Icing of insulated pavements

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Icing of insulated pavements

ISSN 0701-5232,

BUILDING

RESEARCH

NOTE

IGLNG

OF

Division of Building Research, National Research Council of Canada I

mthwa, October 1984

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ICING OF; -7. IMSULmKD PAvE#Wm$

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n

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Considerable information is! available (1) on the i n s u l a t i o n of pavements to reduce the depth oP frost penetration and thus prevent frost-heaving. There are, howear, f ev p u b l i s h e d studies ( * ) on t h e

problems that may be caused by t k e mre frequent occurrence o f surface i c e on insulated pavements. If subgxade cools more quickly when it is over an i n s u l a t i n g layer, then under sui!table weather conditions, surf ace i c e will form mare readily m _{an insulated pavement than on an uninsulated pavement.}

This report reviews the literature on pavement i c i n g and p r e s e n t s some preliminary observations of p a v e b n t surf ace temperatures

,

A study af the problem of i c i n g on insulated pavements requires an understanding of the processes by which I c e can form on pavements.

F i v e main types of pavement: i c i n g can be d e f i n e d :

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G l a z e i s a coating of ice formed when rain or drizzle freezes.

The p r e c i p i t a t i m may or may n o t b e supercooled. Of the f i v e types of i c e discussed here, glaze poses t h e greatest t r a f f i c hazard. It forms a dense, smooth, tenacious sheet of i c e w i t h l o w coefficients of friction,

Closely related t o glaze is rime i c e , formed by the rapid freezing of supercooled water d r o p s when heavy fog passes over cold pavement. It i s usually l i g h t e r , softer and less transparent than glaze, Factors that favour rime formation are small water drops and a high degree of

supercooling with rapid d i s s i p a t i o n of the latent heat of fusion. R i m e is less of a problem than glaze because it occurs less frequently and is m o r e easily broken loose from the pavement by traffic. Nevertheless, rime can be *Since t h i s reporz was written there has been an extensive research

investigation on the i c i n g of pavements. The results have been p u b l i s h e d : GUSTAFSOM, Kent, Road I c i n g on Df fferent Pavement Structures, National Road and Traffic Research I n s t i t u t e , Sweden Wr 216A, 1981, Translated by

serious, particularly s i n c e it often results in "spot" i c i n g , p r e s e n t i n g an unexpected hazard t o motorists;

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Hoar is the feathery sublimation d e p o s i t of inrerlacking ice crystals on pavements. It forms when ajr with a dew p o i n t belaw the freezing p o i n t is brought to saturation b y codling. It can b e hazardous where t r a f f i c is

n o t heavy enough t o break down and remove t h e l i g h t covering of crystals.

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The f r e e z i n g and thawing af compacted n e w snow can result: in the formatim of ice comparable to lglaze in d e n s i t y and slipperiness.

The change from s n o w to ice cay take place in two ways. Either the snow is compacted by t r a f f i c and subsequently thaws and f r e e z e s , or the snrrw melts and is compacted during the a f t

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OaC, and then freezes at nlght., Compacted sncw that has turned to i c e is often consfdered to be a greatek problem than glaze because it occurs more f requen t ly

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The ice that forms when drainage water flows onto a road surface and freezes is dangerous p r i m a r i l y because it occurs at only a f e w l o c a t i o n s along an otherwise bare highway..

Several variations of these general types of pavement i c i n g can occur. Glaze can form on compacted snw turned t o ice; rime can be "soft" l i k e hoar crystals or it can be "hard" like g l a z e ; hoar frost can occur on t o p af

other types of pavement i c i n g . These variations often make classification d i f f i c u l t .

Three b a s i c cmdltions musr be m e t , however, if any type of i c e is to form on pavement:

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(1) A source of moisture is needed. In the case of glaze the source i s p r e c i p i t a t i o n ( f r e e z i n g rain); f o r rime it is supercooled w a t e r

droplets; in hoar formation it i s water vapour; in t h e other cases i t

is surf ace drainage or m e l t e d snaw.

(21 Meteorological conditions must be cMducive ta i c i n g . The r a t e of heat loss t o the atmosphere must be s u f f i c i e n t t o o f f s e t both t h e heat of fusion released when water freezes and any heat conducted to the surf ace from subgrade material.

