Construction of concrete slab

General

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 (IRC:15 2002). Figure-34 presents a photograph of laying polyethylene sheet over DLC.

Figure-34 Laying of polyethylene sheet over DLC ( photo courtesy: Mr. Ashutosh Gautam, Project Director, NHAI, Kanpur )

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 ( Swanlund and Vanikar 2002 ). The working of fixed form and slip form pavers are different - and has been discussed briefly in the following:

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 (O'Flaherty 2002). 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 (O'Flaherty 2002). 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 (O'Flaherty 2002).

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 (O'Flaherty 2002).

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 (O'Flaherty 2002).

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 (O'Flaherty 2002). Figure-35 explains schematically the operation of a typical slip-form paver, and Figure-36 a photograph of concrete pavement construction by a slip form paver.

Figure-35 Schematic diagram explaining operation of a typical slip-form paver

Figure-36 Construction of concrete pavement slab with slip-form paver ( photo courtesy: Mr. Ashutosh Gautam, Project Director, NHAI, Kanpur )