Future Concrete Road Use of Sustainable Macro Synthetic Reinforcement Fibre

Mr. Manoj Didwania (B.E Civil, PGDCTM-CEPT, MBA-OM, IGBC-AP, CIPP) Senior Manager, Adani Total Gas Ltd

Manoj Didwania is a Construction Professional with more than 18 years’ experience in all phases of Project Life Cycle having expertise in Estimation, Procurement, Procurement, Contract Management and Planning. His research interest includes Supplementary cementitious Material (SCM), Sustainable and Green Buildings, Cost effective and durableconstruction Material.

For further details kindly contact: manojdidwania1984@gmail.com

The article is talks about to the use of Macro Synthetic Reinforcement Fibre (MSRF) as the full/Partial replacement of Steel Reinforcement for Concrete Pavement and Floor. This article starts by reviewing the conventional way of Concreting of Pavement and floor with Steel reinforcement with its advantage and disadvantage. This Articles later talk about the Case study where 100% replacement of steel reinforcement was done by Macro Synthetic reinforcement fibre (MSRF) for the Concrete Pavement where the Load requirement was 12 T/m2. Article will suggest about the process, steps, practical application, limitation, advantages of using MSRF for Concrete pavement which was experience and observe during the Case Study. In today’s world where sustainability in construction is core issue, the MSRF plays important role to reduce the embodied carbon and Co2 Emission.

WHAT IS SYNTHETIC REINFORCED FIBERS

Synthetic fibers are manufactured from materials such as acrylic, aramid, carbon, nylon, polyester, polyethylene, or polypropylene. The use of synthetic fibers has been increasing at a steady rate in the past couple of decades. Fibres have been used for thousands of years in mortar construction. With the latest manufacturing techniques, we can take modern synthetic materials and turn them into highly engineered short fibres which act like small reinforcement bars in concrete. Dependent on the application these fibres range in length.

Classification of Micro and Macro Synthetic fibres

 a) Micro Synthetic fibre b) Macro Synthetic fibre

Macro synthetic fibres are defined in EN 14889 – Part 2 Fibres for concrete – Definitions, specifications and conformity and are broadly classified into two Classes.

Class I: Micro Fibre <0.30mm in diameter are fine hair like fibres typically between 6mm and 12mm in length. Micro synthetic fibres do not provide any post crack ductility. They do not control cracking of hardened concrete and cannot be used in lieu of other reinforcement. They cannot be considered in the structural design of concrete.

Class II: Macro Fibre >0.30mm in diameter are short discreet elements (like a tooth pick) and typically between 30mm and 65mm long. Macro synthetic fibres increase the ductility of concrete, improving the post-cracking capacity and can be used as the sole reinforcement in many concrete structures

However, macro synthetic fibres can be used to provide the concrete with significant post-cracking capacity and hence can be used in some designs based on plastic analysis, such as for ground-supported slabs and for rock support using sprayed concrete. Design approaches are not as well developed as for steel fibres (see separate entry Design/Approach/Steel-fibre-reinforced concrete), though in many cases the same methods are used, making allowance for the differences in the stress:strain behaviour and in the long-term performance.

COMPARISON WITH STEEL FIBRES AND MACRO SYNTHETIC FIBRES

In the beginning, steel fibres were mostly used as a substitute for secondary reinforcement or for crack control in less critical parts of the construction. However, over time, SFRC came to be applied in many different construction applications, such as in tunnel linings, ground support in mines, floors on grade, floors on piles and prefabricated elements, to the point where, nowadays, steel fibres are widely used 'as the main and unique reinforcement for industrial floor slabs, shotcrete and prefabricated concrete products. Steel fibres are also now being considered for many structural purposes contributing to the construction 's strength, stability and durability.

Macro-synthetic fibers, have lower tensile strengths and Modulus of Elasticities than steel fibers but will have a much higher fiber count across a potential crack. In effect, the total strength across a crack should be equivalent. This strength is also dependent on the ability of the fiber to bond to the concrete matrix itself. MSF is non-magnetic and non-corrosive making it a very attractive option for exterior paving projects where aesthetics and safety may be a concern. As MSF is mixed, it also becomes somewhat pliable and will not be as abrasive or harmful to pumping lines and equipment.

Synthetic fibres are usually much cheaper than steel fibres, in direct cost comparisons and also because a far lower density of synthetic fibres than steel are required in the concrete mix.

For example, for container combi slabs the density of synthetic fibres required in concrete is around maximum ranges to 3 kg/m3 compared with 35 kg/m3 for steel fibres. So, if the synthetic fibres cost Rs 1500 /m3 and steel fibres cost Rs 3500/m3, there’s a whopping saving of Rs 2000/m3.

CASE STUDY (PRACTICAL APPLICATION)

In this project which we are referring, we used Macro Synthetic fibre for the Concrete Pavement where we replace the steel reinforcement 100% with MSF. The first thing we did as the procedural work is to get the Load calculation being done to identify right dosage for the applied Load. Most important aspect for any project we want to apply the MSF is that the design calculation shall be vetted by structural consultant.

