So you are an engineer and you would like to select a certain concrete mixture over a group of designs. Deciding which materials to use at what percentages is a process that depends on various criteria. The end goal is to optimize the mixture design to produce a concrete that achieves all the required properties while keeping the cost minimum.
Properties of Concrete Mixtures
First let’s have a look at the most popular properties of concrete that engineers should keep in mind when choosing a concrete mixture design:
- Strength characteristics
- Durability properties
- Placeability and lastly and in some cases the most important,
- The cost
as we know, concrete is capable of carrying compressive loads but can’t withstand tensile forces. The ability of concrete to carry loads depends on many factors. For example, increasing the water – to – binder ratio which is the mass of water divided by the mass of the binding materials in concrete, will have a negative effect on concrete strength (both the compressive and the tensile). Also, increasing the amount of cement improves the strength of concrete.
Choosing a specific design strength will depend on the type of project you are working on. However, you should select the proportions of materials such as cement, water, and aggregates to achieve the required design strength. It should be noted that the mark for determining the strength is at 28 days. Thus, in field, specimens should be taken from the concrete patch and tested to check whether it achieved the required strength.
Another factor to consider is do you need a high initial compressive strength or do you want to have the strength gain happens slowly. For example, in case of repair works, you will need to have fast strength gain at the beginning to finish up the work quickly and open the structure for service. While in dams, strength gain is needed to happen slowly to prevent the formation of cracks within concrete.
Depending on the purpose of the project you are working on, you should select the strength that achieves the purpose.
Concrete shall be designed to live a specific service life. In order to do that, it should be durable. This means it has the capability to resist what the surrounding environment is imposing on it. For example, concrete in cold regions should be able to resist the freezing and thawing cycles; concrete foundations that are embedded in soils with high concentrations of salts should be able to resist the damage that might happen due to those salts.
A very important factor to consider in case of durability is the penetrability of concrete which depends on the pore structure of concrete. Concrete with high porosity will make it easy for harmful substances to get into concrete. Thus, concrete should be designed to have a suitable porosity for the purpose it is going to be serving for.
One of the factors of mixture design that has a very powerful impact on the durability of concrete is the water – to – binder ratio. The decreasing of W/C will significantly decrease the porosity of concrete and in turn it will enhance its behavior in terms of durability.
This is the propriety of concrete that gives an indication on how easy it is to place, consolidate, and finish concrete. Of course, you need to have the concrete placeable to fill the form work with the least amount of effort for the labor. However, this should be done without compromising the quality of concrete in general.
The Placeability characteristics of concrete are affected by many factors such as the W/C, shape and size of aggregates, quantity of cement, the use of admixtures, etc.
This an important thing to note is that the placeability properties of concrete should be maintained without the addition of water on the site because this will reduce the concrete strength.
Finally, engineers should seek to achieve that desired properties of concrete at the minimum cost. You shouldn’t seek achieving superior properties of concrete that are way beyond the scope of the project. Instead, try to achieve the desired properties that are required only for the project at hand.
Using supplementary cementitious materials is an option; it will increase the initial cost but on the long run may be beneficial. Your judgment should be based on what is called lifecycle cost analysis which means that your decision should not be based on the initial cost but based on the cost during the lifetime of the structure. In other words, you may use an expensive material at the beginning of the construction but on the long run this material will reduce the lifecycle cost in terms of the repair and maintenance.