Frost action in pavements
Guy Doré Laval University
Context • Environmental factors account for the greatest portion of pavement deterioration, up to 50% of deterioration on high-volume roads and as much as 80% on lowvolume roads. • Premature deterioration of road and runway pavements is related to freeze-thaw cycles, primarily where subgrades are composed of fine-grained, saturated material 1992 Royal Commission on National Passenger Transportation
Frost action in pavements Two main mechanisms responsible for pavement damage caused by frost action: – Frost heave – Thaw weakening
Frost heave Three conditions required: • Cold temperature • Water available • Frost susceptible soils
Frost action in pavements and soils T -
+
Frost heave in pavements can reach 150-200 mm in Québec
The mechanism of frost heave • Frost heave is the result of water movement in soils. • According to Darcy: V=k*i • What causes water to flow upward to the freezing front? • The presence of a layer of partly frozen soil behind the freezing is required for the phenomena to occur
The mechanism of frost heave Hydraulic gradient Can be explained by two theories: •The capillary theory •The thermodynamic theory
pi
ρi
−
pw − π
ρw
= −L
∆T Tf
Hydraulic conductivity •Pores in soils need to be small enough to maximize unfrozen water content •Pores in soils need to be large enough to allow water flow
Hydraulic conductivity
The mechanism of frost heave
Clay
Silt
Segregation potential Temperature +
Pressure +
Sol gelé Frange gelée
Sol dégelé
v = SP * GradT GradT ≅ 5deg.C/m
Soil Sand/grav. Till Silt Clay
SP (mm2/°Cj) 20-50 50-150 150-250 100-300
Sand
Unfrozen water content
dpw kPa = 1220 °K dT
Frost action in pavement granular materials • Frost heave in gramular pavement materials is relatively small but significant • Can induces important problems during spring thaw
Frost action in pavement granular materials What causes « non-frost-susceptible » materials to heave? • Pore water expansion? (0,2 * 0,09 * 500 mm ~ 9 mm) • Development of segregation ice? (20 mm2/°C*d * 10 d* 0,01 °C/mm ~ 2 mm) • Vapor migration (?) • Frost heave caused by deicing salt (?)
Frost heave caused by deicing salt •
• • •
Contamination of pavement granular material by brine seeping through cracks and granular a pocket of unfrozen materials shoulders Presence of allowing seepage of water to the frost front Segregation ice build-up near the frost front Frost heave reaching 60-80 mm near the crack
Thaw weakening
Thaw weakening • Thaw weakening is a transient state of soils and pavement materials going through a consolidation process after frost heaving • Three main factors causing thaw-weakening: • Amount of frost heave • Thaw rate • Rate of consolidation (drainage)
The mechanism of thaw-weakening
Soil type
Clean gravel and sand Silty-Clayey gravel and sand Silt Clay
Range of bearing capacity loss* 0 – 15%
Typical value
20 – 50%
35%
30 – 70% 40 – 60%
50% 50%
10%
Frost action on pavements • Freezing of pavement materials and soils is not a problem if there is no frost heave • Frost heave is not a major problem if it is uniform but his is rarely the case • Te severity of frost heave effects thus depends on: • The intensity of frost heave • The variability of the phenomenon
Differential frost heave Main causes of differential frost heave: • Variabity of soil properties • Geometric effects • Localized pavement damage
Differential frost heave caused by variable soil properties
Longitudinal profiles, St-Augustin (SA-98-02) (Average frost heave = 40mm) 100 Elevatio n (mm)
80
Winter
60 40 20 0
Summer
-20 -40 -60 0
20
40
60
80
Distance (m)
100
120
140
Differential frost heave caused by pavement geometry
Accumulated snow
The effect of thaw weakening on pavements • The main effect of thaw weakening is to amplify pavement damage caused by heavy vehicles • Damage occurs mainly during two critical periods during pavement thawing
Critical periods of pavement thawing Thawed
Frozen
Early pavement thawing
Pavemet damage caused by thaw-weakening
12
Carrelage et fissures en piste de roues (m2/1000m2)
Planche A Planche B
10
10.6
Planche C 9.7
Planche D
8
6
Note: Les droites relient les points entre lesquels l'évolution est attribuable à une situation de dégel printannier ou hivernal.
