This paper aims at investigating the effect of the presence of water on the mechanical resistance of stones. The presence of water, connected to the intrinsic properties of the stone (mineralogical composition, fabric, texture, etc.) and to the conditions of use (anchoring systems, climatic parameters, etc ), is the main cause of stone decay. However, the presence of water alone inside stone could cause a decrease in mechanical resistance . The obtained data could in fact be useful to correct the safety coefficient and should be taken into account in the planning of structural stonework. Moreover, useful suggestions can be drawn for the in situ monitoring, taking into account that non destructive tests, together with conventional mechanical methods, are influenced by the presence of water in stones and should be corrected. Three kinds of stones, which have historically been used for structural purposes, have been studied: Pietra di Luserna (gneiss, Piedmont, Italy), Pietra Verde Argento (gneiss, Piedmont, Italy) and Pietra di Courtil (mica-schist, Aosta Valley, Italy). The flexural strength, rupture energy, open porosity and ultrasonic pulse velocity (UPV) have been determined on specimens in dried and saturated conditions at different accelerated ageing steps. As far as the UPV test is concerned, its results are well correlated with the flexural strength values but, without other parameters, it cannot give information on whether the specimen is filled with water, therefore suitable procedures to correct the UPV value obtained in situ are suggested. Destructive methods, and in particular the flexural strength method, instead, give lower resistance values for saturated specimens than dried ones, thus confirming the weakness of the stone due to the water insid

Marini P, Bellopede R

English

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Paola Marini, Rossana Bellopede     Global Stone Congress 2010 
 1 
 
THE MECHANICAL RESISTANCE OF SATURATED STONES 
 
Paola Marini
1
, Rossana  Bellopede
1 
1
 Land, Environment and Geo-Engineering Department -Politecnico di Torino, Corso Duca degli Abruzzi 24 – 
10129 Torino, Italy, paola.marini@polito.it, rossana.bellopede@polito.it 
 
Abstract  
This paper aims at investigating the effect of the presence of water on the mechanical resistance of 
stones. The presence of water, connected to the intrinsic properties of the stone (mineralogical 
composition, fabric, texture, etc.) and to the  conditions of use (anchoring systems, climatic 
parameters, etc ), is the main cause of stone decay. However, the presence of water alone inside stone 
could cause a decrease in mechanical resistance . 
The obtained data could in fact be useful to correct the safety coefficient and should be taken into 
account in the planning of structural stonework. 
Moreover, useful suggestions can be drawn for the in situ monitoring, taking into account that non 
destructive tests, together with conventional mechanical methods, are influenced by the presence of 
water in stones and should be corrected. 
Three kinds of stones, which have historically been used for structural purposes, have been studied: 
Pietra di Luserna (gneiss, Piedmont, Italy), Pietra Verde Argento (gneiss, Piedmont, Italy) and Pietra 
di Courtil (mica-schist, Aosta Valley, Italy). 
The flexural strength, rupture energy, open porosity and ultrasonic pulse velocity (UPV) have been 
determined on specimens in dried and saturated conditions at different accelerated ageing steps. 
As far as the UPV test is concerned, its results are well correlated with the flexural strength values but, 
without other parameters, it cannot give information on whether the specimen is filled with water, 
therefore suitable procedures to correct the UPV value obtained in situ are suggested.  
Destructive methods, and in particular the flexural strength method, instead, give lower resistance 
values for saturated specimens than dried ones, thus confirming the weakness of the stone due to the 
water inside. 
 
Keywords: water influence, mechanical resistance, saturation 
 
1. Introduction 
In the last few years there have been many cases 
of fault in stone modillions or stone balcony 
slabs (fig. 1, 2 and 3). These faults have mainly 
occurred in historical buildings and the involved 
stones that have been used for centuries, 
normally with very good performances and 
resistant to decay.  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1: The remains of a balcony in Pietra di 
Courtil. 
 
The research has been started in order to explain 
how the anomalous breaking has occurred.  
A database  has been set up pertaining to 62 
cases of stone balconies in Piedmont (Perino 
2008).This database reports the kind of stone and 
its conditions, the dimensional characteristics 
and the structural of the balcony.  
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2: The broken balcony slab of figure 1. 
 