All quality checks and test certificate from recognised Institute were reviewed prior to selection of right kind of Macro Synthetic fibre and their past projects feedbacks were taken to build confidence within the team for surety of Execution

Batching and Mixing of Concrete Containing MSF

MSFs can be added to the concrete at a ready-mix plant during batching and mixing, or it can be added to the ready mix transit concrete truck at arrival on site. In either case, checks should be made to ensure proper fibre dispersion throughout the concrete. If the fibres are mixed at site, the concrete mix should be mixed for at least five minutes at maximum mixing speed of the transit mixer to disperse the fibres throughout the load of concrete.

Most challenging task for the project execution team was mixing the fibre in the 1 bag Mixer, as using the mixer in the batching plant the perspective was it will be more controlled and quality Concrete.

Fig 1 : Mixing of Fibre in the 1 Bag Mixer.

During the Consultation with the Manufacturer’s, it came in discussion that many projects in India was executed and produced in the 1or 2 Bag Mixer successfully and they didn’t find any complain from the Users. The right training with past mixing and laying videos were brief to the Site team

Placing and Finishing of Concrete

No special equipment is required for placing and finishing of the concrete mix containing MSFs. Concrete mix containing MSFs may be placed and compacted using the same methods as used in a plain concrete mix; however, an extra precaution should be taken during finishing to minimise the appearance of fibres on the surface of the slab. Appearance of an excessive number of fibres on the finished surface of the slab may indicate inadequate mix design, fibre type, dosage, and placing/finishing techniques. During broom finishing and tinning, some of the macro synthetic fibres are likely to be seen on the surface of the concrete pavement. Hence, inclined texturing is preferred. However, fibres protruding out will not harm as they will quickly wear off due to abrasion. In some cases, these fibres are cut to match the surface of the pavement.

Precaution and Care

During Concreting, only concern which came was forming of lumps of fibre at different interval (Fig 1) of the pavement. During Pre-execution level training the Manufacturer’s already inform that such lumps may form, and it will get dispersed once the floater is move over the concrete for finishing.

A common concern when using synthetic fibres in concrete is ‘floating’ of the fibres to the concrete surface. We hear this regularly in the market, largely from those who are yet to try fibre technology or those who have used fibre reinforcing without being truly aware of how to mix, dose or place the fibre reinforced concrete.

Finished Concrete Concern

Post hardening of Concrete, there may be some protruded Synthetic fibre on the finish Surface. With this there is no need to worry, the protruded fibres are additional fibres, and it does not have any structural implication and during the course of time by sunlight and initial movement of vehicles 

Fig 2: Protuded Fibre on the finish Concrete

After the concrete has hardened, the fibres give the concrete slab a hairy appearance which are the fibres protruding from the surface. An unconventional method for removing these fibres is burning them off (synthetic) or grind them off (steel).

Instead of dealing with the floating fibres after the concrete has hardened; the concrete mixture can be adjusted to create fluid properties for both finishing as well as limiting the separation and floating of fibres to the concrete surface.

And the End-User shall not worry about the left-over fibres which are protruded as its does not have any structural impact and during the course of time from the rays of Sunlight and some machinery movement this fibre will get detach.

CONCLUSION

This article has brought upon the practical application of the Macro Synthetic fibre reinforcement on the Concrete Pavement where the Steel reinforcement was replaced by 100%. Have talked about the comparison about the steel fibre and synthetic fibre and both plays the significant role as the alternative of Steel reinforcement and Structural designer need to judiciously decide upon the selection.

Over the last two decades, macro synthetic fibres have become a cost-effective viable reinforcement option for design engineers and contractors alike in European Countries, being a proven technology. Indeed, in sprayed concrete applications such as mining, tunnelling and slope stabilization they are rapidly becoming the material of choice and the same trends exist with slabs-on-ground and precast elements.

Fig 3: Finished Pavement Photo

Here the Macro Synthetic fibres are economical and also their dosage value is less which give the industry clear mandate for the experimenting with material. Considering the steel reinforcement rates are still volatile it’s the need of hours for using the proven construction material and also the MSF is Eco-friendly material which eventually helps to reduce the 2.75 Kg/sqm of Carbon Emission on every Concrete pavement and floor.

Cost Comparison

Grade of Concrete: M25 Grade

Trimix Thickness: 150 mm

Steel Reinforcement: 30 kg/cum (10 dia@300 c/c B/w at Bottom)

Panel Size: 6mX 4m

Synthetic fibre: 3 Kg/cum (100% Replacement of Reinforcement) -Material Concrix ES Macrofiber

Concrete Mixing: 1 Bag Mixer.

Reinforcement Rate:  80 Rs/kg (Including Laying) – Total Cost 2400 Rs/Cum (80 X30)

Synthetic Fibre Reinforcement: 500 Rs/kg---Total Cost 1500 Rs/Cum (500X 3)

Saving of 900 Rs/cum (this will change based on sites and its complexities)

The use of fibers as a replacement of wire mesh/rebar in non-structural applications is expected to provide at least a comparable (if not superior) product, as fibers typically provide a 3-D reinforcement configuration. In addition, the steel reinforcement used in these applications is mostly for temperature and moisture crack control (as opposed to load bearing); fibers have had past success in controlling these types of cracking. Finally, in many circumstances, the wire mesh/rebar reinforcement is forced on or near the ground during construction, providing little support. This no longer becomes a problem when fibers are used Considering all aspect discuss in this article, after proper due diligence, care during mixing and finishing the MSF will be successful, shall be utilized at the larger level to attain the benefits associated with it.