透水混凝土蒸发阻力特征及其影响城市热岛的机制

Pervious pavements may hold water in its internal cavity for evaporative cooling and thus have long been thought as cool pavements to mitigate urban heat island (UHI). But pervious pavements may also aggravate the UHI because a porous pervious surface absorbs more solar radiation than dense pavement surface and because the evaporation may be negligible when the near-surface water depletes. Due to this negligible evaporation, it is difficult to adapt the existing experimental setup to correctly measure the evaporative weight loss at the late drying spell. In this project we develop a new, simple apparatus to measure the evaporation from pervious concrete with different water permeability, water retention, and water absorption and under different wind speed, air temperature, relative humidity, and base-drained conditions. We reveal how the evaporative resistance of the pervious concrete correlates to wind speed, air temperature, and relative humidity; to the water permeability and water absorptivity of pervious concrete; and to the water replenishment by artificial or natural precipitation. We will find an optimum pervious concrete mixture proportion that renders the minimum evaporative resistance. On the basis of the measured evaporative resistance, we will model the sensible heat of in-field pervious pavements and compare this sensible heat with the sensible heat releasing from conventional dense pavements. The comparison concludes how pervious concrete contributes to the UHI development and how to optimize pervious concrete to mitigate UHI. Our works will tie the theories of pervious pavement and urban thermal environment and lay technical foundations for adapting pervious pavements to mitigate the UHI.

透水路面或可将雨水保持于其孔隙中并通过蒸发降低路面温度因而被认为可降低城市热岛效应。但其水分蒸发由表向里逐渐减少，后期的蒸发量很小，且其表面反射率比普通混凝土低，透水混凝土可能加剧城市热岛效应。由于蒸发量很小，现有实验方法测量透水混凝土的即时蒸发量容易产生大误差。本项目设计新的实验设备测量不同透水率、吸水率、持水率的透水混凝土在不同气温、风速、湿度、基层排水条件的蒸发速率。揭示风速、气温、空气、基层排水条件影响透水混凝土蒸发阻力的规律，查清透水率、吸水率影响蒸发阻力的数学关系式，查明最小蒸发阻力对应的透水混凝土配合比。数值模拟室外(含雨水补给条件)具有不同吸水率、持水率、透水率、基层排水条件的透水路面传热、散热特征，揭示透水路面蒸发阻力变化特征、显热散失变化规律、影响城市热岛的机制。为完善路面工程与城市热环境的提供科学理论，为依局地气象构建合理透水路面减缓城市热岛效应提供技术支持。

透水路面可将径流排入土层缓解城市内涝，储存于孔隙中的水分蒸发后可降低城市热岛效应。然而蒸发阻力由表及里逐渐增大，后期蒸发量极小。主要研究内容包括：(1) 提出一种使用文丘里管测量多孔介质渗透系数的方法。在双对数图中，柱形单粒径骨料的渗透率和惯性阻力系数随骨料平均粒径的增大而线性增大。新方法有助于有效测试各种渗透率为10-8-10-4 m2的多孔介质(如砾石、团聚土和碎石)的渗透率和惯性阻力系数；(2) 为简化透水路面的水分蒸发传输过程，将透水路面水汽扩散对流过程简化为路面蒸发阻力变化过程。提出了一种用于透水路面蒸发速率的一维有限元模型，发现透水混凝土的蒸发速率主要受路面附近的有效水分(表面阻力)的影响，只有及时补水后透水路面才能降温，水分不足时其温度和普通路面一样甚至更高，该发现对开展冷却路面研究有现实意义；(3) 发明路基模型反照率的测量方法，运用一对已知反照率的参照板可准确测量小至1m2的有限平面的反照率。该成果为进一步量化路基路面真实太阳辐射反照率与吸收量提供了理论基础和计算模型；(4) 发现提高路基边坡反照率可有效降低路基阴阳坡效应。揭示了路基表面太阳辐射吸收规律，为冷却冻土路基提供新思路；(5) 发明一种可测量路面、素土、草地等非均质有限平面反照率的新方法，无需任何参照板对测量结果进行校准。该方法可快速、准确、不间断地测量不可接触表面、斜坡面等表面的反照率； (6) 发现路面日最高温度与日温度振幅随反照率、随热阻的倒数线性降低。该成果统一了路面温度的理论与经验公式，解决了冷却路面路基热传递的核心问题。