Structural Strength Enhancement of Rigid Pavement Using Scrap Steel Fibre Reinforcement

The performance of a pavement or overlay depends on the engineering properties of the material used in construction. The application of Steel Fibre Reinforced Concrete (SFRC) as composite matrix is potentially advantageous from the point of view of its capacity to bear much higher stresses. Under similar loading conditions pavement thickness can considerably be reduced in SFRC, hence reduction in material and cost. Sound SFRC pavement promises an appreciably higher life expectancy, reduced crack growth offer better serviceability and minimum corrosion.

In this study, an attempt has been made to analyze the mechanical characteristics of the waste steel scrap material which is available from the lathe is used as a steel fibre for pavement construction and to optimize the fiber content. The characteristics compressive strength and flexural strength of M30, M35, and M40 cement concrete for various proportions of steel scraps are experimentally found out. The steel scrap is added in various proportions by weight of the cement.

Fiber reinforcement is highly recommended for situations involving impact and fatigue loading. In this study, the drop weight impact test was used to study the impact strength of Scrap Steel Fibre Reinforced Concrete (SSFRC). Abrasion resistance, an important criterion for the pavement material was also tested. The impact and abrasion resistance of SSFRC has been tested for optimized fibre content of M30, M35, and M40 concrete. The experimental results show that the mechanical properties such as compressive strength, flexural strength, impact strength and abrasive resistance of concrete are found to be increased due to the addition of steel scrap fibre in the concrete. This paper focuses attention on the structural strength enhancement of rigid pavements by using the locally available Scrap Steel Fibre Reinforcement and it is compared with the Plain Cement Concrete (PCC) specimens.

Introduction

The inclusion of fiber in concrete, mortar and cement paste can enhance many of the engineering properties of the matrix, such as fracture toughness, flexural strength and resistance to fatigue, impact, thermal shock and spalling. The modern use of fibre reinforced concrete started in 1960s using straight, smooth and discontinuous steel fibres.Applications for SFRC are used in floors, pavements and other plane structures where fibres act as crack distributing reinforcement. The fibres may act as shear reinforcement and also improve the capacity of the bars due to increased crack distribution.

Fibre Reinforced Concrete Pavements

As per IRC:SP:46–1997[1] steel fibres are generally 30–50 mm long, 0.5–1.0 mm in diameter and 50 to 100 mm aspect ratio. According to Gopalaratnam (1991)[2] for a given type of fibre, ahigher volume fraction provides more energy absorption capacity or toughness as long as the fibres can properly be mixed and the composite can be cast and compacted properly. This result should be expected because more fibres provide more resistance, especially in the tension zone. For the given fibre geometry, longer fibres typically provide greater toughness. Balasubramanian et al. (1996)[3] have investigated the impact resistance of the specimens with 0.5%, 1.0%, 1.5%, and 2.0% for each of the three types of steelfibres (viz. straight, crimped andtrough shaped) using Schrader’s test Device. Addition of 0.5% of steel fibres resulted in 3.5 times increase in the number of blowsrequired for failure. Further increase in fibre percentage increases the impact resistance of composites. Crimped fibres give consistently higher impact resistance.

N.Banthia(2002)[4] examines the two major issues related to impact loading on plain and fiber reinforced concrete. It was found that the machine parameters strongly influence the observed material response to impact. Results appear to be far less sensitive to the mass of the hammer than to the hammer drop height. Crimped polypropylene fiber is less effective than steel fiber at quasi state of loading but at higher stress rates, it performs better than steel fibre. Ravishankar (2006)[5] investigated the mix design aspects of steel fibre reinforced concrete and concluded that there is an increase of 42% in modulus of rupture due to addition of fibres in plain concrete. K.Shankar [6] investigated that the addition of scrap steel fibre beyond 1.5% decreases the flexural strength of the FRC.

Objectives

The main objectives of this study are:

• To investigate the use of steel scraps as a Steel Fibre Reinforcement in FRC

• To study the mechanical characteristics of the SSFRC
• To optimize the fibre proportions
• To check the toughness resistance of the SSFRC
• To check the abrasive resistance of the SSFRC
• To find out the cost effective cross section of the pavement.

