Material using for Tsing Ma Bridge

The two anchorages are gravity structures and take the load from the main suspension cables at either end of the bridge as high as 521,000kN. The two towers of 206m high are of reinforced concrete. Each of them was slip formed in about 3 months. Total volume of concrete in towers is 46,000m3.

The successful contractor has decided to adopt conventional aerial spinning method. Each cable consists of 80 strands of 368 no. 5.38mm and 11 strands of 360 no. 5.38mm galvanized high tensile steel wires. Some 28,000 tons of galvanized steel wire are required. After spinning, the wires are compacted into a circular shape approximately 1.1 metres in diameter using special cable compacting machines. The cables are then wrapped and painted. The total length of wires is 160,000km, enough to circle the earth four times.

The bridge is mainly build with reinforced concrete and galvanized steel. Both of them are the common building materials nowadays, they both have their advantages and disadvantages.

Reinforced concrete, concrete in which steel is embedded in such a manner that the two materials act together in resisting forces. The reinforcement is usually, though not necessarily, steel reinforcing rebar) and is usually embedded passively in the concrete before the concrete sets. Thus, it has High relative strength, High toleration of tensile strain, Good bond to the concrete, irrespective of pH, moisture, and similar factors, Thermal compatibility, not causing unacceptable stresses in response to changing temperatures and Durability in the concrete environment, irrespective of corrosion or sustained stress for example.
Galvanized steel is steel that has gone through a chemical process to keep it from corroding. The steel gets coated in layers of zinc oxide because this protective metal does not get rusty as easily. Zinc protects steel in two ways. First, it is highly resistant to rust; iron, a major component of steel, reacts very easily with oxygen and moisture and will eventually disintegrate. The layer of zinc on the surface prevents those elements from reaching the steel so quickly. It also develops a patina — a layer of zinc oxides, salts, and other compounds — that offers further protection. Zinc is also extremely durable and scratch resistant, and has a satiny appearance that many find attractive.

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However, this common material does have it weakness. First and the main factor that increase the cost of project is the “time of construction”. Construction time increases in case of reinforcement. Second, reinforced concrete can corrode easily because of the presence of steel. Steel is heterogeneous material while concrete is a homogeneous material. Therefore, if both materials will be used in combination than quality control will be very difficult. Third, due to reinforcement, probability of cracking in concrete increases, because of shrinkage and creep in freshly lay concrete and hardened concrete. Last but not least, if spacing in aggregates will not be proper, then weak links increases in between steel and concrete and now if either steel or concrete fails then whole structure will be failed.

Galvanized steel that is cooled too slowly or stacked together before they have cooled completely can also have the outer zinc layer peel away. In some cases, small gaps can form between the zinc and the underlying alloy, which can also cause peeling. When a very thick coat of zinc is applied, it can also become brittle and flake off.

Comparing to the others materials, these two king of materials are still the most common building material, due to the cost and sustainability. Reinforced concrete is obviously the most suitable material for the above sea tower as it is hard and corrosion-proof. But the galvanized steel may replace by stainless steel as the main purpose is to resist from corrosion of water. But the cable size have to be changed as they have different hardness and ductility.