Mechanical properties of structural concrete incorporating a high volume of Class F fly ash as partial fine sand replacement

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1 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998, pp SCIENTIFIC REPORTS Mechanical properties of structural concrete incorporating a high volume of Class F fly ash as partial fine sand replacement Dan Ravina National Building Research Institute, Technion, Haifa 32000, Israel Paper received: August 20, 1996; Paper accepted: November 14, 1996 A B S T R A C T A research program was carried out on the utilization of large quantities of Class F fly ash of marginal quality in structural concrete as partial fine sand replacement. The present paper studies the effect of such replacement on the properties of hardened concrete. The effect on the properties of the fresh concrete was presented in another paper [1]. The properties studied were: compressive strength, modulus of elasticity, drying shrinkage and water penetration under pressure. The cement content of the concrete mixtures was maintained constant. Also, the consistency and slump were kept constant by adjusting the amount of the mixing water. The test results show clearly that fly ash of marginal quality, as partial fine sand replacement, has a beneficial effect on the compressive strength of structural concrete, particularly at later ages, and also on the modulus of elasticity. The drying shrinkage of the fly ash mixtures was similar or somewhat lower than that of the reference mix. The maximum penetration depth of water under pressure of the fly ash concrete mixtures was somewhat smaller than that of the reference mix. R É S U M É Un programme de recherches a été entrepris sur l utilisation de grandes quantités de cendres volantes de qualité marginale (Classe F) en remplacement partiel de sable fin. Cet article étudie les effets de tels remplacements sur les propriétés du béton durci. Les effets sur les propriétés du béton frais ont été présentés dans un autre article [1]. Les propriétés étudiées étaient la résistance à la compression, le module d élasticité, le retrait de séchage et la pénétration de l eau sous pression. La teneur en ciment des formules de béton a été maintenue constante. La consistance et l affaissement ont également été maintenus constants par ajustement de la quantité d eau. Les résultats montrent clairement que les cendres volantes de qualité marginale, en tant que remplacement partiel de sable fin, ont un effet bénéfique sur la résistance à la compression du béton, en particulier à des âges avancés, ainsi que sur le module d élasticité. Le retrait de séchage des formules contenant des cendres volantes était égal ou légèrement inférieur à celui du mélange de référence. Dans des formules contenant des cendres volantes, la profondeur maximale de pénétration de l eau sous pression était légèrement inférieure à celui de la formule de référence. 1. INTRODUCTION The use of good quality fly ash as one of the ingredients of concrete is very common nowadays in many countries. Good quality is usually related to the pozzolanic, i.e. cementitious, properties of the fly ash, and to its ability to reduce the mixing water demand. However, only a limited quantity of fly ash produced can be defined as of good quality. High quality requirements, mainly high fineness and low loss on ignition, limit or even rule out some fly ashes for structural concrete or reduce permissible levels. On the other hand, its utilization is of national interest, as unused fly ash raises severe ecological problems and its disposal is quite expensive, not to mention the difficulty in locating dumping sites. Increased use of fly ash can be achieved by considering the utilization of low quality fly ash, namely a material which barely meets the standard specification or does not comply with all quality requirements. Fly ash can act, in the Portland cement system, as a multi-functional ingredient serving as a fine aggregate, a plasticizer, a microfiller and a pozzolanic mineral admixture. Its utilization as a partial substitute for Portland cement brings economic and technological benefits; 15 Editorial note Dr. D. Ravina works at the Faculty of Civil Engineering and the National Building Research Institute of the Technion Israel Institute of Technology, a RILEM Titular Member. He is a corresponding member of TC 094-CHC on Concrete for Hot Countries /98 RILEM 84

