EFFECT OF SUPERPLASTIZER DOSAGES ON COMPRESSIVE STRENGTH OF SELF COMPACTING CONCRETE

Transcription

1 International Journal of Civil, Structural, Environmental and Infrastructure Engineering Research and Development (IJCSEIERD) ISSN Vol.2, Issue 3, Sep TJPRC Pvt. Ltd., EFFECT OF SUPERPLASTIZER DOSAGES ON COMPRESSIVE STRENGTH OF SELF COMPACTING CONCRETE 1 RAHUL DUBEY & 2 PARDEEP KUMAR 1 Research Scholar, Department of Civil Engineering, NIT Hamirpur, HP, India 2 Dean Of Studies Himachal Pradesh Technical University Hamirpur, Himachal Pradesh, India ABSTRACT Self-compacting concrete has to fulfill contradictory requirements of high flowing ability when it is being cast and high viscosity when it is at rest, in order to prevent segregation and bleeding. These requirements make the use of mineral and chemical admixtures essential for self-compacting concrete. High flowing ability is achieved using superplasticizers, while stability against segregation is achieved either by using a large quantity of fine materials, or by using appropriate viscosity modifying agents. This paper presents the results of an experimental research carried out to investigate the effect of dosages of superplasticizer on compressive strength of self- compacting concrete. KEYWORDS: Self-compacting concrete, superplasticizer, compressive strength. INTRODUCTION Self-compacting concrete (SCC), requires no consolidation work at site or concrete plants. The self compacting concrete was first developed in Japan to improve the reliability and uniformity of concrete in 1988 (Okamura, 1999). However, to design a proper SCC mixture is not a simple task. Various investigations have been carried out in order to obtain rational SCC mix-design methods. The establishment of methods for the quantitative evaluation of the degree of self-compactibility is the key issue in establishing the mix design system (Noor et al. 1999). Okamura and Ozawa (Okamura, 1999) have proposed a simple mixture proportioning system. In this method, the coarse and fine aggregate contents are kept constant so that self-compactibility can be achieved easily by adjusting the water/cement ratio and superplasticizer dosage only. Water/powder ratio is usually accepted between 0.9 and 1.0 in volume, depending on the properties of the powder (Noor et al. 1999, Sedran et al.1999). In Sweden, Petersson and Billberg (1999) & Emborg( 1999) developed an alternative method for mix design including the criterion of blocking, void and paste volume as well as the test results derived from paste rheology studies. Many other investigators have also dealt with the mix-proportioning problems of SCC (Sedran et al.1999, Bui et.al.1999, Roshavelov, 1999). Some design guidelines have been prepared from the acceptable test methods (EFNARC, 2002). Self compacting concrete is also made from the same basic constituents as conventional concrete, but mix proportions for SCC differ from those of ordinary concrete. The Self compacting concrete contains more powder content, less coarse aggregates, high range water reducing superplasticizer (SP) in larger amounts and frequently a viscosity modifying

2 99 Effect of Superplastizer Dosages on Compressive Strength of Self Compacting Concrete agent (VMA) in small doses in comparison to ordinary concrete. This paper reports the results of an experimental investigation to determine the effect of dosages of SP on compressive strength of SCC. MATERIALS USED AND MIX PROPORTION An Ordinary Portland Cement (OPC) 43 grade confirmed to Indian Standards (IS: ) reaffirmed 2005 was used in the experimental program. Class F fly ash procured from Guru Gobind Singh Thermal Power Plant, Ropar, India confirmed to Indian Standards (IS: ) along with silica fume confirmed to Indian Standards (IS: ) was used as a mineral powder. The fly ash was used due to its additional strength contributing pozzolanic activity and silica fume was used as a property enhancing material. Because of the very high surface area, concrete made with silica fume is more cohesive and therefore less prone to segregation and bleeding. The chemical properties of fly ash and silica fume are presented in Table 1. Locally available 10 mm downgraded crushed stone of specific gravity 2.65 and fineness modulus of 5.98 was used as coarse aggregate. Locally available river sand of specific gravity 2.6 and fineness modulus of 2.4 was used as fine aggregate. Both coarse aggregate and fine aggregate confirmed to Indian Standard Specifications IS: Polycarboxylate ether based superplasticizer complying with ASTM C 494 type F, with density 1.08 kg/l and ph 4.8 was used in the present investigation as SP. The mix composition was selected as per recommendations of ACI: 237R07 and EFNARC. The quantities of different ingredients used in the preparation of mix are given in Table 2. Table 1: Chemical Properties of Fly ash & Silica Fume Property Fly ash Silica fume (Sio 2 +Al Fe 2 o) % by mass Sio 2 % by mass Mgo% by mass Total Alkali (Na 2 o+k 2 o ) % by mass Sulphuric Anhydride % by mass Al % by mass Fe 2 o 3 % by mass Table 2: Mix Proportions Used in the trials Cement Fly ash Silica Coarse Fine w/p Fume aggregate aggregate (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 )

