Influence of nano silica and silica fume on concrete mechanical properties containing scoria lightweight aggregate and hybrid fibers steel and polypropylene
This study are focused on mechanical properties of hybrid fibers-reinforced scoria light weight aggregate concrete (LWC) and combination of nano silica (NA) particles with silica fume (SF) Different amounts of 3, 5 wt% of nano silica-particles and10, 15wt% SF have been used as a partial cement replacement Bulk density and compressive strength, flexural strength, splitting tensile strength, elastic Modulus and water absorption of hardened concrete were herein investigated. Steel fiber (0.4% - 0.8%) and polypropylene fiber (0.2%) concrete volume with the ratio of length to diameter of 62.5 and 60 wereused respectively.The specimens were cast and then tested at the end of 28 day water curing. The obtained results showed that the use of Na along with SF and fibers significantly increased concrete mechanical capacity due to the addition of steel fibers in hybrid fibers LWC and a reduction in amount was observed by Na and SF.
Keywords: Na, SF, hybrid fibers, lightweight Scoria, mechanical properties
Development of lightweight aggregate concrete (LWAC) has been gained lots of attention since last decade[1–5]. Lightweight aggregates are broadly speaking categorized in to natural and artificial. Pumice, scoria, diatomite, etc. as natural and perlite, tuff, expanded clay or shale (e.g. Lica or Liapor), slate, sintered pulverized fuel ash (e.g. Lytag), etc. are known as some of the artificial ones. Another artificial ultra-lightweight aggregate having low density and rigidity nonabsorbent beads is known as Expanded polystyrene (EPS) [1,6]. The LWAC has several known superiority as compared to normal weight concrete such as economic considerations, low density, reduced Seismic Forces, high strength/weight ratio , low thermal conductivity coefficient, better and durability properties[11,12]. Furthermore, other techniques employed for LWAC production are aerated, cellular and no fine aggregate concrete [7,13,14].
Structural lightweight aggregate concrete [SLWAC] has been defined differently to some extends in different standards. The ACI standard states that cylinder compressive strength and air dry unit weight should not be less than17 Mpa and 1840 kg/m3 for 28- day, respectively. However, some European standard such as DIN- 4219, PN-91, ENV92accountamaximum unit weight of SLWAC as 2000 kg/m3 while, this criterion in PREN 206 is considered 2100 kg/m3 .
In order to have a different desirable mechanical properties and durability it is usually required different kinds of cement replacement materials are to be added to concrete one of the most widely-used pozzolans is SF and Na which lead to increased strength and suppress porosity, permeability and bleeding of concrete because its oxides (SiO2) react with calcium hydroxides produced during the hydration process of Portland cement. The major benefits of pozzolanic reactions can be stated as follow: low heat release and strength growth; consuming calcium hydroxides; dispersion of small pore size .
the use of nano-particles has greatly attained special attention in many fields of applications such as biomedical research as well as for making concrete with new functionalities. By inclusion of ultra-fine particles into Portland-cement mortar or concrete, different characteristics for the materials were obtained from conventional materials [17–19]. Nano-particles of SiO2acting as a nano-filler can fill the existing spaces between C–S-H gel particles Moreover, the increase in amount of C–S–H which is due to the pozzolanic reaction with calcium hydroxide, highly densified the matrix, and hence improved the strength as well as durability of the material[18, 20-25].
The reduction of tensile and flexural strength of LWAC is attributed to the weakness of LWA LWAC due to its brittle nature is in contradiction with the main objective of LWAC requirement that demands ductility in seismic loads. This shortcoming can be well overcome by incorporation of appropriate amount of fibrous materials [26–28]. Brittleness of SLWAC like normal weight concretes can be compensated for by appropriate reinforcing type of fibers in concrete. Effect of the type and properties of reinforcing fibers on the mechanical strength and ductile behavior of the normal weight concrete have been reported in several experimental investigations [29–33]. fibers not only enhances tensile strength, flexural strength, fatigue strength and ability to spalling, but also notably improves impact strength and toughness among other engineering properties. [34-39].
Lightweight Scoria with the basic composition of basalt is viewed physically as irregular, fine to coarse vesicular fragments and generally appeared to be spongy, reddish or black in color. Scoria like pumice which has volcanic origin is usually heavier, darker and more crystalline in comparison to pumice. Scoria is found abundantly in different parts of the world such as in Turkey, Papua New Guinea (PNG)  and Saudi Arabia . Scoria can be utilized in different applications such as in the manufacturing of lightweight concrete, as a source of pozzolan for Portland-pozzolan cement additive, as a heat insulating materials, as low cost fillers, for filtering and absorbent materials as well as other architectural applications.
