Architectural Concrete Consultant David Bennett is the UK’s leading expert on architectural concrete finishes, visual concrete specification writing and on-site training for contractors. He is the author of four major books on architectural concrete, a provider of architectural concrete CPD and a visiting lecturer at twelve UK Universities. Here he reveals the secrets to achieving a fine concrete finish: 1. Uniform Colour of the Mix
The concrete mix must have good colour consistency, good cohesiveness so it does not segregate and good workability for ease of placement.
Colour consistency is achieved by keeping all the constituent materials of the mix the same, with the same proportions and colour and with minimal variation.
The surface colour is governed by the cement colour because they are the smallest particles. There must be sufficient cement in the mix to coat the fine and coarse aggregates and to saturate the surface. A minimum cement content of 350 kg/m3 will be sufficient to do this.
The fine aggregate colour must be the same because it is a pigment that tints the cement colour. If too much fine aggregate is included, it can make the mix very sticky and increases the risk of many fine blowholes on the surface. If there is too little fine aggregate, the mix maybe too harsh, segregate, and form many large blowholes on the surface. The optimum amount of fine aggregate is usually a ratio of 2 parts by weight to I part of cement.
The coarse aggregate grading greatly influences the tonal colour of the mix. It is not seen on the surface but influences the amount of water added to the mix to make it flow. To regulate and to maintain the same water content and therefore tonal colour, the coarse aggregates should have a reasonably constant surface area and to ensure this, specify not more than 30% of the coarse aggregates smaller than 10mm, so that 70% are between 20mm and 10mm.
The water to cement ratio should not exceed 0.5 for a dense, hardwearing surface and the slump maintained between 125 mm and 180 mm for ease of placement and to limit changes in the water content of the mix. For good cohesiveness of the mix, so that it does not segregate when placed or vibrated, the total aggregate to cement ratio must not exceed 6: 1 by weight.
2. Selecting the Form Face
The beauty of the surface finish is the formwork or the shuttering. If the shutter face is rough, split and uneven so will the finish be. For a fine, plain, flat, even surface finish, choose a good film- faced ply.
An MDO (Medium Density Overlay) film-faced ply gives a matt, non-shiny finish and is excellent for slab soffits and walls. A phenolic film-faced ply gives a shiny finish and is good for vertical surfaces but it may highlight minor imperfections and small changes in the mix for flat work, so it is not recommended for soffits.
Use whole sheets with the minimum of site cutting, keep to standard sheet sizes of 1220mm by 2440mm by 18mm thick as far as possible, as these are stock items, and are readily available.
Double sheet the shuttering for soffits and walls to avoid damage to the film-face sheets during assembly and to eliminate lipping, water and grout loss at sheets joints. The underlay sheet is also the decking for the slab, to walk on and to deposit materials and equipment.
Do not nail sheets through the contact face, as it will split the film and create an ugly mark. Back fix vertical elements using screws. Use headless shot fired nails for fixing the top sheets for the slab soffits, for the best results.
Board marked concrete works very well on walls. Choose dense, well formed, smooth timbers that are 125mm to 200mm wide and at least 15 mm thick. Select home grown Douglas Fir, British Larch or Sweet Chestnut for good results. The bare wood has to be sealed first with a water repellent coating and then a release agent applied, otherwise the tannins and resins will stain the concrete finish.
For circular columns specify column formers lined with PVC or GRP for a fine, marble smooth surface finish.
3. Concrete Placing and Compaction
This is critical for walls and columns. Good workmanship, placing and compaction technique will result in a blemish-free surface finish.
Use a hopper and flat hose to place the concrete to avoid it splashing on the form face or striking the reinforcement on the way down and segregating. Compact and vibrate the concrete in 500 mm layers to release all the trapped air. When compacting the concrete, insert the vibrator head quickly into the layer and hold for at least 20 to 30 seconds to release all the trapped air and to consolidate the mix.
Placing rate must be at least 2m/hr vertically for a uniform colour and to avoid horizontal pour planes and cold joints.
4. Formwork Pressures for Wall Shutters
Refer to CIRIA 108 for formwork pressure tables. Design for the correct pressures for concrete mixes with GGBS or PFA replacement cements and with admixtures. Consider the design pressures for a rate rise of 2m/hr vertically in winter at a temperature of 10 deg C and a rate rise of 3m/hr in summer with a temperature of 15 deg C.
Limit the extension of the tie bolt to not more than 2mm across the width of the wall to reduce grout loss, lipping, black eyes and loss of definition of the tie cone hole. This is a frequent problem in concrete construction.
5. Good Workmanship Tips
Apply release agent very sparingly on the form face. Spray it on and wipe it over with a clean cloth and leave just a fine even coating. Excessive use of release agent will remove the surface skin of the concrete and discolour the finish and reveal many clusters of small blowholes.
Detail shadow gaps along the sheet cut lines on the soffits and where they abut columns and walls and stop ends to create a neat clean finish with no grout runs.
Use heavy duty 'dog bone' type concrete spacers at least 28mm thick for supporting bottom reinforcement on soffits. Ensure the shallow curved face is in contact with the form face. Do not use long 'trick trac' plastic spacers or flat faced concrete spacers, as they will be seen.
Punch thick polythene sheets over starter bars at the top of walls and columns to prevent rain bringing rust and free lime, which will stain the wall and column face below.
6. Final Clean and Aesthetic Review
It may take a soffit many months to stabilise in colour and to match the walls and columns for colour.
After many months and as late as possible, clean all the surfaces gently with a soft brush and airline and a rotating Flex machine with a fine abrasive pad for the soffits. This will reveal the true surface colour of the concrete and removes the coating of surface dust, mortar and plaster splashes and reduces the impact of localised surface staining from rust and free lime.
Aesthetic concrete repairs should be done as late as possible to achieve a good colour match and executed with great skill and care. Minimise the repair work to reduce the impact of repairs, as they are never perfect!