Thursday 3 June 2010

Rigid Pavements

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.

No comments:

Post a Comment