Choice of finish
The finish of a concrete floor should be chosen after considering the type of traffic and loading, impact abrasion and chemical resistance, and such factors as hygiene, dust prevention, slipperiness and decorative treatment. Guidance on suitable finishes for various duties is given in a BCA publication Power trowelling and skip floating.
In many industrial situations a structural slab of adequate quality may be direct-finished to give a satisfactory wearing surface. The finish may be produced by power trowelling (or hand trowelling if the operatives are sufficiently skilled) or early-age power grinding. Whichever finish method is used, to achieve good regularity of the final surface it is essential to provide accurately set and levelled square-edged side forms, and to give careful attention to concrete placing, compaction and levelling. Compaction and levelling can be done efficiently and speedily by using a double-beam vibrator or a tri-screed (known as a razorback) and flat surfaces can be achieved, even on steep slopes, with a rotating striker tube.
In some heavy industrial situations, especially where fork-lift trucks with solid wheels are used and where heavy abrasion or impact and chemical attack will occur, special applied wearing screeds may be necessary.
Power trowelling
A power-trowelled finish is obtained by first using a power float to smooth and close the previously levelled concrete surface after it has stiffened sufficiently - about three hours after laying. After a further delay to allow excess surface moisture to evaporate, the slab surface is further smoothed and made dense with a power trowel ( Figure 33). A power float has a rotating circular disc attachment or large flat individual blades. A power trowel has smaller, tilted individual blades and is used for final finishing. The timing of each application is critical.
Power grinding
Power grinding is a finishing technique intended to provide an acceptable and durable concrete wearing surface without further treatment. After the concrete has been fully compacted and accurately levelled with a double-beam vibrator, it is further smoothed with a large, metal skip float attached to a long handle. The concrete is allowed to harden and the surface is then ground to a coarse 'sandpaper' texture using a low-speed grinder to remove the top 1 - 2 mm of laitance and minor irregularities left during finishing.
The grinding is not intended to remove gross inaccuracies of level. The age of the concrete at grinding depends on the gain of concrete strength, but for maximum economy grinding is usually carried out between one and seven days after laying, as soon as the concrete can be ground without tearing sand particles from the surface. Some grinders operate dry, but with the bigger machines the concrete is usually well wetted.
Vacuum dewatering
The problems of timing power trowelling - particularly in cold weather - can be largely overcome by using the vacuum dewatering process immediately after initial compaction and levelling of the concrete slab or wearing screed. The concrete surface is covered with a vacuum mat, incorporating filter layers, which is connected to a vacuum pump. By applying a vacuum for about three minutes per 25 mm depth of concrete, excess water is extracted, causing the concrete to stiffen rapidly. The initial power floating can then take place within an hour of the concrete being placed. The process also improves the wear resistance and general durability.
Applied concrete wearing screeds
Where wearing screeds are considered necessary, it is preferable that they are laid monolithically with the base concrete, i.e. within three hours of placing the base. Where screeds are laid separately and bonded, experience has shown that there is a high failure rate, mainly due to lack of care in bonding techniques. Where separate bonding screeds must be used, an extremely high standard of workmanship is necessary to avoid curling and hollowness of the screed through loss of bond.
Curing
If a hard-wearing surface is to be produced, the concrete surface of a direct-finished slab or wearing screed must be continuously cured for at least seven days after being finished.
An efficient and economical method of curing is to cover the floor with plastic sheeting held down in close contact with the surface. Alternatively, a proprietary sprayed resin curing membrane may be applied, provided it is compatible with any later surface sealing and hardening process which may have been specified to reduce the risk of dusting. Curing techniques are dealt with on page 46 under Methods of curing.