Multi-storey car parks almost always feel colder than the temperature dial in your car would suggest. A welcome hiding place in summer, but less so in winter! Still, repair and refurbishment of these concrete structures often continues apace through the winter and understanding the limitations of the materials we apply to them, when it comes to temperature is critical if we are to achieve the aims of structural refurbishment and maintenance.
In this article, Dr Steven Holmes, Senior Technical Manager from Sika’s Specialist Construction Solutions explains the limitations faced by traditional cement-based materials and how materials with alternative technologies can be used to repair and strengthen your structure, even at temperatures as low as -20°C.
It’s a night shift on a winter’s night and you have placed rapid-set concrete repair materials which will allow you to re-open to the paying public first thing in the morning. The problem is, you’re not convinced the flowable mortar you poured is going to be ready to receive traffic. As 06:00 approaches, you have to make a decision – keep the deck closed for a few hours to allow it to reach strength, with all the potential loss of revenue this brings; or open up and risk it. Neither eventuality is good as, if the temperature was too low for the repair material on application, it may never perform as specified and fail prematurely. Best case – you get away with it! Worst case – the material doesn’t perform and you’re digging everything out and starting again, most likely at your own cost!
Why it happened
It may be that program delays pushed the project back into the winter months, meaning that products were specified for summer/autumn conditions. There may have been a misunderstanding about the materials properties – we’re all human after all, or an attitude from operatives of “we’ve used this in winter before and it was fine”. The risk is that the applied material will never achieve the material’s properties stated on the material’s data sheet – what does this mean for your programme, the structure and the longevity of the repairs?
If you are working on an MSCP project in low temperatures out of necessity and require products which will allow you to progress whatever the weather and give you a quick return to service; there are plenty of products that are specially formulated for just this purpose.
Why traditional cement-based products cannot perform
The curing of traditional concretes, mortars and grouts relies on the hydration of cement in the presence of water. This reaction, its speed and the compounds that result will determine the rate that the material gains strength and cures. As with almost all chemical reactions, the hydration rate is determined by temperature. This means that in low temperature, cement-based products will gain strength more slowly and, in some cases, when the temperature is too low, never gain any discernible strength, never mind their design strength.
What technologies are there for low temperature working?
All of the different technologies used to formulate products which can be used in low temperatures still rely on a series of chemical reactions to gain strength and achieve the materials properties required (hardness, flexibility, etc.). The difference between traditional cementitious products and the more modern materials below is that the rate of these reactions at low temperatures allows them to reach their stated material properties; often very quickly and in some instances at temperatures as low as -20°C.
Alternative solutions include:
- Magnesium Phosphate Cement is a blend of magnesium oxide and ammonium dihydrogen phosphate that reacts with water to rapidly produce heat and strength, significantly quicker than most Ordinary Portland Cement (OPC) based products
- Polymethyl Methacrylate (PMMA) is a thermoplastic resin, requiring an activator to catalyse, and is rapid curing, often achieving full cure in an hour. As these products don’t contain water, they are ideal for use in low temperatures.
- Polyurethane Methacrylate (PUMA), is rapid curing and requires a catalyst. Unlike traditional materials, but similar to PMMA, the formation of bonds at the molecular level is unaffected by time, making it extremely durable. PUMA products display elongation properties, making them useful for crack bridging, even at very low temperatures.
- Reactive Acrylic Resins, when combined with a catalyst, have accelerated chemical reaction rates, generating significant heat and therefore allowing them to be used in very low temperatures. The curing time is extended by lower temperatures, but the chemical reactions do continue, unlike cementitious materials incorporating water, which would simply freeze, meaning the reaction never completes.
- Low Temperature Epoxies are specially formulated to provide positive cures at low temperatures. The resins and their curing agents (‘hardeners’) radiate heat during the reaction phase, enabling the chemical reaction to complete, even in low working temperatures.
- Polyester Resin products require a specific catalyst to effectively polymerise the resin. When combined, an exothermic chemical reaction takes place and curing takes place rapidly, normally within two hours. This enables them to be used in ambient temperatures as low as 0°C.
- Epoxy Acrylate Resins is a technology that can be incorporated in formulations to achieve a very fast curing product. The exothermic polymerisation reaction generates significant heat, which is sufficient to achieve full cure even at low temperatures.
- Modified Cementitious Binders contain chemical ‘accelerators’ which ensure full cure is achieved in temperatures that standard cementitious products could not cope with.
What form do they come in?
The good news for specifiers and quantity surveyors is that the technologies above have been used by Sika® to create an array of products with broad applications across car park structures.
Repair mortars – can be applied at temperatures as low as -20°C and achieve in excess of 70 N/mm2 compressive strength in just 2 hours.
Modified cementitious mortars and concretes – can be applied at temperatures as low as 0°C and achieve in excess of 25 N/mm2 compressive strength in just 2 hours.
Resin grouts and resin anchoring systems - can be applied at temperatures as low as -10°C and achieve in excess of 95 N/mm2 compressive strength in 7 days.
Resin mortars - can be applied at temperatures as low as 0°C and achieve up to 50 N/mm2 compressive strength in just 1 hour.
Structural adhesives - can be applied at temperatures as low as 5°C and achieve up to 55 N/mm2 compressive strength in 1 day.
Rigid bonding adhesives - can be applied at temperatures as low as 5°C and achieve in excess of 50 N/mm2 compressive strength in 1 day.
PMMA-based car park coatings - can be applied at temperatures as low as 0°C and can be used for rapid deck refurbishment in the winter months, whilst offering exceptional crack-bridging properties.
So, when the weather turns cold during refurbishment and maintenance work ensure you are aware of the solutions that will work, even when the temperature falls as low as -20°C.