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

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.

If you have a query, you can ask a question here.