3 Rahul Dubey & Pardeep Kumar 100 PREPARATION OF SCC MIXES For the mix proportion shown in Table 2, required quantities of materials were weighed. Fine and coarse aggregate combinations were mixed for 90 s in a mixer. Cement and filler combinations were added into the aggregate mixes and remixed for 60 s all together in dry state. The water and SP were mixed in a flask and poured slowly into the mixer while mixing. The total mixing time was five minutes with the rotating speed of 45+1 cycle/min of the mixer. The casting immediately followed mixing, after carrying out the tests for fresh properties. The top surface of the specimens was scraped to remove excess material and achieve smooth finish. The specimens were removed from moulds after 24 h and cured in water till testing or as per requirement of the test. The cubes of size 150 mm were cast for determination of compressive strength in accordance with Indian Standards IS 516 (1959). All test measurements were taken as the average of three readings. EXPERIMENTAL STUDIES Six trial mixes were prepared by varying the dosage of SP from 2% to 12% of cementious material with an increment of 2% for each test series. In order to investigate self compacting characteristics in fresh state of mix proportion with varying dosages of SP slump flow test (slump flow diameter and T 50cm time), V-funnel flow test (time taken by concrete to flow through V-funnel), and L- Box test (blocking ratio) were performed according to the procedure proposed by EFNARC. Brief description of these tests are given below. Slump Flow Test The basic equipment used is same as for the conventional Slump test (Figure 1). The test method differs from the conventional one in the way that the concrete sample placed into the mould has no reinforcement rod and when the slump cone is removed, the sample collapses. The diameter of the spread of the sample is measured, i.e. a horizontal distance is measured as against the vertical slump measured in the conventional test. While measuring the diameter of the spread, the time that the sample takes to reach a diameter of 500 mm (T 50 ) is also measured. The Slump Flow test gives an indication about the filling ability of SCC. L-Box Test The L-box test method uses a test apparatus comprising a vertical section and a horizontal trough into which the concrete is allowed to flow on the release of a trap door from the vertical section passing through reinforcing bars placed at the intersection of the two areas of the apparatus (Figure 2). The time it takes to flow a distance of 200mm (T-20) and 400 mm (T-40) into the horizontal section is measured. When the flow has stopped, the height of the concrete at the end of the horizontal section (H2) and that remaining in the vertical section (H1) is measured and ratio H2/H1 is defined as blocking ratio. The L-box test gives an indication as to the filling ability and passing ability.

4 101 Effect of Superplastizer Dosages on Compressive Strength of Self Compacting Concrete Figure 1: Geometric diagram of Slump flow test and test in progress Figure 2: Geometric diagram of L-Box and test in progress V-Funnel Test The equipment consists of a V-shaped funnel (Figure 3). The funnel is filled with the concrete and the time taken by it to flow through the funnel is measured. This test gives account of the filling capacity (flowing ability) of SCC.

5 Rahul Dubey & Pardeep Kumar 102 Figure 3: Geometric diagram of V-funnel and test in progress RESULTS AND DISCUSSION Fresh concrete properties obtained from the tests such as slump test, T 50 Time, L-Box, V-Funnel test are given in table 3. These values are tabulated for different %age dosages of SP. The values have been compared with the recommended values. Table 4 shows the setting time and compressive strength of the mixes with varying %age of SP dosages Table 3 : Fresh Properties of SCC Sr.no. Fresh Concrete Test method 1 Slump flow (Abram s slump Cone) 2 L-box 20 cm 40 cm h 2 /h 1 3 V Funnel Recommended value as per EFNARC mm in 5s Observed value for % dosage of HRWRA in 3.6s s with top layer of s water mm 700mm 710mm 740mm in 3s with In 4.1s with In 3.6s with In 3s with segregation segregation segregation segregati on Levelled with top layer of water Levelled with top layer of water Levelled 8 to 12 s blocked

