Durability of high performance concrete composites containing silica fume

Transcription

1 Technical Note Durability of high performance concrete composites containing silica fume Proc IMechE Part L: J Materials: Design and Applications 227(4) ! IMechE 2012 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / pil.sagepub.com Peng Zhang and Qing-Fu Li Abstract A parametric experimental study has been conducted to investigate the effect of silica fume on the durability of the concrete composites. Four different silica fume contents (3%, 6%, 9% and 12%) were used. The durability of concrete composites includes water impermeability, dry shrinkage property, the carbonation resistance and the freeze thaw resistance. The results indicate that the addition of silica fume has a little adverse effect on the dry shrinkage property of concrete composites. With the increase of silica fume content, the dry shrinkage strain is increasing gradually. However, the addition of silica fume has greatly improved the water impermeability, the carbonation resistance and the freeze thaw resistance of concrete composites. With the increase of silica fume content, the length of water permeability and the carbonation depth of the specimens are decreasing gradually, and the relative dynamic elastic modulus of the specimens has a tendency of increase with the increase of silica fume content. Keywords Concrete composite, durability, silica fume Date received: 24 July 2012; accepted: 16 August 2012 Introduction High performance concrete (HPC) can be defined as the concrete, which meets special performance and uniformity requirements that cannot always be achieved by conventional materials, normal mixing, placing and curing practices. 1 HPC has become popular due to its superior mechanical and durability properties. HPC significantly reduces maintenance costs and enhances service life. Enhanced durability of HPC makes its use attractive in the environments where ordinary concrete would not suffice. The HPC has constantly growing number of applications: marine construction, high-rise buildings, bridge decks and piers, thin-wall shells, airport pavements and many others. 2 Swamy 3 states that HPC is that which is designed to give optimized performance characteristics for the given set of materials, usage and exposure conditions, consistent with requirements of cost, service life and durability. Architects, engineers and constructors all over the world are finding that using HPC allows them to build more durable structures at comparable cost. HPC is being used for buildings in aggressive environments, marine structures, highway bridges and pavements, nuclear structures, tunnels, precast units, etc. 4 The major difference between conventional concrete and HPC is essentially the use of chemical and mineral admixtures. Use of chemical admixtures reduces the water content, thereby reducing the porosity within the hydrated cement paste. 5 Pozzolanic materials are crucial to HPC as far as flowability is concerned. 6 Mineral admixtures, also called as cement replacement materials, act as pozzolanic materials as well as fine fillers, thereby the microstructure of hardened cement matrix becomes denser and stronger. Today silica fume is known as a by-product of silicon metal and ferro-silicon alloy industry instead of a waste product and its utilization in composite technology has increased recently The first utilization of silica fume in concrete was reported in 1952 by a Norwegian researcher. It was only in the late 1970s that silica fume started to be used as a supplementary cementitious material in concrete in Scandinavia. It was not until the early 1980s that it began to be used in this way in North America. 11 Because of a significant improvement attained on interfacial zone of cement paste-aggregate, silica fume is known to improve the early strength and durability of concrete School of Water Conservancy and Environment Engineering, Zhengzhou University, People s Republic of China Corresponding author: Peng Zhang, School of Water Conservancy and Environment Engineering, Zhengzhou University, Zhengzhou , Henan Province, People s Republic of China. zhangpeng8008@yahoo.com.cn

