Role of Fibers for Durable Concrete Construction

  Clinton Pereira, Assistant Manager Technical, Grasim Industries Limited, Mumbai

Introduction

The use of fibers to reinforce concrete materials is a well–known concept. It has been practiced since ancient times, with straw mixed into mud bricks and horsehair included in mortars. However, in our modern day construction practices we have forgotten the ancient practices to control cracks in concrete. Concrete cracking is normal. Portland cement concrete is considered to be a relatively brittle material and is prone to crack in the plastic as well as the hardened stage. Plastic shrinkage occurs when the evaporation of water from the surface of concrete is greater than the rising bleed water. As concrete is very weak in tension in its plastic stage, a volume change causes the surface to crack. As it hardens, the water present in the pores of concrete begins to evaporate. This causes the concrete to shrink due to the volume change, which is restrained by the subgrade and reinforcement. This results in a tensile stress being developed in hardened concrete, again causing the concrete to crack.

Cracks lead to negative perception of quality, durability and serviceability, however in most cases they become only aesthetic problems. Cracks also results in disputes between the owner, Architect, design Engineer and contractor which results in job delays and cost increases due to work stoppages and evaluation which is more severe than the actual consequences of cracking. One of the solutions to this problem is the additions of fibers to concrete. An attempt has been made in this article to provide the advantages and benefits of using fiber reinforced concrete for a variety of applications. The use of fibers help in modifying properties of concrete both in plastic and hardened stage and thus results into a more durable concrete. Incorporating Synthetic fibers help to reduce thermal and shrinkage cracks. Addition of steel fibers enhances the ductility performance, post-crack tensile strength, fatigue strength and impact strength of concrete structures. What is Fiber Reinforced Concrete (FRC)?

FRC is Portland cement concrete reinforced with more or less randomly distributed fibers. In FRC, thousands of small fibers are dispersed and distributed randomly in the concrete during mixing, thus improving concrete properties in all directions. Fibers help to improve the pre- crack tensile strength, post peak ductility performance, fatigue strength, impact strength and minimize thermal and shrinkage cracks.

Which types of fibers are used in FRC?

A wide variety of fibers have been used in concrete. For each application it needs to be determined which type of fiber is optimal in satisfying the concrete application. The different types of fibers used as concrete reinforcement are synthetic fibers and steel fibers. The different types of synthetic fibers used are Polypropylene, Nylon, Polythene, Polyester and Glass Fibers

For architectural and decorative concrete products and for prevention of early age cracking, synthetic fibers may be used. Steel fibers are used for applications where properties of concrete in the hardened stage have to be modified, namely, post crack flexural strength, abrasion resistance, impact resistance and shatter resistance of concrete.

Cracking pattern in concrete panels without fibers and with Fibermesh Polypropylene fibers.

How do fibers work in plastic stage of concrete?

Early age volume changes in concrete causes weak planes and results in the formation of cracks, because the stresses developed in the body of concrete exceeds its tensile strength at that specific time. The growth of these micro shrinkage cracks is inhibited by the mechanical blocking action of both synthetic and steel fibers. The internal support system of the fibers inhibits the formation of plastic settlement cracks. The uniform distribution of fibers throughout the concrete discourages the development of large capillaries, caused by bleed water migration to the surface. Fibers thus lower the permeability of concrete through the combination of plastic crack reduction and reduced bleeding characteristics.

Results of experimental program for reference

The contribution of steel, synthetic and cellulose fibers to the shrinkage-crack reduction potential of cement composites during the initial and final setting of concrete and its evaluation was reported in the ACI Materials Journal, May-Jun 1994, Vol. 91, No. 3, pp 280-288. "Contribution of fibers to crack reduction of cement composites during the initial and final setting period"

The primary variables of the investigation were: fiber types, matrix composition, and test methods. Fiber type consisted of synthetic fibers with lengths varying from a fraction of 1 to 60 mm at volume content of 0.9 kg/m3 and steel fibers of three lengths viz. 30, 50, and 60mm were investigated at volume contents of 45 and 60 kg/m3. The Synthetic fibers were made of nylon, polyethylene, polypropylene and polyester. The longer polypropylene fibers were fibrillated. Nylon and polyester fibers were made of single filaments with lengths varying from 19 to 50mm. The matrix consisted of cement mortar with various cement-sand ratios, concrete containing coarse aggregates, and lightweight concrete. In the case of test methods, the primary variables were specimen thickness and plane dimensions of the test panels. Results indicated that both steel and synthetic fibers made a definite contribution to shrinkage crack reduction during the initial and final setting periods. The microfibers are more effective in rich cement mortar, whereas the longer fibers, are more effective in lean mortars and concrete.

