Strength and capillary water absorption of lightweight concrete under different curing conditions

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 17, April 2010, pp Strength and capillary water absorption of lightweight concrete under different curing conditions Nusret Bozkurt* & Salih Yazicioglu Department of Construction Education, Firat University, Elazig 23119, Turkey Received 15 December 2008; accepted 26 March 2010 This paper reports an experimental study carried out to investigate the influence of addition of pozzolanic materials and curing regimes on the mechanical properties and the capillary water absorption (sorptivity) characteristics of lightweight concrete. A control lightweight concrete mixture made with lightweight volcanic pumice containing only Portland cement (PC) and fly ash lightweight concrete mixture containing 20% of fly ash (FA) and 10% of silica fume (SF) as a replacement of the cement by weight is prepared. The specimens are prepared and cured in three different curing conditions (standard 20 C water, sealed and air cured) for the periods of 3, 7 and 28 days. At the end of each curing period, compressive and tensile strengths and ultrasonic pulse velocity (UPV) values are determined; sorptivity coefficients are recorded at 28 days. The results indicate that SF specimens give higher compressive and tensile strength and lower sorptivity coefficient values than corresponding PC and FA concrete specimens, regardless of curing regime and age of concrete. The results also show a good correlation between the strength development of concrete and its sorptivity, i.e., as the compressive and tensile strengths increased due to the hydration, the sorptivity coefficients decreased significantly. Keywords: Lightweight concrete, Curing regime, Mineral admixture, Sorptivity coefficient, Fly ash, Silica fume Lightweight concrete (LWC) has been successfully used for many structural purposes, such as in modern construction to reduce the self-weight of structures because of earthquake forces that affect such structures. It can be easily produced by utilizing natural lightweight aggregate, i.e., pumice or perlite aggregate. There are numerous studies on durability and mechanical properties of lightweight concrete using lightweight aggregate Rossignolo and Agnesini 1 reported that performance of styrenebutadiene rubber latex (SBR) modified lightweight aggregate concrete (LWAC) exposed to an aggressive environment was better than unmodified one, and SBR modified LWAC led to lower absorption and significant improved resistance to chemical attack and corrosion. Yazicioglu and Bozkurt 2 investigated that mechanical properties of structural lightweight concrete produced with pumice and mineral admixtures. Demirboğa et al. 3 suggested that mineral additions increased the compressive strength of concrete produced with lightweight expanded perlite aggregate. Lo et al. 4 showed that the strength of the lightweight aggregate was a primary factor controlling the strength of high-strength lightweight concrete. Compressive strength of lightweight *Corresponding author ( nuret60@hotmail.com) aggregate (LWA) is relatively low and adsorption capacity is high due to its porous nature. Hence, it needs a large amount of cement paste to achieve suitable workability and compressive strength 6. Altun and Haktanır 7 worked on structural lightweight concrete and normal weight concrete to be used together in composite reinforced concrete members. Haque et al. 8 suggested that compressive strength was comparatively less sensitive to the curing regimes investigated. Both the chloride and sulphate penetration after 12 months exposure were found to be within tolerable limits. Also replacement of lightweight fine aggregate with normal weight sand produces a concrete that is somewhat more durable as indicated by water penetrability and depth of carbonation when concretes are of equal strength. Sufficient curing is essential for a concrete to provide its potential performance 11. It has been generally accepted that curing is more important for concrete with mineral admixtures than for normal concrete 12. Water curing has more effect on the permeability than on the strength of concrete. Bentz et al. 13 reported that curing regimes in early age of concrete have an important role on the degree of hydration of cement. Tasdemir 14 pointed out that the sorptivity coefficient of concrete is very sensitive to