2 344 Proc IMechE Part L: J Materials: Design and Applications 227(4) composites, and produce a high-strength concrete. 12 Silica fume is often used in two different ways: as a cement replacement, in order to reduce the cement content (usually for economic reasons); and as an additive to improve concrete properties (in both fresh and hardened states). 4 The most important region in the microstructure of the concrete is around the aggregate. The addition of silica fume in concrete leads to reduction in porosity of the transition zone between matrix and aggregate in the fresh concrete and provides the microstructure needed for a strong transition zone. 13 Investigation on the aggregate matrix interface concluded that, in concrete with silica fume, the interfacial zone became stronger, more homogeneous and dense, hence, the cracks usually traversed the aggregates; transgranular type of fracture was observed. 14 The durability of concrete is one of its most important properties, aside from its compressive strength, because distresses in concrete are mostly due to durability failures rather than insufficient strength. Permeability is considered to be one of the most important properties affecting concrete durability because many concrete degradation mechanisms are a function of the rate of water or solution flow through the concrete, and it controls the rate of entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing. 15,16 Dry shrinkage, one of the main causes of cracks that directly affect the strength and durability of concrete, usually occurs in hot and dry environments due to the loss of internal water in the concrete to the environment. This results in the reduction of concrete volume and leads to crack formation in hardened concrete. Furthermore, most of this drying shrinkage cannot be regained by rewetting the concrete. 17 Early age cracking of concrete results from the interplay between the volume instability, the mechanical properties of concrete and different types and degrees of restraint. 18 When the concrete behaves like a fluid, i.e. during the very early age, its volumetric changes are usually not of a great concern in terms of stress generation because the concrete deforms plastically without generating stresses. However, once it has transformed a visco-elastic solid, the stress is generated. 19 Carbonation of concrete has great negative impact on durability of concrete. The main harm of concrete carbonation is that it can increase PH of concrete, which will weaken the alkaline protection of the reinforcing rebar in concrete. 20 As a result, the reinforcing rebar has larger possibility of corrosion, and the reinforced concrete is likely to be damaged. Freezing thawing durability of concrete composite has the significant effect on the service life and service quality of concrete structures, and besides, the area coverage of the freezing injury of concrete is quite wide. Freezing thawing durability is so important in the mix design and application of concrete composite, especially when the concrete composite is used in the severe cold region. Studying the durability is certainly necessary and helpful to promote the application of the concrete composites containing silica fume. This article reports the investigation of the effect of silica fume on durability (water impermeability, dry shrinkage, carbonation resistance and freezing thawing durability) of concrete composites. Experimental details Raw materials Ordinary Portland cement (Class 42.5 R) produced by Tongli Factory and silica fume, a by-product of the production of silicon and ferrosilicon alloys were used in this work. The cement and silica fume properties are given in Table 1. Silica fume was mixed in concrete by replacing the same quantity of cement, and the content of silica fume (by mass) varies from 3% to 12%. Coarse aggregate with a maximum size of 30 mm and fine aggregate with a 2.82 fineness modulus were used. The specific gravity and silt content of the coarse and fine aggregates were 2.75 and 0.5%, and 2.62 and 0.9%, respectively. A high range water reducing agent with a commercial name of polycarboxylate HJSX-A was used to adjust the workability of the concrete mixture. This water reducing agent was made of many kink of polymer organic compounds, most of which belong to poly carboxylic acid salt. It has great reducing properties with only a small dosage. With the same slump of the concrete mixture, the water mixed can be reduced 15 40%. Besides, the concrete mixture mixed this reducing agent has little slump loss. Mix proportions of the HPC are given in Table 2. Preparation of fresh concrete composite In order to distribute silica fume uniformly, a forced mixing machine was adopted. The mixing procedure, Table 1. Properties of cement, fly ash and silica fume. Composition (%) Cement Silica fume Chemical compositions SiO Al 2 O Fe 2 O CaO MgO Na 2 O K 2 O SO Physical properties - Specific gravity Specific surface (cm 2 /g)

3 Zhang and Li 345 which was designed by trial and error, was chosen as follows: the coarse aggregate and fine aggregate were mixed initially for 1 min, and the cement and silica fume were mixed for another 1 min. Finally, the high range water reducer agent and water were added and mixed for 3 min. The distribution of silica fume has great effect on the working performance of the mixture and the durability of concrete composite. If silica fume is not distributed well, it will be assembled altogether. From the working performance of the mixture, and the fracture section of the specimen of the concrete composite, it can be seen that the silica fume of this study were distributed well. Water impermeability test A series of frustum specimens with the size of mm were used to determine the water impermeability of concrete composite. The water impermeability of concrete composite can be evaluated by the length of water permeability of the specimen under the action of pressure water. The shorter length of water permeability indicates that the concrete composite has better water impermeability. For water impermeability of concrete composite test, each set of per composition includes 6 specimens, and the average value of the 6 data was computed as the final result. 21 Before testing, the specimens were cured for 28 days at 100% relative humidity and controlled temperature (21 2 C). The tests of water impermeability of concrete composite were carried out by water impermeability instrument in accordance with the Chinese Standard. 21 The side face of the specimens were adhered a layer of sealant, which was made of grease and cement by mixing. The sealant can ensure the tightness between the side face of the specimen and the inwall of the specimen sleeve. The specimen was put from the smaller end through the bigger end of the specimen sleeve and made the specimen and sleeve contact closely with hand. Then the specimen with the sleeve was put on the pad of the hydraulic pressure testing machine. With the pressure of the testing machine, the specimen was installed into the sleeve completely. After the overmuch sealant forced out of the specimen sleeve was removed away, the specimen with the sleeve was put on the water impermeability instrument, and the sleeve was fixed with several screws. The water pressure was controlled at 3.5 MPa, and the floating pressure did not exceed 0.05 MPa. The test should last 24 h before the specimens were taken off from the water impermeability instrument. After the test finished, the specimen was split in half along the longitudinal section on the pressure testing machine. The contour line of permeated water on the longitudinal section of the specimen was drawn with a marking pen. On the longitudinal section of the specimen, the measure points were set every 1 cm along the bottom of the specimen. Each length of water permeability on each measure point was measured with a steel ruler. Finally, the average length of water permeability of the specimen can be measured. Dry shrinkage test The tests of dry shrinkage properties of concrete composite were carried out in accordance with the Chinese Standard. 21 A series of beam specimens with the size of mm 3 were used to determine the shrinkage properties of the concrete composite. For the shrinkage properties of concrete composite test, each set of per mix includes 6 specimens, and the average value of the 6 data was computed as the final result. After the specimens were cast, the moulds should be removed the next day, and the specimens were put into the standard and curing room to be cured at 100% relative humidity and controlled temperature (20 2 C). After the specimens were cured for 2 days, the initial lengths of the specimens should be measured, and then they were put into the shrinkage room to be further cured at 60 5% relative humidity and controlled temperature (20 2 C). There are 10 curing periods, and the length variation of the specimen should be measured by the dial indicator at the end of each period to calculate the dry shrinkage strain. Carbonation test A series of cube specimens with the size of mm 3 were used to determine the carbonation resistance of concrete composite. The carbonation resistance of concrete composite can be Table 2. Mix proportions of the concrete mixtures. Mix no. Cement Silica fume (%) Silica fume Coarse aggregate Fine aggregate Water Water reducer agent

