Prof. Dr Bernhard Elsener from ETH Zurich, Institute for Building Materials details how civil engineering must evolve with a changing society
Civil engineering is traditionally focused on construction – bridges crossing valleys, tunnels under the mountains, high skyscrapers, but also the less spectacular but essential infrastructure for society. Concrete and reinforced/pre-stressed concrete is and will be the main construction material for civil engineering infrastructure. But society is changing, and so the way of construction, maintenance and use of the civil engineering infrastructure, maybe the way to look at civil engineering, must change, too.
Environmental footprint
In many countries around the world, Portland cement has for decades been the most used type of cement to build reinforced concrete infrastructure. For new structures that will be built in industrialised and emerging countries to expand the civil engineering infrastructure, the challenge is to achieve long service life, practical, cost-effective solutions with materials having a reduced environmental footprint. To achieve this, the cement industry made great efforts in substituting clinker (responsible for a great part of the CO2 emissions) with supplementary cementitious materials (SCM). This substitution is ongoing and reflected in the decreasing amount of Portland cement and the increase of blended cement. These modern binder systems containing limestone, fly-ash, geopolymers etc. in a complex blend are suitable for achieving strength similar to Portland cement, thus can be used to build concrete structures. From the point of view of the end-user (engineer, owner of the structure, society) the final product concrete and its durability, especially when reinforced with steel, is more important.
Concrete for a bridge in the Swiss mountains exposed to a harsh climate and de-icing salts must be of much higher quality, compared to concrete inside a building. The term “quality of concrete” includes the care with which it is executed but also its composition – thus water to cement ratio and the cement type. Whereas long experience is available with concrete structures made with Portland cement, new blended cement have in general a much shorter track record. In addition, due to the reduced clinker content, the pH of the pore solution will be lower and questions arise regarding the corrosion protection of the steel, thus the long-term durability of these new structures both regarding the resistance against carbonation and against chloride-induced corrosion. Despite the ongoing massive use of the new blended cements, convincing answers are lacking.
Live-cycle perspective
The civil engineering industry is currently, in many industrialised countries, in a transition phase from building new constructions to maintaining the large stock of valuable assets. These reinforced concrete structures are ageing and very often show premature deterioration due to corrosion of the reinforcement, with increasing costs for maintenance and repair. Taking into account that the average lifetime of a repair is shorter than that of the original, a dramatic increase of structures have to be repaired and the associated costs can be predicted. The indirect costs, energy consumption, pollution, traffic jams etc. are equally important for society. Today engineers and owners try to extend the service life of these assets with minimum interventions, with sophisticated management systems, and maybe with more effective and durable repair methods – but a new approach is lacking. Life-cycle thinking is urgently needed: in education, continuous formation of professionals, for the stakeholders and society. Why, for instance, a car is carefully designed for easy inspection, contains a wealth of monitoring systems – a much more expensive civil engineering structure with an expected service life of 100 years instead not? Why, even at the most exposed points of a structure in harsh environments, do we still use normal reinforcing steel and not a more resistant material like stainless steel that is fully corrosion-resistant? Live-cycle calculations prove the long-term benefit both in durability and costs – but short-term thinking seems to prevail.
New needs of a changing society face civil engineers with new challenges, like a reduced environmental impact and the preservation of ageing infrastructure. The traditional way of construction, the known technologies might not succeed. What we need is a paradigm change towards a life-cycle perspective. Here academics and practitioners, owners and stakeholders need to take a step forward.
Prof. Dr. Bernhard Elsener
ETH Zurich, Institute for Building Materials
elsener@ethz.ch