The study has been organized in two directions: 
non destructive and destructive tests performed 
on three historical stones  that are extensively 
Paola Marini, Rossana Bellopede     Global Stone Congress 2010 
 2 
used for balconies and modillions in Northern 
Italy. It has been decided to test stone specimens 
in both wet and dry conditions because it is 
known in the scientific stone community that 
water has a bad influence on materials in use 
(Luodes et al. 2006, Chen et al. 2004, Winkler 
1994 ).  
 
 
 
 
 
 
 
 
 
 
 
Figure 3: Details of a balcony were the beam leans in 
correspondence to a fracture.  
 
Almost all the ageing methods used on natural 
stones involve water, which together with other 
factor induces weathering: thermal shock (EN 
14066), freeze and thaw (EN 12371), salt 
crystallization (EN12370), resistance to 
humidity and SO2 (EN 13919) resistance thermal 
and moisture cycling (work item of the TC 246). 
Moreover  it is important to consider the loss in 
mechanical resistance due to the wet condition 
of the stone.  
Winkler (1994) on the basis of previous work by 
Colback and Wiid (1965) and by Michalopoulos 
and Triafilidis (1976), proposed a correlation 
between flexural resistance in wet and in dry 
condition, that is useful for a quick evaluation of 
the quality of a stone. This method has been used 
by some authors (e.g. Wasserman, 2002) to 
assess the quality of stones. 
Some catalogues or publications present the 
mechanical properties of stones in wet and dry 
conditions (Yu et al. 2001; Smith 1999) but 
European Standards only foresee tests in dry 
conditions, while ASTM foresees tests in wet 
conditions if necessary.  
On the basis of the results of the research, the 
dimensioning of balconies which have fractures 
and faults has been verified. It has been observed 
that, in most cases, if the data  relative to the 
mechanical resistance, obtained from tests on 
saturated samples, had been inserted into the 
project, rupture of the balcony would have been 
avoided as it would have been dimensioned 
correctly  
The goal of the research is to offer useful 
indications for  the monitoring of buildings by 
means of non destructive tests, taking into 
account that test such as UPV (Ultrasonic Pulse 
Velocity) are influenced to a great extent by the 
presence of water, and to underline the risks  of 
only considering the mechanical strength in dry 
conditions, when the design involves natural 
stones. 
 
2. Materials and method 
Three kinds of stones, which have been 
employed historically with structural functions, 
have been studied.  
Pietra di Luserna: gneiss. This stone outcrop 
over an area of about 50 km
2
 between Val 
Pellice and Val Po (Piedmont, Italy). Some of its 
characteristics, such as its easy splitting and the 
high resistance and durability, have favoured its 
utilization for both inside use (floorings, 
windowsills) and for outdoors (balconies, 
modillions, staircases, coverings, fireplaces and 
coverages) over the centuries. In the 19th 
century The greatest architectural work was the 
Mole Antonelliana in Turin, where Luserna 
stone was employed by Alessandro Antonelli 
(1798-1888) to cover the great dome and the 
steeple above. 
Pietra di Courtil: micascist. This comes from the 
Hône quarry (Aosta Valley, Italy). The stone has 
probably been used since the Middle Ages, but 
its systematic exploitation started at the 
beginning of 19h century. It has been used both 
indoors and outdoors (mainly to cover roofs) 
because of its rustic aspect and its quality and 
hardness characteristics. 
Pietra Verde Argento: gneiss. This is exploited 
in  the Settimo Vittone quarries (Piedmont, 
Italy). This green coloured stone, characterized 
by silver mica, has been used both indoors and 
outdoors for centuries,. 
The following tests have been executed on the 
stone specimens, in both wet and dry conditions: 
flexural strength (according to EN 12372: 2006), 
open porosity (according to EN 1936: 2006), 
UPV in indirect method (according to EN 
14579: 2004). These techniques have been used 
in order to detect the decay of natural stones 
(Bellopede et al. 2005a and b, Kaharaman 2007,  
Marini et al. 2009) and their results are well 
correlated.  
In order to reproduce the real exposure 
conditions of natural stones in buildings , the 
stones have been subjected to artificial ageing by 
means of freezing-thawing cycles according to 
Paola Marini, Rossana Bellopede     Global Stone Congress 2010 
 3 
EN 12371: 2001 standard. A total of 60 cycles 
have been performed. The same specimens have 
been tested at  t0, at intermediate steps and at t60 
in the non destructive tests (open porosity and 
UPV). The flexural strength test has been 
performed on two different sets of specimens, 
one in natural conditions and the other subjected 
to 60 freezing and thawing cycles.  
 