2 Ravina to 25 percent replacement are common for normal structural concrete mixes, namely about 45 to 65 kg of good fly ash per cu.m. of concrete. Fly ash, even of the same class, can vary considerably according to the coal origin and treatment, the combustion conditions and the collection equipment and system used. Hence it is necessary to refer to the specific properties of the fly ash used. Fineness and loss on ignition are the dominant factors affecting the properties of the fresh concrete, while pozzolanicity, with time, affects the properties of the hardened concrete. High volume fly ash concrete indicates the incorporation of large quantities of fly ash in the concrete mix. However, the role of fly ash in the concrete mix should be clearly defined. CANMET, for example, developed a new type of concrete which is now often referred to as highvolume fly ash concrete. This concrete contains a relatively low amount of Portland cement, about 155 kg/m 3, low water content, about 115 L/m 3, with large dosages of superplasticizer, and a high volume of ASTM Class F fly ash, about 210 kg/m 3 (55 to 60 percent by weight of cementitious material). The water-to-cementitious materials ratio is maintained at about 0.32, hence the concrete so produced has adequate early-age strength, high later-age strength [2] and excellent durability [3, 4]. The Class F fly ash used by CANMET [2] can be defined as good. Its fineness (amount retained on 45 µm sieve) is between 15 to 20 percent; loss on ignition 0.30 to 1.5 percent; water requirement 90 to 95 percent; and strength activity index at 28 days: 90 to 99. Hence the fly ash functions as a cementitious ingredient. Another way of incorporating a high volume of fly ash in concrete is as partial replacement of fine beach or dune sand. This replacement, i.e. as a fine material, can be carried out with low quality fly ash, not commonly accepted. Obviously, fly ash as sand replacement is much less valuable than it is as a cementitious material, and its use may actually prove more expensive than that Table 1 Physical properties and chemical analysis of the fly ash Physical Properties Fineness - residue on 45 mm, % 17 Particle Density (t/m 3 ) 2.21 Chemical Analysis, % (wt) Silicon dioxide (SiO 2 ) Aluminum oxide (Al 2 O 3 ) Ferric oxide (Fe 2 O 3 ) 5.76 Calcium oxide (CaO) 7.14 Magnesium oxide (MgO) 1.66 Sulphur trioxide (SO 3 ) 0.77 Sodium oxide (Na 2 O) 0.27 Potassium oxide (K 2 O) 1.03 Loss on ignition 5.96 Strength Activity Index with Portland Cement (ASTM C 311) 28 days 73 of sand in view of the need for quality control, pneumatic tanker trucks for transportation, silos and proper arrangements at the concrete plant. On the other hand, the cost to the power station for the removal of unusable fly ash is very high. Therefore, if these sums are allocated for the replacement, the transition would be worthwhile for the concrete producer, and as the quantities of fly ash thus used, kg/m 3, can be quite significant, it may alleviate considerably the disposal problem of the power station. Fly ash affects the properties of fresh and hardened concrete. The present paper studies the effect of marginal quality Class F fly ash, as partial sand replacement, on the properties of hardened concrete. The effect of such replacement on the properties of fresh concrete was reported in another paper [1]. 2. MATERIALS Cement - Ordinary Portland cement; Fineness (Blaine) 240m 2 /kg; Setting time (Vicat), initial 215 min., final 340 min. Fly Ash - from bituminous coal, ASTM Class F. Its major constituents (XRD analysis) were Mulith and Quartz, and its minor constituents, Hematite, Lime and Calcite. The physical properties and the chemical analysis are given in Table 1. Chemical admixtures - Water Reducer and Retarder, ASTM C 494 type D, GP, chemically based on sodium gluconate. Aggregates - the coarse aggregate, maximum particle size 19 mm (3/4 in.), and the medium and coarse sand, were crushed dolomite stone. The fine aggregate was natural siliceous sea sand; the Fineness Modulus of the sand used was MIX PROPORTIONS The cement content of the Reference mix and the concretes with fly ash, as partial sand replacement, was 270 ± 5 kg/m 3 (455 ± 10 lb/yd 3 ). Also, two mixtures (without fly ash) with 15 percent cement addition or reduction, and one concrete mixture with 15 percent cement reduction and 150 kg/m 3 fly ash, were prepared. The fly ash content varied from 100 kg/m 3 (170 lb/yd 3 ) to 200 kg/m 3 (340 lb/yd 3 ); to maintain the same yield, the fine sand was adjusted respectively. The dosage of the water reducer and retarding (WRR) admixture, GP, was percent (wt.) of the cement. Air entrainment was not employed. The target slump of all the concrete mixes was 125 ± 12 mm (5±1/2 in.), obtained by adjusting the amount of mixing water. 85