4 346 Proc IMechE Part L: J Materials: Design and Applications 227(4) evaluated by carbonation depth of the specimen under the action of CO 2 pressure. The shorter carbonation depth indicates that the concrete composite has better carbonation resistance. For carbonation resistance of concrete composite test, each set of per composition includes 15 specimens, and the average value of the 15 data was computed as the final result. Before testing, the specimens were cured for 28 days at 100% relative humidity and controlled temperature (21 2 C). The test of concrete carbonation was carried out in concrete carbonation box in accordance with the Chinese Standard. 22 Before carbonation test, the specimens were baked for 48 h in the oven under the temperature of 60 C. In the concrete carbonation box, the CO 2 concentration should be controlled at 20 3%, and the humidity and temperature should be controlled at 70 5% and 20 5 C, respectively. The carbonation test should last 24 days. The two side faces which need to be carbonized were marked parallel lines with a marking pen in the space of 10 mm in advance as the measuring points. After the test finished, the specimen was split in half, and the fracture surface was sprayed phenolphthalein alcohol solution with the level of 1%. After about s, the color of the carbonation part of the specimen will be changed. And the carbonation depth of the specimen on each measuring point was measured with a steel ruler. Finally, the average carbonation depth of the specimen can be measured. Freezing thawing test Freezing thawing durability of concrete composite can be measured referring to the standard of ASTM. 23 The freezing thawing durability of concrete composite was evaluated by the relative dynamic elastic modulus of the specimen after 300 freezing thawing cycles. The higher relative dynamic elastic modulus indicates that the concrete composite has better freezing thawing durability. On the contrary, the material has worse freezing thawing durability with lower relative dynamic elastic modulus. Relative dynamic elastic modulus of concrete composite can be measured referring to the standard of ASTM. 24 A series of prism specimens with the size of mm 3 were used to determine the relative dynamic elastic modulus of concrete composite. The initial transverse fundamental frequency of the specimen was measured by the instrument for transverse fundamental frequency before freezing thawing testing. After a certain times freezing thawing, the corresponding transverse fundamental frequency of the specimen was measured. Then the relative dynamic elastic modulus can be calculated with the initial and subsequent values of transverse fundamental frequency. For freezing thawing test of concrete composite, each set of per composition includes 3 specimens, and the average value of the 3 data was computed as the final result. Before testing, the specimens were cured for 28 days at 100% relative humidity and controlled temperature (21 2 C), and the specimens have been soaked in saturated lime water in the last 4 days of the curing period. One freezing thawing cycle should be carried out in 2 to 5 h, and the thawing time should not be less than a quarter of the total time of one cycle. At the end of freezing and thawing, the temperature of the specimen centre should be controlled at 18 C and 5 C, respectively. Results and discussion Effect of silica fume content on water impermeability The variations of the length of water permeability of concrete composite versus silica fume content of the water permeability specimen are illustrated in Figure 1. A considerable decrease for the length of water permeability of the concrete was observed by increasing the content of silica fume. Compared with the mix without silica fume, the decrease was determined as 63% for 12% silica fume content. As seen from the variation curve, the decreasing speed of the length of water permeability is very high before the silica fume content is less than 3%, and the decreasing speed of the the length of water permeability becomes lower with the silica fume content increasing beyond 3%. The main reason of silica fume can improve the water impermeability of concrete composite is that silica fume can take part in hydration reaction in the early hydration of cement to produce more hydrate and decrease the porosity of concrete composite in the early hydration of cement. Therefore, the addition of silica fume can improve the water impermeability of concrete composite evidently. Figure 1. Effect of silica fume content on length of water permeability.

