Fiber Reinforced Concrete in Pavements
Fiber reinforced concrete is considered as a material of improved properties and not as reinforced cement concrete whereas reinforcement is provided for local strengthening of concrete in tension region. Since in Fiber reinforced concrete, fibers are distributed uniformly in concrete, it has better properties to resist internal stresses due to shrinkage. As fibers improve specific material properties of the concrete, impact resistance, flexural strength, toughness, fatigue resistance, ductility also improve.
Fibers generally used in cement concrete pavements are steel fibers and organic polymer fibers such as polypropylene and polyester.
Steel Fiber Reinforced ConcreteSteel fibers have been used for a long time in construction of roads and also in floorings, particularly where heavy wear and tear is expected. Specifications and nomenclature are important for a material to be used as the tenders are invited based on specifications and nomenclature of the items. Such nomenclature is not available in Delhi Schedule of Rates. In a work where steel fiber reinforced concrete was used for overlays just like flooring, the following nomenclature can be adopted for concreting of small thickness.
Providing and laying 40 mm steel fiber reinforced cement concrete in pavement (in panels having area not more than 1.5 sqm) consisting of steel fiber @ 40kg per cubic meter of concrete and cement concrete mix of 1:1.95:1.95 (1 cement: 1.95 coarse sand of fineness modulus 2.42: 1.95 stone aggregate 10 mm and down gauge of fineness modulus 5.99) over existing surface i/c cement slurry, consolidating, tapping, and finishing but excluding the cost of steel fibers which shall be paid separately, complete as per direction of Engineer in Charge (Cement to be used shall be OPC 43 grade and sand and aggregate have to be washed).
Second item of fibers was provided separately as “Providing and mixing steel fibers of dia 0.45 mm in cement concrete duly cut into pieces not more than 25 mm in length.”
The construction was carried out more than a decade back. It isobserved that the performance of the concrete is satisfactory even after many years of construction (Figure 1). Even, no corrosion has been observed in the steel fibers. In fact the concreting has been done just like flooring item in this case over already existing hard surface. In such a case a bonding coat should also be provided like a coat of cement slurry. The fiber reinforced concrete has been provided in small panels considering the workability. Though vacuum dewatered concrete has not been done with steel fiber reinforced concrete but the same is also possible. Vacuum dewatered concrete, though cannot be done in small thickness like 40 or 50 mm but can be used if thickness is 100 mm or more.
Polymer Fiber Reinforced Concrete
The nomenclature can be used in the works as given here.
"Providing and laying ready mix fiber reinforced cement concrete of M35 grade (The concrete shall also have minimum works test beam flexural strength of 40 kg per sqm at 28 days) in required slope and camber in panels i/c shaping at drainage points as required using cementitious materials not less than 435 kg per cum of finished concrete from ACC/L&T/AHLCON/ UNITECH or equivalent batching plant for all leads and lifts with Fibercom-CF/Fibermesh/Recron or equivalent (100 % virgin synthetic fiber size 12 mm long) to be mixed @ 900 grams per cum of concrete i/c finishing with screed vibration, vacuum dewatering process, floating, trowelling, brooming and normal curing etc. complete as per standard manufacturer’s specifications and as per direction of Engineer’s in charge (All related equipment shall be arranged by the contractor. Cost of centering, shuttering, grooving etc. shall be paid separately. Design Mix shall be got approved from the Engineer in Charge).
In both the sites, vacuum dewatered concrete was used. Both the sites are to be used for parking. In a site, fiber reinforced concrete was used over a base cement concrete of lean mix of 1:4:8 (Figure 2) while in other site it was laid over water bound macadam (WBM) (Figure 3).
When dewatered concrete it has no problem of water being coming out on surface during compaction process but when it is done over WBM, a lot of concrete water is soaked by WBM and thus the concrete loses the water to WBM and the water which comes out during dewatering/compaction process is not in same quantity asin case of lean concrete. It appears that it is better to provide base concrete than WBM as the base. The groove was made in one case before setting of concrete and also panels were cast with expansion joints in one direction. No cracks were observed in the direction in which expansion joints were provided assuming this is longitudinal direction. In lateral direction, no joints were provided and the width of such panel was about 12 m. It was later observed that cracks have developed in this direction (Figure 4).
In both the cases, no lateral grooves were made, as working was not a problem due to use of vacuum dewatering process. In both the cases, horizontal line cracks have been observed indicating that the grooves in other direction are also essential. From this, it is imperative that polymer fiber reinforced concrete should be laid in panels or grooves should be provided so that concrete acts like in panels. Cutting grooves is easy as it can be made after casting of the concrete. But it should not be delayed for long and should be made before concrete achieves its desired strength. The size of panels may be kept around 4m x 4m.