Pervious Concrete Freeze-Thaw

透水混凝土是当今土地开发的热点问题之一. As owners, architects, land developers, 具体的专业人士也会熟悉它的好处, the interest in pervious concrete continues to grow. The use of pervious concrete pavements provides a solution to new requirements under Environmental Protection Agency regulations that call for decreasing the amount of surface water runoff and initially treating the runoff.

Pervious concrete is made of cementitious materials, water, admixtures, and narrowly graded coarse aggregate. 混合物中只使用很少或不使用细骨料. With just enough cement paste to coat the aggregate, a system of interconnected voids (15 to 35 percent) is created resulting in a highly permeable concrete that drains very quickly. 让水直接通过混凝土, 地表水径流量显著减少. It can also be used as part of a system to reduce the level of pollution contained in storm water that is captured in the pervious pavement.


Pervious concrete bike path in snow. 透水混凝土路面自行车道莱克伍德公园,莱克伍德,OH. (照片由柯林伍德混凝土公司提供,2006年2月).

Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, some are hesitant to use pervious concrete pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage. 任何混凝土的抗冻性和融性都取决于其渗透性, the degree of saturation, the amount of freezable water, the rate of freezing, 以及从膏体中任何一点到能安全形成冰的自由表面的平均最大距离. 在大多数应用中,冻结的速度由当地气候决定.

Entrained air may help protect the paste as well. Perhaps the most important aspect in designing pervious concrete pavements for freeze-thaw areas is avoiding, or at least limiting, saturation, 尤其是在一年中可能会结冰的时候. It is possible to design pervious concrete pavements to control the degree of saturation and the average maximum distance to a free surface. 合理的基板设计和制备是雨水提取的关键, ice, 并确保路面的融雪和适当的排水. Replacing as little as 7 percent of the coarse aggregate with fine aggregate increases the freeze-thaw resistance; however, 空缺会减少几个百分点(Kevern 2006和Mata 2008).

In addition, the paste (or mortar) should be protected by using air-entraining admixtures to create a sufficient air-void system. Kevern, Wang, and Schaefer (2008) found that the coarse aggregate properties play a large role in providing freeze-thaw durability, with absorption values below 2.5 percent being of greatest impact. The National Ready Mixed Concrete Association (NRMCA 2004) has developed guidelines for using pervious concrete in areas prone to freeze-thaw conditions.

NRMCA Recommendations

Dry Freeze and Hard Dry Freeze Dry freeze areas are those parts of the country that undergo a number of freeze-thaw cycles (15+) annually in which there is little precipitation during the winter. If the ground stays frozen as a result of a long continuous period of average daily temperatures below freezing, the area is referred to as hard dry freeze area. 因为透水混凝土不太可能完全饱和在这种环境, 透水混凝土的成功性能不需要特别的预防措施. However, a 4- to 8-inch thick layer of clean aggregate base below the pervious concrete is recommended as an additional storage for the water. 美国西部海拔较高的许多地区都属于这一类.

Wet Freeze

This includes areas of the country that undergo a number of freeze-thaw cycles annually (15+) and there is precipitation during the winter. 因为地面不会冻结很长时间, 透水混凝土不太可能完全饱和. 透水混凝土的成功性能不需要特别的预防措施, 但建议在透水混凝土下面铺一层4到8英寸厚的净骨料基层. 美国东部的中部地区就属于这一类.

Hard Wet Freeze

Certain wet freeze areas where the ground stays frozen as a result of a long continuous period of average daily temperatures below freezing are referred to as hard wet freeze areas. These areas may have situations where the pervious concrete becomes fully saturated because frozen soil will have very low water permeability. The frost penetration depth (depth at which the temperature is at 32 degrees Fahrenheit varies throughout the country.

 To design the pervious concrete pavement for freeze-thaw resistance the following is suggested by NRMCA : 

1. Calculate the frost penetration depth in your area. In the Washington, D.C., area, for example, it is about 30 inches. 

2. Calculate 65 percent of that. The Federal Aviation Administration (FAA) says that the top 65 percent should contain non-frost-susceptible materials and the bottom 35 percent may be in frost susceptible sub grade. 应该注意的是,美国联邦航空局使用65%的限制来防止冻胀. In this case, the key factor is water infiltration. This is about 19.5 inches for the 30 inches frost penetration depth. 

