The topics that are covered in this article are,
I. Objectives of concrete mix design
II. Detailed procedure of concrete mix design as per IS 10262:2009
III. Computer Aided Design using CeSoftMix software tool
The objective of proportioning concrete mixes is to arrive at the economical and practical combinations of different ingredients to produce concrete that will satisfy the performance requirements under specified conditions of use. The concrete of different qualities can be obtained by using its ingredients namely cement, water, fine and coarse aggregates, mineral additives, in different proportions. Also, the ingredients of widely varying characteristics can be used to produce concrete of acceptable quality. These concrete mixes can be classified into two types, Nominal mix and Design mix.
1. Nominal mix The wide use of concrete as construction material has led to the use of mixes of fixed proportions. These mixes are known as Nominal mixes. However, these nominal mixes do not account for varying characteristics of the constituents and may result in under or over rich mixes. That’s why they are used for relatively unimportant and smaller concrete works of grade M-20 or lower.
2. Design mix: The mix which is obtained by the process of selecting suitable ingredients of concrete and determining their relative quantities with the purpose of producing an economical concrete which has sufficient workability, compressive strength and durability. The required properties of fresh concrete are governed by the methods of placing and transporting and the properties of hardened concrete are specified by the designer of structure, these two sets of requirements will be fulfilled by the use of Design mixes. That’s why they used for important and major projects.
An integral part of concrete mix proportioning is the preparation of trial mixes and if necessary, make the adjustments to such trials until a satisfactory mix has been obtained.
For many works it is desirable to go through the process of mix design, for example, where a large volume of concrete is required, a minimization of the cement content may reduce the cost appreciably or where for technical reasons the type of concrete required necessitates careful selection and proportioning of ingredients.
Most of the available mix design methods are based on empirical relationships, charts and graphs developed from extensive experimental investigations. Some of the commonly used mix design methods are the following
1. Trial and adjustment method of mix design
2. DoE (British) mix design method
3. ACI mix design method
4. Concrete Mix Proportioning according to Indian Standard Guidelines
5. Rapid method for mix design.
IS 10262:2009 - Indian Standard Concrete Mix Proportioning Guidelines, this standard provides the guidelines for proportioning concrete mixes as per the requirements using the concrete materials including other supplementary materials identified for this purpose. It is applicable for ordinary and standard concrete grades only.
The following data are required for mix proportioning of particular grade of concrete:
a. Grade designation
b. Type of cement
c. Maximum nominal size of aggregate
d. Minimum cement content e. Maximum water-cement ratio
g. Exposure conditions
h. Maximum temperature of concrete at the time of placing
i. Method of transporting and placing
j. Early age strength requirements, if required
k. Type of aggregate
l. Maximum cement content
m. Whether an admixture shall or shall not be used and the type of admixture
For the given data, the following step by step procedure has to be applied.
i. Calculation of target mean strength (ft)
It is determined by using the relation
ft = fck + kS
ft is the target mean compressive strength at 28 days
fck is the characteristic compressive strength at 28 days
k is a standard coefficient (k=1.65 as per IS 456:2000)
S is a standard deviation
The standard deviation for each grade of concrete (fck) shall be calculated separately. The total number of test samples for the calculation of standard deviation shall not be less than 30. Where sufficient test results for a particular grade of concrete are not available, the value of standard deviation given in Table no.1 may be assumed for the calculation of target mean strength.
Table no. 1. Standard deviation values
[Source: IS 10262-2009]
ii. Selection of water-cement ratio
The Different cements, supplementary cementitious materials and aggregates of different maximum size, grading, surface texture, shape and other characteristics may produce concretes of different compressive strength for the same free water-cement ratio. Therefore, the relationship between strength and free water-cement ratio should preferably be established for the materials actually to be used. In this context, the selection of water-cement ratio may be based on the designer’s experience. In the absence of such data, the preliminary free water-cement ratio corresponding to the target mean strength at 28 days may be selected from the established relationships (refer figure no. 1), if available. Otherwise, the water-cement ratio given in Table-5 of IS 456 may be used as starting point. However, selected free water-cement ratio should be checked against the limiting water-cement ratio (Table 5 of IS 456) for the requirements of durability and the lower of the two values will be adopted.
Figure no. 1. Compressive strength Vs Water-cement ratio relationships
[Source : IS 10262-1982]
iii. Selection of water content
The water content of concrete is influenced by a number of factors, such as aggregate size, aggregate shape, aggregate texture, workability, water-cement ratio, cement and other supplementary cementitious material type and content, chemical admixture and environmental conditions. An increase in aggregates size, a reduction in water-cement ratio and slump, and use of rounded aggregate and water reducing admixtures will reduce the water demand. On the other hand increased temperature, cement content, slump, water-cement ratio, aggregate angularity and a decrease in the proportion of the coarse aggregate to fine aggregate will increase water demand. The quantity of maximum mixing water per unit volume of concrete for the given nominal maximum size of aggregate may be determined from Table no. 2. This water content value is for angular coarse aggregate and for 25-50 mm slump range.
Table no. 2. Water content
[Source : IS 10262-2009]
If the shape of aggregate or slump value is differing from above, then the following adjustments has to be carried out in order,
The water content in Table no.2 can be reduced approximately by 10 kg for sub-angular aggregates, 20 kg for gravel with some crushed particles and 25 kg for rounded gravel to produce same workability. The modified water content value will be used in further adjustments or calculations.
For the desired workability (other than 25-50 mm slump range), the required water content may be established by trial or an increase by about 3 percent for every additional 25 mm slump. For example, 6 percent increase for the slump of 75-100 mm. The modified water content value will be used in further adjustments or calculations.
