. Normally, the strength at 28 days is used as the design index for structural purposes, but other considerations may dictate the strength at other ages, e.g., formwork demolding time.

Water/cement ratio


where, A is compressive strength in Mpa (usually 96). B is a constant that depends mostly on the cement properties (usually 4).

Although the w/c ratio can be estimated from Abram’s law, based on projected strength of concrete, in concrete design, the w/c ratio required to produce a given mean compressive strength is usually determined from previously established relations for mixes made from similar ingredients, or by carrying out tests using trial mixes made with the actual ingredients to be used in the construction, including admixtures.

𝑓𝑐 =𝐴

𝐵1.5(𝑤/𝑐)

𝑓𝑐 =96

41.5(𝑤/𝑐)

Water/cement ratio


Back to example 1

Water/cement ratio


Durability


Severe exposure conditions require a stringent control of the w/c ratio because it is the fundamental factor determining the permeability and diffusivity of the cement paste and, to a large extent, of the resulting concrete. The adequate cover to embedded reinforcing steel is essential.

If the w/c ratio is determined due to durability requirements, the cement content can be reduced by the use of a larger-size aggregate.


Exposure classes given in EN-206 is recommended to consider in concrete design!


Back to example 1

Workability


Workability


Back to example 1

Workability


After choosing the workability, the water content of the mix (mass of water per unit volume of concrete) can be estimated by considering the workability requirement.

ACI 211.1-81 gives the water content for various maximum sizes of aggregate and slump value (as an index of workability), with and without air entrainment (see Table 3-8).

The values apply for well-shaped coarse aggregates and, although the water requirement is influenced by the texture and shape of the aggregate, the values given are sufficiently accurate for a first estimate.

Advanced Concrete Technology - Zongjun Li 19 Back to example 1

Cement type and content


The choice of the types of cement depends on the required hydration rate

• strength development • chemical attack • thermal considerations

It is needed to choose a cement with a high rate of heat of hydration developed for cold-weather concreting, and with a low rate of heat of hydration for mass concreting, as well as for concreting in hot weather. In the latter case, it may be necessary to use a lower w/c ratio to ensure a satisfactory strength at early ages.

Cement type and content


Because cement is more expensive than aggregate, it is desirable to reduce the cement content as much as possible, provided it can satisfy the strength, durability, and workability requirements. Moreover, low to moderate cement content confers the technical advantage of a lower hydration heat as well as cracking potential in the case of mass concrete, where the heat of hydration needs to be controlled, and in the case of structural concrete where shrinkage cracks should be minimized. The cement content has to meet the minimum requirement by specification from the durability considerations .

Major aggregate properties and aggregate content


Usually, the maximum size of aggregate is determined first, as it has a significant influence on concrete properties.

In reinforced concrete, the maximum size of an aggregate is governed by the geometry of the member and the spacing of the reinforcement.

The maximum aggregate size has to be smaller than 1/4 to 1/5 of the smallest size of the cross section of a member 3/4 of the net spacing distance of reinforcement.

The improvement in the properties of concrete with an increase in the size of aggregate does not extend beyond about 40 mm. For high-strength concrete, the maximum aggregate size is limited to 20 mm.

Major aggregate properties and aggregate content


Another important parameter of an aggregate is its grading. In all cases, dense-graded or well-graded aggregate is preferred, and uniformity has to be achieved.

In the case of a coarse aggregate, uniformity can be obtained relatively easier by the use of separate stockpiles for each size fraction.


The parameters for aggregate content in a concrete mix design that need to be decided include the total aggregate-to-binder ratio and the fine aggregate-to-coarse aggregate ratio. Table 3-11 provides the dry bulk volume of coarse aggregate per unit volume of concrete, which is expressed as a function of both fineness modules of the fine aggregate and the maximum size of aggregate. The mass of the coarse aggregate can then be calculated from the product of the dry bulk volume and the density (or unit weight) of the dry coarse aggregate.

Back to example 1

Fine aggregate content


The fine aggregate content per unit volume of concrete can be then estimated using either the mass method or the volume method. In the former, the sum of the masses of cement, coarse aggregate, and water is subtracted from the mass of a unit volume of concrete, which is often known from previous experience with the given materials. However, in the absence of such information, Table 3-12 can be used as a first estimate; adjustment is made after trial mixes.