Materials Used in the Investigation and their Properties

Materials Used

Chettinad brand Ordinary Portland Cement (OPC) 43 Grade confirming to IS: 4031-1988. Locally available river sand confirms to Zone II of IS: 383-1970 as fine aggregate, Crushed granite aggregate of maximum size 20 mm confirming to IS: 383 as coarse aggregate and Potable water are used. Steel Scraps of length 25 mm to 30 mm, width 1.5 to 2 mm and thickness 0.3 to 0.4 mm which is obtained from the lathe machines as waste or by product are used as reinforcing material in the concrete. Super plasticizer – Conplast SP432 MS supplied by M/S FOSROC India Private Ltd. Is used to improve the workability of the concrete. The dosage of super plasticizer to be added with the concrete is found out from the slump test conducted in the laboratory.

Physical Properties of Materials Used

The physical properties of the material used to prepare the fibre reinforced concrete mix are tabulated in Table 1.

Mix Design

For this investigation, the mix design for M30, M35 and M40 concrete are designed by American

Concrete Institute Method of Mix Design (ACI) and the mix proportions used for the investigation is given in Table 2.

Specimen

The compression and flexural strength of the cube and beam specimens are tested respectively.

From the experiment results optimum dosage of steel scraps used to get the maximum compressive as well as flexural strength is found out. The impact strength specimens and abrasion test specimens are prepared for control concrete and SSFRC with optimum dosage of steel fibres.

No. of cubes (150 mm x 150 mm x 150 mm) prepared for Compression test = 135 No. of beam specimen (500 mm x 100 mm x 100 mm) prepared for Flexure test = 135 No. of cylinders (150 mm diameter, 62.5 mm thickness) prepared for impact test = 54 No. of cylinders (100 mm diameter, 50 mm thickness) prepared for abrasion test = 54

Experimental Programme

Behavior in Compression and Flexure

Extensive research had indicated that there was no appreciable change in the linear part of the stress-strain curve in compression when randomly oriented fibres are added to concrete in different volumes. The variation in compressive strength and flexural strength of SSFRC compared to control concrete with respect to the fibre content is shown in the Table 3 and Figures.1. a

It is observed from the Table 3 and Figures 1.a, 1.b, 1.c and 1.d, the addition of fibres in the concrete increases the Compressive Strength but which is not very significant. In M30 concrete 0.5% steel dosage shows 0.135% of higher strength than control concrete and 1.0% and 1.5% steel dosage shows 2.54% and 2.84% higher strength than control concrete respectively but at 2.0% of steel proportion decreases the compressive strength. In M35 and M40 grades there is an increase in compressive strength up to 2% of fiber dosage. From the experimental results, it is observed that to get higher compressive strength, it is desirable to use 1.5% of steel scrap dosage.

Flexural Test by Third- Point Loading

The specimens (500 mm x 100 mm x 100 mm) are loaded and tested in accordance with the ASTM Test Method C 78. It is evident from Table 3 and Figure 1.e that the rate of gain of flexural strength of SSFRC is 68% more than the control concrete. It is concluded that the addition of 1.5% of steel scrap fibre dosage will give maximum compressive as well as flexural strength. Also the flexural strength of the SSFRC is calculated theoretically using the compressive strength relation of SSFRC and it is shown in the Figures 2.a to 2.c. From the Figures 2.a to 2. c it is seen that the flexural strength obtained experimentally is more than the flexural strength calculated theoretically. The failure of the specimens under compression and flexure is shown in Figure. 3.

Impact and Abrasion Resistance of SSFRC

ACI Drop Weight Impact Test

Toughness is defined as the total energy absorbed prior to complete separation of the specimen. A cylindrical specimen of 150 mm diameter and 62.5 mm thickness is prepared with optimum dosage of steel fibres in three grades of concrete. The equipment consists a standard manually operated 4.55 kg compaction hammer with 423 mm drop. A 62.5 mm diameter hardened steel ball and a flat base plate with positioning bracket similar. The drop hammer is placed with its base upon the steel ball and held vertically. The hammer is dropped repeatedly and the number of blows required for the first visible crack to form at the top surface of the specimen and the ultimate failure are recorded. The impact test results are given in Table 4. and Figure 4.

