Scholarly article on topic 'Properties and mesostructural characteristics of linen fiber reinforced self-compacting concrete in slender columns'

Properties and mesostructural characteristics of linen fiber reinforced self-compacting concrete in slender columns Academic research paper on "Civil engineering"

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{"Linen fiber reinforced self-compacting concrete" / "Rheological properties" / "Mesostructural characteristics" / "Ultrasonic pulse velocity" / "Aggregate distribution" / "Fiber density"}

Abstract of research paper on Civil engineering, author of scientific article — Sabry A. Ahmed

Abstract In this study the linen fibers were used to reinforce self-compacting concrete (SCC) with 2 and 4kg/m3 contents; then their effects on the fresh and hardened properties of SCC were investigated. Furthermore, three circular slender columns were cast using both plain and linen fiber reinforced (LFR) SCC in order to study the variations of hardened properties and mesostructural characteristics along the columns height. The addition of linen fibers to SCC reduced its workability and affected its self-compacting characteristics in a manner depending on the fiber content. Also, noticeable improvement in mechanical properties and slight reduction in unit weight and UPV were recorded. The hardened properties did not vary significantly along the height of columns, however, lower values were observed at the upper end of columns. The aggregate distribution was slightly more homogenous in case of LFRSCC, and the variation of fiber density along the height of columns was relatively high.

Academic research paper on topic "Properties and mesostructural characteristics of linen fiber reinforced self-compacting concrete in slender columns"

Ain Shams Engineering Journal (2013) 4, 155-161

Ain Shams University Ain Shams Engineering Journal

www.elsevier.com/locate/asej www.sciencedirect.com

CIVIL ENGINEERING

Properties and mesostructural characteristics of linen fiber reinforced self-compacting concrete in slender columns

Sabry A. Ahmed *

Materials Engineering Department, Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt

Received 2 May 2012; revised 21 July 2012; accepted 17 August 2012 Available online 25 September 2012

KEYWORDS

Linen fiber reinforced self-compacting concrete; Rheological properties; Mesostructural characteristics;

Ultrasonic pulse velocity; Aggregate distribution; Fiber density

Abstract In this study the linen fibers were used to reinforce self-compacting concrete (SCC) with 2 and 4 kg/m3 contents; then their effects on the fresh and hardened properties of SCC were investigated. Furthermore, three circular slender columns were cast using both plain and linen fiber reinforced (LFR) SCC in order to study the variations of hardened properties and mesostructural characteristics along the columns height. The addition of linen fibers to SCC reduced its workability and affected its self-compacting characteristics in a manner depending on the fiber content. Also, noticeable improvement in mechanical properties and slight reduction in unit weight and UPV were recorded. The hardened properties did not vary significantly along the height of columns, however, lower values were observed at the upper end of columns. The aggregate distribution was slightly more homogenous in case of LFRSCC, and the variation of fiber density along the height of columns was relatively high.

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1. Introduction

Self-compacting concrete (SCC) is a concrete that can be placed and compacted under its own weight without any vibration effort. SCC offers several economical and technical benefits and has been widely used in many countries since it has been developed in Japan in the late 1980s [1]. The three main characteristics of SCC are filling ability, passing ability and segregation resistance. Each one of these properties can be eas-

ily secured separately [2]; however, the first two characteristics are opposite to the last one. To combine these properties it is necessary to have complete control over the rheology of concrete, where the yield value has to be reduced while the plastic viscosity of the fresh concrete has to remain high [3]. In other words, the SCC should have sufficient flowability to ensure an appropriate filling of structural elements and enough viscosity to prevent settlement of coarse aggregate particles and to maintain the uniformity of concrete.

Ordinary self-compacting concrete possesses low tensile strength, limited ductility and little resistance to cracking. Internal micro-cracks are inherently present in the concrete and its poor tensile strength is due to propagation of such micro-cracks, leading to brittle failure of concrete. As the strength of concrete increases, its brittleness also increases. This weakness can be considerably overcome by the inclusion

* Tel.: +20 01120062003.

