Concrete walls are a passive defensive barrier used by both the GDI and Nod during the First Tiberium War, capable of preventing infantry and ground vehicles from going through. Simple, reasonably priced, and vastly superior to sandbags and chain link fences, concrete was the main static defense for many bases. 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.
Though slightly more expensive than sandbags and fences, 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 or barbed wire.
Photographic image from DOS manual
Photographic image from Brady Games' strategy guide