|
Cold Weather Concreting Durability of Flatwork Surface Scaling
- Concrete exposed to deicers in the presence of moisture in a severe freeze-thaw climate must be air entrained to resist surface scaling. That we know. There is another important step, however, that is often underestimated. For air entrainment to be fully effective, the concrete should have al least 30 days of drying under favorable weather conditions before being exposed to deicers. This is at least as important as early curing. In marginal situations it may be helpful to seal the concrete with two coats of linseed oil (cut back with 50% turpentine or kerosene) to protect against deicer scaling for the first winter. It's not a cure-all, however, and certainly no substitute for adequate air entrainment and adequate curing and air drying. There's no room for marginal concrete mixes or marginal construction practices if the concrete being placed at the tag-end of the construction season is to be durable.
Dusting of a Concrete Surface
- A lower concrete temperature (prior to the use of heated materials) and placement on a cold subgrade means slower setting time. There may be more bleeding, and bleed- water may remain on the surface longer. Early troweling of wet surfaces, leaving a weak paste at the surface, can be the result. Later in the season, when concrete is being placed in heated enclosures, the use of unvented combustion heaters can cause severe dusting. Carbon dioxide reacts chemically in fresh concrete, disrupting cement hydration. For that
reason, the combustion gases must be vented to the outside. In a poorly ventilated enclosure, carbon dioxide from gasoline motors on construction equipment or generators may also cause dusting. Carbon
dioxide has no adverse effect on hardened concrete. Compressive Strength In
addition to slower setting times, the early strength of concrete in place will be reduced when concrete temperature, and air and subgrade temperatures are lower. Concrete cylinders that are tested for acceptance
strengths, however, are protected at a specified temperature until time of test (ASTM C31 - 60' to 80' F during job site storage and 73.4´ +/- 3' F during laboratory storage). The actual early strength of concrete
in place will then be lower when compared to the early strength of laboratory cured cylinders than would otherwise be expected. That underscores the need for job cured cylinders -- that is cured at the same
conditions to which the concrete in place is exposed -- to determine the time at which it is safe to strip forms or apply any load to the structure. Concrete Mix Adjustments In both severe and moderate climates, cool weather effects can be countered by:
- adding 1 to 2% calcium chloride by weight of cement--except when prohibited.
- adding non-chloride accelerators.
- adding hot water.
- adding
Hydro admixture.
substituting Type III cement (high early strength) for Type I.
adding about 100 pounds of portland cement per cubic yard.
In fly ash mixes additional possibilities are:
- increased percentage of portland cement
- increase in total cementitious content
In cold weather, just as in hot weather and in all of the temperatures in between, concrete is a versatile, predictable material, always providing every opportunity to achieve the quality desired . |