3. 提供透水混凝土路面加骨料基层等于计算的数量. For a 19.5英寸的计算,6英寸厚的透水混凝土路面超过13.5-inch thick aggregate base would be sufficient. 骨料基层必须由清洁的、排水的、小于1的级配骨料基层组成.5 percent finer than 0.5 mm. If the frost depth is very high, for example 100 inches in North Dakota, 可采取其他措施,以减少完全饱和透水混凝土路面的机会. 一个穿孔的PVC管可以放置在骨料基捕获所有的水,并让它排出. Pervious concrete in a freeze-thaw environment should always be air-entrained to provide additional protection.

High Ground Water Table

Pervious concrete is not recommended in freeze-thaw environments where the ground water table rises to a level less than 3 feet from the top of the surface or where substantial moisture can flow from higher ground.

Deicing

化学用途用于保持不结冰的除冰化学品, safe pavement surfaces for dense pavements may be used on pervious pavements; however, 在许多情况下,可能不需要使用除冰剂来保持防滑表面. Snow fall followed by thawing temperatures allows snow melt to pass through the pavement so rapidly that liquid water is not available at the pavement surface to be refrozen as an ice coating. 路面不结冰有助于行人和车辆的安全. With appropriate plowing and limited use of deicers, moisture is removed from the pavement surface, 再次防止水分在表面结冰,造成结冰的情况. As any melting from the use of deicers occurs, the melt passes downward into the pavement and in many cases leaves behind some un-dissolved deicer making it available to future snow and ice events. 据报道,化学除冰剂的用量减少了70% (Houle 2008)。. This leads to reduced use of deicing chemicals and clear safe pavement surfaces with minimum cost for winter maintenance.

Summary

Colored pervious concrete walkway Example application of pervious concrete: colored pervious concrete walkway in Bainbridege Island, WA. (IMG15586)透水混凝土路面的好处是众所周知的, but concerns over the freeze-thaw resistance may prevent many designers from using pervious concrete in colder climates. There have been several pervious concrete pavement projects in dry and wet freeze areas demonstrating good field performance over several years. 透水混凝土路面抗冻融性能的研究正在美国各地进行. Pervious pavements should be placed by experienced installers and the structure and surrounding details should be designed to accommodate the anticipated water flow and drainage requirements.

References

Freeze Thaw Resistance of Pervious Concrete (2004) National Ready Mixed Concrete Association, 17 pages
There have been several pervious concrete pavement projects in dry and wet freeze areas demonstrating good field performance over several years. Recommendations for successful performance of pervious concrete pavements under the various freeze-thaw conditions have been provided. 在硬湿冻地区,透水混凝土路面的性能研究经验有限. Therefore, in such areas utmost care must be taken. Pervious pavements should be placed by an experienced installer and the pavement structure and surrounding details should be designed to accommodate the anticipated water flow and drainage requirements.

Houle, Kristopher M., 透水路面的冬季性能评价, Masters Thesis, University of New Hampshire, September 2008, 142 pages.  

NRMCA, Karthik Obla,个人通讯,2006年5月18日.

Kevern, John Tristan, 寒冷气候条件下硅酸盐水泥透水混凝土配合比设计研究,硕士论文,爱荷华州立大学,艾姆斯,爱荷华,2006,155页.

Kevern, John T.; Wang, Kejin, and Schaefer, Vernon R., 粗骨料对透水混凝土冻融耐久性的影响, SN3063,冰球突破网站首页,斯科基,伊利诺斯州,美国,2008,29页.

Mata, Luis Alexander, 透水混凝土路面系统的沉降,北卡罗来纳州立大学博士学位论文,罗利,北卡罗来纳州,2008. Also available as 冰球突破 SN3104.

Mindess, S.; Young, J. F.; and Darwin, D.,混凝土,普伦蒂斯大厅,上马鞍河,新泽西州,2003年. NRMCA, Freeze-Thaw Resistance of Pervious Concrete,国家预拌混凝土协会,银泉,马里兰,2004,17页.

Portland Cement Association, 透水混凝土:水文设计与资源 (CD), CD063, Skokie, Illinois, 2006. Portland Cement Association, LEED™认证的东亚特兰大图书馆透水混凝土(视频), CD067, Skokie, Illinois, 2006.

Storm Water Phase II Final Rule: An Overview, EPA 833-F-00-001, Fact Sheet 1.0,美国环境保护署,水办公室,2000年1月,4页. 网址:东南水泥协会,透水混凝土路面网站.

Tennis, P. D., Leming, M. L., and Akers, D. J., Pervious Concrete Pavements, EB302, Portland Cement Association, Skokie, Illinois, and National Ready Mix Concrete Association, Silver Spring, Maryland, 2004, 25 pages.