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 . 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 :
- 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
The following information is generally given to the designer as
- 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 , BS  and IS . 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.