Polymer Fibre Reinforced Concrete
Polymer Fibre Reinforced Concrete
Polymeric fibres are being used now because of their no risk of corrosion and also being cost effective (Sikdar et al, 2005). Polymeric fibres normally used are either of polyester or polypropylene. Polymer fibre reinforced concrete (PFRC) was used on two sites with ready mix concrete and Vacuum dewatering process.
The nomenclature can be used in the works as given here.
"Providing and laying ready mix fibre 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 Fibrecom-CF/Fibremesh/Recron or equivalent (100 % virgin synthetic fibre 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, fibre 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).
Figure-4: A closer view of crack due to no expansion joint provided in PFRC on lean concrete base.
Figure-5: A closer view of crack due to no expansion joint
As it is known that the width of 12 m is too long for expansion/ contraction. It has been observed that almost at about one–third of the panel width, such cracks developed i.e. size of panel from one side is about 4 m and from other side it is about 8m. From the site observation, it is therefore inferred that the panel should have the size of about 4m x 4m in the temperature conditions of Delhi however small variation can also be made as per site conditions. In other case, the contractor delayed the cutting of grooves and thereafter the area was occupied due to some urgent requirements, the cracks in both the directions developed. The cracks were almost in line. Later on the grooves were made through cutters. It has been observed that the distance of cracks in one side was almost near to 4 m and on other side at about 7 to 9 m (Figure 5). Thus from this case study also, inference can be made that grooves if made in panels of 4m x 4m, it would be appropriate.
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 fibre 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.
Thus, fibre reinforced concrete has advantage over normal concrete particularly in case of cement concrete pavements. Polymeric fibres such as polyester or polypropylene are being used due to their cost effective as well as corrosion resistance though steel fibres also work quite satisfactorily for a long time. It appears that fibre reinforced concrete should be laid on base concrete of lean mix such as 1:4:8 cement concrete rather than over WBM and provided with grooves in panels of about 4m x 4m to avoid expansion/ contraction cracks. Grooves can be made after casting of concrete through cutters.