For most projects, earthworks comprising either excavation or filling, or a combination of both these operations, will be necessary to bring the subgrade to the required shape and level.
The general finished surface level will normally be determined by drainage requirements and consideration of such factors as:
- The climatic conditions of the region, particularly rainfall;
- The slope and general level of the existing ground relative to its surroundings;
- The groundwater level and the extent to which it is influenced by seasonal, flood or tidal conditions; and
- the soil profile, the nature of the insitu material and the layer thickness.
The subgrade will generally be constructed to the same shape as the finished surface of the slab. Thus, at any point, the subgrade level is equal to the finished level of the slab minus the total pavement thickness (within the specified tolerance).When imported fill is required, a selected granular material should be used, placed in uniform layers and compacted at or near optimum moisture content to achieve the specified density. Suitable equipment for compacting granular fill includes plate type vibrators, pedestrian-operated vibrating rollers and small tandem rollers (typical examples of which are illustrated in Figures 1.1 and 1.2).
Layer thicknesses should be chosen such that compaction occurs over the full layer, and not exceed 150mm, unless heavier compaction equipment than that noted above is used. Four to eight passes of the equipment will normally be required. Trucks and tracked or wheeled construction vehicles that have low contact pressures with the ground are not suitable for compacting fill.
The strength of the subgrade is not critical, since applied loads are dispersed over large areas by the concrete pavement and bearing pressures transmitted to the subgrade are relatively low. However, it is essential that the upper portion of the subgrade is of uniform material and density, and provides uniform support. In order to achieve the desired uniformity, all top soil should be removed, and soft areas identified and replaced.
In some circumstances, e.g. on good quality natural sands or gravels, it may be possible to build a satisfactory pavement directly on the subgrade, but a subbase is frequently used as a levelling course, or as a means of providing a 'working platform'. Fine-grained subgrade soils in the presence of free water may be 'pumped' through joints and cracks under the action of frequent heavy wheel loads. In this case a non-pumping subbase must be provided.
In constructing the subbase, it is important that the specified density be achieved to avoid any subsequent problems associated with consolidation and non-uniform support. Subbases should be placed in uniform layers, generally not exceeding 150mm thick, and compacted at or near optimum moisture content using appropriate equipment.
The subbase should be finished within the required tolerances to the specified grade and level. In the absence of specified values, a tolerance of +0, -10mm is considered desirable and achievable within reasonable standards of construction. Finished subbase profiles can be achieved by using a scratch template which operates from the top edge of the levelled side forms. Accuracy of subbase profile will help ensure that a uniform concrete layer of the specified thickness is placed.
The use of a blinding layer of fine granular material, eg sand, may assist in grading to the required level, and will reduce the risk of perforation or tearing of the vapour barrier (if used).
Concrete slabs over 100mm in thickness and constructed using good quality concrete that has been well compacted and cured are resistant to the passage of water from the ground. However, concrete slabs, irrespective of their thickness, are not impermeable to the slow passage of water vapour from the soil beneath.
It is for this reason that a vapour barrier should be placed under all interior concrete pavements on the ground, particularly if they are likely to receive an impermeable floor covering, or are to be used for any purpose where the passage of water vapour through the pavement is intolerable.
The most common form of vapour barrier is plastic sheeting (polythene). In order to resist deterioration and punctures from subsequent construction operations, the polythene should have a minimum thickness of 0.25mm and be manufactured from virgin plastic (not from reclaimed scrap polythene).
A vapour barrier placed directly under the concrete also functions as a slip layer and reduces subgrade drag friction. With less restraint to slab movement, the extent of cracking due to volumetric changes of the concrete may well be reduced. The use of a vapour barrier also prevents the loss of mixing water from the concrete down in to the subbase or subgrade.
The vapour barrier is placed directly on the subbase (or subgrade if no subbase course is provided), but if the surface is rough and likely to perforate the plastic sheeting, a blinding layer of fine material should be applied. The sheeting should be continuous under the side forms and lapped at all joints by a minimum of 150mm. There is no need to seal these joints with adhesive tape for vapour-proofing purposes as vapour rises vertically. Furthermore, taping can cause problems by not allowing the plastic to slip as the concrete is placed.
