MECHANICAL PROPERTIES OF HIGH-PERFORMANCE CLASS C FLY ASH CONCRETE SYSTEMS

Transcription

1 MECHANICAL PROPERTIES OF HIGHPERFORMANCE CLASS C FLY ASH CONCRETE SYSTEMS By Shiw S. Singh*, Tarun R. Naik**, Robert B. Wendorf***, and Mohammad M. Hossain**** Abstract The influence of inclusion of Class C fly ash on strength and durability properties of highperformance concrete (HPC) was investigated. Concrete mixtures with and without fly ash were proportioned to attain 28day compressive strength of 6000 psi. Fly ash mixtures were proportioned to have cement replacement of approximately 35, 45, and 55% by weight. One pound of cement was replaced with 1.2 lbs of fly ash (a replacement ratio of 1 to 1.2). Each concrete mixture was tested for compressive strength, splitting tensile strength, flexural strength, abrasion resistance, and chloride permeability. In general, the concrete mixtures up to 55% cement showed adequate performance with respect to all of these parameters for highstrength applications. However, the 35% fly ash mixture (i.e., flyash to cement plus fly ash ratio of 0.40) was found to be especially appropriate for manufacture of HPC. Keywords: abrasion resistance; chloride permeability; compressive strength; concrete; flexural strength; fly ash; modulus of elasticity. * Postdoctoral Fellow, Center for ByProducts Utilization, University of WisconsinMilwaukee, P.O. Box 784, Milwaukee, WI ** Director, Center for ByProducts Utilization *** Laboratory Manager, Center for ByProducts Utilization

2 **** Research Associate, Center for ByProducts Utilization

3 Introduction Highperformance concrete is generally defined as a concrete which possesses highstrength as well as highdurability properties. This type of concrete is desirable as a construction material for use in adverse environments. These applications include seafloor tunnels, offshore piers and platform, sewage pipes, confinement structures for toxic and radioactive materials, heavy duty pavements, etc. A large number of studies (112) have been directed toward development of mixture proportions for production of concrete containing fly ash for structural and highstrength applications. However, very limited amount of research has been done in manufacture of lowcost HPCs containing large amounts of fly ash (11). Therefore, a need exists for developing HPC mixtures incorporating large amounts of fly ash. An extensive research is in progress at the Center for ByProducts Utilization at UWMilwaukee to develop lowcost highperformance concretes (HPCs) containing significant amounts of mineral admixtures including Class F and Class C fly ashes. This study was primarily directed toward studying the effect of Class C fly ash on mechanical properties of concrete. Experimental Program Materials A Type I Portland cement meeting ASTM C 150 requirements was used (Table 1). A highcalcium fly ash obtained from one source was used in this work (Table 1). A natural sand with a ¼ in. maximum size was used as a fine aggregate. A coarse aggregate used in this study was ¾ in. nominal maximum size of crushed limestone obtained from one source. Both the fine and coarse aggregates met the ASTM C 33 requirements. A commercially available melaminebased superplasticizer, and a resin type airentraining admixture were used. Mixture Proportions A total of four different mixtures (C1, D1, D2, and D3) were produced. One of them was a reference mixture without fly ash which was proportioned to have 28day strength of 6000 psi. The other three mixtures contained the Class C fly ash. The mixture proportions were developed for producing concrete on a 1.2 to 1, fly ash inclusion to cement replaced, weight basis. The levels of cement replacement ranged from 35 to 55%. The mixture proportions are given in Table 2. The watertocementitious materials ratio (W/(C+FA)) was maintained at about 0.30 and air content was kept at 6 ± 1%. Each test batch of 5.4 ft 3 was mixed in a power driven tilting mixer according to ASTM C 192. Preparation of Test Specimens

4 Cylinders (6 x 12 in.) were cast from each mixture to evaluate compressive strength, modulus of elasticity, and splitting tensile strength. Prism specimens of 3 x 4 x 16 in. were cast for flexural strength measurements. Slab specimens (12 x 12 x 4 in.) were cast to evaluate abrasion resistance, and cylinders (4 x 8 in.) were cast for measurement of chloride ion permeability. All the test specimens were cast in accordance with ASTM C 192. All the specimens, after casting, were covered with plastic to minimize their moisture loss. After 24 hours storage at room temperature of about 73 ±5 F the specimens were demolded. Then the specimens were moistcured at 73 ±2 F temperature with 100 percent relative humidity until the time of test. Testing of Specimens Slump, unit weight, temperature, and air content for each batch were determined according to applicable ASTM test methods. Compressive strength of test specimens were determined in accordance with ASTM C 39. Static modulus of elasticity of concrete specimens was determined in accordance with ASTM C 469. Tensile strength test was carried out according to ASTM C 496. Flexural strength measurements were made using the thirdpoint loading according to ASTM C 78. Three specimens were tested for each experimental condition for evaluation of the above properties. Previous investigations at the Center for ByProducts indicated that the standard ASTM C 944 test is inadequate to evaluate abrasion resistance of highstrength concrete specimens. Therefore, a modified ASTM C 944 test, developed at the Center for ByProducts, was used in this work. The modified method was equipped with washers having a smaller diameter relative to the dressing wheels. Furthermore, approximately one teaspoon of silica sand ("Ottawa Sand") was added to the concrete surface subjected to abrasion at an intervals of one minute. At each wear location (circle of wear), for each wear time, readings were taken at two points in the circle. At each point, three readings were recorded, and, the average of these six readings were reported as one data point for each wear circle at the measured time of wear. Chloride ion permeability of the mixtures was determined in accordance with ASTM C Three 4 x 8 in. cylinders were cast for each experimental condition. From each cylinder, 4 in. diameter x 2 in. thick slices were cut from the middle portion using a diamond tipped saw. Test Results and Discussion Compressive Strength The compressive strength data were recorded as a function of age and cement

