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Materials Engineer, Roorkee
Concrete mix design is the process of choosing suitable ingredient of concrete and determining their relative quantities with the object of producing as economically as possible concrete of certain minimum properties, notable workability, strength and durability. It should be explained that an exact determination of mix proportions by means of table or computer data is generally not possible. The materials used are essentially variable and many of their properties cannot be assessed truly quantitatively. A Laboratory trial mix does not provide the final answer even when the moisture condition of aggregates are taken into account. Only a mix made and used on the site can guarantee that all properties of the concrete are satisfactory in every detail for the particular job in hand. In fact mix selection requires a knowledge of the properties of concrete and experimental data, and above all the experience of the expert who conduct the mix design. The selection of mix proportions is an art as much as a science. It is not enough to select a suitable concrete mix; it is also necessary to ensure a proper execution of all the operation involved in concreting. It cannot be stated too strongly that, competently used, concrete is a very successful construction material but, in the literal service of the word, concrete is not fool proof. The mix proportions once chosen, cannot expected to remain entirely immutable because the properties of the ingredients (cement, sand, aggregate, water and admixture) may vary from time to time.
CONCRETE MIX DESIGN
IS: 456-2000 page 22 clause 9.2 specified that: As the guarantor of quality of concrete used in the construction, the constructor shall carry out the mix design and the mix so designed (NOT THE METHOD OF DESIGN) shall be approved by the employer.
The above code clearly mentioned that the method of mix design should not be reported, yet in some mix design reports calculations are given. IS: 10262-2009 (Mix Design Code) on last inside page 7th line state that: This standard does not debar the adoption of any other methods of concrete mix proportioning. Accordingly, if a expert conduct mix design as per required specifications and with given materials based on his experiences and published research papers he did not mention the calculations in the report, because the checker could not check the calculations, as the mix design report is based on expert experience and his published research works and these are not with the checker. Accordingly IS: code correctly mentioned MIX DESIGN METHOD SHOULD NOT BE REPORTED. i.e. only mix proportions are to be given. In fact concrete mix design report should not be checked on table, as mix design is not a mathematical problem. It should be practically checked at site itself with the site actual available materials during checking time. If the performance of the mix found not satisfactory, then the mix shall be modified with the consultation of contractor Designer/Expert to get the mix of required properties. Then only the mix should be allowed to be used in the construction. This may be noted any change of materials, site conditions and climate again will required modifications in the mix to give concrete of required properties and specifications.
A Laboratory may conduct many trials before getting a final mix. This finally concluded only one mix is to be reported in the mix design report. It is not justified that 2 or 3 extra trials are reported in the mix design and charged for it from the contractor as these extra reported trials has no use for contractor as well as for the site.
OTHER SPECIFICATIONS OF CONCRETE MIX DESIGN
IRC: SP: 46-2013 page 9. Clause 4.3 specified that: Use of any method to arrive at mix proportioning, which gives the desired performance, is permitted. Past experience and trials can guide the proportioning process.
MSRTH: Specifications for Road and Bridge Works (Fourth Revision) page 463. Clause 1704: specified that: Prior to the start of construction, the contractor shall design the mix in case of “Design Mix Concrete” or propose “nominal mix Concrete” and the proportions of material, including admixture to be used, water reducing admixture (including plasticizer or super-plasticizer), may be used at the contractor option, subject to the approval of the Engineer.
CPWD Specifications 2002 page 88 clause 22.214.171.124 specified that: The producer/manufacturer/contractor of concrete shall submit details of each mix (obtained by trial) of each grade of concrete designed for various workability conditions to the Engineer-in-Charge for his comments and approval. Concrete of any particular design mix and grade shall be produced/manufactured for works only on obtaining written approval of the Engineer-in-Charge.
IS: 10262:2009 last inside page 7th line specified that: This standard does not debar the adsorption of any other method of concrete mix proportioning.
MES, SSR-2009, Part-I Specifications on page 58 clause 4.11.1 specified that: Cement concrete and reinforced concrete shall be provided in accordance with IS: 456-2000, which specified that: the Constructor (contractor) shall carry out the mix design.
This may be noted that in above MES specification volume and also in IS: 456-2000 the reference of IS: 10262 (Concrete Mix Proportioning) is not mentioned.
ACCORDINGLY THE CONCRETE MIX DESIGN SHOULD BE CARRIED OUT BY A KNOWN EXPERT.
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