Concrete Mix Design – Art of Balancing the Conflicting Requirements

Why we need to explore

potentials of Indigenous materials in Pakistan


Concrete, like all other engineering materials, needs to be designed for properties like strength, durability, workability and cohesion. This extremely versatile material can be designed for strength ranging from 10 Mpa to 200 Mpa and workability ranging from 0 mm slump to 250 mm slump. It’s all characteristics including strength, workability and durability, are in our hands. One can make it flow like a liquid, make it light like foam and dense like a stone. One can predict its behavior under any possible circumstances. In all these cases the basic ingredients of concrete are the same, but it is their relative proportioning that makes the difference. Concrete mix design is the science of deciding relative proportions of ingredients of concrete, to achieve the desired properties in the most economical way. With the advent of high-rise buildings and pre-stressed concrete, use of higher grades of concrete is becoming more common. Mix design of concrete is becoming more relevant in this scenario. The most important factor governing the strength and durability of concrete is its water-to-cement ratio (w/c). All time dependent phenomena like creep, shrinkage and elastic modulus are somehow or the other, related to water-to-cement ratio. As a thumb rule, every 1% increase in quantity of water, reduces the strength of concrete by 5% and every extra liter of water per cubic meter will approximately reduce the strength of concrete by 2 to 3 Mpa (290 Psi to 435 Psi) and increase the workability by 25 mm [1]. Hence, the knowledge of water demand of concrete system is the key to a mix designer.


What objectives we want to achieve?

The overall objective of proportioning concrete mixtures can be summarized as “selecting the suitable ingredients among the available materials and determining the most economical combination that will produce concrete with certain minimum performance characteristics”. The


requirements which form the basis of selection and proportioning of mix

ingredients are [2]:

  • The minimum compressive strength required from structural consideration (usually termed as fc’)
  • The adequate workability necessary for full compaction (usually in terms of slump)
  • Maximum water-cement ratio to give adequate durability for the particular site conditions
  • Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete
  • Economy
The following information is generally given to the designer as

requirements.
  • Grade of concrete (the characteristic strength specified at a certain age)
  • Workability requirement in terms of Slump, Vebe Time or Compacting factor
  • Other requirements may include, Retardation of initial set, Slump retention, Pumpability, Acceleration of strength, Flexural strength (normally required

    for concrete pavements)
  • Exposure conditions
  • Degree of quality control at site
After reviewing all the requirements and going through the

complete process of mixture proportioning, the designer is supposed to submit

the following results.
  • Ingredient quantities in Kg/m3 or lb/yd3 of concrete
  • Volumetric and by weight ratio of quantities
  • Results of all tests performed on ingredients including gradation and moisture condition of aggregates
  • Fresh density of Concrete
  • Dosage of admixture
  • Mixing and curing regime adopted in laboratory for trial batches


Why it’s not as easy as it looks?


An obvious constraint is that within a fixed volume, one cannot alter a component independent of others. For example, in a cubic meter of concrete, if the aggregate component is increased, the cement paste component decreases. The task is more complicated by the fact that certain desired properties of concrete may be oppositely affected by changing a specific variable. For example, the addition of water to a stiff concrete mixture with a given cement content will improve the flowability of fresh concrete but at the same time will reduce the strength. In fact, workability itself is composed of different components [i.e., consistency (ease of flow), yield stress, cohesiveness (resistance to segregation) and viscosity], and these tend to be affected in an opposite manner when water is added to a given concrete mixture. The process of mixture proportioning, therefore boils down to the “art of balancing various conflicting requirements”.


Early Approaches and Practices


In ancient times when houses were built with mud mortars, there must be some rules to decide the amounts of mud, clay and water. Mostly the amount of water in mud mortar mixes was established on the basis of “plasticity” or “wetness”, or more precisely the “consistency” of the mix. When concrete was formally adopted as a construction material during the 19th century, compressive strength probably was the only criterion for proportioning the mix. The strength of concrete was supposed to increase with the quantity of cement and better compaction. It was also realized that use of aggregates for having less voids resulted in stronger concrete. Since the amount of cement was always associated with required strength, this association is quantized and standardized in some guidelines by accepting the rule that cement content must be selected by dividing the mean target strength by the “average strength increase per 1 Kg/m3 increase in cement content”. This “average strength increase” is established after testing of a large number of specimens having a wide range of proportions, compressive strengths and workability values.


Perhaps, the earliest approach towards proposing a definite set of rules to decide a mix proportion was “Minimum voids approach” or the “Maximum Density Approach”. The idea is to give main consideration to the density and minimum voids. In early methods proposed based on this approach, all other factors including aggregate grading, resistance to segregation and durability etc. are completely ignored. In this approach, the voids of coarse aggregate and fine aggregate are determined separately. The quantity of sand used should be such that it completely fills the voids of the coarse aggregate. Similarly, the quantity of cement used should be such that it fills the voids of sand, so that a dense mix having minimum voids is obtained. To the mix of cement, sand and coarse aggregate so obtained, sufficient water is added to make the mix workable. However, such methods cannot provide sufficient assurance of getting satisfactory characteristics and problems like bleeding, segregation and lack of workability persists.


