Concrete walls are a constructable passive defensive barrier in Command & Conquer: Red Alert.
Simply put, the concrete wall has seen service as a defensive barricade for almost a century. Simple, reasonably priced, and vastly superior to sandbags, concrete was the main static defense for many bases during the Second World War. They are vulnerable to explosive and cannon based attack, but their thickness barred C4 from effective use against them.
Derived from civilian standard design, the standard concrete walls of most of the wars were fairly tall, preventing enemy infantry from climbing over and stopping direct fire weapons, such as tank cannons, from hitting a target behind it. Steel rebar inserted during construction give it the strength to withstand multiple hits from a tank assault. During the First Tiberium War, these were virtually the ultimate in defensive barricades, being able to halt the progress of any unit, even tanks, for some time.
During the Second World War, concrete was unrivaled as a barrier material. Though slightly more expensive than its competitors, concrete could stop tank shells and halt the progress of any unit.
Despite being able to stop direct-fire weapons, artillery and other indirect-fire weapons could pass over the walls and strike their targets unhindered. Even more so, aircraft were totally unaffected by the presence or absence of walls. Also, despite its superior defensive power, concrete was more expensive than sandbags.