In the first post in this series we differentiated between tension and compression and showed how an arch operates in compression. In this post, we will examine the slab bridge and how it operates.
Introducing the Slab Bridge….
The ubiquitous, ever popular, easily calculated, fast and easily built slab bridge is an interesting study in its own right. The slab bridge uses both compression and tension.
Why? Consider a slab with a weight placed in the center of the span (or better yet build a tiny slab bridge using books to support a thin piece of wood). If you push down on the center of the span, the slab bends. For the slab to bend, there must be both compression and tension. The bottom of the bent slab is being stretched, pulled apart. The top of the slab, however, is being pressed together. (If you build a miniature model slab bridge, if you examine it closely while pushing down on it you can see this. If you start to break it, it will become even more apparent.) Thus, the top half of the slab operates in compression, while the bottom operates under tension. When you let up on the slab, it springs back up, all forces save those induced from self-weight being removed. This is good, for if every time a load was applied the slab sunk a little more without springing back to shape we wouldn’t have a bridge for long.
Stone and unreinforced slab bridges are possible — for small spans. And yes, unreinforced concrete slab culverts have been successfully built, to judge from the appearances of some culverts we have found in southern Kansas. (We were actually looking for stone arch culverts, but have seen all sorts of other strange structures as well.) As mentioned in our previous post in this series, concrete is not terribly strong under tension. At least, it isn’t without some help. A stone or unreinforced concrete slab bridge is, by default, rather limited in span, as it is not terribly strong, being weak in tension. However, there is a easy way to strengthen concrete: rebar. Rebar is simply steel used to reinforce concrete; usually the steel has some texture, however, to ensure the concrete bonds around it nicely. By embedding the steel in the concrete, you can add some serious tensile strength to the concrete. Then you end up with, essentially, a block of stone that is still impervious to the elements, solid and strong, but has good tensile strength. Or almost.
The Reinforced Concrete Problem
The problem with reinforced concrete is simple. A straight-up slab of concrete (no reinforcement) is not very good in tension as stated before. Like most things weak in tension, it can bend but relatively little before it breaks. The steel reinforcement in reinforced concrete, however, has a lot of “give.”
When a basic reinforced concrete slab is placed under load, it bends. It may not bend much, but it bends. Under a serious load, it cracks. It may not crack much, but it cracks. It has to crack for the steel reinforcement to take up much of the tension. But once the cracks form, something happens. The waterproofing of the cement is lost. This results in rebar deterioration. And, over time, the now cracked cement slowly disintegrates.