Study of Strength of Concrete vis-à-vis Grades of Cement

Dr. Renu Mathur, Scientist and Head, Dr. A. K. Misra, Scientist and Pankaj Goel, Technical Officer, Rigid Pavement Division, Central Road Research Institute, New Delhi.

Introduction

Cements of different grades viz. 43 OPC (IS–8112)¹, 53 OPC (IS-12269)² and Portland Pozzolana cement (fly ash based), IS-1489 Part I³ etc. with different brand names are available in the market. However, when tested in the laboratory these cements do not give the strength corresponding to their grades in case of OPCs and markings such as 43 MPa 53 MPa on their bags in case of Portland Pozzolana cements.

To design a concrete mix of the desired strength with available materials, the strength of cement is required at 7 days (IRC:44-2008)⁴ or at 28 days (IS:10262-1982)⁵. Based on the strength values of cement, w/c ratio is directly read from the available charts given in IRC-44 & IS-10262. By opting the guidelines as per the above mentioned standards, a very good approximation of concrete strength was possible in past but for last couple of years difficulty is being faced in obtaining the desired strength of concrete with the present day cements.

There are cases when the cements of certain grades give more than prescribed strength at 7 days and thereafter a very little gain in their strength is observed till 28 days and in some cases the strength does not reach the level as per the grade. There are also cases, where a 43 grade cement gives 28 day strength matching with 53 grade. Earlier, IS – 269⁶ satisfied all the requirements of cement in the country. However, there were certain special requirements relating to strength and composition of cement by Railways and Defense at that time and the same were desired to be covered under standardization so that these could be also made available as ISI marked product. Therefore, high strength cements came into picture through IS-8112, OPC – 43 and IS- 12269, OPC- 53 grade to take care of these specific requirements. All the three grades viz. IS 269 (33-grade), IS – 8112 (43 -grade) and IS – 12269 (53 -grade) were ordinary Portland cements.

Besides these three grades of OPC, now there are number of varieties of cements available in the country covered under the IS – 1489 (part I) – 1991, Specification for Portland – Pozzolana Cement (Fly-ash based). Since these PPCs have not been graded, there is no procedure also to check their quality based on their strength at 3,7 or 28 days. Since these cements are fly-ash based, proportions of which are not specified, they do not follow any strength pattern at different ages as is the case with OPCs. The bags containing these cements have the marking IS – 1489 – part I, 43- MPa or 53- MPa. Since IS: 1489 (part I) is formulated to give strength of the order of 33- MPa only, the marking 43- MPa/53- MPa on cement bags is misleading as the consumer can not reject the consignment in the case of lower strength. The erratic behavior of cement w.r.t. their strength and that of concrete mixes suggested a detailed study on available cements.

Objectives & Scope

• The broad objective of the project was set to study various known brands of cement available in market w.r.t. the strength development of concrete prepared by their use.
• To simplify the procedure of concrete mix design with the available cements.

Work Plan & Methodology

Different brands of cements and cements of same brand name but from different consignments were procured from local market. These were tested for their compressive strength at 7, 28 and 90 days to study the pattern of strength gain.

With three different brands of cement, concrete mixes were designed as per the existing standards using four different quantities of cement per cubic metre of concrete at different water cement ratios.

The results obtained were analyzed, discussed and suggestions have been given for design of concrete mixes with available cement within reasonable period.

Results & Discussion

The compressive strength of different brands of cements and cements of same brand procured from different consignments are shown at Table 1.

From the results it can be seen that these cements do not adhere to the strength requirements as prescribed for either OPCs or PPC (Table 2) by BIS. It can also be seen from the results that though a good number of cements have 7 day Compressive Strength (C/S) below 300 kg/sq. cm, most of them have their 28 day C/S above 400 kg/sq. cm and 90 day C/S up to 500 kg/sq. cm and still above it.

