Home Articles New Suitability of Synthetic Fiber for the Construction of Concrete Pavements

Suitability of Synthetic Fiber for the Construction of Concrete Pavements

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Apavement concrete undergoes dynamic loading and rigorous environmental conditions. Development of shrinkage cracks in plain cement concrete pavements is a major problem especially in tropical regions. To overcome this problem sometime the addition of synthetic fiber to the concrete mix is suggested. This paper briefly discusses the effects of the addition of polypropylene (PP) multifilament and fibrillated fibre on the properties of a paving grade concrete mix of compressive strength 48 MPa and flexural strength 5.4 MPa at 28-day. Concrete mixes containing different dosage of multifilament and fibrillated fiber besides one control mix were used. The important properties of the concrete relevant to its use in pavement such as flexural strength, drying shrinkage, and abrasion resistance etc. were evaluated. The study suggested a significant reduction in drying shrinkage, better resistance to abrasion, and strengths at least at par with controlled concrete for the concrete mixes reinforced with fibre. Further, the comparison of the affects of polypropylene multifilament and fibrillated fibres has indicated similar performance for concrete reinforced with fibrillated fibre.

The concrete used in the construction of road surfaces, bridge decks, airfield runways, and parking lots is generally known as pavement concrete or pavement quality concrete. This concrete has to undergo dynamic loading due to moving traffic and rigorous atmospheric environments. Therefore, this concrete has to possess good strength and durability properties relevant to its use in pavement such as resistance to abrasion, resistance to shrinkage cracking etc. Plain portland cement concrete possesses a low tensile strength as well as a low tensile strain capability consequently, it is prone to have numerous micro- and macro-cracks during its setting and hardening process. Use of synthetic fiber in concrete has been advocated by several researchers1-4 for improving specific properties of the concrete. Thoroughly mixed and dispersed microfibers with a high specific fiber surface area are particularly effective in reducing plastic shrinkage cracking as they are closely spaced. This may delay the process by which the micro cracks coalesce to form large, macroscopic cracks known as macro cracks5. In this way the addition of synthetic fibers modifies the properties of concrete matrix.

Polypropylene (PP) fiber is widely used for this purpose in the construction industry with a dosage of 0.1% by volume of concrete6. Polypropylene fibers are available in three different forms; monofilaments, multifilament and fibrillated7. Monofilament fibers are single strand of fibers having uniform circular cross-sectional area. Multifilament is a yarn consisting of a number of continuous filaments or strands. The diameters of the multifilament fibers depend on the number of monofilament fibers used, and how they are combined to form a yarn. Fibrillated fibers are manufactured in the form of films or tapes that are slit in such a way that they can be expanded into an open network to allow penetration of cementitious materials. In some cases, the fibrillated tape is twisted prior to cutting to enhance the opening of the bundle. Fibers thus, produced are termed fibrillated polypropylene and are cut to desired lengths. This fibre is also known as hi-tech fibre. In this condition, fibers are added to the concrete. During the mixing, due to friction with aggregates, the fibrils are broken that help to enhance the bond with the concrete matrix8.

Several researchers9-10 have reported that finer PP fiber is more effective in reducing the width of plastic shrinkage cracking than coarser fiber. The compressive and tensile strength of the concrete reinforced with low volume of PP fiber are not significantly different from those of the unreinforced matrix6,11-12. Several researchers6,11, 13-14 have reported increase in flexural strength of concrete reinforced with PP fiber. A study by Ramakrishnan et al12 on the use of fibrillated fiber in concrete has shown a slight increase (0.7- 2.6%) in flexural strength of concrete reinforced with fibrillated fiber at the dosage of 0.1% by volume and at 0.2- 0.3% by volume slight decrease in flexural strength have been reported6. In this paper, the effect of the addition of polypropylene multifilament fibers and fibrillated fiber on settlement, compressive strength, flexural strength, drying shrinkage and abrasion resistance of a paving grade concrete mix with respect to unreinforced concrete mix has been presented and discussed.

Experimental Study

Materials

The materials used included ordinary portland cement, well graded crushed quartzite coarse aggregate of nominal maximum size 20 mm, land quarried concrete sand, tap water, polycarboxylate ether-based high range water reducing agent (HRWRA) and polypropylene (PP) multifilament as well as fibrillated fiber of 18 mm average length. The average length of both types of the fibres was 18 mm. Seven concrete mixes that contained different fiber contents (% by volume and in kg/m3) were as listed in Table 1.

