CAPÍTULO III: VALIDACIÓN DE LA PROPUESTA
3.2 Métodos de Validación
OPC 53 grade cement and special cement OPC 53-S are selected from the compressive strength tests as well as the heat of hydration tests of the cement for the investigated cements. The concrete mix should give a target compressive strength of 50 N/mm2 at 28 days.
The High Early Strength to the concrete mix is required in manufacturing of concrete railway sleepers, precast concrete products like pipes, poles and prestressed concrete structural elements in bridge construction with proper dosage of cement content, water cement ratio and super plasticizer. Attempts were therefore to be made to meet the more stringent requirement that the concrete should attain a strength of the order of 15 to 20 N/mm2 in 16 hours (high early strength concrete) without steam curing. In the process it was also attempted to reduce the cement content of 480 Kg/m3 used in the sleeper manufacturing units, to a value in the neighborhood of 400 Kg/m3 in order to reduce the cost of concrete. And also it was aimed that the fresh concrete should have a good workability which was required in the precast industry (slump of not less than 150mm).
Table 6: Basic Mix Proportions for Concrete Trials with OPC 53
Material Content
OPC 53 Grade Cement 400 kg/m³ Sand 0/5mm (Zone II) 607 kg/m³
Aggregates 5/10mm 695 kg/m³
Aggregates 10/20mm 695 kg/m³
Water content 220 kg/m³
w/c-Ratio Varies (0.38-0.43)
Super Plasticizer % as per Manufacturers Recommendation As part of these investigations, trial mixes have been cast using cement OPC 53 grade cement with different cement samples C-I, C-II and C-III with a cement content of 400 Kg/m3 and with super plasticizer. For the w/c ratio in the expected range of 0.35 to 0.45 (based on the experience of sleeper manufacturing) a few trial mixes without super plasticizers were tried. But all of these gave zero slump.
Hence it was decided to have also an appropriate % of super plasticizer added to the mix in order to attain a slump of desirable magnitude. The basic concrete mix proportions used for the trials mixes are shown in table: 6.
The following mixing sequence was arrived at after several trials optimizing the workability. All the ingredients were first mixed in dry condition in the concrete drum mixer for one minute. Then 70% of calculated amount of water was added to the dry mix and mixed thoroughly for one minute. The remaining 30% of
25
0 1 2 3 4 5 6 7 8 9 10111213141516171819202122
Temperature, °C
Elapsed Time, hours
Temperature Development of Cement Paste with and without Admixtures over
Time
No Admixture SC-II 1% SP-XI SC-II 1% SP-IX SC-II 1% SP-X SC-II
Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development water was mixed with the super plasticizer and poured
into the mixer at the final stage and mixed thoroughly for another one minute before the concrete is taken from the mixer. The concrete was filled in the cube moulds of 150mmx150mmx150mm and were compacted by using table vibrating machine. Because strengths of concrete were to be ascertained not only at 28 days but also at other ages, a minimum of 12 cubes were cast in each mix.
A minimum of 3 cubes were tested to ascertain any particular value and the mean thereof was taken as the result.
Test Results on Fresh Concrete Mixes and Hardened Concrete Mixes with Opc 53 Cement
The concrete mixes were prepared by using OPC – 53 grade cements of different brands with different super plasticizers (dosages were selected as per the manufacturers’ recommendation with the water/cement ratio of 0.35 to 0.40). The workability (slump) and compressive strength tests were conducted on the investigated trial concrete mixes and are shown in table 7.
Table 7: Concrete Trials with OPC 53 grade cements with super plasticizer
Compressive Strength at …Days (N/mm²)
1 day 8.0 10.8 12.1
3 days 10.5 22.8 17.9
7 days 19.0 34.1 33.3
28 days 31.1 42.3 49.3
Note: C-I*, C-II* and C-III* were different brands of OPC 53 grade cements
The concrete trials confirmed the perception that common Grade 53 cement i.e. OPC 53 grade cement is not that suitable for the high early strength concrete needed for the Pre-cast elements production. Even with a low w/c-ratio of 0.38 the concrete reached the 1day compressive strength of only about 8 -10 N/mm² ( table 7, mixes T1 and T2). With cement content increased to 450kg/m³ (mix T3) the strength improved slightly to about 12 N/mm² at 24hrs.
