Concrete is one of the most commonly used building material on Earth due to its cost and versatility, it is also good for the environment. So many different grades and admixtures are available, as an engineer it is easy to get caught out when site engineers and contractors start asking questions. A good understanding of concrete mix design will allow you to sleep comfortably at night knowing the grade of concrete that is being used is what you specified in the design.
Why Is Concrete Mix Important?
A concrete mix is similar to that of a cooking recipe, if the latter is done wrong, you go hungry. The stakes are a little higher with concrete however, incorrect mixes can have effects such as:
- Lower mechanical properties
- Cracking of the concrete
- Reduced workability
What is a Concrete Mix Design?
We will now break down the five elements that are used in the production of concrete. Concrete is made up of:
- Aggregates (fine and coarse)
- Admixtures and pozzolanic materials***
*** Admixtures and pozzolanic materials (fly ash, slag etc.) are only used for certain conditions. They are not typically found in concrete used for general construction purposes. We will go into more detail on this further into the post.
To simplify and speed up work on construction sites, ready mix concrete is available to purchase, they strive to achieve the perfect optimised ratios for the desired grade (strength), corrosion protection properties, workability. It is not a simple feat however.
Concrete Design Mix Guide
So we know what the contents or ingredients of the concrete is; Now we need to understand the effects of each of the materials and how they work together.
Cement in Concrete – Cement is the main body of the concrete. The main aspect to note is that cement mixes also exist and need to be taken into consideration, this is more of a niche area though and out of the realm of us engineers.
Water In Concrete – Water content or water ratio is a very critical value in the concrete design mix. To make concrete more workable, more water is required although this is at the expensive of the compressive strength. Therefore a healthy balance needs to be decided, slump tests can ensure the workability of the concrete, a quick and easy test completed on site. A higher slump value means the concrete will flow more.
Air In Concrete – Surprisingly having air in concrete can increase the durability of concrete in certain conditions. In cold weather, concrete is subject to expansion, therefore voids are extremely useful.
Coarse and Fine Aggregates in Concrete – This is dictated by the cross section of the concrete or reinforcement, normally a maximum aggregate size is stated in the design.
Chemical Admixtures in Concrete – Admixtures are used for multiple reasons including to protect the concrete against freeze thaw, reducing the water in the mix. Strength enhancement, corrosion protection, set acceleration and much more. This obviously has a cost attached, so the benefits need to outweigh the expense.
Pozzolanic Materials in Concrete – Pozzolanic materials are ones such as fly ash, silica fumes or slag and can be added to the mix to replace set quantities of cement. This is good for the environment and cost effective, so a win all round. On top of this they can increase the properties of the concrete mix and can slow down the curing of the concrete.
Codes and Standards for Concrete Mixes
Many different standards exist around the world, mainly because of the differences in the climate, for example, New Zealand has a lot of earthquakes and the codes of practise and standards will reflect that. However, in the United Kingdom, we seldom get earthquakes so it is not covered. A breakdown of some of the most common codes for concrete design mixes is below. Also follow local governance but if in doubt, at least follow one of them. One is better than none..
United Kingdom – BS EN 206-1 and its complementary standards BS 8500 parts 1& 2
United States of America – ACI 211, 211.1-91, re-approved -2002
India – Bureau of Indian Standards-I.S.10262-2009
Testing and Quality Control of the Concrete Design Mix
As engineers we control the design mix, but we also need to ensure that the mix that has been requested is to the specification requested, the quality of the materials as well as the volumes that have been used are adequate. These tests are done on site and in a laboratory.
For quality control, records need to be kept of the delivery tickets (a ticket is similar to a barcode) or the truck number, batch numbers, dates, location of pour and weather conditions. This allows us to test the concrete in the state it would be expected to have been at the time of construction.
The tests that are completed on site are:
- Slump Test
- Air Content
The following tests are done in the laboratory:
- Compressive Strength
- Flexural Strength
Software for Concrete Design Mixes
The demand to keep logs and track of a lot of data on construction sites is getting more and more onerous, due to lawsuits, insurance, quality control and general data mining to improve efficiencies.
Software that is available on desktops and mobiles can help massively by aiding the design of the concrete mix and tracking the materials used in the mix. These can then keep records that will be required to satisfy local building regulations and also help with any handovers.