All the concrete mixes were prepared in a tilted drum mixer. All ingredients except water and superplasticiser were mixed in dry state for few seconds in the concrete mixer. Then ¾ of the total required water was added to the mix and mixing was further continued for a couple of minutes. The HRWRA was added in remaining 1/4th of total mixing water and added to the mix in the final stage of mixing. The mix was further mixed for another couple of minutes. Upon completion of mixing, fresh state properties were evaluated.

Mix Proportions

The concrete mix proportions were designed to yield a characteristics compressive strength of 40 MPa and flexural strength of 4.5 MPa. Several trial mixes were used to establish the optimum dose of HRWRA and proportions of different ingredients. The final mix proportions of concrete satisfying required performance are as given below:

C: A: S: W/C:: 1: 2.48: 1.62: 0.38.

It contained 437 kg of cement, 1090 kg of coarse aggregate, 707 kg of fine aggregate and 168 kg of water besides 1.53 kg of HRWRA. The HRWRA dose was 0.35% by mass of cement. The control mix (S-1) was without synthetic fiber while other mixes contained synthetic fiber of different types and dosage as shown in Table 1. The amounts of fiber used were 0.45, 0.90, and 1.35 kg for one cubic meter of concrete. Each concrete mix was batched and mixed in the laboratory in accordance with ASTM C192- 200715. The mixing procedure adopted was as described in earlier section.

Preparation and Curing of Test Specimens

150 mm cube specimens for the evaluation of compressive strength, 100 x 100 x 500 mm beam specimens for flexural strength, 75 x 75 x 285 mm beam specimens for drying shrinkage and 500 x 500 x 100 mm slab specimens for abrasion resistance were cast from each of the concrete mix. The specimens were demoulded after 24 hrs of casting and curing in steel mould. Thereafter, the demoulded specimens were marked for identification and kept submerged in curing tanks at room temperature (270 ± 20C) till the age of testing.

Test Methods

Fresh Mix Properties

The workability of fresh concrete mixes was determined by slump test as per IS- 1199:195917. The settlement of concrete mixes was measured in 150 mm diameter and 300 mm height cylindrical moulds filled with fresh concrete and fitted with two dial gauges for this purpose. The settlement of concrete was determined for mixes S-1, S-3, and S-6 only. Mixes S-3 and S-6 contained a fibre dosage of 0.90 kg/m3 i.e. the prescribed dosage by the manufacturers. The fresh density of concrete was also determined at this fibre dosage. The bleeding of concrete mixes was judged visually.

Compressive and Flexural Strength

The compressive and flexural strength of each concrete mix was determined using the standard specimens as per IS-516:195916. Three specimens from each concrete mix were tested to determine their 28-day average compressive and flexural strength.

Shrinkage of Concrete

Prismatic concrete specimens were cast for conducting drying shrinkage test. The test was conducted at 28 days according as per IS-1199:195917 (Fig. 1). The beams were demoulded after 24 hrs of casting and curing in steel mould. Thereafter, the demoulded specimens were marked for identification and kept submerged in a curing tank at room temperature (270 ± 20C) for the period of 28 days and initial length was measured. After the initial reading, the specimens were dried in at a temperature of 50 ± 10C and 17% relative humidity and then length changes were determined. The specimens were subjected to a cycle of drying, cooling and measurement of length until constant length was attained, that is, when the difference between the two consecutive readings separated by a period of drying of at least 44 hours, followed by cooling for at least four hours, is less than 0.02 mm. The drying shrinkage was calculated as the difference between the original wet measurement and the dry measurement expressed as the percentage of wet length.

Abrasion Resistance

Abrasion resistance of concrete slab was determined at 28 days following the procedures of ASTM C 779-199518. The abrasion machine consists of three discs, which rotate about their vertical axis and at the same time also travels on circular paths at a speed of 12 revolutions per minute in a planetary motion. During the rotation of discs silicon powder falls from the cup (attached at top of the shaft) at the rate of 4 to 6 gm/min which helps in abrading the slab surface. After five minutes of initial charge the abrasion depth is measured with the help of a micrometer, at the each end 5 readings were taken. This represents the initial reading.

Abrasion charge is again applied for the period of thirty minutes and abrasion depths are measured. This process is further continued for a period of thirty minutes and final abrasion depths were measured in mm. Difference between the average initial and average final depths give total abrasion of horizontal slab in mm. Average depths obtained on duplicate specimens are reported here.

Results and Discussion

Properties of Fresh Concrete

The test results of slump and settlement of concrete mixes are listed in Table 2. The results showed that the addition of both types of fiber i.e. multifilament as well as fibrillated has a detrimental effect on the workability (slump) of the mixes. Further, a greater slump reduction for concrete mixes containing multifilament fiber than fibrillated fiber was noted. Increase in slump reduction with an increase in fiber dosage was also seen. This is attributed to the fact that fibre acts as an aggregate. Similar trends were also observed by other researchers1-5,7.