Test Results on Fresh Concrete Mixes and Hardened Concrete Mixes with Opc 53-S Cement
After getting the results of the concrete trials with OPC 53 cement, concrete trials were made with OPC 53-S cement
of samples SC-I and SC-II with cement content of 400 Kg/m3 and with super plasticizer. The basic concrete mix proportions used for the trial mixes with OPC 53-S are shown in table: 8. The mix design proportions by the sleeper factories have been taken as guidelines for arriving at these proportions.
Table 8: Basic Mix Proportions for Concrete Trials with OPC 53-S
Material Content
OPC 53-S Cement 400 to 420 kg/m³ Sand 0/5mm (Zone II) 800 kg/m³
Aggregates 5/10mm 400 kg/m³
Aggregates 10/20mm 800 kg/m³
Super plasticizer 0.7 to 1.8 % by wt. of cement.
Water content 152 kg/m³
w/c-Ratio 0.37 to 0.39
Test results from two trial mixes T4 and T5 made with super plasticizers SP-IV and SP-I are shown in table 9 It can be seen that it was possible to obtain a value as high as 22 N/mm2 concrete strength at 16 hours even with normal curing. But the slump with zero value was unsatisfactory.
Table 9: Concrete Trials with Special Cement OPC 53-S Grade
Compressive Strength at …Days (N/mm²)
16 hrs 16.3 22.4
18 hrs 18.7 24.9
20 hrs 22.4 27.8
24 hrs 27.1 31.7
Note: SC-I and SC-II are the two samples of OPC-53S cement Hence further trails for concrete mix were carried out with the special cement OPC 53-S cement with other super plasticizers. Keeping the cement content as 400 Kg/m3 and water cement ratio as 0.39, trial mixes T6, T7 and T8 with different super plasticizers having appropriate dosage as recommended by the manufacturers, were tried. The mix particulars and the results are shown in table 10. The investigations on fresh concrete have however shown that once again the super plasticizer SP-II, SP-III and SP-IV did not give the
Influence of Super Plasticizers on High Early Strength Concretes Made with Special Cements required slump of an order of 150mm. That the super
plasticizers SP-II and SP-III belonging to the Sulphonated Melamine Formaldehyde (SMF) family did not give satisfactory slump values was in conformity with the results of compatibility tests carried out earlier. Hence these super plasticizers were not considered for further trials.
Table 10: Concrete Trials with Special Cement OPC 53-S Grade
Compressive Strength at ….hours and days (N/mm²)
1 day 35.4 30.5 27.1
3 days 45.7 39.7 28.4
7 days 50.4 45.3 42.1
28 days 53.7 40.0 46.8
Note: SC-I represents the sample of OPC-53S cement
As the Preliminary studies on compatibility of super plasticizers with OPC-53S grade cement have indicated that SP-I belonging to the latest series Multi Carboxylic Either (MCE) group of super plasticizers could be a good choice. The trail mix T5 which used SP-I as super plasticizer gave high early strength of 22 N/mm2 at 16 hours (table 9 but exhibited zero slump. So it was attempted to see whether any modification in the dosages of SP-I could perhaps give better workability. Four trial mixes T9 to T12 (Table 11) have been tried with cement sample SC-I as well as SC-II and with super plasticizer percentage varying between 0.62 to 1.00. But these mixes have also shown inadequate slump characteristics except in the trial mix T-10. Hence the super plasticizer SP-I was also not considered for further trials.
It is relevant to note here that although SP-I showed good compatibility when tested with pure cement, it did not function satisfactorily in the concrete mix. This shows that the compatibility tests on super plasticizers can only be taken as guidelines and final confirmation of the usefulness of the Super plasticizers is to be proved only by a trial mix.