3. Results 
The freezing and thawing cycles did not produce 
an appreciable decay in the three stones. 
The results of the open porosity test are shown in 
table 1as the mean value of 6 specimens. As the 
variation in po from t0 to t60 is very limited, the 
results are reported with a precision of 0,01%, 
and a very slight decay can be appreciated for 
the Pietra di Courtil as can be seen from the 6% 
increase in po. 
 
Table 1. Determination of open porosity at different 
decay steps 
Cycles 
Pietra 
di Courtil 
po(%) 
Pietra 
Verde Argento 
po(%) 
Pietra 
di Luserna 
po(%) 
T0 0,70 0,64 0,83 
T30 0,71 0,64 n.d. 
T60 0,74 0,64 0,84 
 
The results of  UPV for each freeze-thaw step 
are reported in the graph of figure. 4. 
The graph show no variation for the dry 
conditions, while the specimens tested with UPV 
in wet conditions have a higher standard 
deviation and a different trend, probably due to 
the increase in porosity (even though very 
limited) and as a consequence of the presence of 
a higher  amount of water.  
In spite of the open porosity results, the UPV 
measurements do not reveal the limited real 
weathering state to which the stone has been 
subjected.  
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4: UPV (indirect method) on Pietra di Courtil 
in wet and dry conditions for each ageing step. 
As far as the destructive tests are concerned, the 
flexural resistance test performed on the dry 
specimens subjected to the ageing cycles  
compared with the one executed on dry 
specimens in a natural state only varies from to 
2% to 4,8 %. It should be considered that the 
comparison is made between two different sets 
of specimens and the variability of the stone 
should be taken in account.  
The comparison between the results of flexural 
strength on specimens in dry conditions 
compared to wet conditions, shows a decrease in 
the values from 5% to 24%. The flexural 
strength values for the three kinds of stones, in 
dry conditions and in wet conditions, both at  t0 
and  t60 are reported in figure 5.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5: Flexural strength results in dry and wet 
conditions at T0 and T60 ageing steps. 
 
4. Discussion and conclusion 
4.1  UPV and open porosity 
UPV tests are not able to show slight differences 
induced by decay because of the high 
uncertainty in the results. This kind of 
measurement is conditioned by the water 
content, which can cause even higher ultrasonic 
velocity values than those obtained in dry 
conditions, and sometimes even higher than 50% 
(Figure 6). (Rotonda and Ribacchi 1995, Weiss  
et  al.  2002).  
 
 
 
 
 
 
 
 
 
 
Figure 6: Porosity versus UPV in dry and wet 
conditions in function of pores dimensions (from  
Weiss et al. 2002) 
UPV Pietra di Courtil dry-wet
0
1000
2000
3000
4000
5000
6000
7000
T0 T15 T30 T45 T60
m
/s
PC dry
PC w et
 
T 0
 
D
R
YT 0
 
D
R
Y
T 0
 
D
R
Y
T 6
0 
D
R
YT
60
 
D
R
Y
T 6
0 
D
R
Y
T 0
 
W
ET
T 0
 
W
ET
T 0
 
W
ET
T 6
0 
W
ETT
60
 
W
ET
T 6
0 
W
ET
18
22
26
30
34
38
42
46
Pietra di Courtil Verde Argento Pietra di Luserna
Fl
e
x
u
ra
l s
tr
e
n
gt
h 
[M
Pa
]
Paola Marini, Rossana Bellopede     Global Stone Congress 2010 
 4 
Some indication can be found in literature on 
how to correct the UPV results on wet 
specimens: using either a reference stone (with 
controlled temperature and moisture - Bellopede 
et al. 2005), or by means mathematical models 
(Vasconcelos et al. 2008, Kaharaman 2007). 
 