From this study it is revealed that these available cements may be used for cement concrete works including pavement quality concrete. Though Portland Pozzolana cements are designed for 33 grade, none of the PPC tested under this study had the strength of this order. The results obtained are very high. The fact, that it is not possible to design a concrete mix of high strength with cements of low strength, there is a tendency among the manufacturers of Portland Pozzolana cements to produce high strength PPCs with the labels such as IS: 1489 (Part I), 43- MPa & IS: 1489 (Part I), 53- MPa to lure the consumer. The dilemma is that in case of not getting strength 43- MPa/53- MPa, consumer can not reject the PPC and in any case he is to use the supplied cement having minimum 7 days compressive strength, 220 kg/sq.cm and 28 days compressive strength, 330 kg/sq. cm. At present when manufacturers of PPC are not grading their product and also the supply of OPC in open market is highly restricted, the consumer is left with the only option to find out ways to use available cements for design of concrete mixes for various applications.

Thus for any project, it has become a difficult task to design a concrete mix. For making the process of designing a concrete mix less cumbersome, based on the present study, a minimum 7 day compressive strength of 250 kg/sq. cm is recommended for cements to be used for pavement quality concrete (PQC).

Three brands of cement were chosen for the study form the list of tested cements. For detailed investigations, a number of cement concrete mixes were designed using four different quantities of cement (360 kg, 380 kg, 400 kg and 420 kg) and various water cement ratios viz. 0.45, 0.50, 0.55 and 0.60. The results are summarized at Tables 3, 4, 5.

From the results it can be seen that at same water cement ratio, the compressive and flexural strengths go on decreasing as the cement content increase from 360 kg to 420 kg for water contents above 190 litre per cubic meter of concrete. As the cement content increases at the same water cement ratio, the water content per cubic meter of concrete goes on increasing. In order to restrict the maximum water content per cubic meter of concrete to 190 litre considering the guidelines as suggested in IRC-44 & IS- 10262 (for 20 mm maximum size of coarse aggregate), and greater fineness of available cements, w/c ratio may be selected as 0.50 for 360 and 380 kg cement per cubic meter of concrete and 0.45 for 400 and 420 kg cement per cubic meter of concrete.

Based on the study, a concrete mix can be designed with great ease within minimum reasonable time by following the procedure as outlined below:

1. Test the cement for 7 day compressive strength. If the compressive strength is greater than 250 kg per sq. cm, select it for concrete mix design.
2. Design four concrete mixes using
   1. Cement–360 kg; Water–180 litre
   2. Cement–380 kg; Water-190 litre
   3. Cement-400kg; Water–180 litre
   4. Cement-420 kg; Water–190 litre
3. Prepare all four mixes simultaneously in the laboratory. Check the workability and if required, use super plasticizer. Prepare cube and beam specimens from the mixes to be tested at 28 days.
4. From the results, the mix may be selected for the desired compressive and flexural strength. Using very low w/c ratio and excessive use of super plasticizers shall be avoided as besides their use being uneconomical, long term effects of these admixtures are not generally known to common users.

Conclusion and Recommendations

• All the available OPCs and PPCs, irrespective of marking on their bags have 28 days strength above 400 kg/sq. cm . The strength of these cements further increases with time and at 90 days a good number of cements have C/S beyond 500 kg/sq. cm.
• Since the cements have good strength, these can be used for pavement quality concrete.
• Portland Pozzolana cements as per IS: 1489 (part – I) were produced for durability of structure rather than for higher strength. Since all the available PPCs are giving high strength, it is high time that the manufactures grade their PPCs.

References

• IS 8112: 1989 (Reaffirmed 2005) “Specification for 43 grade ordinary Portland cement.”
• IS 12269: 1987 (Reaffirmed 2008) “Specification for 53 grade ordinary Portland cement.”
• IS 1489: Part 1: 1991 (Reaffirmed 2005) “Specification for Portland Pozzolana cement Part 1 Fly-ash based.”
• IS 10262: 1982 (Reaffirmed 2004) “Recommended guidelines for concrete mix design.”
• IRC: 44-1976 “Tentative Guidelines for Cement Concrete Mix Design for Pavements (for Non-Air Entrained and Continuously Graded Concrete) “.(First Revision).
• IS 269: 1989 (Reaffirmed 2008) “Specification for 33 grade ordinary Portland cement.”