The State of the Art

Traditional Waterproofing Systems

Regardless of the structural type of the bridge and the materials that compose it, traditional systems are based on the following elements:

The waterproof layer: placed underneath the railway bed, it was once created on site using cast bitumen and sand. Subsequently, we moved on to the use of prefabricated and heat-welded bituminous membranes. Recently, we have returned to the use of sheaths made on site, with new generation bitumen, reinforced with special fabrics.

The protection layer: placed above the waterproof layer, since this does not have adequate resistance to the stresses transmitted by the stone above. It is generally made of concrete or bituminous conglomerate.

The support: not having adequate resistance to punching, the sheath must be laid on a rigid support, which in deck bridges is made up of the reinforced concrete slab, in arch bridges it is made up of the arch abutment, generally made of lean concrete.

The removal system: by giving an adequate slope to the sheath support, the rainwater that it intercepts is conveyed out of the bridge or discharged underneath it.

Traditional waterproofing systems still represent the optimal solution today in the construction of new bridges, but they present two major limitations for interventions on existing bridges:

the executive methods, which require a prolonged interruption of railway operations. In fact, to re-waterproof a bridge it is necessary to take the railway line out of operation for a period of time that can range from 3-4 days for smaller bridges up to 8-10 days for longer viaducts.
The need for favourable weather conditions. Traditional sheaths, in fact, cannot be installed in unfavourable weather conditions (rain, frost, etc.).

To overcome the limitations of traditional systems, waterproofing systems called ‘loose – laid’ (i.e. laid without gluing) have been tested, both in Italy and abroad.

One of these systems, qualified and used by RFI, involves the use of polyurea sprayed on site on geotextile sheets, previously laid underneath the ballast and protected above by further placed geotextile sheets.

Despite having an undoubted advantage in terms of speed of installation, the afore mentioned systems present the following drawbacks.

Spray installation of polyurea is strongly affected by the weather conditions during installation and in case of rain or frost it may be impossible. However, the weather conditions for installation can never be determined well in advance, as required by the scheduling of interruptions.

The installation also requires particular caution due to the high toxicity of the components used and particular skill of the operators, with consequent repercussions on execution times and costs.

Finally, the waterproofing function is entrusted to a membrane which does not have independent puncture resistance characteristics and which therefore must be protected by layers of TNT.

All this involves longer installation times (3 layers instead of one) and significant water stagnation in the upper TNT which slows down disposal of water.