5 replacement with fly ash (Fig. 1). At 1day age, fly ash mixtures showed lower compressive strength compared to the nofly ash mixture. However, beyond 1day age, the 35% fly ash mixture showed the best results at all test ages. At 28 days, it showed a compressive strength in excess of 8000 psi. The other two fly ash mixtures (45 and 55% cement replacement) attained compressive strengths greater than 6000 psi. at 28 days. In generally, compressive strength of the concrete mixtures increased to a considerable amount when curing was extend to 91 days. Tensile Strength In general, addition of the fly ash caused reduction in tensile strength of the fly ash concrete mixture up to 28 days (Fig. 2). However, all the fly ash mixtures at 91 days outperformed the reference concrete. This was attributed to improved structure of the fly ash mixtures due to significant pozzolanic reactions of the fly ash beyond 28 days. Flexural Strength All the mixture at 28 days attained high values of flexural strength (Fig. 3). The nofly ash mixture showed the highest value of flexural strength of all the mixtures tested at this age. At 91 days, the nofly ash as well as the 35% fly ash mixture showed similar results (Fig. 3). Modulus of Elasticity The modulus of elasticity data are plotted in Fig. 4. At oneday age, the reference mixture attained higher modulus value compared to the fly ash mixtures. However, the difference between the fly ash and the nofly ash mixtures became small between ages of 7 and 28 days. All the fly ash mixtures approached the modulus value of the nofly ash concretes at 91 days due to higher rates of strength gain of the fly ash mixture compared to the reference mixture. Abrasion Resistance The abrasion tests were performed at ages of 28, and 91 days. The average depth of wear for test specimens for all the mixtures are presented in Fig. 5. The results indicated that the concrete mixtures up to 35% cement replacement by fly ash had abrasion resistance similar to that for concrete without fly ash. Beyond 35% cement replacement, abrasion resistance decreased slightly up to 55% cement replacement. Chloride Permeability The chloride permeability data are depicted in Fig. 6. All the mixtures at 28day age showed "moderate" chloride permeability in accordance with ASTM C When duration of curing was increased to 91 days, the chloride permeability values for the mixtures up to 45% cement replacement were rated "low" as per ASTM C At

6 91day age, the 35% fly ash mixture exhibited the highest resistance to chloride ion permeability. At this age, the 45% fly ash mixture attained lower chloride ion permeability value compared to the reference mixture without fly ash. Conclusions The analysis of experimental data led to the following conclusions. 1. Of all the mixture tested, the 35% fly ash mixture (i.e. flyash to cement plusfly ash ratio of 0.40) showed the maximum compressive strength at 7 days and beyond. All the mixtures with and without fly ash attained strength levels in excess of 6000 psi at 28 days. Therefore, all the mixtures are appropriate for highstrength applications. The 35% fly ash mixture is also suitable for manufacture of highperformance concretes. 2. The tensile strength of the fly ash concrete mixtures were lower than the reference concrete up to 28 days. However, at 91day, the fly ash mixtures attained higher tensile strength than the reference concrete. 3. The flexural strengths of the fly ash mixtures were lower than the nofly ash concrete up to 28 days. All the fly ash mixtures up to 55% (i.e., fly ash to cement plus fly ash ratio of 0.60) cement replacement produced flexural strength equivalent to the reference mixture at 91 days. 4. The modulus of elasticity data followed the same general trend as indicated by the flexural strength. 5. The abrasion resistance of the fly ash concrete mixtures up to 35% cement replacements were essentially the same. 6. At 28 days, all the mixtures up to 55% cement replacement showed "moderate" chloride permeability as per ASTM C But the permeability values were rated as low at 91 days for all the mixtures up to 45% (i.e. fly ash to cement plus fly ash ratio of 0.50) cement replacement. The fly ash mixtures showed higher resistance to chloride permeability than the reference mixture at this age.

