civil

The Aim Of Concrete Mix Design

The aim of a concrete mix ratio is to produce the most economical concrete with the required properties in both the plastic and hardened state.

History

The Romans were probably the first to exploit the properties of concrete in a systematic manner, although earlier examples of construction with concrete and mortar are known.
In any event, many Roman concrete structures have endured for 2000 years. The Romans certainly knew the value of producing workable concrete which could be thoroughly compacted. They had recipes for proportioning a concrete mix ratio, they used pozzolanic materials such as volcanic ash and trass, used lightweight aggregates, and utilised hoop iron for reinforcement.
Knowledge of concrete construction was evidently available throughout their area of influence as ruins from all parts of their Empire show. In Roman times, the concrete mix ratio ingredients would undoubtedly have been batched by volume, the volumes of the ingredients being determined by experience.

Nominal Volumetric Proportions Of The 1:2:4 Type

This state of affairs persisted until relatively recently. The second report of the Concrete Committee of the Royal Institute of British Architects (RIBA), issued in 1911, stated that the minimum cube strength of a “1:2:4” cement mixture should exceed 1800 pounds per square inch (psi) (12 MPa), and right up until the 1970’s British Standard Code of Practice 114 continued to prescribe mixes by volume.
CP 114 also reflected minimum concrete strengths and maximum water/ cement mix ratios. The strength of a “1:2:4” mix was taken to be 3000 psi (21 MPa) with a water: cement ratio of 0,60. Nowadays an equivalent cement mixture would have a characteristic strength of around 30 MPa.
This method of proportioning mixes was based on experience with aggregates in the United Kingdom where the coarse aggregates were well-graded gravels with high bulk densities and the sands were fairly consistent. It was based on the volume of a 94-lb. (42,7 kg) cement bag which was taken as 1 cubic foot (28,3 litres).
A 1:2:4 mix was therefore 1 bag of cement to 2 cubic feet of sand to 4 cubic feet of well-graded stone. What is not generally realised is that the sand volume referred to dry sand and that allowance still had to be made for bulking of the sand when damp. CP 114 mentioned this in the small print, as well as the fact that one could adjust the sand to stone ratio as long as the overall ratio of cement to aggregate remained at 1 to 6. The small print was often overlooked which resulted in the batching and placing of harsh, under-sanded mixes which were difficult to compact thoroughly.
A more fundamental problem with this type of mix is that it is impossible to prescribe mix proportions, and water/cement mix ratio, and minimum strength, and slump simultaneously. The reasons for this are that no account is taken of the water requirement of the aggregates or the strength characteristics of the cement. For example a “1:2:4” mix could give characteristic strengths ranging from 20 to 35 MPa depending on the water content of the mix, the binder type and whether or not chemical admixtures are used in the mix.
It follows that the control of concrete quality on site is extremely difficult. A fairly common mistake, still evident today, was mixing 1 bag of cement to 2 wheelbarrows of sand to 4 wheelbarrows of stone. This is in fact a 1:4:8 concrete mix ratio as the volume of a level wheelbarrow is equivalent to the volume of two bags of cement.
In 1918, Duff Abrams published his findings on the relationship between compressive strength of fully compacted concrete and water/cement ratio. This became, in retrospect, the foundation for a more rational method of mix design, leading in turn to a move away from volume batching to batching by mass.
Now, of course, all concrete for important work is ‘batched’ by mass, while volume batching is still used extensively for low strength concrete (and mortar, plaster and floor screeds) in housing.

Tables Of Standard Mixes

A variation on the use of nominal mixes is the use of standard mix design tables which are available from a number of sources. Normally the tables cater for different coarse aggregate sizes, different compressive strengths, sand size and quality, and concrete workability. Generally speaking, standard mixes are given for compressive strength requirements only and durability criteria are ignored. Different tables are required for different cement types and the tables do not, as a rule, cover the use of chemical admixtures.
To use these tables the user needs to be able to distinguish whether the sand to be used is coarse, medium or fine and whether it has a high, average or low water requirement.
Mix design tables are drawn up by making assumptions about the aggregate and binder properties. It is important that the user understands the underlying assumptions.
These tables, which give concrete mix ratio both by volume and by mass, can be very useful at the estimating or tendering stage to get a quick estimate of concrete material costs.

The “Eye-Ball” Method Of Mix Design

Despite the title, this method of mix design can produce good results and is a very useful method if one has to design a mix on-site in a hurry. The only information needed by the mix designer is the required water/binder cement mix ratio of the concrete (and the knowledge that the available aggregates are suitable for use in concrete). The designer does not need to know any of the physical properties of the aggregates.
It is also a useful method for designing unusual mixes. The first trial mixes for the exposed aggregate paving at the V&A Waterfront in Cape Town were carried out on site using this method.
The procedure is as follows:

Weigh out cement (and extender if required) and water to satisfy the water/binder ratio requirement
If applicable, measure the required amount of chemical admixture
Weigh out excess quantities of air-dry sand and stone
Batch the binder, water, admixture, and some of the sand and stone into the mixer and mix thoroughly
Add sand slowly until the slump is estimated to be about 150mm
Add stone and more sand until the slump and stone content appear more or less correct, carrying out slump tests as necessary
Weigh the left-over sand and stone
Calculate the concrete mix proportions