7.3
5.2
4 3.4
3.4
3.6
3.0 2.7
Restrictions: 2 1.6 1.6 0.80.8
0 15-mars-92
0.0
0.0
0.2
15-mars-93
15-mars-94 Date
15-mars-95
14-mars-96
Pavement damage caused by thawweakening
Assessment of relevant engineering parameters • Frost depth • Frost heave • Thaw weakening
Estimation of frost depth Stefan’s equation (modified by Konrad to include the effect of segregation potential) can be used to estimate frost depth in a homogenious soil layer
2(k f − (SP × L ))
X ss =
Ls
× FI t
Where: FIs is surface freezing index (°C*j) k is the thermal conductivity of the frozen soil layer (W/m°C) L is the latent heat of fusion (334 MJ/m3) Ls is the latent heat of fusion of soils
Ls = ρ d × ω × 334
Freezing index (air)
t
FI a = ∫ − T− dt 0
or t
FI a = ∑ − MDAT− 0
Freezing index (surface) Material
nf Suggested practical range
Range
nf = nt =
FI s FI a TI s TI a
nt Range
Asphalt concrete
0.252.50
0.8-0.95
1.60-3.00
Gravel
0.601.50
0.9-1.0
1.10-2.00
Trees and 0.250.30-0.35 0.37-0.80 brush, 0.50 moss and peat soil Snow 1.00 Compiled from: Dysli et al. (1997), Zarling and Braley (1988), Lunardini (1978), Ladanyi (1996), Andersland and Ladanyi (2004).
Freezing index (transmitted) FIa
(Corté et al, 1995) n
FIs
FI s − be D FI t = 1 + aD
∑b D i
2
be =
i
i =1 n
∑D
i
i =1
FIt
Material Asphalt concrete Asphalt stabilized base Granular material
a (cm-1)
b (°C·day)0.5/cm
0.008
0.06
0.008
0.10
D: layer thickness (cm)
Thermal conductivity Clay and silt: 1.373 ρ d
k f = 0.001442(10 )
0.4994 ρ d
+ 0.01226ω (10 )
Sand and gravel: 0. 8116 ρ d
k f = 0.01096 (10 ) Soil or material Fresh snow Compacted snow Asphalt concrete Granular material Polystyrene insulation Peat Sand-gravel Silt Clay
0 .9115 ρ d
+ 0.00461ω (10 )
Thermal conductivity k (W/m°C) 0.06-0.10 0.3-0.6 1.50 1.3-1.7 0.03-0.06 0.6 1.2-3.0 1.2-2.4 0.9-1.8
Volumetric heat capacity Cv (MJ/m3 °C) 0.21 0.42-1.05 2.0-2.5 2.0 0.04-0.06 3.0 2.4-3.0 2.5-3.1 2.6-3.4
Segregation potential •Segregation potential of •Segregation potential can be soils can be determined using estimated based on typical a freezing test values •Segregation potential of a given soil varies with Soil SP (mm2/°Cj) saturation level and applied Sand/grav. 20-50 pressure Till 50-150
Essai de gel Détermination du potentiel de ségrégation
Soulèvement
h
-6°C
150-250 100-300
Profondeur
Silt Clay
v = dh/dt
T°
+2°C
sp=
v gradT°
Profondeur
Temps
gradT° 0°C
Estimation of frost heave • Simplified method proposed by Saarelainen • Based on the principle that frost heave is proportional to the thickness of the frozen soil layer and its segregation potential
h=
2 SP( X − D ) X FI s
2
Where: h is average frost heave (m) SP is the segregation potential (m2/°C*d) X is total frost penetration (m) D is total pavement thickness (m) FIs is surface freezing index
Estimation of thaw weakening: The thaw-weakening index •
h x TWin = × • D S ∆M = 0.25 ln (TWin ) + 0.33 Ms
Mitigation of frost-heave and thaw weakening • Cases requiring special care – Highly frost susceptible soils (SP > 150) – Geological contacts (sol gélif/roc; sol gélif/sol nongélif) – Heterogeneous frost sensitive soils (Glacial tills) – Varved clays – Clayey soils with liquid limit > 0,9
Frost protection Total frost protection for severe cases – Transitions (granular wedge) (localized problem) – Pavement insulation (problem affecting large surfaces) Partial protection for pavements subject to frost action – Increased thickness of granular materials – Drainage or capillary barrier – Homogeneisation – Chemical treatment (lime, cement, salt, etc.)
