During winter, Five Star Repair Concrete will have a low temperature, having been prepared from cold materials; heat losses from shutter, cold reinforcing bars or when poured between larger material masses will also reduce the heat development of the setting cement. In many cases the substrate may have a temperature below that of the Five Star and thus act as a heat sink causing further heat loss.
The bags of concrete should be stored in a warm dry environment, off the ground. Temperature of storage plays a major role in keeping the materials from freezing in cold weather. It is a common fallacy to mix cement and aggregates with warm water and expect the concrete mix to warm up in the process. Work carried out by the C&CA many years ago and repeated by Weber using Five Star Repair Concrete as the test product showed clearly that it is the powder that governs the temperature of the mixed material, not the water. The heat capacity of the powder and its relatively larger volume compared to that of water is the governing factor.
On no account should the Five Star Repair Concrete be placed on frozen concrete or adjacent to cold steel. Provide heating to raise the temperature of the substrate concrete, steel etc to above 5°C, preferably 10°C for 24 hours prior to application. The use of tenting and heating is an accepted process in many cold countries in order to keep contracts running even in winter.
We recommend mixing the product for at least 3 minutes to ensure full dissolution of components and good dispersion and a water content of 3.1 to 3.3L per 25kg bag and no more than 3.3 litres maximum for a flowable mix. A pumpable mix needs about 3.1L water per 25kg bag, but this depends on site temperature and the equipment used. It would be helpful if hot water is used as it will tend to raise the temperature of the mixing equipment slightly. For mixing large volumes of Five Star Repair Concrete, we would recommend the use of a mixer pump such as a Putzmeister P13, which is a twin piston pump. Worm-drive pumps are not as good as they tend to get worn down by the sharp aggregates in the concrete.
Protection from frost
Maintain temperature during application
It has already been stated that concreting should not continue when air temperatures reach 5°C on a falling thermometer. When leaving concreted areas overnight in winter it is safest to assume that frost will occur and take the necessary precautions. Leave the shuttering in place as long as possible and cover any exposed concrete with strawboard, expanded polystyrene or similar insulating material. Wrap the whole area in dry sacking and, if possible, keep enclosed areas gently heated to a temperature not exceeding 25°C. Heaters are available that exhaust to the outside of an enclosure or building and just blow in warm air. That eliminates the carbonation problem. When using hot, dry air in an enclosure, the concrete surface can dry out quickly, leading to crusting or plastic shrinkage cracking. Adequate ventilation should be provided for the workforce.
Maintain temperature during curing
Temperatures must be maintained above 5°C for at least 24 hours if not 48 hours. This depends on the ambient temperature. The continued use of the tenting and heating system is a common practice but the use of insulated blankets or heated blankets is better recommended as this method does not dry out the concrete surface and lead to plastic shrinkage.
Five Star Repair Concrete is based on rapid hardening cement and, as such, will mature more rapidly than materials mixed on site with ordinary Portland cement. In order to study the rate of development of strength at various temperatures 100m cubes of Weber Five Star Repair Concrete were made using 3.3 litres of water per 25kg of dry mix. Compressive strength was determined at various periods of time and the results plotted to produce the graphs below.
The graph may be used as a guide to predict strength. Values for intermediate temperatures may be estimated by inspections or an alternative method such as the Sadgrove method may be used. This uses the principle that since cement hydrates more slowly at lower temperatures its strength is equivalent to that of similar material at a shorter time duration. Five Star Repair Concrete that has attained a strength of 10 N/mm2 is normally considered to be sufficiently mature to withstand further damage from frost.