CONCRETE MIX DESIGN BY Dr. B.N. KRISHNASWAMIFormer Faculty, DEPT OF CIVIL ENGG., NIT, TRICHY
TECHNICAL MEETING ON 09-05-2009 ORGANISED BY CIVIL ENGINEERS & ARCHITECTS ASSOCIATION, KUMBAKONAM
INTRODUCTION Concrete is the second largest material consumed by human beings after food and water as per WHO. It is obtained by mixing cement, fine aggregate, coarse aggregate and water in required proportions. The mixture when placed in
forms and allowed to cure becomes hard like stone. The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age. The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, proportion of the mix, the method of compaction and other controls during placing, compaction and curing.
Concrete possess a high compressive strength and is usually more economical than steel and is non corrosive which can be made with locally available materials. Hence concrete is used widely in all present-day constructions. The concrete is good in compression and bad in tension. Hence liable to be cracked when subjected to tensile load. In situations where tensile stresses are developed concrete is strengthened by steel bars forming a composite construction called Reinforced Cement Concrete (RCC).
Mix Design Design of concrete mixes involves determination of the proportions of the given constituents namely, cement, water, coarse aggregate and fine aggregate with admixtures if any. Workability is specified as the important property of concrete in the fresh state. For hardened state compressive strength and durability will be considered
Methods of concrete mix design The mix design methods being followed in different countries are mostly based on empirical relationships, charts and graphs developed from extensive experimental investigations. Following methods are in practice 1. ACI Mix design method 2. USBR Mix design method 3. British Mix design method 4. Mix design method according to Indian standard Since ACI Mix design method is an originator for all other methods, including Indian standard method, wherein every table and charts are fully borrowed from ACI, so we follow the ACI Mix design method in practice
Factors to be considered in Mix design (as per SP23-1982)
The design of concrete mix will be based on the following factors.
(a) Grade of concrete: This gives the characteristic strength requirements of concrete. Depending upon the level of quality control available at the site, the concrete mix has to be designed for a target mean strength which is higher than the characteristic strength.
(b) Type of cement: The type of cement is important mainly through its influence on the rate of development of compressive strength of concrete as well as durability under aggressive environments ordinary Portland cement(OPC) and Portland Pozzolona cement (PPC) are permitted to use in reinforced concrete construction.
Table1 GRADES OF CONCRETE
Grade Designation MIO MI5 M20 M25 M30 M35 M40 M45 M50 M55 M60
Specified Characteristic Compressive Strength In N/mm2 At 28 Days Curing 10 15 20 25 30 35 40 45 50 55 60
In the designation of a concrete mix M refers to the mix and the number to the specified characteristic compressive strength of 15 cm cube at 28 days curing expressed in N/mm2.
MI5 and less grades of concrete may be used for lean concrete bases and simple foundation for masonry walls.
Grades of concrete lower than M20 shall not be used in reinforced concrete structure as per IS 456-2000.
Grades of concrete lower than M30 shall not be used in pre stressed concrete structure
Maximum nominal size of aggregate: It is found that larger the size of aggregate, smaller is the cement requirement for a particular water cement ratio. Aggregates having a maximum nominal size of 20mm or smaller are generally considered satisfactory.
Minimum water cement ratio: The minimum w/c ratio for a specified strength depends on the type of cement.
Workability: The workability of concrete for satisfactory placing and compaction is related to the size and shape of the section to be concreted.
Out line of mix design procedure
(a) Arrive at the target mean strength from the characteristic strength required.
(b) Choose the water cement ratio for target mean strength.
(c) Arrive at the water content for the workability required
(d) Calculate the cement content
(e) Choose the relative proportion of the fine and coarse aggregate
(f) Arrive at the concrete mix proportions
Target mean strength: Considering the inherent variability of concrete strength during production it is necessary to design the mix to have a target mean strength which is grater than characteristic strength by a suitable margin ft = fck + k x s Where ft = Target mean strength fck= Characteristic strength k = a constant depending upon the definition of characteristic strength derived IS from the mathematics of normal distribution S = Standard deviation of the particular mix which is available in IS 456-2000 The value of k is equal to 1.65 as per IS 456-2000 where not more than 5% of the test results are expected to fall below the characteristic strength. and is
Table 2 Values of statistical constant (K)
Percentage of result below the characteristic strength Constant k
Table 3 Assumed standard deviation (S)
Grade of concrete Standard deviation assumed (N/mm2)
MATERIAL TESTING RESULTSFINENESS MODULUS TEST COARSE AGGREGATESerial No. Sieve Opening Size (mm) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 80 40 20 10 4.75 2.36 1.18 0.6 0.3 0.15 Weight of soil retained (g) 0 0 1815 1738 1220 197 30 0 0 0 Cumulative weight of soil retained (g) 0 0 1815 3553 4773 4970 5000 5000 5000 5000 Cumulative percentage of soil retained (%) 0 0 36.3 71.06 95.46 99.4 100 100 100 100 Percentage of fine aggregate passing (%) 0 0 63.7 28.94 4.54 0.6 0 0 0 0
= Cumulative percentage of soil retained/100 = (36.3+71.06+95.46+99.4+100+100+100+100)/100 = 7.02
FINENESS MODULUS TEST FINE AGGREGATE Serial No. Sieve Openi ng Size (mm) 1. 2. 3. 4. 5. 6. 7. 4.75 2.36 1.18 0.6 0.3 0.15
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