Special care should be taken to avoid damage to the vapour barrier prior to and during concreting, and any tears or perforations should be patched immediately. Placing the sheeting as late as possible will assist in avoiding damage.
Many of the problems associated with the performance of concrete pavements are caused by poor finishing procedures. During the compacting, levelling and power floating of a pavement, a layer of cement-rich mortar is inevitably brought to the surface. This surface laitance should not be allowed to become too thick by excessive working of over-wet concrete. A slab with a thick layer of surface laitance will wear rapidly, possibly craze, and dust badly. The use of fully compacted, low-slump concrete followed by the floating and trowelling operations at the correct times will avoid the production of an excessively thick layer of laitance, and result in a durable pavement surface.
It is essential in the direct finishing of concrete pavements that no floating or trowelling operations be commenced while bleed water continues to rise or remains on the surface. The incorporation of bleed water into the surface layer will significantly increase the water-cement ratio of the concrete in that surface layer, resulting in a weakened surface prone to dusting. The use of a mixture of cement and stone dust (known as driers) to absorb bleed water will also produce a very poor wearing surface, and this practice should be banned for industrial pavements.
It is important that the concrete surface be brought to the final specified level prior to the commencement of any finishing operations, and this will generally be achieved by one or two passes of the vibrating beam. Floating and trowelling should not be considered as methods of correcting inaccuracies in level or profile.
Where a pavement is to be finished by power floating and trowelling, the surface left by the double-beam vibrating screed will be level enough to be followed by initial power floating after a suitable delay.
Floating & Trowelling
General floating and trowelling for large pavement areas is normally undertaken using powered equipment. Power floating and trowelling will not necessarily achieve a better quality of surface finish than good hand floating and trowelling, but will be more economical.
A power-trowelled pavement finish is obtained in two stages:
Stage 1 - Power-floating the stiffened concrete to even out any slight irregularities left by the vibrating beam. A power float is a machine with large horizontal steel rotating blades, used for the initial floating operations only.
Stage 2 - Final power-trowelling to close the surface, making it smooth and dense. A power trowel is the same or similar machine to a power float, but fitted with small individual steel trowel blades that can be progressively tilted during the trowelling operations. The power-trowel should be used only for the final trowelling operation.
It is important that power-floating is not begun until the concrete has stiffened sufficiently. The time interval before the initial power floating can commence depends on the concrete mix and the temperature. In cold weather it may be three hours or more after the concrete is placed. In hot weather the concrete may stiffen rapidly, and it is then important that concrete is not placed faster than it can be properly power-floated and trowelled with the available resources.
As a general guide, when an average-weight man can stand on the surface and leave footprints not more than about 3mm deep, the surface is ready to power float. The power-float should be systematically operated over the concrete in a regular pattern leaving a matt finish (see Figure 1.5).
Concrete close to obstructions or in panel corners that cannot be reached with a power-float must be manually floated before any power-floating starts.
A steel hand-trowel may be used to give an improved finish near the panel edges (see Figure 1.6). The concrete must always be kept level with the side forms.
If power-trowelling is started too early, the trowel blades will leave ridges. Power-trowelling should be commenced when most of the moisture brought to the surface by the initial power-floating has disappeared and the concrete has lost its stickiness. Whilst high concrete strength assists in providing surface abrasion, resistance power trowelling also increases surface abrasion.
A practical test to check the readiness for each trowelling operation is to press the palm of the hand onto the concrete surface. If mortar sticks to the palm when the hand is taken away from the surface, the pavement is not yet ready for trowelling.
Power-trowelling of the full pavement bay is undertaken in a systematic pattern with the trowel blades set at a slight angle; the angle depends on the concrete stiffness but should be as steep as possible for the particular surface (see Figure 1.7). If the tilt on the blades is too great, the concrete surface will be marked.
Where a second power-trowelling is specified, it should not be commenced until the excess moisture brought to the surface during the initial trowelling has disappeared.
Again, the practical test described above may be used. The tilt of the trowel blade should be gradually increased to match the concrete stiffness.