4.2  Flexural strength and open porosity 
From the results obtained from the flexural 
strength tests in dry and wet conditions, stones, 
even when very durable, show the effects of 
water inside, which considerably lowers their 
resistance. The flexural strength on wet 
specimen of all the stone tested is much lower 
than the values obtained on dry specimens, for 
the corresponding ageing steps (Table 2). This 
effect has been faced in different ways (Winkler, 
1994; West 1994), only considering  it for 
porous stones. However, taking into account that 
it happens for stones with low porosity, it should 
be further explained.  
The decrease in flexural strength from T0 to 
T60of specimens in dry conditions, correlated to 
the increase of porosity, does not occur in wet 
condition (Table 3). 
 
Table 2: Flexural strength variations from dry to wet 
condition at T0 and T60 
 Flexural strength variation from dry to wet 
condition  [%] 
Cicli 
Pietra di 
Courtil 
Pietra Verde 
Argento 
Pietra di 
Luserna 
T0 -24 -17 -21 
T60 -17 -16 -15 
 
Table 3: Flexural strength and open porosity 
variations from T0 to T60 
Pietra di 
Courtil 
Pietra V. Arg. 
Pietra di 
Luserna 
Tests ∆T0-T60 
dry [%] 
∆T0-T60 
wet [%] 
∆T0-T60 
dry [%] 
∆T0-T60 
wet [%] 
∆T0-T60 
dry [%] 
∆T0-T60 
wet [%] 
Fl.Str. 
 
-4,6 +4 -2,1 -0,9 -5 +2 
Op.por +6 +1 +1,3 
 
5. Conclusions 
The three kinds of stones during both non 
destructive and destructive tests show a good 
resistance to weathering.  
Of the three stones that have been analysed, the 
micascist Pietra di Courtil is the one that is most 
influenced by freeze-thaw cycles, and it suffers 
more than the others two gneiss from the 
presence of water probably because it has a 
higher percentage of mica. This can be see from 
the flexural strength and open porosity results, 
while the UPV test cannot detect the very limited 
change in the structure of the stone. 
The flexural strength values obtained from 
specimens tested in wet conditions are surely 
useful to dimension buildings where such stones 
will be employed, as they allow the project 
resistance to be calculated. By recalculating the 
dimensions of the balcony of figure 1 and 2, it 
has been possible to demonstrate the previously 
mentioned assumption. By using the results 
obtained from the flexural strength in wet 
condition in the calculation, it has been observed 
that the modillions should have been positioned 
at a closer distance in order to prevent the slab 
from being subjected to high flexural load. 
 