From the Table 4 and Fig. 4, it is observed that M35 concrete requires more number of drops for failure when compared to M30 and M40 grade of concrete forboth SSFRC and control concretespecimens. In all the three grades of concrete SSFRC shows increased impact strength (energyabsorption). The failure of the specimens under impact is shown in the Figures. 5 and 6.

Abrasion Resistance Test

This equipment is specially developed by the Department of Transportation, Engineering Service Centre, California [7]. The test specimens shall be cylindrical in shape, 100 ± 1 mm in diameter and 50 ± 3 mm high. They shall be soaked in water for a minimum of 2 hours prior to testing. The abrasion loss in grams is calculated by subtracting the mass of the saturated surface dry specimen after the test from the mass of the surface dry specimen before test. The results of the abrasion test are tabulated in Table 5.

From Table 4 & 5, it is observed M35 SSFRC requires more number of drops for failure compared to other two grades of SSFRC. Comparing the Figure 5 and Figure 6 the crack formation and crack width are also minimum in SSFRC specimens compared to the plain concrete specimens. In the abrasion resistance test M40 SSFRC shows less loss in weight compared to other two grades of SSFRC and plain cement concrete.

Pavement Design and Analysis

Pavement slab is designed as per IRC 58:2002[8]. The flexural strength is directly taken from the beam flexural test. The design details are tabulated in Table 6. The Axle load spectrum is taken from IRC: 58 -2002 and other data used in this design is given below:

• Elastic modulus of concrete = 3 x 10-5 N/mm2
• Tyre pressure = 8 kg/cm²
• Spacing of contraction joints = 4.5m
• Design life = 20 years
• Poisson’ ratio = 0.15
• Rate of traffic increase = 0.075
• Present traffic =1000 cvpd
• Elastic Modulus of Sub grade
• Reaction of the DLC sub-base = 8 kg/cm³
• Coefficient of thermal Expansion of concrete = 10x 10-6 /ºC

From Table 6, it is clear that the addition of small amount of fiber will also reduce the thickness of the pavement slab. For M30 Concrete thickness saved in construction with SSFRC is 41%, for M35 concrete thickness saved with SSFRC is 38% and for M40 concrete thickness saved in SSFRC is 33%.

Conclusion

From the experimental studies and subsequent pavement analysis carried out as per IRC: 58-2002, it is concluded that the compressive strength of SSFRC increased when compared to plain cement concrete. Addition of steel scraps increases the flexural strength of SFRC to great extent.

The mechanical properties of the concrete are increased by increasing the proportion of the steel scrap up to 1.5%. From 1.5% to 2.0%, it shows slight decrease in mechanical strength. At 2.0% of steel proportion, there is considerable reduction in the mechanical strength of SSFRC. It the pavement thickness is decreased by 41% and which is economical when compared to plain cement concrete slab.

References

• IRC:SP:46-1997, ‘Steel Fibre Reinforced Concrete for Pavements’.
• V. S. Gopalaratnam, S. P. Shah, G. B. Batson, M.E. Criswell, V. Ramakrishnan and M. Wecharatana, ‘Fracture Toughness of Fiber Reinforced Concrete’. ACI Material Journal, 88 4 (1991), pp. 339–353.
• K. Balasubramaniam, B. H.Bharat Kumar, S. Gopalakrishnan and V.S. Parameswaran, ‘Impact Resistance of Steel Fibre Reinforced Concrete,’ The Indian Concrete Journal (May, 1996), pp. 257-262.
• N. P. Banthia, S. Mindess and A. Bentur, ‘Impact Behavior of Concrete Beams,’ RILEM, Mater.Struct.20(1987) pp. 293- 302.
• U. Ravisankar, H.V. Venkata Krishna and Sures ‘Mix Design Aspects of SFRC PavementDesign’ Indian Highways (May 2006) Vol.34, No.5 and pp 44-50.

• K. Sankar, ‘A Study on the Effect of Fiber Reinforcement in Concrete Pavements,’ M.TechThesis, Department of Civil Engineering, National Institute of Technology, Tiruchirappalli –620-015.
• ‘Method of Test for Determining Pavement and Structures’, Department of Transportation,Engineering Service Centre, Sacramento, California 95819– 4612 (Feb.2000).
• IRC-58:2002, ‘Guidelines for the Design of Rigid Pavements.’ Figure 5: Control Concrete specimens failed under Impact Figure 6: SSFRC specimen failed under impact