A Guide to managing a pour

And getting the best results from your concrete

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An awareness of the process and associated difficulties is required to effectively manage a concrete pour. Often the project manager is ultimately responsible to ensure all variables are covered.

Please contact ALLIED CONCRETE for more in-depth or detailed information. PH 0800 4 ALLIED

1: Setup & Ordering

• Ensure safe & clear access to site for concrete trucks.
• Reinforcing: is it required? – ask us about Allied Ready- Floor as an option.
• Sub-grade preparation, level & flat – Is a vapour barrier such as polythene necessary?
• Type of concrete, slump (workability ) and volume required. Check the weather, extra products may be required to cope with hot or cold conditions.
• Speed of supply, how close together the concrete trucks are required – consider travel time, a retarder may be required to slow the setting time.
• Have your account details or method of payment, contact phone number & site address available.
• Concrete Cutting required? – if cutting is required ensure that it is done as soon as possible after placing.

2: Placing

• Choose an experienced concrete placer, ask to view some of their recent work.
• Monitor water added to concrete on site, (check with the batcher to ensure that the maximum water content is not exceeded).Adding extra water will reduce the concrete strength.
• Vibrate or rod the concrete to ensure adequate compaction.
• Screed the concrete to the correct level.
• Float off to close the surface (smooth off).
• Take care not to overwork wet concrete bringing too much cement paste to the surface and do not add water to the surface when troweling , either action will weaken the final surface.

3: Finishing

• The finishing process can only begin when the concrete has stiffened sufficiently and the bleed water has evaporated from the surface. The timing of this will depend on the weather and concrete temperature.
• The finishing process will determine how dense and hard wearing the concrete surface will be. Depending on the requirements different methods are available.
• Mechanical power floating – for a hardwearing smooth surface, standard internal finish.
• Bull Float – applicable for most external surfaces, can be broomed to add grip (will also increase surface area, not as hard wearing).
• Decorative finishes, exposed aggregates, ground or polished. These are specialised finishes that require training and expertise to achieve a high level of finish.

4: Curing

• Concrete should be protected from early loss of moisture; this loss can cause shrinkage cracking as little strength development has occurred to withstand the stresses resulting from the volume change (due to loss of water).
• A good curing environment should exist immediately after finishing, and for best results for seven days.
• After finishing and before curing can take place without damaging the surface, an evaporation retardant maybe required to slow moisture loss in hot weather.

Curing methods

• Ponding – Build a sand bund around the perimeter and fill with water to cover slab
• Spraying – The use of sprinklers to keep slab continuously wet.
• Covering – Impermeable covering such as plastic sheet will trap moisture on concrete surface and minimise evaporation.
• Curing Compounds – Apply after finishing when bleed water disappears. (Use with caution, these products may affect follow on trades like paint, tiles, vinyl, adhesives )

Minimise Cracking

Control Joints

A shrinkage control joint is defined in NZS3604 as “a line along which the horizontal strength of a slab is deliberately reduced so that any shrinkage in the slab will result in a crack forming along that line”.

• All concrete cracks. Cutting or other methods try and control where.
• Concrete should be cut as soon as possible without saw-cuts chipping on edges. ( within 12 hours in Summer, 24hours in Winter ) Soft-cut saws can be 6 to 8 hrs after placing.
• Cuts should be 1/3 of the depth of the concrete, try and keep sections as square as possible to equalise shrinkage in each direction.
• Control joints can also be made at the time of placing with a specially shaped trowel or by adding plastic or steel inserts.
• Cracks may occur where dimensions change in the slab or around obstructions and re-entrant corners i.e. columns, waste pits, drainage sumps etc. Use diagonal bars at the corners or put in joints to prevents cracking. Make sure that any saw cuts are not too far apart to relieve stress.

Plastic shrinkage cracks

• Plastic shrinkage cracks are formed in the surface of the concrete before it has set (or in a plastic state); often they may not become visible until some time later.
• As a general rule : If it is good weather to hang out washing to dry it is ideal weather for plastic cracking to occur as well. Our web site www.alliedconcrete.co.nz has a graph to help you assess the risk of cracking on any given day.
• Factors which lead to plastic cracking
• Sunny and or windy days. (High evaporation rate )
• Low humidity (Higher evaporation rate )
• Exposed sites (Higher evaporation rate )
• Broom finishes (Increase surface area and evaporation rate )

How to minimise the risk of plastic shrinkage cracking.

• Do not place concrete when the weather conditions are too severe for you to control the evaporation rates of water from your concrete.
• Moisten the sub-grade and forms prior to starting the pour.
• Use polypropylene fibres in your mix – Allied concrete stock a wide range of poly fibre options.
• Use an anti-evaporative spray to prevent excessive water loss from slab. We supply and recommend SIKA- film. (Use in accordance with manufacturers instructions)
• Cover the concrete with polythene or membrane cure immediately after finishing.

Hot weather concreting

• Assess the risk of plastic cracking ? Check weather conditions.
• Organise workers & mixer trucks to avoid / minimise delays.
• Use Antivap spray & cure the concrete.
• Consider early morning or evening placement.
• Retarder may be necessary to slow the setting time or to allow for travel.

Cold weather

• Low temperatures drastically slow the setting process (less than 5 degrees is considered unfavourable)
• Do not place onto frozen ground or if snow is forecast • Order concrete with accelerator in it.
• Consider using a higher grade of concrete & a lower slump (lower water content)
• Do not attempt to finish concrete until all bleed water has evaporated, this will take longer on cold days.
• If frost is expected cover the slab

Decorative Concrete

Decorative concretes cost a premium so it is important that proper care and planning are taken into account.

• Not all colours are equal, Allied stock a premium range of integral colours that will not leach out of the concrete.
• Recommended minimum strength for decorative concrete is 20MPa.
• Areas in shade, time of day, and concrete poured in stages on different days can affect colour.
• Accelerators will cause coloured concrete to produce a variable colour.
• Coloured concrete often requires an acid wash to remove efflorescence (white minerals) from the surface. This is a specialised operation and requires experience.
• Exposed or polished concrete will show any foot prints or marks that have been filled in by screeding slurry across them, should rather be filled with a shovel of mixed concrete.
• Sealing concrete: don’t seal when concrete temp is below 12 degrees or when it is damp. ( Moisture will make the sealer go ‘milky” ) Minimum of 28 days from placing until concrete can be sealed.

Ask about our READY Solutions

WE MAKE WHAT’S HARD EASY

PH 0800 4 Allied

• READYfloor – Steel fibre reinforced concrete is an economic alternative to traditional crack control mesh reinforcement and can be used for any lightly loaded concrete ground floor
• READYcolour – For internal floors, driveways, patios, paths and pool surrounds think coloured concrete
• READYexposed – Combine the strength, durability and affordability of concrete with the natural beauty of New Zealand’s spectacular riverbeds.

READY PRODUCTS

• READYsealer, READYacid & READYthinner
• Sika Rugasol® C - For exposed aggregate finish. Sika® Fibre - Reduction of plastic shrinkage cracks, reduced permeability and bleed water. SikaFilm® - Anti-evaporative spray, protects concrete from rapid moisture loss, helps reduce risk of plastic cracking. Sika® Accelerators - For faster setting
• Sika® Retarders - To slow down the setting time. Sika® -1+ - To waterproof concrete.