E-mail address: asabry_2003@yahoo.com

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of linen fibers in the concrete mix. These fibers enhance cracking resistance, thus improving the mechanical properties and extend the possible fields of application of SCC [4,5]. The improvements in the properties of linen fiber reinforced self-compacting concrete (LFRSCC) depends primarily on the distribution of fibers through SCC; where a good distribution is required to achieve optimum benefits of the fibers [6]. One difficulty in adding such fibers, however, is that it can hinder the flowability of fresh SCC mix [7-9]. The possible applications for LFRSCC are in concrete repair, pavement slabs, thin shells, walls, pipes and manholes.

Many types of fibers such as steel, glass, carbon, polypropylene, nylon and polyester, were widely used in concrete for their advantages; however, the steel fiber was the most popular type of fiber used to reinforce SCC [10-12]. In the innovative work of Torrijos et al. [13], the mesotructural homogeneity of steel fiber reinforced self-compacting concrete (SFRSCC) and its effect on the physical and mechanical properties of the material were studied. They found that homogeneity of concrete constituents and distribution of fibers are very important parameters in SFRSCC, where the correct filling and compacting of concrete are specifically depend on its rheological properties. Nevertheless, it is still necessary to study properties and mesostructural characteristics of LFRSCC; due to the big difference in the densities of steel and linen fibers and so contents of fibers that can be used in SCC. Also, it is important to mention that this is the first trial to use linen fibers as reinforcement in SCC. At the mesostructural level, the LFRC consists of mortar matrix, coarse aggregate, interfacial transition zone and linen fibers.

The objective of present study was to analyze the meso-structural characteristics of plain and linen fiber reinforced self-compacting concrete used in slender columns. This type of elements was selected to check the segregation resistance and the capability of LFRSCC to distribute evenly along the column height. Also, the rehological properties of plain SCC and LFRSCC as well as the variations of hardened properties along the columns height were measured.

2. Experimental work

2.1. Materials, mixing and casting of prototype columns

The cement used in all mixes was ordinary Portland cement (OPC); its specific gravity and specific surface area were 3.15 and 3250 cm2/gm, respectively. The fine aggregate was natural siliceous sand with specific gravity, fineness modulus and water absorption of 2.64, 2.8 and 0.6%, respectively. The coarse aggregate was dolomite with a NMZ of 14 mm, specific gravity of 2.67 and water absorption of 0.92%. The limestone powder was used as a mineral admixture to increase the paste volume and enhance deformability and stability of the concrete mix; it had a specific gravity and specific surface area of 2.70 and 4750 cm2/gm, respectively. Synthetic polymeric high range water reducing (HRWR) admixture, RHEOBUILD 1100, was used to obtain the required workability without increasing the w/c ratio, its specific gravity is 1.190 at 24 0C.

The linen chopped fibers, shown in Fig. 1, were used to reinforce SCC. Linen is a bast fiber, derived from the flax plant, with a natural smell and pale yellow color. It consists of parallel regular fibers not less than 600 mm length with

Figure 1 Linen chopped fibers.

cleanness not less than 98% and moisture not more than 12%. The linen short fibers used in this study were manually cut to a length of 30 mm.

Three concrete mixes, with the proportions shown in Table 1, were prepared. One of them did not contain linen fibers; representing the plain SCC mix, while the other two mixes contained moderate and maximum contents of linen fibers. After many trials, it was found that 4 kg/m3 was the maximum amount that can be added without adverse effect on both fibers balling and self-compacting. Half this value, i.e. 2 kg/m3, was taken as the moderate content. In the suggested mixes, the mix ingredients, w/p ratio and HRWR% were kept constants, see Table 1.

For optimum mixing, the following steps were followed:

1. All dry ingredients were mixed for 1 min.

2. 70% of the calculated amount of water was added to the dry mix and mixed thoroughly for 1 min.

3. The remaining 30% of water was mixed with the required amount of HRWR and was poured into the mixer and mixed for 5 min.