2 146 INDIAN J. ENG. MATER. SCI., APRIL 2010 the curing condition and its effect on the sorptivity coefficient of concrete was higher in low-strength concretes. Dinku and Reinhardt 15 have shown that gas permeability is sensitive to changes in curing duration, water/cement ratio, age of testing and moisture history of concrete. According to their findings, it is possible to predict the gas permeability from the capillary sorptivity measurements. To examine the durability characteristics of concrete, permeability (either of gas or liquid) is generally employed. This method can be considered significant where concrete is exposed to water pressure, such as water retaining structures. Capillary water absorption (sorptivity) characteristics of concrete for structures located above the ground level would be more appropriate. Sorptivity coefficient can be determined by means of a simple test allowing one face of a concrete specimen to be in contact with water and the mass (non-destructive) or height (destructive) of water absorbed by capillary suction measured at predefined intervals 16,17. The objective of this study is to examine the influence of different pozzolanic additions (fly ash and silica fume) and curing conditions (standard 20 C water, sealed and air cured) on the mechanical properties and capillary water absorption (sorptivity) of LWC. Experimental Study Materials CEM I 42.5 Class N, Normal Portland cement 42.5, which was provided by Elazig in Turkey was used in this study. Silica fume and fly ash were obtained from Antalya Electro Metallurgy Enterprise and Soma Power Plant in Manisa in Turkey, respectively. The chemical composition and physical properties of Portland cement, silica fume and fly ash as well as the pumice aggregates are given in Table 1. The fine Table 1 Chemical composition of cement, silica fume, fly ash and pumice Oxide composition Cement(%) Silica Fly ash Pumice fume(%) (%) (%) SiO Al 2 O Fe 2 O CaO MgO SO LOI Specific gravity (g/cm 3 ) aggregate was river sand from Murat River in Turkey. Crushed volcanic pumice was used as the coarse aggregate in the production of lightweight concrete. Volcanic pumice was obtained from natural deposits in Elazığ city (Turkey). Its apparent reserve is about 20 million m 3. Bulk dry unit weight, water absorption and compressive strength of the volcanic pumice were 1290 kg/m 3, 21.3 ±3% and 14.09±4 MPa, respectively. The relative density of volcanic pumice was The aggregate used was 20 mm nominal maximum size of volcanic pumice and its chemical composition is given in Table 1. Concrete mixture composition and sample preparation Table 2 presents the dry unit weight and composition of concrete mixtures produced and tested. A control lightweight concrete mixture made with lightweight volcanic pumice containing only Portland cement (PC), and lightweight concrete mixture containing 20% fly ash (FA) and 10% silica fume (SF) as a replacement of the cement by mass were prepared. Cube concrete specimens ( mm) were prepared to determine compressive strength and ultrasonic pulse velocity, and cylindrical concrete specimens, 150 mm in diameter and 300 mm high, were prepared for splite tensile strength tests. Furthermore, cube concrete specimens ( mm) were also prepared to determine capillary water absorption. The concrete constituents were mixed in a revolving drum type mixer for approximately min to obtain uniform consistency, because porosity ratio of volcanic pumice is higher than other coarse aggregate types. After mixing, concrete mixtures prepared were filled in cubic and cylindrical moulds in three layers by tamping. Later, all the test specimens were stored at temperature of 22±3ºC (about 23ºC) in the laboratory. On the day following casting, the specimens were de-moulded and placed in three different curing conditions, namely standard 20 C water, sealed and air cure for periods of 3, 7 and 28 days. At the end of each curing period, a total of 3 Table 2 Approximate concrete mixture composition (kg) and densities (kg/m 3 ) of a cubic meter concrete Mix Cement (kg/m 3 ) FA (kg/m 3 ) SF W/B Sand Aggregates (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) Density (Air dry) (kg/m 3 ) PC ±20 SF ±25 FA ±22

3 BOZKURT & YAZICIOGLU: LIGHTWEIGHT CONCRETE UNDER DIFFERENT CURING CONDITIONS 147 specimens were tested for each concrete property. The compressive strength and ultrasonic pulse velocity (UPV), tests were carried out on the cubic specimens, whilst the splitting tensile tests were carried out on the cylindrical specimens. All the tests performed were conducted at 3, 7 and 28 days for all curing conditions. Water absorption tests were carried out to determine the sorptivity coefficient of concrete specimens, which were preconditioned in oven at 100±5ºC for 24 h and then cooled in a desiccators for 24 h to achieve a constant moisture level. Four sides of the concrete specimens were then sealed by paraffin to avoid evaporative effect as well as to maintain uniaxial water flow during the test, and two opposite faces were left unsealed (see Fig. 1). Before Fig. 1 The measurement of water capillary sorption the specimens were located in water, their initial weights were recorded. One face of the specimen was in contact with water. The water absorption at predefined intervals was measured to an accuracy of 0.1 g. The sorptivity coefficient 14,18,19 can be calculated by the following expression: S = (Q/A)/ t... (1) where S is the sorptivity (cm/s 1/2 ), Q is the volume of water absorbed (cm 3 ), A is the surface area in contact with water (cm 2 ) and t is the time (s). S was obtained from the slope of the linear relationship between Q/A and t. Results and Discussion Compressive strength The results obtained from compressive strength tests for PC, SF and FA concretes for all concrete ages and curing conditions are given in Figures 2a, 2b and 2c, respectively. It can be seen that the highest compressive strength values were obtained from water cured specimens followed by the sealed and air cured specimens regardless of the concrete types. This shows the role of curing methods on the early age compressive strength of concretes, i.e., higher the moisture level the specimens were exposed to the higher temperature and the higher compressive strength achieved. As can also be seen, the highest Fig. 2 Compressive strength of PC concrete, FA and SF for all curing methods (a) water, (b) sealed and (c) air

