Road Construction
Construction can be either: Flexible Pavement or Rigid pavement
RIGID PAVEMENTS
Introduction
· Specialist operation
· Requires complex and expensive machinery
· The concrete slab is equivalent to wearing course, base course of a flexible pavement
· ‘Rigid’ because it does not deflect under traffic loads
· Designed to last 40 yrs
· Constructed of high strength and high quality concrete
· Great attention placed on surface finish
· Expansion and contraction of concrete must be accommodated
· Transverse joints used - contraction and expansion joints, every 3rd being expansion
· Expansion joints used - as much as 30m apart
· The concrete carriageway, for construction purposes, is divided into individual panels by joints in both transverse and longitudinal directions
The Rigid Road
· Unreinforced
· Jointed reinforced
· Continuously reinforced
· A separation membrane must be provided between the road slab and the sub-base
· For continuously reinforced slabs, a bituminous spray is used as a waterproof membrane
Foundation Requirements
· Based on the number of commercial vehicles usage per day
· Design based on DoT HD 14/87
· Foundation requirements mean that the sub-grade must be tested to determine its CBR value
· If CBR below 15% then capping layer required
Sub-base materials
· Sub-base material: hard, durable, chemically inert, compacted to high density and not be susceptible to frost heave
· If capping layers needed, then cement-bound or wet-lean concrete must be used
Road Slab thickness
· Determined by quantity and type of traffic likely to use it
· 150 mm for residential estates
· 300 mm for heavily trafficked trunk roads and motorways
Concrete Quality
· Increased traffic meant that designers had to increase the strength of concrete
· OPC used, Grade 40 concrete which requires a minimum 320 Kg cement per cubic metre of concrete.
· Coarse and fine aggregates need to be chosen carefully: natural (to BS 882), crushed air-cooled blast furnace slag (to BS 1047)
· Workability must be sufficient to allow concrete to be fully compacted and finished without undue flow
· Concrete for the top 50 mm must be air-entrained.
Strength of Concrete
· C40 concrete at 7 days should give 31 N/ mm2 (with OPC cement)
Reinforcement
· Hot rolled steel bars or mesh (Grade 250 or Grade 460)
Surface finish
· Must be brush-textured in a direction at right angles to the longitudinal axis of the road
· Applied evenly by wire brush 450 mm wide
· Minimum texture depth should be 0.75 mm, measured between 24 hrs and 7 days after construction
Curing
· Curing essential to prevent protection from evaporation and against heat loss by radiation
· Cured by keeping concrete damp by: covering with plastic sheeting, spray on plastic, spray resin based aluminised curing compound
Joints in Pavements
· Rigid pavement divided into panels by joints in both transverse and longitudinal directions
Transverse Joints
· Formed at right angles to longitudinal axis: see sketches
· Spacing (distance between them) depends on weight of longitudinal reinforcement
· If concreting done in summer 21 April to 21 October then not necessary to provide expansion joints, at other times every 3rd joint must be expansion.
Longitudinal Joints
· Where carriageway is more than 4.2 m wide it is necessary to provide longitudinal joints: see sketches
· The tie bars prevent the joint from opening by more than a fraction of the mm, so maintaining the interlocking of aggregate particles on the two sides of the crack
· Arranged to coincide with lane divisions
· Manholes and gullies need to be treated as localised slabs
Machine Laid Concrete
· Slip-form pavers: conforming plate or oscillating beam models
· Concrete trains
Conforming Plate
· Built around a pair of parallel side forms which are linked together by a horizontal top plate
· Ready mix concrete is fed into the hopper at the front of the machine
· The waterproof underlay is laid from rolls immediately in front of the machine
· As machine moves forward the weight of the concrete in the hopper forces the material under the conforming plate.
· Vibrated by a row of vibrating pokers
· Adjustable to lay pavement widths maximum 13m
· Automatic dowel bar insertion mechanism
Oscillating Beam Slip-form paver
· In this machine side forms give shape and the oscillating beams compact and shape the top of the road slab
· Initial compaction by vibrators in the hopper
· Longitudinal joints cut mechanically as machine proceeds
Control System
· By two tensioned guide lines or wires on each side of slab
· At constant height and parallel to one edge
· Paver has electronic sensor which picks up signals from the wires
· These signals initiate the alterations to line and level
Concrete Trains
· Consist of a series of machines travelling in sequence along prepared side forms
· Jobs must be extensive for economic use
· Large scale batching plants required
· Rails to support the train have to be fixed - over several kilometres ?
· e.g. M6 Preston northwards: 20K, 11m wide c/way, 0.8 km per day, 8 No 10m3 mixer trucks and a batcher capable of producing 8000 tonnes per day.
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