4. Finally, the required amount of linen fibers was sprinkled over the concrete mix and mixed for 1 min to get a uniform mix.

The total mixing time reached 7 min. From each concrete mix, six cylinders of 150 mm diameter and 300 mm height were cast in one layer without any compaction. After 24 h, the specimens were cured with clean tap water for 7 days, then stored in the laboratory till the testing date. These cylinders were used to determine the hardened properties of SCC; unit weight, ultrasonic pulse velocity and com-pressive and splitting tensile strengths.

Three round slender columns with 150 mm diameter and 2500 mm height were cast using plain and linen fiber reinforced SCC of two contents (2 and 4 kg/m3). Three plastic tubes with the required diameter and length were opened from one side and supported vertically using the wooden form shown in Fig. 2. Steel wires were used as clamps at short spaces to provide additional restrictions to opening of the plastic tubes. Placing of concrete columns was performed in a continuous manner without any type of compaction. After 24 h of placing, the plastic tubes were removed and the concrete columns were

Table 1 Constituents of the three investigated mixes of SCC.

Cement (kg) Limestone powder (kg) Coarse aggregate (kg) Fine aggregate (kg) W/P HRWR (%)

400 100 800 900 0.42 1.2

Form During curing After curing

Figure 2 Casting of the prototype columns.

left inside the wooden form, then they were covered completely using burlaps. As shown in Fig. 2, a water pipe was used to ensure continuous droplets of water on the columns for 7 days, to simulate curing of standard cylinders in water tanks. Fig. 2 shows also the concrete columns after curing.

2.2. Tests of fresh concrete

For fresh concrete, slump flow, J-ring and V-funnel tests were carried out according to EFNARC [14,15]. The slump flow test is used to assess the horizontal free flow (deformability) of SCC in the absence of obstructions. The average diameter of spread was determined as the slump flow diameter. The slump flow test was also used to measure the flow rate and segregation resistance of concrete. These properties were measured by recording the time it took to reach a 500 mm spread circle (T500 mm), and visually checking any segregation border between the aggregates and mortar around the edge of spread.

The J-ring (Japanese ring) test is used to determine the passing ability of concrete between reinforcing bars. The difference in height between the concrete just inside and just outside the bars was measured at four different locations and the average value (DD) in mm was calculated. The V-funnel test measures the passing ability of concrete in restricted areas. The V-funnel flow time is the elapsed time in seconds between the opening of the bottom outlet and the time when the light becomes visible from the bottom, when observed from the top. Good flowable and stable concrete would consume short time to flow out.

2.3. Tests of hardened concrete

The adopted criteria used for testing hardened concrete and the used methodology are practically the same as that used early by Torrijos et al. [13]. To study the mesostructural characteristics and the variation of concrete properties along the column height, the columns were transversally sawed in a

manner shown in Fig. 3. Seven cylinders (C1-C7) of 300 mm height, eight slices (S1-S8) of 20 mm thickness intercalated between cylinders and two ends (E1, E2) of 120 mm height were extracted from each column. To avoid the effect of extra compaction at the bottom end and the effect of bleeding at the top end, the two ends (E1, E2) were excluded from the analysis. The cylindrical specimens were used to evaluate the concrete surface finish and to measure the dry unit weight, ultrasonic pulse velocity and compressive strength. The ultrasonic pulse velocity was measured using PUNDIT equipment with a frequency of 54 kHz.

The slices specimens were used to study the variation in the distribution of coarse aggregate and fibers along the column height. The cut planes of slices were polished to improve the digital scanning of the cross-sections and facilitate the counting of coarse aggregate particles and fibers. At each cut plane, only the particles with at least one dimension P 5 mm were considered as coarse aggregate for counting. To avoid wall effects, the analysis was carried out on a concentric circle of 100 mm diameter. Once the images of the cut planes were digitally scanned, the perimeter of aggregate particles were differentiated and the delimited areas quantified by means of image analysis software. At each cross-section, the total number and density of coarse aggregate (mm2 of aggregate/mm2 of cross-section) were recorded (Fig. 4).

