CONCRETE & CONCRETE MIX DESIGN – PART 1
Concrete, Concrete Mix design and its production cover one of the most important and integral part of the construction industry. Technological growth over the years has had its impact on the concrete industry too with the invention and introduction of technology in different materials, production techniques, and different types of concrete production suitable for various constraints. The basic idea of concrete production is more or less unique, and this post is the first one of the same topic to explain some fundamentals of concrete and its production.
What is a concrete mix design?
The process of selecting the right quantity of ingredients – Cement, Course and fine aggregates, water and admixture – for the selected properties of concrete is known as Concrete mix design. Though the word used is cement in mix design it is in fact cementitious materials that include fly Mineral admixtures viz: fly ash, micro-silica or silica fume, rise ash husk, and GGBS.
The properties of concrete that factor in concrete mix design is compressive strength, workability (Slump or flow), retention time required, placing method, and durability. An invisible ingredient that plays a prominent part in the mix design, as well as the finished product, is air content in a given volume of concrete.
The challenge in designing a mix is to arrive at the perfect combination between the cementitious materials, CA, FA, Water, and admixture such that the resulting mix is cost-effective as well as the mix have all the desired properties for which or where it is intended to be used such as strength, workability, and durability, etc. The term for cost-effective concrete in construction is called optimized concrete or optimization. Click here to know more.
Pre Requisites-Before Designing Of Mixes
Since the properties and performance of concrete produced are directly related to a number of different characteristics/properties of its different ingredients. Hence it is essential to ensure that those different materials are tested as per the various clauses of standards/codes pertaining to the materials to ensure that results are satisfactory for concrete mix design.
Materials For Concrete
Cement
Different types of cement are available in the industry with varying properties of their own. The selection of particular cement for a concrete mix depends on the intended use and properties of concrete that are required to fulfill the structure it is going to be used.
Classification of cement is as below
- 33 grade ordinary Portland cement conforming to IS 269-2013.
- 43 grade ordinary Portland cement conforming to IS 8112-2013.
- 53 grade ordinary Portland cement conforming to IS 12269-2013.
- Rapid hardening Portland cement conforming to IS 8041-1990, Reaffirm Apr 2014
- Portland slag cement conforming to IS 455-1989, Reaffirm Apr 2014.
- Portland Pozzolana cement (fly ash based) conforming to IS 1489 (Part 1)-1991, Reaffirm Apr 2014.
- Portland Pozzolana cement (calcined clay-based) conforming to IS 1489 (part 2)-1991, Reaffirm Apr 2014.
- Hydrophobic Portland cement conforming to IS 8043-1991, Reaffirm Apr 2014.
- Low heat Portland cement conforming to IS 12600-1989, Reaffirm Apr 2014.
- Sulphate resisting Portland cement conforming to IS 12330-1988, Reaffirm Apr 2014.
- White cement conforming to IS 8042-1989, Reaffirm Apr 2014.
The precautions, provisions, and instructions for each grade of cement are to be followed while using it for concrete to achieve the best results. Also, it is important that the characteristics and properties of cement be confirmed with respect to the respective standards and codes. Different types of cement shall not be mixed together. In case more than one type of cement is used in any work
Mineral Admixtures
Some industrial by-products or materials derived from other substances are suitable for use in the production of concrete provided it meets some criteria put forth by respective standards and codes. These substances are collectively known as mineral admixtures. The different materials are fly ash, rice ash husk, and Metakaolin which are under the pozzolanic category and GGBS.
1)Fly ash.
It is a by-product of thermal power plants produced from pulverized coal. With its pozzolanic property, fineness, and easiness in which it can be blended with concrete, it is considered to be the primary cementitious material for concrete.
2) Silica Fume
A by-product from the production of silica and silicon-based compounds, that is very fine and pozzolanic. Due to its easily blending property, it is also considered to be a replacement material for cement in concrete in specified percentage,
3) Rice Husk Ash
Ash obtains from rice husk containing large properties of silica. If it is burnt at a controlled temperature amorphous state can be achieved. Careful study and analysis of properties is essential for the proposal of including this in the concrete,
4) Metakaoline
Calcination of pure Kaolin clay at a temperature between 650 to 850 or thermally activating ordinary and kaolin clay produces Metakaoline. Grinding of this product to achieve a fineness of 700 to 900 will result in high pozzolanic character which can be used in concrete.
5) Ground Granulated Blast Furnace Slag – GGBS
A by-product of the iron extraction process from iron ore. Out of many types of slag, only granulated slag is mainly used as a mineral admixture in concrete. It also processes both pozzolanic and cementitious properties. It is usually prepared by quenching to form a hardened matter which is then grounded into particles of fineness similar to the cement, thus the name ‘Ground Granulated Blast Furnace Slag’.
Aggregates
Aggregates form the major portion of concrete which are of mainly two types. Course and fine aggregates. For most work, 20mm aggregate is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be used.
In concrete elements with thin sections, closely spaced reinforcement, or small cover, consideration should be given to the use of a 10mm nominal maximum size.
Fine aggregates are normally river sand or crushed stone sand. The properties of aggregates shall conform with IS 383.
Water
Water used for mixing, and curing shall be clean and free from injurious amounts of oils. Acids, alkalis, salt, sugar, organic materials, or other substances that may be deleterious to concrete or steel. Potable water is generally considered satisfactory for mixing concrete.
Admixtures
Admixtures are additives that are added during the production of concrete to enhance different properties of concrete. Several types of admixtures are available in the market manufactured and supplied by many reputed companies. Admixtures may be any one of the following classes for use in concrete:
- Water Reducing Admixture
- Retarding Admixtures
- Accelerating Admixtures.
- Water Reducing and Retarding Admixtures.
- Water Reducing and Accelerating Admixtures.
- Permeability Reducing (waterproofing) Admixtures.
Standards and codes on concrete mix design are available as guidelines to showing detailed steps to be followed for different kinds of mix which will be covered in subsequent posts. Click here to download Indian standard for Mix Design
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