Transition
Pavemet insulation
Drainage/capillary barriers
Homogeneisation/chemical treatment
Increased thickness of granular materials
Designing pavements for frost action Design based on empirical rules • 0,5 X • Transport Canada design chart Protection contre le gel (MTQ)
Correction factor É p a is s e u r to ta le d e la chaussée
1100
Soil type GM, GC SM, SC ML, CL, MH CH, SM (fine)
1000 900 800 700 600 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500 Indice de gel normal (deg. C*jours)
FS 0,85 1,00 1,15
Road class Freeway Major Highway Reg. and Coll. Local
Required protection = P X FS X FR
FR 1,10 1,00 0,90 0,80
Designing pavements for frost action Design based on allowable frost heave Allowable frost heave (mm) Road classification FinnRA1 MTQ2 Freeways 30 50 Main highways 50 55 Regional roads 60 Local roads 100 70-80 1 Tammirinne et al. 2002 2 St-Laurent 2006
Designing pavements for frost action Design based on allowable frost heave : step by step approach 1. Select appropriate level of allowable frost heave 2. Compute allowable transmitted freezing index based on allowable frost heave 3. Compute required layer thickness to limit transmitted frost heave under allowable value
Designing insulated pavements General principles • Used in cases of severe differential heaving affecting large pavement surfaces • The insulation layer impedes heat loss => Limits soil cooling and consequently soil freezing ⇒Limits proportionnally frost heaving and consequently differential frost heaving • The insulation layer typically weakens the pavement structure
Designing insulated pavements Design based on three criteria: • Mechanical considerations • Thermal considerations • Considerations for differential icing
Designing insulated pavements Mechanical considerations The thickness of granular material required above the insulation layer to avoid excessive stress on the insulation material can be calculated using :
Designing insulated pavements Thermal considerations Thickness of the insulation layer calculated based on the air freezing index A conservative design should be based on a rigorous winter MTQ uses a return period of ten years (average freezing index of the three coldest winter in 30 years) IGr = 1,143 IGn + 220 (°C*d) IGr and IGn are rigorous and normal freezing indices
Designing insulated pavements Thickness of the insulation layer usually calculated for full protection against frost Methods: – Design charts (figure) – Multilayer thermal calculation models: These methods require a good understanding of physical principles involved and a good knowledge of material properties: - Heat capacity - Thermal conductivity - Latent heat of fusion Models:
Berg Mut1D
Designing insulated pavements
Designing insulated pavements Considerations for differential icing Since the insulation layer impedes geothermal heat flow, there is a risk that the pavement surface above the insulation layer becomes colder than the surfaces of adjacent uninsulated surfaces during fall There is thus a risk of hoarfrost formation at the surface of insulated pavements while adjacent sections remain unfrozen => Important risk for road users
Designing insulated pavements Required precautions to minimize the risk of hoarfrost formation: – Avoid pavement insulations in these conditions: • In braking zones (for example, near intersections or railway crossings) • In curved areas • In steep slopes – Insulation layers should be covered by at least 450 mm of compacted gravel
Designing insulated pavements Considerations for transitions Transitions are required to minimize the risk of differential behaviour at the end of insulated sections Transitions are done by reducins progeressively the thickness of the insulation layer Longitudinal and lateral transitions are required. Lateral transition should be placed beyond the edge of the paved surface
Lateral transitions
Longitudinal transitions
Designing pavements for thaw weakening Two main approaches: • The use of the effective modulus in the AASHTO design procedure (empirical procedure) • The computation of seasonal pavement damage for mechanistic-empirical pavement design methods
The use of the effective modulus in the AASHTO method • Principle: find the modulus that would cause the same amount of annual damage than modulus varying across the seasons • Applied only at the subgrade soil level
Winter
Spring
Summer
Fall
Damage
Resilient modulus
uf u f = ∑ =3,72=0,31 n 12
u f =1,18×108×M R−2.32
Solution par abaque de l’équation AASHTO
Design objectives (design life, traffic volume and level of service)
εadm
Empirical transfer function
Log N Allowable strains (bottom of AC and top of subgrade)
Mechanistic model
εadm. εadm.
Caractéristics and conditions •Soils •Materials •Climate
Thickness of pavement layers
Principle of mecanistic-empirical pavement design
Variation in soil and material properties
Compute seasonal damage in mechanistic-empirical methods Computation steps o o
The year is divided into representative seasons For each season: 1. Typical resilient and dynamic modulus are attributed to each layer of the pavement 2. Critical strains are computed for a reference load for each combiation of material properties using the mechanistic model 3. Number of allowable load repetition is computed for each value of strain obtained using empirical damage models 4. The proportion of design life consumed is calculated for each season considering the proportion of total traffic circulating during the season o The summ of damage consumption consumed for each season must not exceed 100%
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