The analysis of fibers was carried out on the same concentric circle of 100 mm diameter using ERDAS IMAGINE software. The model maker included in the program read the digitally scanned photo and classified the data representing the cross-section of the slice into group of inputs; all of them were the material ingredients of the SCC mix. Then the model gave a color level to each of the input data, and accordingly counting of fibers became very simple through giving an order to the program to count all points carried the same color specified for the fibers. The density of linen fibers was calculated as the number of fibers per mm2 of the cross-section.

E1 C1 C2 C3 C4 C5 C6 C7

7x300+8x20+2x120= 2500 mm (Column height)

Figure 3 Sawing of concrete column and samples.

Figure 4 Sample of slices and typical image used in the analysis of aggregate distribution.

3. Results and discussion

3.1. Fresh concrete properties

Results of the fresh concrete tests in terms of slump flow diameter and time, J-ring test and V-funnel flow time are given in Table 2. Moderate content of linen fibers caused a slight negative effect on the workability and self-compacting characteristics of SCC. This effect increased with increasing linen fiber content; at maximum content the slump flow diameter was

considerably reduced from 735 mm to 600 mm. According to EFNARC limits [15], this diameter was out of limits accepted for SCC, but according to Nagataki and Fujiwara [16] the slump flow diameter required for SCC ranging from 500 to 700 mm. Therefore, the concrete mix contained maximum content of linen fibers was still within the range recommended for SCC. Also, the other self-compacting characteristics of SCC mix containing maximum content of linen fiber, i.e. slump flow time, J-ring test value and V-funnel time were highly increased but still at the boarder of the upper limits recommended by

Table 2 Fresh properties of the three investigated mixes of SCC.

Mix type Test Slump flow diameter T500 mm slump J-Ring (AD) V-Funnel time

EFNARC limits [14] 650-800 mm 2-5 s 0-10 mm 6-12 s

SCC (without fibers) 735 3.3 5 6.8

LFRSCC (with 2 kg/m3 fibers) 683 3.9 7 8.4

LFRSCC (with 4 kg/m3 fibers) 600 4.8 10 11.5

EFNARC for SCC [15]. Therefore, all concrete mixes were considered as SCC. In all studied mixes, there was no segregation of aggregate near edges of the spread-out concrete as observed from the slump flow test. No problems in mixing have been encountered and the fiber distribution was uniform.

3.2. Hardened concrete properties

The results of hardened concrete standard cylinders are presented in Table 3, which included the unit weight, ultrasonic pulse velocity (UPV), compressive strength (fc) and splitting tensile strength (ft). All of these properties were measured at 35 days age, 7 days curing in water and 28 days in lab atmosphere, at the same time of testing specimens extracted from the columns. The unit weight of SCC without fibers was slightly higher than that of LFRSCC. This indicates higher compaction in the first case and also may be due to lower density of linen fibers. However, the maximum difference between the unit weights of the two cases of LFRSCC and that of SCC was less than —2%. Similar trend was also shown for the ultrasonic pulse velocity; the values were higher for SCC without fibers followed by LFRSCC with 2 kg/m3 fibers, then LFRSCC with 4 kg/m3 fibers (see Table 3). The very small differences observed in the unit weights and ultrasonic pulse velocities seem to be an indication of the uniformity of concrete in all mixes.

Somewhat different trend of results was observed in case of compressive and splitting tensile strengths, where fiber inclusion improved the mechanical properties of SCC. Two noticeable observations can be seen in Table 3: first, at moderate fiber content the improvement in the property was higher than that at maximum fiber content, irrespective of the type of mechanical property. Second, the improvement in the splitting tensile strength was higher than the improvement in the com-pressive strength. The percentages of improvements at moderate and maximum fiber contents were 8.3 and 6.25 for compressive strength and were 17.6 and 11.8 for splitting tensile strength, respectively. A higher improvement may be expected if the flexural strength of this material is measured; as a result of bridging of cracks by fibers. When micro-cracks develop in the matrix, the fibers in the vicinity of such microcracks will arrest them and prevent further cracks propagation. Thus the cracks will display a meandering path resulting in the demand for more energy for further cracks propagation, which in turn increases the ultimate load resistance.

