The Influence of Building Materials

he traditional building materials for bridges are stone, timber and steel, and more recently reinforced and prestressed concrete. For special elements aluminium and its alloys and some types of plastics are used. These materials have different qualities of strength, workability, durability and resistance against corrosion. They differ also in their structure, texture and colour or in the possibilities of surface treatment with differing texture and colour.

For bridges one should use that material which results in the best bridge regarding shape, technical quality, economics and compatibility with the environment.

Natural Stone

he great old bridges of the Etruscans, the Romans, the Fratres Pontifices of the Middle Ages (since about 1100) and of later master builders were built with stone masonry. The arches and piers have lasted for thousands of years when hard stone was used and the foundations constructed on firm ground. With stone one can build bridges which are both beautiful, durable and of large span (up to 150 m). Unfortunately, stone bridges have become very expensive, if considered solely from the point of view of construction costs.

Over a long period, however, stone bridges, which are well designed and well built, might perhaps turn out be the cheapest, because they are long-lasting and need almost no maintenance over centuries unless attacked by extreme air pollution. Stone is nowadays usually confined to the surfaces, the stones being preset or fixed as facing for abutments, piers or arches. Of course, sound weather-resisting stone must be chosen, and fundamental rock like granite, gneiss, porphyry, diabas or crystallized limestone are especially suitable. Caution is necessary with sandstones, as only silicious sandstone is durable.

In Western Germany basalt-lava from the Eifel Mountains is popular. In choosing the stone one should respect any local experience gained from old buildings and bridges. Stone is worked upon in different ways, depending upon the direction of the natural strata occurring in the quarry and on the requirements in the bridge Very different effects can be produced with stone by the choice of the type of masonry, the height of the courses, the proportion of the stones (length to height), the arrangement of the joints, the surface treatment etc., and especially the overall scale.

The choice of colours of the stone is also relevant. Granite of a uniform grey colour and sawn surface can look as dull as simple plain concrete. A harmonious mixture of different colours and slightly embossed surfaces can look very lively, even when the masonry areas are extensive. Surfaces can also be enlivened by bright or dark joint-filling. The sizes of the stone blocks and the roughness of their surfaces must be harmonized with the size of the structure, the abutments, the piers etc. Coarse embossing does not suit a small pier only 1 m thick and 5 m high, but large sized ashlar masonry is suitable for large arch bridges such as the Saalebrucke Jena or the Lahntalbrucke Limburg. Granite masonry was preferred for piers of bridges across the River Rhine, because it resists erosion by sandy water much better than the hardest concrete.

Artificial Stones, Clinker and Brick

mongst the artificial stones, clinker and hard-burned brick are used in bridges both as liners and for bearing vaults. They were often used in northern Germany, the Netherlands, Belgium and Denmark, because there is no suitable natural stone available. The warm colours of clinker or brick blend happily into the landscape. Also in an urban environment, they are preferable to plain concrete, if brick is the regional construction material.

The sizes of these stones are standardized, and one can only choose between different types of joint arrangements. Small differences in colour and a pleasing treatment of the joints can embellish the surfaces. Finally, one can also use split concrete blocks for facing. If the concrete is made with colourful aggregates, which break when being split, then masonry-work produced with these artificial blocks can also look good - similar to masonry of natural conglomerates, which are in fact nothing else but natural concrete.

Reinforced and Prestressed Concrete

oncrete is an all-round construction material. Almost every building contains some concrete, but its questionable application in certain buildings-for example in its use in the style of brutalism - has brought it into discredit. Its dull grey colour has contributed to the fact that the word concrete has become a synonym for ugly. In the field of bridges, concrete deserves a more favourable judgement.

Not all concrete bridges have turned out to be beauties, but pleasing bridges can be built with concrete if one knows the art. Concrete is poured into forms as a stiff but workable mix, and it can be given any shape; this is an advantage and a danger. The construction of good durable concrete requires special know-how - which the bridge engineer is assumed to have.

Good concrete attains high compressive strength and resistance against most natural attacks though not against de-icing saltwater, or CO2 and SO2 in polluted air. However, its tensile strength is low, and the use of concrete alone is therefore limited to structures which are only subject to compressive stresses. But tensile stresses also occur in abutments and piers due to earth pressure, wind, breaking forces and to internal temperature gradients.

To resist these tensile forces, steel bars must be embedded in the concrete, the so-called reinforcing bars, and this has lead to the development of reinforced concrete. The steel bars only really come into play after the concrete cracks under tensile stresses. If the reinforcing bars are correctly designed and placed, then these cracks remain as fine "hair cracks" and are harmless. A second method of resisting tensile forces in concrete structures is by prestressing.