Water reducing admixtures like plasticizers and super-plasticizing admixtures usually decrease water content by 5 to 10 percent and 20 percent and above respectively at appropriate dosages in trials.
iv. Calculation of cementitious material content
The cement content per unit volume of concrete may be calculated from the free water-cement ratio and the quantity of water per unit volume of concrete. The calculated cement content shall be checked against the minimum cement content for the requirements of durability (Table no. 3) and greater of the two values will be adopted. The maximum cement content shall be 450 kg/m3 in accordance with Clause 188.8.131.52 of IS 456.
If fly ash or ground granulated blast furnace slag is used as supplementary material, percentage of supplementary material is based on project requirement and quality of materials. In certain situations increase in cementitious material content may be warranted. The decision on increase in cementitous material content and its percentage may be based on experience and trial. In this case, the maximum cementitious material content shall be 500 kg/m3.
Table no. 3. Exposure conditions and limiting values
[Source : IS 456-2000]
v.Estimation of Total Aggregate Volume
The absolute volume of total aggregate (coarse and fine) is calculated by following relation,
Va = 1 – ( Vc + Vw + Vadm )
Vc = absolute volume of cementitious material, m3
Vw = absolute volume of water, m3
Vadm = absolute volume of admixture, m3
These values can be determined by dividing their mass by their respective specific gravity, multiplying by 1/1000.
The mass of admixture will be calculated as the percentage of cementitious material content. For example, 2 percent of cement.
According to IS 10262:2009, the volume of air content has been neglected.
v. Estimation of coarse aggregate and fine aggregate proportions in total aggregate volume
The coarse aggregate used shall be confirmed to IS 383. Coarse aggregates of different sizes may be combined in suitable proportions so as to result in an overall grading confirming to Table 2 of IS 383 for particular nominal maximum size of aggregate. Based on nominal maximum size of aggregate and grading zone of fine aggregate, the volume of coarse aggregate per unit volume of total aggregate is determined from Table no. 4 for a water-cement ratio of 0.5. If the selected water-cement ratio is differing from 0.5, volume of coarse aggregate is required to be increased or decreased. For every increase in water-cement ratio by 0.05, the volume of coarse aggregate will be reduced by 0.01. Similarly, for every decrease in water-cement ratio by 0.05, the volume of coarse aggregate will be increased by 0.01. For example, if the selected water-cement ratio is 0.40, as the water-cement ratio is lower by 0.10, the proportion of volume of coarse aggregate is increased by 0.02.
Table no. 4. Volume of coarse aggregate per unit volume of total agggregate
[Source : IS 10262-2009]
When the method of placing of concrete is by pumping or when the concrete is required to be worked around congested reinforcing steel, it is desirable to reduce the volume of coarse aggregate up to 10 percent.
As the coarse aggregate proportion is already determined, the remaining proportion will be the fine aggregate proportion. For example, if the coarse aggregate proportion is 0.64 then the fine aggregate proportion will be 0.36 in total aggregate volume. The zone of the given fine aggregate shall be confirmed to Table 4 of IS 383.
vi. Estimation of coarse aggregate and fine aggregate contents
The coarse and fine aggregate contents are determined by multiplying respective volumes with their respective specific gravities and multiplying by 1000.
The total volume of aggregate, Va
The coarse aggregate volume in total aggregate, Vca
The fine aggregate volume in total aggregate, Vfa
The coarse aggregate content = Va X Vca X Specific gravity of CA X 1000
The fine aggregate content = Va X Vfa X Specific gravity of FA X 1000
vii. Trial mixes
The calculated mix proportions (Trial Mix No. 1) shall be checked by means of trial batches. The workability of the Trial Mix No. 1 shall be measured. The mix shall be carefully observed for freedom from segregation and bleeding and its finishing properties. If the measured workability of Trial Mix No. 1 is different from the stipulated value, the water and/or admixture content shall be adjusted suitably. With this adjustment, the mix proportion shall be recalculated by keeping the free water-cement ratio at the pre-selected value, which will comprise Trial Mix No.2. In addition two more Trial Mixes No.3 and 4 shall be made with the water content same as Trial Mix No.2 and varying the free water-cement ratio by ±10 percent of the preselected value. Mix No.2 to 4 normally provides the relationship between compressive strength and water-cement ratio from which the optimum mix proportions for field trials may be obtained.
A tool has been developed to calculate the design mix proportions keeping in mind of practical considerations based on IS: 10262 – 2009. This tool will surely be helpful to achieve these strategies in line with IS 10262:2009 concrete mix design. Results reported can be achieved in the best manner pertaining to this tool. This software tool was prepared in such a way that it can fulfill all the codal provisions IS 10262:2009, IS 456:2000 and IS 383:1970 while deriving proportions. It eliminates the cumbersome procedures of referring to charts, tables etc. It contributes a good scope to analyze the things in a better way. Hence, it can help the designer to make more rational and efficient design. This tool can be used for people working in government organizations and private sector of concrete industry.
* The software is interctive, user friendly
* Accurate and efficient procedure implemented
* Flexibility to change input conditions during run-time by using Modify option and the corresponding calculations will be modified
* Project details and Input data can be Saved and Retrieved
* User can visit IS code clauses, tables and figures which are embedded in software tool
* Calculations related to preliminary tests of concrete mix design such as Sieve analysis, fineness modulus, specific gravity and moisture content are available
* User can combine graded aggregates upto three size fractions
* Fly ash and water reducing admixures are incorporated in design procedure
* Trial mix calculations and plots of compressive strength vs water-cement ratio can be generated for 7 days and 28 days
* Detailed report in PDF format will be generated at the end
Demo Video link - Click here
Sample report link - Click here
User Manual link - Click here
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Article by Vijay Prakash Kondeti
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