Fig. 5 shows surface finish of the three SCC columns at three different locations along the height; lower end, middle and upper end. As notably observed surfaces of the three columns were smooth and homogenous and free from defects and air bubbles. This indicated that although diameter of the tubes used for casting the columns was relatively small (150 mm), the implemented casting conditions were optimum for the three cases of SCC.

At the upper end

At the middle

|c,(ïïf V C (1) f

w ' äi" Wr 1

At the lower end Figure 5 Surface finish along the height of SCC columns.

The above mentioned results of the unit weight, compressive strength and also surface finish of LFRSCC was different from the corresponding results obtained by Torrijos et al. [13] for SFRSCC. The unit weight of SCC contained maximum content of steel fibers was considerably higher than that of plain SCC, due to the high density of fibers used and its considerable amount. Also, the concrete mix used in the present study was normal strength concrete with about 30 N/mm2 compressive strength, while that used by Torrijos et al. [13] was high strength concrete with about 53 N/mm2 compressive strength. Furthermore, the specimens cast by Torrijos et al.

Table 3 Hardened properties of standard concrete cylinders.

Mix type Unitweight (kg/m3) UPV (km/s) fc (N/mm2) f (N/mm2)

SCC (without fibers) 2229 4.13 28.80 3.40

LFRSCC (with 2 kg/m3 fibers) 2214 4.09 31.20 4.00

LFRSCC (with 4 kg/m3 fibers) 2190 4.03 30.60 3.80

Unit weight (kg/m )

UPV (km/s)

Comp. Strength (N/mm2)

Figure 6 Variation of the hardened properties of SCC along the height of columns at different fiber contents.

[13] were not strictly homogenous and there were air bubbles at their surfaces, due to low workability of concrete mix used.

Variations of the unit weight, ultrasonic pulse velocity and compressive strength of SCC along the height of columns are illustrated in Fig. 6. These properties were measured on the 150 x 300 mm cylinders extracted from the three SCC columns at different heights (see Fig. 3). The abbreviations showed in legend of figures were 0 for SCC without fibers, 2 for LFRSCC with 2 kg/m3 fibers and 4 for LFRSCC with 4 kg/m3 fibers. As previously obtained in case of standard cylinders, the unit weight and UPV were slightly higher in case of SCC without fibers, followed by LFRSCC with 2 kg/m3 fibers and then LFRSCC with 4 kg/m3 fibers, as shown in Fig. 6a and b. The mean values of unit weight were 2260, 2248 and 2230 kg/m3, and the mean values of UPV were 4.11, 4 and 3.91 km/s for 0, 2 and 4 SCC cases, respectively. Variations of the unit weight and UPV along the heights of the three SCC columns were low, with maximum differences with respect to the mean values of ±1.65% and ±3.84%, respectively, irrespective of the SCC case. Also shown in Fig. 6a and b are the lower values of the unit weight and UPV at the upper end of columns compared with the corresponding values at the lower end, may be due to the relatively lower compaction at this part of columns.

Fig. 6c shows the variations of compressive strength along the height of SCC columns. The results of compressive strength were also consistent with those obtained in case of

standard cylinders, where again the addition of fibers to SCC improved its mechanical properties. The mean values of com-pressive strength were 29, 32.9 and 31.2 N/mm2 for 0, 2 and 4 SCC cases, respectively, with maximum difference of variation along the heights of columns with respect to the mean values of ±5.77%, irrespective of the SCC case. Fig. 6c shows also how the relatively lower compaction of the upper part of columns produced a decrease in the compressive strength, which was consistent with the unit weight and UPV results. The percentages of reductions in the compressive strength of the upper part compared to that of the lower part were 8.3, 9.2 and 12.2 for 0, 2 and 4 SCC cases, respectively. The higher reduction observed in the compressive strength of LFRSCC cases may be due to the decrease in the concrete workability as a result of fiber inclusion, which in turn affected the self-consolidation of concrete in this part of column.

