Mix Proportioning

The very first task.

Is concrete mix proportioning a Science or an Art? We know that concrete mix proportioning can not just be left to the judgment or guess work of the concrete mixer operator or a contractor.

We also know that exact mathematical rules do not and can not obtain to get the precise proportions of a concrete mix as the variables are too many.

It is both a Science and Art. These two are dovetailed intricately – we can not say when is which. For a concrete mix design or proportioning, the very first question one should ask is concrete for what? Is it for a storage shed or for the ground floor column of a 50 storeyed building? Is it for a gravity dam or for a flowing and self consolidating concrete? Or for what else ? This question needs a precise answer. It is important to know the location of the structure for which concrete mix is required to be proportioned. Location decides the exposure conditions. A structure being built in a coastal area falls under severe/very severe exposure conditions. If the mix is to be proportioned for a footing in a non-aggressive soil, it shall be considered as moderately exposed. There can not be the same mix for the ground floor column of a 50 storeyed building and for a small span slab in the same building.

The second Question is, does the concrete need slow setting characteristics or does it need rapid hardening? Rapid hardening would also normally imply some what rapidly setting characteristics though with in limits. Either hydration of cement is slow or fast but it can not be both at the same time!

The third question is “who is going to place it and consolidate the concrete? How will these processes be done? If it has to be done by a semiskilled person controlling a gang of unskilled labour as we see in many places, we know what type of quality control that will be possible despite the high intentions of the mix designer. But if pumped concrete is placed by articulated vehicles by experienced operators one can achieve a very good quality control. This question will decide the margin of safety needed for a mix design. It will also indicate the extent of risk of failure of the required concrete quality.

The fourth question is, from where the materials are going to come? Though all cement bags may have an ISI mark, cements vary a lot from factory to factory, batch to batch. It is necessary to understand the characteristics of cements coming from a particular factory at a particular time. Grinding balls of the ball mill in the factory may have worn out. Cement kiln temperatures may have been affected by the type to fuel used. Raw materials may not be excellent or correctly prepared. Gypsum may be more a commercial byproduct than of pure quality. As such the cements that we are going to use needs to be assessed thoroughly. Not just by the setting times and the so-called 7 day or 28 day strength of 7 cm standard sand and mortar cubes but by understanding fully the characteristics of the cement that is that is likely to be used.

Fifthly, aggregates may come from crushers that yield a lot of flaky and elongated particles. Aggregates may have friable nature. May also contain on them lot of dust. Sand may be contaminated with mud or chemicals. It may be poorly graded. It may contain too many fine particles (in the form of silt). It may show high water absorption. These need to be investigated.

Lastly, water that is likely to be used may contain many salts. It may have undesirable acidity or alkalinity.

So, these questions need through answers before we start mix proportioning for an important project. A lot of information needs to be obtained, a task normally not under taken by a contractor or a field engineer. But the concrete mix designer will be concerned with these as they influence the product that he has designed. He has to demand this information. One may ask: Is concrete mix design so difficult? Answer is No. This information can be systematically and routinely obtained and kept in the office or laboratory of the mix designer. That will be a part of his information system.

Role of Admixtures :

When it comes to our body’s health, the days of eating one omnibus ‘leha’ or ‘churan’ are over. We go to different specialists depending on the nature of the problem. And we take different drugs for different needs. So also, now a day, concrete needs to be precisely tailored for specific purposes.

For this purpose of specifically tailoring a concrete for a particular need, especially to achieve high strength concretes, or highly durable concretes or self consolidating concrete or economical concretes we need an extra ingredient, which is the admixture. IS 456 – 2000 recognises use of mineral admixtures such as:

fly ash
blast furnace slag
condensed silica fume
rice husk ash
meta-kaolin

in making standard concrete mixes to high performance concrete mixes. The use of mineral admixtures is likely to enhance concrete properties through physico-chemical interaction. Among the mineral admixtures that are permitted to be used in concrete, fly ash stands out due to its availability, chemical interaction for long term durability, physical interaction for workability enhancement. However, there suitability should be established at least through physical- chemical tests such as fineness, lime reactivity, loss on ignition, soluble silica content etc., before use.

Chemical admixtures have revolutionized concrete technology all over the world. They have come to stay. A thorough understanding of what an admixture can do and what it cannot do is very much necessary. A concrete mix designer must be fully aware of the various types of admixtures, their availability in the local market and relative advantages and disadvantages of competing commercial products. He should not be carried away by commercial information only but must be aware of published literature and experiments conducted by unbiased laboratories. This is very important since getting a favourable result in one property may sometimes mean unfavourable feature in another.

It is really a specialist’s job. Otherwise, it will be like an ill qualified medical practitioner prescribing highly specialized neurological or cardiac drugs to a patient without knowing their implications fully.