6 103 Effect of Superplastizer Dosages on Compressive Strength of Self Compacting Concrete Table 4 : Results for Setting Time and Compressive Strength % dosage of HRWRA Setting time in no. of days Compressive Strength (Mpa) at no. of days Not set even after 11 days Various fresh concrete properties, setting time & compressive strength results of the SCC mixes can be summarized as follows 1. At 2% dosage of SP in the mix proportion, setting time of concrete was 24hours and compressive strength of 7Mpa at 3days, 14.2 Mpa at 7days, 22.6 Mpa at 28 days, 28.5 Mpa at 56 days, 30.1 Mpa at 90 days, 34 Mpa at 120 days was achieved. 2. At 4% dosage of SP in the mix proportion, setting time of concrete was 72 hours and compressive strength of 5Mpa at 3days, 11.8 Mpa at 7days, 18.9 Mpa at 28 days, 20.7 Mpa at 56 days, 21.0 Mpa at 90 days, 23.3 Mpa at 120 days was achieved 3. At 6% dosage of SP in the mix proportion, setting time of concrete was 7 days and compressive strength of 4.5 Mpa at 3days, 10.2 Mpa at 7days, 15.7 Mpa at 28 days, 18.4 Mpa at 56 days, 20.1 Mpa at 90 days, 21.8 Mpa at 120 days was achieved 4. At 8% dosage of SP in the mix proportion, setting time of concrete was 7days and compressive strength of 7Mpa at 3days, 13.2 Mpa at 7days, 22.6 Mpa at 28 days, 28.7 at 56 days, 37.4 Mpa at 90 days, 41 Mpa at 120 days was achieved Mpa. 5. At 10% dosage of SP in the mix proportion, setting time of concrete was 11days and compressive strength of 3.7 Mpa at 3days, 8.1 Mpa at 7days, 12.4 Mpa at 28 days, 14.7 Mpa at 56 days, 17.4 Mpa at 90 days, 19 Mpa at 120 days was achieved. 6. On increasing the dosage of HRWRA beyond 10 % the trial mixes were not set even after 11 days. CONCLUSIONS Experimental, investigations lead to following conclusions:- 1. With incorporation of SP up to 4% there was significant increase of compressive strength of SCC mix at all ages. 2. When SP was added beyond 4% and upto 8%, although there was increase in compressive strength at all ages but the increase was marginal.

7 Rahul Dubey & Pardeep Kumar Beyond 8% and upto 10% the increase in compressive strength with aging was further reduced. 4. With the increase of dosage of SP setting time was also increased. When SP was added more than 10% trial mix was not set even after 11 days. 5. This trend of compressive strength and setting time suggests that in the presence of increased SP the hydration of cement did not take place appropriately. REFERENCES 1. Okamura, H., Ouchi, M. (1999). Self Compacting Concrete. Development, Present Use and Future. Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp Noor, M. A., Uomoto, T. (1999). Three-Dimensional Discrete Element Simulation of Rheology Tests of Self-Compacting Concrete. Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp Sedran, T., De Larrard, F. (1999). Optimization of Self-Compacting Concrete Thanks to Packing Model, Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp Petersson, O., Billberg, P. (1999). Investigation on Blocking of Self- Compacting Concrete with Different Maximum Aggregate Size and Use of Viscosity Agent Instead of Filler, Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp Emborg, M. (1999). Rheology Tests for Self-Compacting Concrete How Useful are they for The Design of Concrete Mix for Full-Scale Production?, Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp Bui, V. K., Montgomery, D. (1999). Mixture Proportioning Method for Self compacting High Performance Concrete with Minimum Paste Volume, Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp , T. T. (1999). Concrete Mixture Proportioning with Optimal Dry Packing, Proceedings of the lst International RILEM Symposium on Self Compacting Concrete, Sweden, pp EFNARC. (2002). Specification and Guidelines for Self-Compacting Concrete.Retrieved from http;// 9. IS: (2005). Indian Standard 43 Grade Ordinary Portland cement Specifications, Bureau of Indian Standards, New Delhi (India).

8 105 Effect of Superplastizer Dosages on Compressive Strength of Self Compacting Concrete 10. IS: Indian standard on pulverized fuel ash specification. Part 2.For use as admixture in cement mortar and concrete. Bureau of Indian Standards. New Delhi (India). 11. IS: (2003). Indian Standard Silica Fumes - Specifications, Bureau of Indian Standards, New Delhi (India). 12. IS: 383. (1997). Indian Standard Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete, Bureau of Indian Standards, New Delhi (India). 13. IS: 516. (1959). Indian Standard Method of Tests for Strength of Concrete, Bureau of Indian Standards, New Delhi (India)