4 148 INDIAN J. ENG. MATER. SCI., APRIL 2010 compressive strength results were obtained from the SF concretes followed by PC and FA concretes for all curing methods. According to literature 20-22, silica fume influences the microstructural development of hydrating cement paste in concrete through at least three mechanisms. First, because of its small particle size, silica fume packs more efficiently do in the interfacial transition zone (ITZ) region than the larger cement particles. The very fine silica fume particles are more or less evenly distributed within this water phase and therefore, actually exhibit an increase in volume fraction in the ITZ relative to the bulk paste. Thus, the ratio of silica fume to cement is much higher in the ITZ region than it is in the bulk paste This extra silica fume in the ITZ region will participate in pozzolanic reactions, resulting in a denser, more homogeneous ITZ microstructure. Second, silica fume reduces the overall porosity in concrete. Silica fume has a third extremely significant effect on the microstructure of hydrating cement paste. According to Bentz 20, the pozzolanic C-S-H gel produced from silica fume appears to have an inherent chloride ion diffusivity that is approximately 25 times less than that of C-S-H gel formed from conventional cement hydration. Therefore, inclusion of silica fume in concrete mixture, mainly affects strength of concrete 20,23. At 28 days, the compressive strength of concrete containing 10% silica fume was about 20% more than that of control concrete for water and sealed cured specimens. As for air cured specimens, it was 7%. The compressive strength was found to decrease with 20% FA replacement by mass of cement at all ages and curing conditions. These decreases were 20%, 21% and 22% for water, sealed and air cured specimens, respectively. Fly ash concrete mixes typically result in lower strengths at early ages (usually up to 28 days). Moreover, in this study, the fly ash had a bigger particle size than Portland cement used. The filler effect of fly ash in this case might not be of any significance. However, the developed compressive strength for 28-day samples of lightweight concrete was in the range of MPa that is quite above the requirement for the structural lightweight concrete except FA concrete specimen air cured. All these results are found to be satisfactory, because the minimum compressive strength in structural lightweight concrete is known to be 17.0 MPa for 28-day test 10,24. Tensile strength The tensile strength results for the three types of concrete, under the three different curing methods for the 3, 7 and 28 days curing periods are shown in Figures 3a-c, respectively. As indicated in these figures the development of tensile strength of FA concrete was the lowest followed by PC and SF concretes at 3,7 and 28 days. The figures show that Fig. 3 Tensile strength of PC concrete and FA, SF for all curing methods (a) water, (b) sealed and (c) air

5 BOZKURT & YAZICIOGLU: LIGHTWEIGHT CONCRETE UNDER DIFFERENT CURING CONDITIONS 149 use of SF in concrete contributed to the significant increases in the tensile strength at all ages. When the influence of curing methods on the tensile strength of concretes is examined, it can be seen that the highest values were obtained from water cured specimens followed by sealed and air cured specimens, regardless of concrete types. Ultrasonic pulse velocity (UPV) Figures 4a-c give the UPV test results for PC, FA and SF concretes, respectively, at 3, 7 and 28 days for all curing conditions. The highest UPV values were obtained from the SF followed by PC and FA concretes at 28 days, which indicates the effect of the filling and packing capacity of silica fume particles on UPV values. Since SF particles were much finer than PC and FA, they filled the micropores in cement paste and the mechanical properties and durability of concrete were improved, by reducing permeability and porosity 2,16. Water cured specimens for all concrete types gave the highest values, followed by sealed and air cured specimens again indicating the role of moisture level on the hydration and strength development. Sorptivity The results of sorptivity tests are given in Fig. 5 on the basis of curing conditions and concrete types. SF concrete gave the lowest sorptivity values followed by PC and FA concretes under all curing conditions. The highest sorptivity value ( cm/s 1/2 ) was obtained from the FA concrete cured in air, whilst the lowest sorptivity value ( cm/s 1/2 ) was obtained from SF cured in water. Since SF is very fine, pores in the bulk paste or in the interfaces between aggregate and cement paste is filled by this kind of mineral addition, hence, the capillary pores are reduced. The beneficial role of mineral addition causes an increase in the strength and a reduction in the capillary sorption of concrete. The high early strength development in SF concrete can be attributed to an early pozzolanic reaction. In concretes with fly ash, the average particle size of the mineral admixture is higher compared to SF and PC, and the pores in bulk paste and interfaces are not filled completely. Thus, concrete has larger capillary pores, and lower compressive strength as a result of the higher capillary sorption in concrete is obtained 14. Comparison of tensile and compressive strength The tensile and compressive strength results of PC, FA and SF concretes are compared in Fig. 6, regardless of the curing method applied. According to Neville 24, tensile strength has a close relationship with compressive strength, but there is no direct proportionality. The ratio of the two strengths depends on the general level of the strength of concrete. It is indicated in Fig. 6 that as the compressive strength of concretes increases, the Fig. 4 Ultrasonic pulse velocity of PC concrete and FA, SF for all curing methods (a) water, (b) sealed and (c) air