( 3 ) The surface temperature mugt be below O°C.

A n insulating layer in the pavement subgrade can influence the amount of ground heat a v a i l a b l e for transfer to the pavement; surface and the micrometeoro~ogical conditions t h a t control the rate of cooling of the surface. Since surface ice formation is very s e n s i t i v e t o the amount of heat: s u p p l i e d from the subgrade,

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by an insulating layer, n o t just glaze or hoar as has been suggested in t h e literature.

The s i t u a t i o n is c o m p l i c a t e d by the many other variables t h a t can influence pavemenr i c i n g such a s solar r a d i a t i o n , w l n d , topography (whether

icing occurs in a frost hollow or n o t ) , proximity to water bodies and

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t r a f f i c . Plny f i e l d study or analytical method f o r assessing the influence of buried i n s u l a t i o n on paveme -t i c i n g must c o n s i d e r t h e s e variables.

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PREDICTIHG SURFACE m E U T U R E

To determine the effect o f ' buried insulation on pavement i c i n g , i t s influence on surface tereperatu* rmst b e examined.

If

temperature of the insulated pavement cools below

O"C

Much of t h e work on predicring mintmum nighttime temperatures for

agricultural crops s u b j e c t to freezing is directly related to the problem o f determining the effect of ground thermal cmditicms on surface temperatures. Bront (2) developed a f o m l a for predicting the drop i n a i r tenperarure that can be expected over soils; with different thermal properties:

where

AT = the drop i n a i r temperature from sunrise to sunset in time

(TI.

Cv = conducrive cagaci t y of s o i l , a = thermal diffusivity.

The chfef difficulty in this type of formla is obrafning reliable values for Cv for s p e c i f i c sites. In assessing Brunt's formla, &ighting ( 3 )

recognized t h i s problem and suggested that t h e measured temperature f a l l at the surface be used t o calculate t h e s o i l parameters, rather than u s i n g e s t i m a t e d s o i l parameters t o estimate d b i m m temperatures. Another limitation t o the formla is that it is based an the assumption that t h e transfer of heat from t h e a i r to the ground by convection can be ignored. Brunt recognized t h i s weakness but considered t h a t in most cases the major

form of heat loss during nighttime cooling would be r a d i a t i v e .

More sophisticated methods of estimating ground surface temperatures, p a r t i c u l a r l y pavements, have bekn tried. These methods attempt to measure or calculate the d i f f e r e n t compopents of the surface energy balance equation (including the heat transfer from the ground). The main problem is

measuring or calculating the coraponeu ts of the surf ace heat balance equation with s u f f i c i e n t accuracy. One study (4) suggests that "heat l o s t by the

combined fluxes of convection, evaporation and sublimation f s nearly 2 2 . 5 %

d i f f e r e n t from measured values and causes t h e greatest portion of error between calculated and measured heat fluxes out of the ground." h o t h e r

study (5) on t h e p r e d i c t i o n of p-avement surface temperatures, using a d i g i t a l s i m l a t i m model of heat flow with a computer solution, obtained reasonable results for w e l l - d e f i n e d summer conditions but w a s n o t as

obtained f o r winter (no sun) days but were not successful, probably because small heat flaws are d i f f i c u l t I to preaict and minor errors have large

temperature effects,"

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The most comprehensive a n q l y s i s of the problem of predicting

temperature variations in soils, including surf ace temperature, I s reported in a study that p r e s e n t s analyef c a l solutiws f o r the temperature variations

in layered sails with d i f farent thermal properties (4). The authors

conclude that "changing the phqsical properties of upper layers of soil can cause several degrees Ce l s f u s dif f erenee in d i u r n a l variation of surf a c e temperature, particularly at the. . t - s e t of a w a r m or c o l d s p e l l , "

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Methods of p r e d i c t i n g pavement tenperatures ta gfve warnings of i c i n g c o n d i t i o n s have been developed lin Japan (71, The prediction method depends not o n l y an the calculation of the various terms in the surf ace heat balance

equation but on a s t a t i s t i c a l analysis of the variables affectirlg surface temperatures. The method requires mch d e t a i l e d information on surface and subsurface pavement temperatures and meteorological variables to make

predictfons possible.