Fig. 7 presents the variations in the distribution of coarse aggregate and fibers along the height of columns, which were considered for the analysis of mesostructural characteristics. The variations of coarse aggregate distribution along the columns height did not follow a particular patron, see Fig. 7a and b. However, the maximum variations along the heights of columns of 0, 2 and 4 SCC cases with respect to the average values were ±12.7%, ±5.8% and ±7.6% for the number of coarse aggregate and ±9.1, ±4.5 and ±6.3 for the density of coarse aggregate, respectively. These variations were calculated as the maximum variations, irrespective of the location

50 60 70 80 No. of agg. particles

Agg. density (mm2/mm2)

Fibers density (No./mm )

Figure 7 Variation of the mesostructural characteristics of SCC along the height of columns at different fiber contents.

along the column height. As seen, the maximum variations were higher in case of SCC without fiber than the fiber mixes. This can be attributed to the presence of fibers in the SCC mix, which makes the concrete more stable and has a relatively high resistance to segregation. The somewhat reversed effect shown at maximum content of fibers may be due to the relatively lack in the self-compacting characteristics of this type of mix, as described earlier in the analysis of fresh concrete properties.

Variations of the density of fibers, represented as number of fibers per mm2, along the height of LFRSCC columns are shown in Fig. 7 c. Both columns showed a great reduction in the density of fibers with their heights. The maximum reduction in the density of fibers was happened at the upper end of columns, with percentages reaching about 32 and 28 with respect to the average values for 2 and 4 kg/m3 fiber contents, respectively. The average values of fibers density were consistent with the increase of fiber content from 2 to 4 kg/m3. Trends of the mesostructural characteristics of LFRSCC agreed with those obtained by Torrijos et al. [13] for steel fiber concrete.

4. Conclusions

From the results of present study, the following conclusions may be drawn:

1. Adding linen fibers to plain SCC mix reduced its workability to a degree depended on the content of fibers. At 2 kg/ m3 fiber content there were no substantial variations in the workability and self-compacting characteristics of SCC mix. However, at 4 kg/m3 fiber content there was considerable reduction in the slump flow diameter from 735 to 600 mm, and slump flow time, J-ring test value and V-fun-nel time were highly increased but still at the boarder of the upper limits recommended by EFNARC for SCC.

2. Addition of linen fibers into SCC showed slight reduction (<2%) in the unit weight and UPV, while noticeable improvement in the compressive and splitting tensile strengths reached to 8.3% and 17.6% at 2 kg/m3 fiber content, respectively.

3. Surface finish of the studied columns was smooth and homogenous and free from defects and air bubbles; indicating that the implemented casting conditions were optimum.

4. The unit weight, UPV and compressive strength of the three investigated SCC columns did not vary significantly along their heights; the maximum differences with respect to the mean values were ±1.65, ±3.84 and 5.77%, respectively. The lower values of such properties were observed at the upper end of columns.

5. The mesostructural analysis illustrated that there was no general trend for the distribution of coarse aggregate along the height of columns, however, the variations in aggregate distribution were higher in case of plain SCC.

6. The density of fiber was significantly reduced with the height of both LFRSCC columns. The maximum reduction was observed at the upper end of columns; with percentages reaching about 32 and 28 with respect to the average values for 2 and 4 kg/m3 fiber contents, respectively.

References

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Sabry A. Ahmed is an Associate Professor in Engineering Materials Department, Faculty of Engineering, Zagazig University, May 2003-Now. The author graduated from Zagazig University in 1987 and received his M.Sc. Degree in 1991. He obtained his Ph.D. Degree in 1998 from Cairo University. His Fields of interest are in self-compacting concrete, special concretes and repair of concrete structures.