7 REFERENCES 1. M. A. Gillott, T. R. Naik, and S.S. Singh, "Microstructure of Fly Ash Containing Concrete with Emphasis on the AggregatePast Boundary", Proceedings of the 51st Annual Meeting of the Microscopy Society of America, August T.R. Naik, and S.S. Singh, "Superplasticized Structural Concrete Containing High Volumes of Class C Fly Ash." ASCE Journal of Energy Engineering. Vol. 117, No. 2, pp. 8795, (Aug. 1991). 3. T. R. Naik, and S.S. Singh, "Fly Ash Generation and Utilization An Overview", published in Recent Trend in Fly Ash Utilization, Ministry of Environment and Forestry, Government of India, (June 1993). 4. P. K. Mehta, "Pozzolanic and Cementitious ByProducts in Concrete Another Look", Proceedings of the Third International Conference, Trondheim, Norway, V.M. Malhotra, Ed., ACI SP114, pp. 143 (1989). 5. T.R. Naik, and B.W. Ramme, "Low Cement Content High Strength Structural Grade Concrete with Fly Ash." International Journal of Cement and Concrete Research. Vol. 17, pp (1987). 6. T.R.Naik, and B.W. Ramme, "High Strength Concrete Containing Large Quantities of Fly Ash." ACI Materials Journal. Vol. 86, No. 2, pp (MarchApril 1989). 7. T.R. Naik and B.W. Ramme, "High Early Strength Fly Ash Concrete for Precast/Prestressed Products." PCI Journal. pp (Nov.Dec. 1991). 8. T.R. Naik, and S.S. Singh, "Superplasticized Structural Concrete Containing High Volumes of Class C Fly Ash." ASCE Journal of Energy Engineering. Vol. 117, No. 2, pp (August 1991). 9. T.R. Naik, V.M. Patel, and L.E. Brand, "Properties of Fresh and Hardened High Strength Concrete", CBU Report No. 191, Center for ByProducts Utilization, University of WisconsinMilwaukee, 40 pages (January 1991). 10. T.R Naik, W.C. Collins, V.M. Patel, and J.H. Tews, "Rapid Chloride Permeability of Concrete Containing Mineral Admixtures," Proceedings of the CBU/CANMET International Symposium on the Use of Fly Ash, Silica Fume, Slag, and Other ByProducts in Concrete and Construction Materials, Milwaukee, WI, (November 1992). 11. T.R. Naik, W.A. Olson, and S. S. Singh, "Effects of Temperature on HighPerformance Concrete," to be presented at the ACI 1994 International Conference on HighPerformance Concrete, Singapore, (November 1518, 1994).

8 Table 1: Properties of Cement and Fly Ash Chemical Composition Cement, % ASTM C 150 Type I, % Fly Ash, % ASTM C 618, % Silicon dioxide, SiO Aluminum oxide, Al 2 O Ferric oxide, Fe 2 O Total, SiO 2 + Al 2 O 3 + Fe 2 O min. Sulfur trioxide, SO max. 5.0 max. Calcium oxide, CaO Magnesium oxide, MgO max max. Titanium dioxide, TiO Potassium oxide, K 2 O Sodium oxide, Na 2 O max. Moisture content max. Loss on ignition max max. Physical Properties of Cement Air content (%) max. Fineness (m 2 /kg) min. Autoclave expansion (%) max. Specific gravity 3.16 Compressive strength, psi 1day 3day 7day 28day min min. Vicat time of initial Set (min) min., 375 max. Physical Properties of Fly Ash Fineness retained on No. 325 Sieve (%) Strength activity index with cement, 7day (% of control) Pozzolanic activity index with cement, 28day (% of control) max min min. Water requirement (% of control) max. Autoclave expansion (%) max. Specific gravity 2.58 Note: 1 psi = MPa

9 Table 2: Mixture Proportions Using Dairyland Power Plant Class C Fly Ash Mixture Number C1 D1 D2 D3 Specified design strength (psi) Cement (lb/yd 3 ) Fly ash (lb/yd 3 ) Water (lb/yd 3 ) [W/(C + FA)] Sand, SSD (lb/yd 3 ) ¾ in. Aggregates, SSD (lb/yd 3 ) Slump (in.) 5 5 3½ 4¾ Air content (%) Superplasticizer (liq.oz./yd 3 ) Air entraining admixture (liq.oz./yd 3 ) Air temperature ( F) Concrete temperature ( F) Fresh concrete density (lb/ft 3 ) Hardened concrete density, SSD (lb/ft 3 ) Note: 1 psi = MPa; 1 lb/yd 3 = kg/m 3 ; 1 in. = 25.4 mm; 1 liq.oz. = 29.57x10 3 L; t F = 1.8 t C + 32; 1 lb/ft 3 = kg/m 3.