Discussions & Remarks

From the above experiment, it is clearly observed that the addition of fibers to concrete helps in minimizing plastic shrinkage cracks which eventually helps in enhancing the durability of the structure. Reduction in surface and internal cracks prevents the entry of moisture and other harmful chemicals which can have a devastating effect on concrete. As fibers arrests the formation of micro cracks the permeability of concrete is reduced; this property is of prime importance for the manufacturing of waterproof concrete.

How do fibers work in hardened stage of concrete?

The early age benefits of using synthetic and steel fibers continue to contribute to hardened concrete. Hardened concrete attributes provided by synthetic and steel fibers are lowered permeability and the enhanced resistance to shattering, abrasion, and impact forces.

Shatter resistance test for concrete without fibers and with Fibermash Polypropylene fibers

The ability to resist shattering forces is greatly enhanced with the introduction of fibers to the concrete. When plain concrete is compressed, it will shatter and fail at the first crack. Fibers manufactured specifically for concrete prevent the effect of shattering forces by tightly holding the concrete together. Abrasion resistance of concrete is enhanced when fibers are used because the water-cement ratio at the surface is not lowered by variable bleed water. This improvement is assisted by the internal settlement support value of the fibers contributing to uniform bleeding. Resistance to impact and other suddenly applied loads is also one of the significant improvements in the properties of fiber reinforced concrete. Fibers help in distributing the impact forces to the entire body of concrete, thus reducing the concentration of the braking forces.

Results of experimental program for reference

The contribution of synthetic and steel fibers to enhance the "Flexural behavior and toughness of fiber reinforced concretes" was reported by the Transportation Research Record, 1989, No. 1226, pp 69-77.

This above referred article presents the results of an extensive investigation to determine the behavior and performance characteristics of the most commonly used fiber reinforced concretes (FRC) for potential airfield pavements and overlay applications. A comparative evaluation of static flexural strength is presented for concretes with and without four different types of fibers: hooked-end steel, straight steel, corrugated steel, and polypropylene. These fibers were tested in four different quantities (0.5, 1.0, 1.5, and 2.0 percent by volume), and the same basic mix proportions were used for all concretes. The test program included (a) fresh concrete properties, including slump, vebe time, inverted cone time, air content, unit weight and concrete temperature, and hardened concrete properties; (b) static flexural strength, including load-deflection curves, first crack strength and toughness, toughness indexes, and post-crack load drop; and (c) pulse velocity. In general, placing and finishing concretes with less than 1 percent by volume for all fibers using laboratory-prepared specimens was not difficult. However, the maximum quantity of hooked-end fibers that could be added without causing balling was limited to 1 percent by volume. Corrugated steel fibers performed the best in fresh concrete; even at higher fiber contents (2 percent by volume), there was no balling, bleeding, or segregation. Higher quantities (2 percent by volume) of straight steel fibers caused balling, and higher quantities of polypropylene fibers (2 percent by volume) entrapped a considerable amount of air. Compared with plain concrete, the addition of fibers increased the first crack strength (15 percent to 90 percent), static flexural strength (15 percent to 129 percent), toughness index, post-crack load-carrying capacity, and energy absorption capacity.

Discussions & Remarks

From the above experiment, it is clearly observed that addition of fibers to concrete helps in enhancing resistance to shattering, abrasion, and impact forces, thus improving the mechanical properties of concrete. The increase in the flexural toughness/residual strength of concrete increases the load bearing capacity of concrete, which can potentially reduce the depth of slabs on grade. The increased impact and abrasion resistance increases the durability and reduces the maintenance cost of the concrete structure in use.