Therefore further investigations were carried out with the super plasticizers SP-VIII of Sulphonated Naphthalene Formaldehyde (SNF) group and SP-IX of Multi Carboxylic Either (MCE) group. The early strength development of concrete mixes with cement OPC 53-S
grade with super plasticizers SP-VIII and SP-IX are shown in table 12.
Table 11: Concrete Trials with Special Cement OPC 53-S Grade
Compressive Strength at ….hours and days (N/mm²)
1 day 24.2 24.9 30.0 31.7 3 days 33.3 33.5 39.0 35.0 7 days 37.3 38.0 44.1 51.3 28 days 39.3 51.5 47.4 56.3 Note: SC-I and SC-II are the two samples of OPC-53S cement
Table 12: Concrete Trials with Special Cement OPC 53-S Grade
Compressive Strength at ….hours and days (N/mm²)
16 hrs 16.7 21.3
From the table 12 it can be observed that it was possible to get the necessary high early strength concrete even by reducing the cement content from 480 Kg/m3 to 400 Kg/m3 using the super plasticizers SP-VIII and SP-IX.
The slump values of 170/180mm and the 28 day compressive strengths were also as per required target values.
Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development The mix proportions finally arrived at for high early
strength concrete mix are shown in table 13.
Table 13: Mix proportions for high early strength Concrete Mix
Material Content
OPC 53-S Cement 400 kg/m³
Sand 0/5mm (Zone II) 800 kg/m³
Aggregates 5/10mm 400 kg/m³
Aggregates 10/20mm 800 kg/m³ Super plasticizer 0.9 % by wt. of cement of
SP-IX of MCE group (or)
1.5 % by wt. of cement of SP-VIII of SNF group
Water content 152 kg/m³
w/c-Ratio 0.38
CONCLUSIONS
1. When it is attempted to design high strength concrete mixes with high early strength using super plasticizer, the Sulfonated Melamine Formaldehyde (SMF) super plasticizers do not give required slump and strength even when added in high dosages.
2. The Sulfonated Naphthalene Formaldehyde (SNF) based super plasticizers require higher dosage compared to Multi Carbaxylat Ether (MCE) group for the same slump, keeping all other parameters same for the required strength.
3. For making high early strength concretes of the order of 15 to 20 N/mm2 in 16 hours without steam curing,
OPC 53-S grade cement with a Blaine’s fineness value of 375 m2/kg is better suited than OPC 53 grade cement with a Blaine’s fineness of 225 m2/kg
4. The mix proportions developed for making high strength/high early strength (50 MPa at 28 days/ 15 to 20 at 16 hours) concretes using OPC 53-S Grade Cements with cement consumption as low as 400 kg/m3 (compared to 480 kg/m3 used by sleeper manufacturer) can be used as guidelines by the construction industry for developing their own trial mixes.
5. It is possible to make concretes of M50 also by using OPC 53 grade, with Blaine’s fineness around 225 m2/kg, but the corresponding 16 hours strength will be only around 10 N/mm2.
REFERENCES
[1] IRS/T-40-1985, Indian Railways Standard Specification for Special grade cement for use in concrete sleepers.
[2] IS 12269:1987, Specification for 53 grade ordinary Portland cement, Bureau of Indian Standards, New Delhi, First Reprint, September, 1993.
[3] Code of Practice for Precast Concrete Conctruction-2003, Buildings Department, the Government of the Hong Kong Special Administrative Region, Hong Kong, October, 2003.
[4] Shetty.M.S., Klaus Muenz, Norbert Gall, “ Delhi Metro:
Quality Control of Concrete for under Ground Secton”
The Indian Concrete Journal, April 2005, pp. 11-21.
[5] Portland cement Association, Concrete Technology Today, Vol.15, No.1, March,1994.
[6] Collepardi M., “Admixtures used to enhance placing characteristics of concrete”, Cement Concrete Composite Vol.20, 1998, pp.103-112.
Proceedings of the National Conference on Advances in Civil Engineering and Infrastructure Development (ACEID-2014), Vasavi College of Engineering, Hyderabad, A.P. 6 - 7 February, 2014. pp.71-75.