Concrete resembles masonry in its resistance to compression and its comparative weakness in tension. So it is not surprising that that some early, and some not so early, concrete bridges were built like masonry ones. That's not a bad strategy; if you don't know what to do, do something that has worked before. The Iron Bridge in Coalbrookdale was made like that; some of its detailing owes more to carpentry than to engineering. To innovate in material presents difficulties, as does innovating in construction. Perhaps doing both at once seemed to risky. Sometimes, however, one cannot be done without the other.

Concrete has one huge advantage over masonry - you can pour it into moulds made with wooden shuttering, and make any shape you like, subject to the cost of making the shuttering. No more do stone blocks have to be cut to shape and laboriously carried to their final resting place. And concrete can be modified in two ways that are not practical with masonry - it can contain steel reinforcing bars; and it can contain steel pre-stressing wires. It is the inclusion of steel that has revolutionized the use of concrete and made it ubiquitous.

The moulding of concrete confers on it a feature that no previous structural material had possessed - the ability to take on its surface the imprint of any shape that can be carved into the shuttering, from an aardvark to a zebu. The appearance of most engineering structures in other materials is largely determined by structural requirements, and the larger or more extreme the system, the smaller the possibility for adornment. Some bridges, after having been designed for extraneous covering, have remained bare. Such are the Clifton suspension bridge and the George Washington bridge. Tower Bridge in London does have a coat of masonry, which serves only to emphasise its already bizarre appearance, with trussed suspension chains which would resist wind strengths that never occur there.

This is not a trivial question: on the contrary, it is one of great importance. Because concrete can be poured in huge slabs, it sometimes is, and many cities contain vast areas of bleak, bare concrete, bad enough in bright sunshine, but unbearable in the grey of northern winter.  People have to live with the work of architects and engineers. Does this mean that artificial decoration should be added to concrete? If some kind of pattern is moulded in, the effect when repeated hundreds of times in a concrete housing area is about as depressing as the bareness it was intended to cure. So what is the answer? What do you think?

The Palisades Parkway in New York State is furnished with some concrete bridges which have been dressed up to look like masonry, and have attracted criticism for this reason. This road was constructed from 1947 to 1958, so concrete was already well established. The builders clearly made a tremendous effort to create structures that did not look too incongruous with the surroundings.

The very versatility of a moulded material presents a problem - just what do we do with it? In stone masonry, you can provide a restrained form of decoration be making alternate blocks project slightly, or by using several different sizes. In brick you can also use different colours of brick. These methods, used sparingly, can seem perfectly natural, and they can breathe life into large monotonous areas. But with concrete, any such surface treatment is likely to seem arbitrary, because there is no scale of size for reference. In the first picture below, see how many details you can find where the builder has enlivened the structure by simple means.

Let's return to structural matters and the difference between concrete and masonry. With masonry, all you can do with the spandrels is to fill them in with masonry, possibly with a hole or two, or to lighten them with subsidiary arches, as in these examples. With masonry, you cannot make beams, and you cannot make cantilevers.

But you can with concrete. Here are some examples in which the deck is wholly or in part in the form of a beam. There was no need to use subsidiary arches in the spandrels.