Stonehenge inspires

Stonehenge has survived largely because it was over-engineered by its prehistoric designers. Modern builders do not have the same luxury .


Stonehenge has survived largely because it was over-engineered by its prehistoric designers. Modern builders do not have the luxury of such leeway and that is why buildings and structures made of modern materials can deteriorate in far less time than 4,000 years. They age and need help and Dr Paolo Casadei aims to prove that new techniques can do the job.

He believes that reinforced concrete can be repaired by the careful addition of prestressed fibre-reinforced polymers (FRP). Steel is sometimes used for this, by bonding plates to the surface of the structure. However, it does have problems as, when the steel corrodes, the bond gets weaker. Other FRP solutions have been tried but none has exploited the material’s high-tensile properties.

Casadei believes prestressed FRP systems can provide several advantages, besides the direct economic benefit of fully employing the high-tensile strength of the materials. These include: increasing live load capacity; reducing dead load deflections; reducing crack widths and delaying the onset of cracking; reducing serviceability problems such as excessive deflection, cracking of the concrete and tensile steel stresses at serviceability; and improving fatigue strength by reducing tensile steel stresses.

He is developing a method to bond bars of prestressed FRP to deteriorating reinforced concrete. He began his work in the US before continuing it at BathUniversity and later this year he will oversee it from a firm of consulting engineers in Milan, Italy.

The technique is straightforward. A groove is cut in the concrete and some epoxy is laid inside it. Then the bar, up to 5m long, is also placed in the groove and held in place by the epoxy. Hydraulic actuators with grippers take hold of each end of the bar and stress it with the desired force and more epoxy is used to cover the bar and fill the groove completely. The bar is held by the grippers until the epoxy has set and then they are removed.

Casadei has demonstrated the technique on an I-beam similar to those used on bridges and now wants to apply it widely. ‘We know now that it is do-able,’ he said. If he succeeds, some modern decaying structures may well enjoy life spans longer than our own.