Cold weather concreting problems
Few areas in Australia experience temperatures low enough to warrant elaborate and expensive protection of freshly-placed concrete which are common practice in Europe and North America. However occasional frosts, abrupt drops in ambient temperature, and/or prolonged periods of cold weather, do occur in our winter seasons. Harmful effects of these conditions on fresh concrete can be avoided by relatively simple measures in ordering, placing and curing.
Hardening of concrete is a chemical process and as in many chemical reactions the rate is temperature dependent. The lower the temperature, the slower is the process of hardening or setting of concrete.
At an ambient temperature just above 0°C the development of strength in unprotected freshly-placed concrete is very slow. If the ambient temperature drops below 0°C some of the water in the concrete may freeze; setting will virtually stop until it thaws, and this interruption of hydration increases porosity and reduces strength and durability.
Because some heat is generated during the hydration process, ordinary concrete has a minor inherent resistance to the freezing of its water after placing. But when the temperature of the concrete surface itself falls below freezing point, the water near the surface will solidify, increasing in volume and causing high pressures in concrete, which is no longer plastic. Scaling or spalling will follow, and will be severe if several freezing and thawing cycles occur.
Of all the factors affecting freeze resistance of concrete, permeability plays by far the most important role. Impermeable concrete has only small amounts of free moisture in its pores and thus the destructive action of freezing and expanding water is largely eliminated. There are three basic methods of reducing permeability and hence increasing freeze resistance of concrete, viz:
- Use of air-entraining admixtures. These prevent formation of continuous capillary passages by replacing them with minute, discrete (not interconnected) air voids.
- Reduction in water to cement ratio, which in turn reduces the bleeding rate (and bleeding capacity) of concrete. The presence of relatively large and continuous capillaries is usually closely related to bleeding of concrete.
- Use of pozzolans, such as fly ash, in order to replace part of the cement (generally fifteen to twenty per cent) resulting in a slight increase in the amount of hydraulically active material. Pozolans react with soluble products of cement-water reaction and form water-insoluble and hence water-impermeable substances. With proper use of pozzolans, permeability of concrete can be reduced by a factor larger than ten. However, as pozzolanic reaction is very temperature- sensitive, use of fly ash can reduce the rate of strength gain (depress early strength) in cold water concreting.
Water is at its maximum density at approximately 4°C, i.e. it has minimum volume for a given mass at that temperature. Therefore disruption to hardened concrete structure due to the increase in volume of freezing water (or ice) is possible at very low temperatures only. Hence, at temperatures above 5°C, long-term durability and strength of concrete are not going to suffer (ultimate strength of concrete moist cured in cool storage is generally superior to conventionally cured concrete).
Early age strengths
However, the rate of strength gain of concrete at low temperatures is relatively slow (refer to the graph above) and this can adversely affect construction pace (delay in removal of formwork, disruption to "critical path" etc...). To overcome this problem, several methods of producing higher early strength can be employed.
The methods of achieving faster setting times and high early strengths of concrete vary with particular applications, viz: local climatic conditions in different regions, availability of certain raw materials (e.g. cement and admixture types etc.), as well as layout of plant and machinery. Therefore it is important to discuss all the special requirements of cold weather concreting with Holcim production or technical staff.
- These methods include:
- The use of accelerating admixtures
- The introduction of hot water at the concrete batch plant
- Covering or heating of form areas prior to pouring.
This particularly applies to the inclusion of set accelerating admixtures such as calcium chloride, improper use of which can produce an adverse effect both in plastic and in hardened concrete.
Preparing for pouring
Small oversights at the pouring stage can result in disputes and dissatisfaction over cold-weather concreting.
After rain, free water lying on the surface, or lying in porous sub-grades, will be slow to evaporate, and its total volume may be substantial. If the concrete pour causes much of the free (and cold) water to accumulate in one end or corner of formwork and combine with low-slump concrete there, a critical weakness may develop.
Accumulations of ice at the bottom of holes prepared for concrete piers may be overlooked, and structural movement may follow.
Concrete should not be poured on frozen ground, or on reinforcing steel or formwork which has a temperature near freezing point. Covering or heating of form areas prior to concreting, a not uncommon winter practice in Hobart, parts of Victoria, the Snowy Mountains area, and Canberra, is less usual in coastal areas further north. But successive frosts in Sydney's western suburbs can cause ground temperatures there to drop to low levels, particularly where ground is shaded throughout the day by adjacent buildings or trees. In Sydney generally, if form areas are covered overnight, frosts will not delay pouring the following morning. Wherever possible, monolithic floor finishes should be placed after walls and roof enclose the area.
After placing concrete in cold weather its temperature must be maintained at a consistent high level if strength gain is to be normal.
Where ambient temperatures can be expected to be near or below freezing point for several days, insulation by batts or commercial blankets is indicated. Such insulation should be in close contact with surfaces and forms, and should itself be covered with strong, moisture-proof material. Steel projecting from forms should also be covered where possible.
Where minimum daily temperatures are unlikely to fall much below 5°C, less elaborate means of maintaining concrete temperatures can be used. It may be sufficient to lay waterproof paper on the form area, cover the paper with straw or sawdust to a depth of three or four inches and cover this with more waterproof paper, or sufficient merely to create dead air space between the form area and tarpaulins suspended above it. Heating and curing by exhaust system requires the building of an enclosure to keep cold air out. Any breakdown in the process may permit surface icing or rapid temperature changes in the concrete, with subsequent cracking.
Heating by fires placed at intervals provides uneven temperatures and is not favoured. The absorptive ability of cold air is low but increases rapidly as the air is heated. If heated air causes excessive evaporation from the concrete surface, shrinkage cracks will occur. Also, carbon dioxide produced by fires may carbonate the concrete surface, causing it to become chalky.
Five or six days after pouring, insulation should be removed at a time of day and in a manner which will allow the drop in temperature of any area of the concrete to be gradual. Membrane-curing compounds can be applied at this stage if necessary.
Strength gains of concrete will vary with the type of cement and type of mix, the use of accelerators, the ratio of mass to surface area, and other factors apart from temperature.
Generally it will be advantageous to leave formwork in position longer than the minimum period specified. Formwork will foster rather than retard curing in cold weather, and while it remains in position it is a reminder that any one section of a new slab should not be loaded too early or too abruptly.
Concrete mixes with some air entrainment, with the minimum practical water content and adequate cement, minimise problems in cold-weather work.
The ability of the concrete supplier to design and supply consistent batches of such mixes is important. The need to ensure that the mixes are at or close to ideal curing temperature is no less important.