Advantages and benefits of Fibers in concrete:-

1. Fibers inhibit and controls the formation of intrinsic cracking in concrete caused both in the plastic and hardened stage of concrete, thus ensuring a more durable concrete construction.
2. Fibers reinforce concrete against impact forces, thereby improving the toughness characteristics of hardened concrete.
3. Fibers improve the resistance to shattering forces caused due to earthquake loads and vibrations induced in machine foundations, thus making concrete a more versatile material for such critical applications.
4. Fibers enhance the hardness of the surface of concrete against material loss due to abrading forces caused by frequent movement of wheel loads. This enhances the service life and safety of concrete pavements.
5. Fibers reduce the permeability and water migration in concrete, which ensures protection of concrete due to the ill effects of moisture.
6. Fibers reduce plastic shrinkage and settlement cracking when concrete is still green, thus enhancing the overall life of the structure and reducing the maintenance cost.
7. Fibers can replace the secondary reinforcement or crack control steel used in grade slabs, thereby reducing the overall cost of the structure.

Construction of road pavements using FRC & Use of FRC for construction of precast panels

Typical recommended applications of FRC

Fiber reinforced concrete is recommended in all types of concretes which demonstrate a need for enhanced toughness characteristics, resistance to intrinsic cracking and improved water tightness such as:-industrial floorings, canal linings, driveways, bridge decks, pavements, precast structures, water tanks, overlays/toppings, tilt-up panels, RCC elements, composite decks, mass concretes, sloping slabs, walls, thin sections, shotcrete and terrace slabs.

Conclusion

An important focus of our vision should now be on increasing the durability and longevity of various structures. The life of all bridges, pavements and other concrete structures should double in the next century as our country's major financial resources are invested in the construction sector. Carefully selecting materials to optimize and control their properties and use of more performance based specifications will result in advances in the durability of concrete.

Concrete is an inherently brittle material with a relatively low tensile strength as compared to its compressive strength. Reinforcing with randomly disturbed fibers presents an effective approach in controlling cracks and improving the ductility and flexural strength of concrete. A variety of materials such as polypropylene, nylon, polyester, glass, carbon and steel fibers can be used in fiber reinforced concrete. The applications of fibers are wide; however the appropriate fiber has to be used in order to meet the requirements of the structure and achieve maximum effectiveness.

FRC can be used in almost all types of civil structures where durability of the structure and a lower maintenance cost becomes the important factor in the selection of concrete materials. For corrosion protection of structures in the coastal belt cracks should be practically minimized, which can be achieved by the additions of fibers. As FRC provides a higher shatter resistance to concrete, structures in earthquake prone areas would minimize the risk of human casualties. Addition of fibers to concrete pavements enhance various toughness properties and thus reduce maintenance costs, enhance driving safety, controls surface erosion and increases the overall life and durability of the structure. In lieu of the same, it is felt that Architects & consultants should take strong initiatives in promoting the use of FRC by way of specifying the same in tender documents.

In conclusion, the use of fibers help in modifying properties both in the plastic as well as hardened stage of concrete, thus making concrete a more versatile material to be used for variety of applications.

Acknowledgment

I would like to thank Mr.S.B.Kulkarni-Asst. Vice President-Technical, Grasim Industries Ltd-Cement Business who has motivated me to write my first article.

I would also like to thank my colleague Mr. Hemendra Shribatho, Dy.Manager-Technical, Grasim Industries Ltd-Cement Business for proofreading the final version of the article and offering suggestions for improvement.

References

• R. Brown, A. Shukla and K.R. Natarajan, "Fiber reinforcement of concrete structures" University of Rhode Island URITC project No. 536101, September 2002.
• ACI Materials Journal, May-Jun 1994, Vol. 91, No. 3, pp 280-288. "Contribution of fibers to crack reduction of cement composites during the initial and final setting period"
• Transportation Research Record, 1989, No. 1226, pp 69-77. "Flexural behavior and toughness of fiber reinforced concretes"
• Specification for Concrete Cracking" by Juan Pablo Covarrubias, Concrete International, September2007.
• Photographs courtesy: Nina Concrete Systems Pvt Ltd, Mumbai. Distributors for Fibermesh Polypropylene fibers in India.