The zones of concrete girders which are under tensile stress due to loads or other actions are first put under compression - are pre-compressed - so that the tensile forces must first reduce these compressive stresses before actual tensile stresses come into being. This pre-compression is obtained by tensioning high strength steel bars or wire bundles, which are in ducts inside the concrete girder.

Tensioning elongates the steel bars and they are anchored in this state at the ends of he girder, transferring this tensioning force as a compressive force onto the girder. These girders, prestressed with 'active steel" (prestressing steel) are in addition reinforced with "passive steel" (non-stressed steel bars) for various reasons. Prestressed concrete revolutionized the design and construction of bridges in the fifties. With prestressed concrete, beams could be made more slender and span considerably greater distances than with reinforced concrete.

Prestressed concrete - if correctly designed - also has a high fatigue strength under the heaviest traffic loads. Prestressed concrete bridges soon became much cheaper than steel bridges, and they need almost no maintenance - again assuming that they are well designed and constructed and not exposed to de-icing salt. So as from the fifties prestressed concrete came well to the fore in the design of bridges.

All types of structures can be built with reinforced and prestressed concrete: columns, piers, walls, slabs, beams, arches, frames, even suspended structures and of course shells and folded plates. In bridge building, concrete beams and arches predominate. The shaping of concrete is usually governed by the wish to use formwork which is simple to make. Plain surfaces, parallel edges and constant thickness are preferred. This gives a stiff appearance to concrete bridges, and avoiding this is one task of good aesthetic design.

The extra cost for one-way curved surfaces, for tapering piers, for varying depth of beams or arch ribs is as a rule comparatively small. Therefore one should not hesitate to choose such divergences from the most primitive and simple forms in order to improve appearance.

There is one great disadvantage to concrete as it emerges from the forms: the inexpressive, dull grey colour of the cement skin. The surfaces frequently show stains, irregular streaks from placing the concrete in varying layers, and pores or even cavities from deficient compaction, which ire then patched more or less successfully. These deficiencies have lead to a widespread aversion to concrete, As well as to efforts for improvement. Some of the methods used to achieve a good concrete finish in buildings, like profiles and patterns on the formwork, ribs or accentuated timber veins etc are not generally suitable for bridges.

The best effect is obtained by bush hammering as was usual between 1934 and 1945 for the bridges of the German autobahn system. The concrete coating of the rein- forcement is increased by 10 to 15 mm, so that a thin layer together with the cement skin can be taken off by fine or coarse bush hammering. The aggregate is then exposed with its structure and colour.

The protection of the embedded steel is not damaged, because the exterior cement skin is in any case the worst part of concrete. It is very porous, because mixing water collects at the forms by vibrating the concrete, and it is the porosity of the cement skin which makes it so susceptible to collecting the dirt of polluted air. With bush hammering one can adapt the degree of roughness to the size of the surfaces. Piers of viaducts, for example, were chiselled very roughly, taking off pieces 20 to 30 mm in depth by oblique chisel work.

The colour can be favourably influenced by the choice of coloured aggregates like red porphyry or yellow limestone. Such surfaces age as well as natural stone masonry, and they retain their texture over a long period of time. The cement skin can also be washed off by special means after the concrete has hardened - such "exposed aggregate" surfaces can look pleasing, depending on the colour and size of the aggregates. Bush hammering was given up after about 1950 due to the high labour cost. At that time suitable machines were not yet available, but with modern machinery this treatment should now be taken up again to embellish concrete surfaces.

Another possibility is colouring the concrete it has been well developed during the last decade. By the use of mineral colour pigments natural warm tones can be attained - earthy colours with tones of ochre, reddish-brown sepia. umber, greyish-green, slate-grey. Dark toned piers of a viaduct often look better in the landscape than with a light grey colour. Bright coloured concrete-with white cement-can for example be chosen to emphasize a fascia beam.

Fritz Leonhardt has often recommended the painting of bridges in the same way that steel bridges are painted for corrosion protection. At the same time the dreary grey of normal concrete is converted into a harmonious colourful statement. For painting, soft colours should again be chosen and not bright loud colours. Before painting, the porous cement skin must be removed, so that the paint will not peel off later.

Mineral colours, especially those with fluor- or silicious compounds, can also give an additional protection to the concrete. The colourfilm must be hygroscopic, so that it does not prevent the change of moisture content in the concrete. If the choice of colour and type of paint is based on the most up-to-date information, then these paints can last long and keep their colour like the paintwork of many old houses and churches, particularly in the Alps, which is often more than 200 years old and still beautiful. Colour painting of concrete bridges has already been used in several places. A most striking example is that of the long bridges along the riverbanks in Brisbane, Australia.

