Protecting Concrete in Cold Weather

保护混凝土在寒冷的天气是连续的s challenge for concrete contractors and site supervisors. Placing concrete in the cold weather condition requires special preparation and protection. All necessary precautions should be taken in order to to alleviate the negative impacts of cold weather. Special curing and protection is required in most cases. In my previous article, we reviewed what is considered cold for concrete construction, and what should be done prior to placing concrete. In this article, I will review some of the widely used protection techniques and strategies and the challenges in protecting the concrete from extreme cold. But first, let’s see what cold means for concrete:

为什么寒冷的温度至关重要

这hydration of cement is a chemical reaction. Extremely low temperatures as well as freezing can significantly slow down the reactions, thus, affecting the strength growth. In fact, freezing temperatures within the first24 hours（或者当混凝土仍然处于塑料状态时），可以降低强度超过50％。

Protecting Concrete in Cold Weather

这CSA A23.1specifies that protection shall be provided by means of:

1) Heated enclosures
2) Coverings
3) Insulation

应继续保护，直到所需的结构性质如强度实现。将混凝土暴露在极端寒冷之前的最小强度500 psi (3.5 MPa)。CSA A 23.1.指定了压缩强度7.0 MPA.被认为是安全暴露在冻结。传统上，铸造打孔缸用于以某些间隔估计强度。成熟度方法越来越受到无线传感器技术的最新进步的普及。应避免在此期间的湿固化。

Covering – Curing Blankets w/o Insulation

用固化坯料覆盖在寒冷的季节期间广泛应用于建筑工地。从水泥的水合产生的热量通常足够了，对于许多情况而言，如果毯子应该适当使用。毯子应该留下几天。所需的绝缘值取决于混凝土的厚度，水泥的量和预期的寒冷。咨询ACI 306第7章有关保温的详细信息。

Site managers, and engineers are responsible to assess if concrete has reached desired strength. Temperature monitoring using infrared thermography from concrete surface or到期methodcan be used for better decision making.

---

这heat generated from hydration process should suffice in most cases, if appropriate insulating blankets of polyethylene sheets are used. Additional source of heat might be required based on area and temperature.

---

When covering with blankets, special attention should be given to the corners and edges of the slabs. These area often require further insulating layers. If covering can not keep the concrete temperature at desired levels, external heat source such as electric heating blankets, or hydronic heating pipes should be used.

加热的外壳

如果毯子没有提供足够的保护，或者天气极为冷（甚至在放置混凝土之前），则应使用加热的外壳。该技术包括封闭施工部位（例如，正在建造的楼层），并加热空间。应解决某些挑战：

1-二氧化碳 - 碳酸化

One common challenge with heated enclosure is the problem of carbonation. The carbon-dioxide produced by some of commercially available heaters increases the chance of carbonation of freshly placed concrete. This can lead to formation of a weak concrete layer which is often unacceptable. It is recommended to use heating systems that exhaust to the outside of the enclosure.

2- Rapid Drying / Uneven Heating

这use of heaters can result in very rapid drying of concrete, which will increase the chance of plastic shrinkage, and might lead to poor quality concrete (if water required for hydration process evaporates). It is recommended to move the location and direction of heat source for a more uniform heating pattern.

3- Fire

Special attention should be paid to the heaters that use propane gas.