CE 308 Plain and Reinforced Concrete I

Mix Design of Concrete

Dr. Wasim Khaliq

Process of Mix Design

Consists of two interrelated steps:

1. Selection of the suitable ingredients (cement, aggregate, water, and admixtures) of concrete

2. Determining their relative quantities ("proportioning") to produce, as economically as possible, concrete of the appropriate workability, strength, and durability.

Basic Considerations

Economy– Cement is much more expensive than aggregate, it is clear that minimizing the

cement content is the most important single factor in reducing concrete costs Workability

– Properly designed mix must be capable of being placed and compacted properly with the equipment available.

– Finish-ability must be adequate, and segregation and bleeding should be minimized.

– A deaf ear should be turned to the frequent pleas from any job site for "more water."

Strength, and Durability– concrete specifications will require a minimum compressive strength– Specifications may also require that the concrete meet certain durability

requirements, such as resistance to freezing and thawing or chemical attack.

Economics of Mix Design

Goals: Complying with the Design considerations at minimum cost

How to reduce cost : Minimize the material cost Minimize cement cost Use the available materials

Minimum cement content can be achieved by Using lowest slump that will permit handling Using good ratio to coarse to fine aggregates Possible use of admixtures

Mix Design by ACI Method

The most common method used in North America (and elsewhere) - established by ACI Recommended Practice 211.1

Provide a first approximation of the proportions and must be checked by trial batches

Local characteristics in materials should be considered

ACI Method - Sequence of Steps

Pre-determined parameters, Steps 1 - 3

Pre-determined parameters, Steps 1 - 3

Estimation of Mixing Water and Air Content

Estimation of Mixing Water and Air Content

Determination of w/c – based on Strength

Table is based on OPC, for other cements use data obtained from experiments

Determination of w/c – based on Durability

Calculation of Cement or Cementitious Material Content

• Slump, w/c ratio, water content – Already decided• Cement or cement plus mineral admixture

− Simplest by dividing the estimated water requirement by the w/c ratio.• However, many specifications, in addition, require a minimum

cement content. Such a requirement may be used to ensure − Satisfactory finish-ability, − Quality of vertical surfaces, − Workability; − Ensure specified strength at the job site.

• When a portion of the cement is replaced by a mineral admixture, the replacement can be made on the basis of mass (weight) or volume, mass method is most commonly used.

• Since portland cement and the admixture have different specific gravities, the values of the weight and volume percentages will differ.

Cementitious Material Content

Cement Replacement of cement with mineral admixture (weight fraction)

Cement content

• Weight of pozzolan P = FwCT (As per desired weight replacement %age)

• Weight of cement C = (1 - Fw)CT

• CT = P+C = Total cement content (100% of Cement)Note: All common mineral admixtures have specific gravities lower than portland cement, so that when replaced on a mass basis, they occupy a larger volume than the cement they replace.

Estimation of Coarse aggregate content

Volume of Coarse Aggregate (CA) depends on max size and fineness modulus of fine aggregate. The OD weight of CA per m3 of concrete = value * dry rodded unit weight of CA in kg/m3

First Estimate of Fresh Concrete

Exact Calculation of Weight of Fresh Concrete

The weight of fine aggregate is then the difference between the total weight of the fresh concrete and the weight of the other ingredients.

Estimation of fine aggregate content Volume Method

Volume Method. This is the preferred method, as it is a somewhat more exact procedure, which requires a knowledge of the volumes displaced by the various ingredients.

The volumes of the cementitious material, water, air, and coarse aggregate are subtracted from the total volume; the difference is the volume of fine aggregate.

The weight of fine aggregate can then be obtained by multiplying this volume by the density of the fine aggregate.

Adjustment for Moisture in the Aggregates

Trial Batch

Trial Batch

ACI Mix Design Example

ACI Mix Design Example

10.2 10.3

ACI Mix Design Example

10.8

ACI Mix Design Example

10.9

ACI Mix Design Example

ACI Mix Design Example

ACI Mix Design Example

ACI Mix Design Example

ACI Mix Design Example

ACI Mix Design Example

ACI Mix Design Example

Required Average Compressive Strength

ACI Committee 301