Highway Gantries: The "Material Contest" Between Steel and Aluminum Alloys

When driving at high speed on the highway, the gantries overhead always catch our eye inadvertently—whether they carry ETC toll collection equipment, suspend speed limit signs, or support high-definition surveillance cameras, they are the "airborne guardians" ensuring orderly highway traffic. These seemingly similar gantries, however, hide great knowledge in material selection. As the mainstream options, steel and aluminum alloys each have unique advantages and applicable scenarios. Today, we will decode the "personality traits" of these two materials.

1. Material Basics: Core Differences Between Steel and Aluminum Alloys

The material selection for highway gantries essentially boils down to a trade-off among four core factors: "strength, weight, weather resistance, and cost". Steel and aluminum alloys perform vastly differently in these dimensions, which directly defines their application boundaries.

As a traditional structural material, steel inherently boasts "sturdy" characteristics due to its nature as an iron-carbon alloy—with a density of approximately 7.85g/cm³ and a tensile strength generally above 235MPa (even up to 690MPa for high-quality steel), it is like a "strongman" in construction. On the other hand, aluminum alloys, based on aluminum and modified with alloying elements such as magnesium and silicon, have a density of only about 2.7g/cm³, less than one-third that of steel, and a tensile strength ranging from 100MPa to 300MPa, making them "lightweight powerhouses". These differences in basic properties pave distinct application paths for them.

2. Steel Gantries: Stalwarts for Heavy Loads and Harsh Environments

In the application of highway gantries, steel often appears in scenarios with the "most arduous tasks", thanks to its three prominent advantages.

First is its superior load-bearing capacity. When a gantry needs to simultaneously mount "heavy-duty equipment" such as ETC antennas, toll amount displays, large variable message signs (over 3 meters in size), and multiple sets of monitoring devices, the high-strength characteristic of steel comes into play. For example, in the reconstruction projects of former provincial boundary toll stations on highways, some gantries need to carry the entire set of composite toll collection systems, and the main structure of steel welded together can easily cope with long-term loads and avoid deformation and cracking.

Second is its ability to withstand extreme environments. After anti-corrosion treatments such as hot-dip galvanization and plastic spraying, steel can resist freeze-thaw cycles in the frigid northeastern regions with temperatures as low as -40℃ and endure strong sand erosion in the Gobi Desert of northwestern China. In contrast, low-temperature environments may reduce the toughness of aluminum alloys, while steel has more stable low-temperature mechanical properties. Hence, steel gantries account for a significantly higher proportion in the high-latitude highway sections in northern China.

Third is its cost advantage. For gantries with large spans (over 10 meters) and heavy loads, the raw material and processing costs of steel are lower than those of aluminum alloys. Particularly in the mass construction of main highway lines, steel can effectively control the total project cost.

However, steel also has shortcomings: its heavy weight requires large equipment (such as cranes) for transportation and installation, resulting in a relatively long construction period; moreover, if anti-corrosion treatment is not in place, it is prone to rust in humid environments for a long time (such as the plum rain areas in southern China and coastal highways), necessitating regular maintenance.

3. Aluminum Alloy Gantries: A New Option for Lightweight and Convenience

With the maturity of aluminum alloy processing technology, this lightweight material has been increasingly widely used in highway gantries, especially suitable for scenarios requiring "flexibility, efficiency, and low maintenance". Its core advantages focus on three aspects.

First is the construction convenience brought by lightweight. The weight of aluminum alloy gantries is only 30%-40% of that of steel gantries of the same specification. No large trucks are needed for transportation, and small hoisting equipment can complete the installation. Even in some narrow construction sections (such as mountain highways), assembly can be achieved with manual assistance. This not only shortens the construction period but also reduces the impact on highway traffic—installing a steel gantry may require closing half of the lane for several hours, while the core operation of an aluminum alloy gantry can often be completed in 1-2 hours.

Second is its natural corrosion resistance. Aluminum forms a dense oxide film in the air, which can prevent further corrosion of the internal material. It can be used for a long time in coastal highways and humid southern areas without complex anti-corrosion treatments. For instance, in the highway sections of coastal provinces such as Guangdong and Fujian, the service life of aluminum alloy gantries can reach 15-20 years, while steel gantries without special anti-corrosion treatment may need component replacement after 10 years.

Third is its aesthetics and environmental friendliness. The surface of aluminum alloy can present various appearances such as silver-white and bronze through anodizing treatment, which can blend more harmoniously with the natural environment in highways around scenic spots (such as the highways leading to Huangshan and Zhangjiajie). Meanwhile, aluminum alloy can be 100% recycled and reused, conforming to the development concept of "green transportation".

The limitations of aluminum alloys are also obvious: their load-bearing capacity is relatively weak, making them unsuitable for mounting large equipment; the raw material price is high, so the cost advantage disappears in scenarios with large spans and heavy loads; under high-temperature exposure, the thermal expansion coefficient of aluminum alloy is larger than that of steel, requiring more expansion space reserved in structural design.