Various Methods of Mix Proportioning


This article will briefly discuss some of the limitations of 3 most commonly used mixture proportioning methods i.e. ACI [3], BS [4] and IS [5]. The basic assumption made in all these methods is that the compressive strength of workable concretes, by and large, governed by the water/cement ratio. Also it is assumed that for a given type, shape, size and grading of aggregates, the amount of water primarily determines the workability. However, there are various other factors which affect the properties of concrete, for example the quality & quantity of cement, water and aggregates, batching, transportation, placing, compaction and curing etc. Therefore, the specific relationships that are used in proportioning concrete mixes should be considered only as the basis for trial, subject to modifications in the light of experience as well as for the particular materials used at the site in each case. No mix design method directly gives the exact proportions that will most economically achieve end results. These methods only serve as a “base to start” and achieve the end results in the fewest possible trials.


(a) ACI method


This method is recommended by ACI Committee 211 [3] and is based on determining the coarse aggregate content (in terms of percentage of concrete volume) from dry rodded bulk density and fineness modulus of sand, thus taking in to account the actual voids in compacted coarse aggregates that are to be filled by sand cement and water. The committee report provides two methods for calculating aggregate quantities i.e. Weight method and Absolute Volume method. The weight method is considered less exact but does not require the information on the specific gravity of the concrete-making materials. The absolute volume method is considered more exact as well as easy to use in site conditions using known-volume containers and buckets. In the context of applying to local materials in Pakistan as well

as in general, the following limitations can be observed [8].
  • It gives coarse aggregate contents for sand with FM range of 2.4 to 3.0 (Table 2, Appendix B in committee report). It is found that sands available in many parts of Pakistan including Lawrencepur and Ghazi are generally very fine and have fineness moduli less than 2.4.
  • In this method the density of fresh concrete is not given as function of specific gravity of its ingredients. In British Method (also referred to as DOE method) [4] and IS [5] methods, the

    plastic density or yield of concrete is linked to specific gravity of ingredients.
  • The ACI method also does not take into account the effect of the surface texture and flakiness of aggregate on sand and water content, neither does it distinguish between crushed stone aggregates and natural aggregates.
  • The ACI method does not have a specific method of combining two different aggregates sizes.
  • The fine aggregate content cannot be adjusted for different cement contents. Hence the richer mixes and leaner mixes may have same sand proportion, for a given set of materials.

(b) British (DOE) Method

The British method of concrete mix design, popularly referred to as the “DOE method”, is used in the United Kingdom and other parts of the world and has a long established record. In 1975 “Design of Normal Concrete Mixes” was published by the British Department of the Environment (DOE). The DOE method utilizes British test data obtained at the Building Research Establishment, the Transport and Road Research Establishment, and the British Cement Association. The aggregates used in the tests conformed to BS 882 and the cements to BS 12 or BS 4027. In this method, the fine aggregate content is taken as a function of 600 micron (0.6 mm) passing fraction of sand and not the gradation zone of sand. The 600-micron passing fraction emerges as the most critical parameter governing the water demand, cohesion and workability of concrete mix. Thus sand content in DOE method is more sensitive to changes in fineness of sand when compared to the ACI method. The sand content is also adjusted as per workability of mix. It is well accepted that higher the workability greater is the fine aggregate required to maintain cohesion in the mix. The water content per cubic meter is recommended based on workability requirement given in terms of slump and VeBe time. It recommends different water contents for crushed aggregates and for natural aggregates. The quantities of fine and coarse aggregates are calculated based on plastic density relationships. The DOE method also suffers from some limitations [8].
  • The fine aggregates content calculated from DOE method often is on the higher side resulting in over sandy mixes as compared to ACI method.
  • Like ACI method, in BS method, the fine aggregate content cannot be adjusted for different cement contents. Hence a rich mix with cement of 400 Kg/m3 will have the same fine aggregate

    proportion, as a lean mix with 300 Kg/m3 cement for given sand. Thus richer mixes may not be as workable because of higher fines, when compared to mixes obtained from the IS method.
  • Like ACI, the DOE method also does not take into account the effect of the surface texture and flakiness of aggregate on sand and water content although it distinguishes between crushed

    stone aggregates and natural aggregates.

