Environmental Engineering Reference
In-Depth Information
8.3. Extrinsic degradation factors
Most extrinsic degradation factors are linked to water, either directly (frost) or
indirectly (salts, atmospheric pollution and biological actions).
8.3.1
. Frost
Frost is a rock deterioration agent that has been studied for many years [BOU 79,
LET 87, TOU 70] and two categories of key parameters have been defined for frost
resistance. The first category includes intrinsic porous rock properties, such as total
porosity, morphology size and connectivity of porous network, and finally the
occurrence of cracks. The second group of parameters includes climatic variables,
such as duration and intensity of the frost event or rock moisture. These authors
propose to subdivide the frost damage process in to three categories:
- bursting
, linked to the progression of the frost front in a water-saturated rock:
water pushed while ice is forming generates high pressures within the rock, which
leads to failure;
- scaling
, linked to the phenomenon called “frost suction”, corresponding to the
transfer of water towards zones where ice is forming; as a consequence, ice lenses
form and cause the detachment of scales at the frost front;
- frost disruption
, which consists of rupture due to ice formation in an already
existing crack network.
In 1984, Letarvernier classified the frost susceptibility of stones in four
categories:
-
Class 1: stones with microcracks.
If the porosity corresponding to microcracks
is higher than 0.3%, the stone will develop frost disruption.
-
Class 2: stones with medium pore size
(0.1µm <
r
< 1µm). In this class, stones
have a fairly high total porosity
N
t
(15% <
N
t
< 40%), a low permeability and a very
high saturation coefficient (see sections 9.2.1 and 9.2.3). Chalks are representative
of this class (
N
t
= 40%;
k
nitrogen
≤ 1 mD; 0.4 µm <
r
< 0.8 µm). These stones are the
most sensitive to frost deterioration.
-
Class 3: macroporous or permeable stones.
These stones have a high
permeability (
k
nitrogen
> 10 mD). This characteristic is linked to a bimodal
morphology of the porous network, with two pore radius accesses (<1 µm and
>1 µm respectively). These stones have a low saturation coefficient (50 <
S
< 70%):
water sucked into the stone is located in the micropores; the macropores remain
empty and constitute expansion sites in the case of ice formation. These stones are
frost resistant unless they become fully saturated with water, a situation that occurs
