R.P.J. van Hees

Professor of Heritage & Technology - Department of Architectural Engineering + Technology

The restoration of historical buildings demands a sound knowledge of historical construction techniques as it could otherwise cause more damage than good. In his capacity as chair of Heritage & Technology, Professor Rob van Hees is researching these technical aspects of architectonic heritage.

The Romans built the Pantheon nearly two thousand years ago. Its Roman concrete dome has a free span of over 43 metres, a feat that would not be achieved again until the Renaissance period. To limit the weight of the dome, the builders used a different composition of concrete according to its position in the dome, using a lighter aggregate for the top and a heavier one for on the underside. "A wonderful example of historical knowledge that can we can still learn from today", says Van Hees. "But we also need to fully understand such constructions if we are to preserve them."

Van Hees (born in 1952) has always been fascinated by historical architecture and the underlying technical competence. His work as Professor of Heritage & Technology, in terms of both teaching and research, focuses on existing structures. The key objective is to find methods of sustainable and compatible restoration.

Brick and mortar structures have been used for thousands of years, but that does not make every kind of mortar suitable for every restoration. Existing buildings each have their own specific material properties, which can vary significantly depending on their age. Interesting is however that The Romans, used pozzolana mortars, based on volcanic ash, as a waterproof cement while, for early Dutch structures a similar material, ground tuff stone - trass - was used as an aggregate to make lime mortars waterproof.

Commencing restoration without thorough research beforehand is a recipe for problems, Van Hees observes. "When it comes to restoration there are two important rules of thumb to bear in mind. Firstly: make sure you understand why a particular technical measure is needed - what has caused the deterioration of a building or material? Secondly: make sure that a 'compatible' material is used for the restoration, one which will not damage the existing historical material and which is itself as sustainable as possible."

If a monument has deteriorated due to shortcomings of the historical materials used, the restoration specialist will need to make difficult choices. Should they use a modern restoration material that would cause fewer problems, with moisture movement for instance? Or would that perhaps cause even more problems elsewhere? The search for the best option can sometimes lead to surprising innovations. Research into salt damage mechanisms in buildings in Zeeland, for example, prompted currently ongoing PhD research into self-healing mortars that are resistant to saltwater intrusion. "But you could also opt to use a self-sacrificing material to preserve an existing material", Van Hees explains.

The chair in Heritage & Technology has previously collaborated on research at the Delft Centre for Materials into the self-healing behaviour of historical lime mortars. Cracks that appear can, under certain conditions, subsequently close again 'automatically'.
Earlier PhD research into cracking patterns in historical brickwork could also be relevant to present issues such as the earthquake damage in the north of the Netherlands.

One of the biggest challenges for the coming years is making monumental buildings more sustainable. This involves more than developing ways of making monuments more energy-efficient. Van Hees: The objective is: durable and sustainable monument preservation: making monuments more sustainable in such a way that restoration measures will not be needed again soon. Plus the use of such materials and techniques that the monumental value remains intact."

Rob van Hees also works in the Building Materials department at TNO, where he coordinates national and international research projects in the field of restoration and renovation. Among his other roles, he is a member of the programme management team of MonumentenKennis, the collaborative partnership established this year between TU Delft, TNO and the Netherlands Cultural Heritage Agency (RCE) in the field of monument preservation.

Rob van Hees