Concrete Pavement Construction Process

The construction of concrete pavement involves sequential construction of subgrade, sub-base/ base and the concrete slab. These are discussed in the following.

Subgrade preparation

Subgrade preparation involves cleaning, earthwork (excavation or filling of soil, replacement of weak soil, soil stabilization etc.) and compaction.

Where the concrete layer is laid directly over the subgrade, the subgrade is moist at the time concrete is placed. If the subgrade is dry, water could be sprinkled over the surface before laying any concrete course, however, care should be taken so that soft patches or water pools are not formed at the surface. As an alternative arrangement, concreting could be done over a water proof polyethylene sheet, and in that case moistening the subgrade surface becomes redundant. This polyethylene sheet acts as a capillary cut-off layer.

Figure shown below presents a photograph of subgrade construction in progress.

Construction of Base/ Sub-base

A base/ sub-base to the concrete pavement provides uniform and reasonably firm support, prevents mud-pumping , and acts as capillary cut-off. Sub-base for concrete pavement could be constituted with brick flat soling, WBM, granular aggregates, crushed concrete, slag, stabilized soil etc. As per IRC:15, sub-base could be of three types with

(i) Granular material (for example, brick soling with one layer of sand under it, WBM, well graded granular materials etc.)

(ii) Stabilized soil

(iii) Semi-rigid material, (for example, lime burnt clay pozzolana concrete, lime fly-ash concrete, lean cement concrete roller compacted concrete etc.).

Following contains a brief discussion on dry lean cement (DLC) concrete as sub-base, which is popularly being adopted for the current concrete pavement construction in India.

Dry lean cement concrete as sub-base

The thickness of DLC, generally recommended is 100mm or 150mm. The maximum aggregate to cement ratio is 15:1. The average compressive strength of DLC cubes at 7 days, as recommended by Indian guidelines should not be less than 10 MPa, tested on 5 samples and individual compressive strength should not be less than 7.5 MPa, at 7 days.

• (A) is the outer edge, and (B) is the inner edge of the pavement

Before construction of DLC sub-base, the prepared subgrade is sprinkled with water to moisten the surface. The material is to be laid uniformly by a paver without any segregation. The paving machine should have high amplitude paving bars. The curing of DLC can be done by spraying liquid curing compound, or by covering the surface by gunny bags. As per Indian guidelines, the construction of cement concrete pavement can only start after 7 days of DLC construction.

Construction of concrete slab

The concrete surfacing could be made up of plain concrete or reinforced concrete. Reinforced concrete has been discussed later in a different lecture. The proportions between cement, aggregate and water is determined by standard concrete mix design technique.

Premature setting and segregation is to be avoided while transporting the concrete mix through the access haul road and continuous stirring may be helpful in such a case. The spreading of concrete should be done uniformly such that no segregation of materials takes place. A separation membrane, made up of impermeable plastic/ polyethylene sheet (of thickness of the order of 150 micron) is sometimes laid over the sub-base, without creases, on to which concrete slab is laid.
Following figure presents a photograph of laying polyethylene sheet over DLC.

Compaction and surface finishing

Concrete is spread evenly and is rodded with suitable equipment such that formation of honey-combing or voids can be avoided. At the same time, over-compacting needs to be avoided, which can cause segregation and loss of entrained air. The working of fixed form and slip form pavers are different – and has been discussed briefly in the following paragraphs.

Fixed form paving system

In fixed-form paver system, generally, separate powered machines for spreading, compacting and finishing are used. The spreader spreads concrete evenly through reversible auger to the desired surcharge level. The rotary strike-off paddles trim minor irregularities in the surface of the surcharge concrete and adjusts with the carriage-way cross-slope. The compaction beam applies vibration to the concrete with pre-designed amplitude and frequency. This vibration also helps to put the dowel and tie bars at their desired positions (for a single layer construction).

The wet formed joint groove is made by introducing vertical cut immediately after compaction is over and inserting a preformed cellular permanent strip. As an alternative, saw joint groove can be made after the concrete is sufficiently hardened and can maintain the sharp edge itself.

The finishing of the surface is made, generally, with a pair of finishing beams. The leading beam vibrates and smoothens the surface, and the rear beam acts as float. The beams are oriented obliquely so that it causes less damage to the joints.

Slip form paving system

Slip-form paving machine is a self-propelled system that can automatically spread, trim, compact and finish the surface in a synchronized manner through its feedback sensors. Placing of dowel/ tie bars at their pre-designed locations are done by the slip-form pavers. The introduction of joint grooves, surfacing texturing and spraying of curing compound etc. are done by the equipment  those follow the paver.

Slip-form paver requires guide-wires, parallel to edge of construction and maintained at fixed height, installed on the both side. The alignment of the slip-form paver is controlled automatically with respect to the guide-wires. Correct and precise alignment of the guide-wires is therefore extremely important. The hopper/ spreader maintains a constant surcharge of the concrete above the conforming plate level. The conforming plate, vibrators, strike off paddles and the finishing screed gives the final shape of the concrete pavement.

Following figure explains schematically the operation of a typical slip-form pavement

Texturing

Finished concrete has a smooth surface; texturing of concrete surface is done to impart required skid resistance to the concrete surface. The texturing is done by means of wire brushing or grooving along the transverse direction. Initial texturing may be done at the time of construction of the paver itself. Final texturing is done no sooner the sheen of the concrete surface goes off.

Concrete curing

Curing is a process in which requisite moisture content and temperature is maintained so that concrete achieves its design strength through hydration of cement. For initial curing, curing compound with high water retentivity may be spread over the finished surface to prevent rapid drying of water. For final curing, continuous ponding or moistened hessain/ gunny bags should be kept for about a fortnight. As an alternative arrangement to ponding, impervious liquid maybe spread over the surface so as to restrict evaporation of water from the laid concrete. Forms are removed from the freshly prepared concrete layer after about curing of fourteen hours.

Recapitulation

 The construction of concrete pavement involves construction of subgrade, sub-base/ base and the concrete slab.

 Subgrade preparation involves cleaning, earthwork (excavation or filling of soil, replacement of weak soil, soil stabilization etc.) and compaction.

 The various steps involved in cemented/concrete layer construction, depending on requirements, are spreading, trimming, compacting, jointing, finishing, curing, texturing etc. The construction cemented/ concrete is generally done by automated equipment, called paver.

 Pavers are of two types, fixed form and slip form. Slip-form paving machine is a self-propelled system that can perform paving in a synchronized manner through its feedback sensors.

Reference

IRC:SP-49 1998

IRC:15 2002

Source

NPTEL