Concrete mix design overview
Concrete is the most important material in the infrastructure projects and the concrete mix proportions are the most important aspects of working with the concrete. Designing a concrete mix needs to follow dedicated and judicious approach which requires detailed analysis of available raw material (concrete ingredients) and composition thereof. Concrete mix is a combination of coarse aggregate, fine aggregate, cement and water. Admixtures are used to increase workability and setting time of concrete mix without affecting its durability, consistency and other strength parameters. Some project specification allows replacement of cement content partially with pozzolanic material such as fly ash, Ground Granulated Blast Furnace Slag (GGBFS), silica fumes etc. to give increased impermeable and durable concrete. In this blog we are looking at some important things that we need to keep in our mind while designing concrete mixes. As we all know, during the mix design trials we attempt to find out perfect proportion of concrete ingredients which gives desired strength and achieve durability requirements. For getting perfect mix all the raw material quality test results shall satisfy the technical specification and IS code requirements for the respective test.
How to design concrete mix
In India IS: 10262 ‘’ Concrete mix proportioning- Guidelines’’ standard has been followed to design concrete mixes. Addition to the Indian standard guideline, requirements set in the technical specification for a particular project shall be taken into consideration. Before designing concrete mixes, we should know characteristic strength, target mean strength, slump requirement at pouring location, distance of pouring site to concrete mixing plant, placing method pump/manual, concrete pouring time etc. Accordingly, proportion of coarse and fine aggregate, cement content, water cement ratio, percentage of admixture shall be arrived.
Finding a good raw material source is bit tricky process particularly aggregates (whether coarse or fine). At many construction sites to get good quality aggregate one need to travel for an average distance of 40 to 50 Km from site location unless the construction firm install its own crusher unit near site. Once the aggregate and other raw material sources finalized, all the material needed shall be tested for a material specification set in the technical specification for respective project. This preliminary test results will be used during preparation of mix design calculations. We are not discussing about mix design calculation part here as it has given in the IS code with example and can be referred from the code.
Things to keep in mind while designing concrete mixes
- All the measuring instruments / equipment shall be calibrated prior to use for concrete mix designs.
- Concrete raw material shall be stacked/stored as per standard industry practice or provisions made in IS code/technical specification. Perform quality tests for raw material at site laboratory / external laboratory for their set frequency intervals. This will assure consistency in raw material quality.
- After finalizing all the raw material proportion and batch quantity, always start with control mix (without adding admixture). This will give a brief visual inspection to observe combination of coarse and fine aggregate in the concrete mix and thus allows to fine tune proportions of concrete ingredients.
- Frequently check flakiness index of the coarse aggregate as increased flakiness index will result in higher breakage of aggregate thus affecting strength and durability parameters. Also, higher surface area and more voids due to flaky material will result in reduced cement paste in the concrete mix.
- It is a myth, that increased sand content in the mix always give smooth and workable concrete. However, sand quantity beyond its required quantity increases fine content in the total mix and therefore reduces compressive strength and increases demand of cement content to form required cement paste.
- It is recommended that compatibility test of admixture with proposed cement grade need to be conducted to check saturation levels of admixture dosages. This will optimize admixture dosage. Marsh cone test apparatus is used to find out saturation limit and optimum dosage in the compatibility test.
- Water cement ratio (or W/C ratio) plays important role in designing concrete mixes. Increased W/C ratio will reduce compressive strength and affect other durability parameters also. The water requirement of the mix can be reduced by using admixtures with suitable dosage.
- Adhesion (cement) and compressive strength are two different factors in the quality of concrete. More cement does not mean more compressive strength. Cement content to a certain proportion (weight per cubic meter) will enhance compressive strength and after that increased cement content will result in increased heat of hydration in the concrete that leads to surface cracks and reduced concrete strength due to increase in fine content of the mix.
- During rainy season or excessive cold where fog can form, always adjust batching quantity to the actual water requirement as per designed water cement ratio. By doing this we are maintaining W/C ratio as per approved mix design. As bulking of sand takes place after increase in moisture content, this will change batch quantity in case of volumetric batch. Conduct the concrete mix design with two different brands of cement by keeping same batch weights of the mix. This will help when supply of one cement brand gets stuck due to any reason.
- After collecting monthly concrete compressive strength data, always check grade wise standard deviation to analyze the mix performance and control over the raw material.
In addition to the above points, I encourage readers to experiment concrete mix behavior in different exposure conditions that will surely unfold exciting facts about concrete mixes.