References 
Bellopede R, De Regibus C, Manfredotti L, 
Marini P. & Tiano P. (2005a) Monitoraggio 
del paramento di facciata in marmo del Duomo 
di Santa Maria del Fiore (Firenze): tre metodi 
a confronto. Workshop Patrimonio 
monumentale. Monitoraggio e conservazione 
programmata. Torino. 25 Novembre 2005. 
Nardini Ed. Kermes quaderni. pp. 106-113. 
Bellopede R, De Regibus C, Manfredotti L, 
Marini P, (2005b). Water absorption and 
ultrasound pulse velocity to evaluate the decay 
of stones. ART '05 Lecce maggio 2005, 11 pp. 
ISBN/ISSN: 88-89758-00-7. CD ROM. 
Marini P, Bellopede R, (2009). Bowing of 
marble slabs: evolution and correlation with 
mechanical decay. Construction And Building 
Materials, ISSN: 0950-0618, doi: 
10.1016/j.conbuildmat.2009.02.010 
Chen T C, Yeung M R, Mori N, (2004) Effect of 
water saturation on deterioration of welded 
tuff due to freeze-thaw action. Cold regions 
science and technology. Elsevier. 38. 127-136. 
Colback PBS, Wiid BL, (1965) The influence of 
moisture content on compressive strength of 
rocks. Proc. 3
rd
 Can. Symp. On Rock 
mechanics, Toronto, Mines Branch, Dept of 
Mines and Technical survey, Ottawa, pp65-83. 
EN 12372: 2006 Natural stone test methods - 
Determination of flexural strength under 
concentrated load. 13p. 
EN 1936: 2006 Natural stone test methods - 
Determination of real density and apparent 
density, and of total and open porosity. 9p. 
EN 14579: 2004 Natural stone test methods - 
Determination of sound speed propagation. 
10p. 
EN 12371: 2001 Natural stone test methods - 
Determination of frost resistance. 16p. 
Paola Marini, Rossana Bellopede     Global Stone Congress 2010 
 5 
Kaharaman S, (2007) The correlation between 
the saturated and dry P-wave velocity of rocks. 
Ultrasonics. Elsevier. 46. 341-348 
Luodes NM, Bellopede R, De Regibus C, Marini 
P, (2006) The influence of the humidity on the 
decay of marbles. Heritage, Weathering and 
Conservation. Fort, Alvarez de Buergo, 
Gomez-Heras & Vazquez-Calvo (eds). Taylor 
& Francis Group. 21 -24 giugno 2006. 
London. ISBN 0-415-41272-2. pp.491-496 
Michalopoulos AP, Triafilidis GE, (1976) 
Influence of water on hardness, strength and 
compressibilità of rocks. Bull.Ass. Eng. Geol. 
XIII (1) , 1-21 
Perino L, (2008) Balconi in pietra: casi studio e 
prove di durabilità. Tesi di laurea. Politecnico 
di Torino. 139 p. 
Rotonda T,  Ribacchi  R,  1995.  Caratteristiche  
dinamiche  di  rocce  porose  e fessurate  
Estratto  da Rivista Italiana di Geotecnica 
Anno XXIX N.1 Scientifiche Italiane Ed., 18-
36  
Smith M R, (1999) Stone: Building stone, rock 
fill and armourstone in construction 
Appendix C: Stone and rock properties 
Geological Society, London, Engineering 
Geology Special Publications ,v. 16; p. 451-
470 doi:10.1144/GSL.ENG.1999.016.01.12;  
Vasconcelos G, Lourenço P B, Alves C A S, 
Pamplona J, (2008) Ultrasonic evaluation of 
the physical and mechanical properties of 
granites. Ultrasonics. Elsevier. 48. 453-466. 
Wasserman I, (2002) Assessment of the 
durability of two natural stones intended for 
the conservation of the historical masonry sea 
wall in the old Town of Acre 
www.arcchip.cz/w10/w10_wasserman.pdf  
Weiss T, Rasolofosaon PNJ, Siesgesmund S, 
2002. Ultrasonic wave velocities as a 
diagnostic tool  for  the  quality  assessment  of  
marble  in  Natural  Stone,  Weathering  
Phenomena,  Conservation Strategies and 
Cases Studies. Geological Society. London. 
Special Publications. 205:149-164.  
West, G, (1994). Effect of  suction  on  the  
strength  of  rock. Quarterly  Journal of 
Engineering Geology, 27,  51-56. 
Winkler EM, (1994) Stone in architecture: 
properties, durability. Berlin. Springer-Verlag 
(ed). 313 p. 
Yu JYH, Chan SL, (2001) Practice and testing of 
stone cladding in Hong Kong  
www.hkisc.org/.../8_Joe%20Yu_2006%20Samin
ar%20stone%2001.pdf 


Bowing of marble slabs: evolution and correlation with mechanical decay. Construction And Building Materials, ISSN: 0950-0618, doi: 10.1016/j.conbuildmat.2009.02.010

Caratteristiche dinamiche di rocce porose e fessurate Estratto da Rivista Italiana di Geotecnica Anno

Influence of water on hardness, strength and compressibilità of rocks.

Monitoraggio del paramento di facciata in marmo del Duomo di Santa Maria del Fiore (Firenze): tre metodi a confronto. Workshop Patrimonio monumentale. Monitoraggio e conservazione programmata.

Natural stone test methods -Determination of flexural strength under concentrated load.

Natural stone test methods -Determination of frost resistance.

Natural stone test methods -Determination of real density and apparent density, and of total and open porosity.

The mechanical resistance of saturated stones

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