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.