(c) Indian Standards (IS) Method

There are several reasons of discussing this method for comparison in this article. Firstly, the aggregate mineralogy of India and Pakistan is almost identical. Both countries also share identical weather exposure and comparable construction practices. Indian standards [6] also classify various cement types in terms of “Grade 43” and “Grade 53” just like Pakistan standard PS 232-2008(R) [7] developed by Pakistan Standards and Quality Control Authority (PSQCA). IS 456-2000 [6] has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day 6 inches cube strength of mix in N/mm2. The IS method treats normal mixes (up to M35) and high strength mixes (M40 and above) differently. The method also gives correction factors for different w/c ratios, workability and for rounded coarse aggregate. The quantities of fine and coarse aggregate are calculated using yield equation, which is based on specific gravities of ingredients. Thus plastic density of concrete calculated from yield equation can be close to actual plastic density obtained in laboratory, if specific gravities are calculated accurately. Thus actual cement consumption will be close to that targeted in the first trial mix itself. The water cement ratio is calculated from cement-specific curves based on 28 days strength. The IS method suffers from following limitations [8].
  • The IS method considers compacting factor as measure for workability, to calculate the water demand. Compacting factor may not correctly represent workability therefore the revised IS 456 2000 [6] has excluded compaction factor as a measure of workability. Now, it recommends use of slump as a measure for workability.
  • The IS method does not recommend any corrections when crushed fine aggregate is used against natural fine aggregate as in case of DOE method.
  • The IS method gives water demand and fine aggregate content for 10 mm 20 mm and 40 mm down aggregate. In practice the maximum size of coarse aggregate is often between 20mm and 40 mm, the estimation of water and sand content is difficult.
  • The quantities of fine aggregate and coarse aggregates are calculated from the Yield equation. The yield equation is based on the idea that volume of concrete is summation of absolute volumes of its ingredients. Absolute volume of ingredients is function of specific gravities of ingredients .The plastic density of concrete if theoretically calculated on the basis of specific gravities, may not match with that actually measured from concrete. This may induce some uncertainty in determined quantities.
Design Practice in Pakistan – The need for improvement


The method proposed by ACI 211 [3] Committee for mix design of normal concrete is widely used by practicing engineers, contracting firms as well as academicians in Pakistan. However it is found in many cases that quantities recommended by this method as a first trial batch were quite far from quantities which gave desirable characteristics at the end of all trials in laboratory. The solution for this cumbersome process of making trials and waiting for 28 or so days was found in development of some thumb rule proportions by contractors for each strength level of concrete. Common proportions (by weight) of Cement, Sand and Crush used in small and medium level projects in Pakistan are listed below.
Common ingredient ratios (by weight) used in local projects in Pakistan
Design Strength (Psi)
Ratio (by weight) of Cement, Sand and Crush
Less than 2500
1 : 3 : 6
3000
1 : 2 : 4
4000
1 : 1.5 : 3
5000
1 : 1 : 2
Greater than 5000
1 : 0.8 : 1.7
A general practice is to make a small change in these proportions according to site conditions in the name of so called “Past Experience”. Some firms have developed their own recipes and ready-to-use mix designs which are considered quite reliable and often not validated through laboratory trials.
The specific relationships constituting figures and tables given in American and British methods are based on results obtained from extensive testing regimes applied on natural aggregates and materials with certain properties. Applying these relationships to local materials (with different specific gravities, absorption values, bulk densities etc.) and expecting the same result will be an erroneous approach. A common observation during local construction projects is that there is a reasonable difference in the amount of required cement quantities recommended by these methods and the final amount determined from laboratory trials. This results in a reasonably higher number of required trials and waste of time. Keeping in view all these factors, there is a need of serious attempts to address the absence of data regarding effect of different properties of indigenous aggregates and cement types on desired properties of concrete in both fresh and hardened states. Detailed, systematic and comprehensive studies are required, exploring and evaluating the performance of local materials and resulting in the development of consistent relations between various involved variables for indigenous materials. There is also a grave need to unify and summarize the already available data in order to ensure the practicality and convenience in understanding the local aggregate and cement types for the benefit of end users including postgraduate students, practicing engineers and consultants.


References


[1] Neville, A.M., and Brooks, J.J. “Concrete Technology”, ELBS, 1987.
[2] P. Kumar Mehta and Paulo J. M. Monteiro, “Concrete – Microstructure, Properties and Materials”, Third Edition, McGraw-Hill, 2006.
[3] American Concrete Institute Committee 211 report, “Standard practice for selecting proportions for normal, heavyweight, and mass concrete”. ACI Manual of Concrete Practice, Part 1-1996.
[4] Design of Normal Concrete Mixes, Second Edition, Building Research Establishment Ltd, Garston Watford, ISBN 1860811728, Copyright BRE 1997William J. Palm III,
[5] IS SP23-1982, “Handbook on concrete mixes”, Bureau of Indian Standards, New Delhi.
[6] IS 456-2000, Indian Standard code of practice for plain and reinforced concrete” Bureau of Indian Standards, July 2000.
[7] PS: 232-2008 (R) Pakistan Standard Specification for Portland Cement (Ordinary, High Strength and Rapid Hardening), Pakistan Standards & Quality Control Authority (PSQCA) Standards Development Centre (SDC/PSQCA)
[8] Durocrete Engineering Services Pvt. Ltd., “Mix Design Manual” accessed from http://www.durocrete.com/.