n A minimum cement content of 320 kg/m3, but preferably not less than 350 kg/m3
n A sand content of not more than twice the weight of cement n Total aggregate not more than six times the weight of cement n Avoidance of an excessive quantity of 10 - 5 mm aggregate
n Free water/cement ratio of 0.50 or less
n Consistence class normally S2 (50 - 90 mm slump).
The proportion of 1 : 2 for the cement: sand fraction is based on M grade sand. In the case of finer sand, the quantity should be reduced to compensate for its increased surface area.
Trial mixes and sample panels are essential to determine the suitability of the concrete for the particular job in question.
Supervision and workmanship
A high level of supervision is essential, with special attention to the following:
n The formwork should be checked for alignment, level and plumb, for bracing and tightness of all fixings, joints must be watertight to prevent leakage. The formwork should also be checked for accuracy of dimensions
n Reinforcement needs to be checked so that the correct cover will be achieved
n The first part discharged from a truck-mixer should either be used in a visually unimportant part of the work, or set aside and used later after some of the more homogeneous concrete has been placed
n Once begun, placing should be continuous until the pour has been completed; continuity in the supply of concrete is therefore essential
n Placing should proceed at a uniform rate of not less than 2 m height per hour. This requires careful consideration and planning of pour sizes
n The actual rate of placing needs to be restricted by the rate at which the concrete can be compacted.
Plain smooth finishes
Contrary to a widely held belief, plain smooth surfaces are the most difficult to produce to a consistently high standard, because of the inherent variation in the constituent materials and the fact that even the smallest blemishes are readily visible on plain surfaces.
There is still need for a good finish, even when grit blasting or bush hammering is to be used to expose the aggregate. It is not true that bush hammering will make a poor finish acceptable; in practice, any tooling of a poor finish only serves to accentuate the surface defects.
Textured and profiled finishes
The simplest textured finishes can be obtained by using formwork made from rough-sawn boards, so that the imprint of the wood grain is reproduced on the surface of the concrete. To be effective, great care is necessary in the design and fabrication of the formwork, particularly the joints and any fixings.
Formwork linings are made in a variety of patterns from materials such as expanded polystyrene (which can only be used once) and flexible rubber-like plastics (which will give repeated use). Manufacturers of these materials usually provide guidance as to their use.
Ribbed finishes are usually produced by fixing timber battens securely to the formwork; fixing should be by both gluing and screwing to prevent grout loss under the battens. The battens must have a generous taper ('draw') to enable them to be struck from the hardened concrete. The formwork for intricately modelled surfaces is often made from glass fibre reinforced plastics.
Guidance on textured and profiled finish is given in the BCA
publication Appearance Matters - 7.
Exposed-aggregate finishes
Coarse aggregate is exposed by removing cement and sand mortar from the face of the concrete. The variability in the distribution of the coarse aggregate in ordinary concrete tends to give an uneven appearance when that aggregate is exposed. It is therefore necessary to use a special prescribed concrete using gap-graded aggregates containing as large a proportion as possible of the coarse aggregate, and it should be noted that such a concrete will require particular care in transporting and placing. For example, all the coarse aggregate should be between 20-1 0 mm in size instead of 20 - 5 mm. Trial mixes will have to be produced to determine the mix design, firstly to achieve the finish and secondly to satisfy durability and compressive strength requirements.
One method of exposing the aggregate is to coat the formwork with a chemical retarder, which prevents the cement in contact with it from hardening. When the formwork is removed the surface mortar is brushed away to uncover the aggregate embedded in the hardened concrete. Timing of the operation is fairly critical since some retarders will delay the hardening only while the surface is tightly covered. When the formwork is removed and air gets to the surface, the mortar soon hardens. It is therefore important to organize the work so that the surface can be treated within a short time of the formwork being stripped. Brushing should begin near the bottom of the wall or column, because the concrete will be harder there than towards the top.
The surface mortar can also be removed from hardened concrete by abrasive blasting. In this technique, compressed air is used to carry a stream of selected grit along a flexible hose to a nozzle, where it emerges as a jet and is directed at the surface of the concrete. The operator must wear a helmet with a piped air supply to protect him from the dust which is created, and the working area has to be screened off to avoid endangering other site personnel and passers-by. A deep or heavy exposure, in which the coarse aggregate is exposed by a third of its depth, is best carried out when the concrete is about two days old. If it is left until the concrete is harder, it will take much longer to produce the finish. A medium-depth finish can be carried out at three to four days after placing the concrete, and a light abrasive-blast finish at seven days or later. The operator must have considerable experience of working on concrete but, given this, abrasive blasting is often quicker and less sensitive to variations in the finish than other methods of exposing the aggregate.
Guidance on exposed aggregate finish is given in the BCA publication Appearance Matters - 8.
Tooled concrete finishes
Tooled finishes are not produced until the concrete has achieved a compressive strength of at least 20 N/mm2, so this particular operation can often be left until near the end of a contract. Tooling removes a layer of concrete from the surface - typically 5 mm with bush hammering and 10 mm or even 20 mm with point tooling - and reveals the colour but not necessarily the shape of the aggregate. Care is required when working near the edge of an element, and it is usual to provide a plain margin that is left untooled to minimize the risk of breaking the edges.
Guidance on tooled finishes is given in the BCA publication Appearance Matters - 9.
Remedial work
Except for the need to cure the concrete, plain and textured finishes should not need any further attention once the formwork has been removed. However, it will be necessary to fill any tie-bolt holes, and these are best dealt with as soon as possible so that the mortar gains strength at the same rate as the concrete itself. Similarly, if blow-holes are to be filled, these should be 'made- good' as soon as possible.
Making-good is a skilled job that should not be entrusted to a general labourer. The mortar that is used has to be blended by introducing a proportion of white cement, or some white limestone sand, to match the grey of the concrete and smoothness of a formed face.
Remedying a fault such as a locally honeycombed area, which may have to be cut back to sound, well compacted material before being reinstated, may be satisfactory from a structural point of view, but is unlikely to be acceptable in terms of appearance because the patch will often tend to weather differently and become more obvious with time. It is therefore all the more necessary to take particular care in the production of visually important sections of the work. Further information is available in the BCA publication Concrete on site - 8: Making good and finishing.