3 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars TEST METHODS Compressive strengths were evaluated at 1, 7, 28, 90 and 180 days, using for each test three 10 cm cubes. The static modulus of elasticity was evaluated at eight or nine months. The measurements were performed with a demountable mechanical strain gauge of 100 mm gauge length. Drying shrinkage was evaluated at 14, 21, 28, 56, 90 and 180 days, using mm concrete beam specimens; zero measurements were taken at the age of 7 days, when the specimens were taken out of the water. Determination of the depth of penetration of water under pressure was done at the age of 90 days using two mm concrete specimens. Table 2 Concrete mixtures data and the amount, L/m 3, of water required at 21 C (70 F) and 32 C (90 F) to obtain the target slump, 125 ± 12 mm (5 ± 1/2 in.) and the difference, L/m 3 and percent, versus the Reference mix Mix Non-Fly Ash Concretes Fly Ash Concretes designation Reference +15%C -15%C %C Concrete Temperature 21 C (70 F) Cement Fly Ash Water Dif. vs. Ref. L/m Percent Concrete Temperature 32 C (90 F) Cement Fly Ash Water Dif. vs. Ref. L/m 3 (-19) Percent (90) Table 3 Compressive strength, MPa, of concrete mixtures with and without fly ash, at different testing ages: casting temperature 21 C (70 F) Age at 150 FA test, days Reference 100 FA 125 FA 150 FA 175 FA 200 FA -15%C +15%C -15%C Table 4 Compressive strength, MPa, of concrete mixtures with and without fly ash, at different testing ages: casting temperature 32 C (90 F) Age at 150 FA test, days Reference 100 FA 125 FA 150 FA 175 FA -15%C +15%C -15%C After brushing the testing face (beside the casting face) with a steel brush, the specimens were mounted in the test apparatus and a pressure head of 1 Bar was applied for 3 days. After testing, the specimens were split and the penetrating fronts were measured. 5. TEST RESULTS AND DISCUSSION 5.1 Water requirement and workability The required mixing water and the comparison of the various concretes with the Reference mix at 21 C (70 F) and 32 C (90 F), is presented in Table 2. It shows that the mixes with 100 to 150 kg/m 3 fly ash practically did not change the water requirement for the target slump; their workability, cohesiveness and finishability were much better than that of the Reference mix. By contrast, more mixing water was required for the mixes with 175 and 200 kg/m 3 fly ash; their workability was lower as the concretes were sticky. At the elevated temperature, additional mixing water of 5 to 9 liters was required in some mixes. 5.2 Compressive strength The compressive strength, average of three cubes, of the various concrete mixtures cast at 21 C and 32 C is given in Tables 3 and 4. The data presented show that, although more mixing water was required (to obtain the target slump) in some of the mixtures with fly ash as partial fine sand replacement, the compressive strength of all the fly ash concretes was either similar or higher than that of the Reference mix. At the ages of 1 and 7 days the compressive strength was practically the same or somewhat higher. From 28 days on, the compressive strength of all the fly ash concretes was significantly higher, Figs. 1, 2. The average compressive strength of the fly ash concrete mixtures (fly ash from 100 to 200 kg/m 3 ) was higher compared with the Reference mix by about 0.8 MPa at 1 day, 2 MPa at 7 days, 7 MPa at 28 days and by about 14 MPa at 90 and 180 days, at a casting temperature of 21 C. The beneficial effect of fly ash on the compressive strength may be related to different mechanisms. At an early age, the filler effect is

4 Ravina Fig. 1 Compressive strength, MPa, of concretes with and without fly ash, at different testing ages; casting temperature 21 C. Fig. 2 Compressive strength, MPa, of concretes with and without fly ash, at different testing ages; casting temperature 32 C. probably the major factor contributing to the compressive strength. Fillers, i.e. finely divided materials, were found to exert a positive effect on concretes, for reasons of both particle packing and physicochemical reactions. The fine particles can serve as precipitation sites of the hydrates from the cement, thus accelerating the chemical hydration process. Moreover, precipitation for Portland cement occurs close to the cement particles, while in the presence of fly ash precipitation will also occur in the open space. Hence, fly ash may therefore modify the microstructure of the hardened cement paste. The continuous increase of strength at later ages can be attributed to the pozzolanic nature of fly ash. Although the pozzolanic activity of the fly ash used is low, nevertheless, particularly with the high quantity of the fly ash, pozzolanic reactions take place. Fig. 3 Compressive strength increments, MPa, between testing ages; casting temperature 21 C (70 F). Fig. 4 Compressive strength increments, MPa, between testing ages; casting temperature 32 C (90 F). 87

