Concrete mixed designs aids a foundation to achieve sound infrastructure. Concrete mix design involves a process of preparation in which a mix of ingredients creates the required strength and durability for the concrete structure. Because every ingredient in the mix consists of different properties, it’s not an easy task to create a great concrete mix. It is imperative that all ingredients be tested to determine their physical properties and the bearing capacity of the project location. The ingredients to be tested: water, fine aggregate (sand), coarse aggregate, cement, chemicals, reinforcement, and soil.
The design of concrete mix is not a simple task on account of the widely varying properties of the constituent materials, the conditions that prevail at the site of work, in particular the exposure condition, and the conditions that are demanded for a particular work for which the mix is designed.
The values of the physical properties obtained after testing will be used as the basis for all concrete mix design considerations. This will ensure the structure will be sound and prevent failure of the mix. It is important to note that the ingredients for the mix might vary from one project location to another, so the physical properties must be tested for the requirements specified for each location.
Selection of concrete mix designs
Modern concrete mix designs can be complex. The choice of a concrete mix depends on the need of the project both in terms of strength and appearance and in relation to local legislation and building codes.
The design begins by determining the requirements of the concrete. These requirements take into consideration the weather conditions that the concrete will be exposed to in service, and the required design strength. The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples.
Many factors need to be taken into account, from the cost of the various additives and aggregates, to the trade offs between the “slump” for easy mixing and placement and ultimate performance. A mix is then designed using cement (Portland or other cementitious material), coarse and fine aggregates, water and chemical admixtures. The method of mixing will also be specified, as well as conditions that it may be used in.
This allows a user of the concrete to be confident that the structure will perform properly. Various types of concrete have been developed for specialist application and have become known by these names. Concrete mixes can also be designed using software programs. Such software provides the user an opportunity to select their preferred method of mix design and enter the material data to arrive at proper mix designs.
Types of concrete mix designs
There are two types of concrete designs, each of them is explained below.
Regular concrete
Regular concrete is the lay term for concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. The ingredients in any particular mix depends on the nature of the application. Regular concrete can typically withstand a pressure from about 10 MPa (1450 psi) to 40 MPa (5800 psi), with lighter duty uses such as blinding concrete having a much lower MPa rating than structural concrete. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.
High-strength concrete
High-strength concrete has a compressive strength greater than 40 MPa (5800 psi). In the UK, BS EN 206-1defines High strength concrete as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond. Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
The best concrete mix design is the one which satisfies all the aspects for which it was designed
Strength- The strength of 95% cube casted after 28 days of curing should be greater than the characteristic strength for which concrete has been designed.
Workability & placing- As working condition changes so does the properties desired from concrete also changes, the concrete which can be easily placed without segregation and with least compaction required.
Durability- The concrete must be durable enough to face harsh conditions of atmosphere for which it has been designed.
These are the main properties considered while designing concrete and the concrete designed satisfying such conditions can be called as best concrete mix design. Mix design of concrete depends on the grade of concrete. Moreover, the minimum grade of concrete is decided keeping in view the strength and durability requirements. Under a particular exposure condition grade of concrete may vary for different structural components. IS 456 specifies minimum grade of concrete for mild, moderate, severe and very severe exposure conditions.
Factors affecting the choice of mix proportions of concrete mixed designs
The various factors affecting the mix design are:
Compressive strength
It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction.
Workability
The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site.
Durability
The durability of concrete is its resistance to the aggressive environmental conditions. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used.
Maximum nominal size of aggregate
In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in size of aggregate. IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible.
Grading and type of aggregate
The grading of aggregate influences the mix proportions for a specified workability and water-cement ratio. Coarser the grading learner will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which can be achieved by mixing different size fractions.
Quality Control
The degree of control can be estimated statistically by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and minimum strengths of the mix lower will be the cement content required. The factor controlling this difference is termed as quality control.
Factors to be considered for concrete mix designs
Strength and durability of the concrete mix design are dependent upon the following factors:
Grade designation: Concrete’s strength is measured in N/mm2 when subject to test after curing in any curing medium. The choice of concrete grade depends on its usage.
Choice of cement: Cement choice varies depending on usage. The cement should be tested for performance required by their usage before being tested in the design mix.
Choice of aggregate size: Aggregates needed for each mix is dependent upon the physical properties needed for the design. All aggregates must be quality sized before use.
Type of water: Any water used for concrete mix design should be tested before use to ensure it is within the range of water required for concrete. Most all consumable water is good for concrete work, but should still be tested.
Water to cement ratio: The ratio of water to cement should be tested for consistency, initial and final setting, soundness of the cement, workability, slump of the concrete and compacting factor.
Workability: This is the measure of ease of mixing concrete without segregation or bleeding. It mostly depends on the designed slump of the concrete.
Durability: This is the measure of the required strength (N/mm2) of any concrete grade after 28 days of curing. Durability should be control tested on site.
This method of proportioning was first published in 1944 by ACI committee 613. In 1954 the method was revised to include, among other modifications, the use of entrained air. In 1970, the method of ACI mix design became the responsibility of ACI committee 211. We shall now deal with the latest ACI Committee 211.1 method.
Purpose of concrete mix-designs
Concrete mix design is economically proportioning of concrete ingredients for better strength and durability based on construction site. While the nominal concrete mix may have a higher amount of cement, when it is designed mix, (Here nominal concrete mix and design concrete mix are the two types of concrete mix design) the cement requirement may below for the same grade of concrete for a given site. The proportions resulting from concrete mix design are tested for their strength with the help of compressive strength tests on concrete cubes and cylinders.
A well-designed concrete mix will ensure that the finished structure is as strong as it can be. It also maximises the efficiency of ingredients by reducing wastage and getting the most concrete out of the wet and dry ingredients.
Mix design of concrete is done to produce a concrete of your choice with local available material with you. Strength, durability and workability are the main criteria which are to be stipulated before mix designing of any concrete. Water to cement ratio and aggregate to cement ratio are main factors which affects the properties of concrete. Therefore mix designing is the process to find out the best suited proportions of water : cement : fine aggregate : coarse aggregate : admixture for a concrete you want.
The object of mix design is to decide the proportions of materials which will produce concrete of required properties. The mix proportions should be selected in such a way that the resulting concrete is of desired workability while fresh and it could be placed and compacted easily for the intended purpose. The fresh concrete should be fluid enough to fill the formwork and surround the reinforcement fully and the hardened concrete should develop required strength and durability.
All ready-mix producers strive to find the perfect proportions of these ingredients in order to optimize their concrete mixes and give their concrete strength, durability, workability, and other desirable properties. It’s important to optimize concrete to ensure the lowest cost while maintaining the highest strength of your mixture. This is far from easy, as every addition or subtraction to the concrete mix entails adjustments to the components, making the process very complicated and inefficient.
Conclusion
The rules and mixed proportions should be taken care of extensively as a slight mistake may result in adversity of structure and desired goal could not be fulfilled. Choose the suited proportions of water : cement : fine aggregate : coarse aggregate : admixture for a concrete you want with expert guidance.