Fine aggregate content


W = mixing water requirement, kg/m3

C = cement content, kg/m3

γ = specific gravity of cement (generally 3.10 for Portland cement)

A = air content, %

PROCEDURES FOR CONCRETE MIX DESIGN


• Sieve analysis results and fineness modulus of fine and coarse aggregate

• Dry-rodded density (unit weight) of coarse aggregate • Bulk specific gravity of each raw material • Absorption capacity or moisture content of the aggregates • Variation of the approximate mixing water requirement with

slump, air content, and grading of the available aggregates • Relationships between strength and water/cement ratio for

available combinations of cement and aggregate • Job specifications, if any, e.g., maximum water/cement ratio,

minimum air content, minimum slump, maximum size of aggregate, and strength at early ages (normally, 28-day compressive strength is specified)

Concrete mix design steps


Step 1: Choice of slump Step 2: Choice of maximum size of aggregate Step 3: Estimation of mixing water and the air content Step 4: Selection of water/cement ratio Step 5: Calculation of cement content Step 6: Estimation of coarse aggregate content Step 7: Estimation of fine aggregate content Step 8: Adjustment of amount of free water Step 9: Trial mixes

Adjustment of the design


Due to so many assumptions underlying the foregoing theoretical calculations, the mix proportions for the actual concrete have to be checked and adjusted by means of laboratory trials consisting of small batches (e.g., 0.02 m3 or 50 kg of concrete).

Fresh concrete should be tested for slump, cohesiveness, finishing properties, and air content, as well as for unit weight.

The specimens of hardened concrete cured under standard conditions should be tested for strength at specified ages.

Adjustment recommendations


If any property cannot meet the design requirement, adjustments to the mix proportions have to be conducted.

If the correct slump is not achieved, the estimated water content is increased (or decreased) by 6 kg/m3 for every 25mm increase (or decrease) in slump.

If the desired air content is not achieved, the dosage of the air-entraining admixture should be adjusted to produce the specified air content. The water content is then increased (or decreased) by 3 kg/m3 for each 1% decrease (or increase) in air content.

Adjustment recommendations


If the estimated density (unit weight) of fresh concrete by the mass method is not achieved and is of importance, the mix proportions should be adjusted, with allowance being made for a change in air content. If the projected strength cannot be met, w/b should be reduced at a rate of 0.05 for every 5 MPa.

Mix proportion adjustments After several trials, when a mixture satisfying the desired criteria of workability and strength is obtained, the mix proportions of the laboratory-size trial batch can be fixed and scaled up for producing large amounts of field batches.

Example


Concrete is required for a column that will be moderately exposed to freezing and thawing. The cross section of the column is 300 × 300 mm. The smallest spacing between reinforcing steel is 30 mm. The specified compressive strength of concrete at 28 days is 40 MPa with a slump of 80 to 100 mm. The properties of materials are as follows: (a) Cement used is type I Portland cement with a specific gravity of 3.15.

(b) The available coarse aggregate has a maximum size of 20 mm, a dry-rodded unit weight of 1600 kg/m3, a bulk specific gravity (SSD) of 2.68, absorption capacity of 0.5%, and moisture content (OD) of 0.25%.

(c) The fine aggregate has a bulk specific gravity (SSD) of 2.65, absorption capacity of 1.3%, a moisture content (SSD) of 3%, and a fineness modulus of 2.60. The aggregates conform to the ASTM C33-84 requirements for grading.


Step 1: Choice of slump. The slump is given and consistent with Table 3-7. Step 2: Maximum aggregate size. The maximum aggregate size is 20 mm, which meets the limitations of 1/5 of the minimum dimension between forms and 3/4 of the minimum clear space. Step 3: Estimation of mixing water and air content. The concrete will be exposed to freezing and thawing; therefore, it must be air entrained. From Table 3-8, the recommended mixing water amount is 180 kg/m3, and the air content recommended for moderate exposure is 5.0%. Step 4: Water/cement ratio (w/c). According to both Table 3-1 and Table 3-3, the estimate of the required w/c ratio to give a 28-day compressive strength of 40 MPa is 0.35. Step 5: Calculation of cement content. Based on the steps 3 and 4, the required cement content is 180/0.35 = 514 kg/m3.


Step 6: Estimation of coarse aggregate content. From Table 3-11, for fineness modulus of the fine aggregate of 2.60, the volume of dry-rodded coarse aggregate per unit volume of concrete is 0.64. Therefore, there will be 0.64m3 coarse aggregate in per volume concrete. And, the OD weight of the coarse aggregate is 0.64 × 1600 = 1.024 kg. The SSD weight is 1024 × 1.005 = 1029 kg. Step 7: Estimation of fine aggregate content. The fine aggregate content can estimated by either the weight method or the volume method. Volume method. Based on the known weights and specific gravity of water, cement, and coarse aggregate, the air volume, the volumes per m3 occupied by the different constituents can be obtained as follows:


Step 9: Adjustment for moisture in the aggregate. Since the aggregates will be neither SSD nor OD in the field, it is necessary to adjust the aggregate weights for the amount of water contained in the aggregate. Since absorbed water does not become part of the mix water, only surface water needs to be considered. For the given moisture contents, the adjusted aggregate weights become

chapter vi mix design of concrete - mert yücel yardimci · civ415 concrete technology chapter vi...

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