The concrete surfaces of Bridges, Flyovers and Viaducts are subjected to continuous traffic movement on deck slabs causing damage, which can be prevented if they can be kept away from direct contact with traffic, water and salt. This is often achieved by providing a sacrificial overlay on the deck slab that will abrade itself against the traffic movement, provides waterproofing layer and also a barrier against salt attack. Conventionally usage of flexible asphalt concrete based overlay or thin polymer concrete overlay (TPO) for this purpose is in vogue worldwide. Of late, with the advent of fiber reinforced concrete, high abrasion resistant, fiber reinforced high performance concrete overlays are becoming popular.
Fiber Reinforced Concrete overlay (FRCO)
FRCO consist of binders, non-metallic fibers, aggregates and additives. They have provided long-lasting wearing surfaces for bridge decks with many advantages, including adding very little dead load; very fast cure times; shallow depths that eliminate the need for raising approach slabs; ability to transition from overlaid lane to non-overlaid lane during construction; low permeability; and good frictional resistance. FRCO have become an accepted construction method for deck preservation, restoring surface friction and extending the durability of decks. When constructed properly on sound decks, FRCOs should provide a service life of 20 or 25 years.
Experience shows that establishing efficient protection of bridge decks is costly. It is therefore, important that the overlay concrete shall be as abrasion resistant as possible and the numbers of vulnerable issues there in are addressed.
Some of the important attributes of the concrete overlay are as follows:
– The overlay shall be abrasion resistant for the traffic movement
– The overlay concrete shall be impact resistant
– The overlay concrete shall be resistant to shrinkage cracking
– The overlay concrete shall be impermeable to water
– There shall not be ferrous material as reinforcement and fillers
Generally the overlay concrete wearing course will cast many days after the deck slab concrete is cast due to the construction schedules. In view of this, establishing a very good bond with the deck slab concrete and the fiber reinforced concrete overlay is essential.
The surface texture of deck slab shall be such that it provides enough mechanical key with the overlay to resist shear forces created by the moving traffic on the bridge. Sometimes the top of the deck slab is also grooved transversely. The grooving should be done as early as practicable after the concrete is cast, followed by curing with water or with a subsequent application of curing membrane.
The fiber reinforced concrete overlays are designed keeping in mind the above critical parameters.
The FRCO can be laid on the bridge decks using some of the conventional concreting methods. It can be placed using slip form paver with steering wires or screed rails for rail guided levelling equipment. Controlling the FRCO thickness uniformly on the profile of the bride deck is essential. Any cambers or slopes towards the water drainage systems should be observed before laying the FRCO layer
Although, the mechanical keys provided on the deck slab ensure necessary bond and slip resistance of FRCO, as a second line of defense it is prudent to apply a suitable bonding agent along borders and casting joints. If the surface is moist and an epoxy bonding agent is chosen, the surface must be dried with suitable burner combined with compressed air before applying the bonding agent. Epoxy bonding agent shall be applied in stripes of about 30 cm wide along longitudinal borders and 50 cm along transverse borders or the casting joints. All vertical-casting joint s shall, moreover, be given an application of the bonding agent. If epoxy bonding agent is not preferred, application of cement/latex bonding agent on overlay areas can be used.
Uniformity of the thickness
If unevenness results in the FRCO, locally having less than the required minimum thickness, this may be acceptable (up to about 2 – 3 m bridge lengths) provided it is fully bonded with the deck. It is to be kept in mind that form paver requires a certain minimum overlay thickness to be able to pass.
Cleaning of the Deck
FRCOs are usually placed along one lane, or possibly along half width of the bridge at a time. When casting there will always be some spills outside the overlay area. Therefore it may be advantage to clean half the bridge width at a time.
Normal procedure when using FRCO is:
1. Old concrete is pre-treated with sandblasting or sand washing.
2. The concrete is well pre -wetted before application of FRCO, no free water shall remain on the surface, SSD condition of the deck is ideal.
3. The FRCO is poured onto the deck slab and is spread with concrete spreaders and finished with travels of floats. Mechanical floats can also be employed for larger areas.
FRCO can be used in on bridges in varying conditions of soundness. It can even be used for the repairs of the bridge deck potholes and maintenance of the carriageway. Factors that influence performance of FRO are: (1) Soundness of the substrate, (2) Surface preparation, (3) Compatibility of FRCO and substrate (4) overlay thickness, (5) bridge girder flexibility, (6) environment, and (7) Workmanship.
FRCOs have should become the accepted construction method for deck preservation, restoring surface friction and extending durability of decks. These overlays give good performance when placed on decks that are in good condition and when constructed in accordance with sound engineering practices when constructed properly on sound decks, FRCOs could provide a service life of 20 or 25 years.
For further details:
Talrak Construction Chemicals Pvt. Ltd.
#148 (First Floor), Sri Gururaghavendra Complex,Basaveshwara Circle, BEML 3rd Stage, Rajarajeshwari Nagar, Bangalore 560098, India