In this study attempts have been made to confirm that by using nano-materials with SF and hybrid steel and polypropylene (PP) whether it is possible to obtain high performance and strength fiber reinforced LWC.
2. Experimental plan
An ordinary Portland cement (OPC), conforming to the ASTM C150with a specific area of 2900(cm2/g) and a specific gravity of 3.15 (g/cm3) is used. The SF with a 2120 kg/m3 from Iran ferroalloy industries was used as pozzolanic material in all designs.Nano-SiO2 with 1.2 g/cm3 density has been used in combination with SF. The chemical properties of cement, SF and Na are given in Table 1.Fine and coarse aggregates properties are shown in Fig 1,2 and Table 2, 3. Poly carboxylic ether based high range water reducer (HRWR) namely P10-3R with density 1.1g/cm3 was used to enhance the flow ability of the mixtures. Two types of reinforcing fibers of polypropylene, steel (Fig 3 and Table 4 for more details) were utilized.
2.2. sample preparation and mix designs
Totally, 17 mixing designs with Na and SF replacement of cement were used. The volume fraction of steel 0.4% and 0.8% and PP fibers varied 0.2% and 0.4% of concrete, respectively. The incorporation of fiber in concrete is usually carried out in two ways. Some researchers have incorporated dry aggregates with fiber and then followed by adding cement and water[42,43].However, some others made concrete without fibers first and then added fiber[44,45]. In this study, first of all dry LWA were mixed with incorporated water for 30 minutes in order to pre-wet the LWA. After that, sand was added to the LWA. Afterward, cementitious material including cement, SF and Na were added to the mixture and next some more water and super lubricant were added. Finally, the remaining water was also added. In mixed design containing fiber, fiber was poured in to the cycling mixture for gaining dispersion and uniformity throughout the concrete. Incorporation was carried out in a way that the amount of included absorbing water be actually absorbed in half an hour by the materials. The samples were kept in a temperature of 22–25°C in laboratory environment for 24 hours. Table 5 shows the concrete mix compositions for the samples. Also a summary of type, dimensions and standard of testing is presented in Table 6.
3. Hardened concrete results
3.1. Dry density
The dry densities of hardened concretes which were measured at 28 days are listed in Table 7 and Figure 4. It is noted from the bulk density results that the PP fibers have insignificant effect on the density of concrete specimens. But, that does not apply to steel fibers in which case concrete density is greatly affected by it. All the designs are within the permitted weight limitation (less than 2000 kg/m3). Therefore, it would be possible to make SLWAC with 62.8 kg/m3 steel fibers.
3.2. Compressive Strength
The 7, 28days compressive strength of LWC are shown in Figure 5. From these figures, adequate structural compressive strength (above17 MPa) was achieved for all mixes. PP fibers reduced the compressive strength and the combined effect of steel and PP fibers increased the strength. A considerable increase of the compressive strength for the concretes without fibers was observed by increasing the SF and Na content. The increase of compressive strength is reached to a maximum of about 32% for 10% SF and 43% for 3% Na with 10% SF contents, respectively. This result can be clearly depended on increasing bond strength between cement paste–aggregate interface by means of filling effect of SF and Na. The compressive strength of the concretes with Pozzolan and hybrid fibers were obtained higher than that of concretes containing only Pozzolan. The presence of PP fiber reduced the compressive strength about 4.7% and its combined effect with steel fibers has increased the strength to a maximum of 11%.
3.3. Splitting Tensile Strength
Figure 6 and Table 8 show the tensile strength behavior of SF and Na reinforced concrete sample with different contents of reinforcing steel, PP fibers.
Increasing SF, Na and fibers contents increase the splitting tensile strengths of the concretes. The increases in the splitting tensile strengths of the concretes without fiber were determined as 25%, 12% for the 10%, 15% SF and 36.5%, 15.7% for the 3%, 5% Na contents, respectively. The maximum increase in the splitting tensile strength of the concrete contained 0.8% steel fiber and 0.2% PP fiber with 10% SF content was 27% which shows the effective presence of steel fibers in improving tensile strength. This increase in strength was also reported in other studies [5,46]. The cause of increase in strength using steel fiber can be attributed to an increase in the strain resistance of the fiber during splitting of fibers through tension transmission from matrix to fibers.
Figure 7: Correlation of compressive strength and tensile strength: (a) SF and fibers, (b) Na and fibers
The variation of tensile strength with the compressive strength is given in Figure 7. From this,
it can be seen that the tensile strength is increased with an increase in compressive strength.Thebest fit relationship obtained from the regression analysis of the experimental results is presented in the figures. The results show relatively a good correlation between the compressive strength and tensile strength.
