Strength of Concrete vs 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.

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.

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.

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

Table 1:
S.No.	Details of cement	Compressive strength kg/ sq. cm
		7 days	28 days	90 days
1.	Birla Cement 53 MPa	248	425	468
2.	Ambuja Cement 53 MPa	335	510	597
3.	Sri Ultra 53 MPa	247	348	400
4.	J.K. Cement OPC 43	315	400	425
5.	Birla Nirman Cement 53 MPa	361	490	530
6.	Birla Samrat 53 MPa	251	530	550
7.	Rajshree OPC 53	363	523	545
8.	ACC 53 MPa	355	573	590
9.	Ambuja 53 MPa	387	527	540
10.	Lafarge 53 MPa	227	423	525
11.	Lafarge 53 MPa	229	375	470
12.	Lafarge 53 MPa	230	350	400
13.	Lafarge 53 MPa	250	435	500
14.	Lafarge 53 MPa	360	450	475
15.	Sri Ram OPC 53	359	519	519
16.	Sri Ram OPC 53	377	517	535
17.	Sri Ram OPC 53	468	492	500
18.	Sri Ram OPC 53	360	400	430
19.	Sri Ram OPC 53	448	570	603
20.	Sri Ram OPC 53	472	580	590
21.	J. K. Sarva Shaktiman OPC 43	268	438	580
22.	J. K. Sarva Shaktiman OPC 43	247	430	480
23.	J. K. Sarva Shaktiman OPC 43	300	400	490
24.	J. K. Sarva Shaktiman OPC 43	370	450	475

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.

Table 3:
J K CEMENT OPC - 43
Cement content per cum
W/c ratio	360 kg				380 kg				400 kg				420 kg
	Water content	Strength Kg/sq.cm			Water Content	Strength Kg/sq.cm			Water content	Strength Kg/sq. cm			Water content	Strength kg/sq. cm
		C/S		F/S		C/S		F/S		C/S		F/S		C/S		F/S
		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day
0.45	162	-	-	-	171	490	520	57.0	180	480	512	56.0	189	455	490	51.0
0.50	180	427	480	48.4	190	421	460	50.7	200	396	426	44.0	210	405	445	45.0
0.55	198	358	388	39.5	209	354	384	39.0	220	-	-	-	231	-	-	-
0.60	216	295	333	33.0	228	-	-	-	240	-	-	-	252	-	-	-

Table 4:
BIRLA NIRMAN 53 MPa
Cement content per cum
W/c ratio	360 kg				380 kg				400 kg					420 kg
	Water content	Strength Kg/sq.cm			Water Content	Strength Kg/sq.cm				Water content	Strength Kg/sq. cm			Water content	Strength kg/sq. cm
		C/S		F/S		C/S		F/S			C/S		F/S		C/S		F/S
		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day			28 Day	56 Day	28 Day		28 Day	56 Day	28 Day
0.45	162	-	-	-	171	470	495	52.0		180	475	500	54.0	189	470	490	52.0
0.50	180	439	480	48.0	190	430	475	46.6		200	398	427	42.0	210	390	420	41.0
0.55	198	393	469	43.6	209	328	393	38.5		220	-	-	-	231	-	-	-
0.60	216	350	398	36.5	228	-	-	-		240	-	-	-	252	-	-	-

Table 5:
BIRLA SAMRAT 53 MPa
Cement content per cum
W/c ratio	360 kg				380 kg				400 kg				420 kg
	Water content	Strength Kg/sq.cm			Water Content	Strength Kg/sq.cm			Water content	Strength Kg/sq. cm			Water content	Strength kg/sq. cm
		C/S		F/S		C/S		F/S		C/S		F/S		C/S		F/S
		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day		28 Day	56 Day	28 Day
0.45	162	-	-	-	171	480	525	56.0	180	480	527	57.0	189	448	490	50.0
0.50	180	409	466	47.0	190	451	533	54.0	200	405	501	43.5	210	425	445	44.0
0.55	198	381	449	40.0	209	401	468	45.0	220	-	-	-	231	-	-	-
0.60	216	364	430	38.5	228	-	-	-	240	-	-	-	252	-	-	-

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: