Two different temperatures have to be considered when working with concrete in cold weather, firstly the ambient air temperature and secondly the concrete temperature at time of delivery. The definition of cold weather is a period during the day or night when the ambient air temperature will fall below 2°C. The BS and EN standard states that the temperature of the concrete should not fall below 5°C until the concrete achieves a strength of 2 N/mm² and that the temperature of fresh concrete shall not be less than 5°C at the time of delivery.
Therefore general good practice is to only place concrete when the ambient air temperature is 2°C and rising. However, following best practice and through early engagement with the supply chain, concrete could be placed down to 0°C with not unforeseeable risk.
Ensure batching facilities have heated water and heated aggregate bins.
Review sequence, logistics, materials and methodologies to ensure best practice is followed.
Remove frost from rebar and formwork before pouring.
Monitor the concrete temperature as it arrives on site. Reject any that is below 5°C.
Temperature must be 2°C and rising (sometimes 4°C and rising, check specification).
Apply frost blankets quickly at the end of the pour; avoid thermal shock when removing.
Concrete will be frost damaged unless 2 N/mm² or stronger.
Measure the temperature gradient for large pours; thermal gradient control (insulated shutters, quilts etc.) must be designed to control the thermal gradient.
Concrete will ‘go off’ slower. Allow more time for slabs and beams to cure, and measure strength before striking.
In wall pours control the rate of rise to keep shutter pressures as designed. Masonry and Render.
Temperature must be 2°C and rising (sometimes 4°C and rising, check specification).
Provide frost protection for minimum 24hrs.
Avoid retarded mortar.
Special admixtures can be used to accelerate the set.
For rendered elevations, ensure the cement particle board, insulation and render are applied in quick succession; the CPB and insulation will deteriorate in a day or less if exposed to bad weather.
Maintain moisture in wet mortar.
Cover completed work with polythene to avoid desiccation.
Again with concrete pours that take place in warm weather the same two different temperatures need to be considered, the ambient air temperature and the concrete temperature at time of delivery. Ambient temperatures up to about 20°C should not on their own cause significant problems, especially in damp or humid conditions but when the ambient temperatures of 20°C and above are in partnership to low humidity and drying winds consideration of a more efficient curing regime is needed.
The target temperature of concrete on delivery from the chute should not be more than 30°C.
Make appropriate modifications to concrete mixtures to manage rate of slup loss, setting time and other characteristics. Retarders, extended set control admixtures, synthetic fibers or other proven local solutions may be helpful.
The consistence of the material is affected. The slump/flow/slump flow of concrete reduces more rapidly. If there is water added to improve the consistence it decreases the concretes compressive strength, potentially increases permeability and ultimately affects the durability of the structure.
The hot weather will accelerate the loss of moisture from the surface and therefore increase the risk of plastic shrinkage cracking. Consideration of some form of protection is needed. If using hessian and water, you will need to rewet the hessian frequently (up to four times per hour) to ensure that it is effective.
On hot or dry days when the conditions are conducive to plastic shrinkage consider dampening the sub-grade, pans and forms and reinforcement prior to pouring concrete, do not allow water to pond however during and after this process.
Check the weather forecast to pick the best time to pour. Consider starting large pours at 6am or earlier avoiding working during the hottest part of the day, often between noon and mid-afternoon.
As the concrete temperatures increase the setting time reduces and impacts the time to place, compact and finish the concrete. Consideration of more labour is needed to achieve the work and schedule the rate of concrete delivery to avoid overloading the labour and/or equipment.
Consideration of how to maintain the labour in hot conditions. On a hot day you need to have access to drinking water at all times to avoid exhaustion during manual work. Also, sunscreen should be applied before starting the pour.
Keeping equipment out of direct sunlight until it is required to lay the concrete is a small but effective measure. This will stop the equipment from heating the concrete while it is being poured.
Concrete will go off quicker. Begin final finishing operations as soon as the water sheen has left the surface. Apply curing measures immediately on finishing an area; on a large slab this will be while pouring is still in progress in other areas.
In large elements or rich mixes faster hydration of cementitious materials down to high ambient temperatures can result in higher maximum concrete temperatures and thermal cracking.
Changes in temperature of the concrete may also result in cracking particularly where concrete is placed on a hot day followed by a cool night.
With the increased rate of hydration the surface of the concrete will dry quicker which leads to premature finish being applied, trapping of bleed water and possible debonding of the top surface with subsequent flaking/de-lamination
Delays in transport should be minimised although in numerous cases this is a difficult task.
None the less when it comes to planning any pour with any amount of concrete you will need the knowledge and advice of an expert. Whether that is you, a contractor or your concrete supplier depends on your project, what you are doing and how much risk it entails. To avoid pouring concrete incorrectly careful consideration and forethought is a must.
In-situ reinforced concrete is expensive. Prior to starting there are some key checks, areas to set up, materials and pre-work processes to be considered but foremost to avoid any quality issues on site we should ask, “Has pre-cast concrete been considered as alternative?“.
Despite this we can’t always go with the factory controlled alternative so pre-start and pre-work considerations are essential here I list a few that I consider;
Ensure a complete specification for the site has been received addressing strength and durability; ensure mix designs have the correct cement type, water/cement ratios, work-ability. Its also important to check that all mix designs have been approved by structural engineer, materials engineer and designer, before works commence. Test the mix… if not then you’re not ready.
Ensure an adequate clean storage area is available before reinforcement is delivered. Rebar can get dirty, bent, lost and damaged from poor storage…
Ensure that the cement type and delivered concrete temperature have been considered in the shutter design and the rate of pour. – We can look at winter working and summer working in more detail later but all these things factor into a successful defect free pour.
Have the number of mix designs been kept to a minimum? Its all well that you have multiple options and are they clearly named as more than once a wrong mix has gone into a pour only to have it broken out at a later date when you discover that the blinding mix is in a column.
Is testing equipment on site and operational and has the Lab been certified and checked? Do you have a cube tank and is that heated and operational?
If you have not carried out a plant inspection then it is well worth doing so as seeing the set up, location, routes to site and facilities is vitally important.
Right set up for the job… go and look and check it out before you start. Remember if you have not carried out a plant inspection then it is well worth doing so as seeing the set up and facilities is vitally important and the purchaser assumes the responsibility for ensuring technical correctness
Plan large pours meticulously and early with concrete suppliers, your onsite batching facility, plant suppliers and subcontractors, etc. Be aware of the time between placing fresh concrete on already placed fresh concrete, taking into consideration heat developing in the concrete during curing. as the last thing that is needed is a defect.
Make sure that your method statements take into account precautions to protect against cold/hot weather, rain and drying wind and ensure the operatives and supervisors are aware of what needs to be done to protect the work.
This isn’t an exhaustive list and nothing can prepare you more than knowledge, experience and planning and just one last thing if that wasn’t enough… it is also important to note that it is the purchaser that assumes the responsibility for technical correctness of the concrete specification.
Poor quality in the construction industry is one of the most common issues that results in lost time and additional costs to a contract. Year on year it costs contractors millions of pounds, damages relationships with Clients, places additional pressures on Site Teams attempting to complete a contract or results in further intervention from Designers through lengthy reports. The implication of finding such workmanship issues can be carefully navigated through controls and an understanding of what is required and what is expected from the specification/design prior to putting people to work. As with most things any defects in construction require re-work to correct the problem and the longer defects remain uncorrected, the more rework we are likely to encounter and the more rework there is likely to be involved. Ensuring that the work force and supervision have the right skills and knowledge, right tools and materials and, following completion, the right protection will all help prevent this from happening.
In the next series of posts I have decided to look at specific issues with different activities and to compile a list of checks that can be undertaken by the Engineer or Supervisor, with some examples, to help check and ensure that we are identifying the major and sometimes minor details that people miss when planning or executing an activity. As suggested earlier defects can be avoided by sufficient planning and adequate monitoring. Allowing enough time for the work to be done properly and in the correct sequence is always particularly important.
Excessive distance to the top step from finished level (maximum 675mm)
Poor control with cutting joints damaging permanent mesh on a parapet rail
Structures settlement to the parapet due to poor compaction
The first at avoiding common defects will look at one of the most common and versatile construction materials used in civil engineering – reinforced concrete. Following on from that I will go through various other trades and activities which include;
Falsework and Temporary Works
Earthworks Planning, Bulk Cut/Fill Operations and Finishes
Drainage Planning, Pipelaying and Chambers
Roadworks – Type 1 Sub-base
Roadworks – Kerbs and Finishes
Sheet and Bearing Piles
Setting Out Checks
Whilst we review the checks that we should look in the coming weeks there are some simple first steps that can be implemented to ensure the right ethos is achieved with the team to achieve good planning, good leadership and a culture that promotes good quality.
Ensure everyone knows and understands the contractual project requirements and specifications from the designer and the client or clients representative. No client wants a bad job.
Always read the label – People should be encouraged to refer to British Standards, European Standards and Trade or Manufacturer literature. Instructions are there for a reason, if you don’t follow them it will not go well.
Learn to recognise what good looks like. If it doesn’t look right, it probably isn’t and don’t be afraid to stop the work if anything looks wrong. Allowing it to continue could cost more money.
Make sure that everyone is qualified, competent and capable for the activity that you are planning to execute. Hiring a bricklayer when you need a joiner will never work. You should never assume specialist trades know what acceptable quality is, just because they come in a van with a logo on it does not mean they know what they are doing.
Don’t assume a manufactured product is correct, always check what you pay for. Quality is still an issue in a factory, even where quality controls are stricter.
Follow the Inspection and Test Plan set out by the Quality Team and Quality Manager, get involved with its production and complete quality inspection records as the work progresses. Quality can be as onerous as you like and sometimes those with the most letters after their name don’t always realise the workload they are enforcing on people.
Ensure everyone understands the activity, the sequence and what is expected and always take photographs of the works as they progress and get sign off from those accountable or executing the work.
Making sure that we have enough competent supervision for the normal working week is essential before we move to increase working time to either weekend and/or night work as these are times when defects occur most frequently. We should also scrutinise our reliance on our supply chain and their ability to supervise and correct defects in the manor that we require, from painful experience there will always be a percentage that will not return to undertake the work so early intervention is essential.
Effectively its down to you to ensure that you are right first time because if you think quality is expensive…try building it twice.