Asphalt is a ubiquitous paving material, forming the surfaces of roads, parking lots, and playgrounds worldwide. A central characteristic of asphalt is its capacity to absorb and retain heat, leading to significantly elevated surface temperatures. This is largely attributed to its dark coloration, which efficiently absorbs solar radiation. This property makes asphalt a significant contributor to heat challenges in paved areas.
Factors Influencing Asphalt Temperature
Asphalt’s elevated temperatures stem from several factors. Solar radiation serves as the primary heat source, with asphalt’s dark color playing a significant role in absorbing this energy. Darker materials, like traditional asphalt, absorb a greater percentage of sunlight—up to 95%—compared to lighter surfaces, converting this light energy directly into heat.
Ambient air temperature also contributes to the overall heating, but it is not the sole determinant of asphalt’s surface temperature. Asphalt is a dense material that efficiently absorbs and stores heat from the sun, allowing its surface to become considerably hotter than the surrounding air. Thermal conductivity is relatively low for asphalt, meaning it absorbs heat faster than it can release it. This property contributes to heat buildup within the material.
Asphalt’s specific heat capacity influences how much energy it can store for a given temperature increase. Once heated, asphalt retains high temperatures for extended periods. While wind speed can offer some cooling by facilitating convective heat loss, the material’s intrinsic properties ensure substantial heat retention.
Measuring Asphalt Temperatures
Measuring asphalt temperatures involves various tools and techniques. Infrared thermometers, often called thermal guns, are widely used for rapid, non-contact surface temperature readings. While effective for surface checks, they may not accurately reflect internal asphalt temperatures. For more precise internal monitoring, especially during paving, contact thermometers with probes, such as thermocouple thermometers, are employed.
Embedded sensors offer a more advanced method, allowing for continuous, real-time data collection at various depths within the pavement structure. These sensors provide valuable insights into the material’s thermal behavior.
Asphalt surface temperatures routinely exceed ambient air temperatures. For example, on a day with an air temperature of 95°F (35°C), blacktop can reach 140°F (60°C). In hotter climates, asphalt surface temperatures have been recorded up to 166°F (74.4°C) when the ambient temperature was 120°F (49°C).
Impacts of High Asphalt Temperatures
High asphalt temperatures present several concerns for public safety and urban environments. One significant danger is the risk of burns to skin, especially for children and pets. When air temperatures reach 77°F (25°C), asphalt can soar to 125°F (52°C), and at 87°F (30.5°C), it can reach 143°F (61.5°C). Burns can occur within a minute of contact with surfaces around 125°F (52°C). Pets, whose paw pads are vulnerable, can suffer severe burns and blisters, making it crucial for owners to check pavement temperatures before walks.
High asphalt temperatures contribute significantly to the urban heat island effect, a phenomenon where urban areas become warmer than surrounding rural areas. Asphalt’s dark color and ability to absorb and retain heat mean it emits this stored heat into the atmosphere, raising ambient temperatures in cities. This elevated heat increases energy consumption for cooling and exacerbates air pollution, negatively impacting human health.
High temperatures also affect vehicles and the integrity of the asphalt itself. Hot asphalt can cause tires to wear out faster, as the heat from the road transfers to the tires, increasing their internal temperature and making the rubber more pliable. This accelerated breakdown of rubber compounds can lead to quicker tread wear. Additionally, extreme heat can cause asphalt to soften, leading to issues like rutting, where depressions form in the pavement under heavy traffic, and thermal cracking due to expansion and contraction.
Strategies to Reduce Asphalt Heat
To mitigate the heat challenges posed by asphalt, various strategies are being developed. “Cool pavement” technologies aim to reduce surface temperatures by increasing reflectivity or promoting evaporative cooling. Reflective coatings or lighter-colored asphalt mixes reflect more solar radiation, preventing heat absorption. These materials can significantly lower surface temperatures, with some studies showing a reduction of 10 to 16°F (5 to 9°C) compared to conventional asphalt.
Shade from trees or structures offers a natural way to reduce direct sun exposure. Trees lower surface temperatures by blocking sunlight and through evapotranspiration. Shaded surfaces can be 20 to 45°F (11 to 25°C) cooler than unshaded areas.
Permeable pavements, allowing water to seep through, also contribute to cooling. Water stored within these pavements can evaporate, absorbing heat and lowering surface temperatures through evaporative cooling.