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An analytical method for predicting the teraperature distribution in i n s u l a t e d pavements has been developed ( 8 ) but the technique, d e s i g n e d t o estimate temperature on a seasonal b a s i s , does not appear suitable for short-term p r e d i c t i o n s .

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It is clear from the literature on predictfng ground surface

temperatures that the main valuk of analytical solutims is to develop an understanding of the p h y s i c a l processes involved and the effect of d i f f e r e n t variables, such as the role of buried i n s u l a t i o n , in controlling surface temperature. Analytical solutions, however, cannot substitute f o r f i e l d observations. The authors of the study on temperature variation in layered s o i l s ( 6 ) conclude that " t h i s example may show how important b o w l e d g e of t h e theoretical background is in performing field experiments". They a l s o conclude that "the agreement between f i e l d measurement and theory is f a i r l y good and could be improxd upon by adopting s l i g h t l y different t h e m 1 properties for the soil but it is n o t considered worth the effort. So many assumptions have t o be made that t h e results m s t b e considered as a

qualitative check rather than as a quantitative one."

FIELD OBSRVBTIOlilS

!l%e observations of surface temperature of insulated concrete s l a b s reported here may be of value i n planning further s t u d i e s of f cing problems

on i n s u l a t e d pavements. Observatiws were made for two types of concrete surfaces: 90 x 9 0 cm slabs e l e v a t e d above t h e ground and 4.4 x 4.8 m

Observatloas of Surface Teqeratureci of Elevated Slabs

The three concrete s l a b s , e l e v a t e d about 90 cm above ground l e v e l , were intended as models for subsequegt observations on elevated roadways subject to pavement: i c h ~ g .

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(1) Slab No. 1 had 7.5 cm of concrete and no i n s u l a t i o n .

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(2) S l a b No. 2 had 5 cm of c a c f e t e and 2.5 cm of i n s u l a t i o n . ( 3 ) S l a b No. 3 had 2.5 cm of cmcrete and 5 cm of i n s u l a t i o n .

The s i d e s of a l l three slabs had 2.5 ern of insulation. Temperature was measured a t t h e surface and t h e (interior of the s l a b s by thermcorlples. The temperatures were recorded continuously a n d compared w i t h air temperatures recorded in a nearby Stevenson Screen.

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During a clear n i g h t with l f g h t t o moderate w i n d s , the surface

tenperatures of a l l three slabs dropped well b e l o w t h e a i r temperature. The surface temperature of slab No. 1 was aboue 2.8*C below the air temperature through most of the n i g h t . Slab No. 2 cooled close t o t h e dew point

temperature, about 4.4'C _{below t h e air temperature, Slab No. 3 cooled to} the dew point temperature r a p i d l y , w l t h a surface temperature about 6.1 "C

below t h e air temperature. Condensation probably occurred on the surface of thls s l a b during the n i g h t .

The relative magnitude of t h e campmen t s of the surface heat balance

were campared -for t h e three slabs f o r a clear n i g h t , 7-8 February. The change in heat storage (Qs) w a s estimated by as8uming a value for the beat capacity of concrete and using measured change in temperatures., The n e t long-wave radiation (Qn) and convective heat losses [Qc) were estimated f r o m fornulas. The f o l l o w i n g is the estimated r a t e of heat l o s s or gain for the p e r i o d 1600 t o 0700 h,

Number o.f degrees surface temperature

rn

Qs

cal/cm2 cal/cm2 cal/cm2 temperature Slab No. 1 -106

S l a b N o . 2 -110 Slab No. 3 -120

These approximate values Illustrate t w o p o i n t s :

( 1 ) Qs was much greater for slab No. 1 than for the other two s l a b s , and was sufficient to maintain the surface temperature of s l a b No. 1 w e l l above the dew point temperature during the n i g h t .

The variation in surface demperature during cloudy periods f o r t h e s e three s l a b s showed a d i f f e r e n t pattern. ?he surface teqerature of a l l the slabs remained above the air temperature during t h e early n i g h t when the s k y was cloudy. When t h e w i n d spedd decreased and clearing started, the surf are temperature began to coal b e l d , the air temperature. Ihder cloudy

c a n d i t i m s with high w i n d s {wh? heat loss from langwave radiation i s l o w ) , heat l o s s or gain from c o n v e c t i ~ is the major factor i n controlling surface

temperatures.