3.4. Flexural Strength
The flexural test results for determining the modulus of rupture in concrete specimens are carried out and shown in figure 8 and table 8.Maximum increase in flexural strength reaches 38.75% for 10% SF and 42% for 5% Na with 10% SF when hybrid fibers is increased to 0.8% in steel and 0.2% in PP, respectively.As it can be observed from Figure8, increase of combined effect of SF to 10¬% and Na content up 3%, increased the flexural strength continuously in a linear manner to a peak value.
It should be emphasized that the flexural strength of samples consistently increased up to the steel fiber volume of 0.8%.The significant effect of steel fibers on the flexural strength improvement is probably due to the fact that these fibers delay the unstable growth of cracks which usually occurs in flexure and prevent the crevices from spreading the tension and increase in the width of cracks. This increases the modulus of rupture by reducing the depth of cracks. Similar trends were also reported by other researchers[45, 47,48,49].
Figure 9 shows the compressive strength in terms of flexural strength. Determining the compressive and flexural strengths relationship is useful because core sampling in a prismatic shape would not be as easy as coring cylindrical sample on the site. So predicting the flexural strength as a criteria for design is of great importance. Regarding the figure, it could be mentioned that correlation coefficient of compressive and flexural strengths is relatively fine, because the flexural strength is increased by an increase in compressive strength. However, a bit increase in compressive strength and a considerable increase in the flexural strength is the reason for a little reduction in correlation coefficient.
3.5. Modulus of Elasticity
The modulus of elasticity is a function of the compressive strength. The increase in the compressive strength of the sample also increases the modulus of elasticity .Increasing pozzolan content, increases the elastic modulus of concretes[51,52]. For all steel fiber contents, elastic modulus of concrete incorporated pozzolan have higher values than elastic modulus of control concrete. Also elastic modulus of the concretes including only SF with increasing Na, is increased by the addition of the steel fiber and decreased by the addition of the PP fiber. At a single SF and Na content, the modulus of elasticity of the concretes is found to be increasing with the increasing steel fiber content. Increases in elastic modulus of concrete using pozzolan is an expected and well-known because that pozzolan makes the concrete structure more brittle. Table 9 and Figure 10 show the amounts of modulus of elasticity of specimens.
Figure 11 presents the compressive strength in terms of modulus of elasticity. As it is shown, correlation coefficient of compressive strength and modulus of elasticity is very good, because the elasticity modulus increases by an increase in compressive strength. It should be noted that a reduction in compressive strength by PP fiber has also led to a decrease of elasticity modulus which in turn has improved correlation between the two.
3.6. Water Absorption
The amount of water absorption is high in LWC, which has been reported in most cases to be above 10%. However, using SF and Nano-sio2 can reduce this amount. The quality of concrete has respectively been categorized by CEB in three levels of good for a water absorption of 5% and higher, medium between 3 to 5% and weak below 3%. The presence of fiber increased water absorption and this is due to increased porosity arising from fiber. Nevertheless, based on the presented category by CEB, most samples are categorized as good. The appropriate amounts of water absorption can be related to the use of SF and Na. The existing silica in SF and Na reacts with the hydration products and fills the pores in the concrete .
The presence of steel fiber increases the density of concrete. All the designs are within the permitted weight limit (less than 2000 kg/m3). Therefore, it would be possible to make structural lightweight concrete with 62.8 kg/m3fiber.
The compressive strengths of concretes produced by additions of steel fiber and pozzolan had higher than the ones containing only pozzolan. On the other hand, elastic modulus of concrete is increased by increase in steel fiber content, but contrary PP fibers tend to decrease compressive strength and elastic modulus of concrete.
0.2% PP fibers increased flexural strength and tensile strength to some extent such that maximum increase in the order of about 10%, 8% is obtained respectively. Steel fibers significantly affect both flexural and tensile strength. But, the effects on flexural strength(up to about 42%) are rather different from those on tensile behavior, but they have much smaller effect on increasing the compressive strength (up to about 27%).
Mechanical properties are initially increased by the increase of Na with SF content up to 3%, 10%, respectively and then decreased. Also In this study, the optimum content of SF in concrete in order to increase Mechanical properties was 10 %.
The water absorption of the specimens is decreased by increasing the NS and SF content. Also it can be seen that fibers increase the water permeability of concrete. Nevertheless, based on the presented category by CEB, most samples are categorized as good.
This research was done and funded by Yaser Ghorbani during the course of his master’s level at Deylaman higher education institute under the guidance and technical supervision of S. H. Ghasemzadeh Mosavinejad. The authors have no conflicts of interest.
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