Concrete Pavement by Slipform, Muscat Airport

The process consists of placing and spreading low slump concrete on the ground in front of a paver that slips over the concrete and extrudes strips of slabs. Vibrators fixed along the front of the paver consolidate and fluidize the concrete. The concrete is then shaped by passing between the vertical side forms and underside of the horizontal profile pan as the paver moves forward. Finally the top surface is brushed to provide a texture and curing agent is sprayed to the top and side surfaces.
Sequence of Work.
Concrete strips of slabs (also known as lanes) were paved to a “hit and miss” lane pattern. The hit lanes (also known as pilot lanes) were slipformed first with the infill lanes completed later after a minimum of 14 days to avoid damaging the pilot lane concrete. Timber stop-ends were used at the start and finish of the lanes and where the operation stopped unexpectedly. At expansion joints along the lanes, it isn’t possible to insert dowels and expansion filler board with the slipform process so a slab was left out that was hand laid at a later date. Stop-ends for transverse joints had to be made smaller than the slab profile to allow the paver to move over without dragging the stop-end, therefore the stop-ends were set 500mm from the required locations and the concrete was cut at the required location, this is known as “Cut Back” construction. Slabs containing inserts were identified beforehand and left out to be in-filled later by hand work.
Transporting Concrete.
The freshly mixed concrete was transported in tipper lorries that were covered by sheeting to prevent evaporation of moisture. The lorries were fitted with electrical vibrators fixed externally to the underside of the tipper body to aid discharge. Eight number lorries were used to ensure a continuous supply to the paver. Tipper lorries were used as the low slump concrete would be difficult and slow to discharge using mixer lorries.
Placement of Concrete.
The concrete is placed and spread using a Gomaco PS-2600 Placer / Spreader a self propelled machine. Concrete is tipped by lorry onto the side conveyor belt that discharges the concrete on the ground in the centre of the lane. Augers spread the concrete across the width of the lane. Side-forms and a beam roughly shape the concrete for the paver that follows.
Slip forming.
A Gomaco GP-2600 Slipform Paver machine supported on four tracks compacts the concrete by means of internal vibration and shape it between sliding side forms by means of a profile pan. The vibrator frequency and elevation were set to ensure proper compaction. The level of the profile pan is controlled automatically from string-lines set up on either side of the pilot lanes and by sensors attached at the four corners of the slip form paving machine that use the stringlines as a level reference. The alignment of the machine is controlled automatically by sensors also using the stringlines as a reference. The slipform paver also floats the concrete surface by the automated float attached to the rear of the paver. Working off a footbridge, concrete finishers make good any areas not satisfactorily finished by the automated float. Edge slump where the edge of the formed slabs after the paver tend to slump had to be closely monitored and corrected. Internal hydraulic 75mm dia poker vibrators are set horizontally at 400mm centres along the front of the profile pan, the outside vibrators are set at 150mm from the slab edges, the bottom of the pokers are set level the profile pan i.e. the top surface of the slab. The vibration level was set at 8,000 to 11,000vpm (vibrations per minutes) which along with the rate of pave gave the best results for compacting and fluidising the site concrete mix without causing segregation of the mix.
Texturing & Curing.
The slip formed slabs are textured using a Gomaco TC 400 Texturing / Curing Machine that works off stringlines for accurate texturing of the slab. The texturing system travels longitudinally with the machine, a wire comb transverses the width of the slab in one pass at a uniform speed and pressure to give an even texture. Finally the machine sprays curing agent at a target rate of 5m2 per liter to top surface. By hand spraying curing agent is applied to the slab edges in two coats.
Joints.
Contraction joints at 5m centre’s were formed by fixing dowel pre-fabricated joint assemblies to the underlying surface, slipforming the concrete thought the joints and saw cutting the concrete to a third of the slab depth after 12 hrs or when the concrete initial strength allowed. Holes for dowel bars and tie bars in the longitudinal and expansion joints were drilled using a purpose made drilling rig that drills four holes at a time. Sure anchor epoxy J-50 supplied by Dayton Superior is injected into the holes to fix the dowels and tie bars.