6 150 INDIAN J. ENG. MATER. SCI., APRIL 2010 Fig. 5 Sorptivity values of concretes in different curing conditions tensile strength also increases but at a smaller rate 25,26. It is also shown in Fig. 6 that at the lower strength levels (at the early ages), there was little difference between the tensile strength values of different concretes. However, as the compressive strength increased (typically beyond 15 MPa), differences between the tensile strength values increased. Figure 7 gives a comparison between sorptivity and both compressive and tensile strengths of concretes, regardless of curing conditions. In general, very good correlation is observed between the sorptivity and the strength values, i.e., as the strengths of concretes increased due to hydration, the sorptivity reduced significantly indicating a denser microstructure. Fig. 6 Comparison of compressive and tensile strength of concretes Fig. 7 Results of capillary absorption of water through concrete surface

7 BOZKURT & YAZICIOGLU: LIGHTWEIGHT CONCRETE UNDER DIFFERENT CURING CONDITIONS 151 Conclusions On the basis of the experimental investigation carried out, the following conclusions can be drawn: (i) The compressive and tensile strengths of SF lightweight concretes were higher than those of FA and PC concretes for all testing ages and for all curing conditions. (ii) For all concretes, the water cured specimens gave highest compressive strength values followed by sealed and air cured specimens. (iii) Proper curing and use of pozzolanic additives in the form of SF enhanced the resistance of concrete against water absorption significantly. (iv) A good correlation was observed between the sorptivity and the strength values, i.e., as the strengths of concretes increased due to hydration, the sorptivity reduced significantly indicating a denser microstructure. References 1 Rossignolo J A & Marcos Agnesini C V, Cem Concr Compos, 26 (4) (2004) Yazicioglu S & Bozkurt N, J Faculty Eng Archit Gazi Univ, 21 (2006) Demirboğa R, Örüng İ & Gül R, Cem Concr Res, 31 (2001) Lo T Y, Cui H Z & Li Z G, Waste Manage, 24 (2004) Topcu İ B, Cem Concr Res, 27 (1997) Chia K S & Zhang M-H, Cem Concr Res, 32 (2002) Altun F & Haktanir T, J Mater Civ Eng, 13 (2001) Haque M N, Al-Khaiat H & Kayali O, Cem Concr Compos, 26 (2004) Sari D & Pasamehmetoglu A G, Cem Concr Res, 35 (2005) Yasar E, Atis C D, Kilic A & Gulsen H, Mater Lett, 57 (2003) Khatri R P, Sirivivatnanon V & Yu L K, Mag Concr Res, 49 (1977) Bentur A & Goldman A, J Mater Civ Eng ASCE, 1 (1989) Bentz D P, Snyder K A & Stutzman P E, Hydration of Portland cement: The Effect of Curing Conditions,, 10 th Int Congress on Chemistry of Cement, vol. 2, Sweden, Tasdemir C, Cem Concr Res, 33 (2003) Dinku A & Reinhardt H W, Mater Struct, 30 (1997) Kelham S A, Mag Concr Res, 40 (1988) Khatib J M & Clay R M, Cem Concr Res, 34 (2004) Dias W P S, Cem Concr Res, 30 (2000) De Beer F C, le Roux J J & Kearsley E P, Nuclear Instrum Methods Phys Res, 542 (2005) Bentz D P, Cem Concr Res, 30 (2000) Bentz D P, Jensen O M, Coats A M & Glasser F P, Cem Concr Res, 30 (2000). 22 Bentz D P & Garboczi E J, ACI Mater J, 88 (1991) Gonen T & Yazicioglu S, Build Environ, 42 (2007) Neville A M, Properties of concrete, (Longman Group, England), Zaina M F M, Mahmud H B, Ilham A & Faizal M, Cem Concr Res, 32 (2002) Marzouk H & Chen Z W, J Mater Civil Eng ASCE, 7 (1995) 108.