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1 Center for By-Products Utilization EFFECT OF DOSAGE OF SUPERPLASTICIZER ON CONCRETE MICROSTRUCTURE AND STRENGTH By Tarun R. Naik and Shiw S. Singh Report No. CBU September 1997 Department of Civil Engineering and Mechanics College of Engineering and Applied Science THE UNIVERSITY OF WISCONSIN - MILWAUKEE

2 ABSTRACT This work was conducted to evaluate the amount of superplasticizer on concrete microstructure and compressive strength. A melamine-based superplasticizer was used for this investigation. Four dosages of the superplasticizers were used. When the normal dosage of the superplasticizer was used, concrete strength improved considerably. However, when superplasticizer dose was increased beyond normal dose, concrete performance deteriorated. This was attributed to the fact that at high dosages of the superplasticizer substantial segregation in the concrete mixture was observed. This resulted in the poor microstructure which adversely affected concrete strength. INTRODUCTION Water-reducing admixtures are used either to improve concrete flexibility or decrease cement demand for a given workability. Use of normal water-reducing admixtures (NWRA) at a dose of 0.05 to 0.1% of total cementitious material results in reducing water demand by 5 to 12% [1,2]. However, use of a NWRA at high dosages causes excessive set retardation, increased air entrainment, and reduced strength[1,2]. Consequently, they are not suitable for application where large amounts of water reduction are required, especially in manufacture of high-strength concrete (HSC) or high-performance concrete (HPC). To offset the adverse affects of normal water-reducing admixtures, high-range water reducing admixtures (HRWRAs), called superplasticizers, are used. These HRWRAs can reduce water demand by 20% to 30% without significantly increasing setting and hardening time, air entrainment of concrete, and decreasing strength. Superplasticizers are composed of long chain high-molecular

3 weight (20,000 to 30,000) anionic surfactants [3]. These surfactants cause reduction in surface tension of the water surrounding the cement particles. HSC/HPC are manufactured at low water-to-cement ratio for improving strength and durability-related properties. However, excessive amounts of superplasticizer can cause negative effects on the concrete properties due to the increased segregation, resulting in deterioration of concrete microstructure. This research was carried out to evaluate the effects varying amounts of a superplasticizer on concrete microstructure and compressive strength. EXPERIMENTAL PROGRAM Non-air entrained concrete mixtures were proportioned to have the 28-day strength of 5,000 psi. The fine aggregate was natural sand with a 1/4-inch nominal maximum size. The coarse aggregate used in this study was 3/4-inch nominal maximum size crushed limestone that met ASTM C 33 requirements. All concrete mixing, casting, and curing of test specimens was performed in accordance with ASTM C 192. An Hitachi S-570 scanning electron microscope was used to obtain micrographs of concrete using polished specimens. MICROSTRUCTURE OF CONCRETE As evident from Fig. 1(a), 2(a), 3(a), and 4(a), on microstructure level concrete is composed of two major phases: a matrix and a particulate filler. The particulate filler is composed of coarse and fine aggregates differing in shape and size. The matrix, called

4 hydrated cement paste (HCP), acts as a binding medium for the aggregates to form a stable structure. At microscopic level, details of internal structure of concrete were observed (Fig. 1 through 4). Generally, concrete microstructure is quite different at the interfacial region between aggregate and HCP compared to bulk cement paste. The interfacial region is called transition zone (TZ). This zone is much more porous compared to bulk HCP (away from TZ). This is due to the fact that water accumulates in TZ due to the wall effects of aggregates, leading to increased water to cementitious materials ratio. Consequently, there are decreased amounts of hydration products and increased porosity (unfilled spaces in the structure). The interfacial region is marked on the micrograph.

5 REFERENCES 1. Ramachandran, V.S., "Recent Progress in the Development of Chemical Admixtures," in Advances in Concrete Technology, Second Edition, V.M. Malhotra, Ed., CANMET, Natural Resources Canada, Ottawa, Ontario, Canada, 1994, pp Collerpardi, M., "Advances in Chemical Admixtures for Concrete," in Advancement in Cement and Concrete," Proceedings of an Engineering Foundation Conference, M.W. Civil Engineers, New York, NY, Grutzeck and S.L. Sarkar, Eds., American Society of July 24-29, 1994, pp Mehta, P.K., and Monterio, P.J. M., "Concrete Structure, Properties, and Materials," Prentice-Hall, Inc., Englewood Cliff, New Jersey, Second Edition, 1993, pp REP-326

6 Table 1 Mixture Proportion Parameter Mix 1 Mix 2 Mix 3 Mix 4 Cement (lb/yd 3 ) Water (lb/yd 3 ) Sand (lb/yd 3 ) /4-in. SSD Coarse Aggregate (lb/yd 3 ) Slump (in.) 1/ Air Temperature ( 0 F) Concrete Temperature ( 0 F) Unit Weight (lb/yd 3 ) *Superplasticizer (%) * Percent of cementitious material Table 2: Compressive Strength of Test Mixtures Age, Day Compressive Strength, psi* Mix 1 Mix 2 Mix 3 Mix *Each data point represents the average of three observations. 145 psi = 1 MPa

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8 (a) (b) (c) (d) Fig. 1 Micrographs of Mix 1

9 (a) (b) (c) (d)

10 Fig. 2 Micrographs of Mix 2

11 (a) (b) (c) (d)

12 Fig. 3 Micrographs of Mix 3

13 (a) (b) (c) (d)

14 Fig. 4 Micrographs of Mix 4