Steel and Aluminium

mongst bridge materials steel has the highest and most favourable strength qualities, and it is therefore suitable for the most daring bridges with the longest spans. Normal building steel has compressive and tensile strengths of 370 N/mm2, about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks, because it begins to yield above a certain stress level. This yield strength is used as the first term in standard quality terms.

For bridges high strength steel is often preferred. The higher the strength, the smaller the proportional difference between the yield strength and the tensile strength, and this means that high strength steels are not as ductile as those with normal strength.

Nor does fatigue strength rise in proportion to the tensile strength. It is therefore necessary to have a profound knowledge of the behaviour of these special steels before using them. For building purposes, steel is fabricated in the form of plates (6 to 80mm thick) by means of rolling when red hot. For bearings and some other items, cast steel is used. For members under tension only, like ropes or cables, there are special steels, processed in different ways which allow us to build bold suspension or cable-stayed bridges.

The high strengths of steel allow small cross-sections of beams or girders and therefore a low dead load of the structure. It was thus possible to develop the light-weight "orthotropic plate" steel decks for roadways, which have now become common with an asphalt wearing course, 60 to 80 mm thick.

The pioneers of this orthotropic plate construction called it by the less mysterious and less scientific name "stiffened steel slabs". Plain steel plate, stiffened by cells or ribs, forms the chord of both the transverse cross girders and the longitudinal main-girders. Simultaneously it acts as a wind girder. This bridge deck owes its successful application mainly to mechanized welding, which is now in general use and which has greatly influenced the design of steel bridges.

So plate girder construction now prevails, in which large thin steel plates must be stiffened against buckling. Previously, vertical stiffeners were placed by preference on the outer faces; longitudinal stiffeners were then arranged on the inside.

Today all stiffeners are placed on this inside so as to achieve a smooth outer surface allowing no accumulation of dust or dirt deposits that retain humidity and promote corrosion - the "Achilles heel" of steel structures. Modern steel girder bridges now hardly differ from prestressed concrete bridges in their external appearance - except perhaps in their colour. This is perhaps regrettable, because stiffeners on the outside enliven the plate-faces, give scale and make the girder look less heavy. In addition to plate girders, trusses also take full advantage of the material properties of steel. Very delicate looking bridges can be built by joining slender steel sections together to form a truss.

Again welding has improved the potential for good form, because hollow sections can be fabricated and joined without the use of big gusset plates. In this way smooth looking trusses arise without the "unrest" which occurs by joining two or four profiles of rolled section with lattice or plates. Steel must be protected against corrosion and this is usually done by applying a protective paint to the bare steel surface. Painting of normal steels is technically necessary and can be used for colour design of the bridge.

The choice of colours is an important feature for achieving good appearance. There are steels which do not corrode in a normal environment (the stainless steels V2A and V4A to DIN 17440), but are so expensive that they are used only for components that are either particularly susceptible to the attacks of corrosion or that are very inaccessible.

From the USA came Tentor steel, alloyed with copper, its 'first corrosion layer being said to protect it against further corrosion. This protective rust has a warm sepia-toned colour which looks fine in open country. This type of protection, however, does not last in polluted air and the corrosion continues. For steel bridges, good use should be made of the technical necessity of protecting the steel with paint to improve appearance and to achieve harmonious integration of the structure within the landscape.

Aluminium was occasionally used for bridges and the same form was used as for steel girders. Aluminium profiles are fabricated by the extrusion process which allows many varied hollow shapes to be formed, so that aluminum structures can be more elegant than those of steel. Aluminium profiles are popular for bridge parapets because they need no protective paint.

Timber

imber has favourable qualities of strength for resisting compression, tension and bending. Rough tree trunks or sawn timber beams have been used since primitive times for beam bridges; raking frames and arches soon allowed larger spans. The Swiss carpenters, the brothers Grubennann reached a 100 m span with the timber bridge across the River Rhine near Schaffhausen. Timber should be protected against rain and therefore covered bridges with a roof and sidewalls with windows evolved, and many of these are rightly preserved in the Alpine countries, testifying to the high standard of their craftsmanship.

Many now only serve pedestrians. Recently timber bridges have been given a new impetus by glue technology which allows larger cross-sections and larger lengths of beams to be made than grow naturally. Moreover timber can now be better protected against weather and insect attack. So new possibilities have arisen or the choice of structure, for its shaping and for the size. Large timber trusses and even folded space trusses have been built using steel gusset plates for jointing the members. Timber bridges, however, have limits of span and carrying capacity, confining them mainly to bridges for pedestrians or for secondary roads.