14/03/2017

16:30:00-18:30:00

Aula Arduino

Siegfried Siegesmund

University of Goettingen, Germany

Marble bowing

For about four decades marble has been used as thin veneer cladding. The

durability of such thin slabs (mostly 30-40 mm) is satisfactory at most

constructions. However, on numerous buildings all over the world, the long-term

deformation (expansion, bowing) of some claddings is associated with structural

weakening and strength loss which led to an inevitable restoration of the affected

facades and to an image problem of marble used as a structural building stone.

Up to now, the existing knowledge about the causes for this special kind of

marble weathering was mostly restricted to research on Carrara marble and to

the effect of individual parameters. This talk summarises the combined influence

of extrinsic (environmental) and intrinsic (material specific) parameters on

marble bowing. The respective impact of these factors on the weathering process

is elucidated by damage mapping of different marble facades. The work focuses

on the influence of the microfabric on the damage dynamics of veneer marble

and particularly considers moisture as an essential climatic parameter.

Generally, the weathering process is promoted by thermal-hygric microcrack

propagation. In case of temperature variations, as occuring during day-night

cycles, the anisotropic strain properties of the main components calcite and/or

dolomite generate stress within the interconnection of grains. This finally leads

to microcracking along fabric discontinuities such as grain boundaries, cleavage

planes or pre-existing cracks. Irreversible residual strain serves as an indicator of

the accumulated extension of microcracks.

Exemplary long-term field data from the facade of the University Library

building in Göttingen as well as weathering simulations of the concerned veneer

marble under laboratory conditions reveal an unbroken progress of slab bowing

there. The detailed knowledge of long-term bowing dynamics and its correlation

with strength loss of veneer panels can provide the basis for risk assessment and

prediction of the service life of damaged marble facades.

15/03/2017

16:30:00-18:30:00

Aula Arduino

Siegfried Siegesmund

University of Goettingen, Germany

Marble decay and ultrasonic testing

Numerous case studies of damage on sculptures. architectural heritage or facade

stone made from marble indicate that the deterioration of building stones

depends mainly on climate. Chemical mechanisms have recieved much

attention in recent years with a special emphasis on the effect of acid rain or

biofilms. It has been shown that the initial reaction of calicte surfaces to incident

rainfall produces clear morphological alteration even within a short term of

exposure. Recently, the physical weathering is discussed to be the initial stage of

deterioration of marbles. Durability is an important issue to consider specifying

stones as cladding material for exterior exposure.

In the seminar a recent study will also document the results of an

interdisciplinary model project that was planned with the aim of developing an

innovative winter covering system for marble statuaries. Such a system would

need to fulfil the various requirements for structural stability, aesthetics, climate

and practical use.

The degree of destruction can be measured by means of ultrasonic velocities that

drops from more than 5km/s for fresh material down to less than 1km/s for

totally dstructed material. The predominantly strong decay of the marbles can be

determined exactly in the laboratory or on-site, in additon via ultrasonic

tomography. Since on-site the influence of water leads to an increase of

ultrasonic velocities, the water content has to be taken into account when dealing

with ultrasonic analysis.