If

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convective heat l o s s will d r i v e the surface temperature down; if the air

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temperature s t a r t s t o exceed t+ surf ace temerature, convective heat gain

from the a i r tends to maintain f surface temperature closer to t h e a i r temperature. It is only when $nd speed drops and convective heat l o s s or

gafn becomes less important tha,t t h e heat stored i n the s l a b becomes s i g n i f i c m t enough to affeet su face temperature.

Observa-s of Surface Te-tmms of Zngrmmd Slabs

Observations of surface temperatures were taken at an insulated section of concrete pavement and at a nearby uninsulated asphalt pavemnt on NRC grounds, during t h e late wifiter '

of

recorded at t h e centre of a 4.8 x 4.8 m concrete slab, w i t h approximately 17.5 cm of concrete over an fnsiilated lager 5 cm t h i c k and at anearby uninsulated asphalt roadway about 5.0 cm t h i c k on a crushed rock subbase.

These preliminary observations showed that:

(1) The heat stored dur1n.g t h e day (during p e r i o d s of r e l a t f v e l y hf gh s o l a r radiatian) was s u f f i c i e n t t o maintain the surf ace temperatures above the a i r temperature during the a f g h t ;

(2) There were no large differences in surface temperature between the i n s u l a t e d and uninsulated s l a b s .

It is concluded that i c i n g of insulated pavements is not: likely ta be a problem during late wfnter.

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Observatbns made during early winter i n d i c a t e d , however, that there c o u l d b e lower surface temperatures over the insulated pavenent that would

be conducive t o a greater incidwce of surface icing* Several consecutive clear nights w i t h high rates of coaling are necessary t o produce t h i s effect. This suggests that the problem is not one of simply predicting surface temperatore during a s i n g l e n i g h t b u t rather one of accounting for the c u m l a t i v e e f f e c t of several nights of cooling. The difference in the amount of heat s t o r e d between insulated and uninsulated pavements becomes s i g n i f i c a n t enough to influence surface icing.

TKHTATIVE COHCLUSIOHS

This preliminary study cm?f rms that under certain c m d i t f ons surf ace i c e will form mare r e a d i l y an idsulated paveraent than cm uninsulated

pavement. A conclusive s t u d y s h o u l d be carried out over several winters The thermal properties of the subgrade rnatarlal in both the insulated and conventional pavements need t o b e nseasured accurately. A i r temperature should b e measured at more than one l e v e l immediately above the pavement surfaces so that the role of c ~ v e c t i v e heat in controlling surface

temperatures can b e assessed. N e t r a d i a t i m should be measured on both the i n s u l a t e d and u n i n sulated surfaces. These measurements are needed for t h e theoretical analyst6 of the d i f f e r e n c e in i c i n g potentfal at the t w o pavement surfaces.

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Experimental s3tes should

bb

~ C R S

( I ) Penner, E., Oosterbaan, M.?., and R.W. kdman, Ferfammce of C i t y Pavement Structures Gontafrring Foamed Plastic Insulation, National

Research Coun cll (U. S

.

,

4

(2) B r u n t ,

D.,

( 3 ) Knighting, E., A Note of Moicturnal Cooling, mart J. of the Royal Met. S o c i e t y , Vol. 76, No. 328, k p r i l 1950.

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( 4 ) Berg, R.L., &ergy Balance cm a Paved Surface, Corps of h g i n e e r s , U.S. Army, CRWL, Tech. R e p o r t 226, June 1974.

(5) Straub, A.L., Schenck, N.

Jk.,

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( 6 ) van Wijk, W.B., ed., Physics of Plant Fnvironment, North-Holland P u b l i s h i n g Co,

,

(7) Motoya Inoue, Kozaburo Baba, and Yoshiharu Takada, "Ice Detection,

Prediction and Warning System on Highways", %ow Removal and Ice Control &search, National Resaarchi Council

(U.

Proceedings of an In ternatf ma1 Symposium h e l d at Hadover,

N.H.,

(8) Ho, D.M., k r r , M.E., and G.A. Liewards, Tkansient Temperature Distribution in I n s u l a t e d Pavements, Predictims vs Observatims, Can. Geo. Journal, V o l , 7, No. 3, August 1970.