Criteria for Mix Design or Mix Proportions:

The 28 day cube strength is usually the principal criterion. But this is not enough. For example, criteria for a high performance concrete shall satisfy strength criteria and any one or all the laid durability criterion such as:

Chloride ion diffusion
Water permeability
Freeze-Thaw resistance
Sulphate attack
Early (one day) strength etc.

For every project the criteria needs to be clearly obtained from

structural engineer
project engineer and
contractor before a concrete mix design is embarked on.

Differences in their opinions or needs require to be reconciled and clarified. There is usually a conflicting thought between a contractor working for better productivity from labour and materials and project engineer looking for assured quality at all costs.

Variables in Concrete Mix Design and Mix Selection Process:

With concrete-making materials of given characteristics and with given job conditions, the variables generally under the control of a mix designer are as follows: the cement paste-aggregate ratio in the mixture, the water-cement ratio in the cement paste, the sand-coarse aggregate ratio in the aggregates, and the use of admixtures.

The task of mixture proportioning aims at optimizing several conflicting requirements such as workability, strength, durability and cost applying sound technical principles governing mixture proportioning procedures. It is complicated in the sense if a specific variable is changed, certain desired properties may be oppositely affected. For example, the addition of water to a stiff mix with given cement content will improve the flowability of fresh concrete but at the same time will reduce the strength. In fact, workability if self is composed of two main components namely, consistency (ease of flow) and cohesiveness (resistance to segregation), and both tend to be affected in an opposite manner when water is added to a given mix. The process of mix proportioning boils down to the art of balancing various conflicting requirements.

Numerous procedures for computing the concrete mixture proportions are available in most countries of the world. Mathematical approaches to determine the correct proportion of component materials of a concrete mixture meting a given set of specifications generally do not work because the materials vary widely in their characteristics. This explains why there is a large number of empirical methods based on extensive test data developed from local materials.

Concrete mixture proportions can be obtained by the widely used recommended methods of ACI Committee 211 and British Method of Mix Design. Our own BIS method (IS:10262-2009) gives reasonably good proportions when used judiciously. Most methods take in to account the m.s.a and slump requirements to choose the water content in the mix. The water-cement ratio is chosen based on the strength requirement and now on durability. The aggregates are chosen either based on fresh density of concrete or bulk density of aggregates and its volume fraction in unit volume of concrete which takes in to account the shape and size factor and sand grading.

There is renewed interest in the least voids grading of aggregates as it gives maximum packing density and hence minimum cement. There are number of theoretical studies on packing density of granular materials, which is defined as the solid volume in a unit total volume. The objective of particle packing studies has been to obtain mathematical modeling or ideal grading curves that help to determine the ideal combination of different size fractions of aggregate particles to produce minimum void space.

There are many methods for determining the mixture proportions of concrete when compressive strength is the design criterion. However there is not much information when other criteria, such as modulus of elasticity, fracture energy, or durability aspects are specified. Monteiro P.J.M and Helene P.R.L have suggested developing mix design nomogram from well- established concrete relationships. Fig 4.1 and Fig 4.2 show two such typical nomogram for constant consistency but different design criterion. They typically follow Abrams’ law between strength/modulus of elasticity Vs W/C, Lyse’s law between W/C Vs Aggregate- cement ration and Molinari’s law between cement content Vs. aggregate to cement ratio. Nomogram for different design criteria using locally available materials can serve as a very handy tool to mix design engineers.

Lastly, the admixture dosage is an issue while proportioning mixes using any of the available methods. Compatibility studies and determination of optimum dosage has to be carried out based on the prevailing codal practice which is quite cumbersome at times. However, Marsh cone has emerged as a handy tool to determine the optimum dosage and also can give information on compatibility.

Importance of Trial Mixes and Laboratory Castings:

The calculated mix proportions shall be checked by means of trial batches. The concrete for trial mixes shall be produced by methods of actual production and with the materials that are used in actual production. Pan mixer is required to be used to simulate the site conditions.

The trial mixes tell the real problems likely in compatibility of cement and admixtures, harshness due to aggregate shape, bleeding and segregation etc.

Importance of Placing, Compacting and Curing Methods. With out proper placing, compaction and curing, all the care taken in design will be lost. How to ensure them?

Proper placing of concrete using pumps, articulated vehicle delivery, buckets in cranes etc.,
Continuous moist curing is a must for all cementitious materials. Many options are available. Curing compounds, which prevent evaporation of moisture, are gaining importance in tropical climates.

Will be Continued in Issue 9..

References:
REFERENCE MANUAL FOR FIELD ENGINEERS ON BUILDING CONSTRUCTION

Published by:
Task Force for Quality Assurance in Public Constructions
GOVERNMENT OF KARNATAKA ISBN

Important Note:
This reference manual is intended for the use of Government Engineers.

Disclaimer:
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.