The settlement of concrete was determined only for the fibre content of 0.9 kg/m3 as this is the normal prescribed dose of fibre by its manufacturers. The results showed the maximum settlement for control concrete mix than the mixes with fiber. Fibrillated fiber reduces concrete settlement more effectively than the multifilament fiber. This reduction in settlement of concrete containing fiber may be attributed to the action of fibers in the mix similar to the formation of a three- dimensional sieve, stopping the air passing up through the sieve and preventing the aggregate from pass down19. No bleeding was noticed in the concrete mixes. The fresh density of concrete mixes containing fibre was slightly less than control concrete.

 

 

Compressive and Flexural Strength

Table 3 shows 28-day average compressive and flexural strength of the concrete mixes. It is obvious from the results that the addition of synthetic fiber to the concrete mixes has not significantly affected the compressive strength of concrete in comparison to the control mix. This trend confirms the earlier reported work6,11,12.

The flexural strength of concrete mixes containing fiber is slightly higher than control concrete mix as reported by other reserachers6,11,13,14. Concrete mixes with multilament fiber developed flexural strength slightly higher than concrete containing fibrillated fiber for the same fiber content. This increase in flexural strength may be due to the crack bridging action of the multifilament fibre. Further, the fibrillated fiber has higher diameter and its fibrils are broken during the mixing resulting in a lower effective aspect ratio of fibrillated fiber than multifilament fibre. So in case of multifilament fiber, effective fiber reinforcing index (product of aspect ratio to volume fraction of fiber) is higher than effective fiber reinforcing index of fibrillated fiber. However, the increase in flexural strength is less than 10%. In general it may be concluded that concrete containing fibrillated fibre performs similar to concrete containing multifilament fiber in the development of flexural strength within the fibre dosages used in this study.

Drying Shrinkage

Figure 2 shows the drying shrinkage test results. A reduction in drying shrinkage up to 40% of that of the control concrete may be observed for both the fibers. This reduction in shrinkage is attributed to the higher tensile strength of fibers that enable it to carry more tensile stresses in comparison of plain concrete. Further, early age volume changes in concrete cause weakened planes and cracks to form the growth of these micro shrinkage cracks is inhibited by mechanical blocking action of the fibers. A better performance for the fibrillated fiber in controlling drying shrinkage than multifilament fiber is obvious. This may be due to better stabilization of matrix by the net of fibrillated fiber.

Abrasion Resistance

Abrasion resistance was measured in term depth of the abraded concrete surface. The results obtained on duplicate specimens are shown in Figure 3. The results show a significant reduction (up to 29%) in abrasion depth for concrete containing fiber in comparison to control concrete indicating an increased abrasion resistance for those mixes. This increased abrasion resistance for the concrete containing fiber is mainly due to discourage of the development of large capillaries pores due to bleed water migration to the surface leading to improvement in microstructure of surface zone concrete. Another reason for the enhancement in abrasion resistance could be the bonding between the fibers and the concrete matrix which might have not allowed the particles to move away during the testing. It may be concluded that for the same volume fraction of fiber, multifilament as well as fibrillated fiber have similar effect on the abrasion resistance of the concrete.

Conclusion

The major conclusions that emerged from the experimental study are as given below:

  • The addition of synthetic fibre reduces the slump of concrete mix. At the same fiber content the reduction in slump is more in case of concrete containing multifilament fibre than fibrillated fiber.
  • Fibrillated fibre is more effective in reducing the settlement of concrete than multifilament fiber.
  • The addition of synthetic fiber has insignificant effect on compressive and flexural strength of concrete.
  • Fibrillated fiber performs better than multifilament fiber in controlling drying shrinkage of concrete.
  • The addition of synthetic fibre to the concrete increases its abrasion resistance. However both the fibrillated and multifilament fiber has similar performance on abrasion resistance of concrete.
  • The addition of synthetic fibre can be used in a pavement concrete for specific purpose such as to minimize the growth of plastic shrinkage cracks and to improve the abrasion resistance.

Acknowledgement

The authors are grateful to the Director (Dr S. Gangopadhyay), Central Road Research Institute for his permission to publish the research paper.

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Rakesh Kumar, Pankaj Goel & Renu Mathur – Rigid Pavements Division, CSIR-Central Road Research Institute, New Delhi

B Bhattacharjee – CED, IIT Delhi, Hauz Khas, New Delhi

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