What is Concrete Strength and what are the factors affecting it?

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By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

Concrete Strength
Cement like water, aggregates and some times admixtures is one of the ingredient of concrete. The mixing of these materials in specified proportions produces concrete. Accordingly cement alone is not a building material, it is the concrete which is a building material. For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregates, and water (under same mixing, curing and testing conditions) is influenced by the :

a) Ratio of cement to mixing water
b) Ratio of cement to aggregates, the strength of the mortar, the bond between the mortar and the coarse aggregate.
c) Grading, surface texture, shape, strength, and stiffness of aggregate particles.
d) Maximum size of aggregate.

Strength of concrete is directly related to the structure of the hydrated cement paste. Air in concrete produces voids. Excess of water in concrete evaporate leave the voids in the concrete. Consequently, as the W/C ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete decreases.

STRENGTH OF CEMENT V/S STRENGTH OF CONCRETE
It is not possible to design a concrete mix of high strength with cement of low strength. The variation in strength of cement is due largely to the lack of uniformity in the raw materials used in its manufacture, not only between different source of supply, but also with in a quarry. Further, differences in details of the process of manufacture and above all, the variation in the ash content of coal used to fire the kilin, contribute to the variation in the properties of commercial cements. This is not to deny that the modern manufacturing of cement is a highly sophisticated process.

Upto 1975, the mass production of cement in India was only OPC-33 Grade. It was found difficulty in obtaining high strength concrete with this cement. The consumer has been normally finding it difficult to get consistent and ensured supply of high strength cement for prestressed concrete and certain items of precast concrete. For these special requirements BIS published IS:8112, Specification for OPC-43 Grade cement. Now, the varieties of cement manufactured in India are:

1. Ordinary Portland Cement (Grade OPC-33, OPC-43 and OPC-53. OPC-33 Grade almost vanished from Indian market)
2. Portland Pozzolana Cement (PPC)
3. Sulphate Resistance Cement (SRC)

Test results of different brand of cement minimum to maximum compressive strength are given in the table-1.

Due to variation of cement strength, the concrete made from these cement will also have variable strength. For a correct approach in the Concrete Mix Design, if the facilities at site are available, with the given set of materials, requirements and site conditions own W/C ratio v/s compressive strength of concrete curve should be developed at site itself.

It is often observed that cement bags marked as OPC-43 Grade may really be containing cement of much higher grade. PPC cement as per IS Code is only of 33 Grade. Where as on bags it is marked as 43 MPa or 53 MPa. Site cement samples should be tested for its actual strength and other properties. There are instances where higher grade cement is being used even for low strength concrete, as mortar or even for plastering. This can lead to unnecessary cracking of concrete/surfaces.

In low grade OPC, the gain in strength will continue beyond 28th day. Due to early strength gain of higher grade of OPC the concrete strength do not increase much beyond 28th day. The heat of hydration of higher grade OPC being higher, the chances of micro-cracking of concrete is much greater. Thus during initial setting period of concrete, the higher head of hydration can lead to damaging micro-cracking with in the concrete which may not be visible at surface. The situation can be worse when we tend to increase the quantity of the cement in concrete with a belief that such increase are better for both strength and durability of concrete.

Table-1 : Compressive Strength of different Grade of Cement:

Test

Grade/ Specifications

Test results as shown on test report of the cement companies

Test results obtained at independent testing laboratory

Brand1

Brand 2

Brand 3

Brand1

Brand 2

Brand 3

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

7-days CS N/mm2

OPC-43 Grade IS:8112

(33.0 Min.)

41.0

45.1

57.7

39.5

42.0

24.7

37.0

28-days CS N/mm2

(43.0 Min.)

50.5

56.1

74.5

41.0

53.5

40.0

45.0

7-days CS N/mm2

OPC-53 Grade IS:12269

(37.0 Min.)

57.8

37.8

43.5

28-days CS N/mm2

(53.0 Min.)

73.9

52.5

57.8

7-days CS N/mm2

PPC

IS:1489 (P-1)

(22.0 Min.)

26.5

38.0

49.4

52.6

34.5

35.0

25.5

38.7

28-days CS N/mm2

(33.0 Min.)

48.0

50.0

63.7

67.0

51.0

54.0

51.0

52.7

The variation in cement strength is given below : (in N/mm2)
OPC-43 Grade : 7-days from 24.7 to 57.7 and 28-days from 40.0 to 74.5
OPC-53 Grade : 7-days from 37.8 to 57.8 and 28-days from 52.5 to 73.9
PPC : 7-days from 25.5 to 52.6 and 28-days from 48.0 to 67.0
Note: Compressive Strength (CS) of cement given are average values.

We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting this research paper and helping all civil engineers understand the concept of Concrete Strength and factors affecting it.

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Comments
  • Zabihullah ibrahimi December 26, 2011 at 9:34 am

    i am very thanks from this web site chief. thanks again and again

    engineer zabihullah ibrahimi from Kabul Afghanistan

  • kumar December 31, 2011 at 2:12 pm

    in nominal mix for M15 we used to adopt 331kgs of cement,0.92 cum of coarse agreegate,0.46 cum of fine aggregates where as in M 25 mix also the quantity of cement used is 300 kgs only.then for less cement high strength please elobarat.

    • Kaushal Kishore January 21, 2012 at 8:33 am

      Concrete strength depend upon many factors in which W/C ratio is most importnat. Now see what
      W/C ratio you are taking for M-15 and M-25 Kaushal Kishore

  • Bose January 30, 2012 at 10:08 am

    Is it advisable to do mortar mixing for RCC roofing on the another RCC roof

    • Kaushal Kishore February 1, 2012 at 7:23 am

      No matter where you mix the mortar. The results obtained will speak itself.

    • Kaushal Kishore February 1, 2012 at 9:57 pm

      Compare the Test Reports issued by Indian Cement Companies with the same Brand of cement Test Reports issued by reputed Laboratories of India. Also see the final results obtained on testing Design Mixes concrete cubes with the same Brand of cement.

  • E.Prasad April 28, 2012 at 12:34 pm

    what is the difference between 53 grade cement & 43 grade cement

  • E.Prasad-quality control engineer April 28, 2012 at 12:36 pm

    how to find out silt content for fineaggregate as per is specification how much

  • tarun jain February 20, 2013 at 7:18 am

    the grade is M30,pls answer me
    am gettin 7day strength as 45 by using opc 53 grade. is it correct

  • Jan F. April 16, 2014 at 7:24 am

    Hi,
    I´m looking for a book or thesis that tries to explain how each of concrete components affects the compressive strength and inner structure of concrete mixture (including things like additives for HPC, microsilika, etc.). I would be grateful for any reccomendations. With a wish of a nice day,
    J.F.

  • Durgesh Shukla September 2, 2014 at 11:16 am

    What is the relationship between concrete and its age or time? For ex. 28 days of curing for strength…

  • Post a comment

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