5 Zhang and Li 347 Effect of silica fume content on dry shrinkage Dry shrinkage tests can provide necessary information on how the drying shrinkage stresses develop although they cannot offer sufficient information on the behavior of concrete structure. Dry shrinkage with curing age is shown in Figure 2 for different concretes with different silica fume content. It can be seen that the shrinkage values yielded approximately comparable values at early ages of drying period. However, a clear distinction was observed for different concretes after about ten days. From Figure 2, it can be seen that the dry shrinkage strain in the concretes containing silica fume was more than that in the concrete without silica fume, and a considerable increase for the dry shrinkage strain of the concrete was observed by increasing the dosage of silica fume. As the silica fume content is increased from 0 to 12%, the maximum dry shrinkage strain of 90 days ages increases 22% from 693 me to 845 me. In other words, the addition of silica fume may have adverse effect on the anti-dry-shrinkage cracking property of concrete composite. This adverse effect appeared to be more pronounced with increasing replacement level of silica fume. Effect of silica fume content on carbonation depth The variations of the carbonation depth of concrete composite versus silica fume content of the carbonation specimen are given in Figure 3. A considerable decrease for the carbonation depth of the concrete was observed by increasing the content of silica fume. Compared with the mix without silica fume, the decrease was determined as 30% for 12% silica fume content. The main reason of silica fume can improve the carbonation resistance of concrete composite is that the grain size of silica fume particle is smaller than that of the cement. As a result, the silica fume particles can fill the interspaces around the cement particles to make the hardened concrete much more compact. Therefore it is difficult for the CO 2 in the atmosphere to permeate into concrete. Effect of silica fume content on freezing thawing durability Figure 4 illustrates the variations of the relative dynamic elastic modulus of concrete composite versus silica fume content. A considerable increase for the relative dynamic elastic modulus of the concrete was observed by increasing the content of silica fume. Compared with the mix without silica fume, the increase was determined as 8.1% for 12% silica fume content. The particles of silica fume can react with Ca(OH) 2 generated from cement hydration to produce C S H, which can be filled into the interspaces around the cement particles. With the addition of silica fume, the bubble structure in the concrete composite was improved for the number of little bubbles and gel holes was increased, and the number of large bubbles Figure 3. Effect of silica fume content on carbonation depth. Figure 2. Effect of silica fume content on dry shrinkage. Figure 4. Effect of silica fume content on relative dynamic elastic modulus.

6 348 Proc IMechE Part L: J Materials: Design and Applications 227(4) was decreased. As a result, the space between two bubbles was reduced, which has positive contribution on the freezing thawing durability of concrete. In addition, the pozzolanic effect of silica fume can improve the compactness of concrete. So the amount of free water permeating into the concrete composite is less, and the amount of water likely freezing is less. Therefore, the addition of silica fume can improve the freezing thawing durability of concrete composite evidently. Conclusions This article reported experimental results of durability studies conducted on concrete composite containing silica fume. The following conclusions can be draw from the results presented in this study.. Effect of silica fume on the length of water permeability of concrete composites is significant, and the addition of silica fume can reduce the length of water permeability and improve the water impermeability of the concrete composite evidently, and the water impermeability is becoming better and better as the silica fume content is increasing gradually.. Addition of silica fume has adverse effect on the dry shrinkage property of concrete composite. The dry shrinkage strain in the concretes containing silica fume was more than that in the concrete without silica fume, and a considerable increase for the dry shrinkage strain of the concrete was observed by increasing the dosage of silica fume.. Presence of silica fume in concrete composites reduces the carbonation depth considerably. Carbonation depth of the concrete is decreasing gradually as the silica fume content is increasing gradually. The addition of silica fume can improve the carbonation resistance of the concrete composite evidently.. Freeze thaw resistance of concrete containing silica fume was found to evidently increase when compared to concrete without silica fume. Moreover, there is a tendency of increase in the freeze thaw resistance with the increase of silica fume content. Funding This work was supported by China Postdoctoral Science Foundation (grant no ), Open projects funds of the dike safety and disaster prevention engineering technology research center of Chinese Ministry of Water Resources (grant no ), and Foundation for University Key Teacher by Zhengzhou University (grant no. 2012). References 1. Bharatkumar BH, Narayanan R and Raghuprasad BK. Mix proportioning of high performance concrete. Cem Concr Compos 2001; 23(1): Zhutovsky S and Kovler K. Effect of internal curing on durability-related properties of high performance concrete. Cem Concr Res 2012; 42(1): Swamy RN. 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