5 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars 1998 Table 5 Compressive strength increments, MPa, at testing ages intervals, and differences between fly ash mixtures and Reference mix. Casting temperature 21 C (70 F) Testing ages Concrete mix designation Difference, MPa, between Intervals, fly ash (average) mixtures Days Reference 100FA 125FA 150FA 175FA 200FA and Reference mix Average FA increment Average FA increment Average FA increment Average FA increment Table 6 Compressive strength increments, MPa, at testing ages intervals, and differences between fly ash and Reference mix. Casting temperature 32 C (90 F) Testing ages Concrete mix designation Difference, MPa, between Intervals, fly ash (average) mixtures Days Reference 100FA 125FA 150FA 175FA 200FA and Reference mix Average FA increment Average FA increment Average FA increment (2.2) Average FA increment 1.9 Evidence of the fly ash pozzolanic effect can be seen by comparing the increments of the compressive strength during the intervals between the testing ages, between the fly ash mixtures and the Reference mix, as presented in Tables 5 and 6 and in Figs. 3 and 4. The test results show clearly that marginal quality fly ash, as partial fine sand replacement, has a beneficial effect on the compressive strength of structural concrete, particularly at later ages. However, as the reactions of cement, fly ash and chemical admixtures are interrelated, the magnitude of the positive effect found in this study can be different when using a different set of materials. Moreover, in order to maintain the same variation limits of properties as of that with non-fly ash concrete, fly ash used for sand replacement should also comply with the conformity criteria specified in EN 450 (Clause 6), ASTM C 618, or any other national Standards. An overall view regarding the compressive strength of the different concrete mixtures, at the various testing ages, is illustrated in Fig. 5. The strength development of the fly ash concretes (presented as an average value of the fly ash mixtures with 100 to 200 kg/m 3 fly ash), the Reference mix (the same cement content but without fly ash), the concrete mixtures with a 15 percent addition or reduction of Portland cement (no fly ash) and the concrete mixture with 150 kg/m 3 fly ash and 15 percent cement reduction is plotted. The difference between the fly ash concretes and the Reference mix can be seen, as already discussed. It can also be seen that the fly ash mixtures after 28 days developed higher strength than even the concrete mix with addition of 15 percent Portland cement. Moreover, even the concrete mixture with a 15 percent reduction of cement, but with 150 kg/m 3 fly ash, developed, at later ages, higher compressive strength than that of the Reference mix. 5.3 Modulus of elasticity Fig. 5 Compressive strength, MPa, of concrete mixtures with fly ash (average value of 100 to 200 kg/m 3 ) and without fly ash. The modulus of elasticity data are plotted in Figs. 6 and 7. The measurements were performed at a late age, eight (21 C) or nine (32 C) months, i.e. at an age when the fly ash, according to the compressive strength data presented before, contributed significantly to the properties of the hardened concrete. The tests show that the modulus of elasticity of all the concrete mixtures with fly ash, as partial sand replacement, was higher than that 88

6 Ravina Fig. 6 Modulus of elasticity. Casting temperature 21 C. Fig. 7 Modulus of elasticity. Casting temperature 32 C. of the Reference mix by about 8 to 17 percent. Also, the concrete mixture with a 15 percent reduction of cement, but with 150 kg/m 3 fly ash, obtained about the same modulus of elasticity as the Reference mix. 5.4 Drying shrinkage The drying shrinkage from 7 days to 180 days of the concrete prisms cast at a temperature of 21 C and 32 C is presented in Figs. 8 and 9. The measurements of the specimens cast at 21 C show that increasing the fly ash content, as partial fine sand replacement, increases the drying shrinkage. Nevertheless, the shrinkage of the fly ash mixtures was less than that of the Reference mix. At a casting temperature of 32 C, the drying shrinkage of the fly ash mixtures was either similar to or somewhat higher than that of the Reference mix. 5.5 Water penetration under pressure The maximum depth of water penetration after 3 days under a pressure of 1 Bar at the age of 90 days is presented in Fig. 10. The test results show that the maximum depth of the water penetration of the fly ash con- Fig. 8 Drying shrinkage of concrete mixtures with fly ash, as partial fine sand replacement, and of reference mix; casting temperature 21 C. Fig. 9 Drying shrinkage of concrete mixtures with fly ash, as partial fine sand replacement, and of reference mix; casting temperature 32 C. 89

7 Materials and Structures/Matériaux et Constructions, Vol. 31, Mars CONCLUSIONS The test results show clearly that fly ash of marginal quality, as fine sand replacement, has beneficial effects on the properties of the hardened concrete. Compressive strength, particularly at later ages, and modulus of elasticity were higher than that of the Reference mix. Drying shrinkage of the fly ash mixtures was similar or somewhat lower. Also, the maximum depth of water penetration under pressure of the fly ash mixtures was somewhat less than that of the Reference mix. REFERENCES Fig. 10 Water penetration, max mm. crete mixtures was somewhat less than that of the Reference mix. Generally, all the concrete mixtures showed moderate water penetration, except the concrete mixture with 15 percent reduction of cement which showed high depth of water penetration. However, the companion mix, i.e. 15 percent cement reduction but with 150 kg/m 3 fly ash, showed water penetration similar to the Reference mix. The test results do not indicate any trend regarding the fly ash content or the casting temperature. [1] Ravina, D., Properties of fresh concrete incorporating a high volume of Class F fly ash as partial sand replacement, Mater. Struct. 30 (202) (1997) [2] Carette, G.G., Bilodeau, A., Chevrier, R.L. and Malhotra, V.M., Mechanical properties of concrete incorporating high volumes of fly ash from sources in the U.S., ACI Materials Journal 90 (6) (Nov.-Dec. 1993) [3] Bilodeau, A., Sivasundaram, V., Painter, K.E. and Malhotra, V.M., Durability of concrete incorporating high volumes of fly ash from sources in the U.S., ACI Materials Journal 91 (1) (Jan.- Feb. 1994) [4] Sivasundaram, V., Carette, G.G. and Malhotra, V.M., Mechanical properties, creep, and resistance to diffusion of chloride ions of concretes incorporating high volumes of ASTM Class F fly ashes from seven different sources, ACI Materials Journal 88 (4) (1991)