Moisture Content in Road
Pavements
State of the art and development of a simple
moisture measurement equipment p-) ) qqued] <C ) Ca) CO D I= ame Ain 3 C Jorgen Svensson ad D =
Swedish National Road and Transport Research Institute
VTI meddelande 809A - 1997
Moisture Content in Road Pavements.
State of the art and development of a simple moisture measurement equipment
Jorgen Svensson
Swedish National Road and
Publisher Publication
VTI meddelande 809A
Published Project code
, , 1997 60141
Swedish National Road and Project
A Transport Research Institute Structural pavement design Printed in English 2000
Author Sponsor
Jorgen Svensson Swedish Transport and Communications
Research Board
Title
Moisture Content in Road Pavements. State of the art and development of a simple moisture measurement equipment.
Abstract (background, aims, methods, result)
Unfavourable combinations of soil, water and climate factors lead to bearing capacity problems and damage to structures such as roads. It is therefore important to expand knowledge of moisture measurement in roads and the influence of variations in water content on road condition.
Moissture Content in Road Pavements. State of the art and development of a simple moisture measurement equipment is a subsidiary project under the theme Structural pavement design financed by the Swedish Transport and Communications Research Board.
The project aims at increasing knowledge of moisture measurement in the unbound material layers of the road structure and subgrade, primarily in the unsaturated soilzone, and also to design simple and reliable measuring equipment for installation in the road structure.
A study has been made of reports and specialist literature dealing with moisture measurement, mainly in various road structures. In addition, contacts have been established and information exchanged with several technical institutions both in Sweden and abroad. Prototype equipment for moisture measurements has been developed and constructed at the VTI.
The project has led to increased knowledge and understanding of the way in which water behaves in soil and unbound road materials. Furthermore, information has been acquired stating which methods and quipment are currently used for determining water content in situ, for example in road structures.
ISSN Language No. of pages
0347-6049 English 46
Preface
This state of the art includes a description of the development of simple measuring equipment and forms a sub-project in the theme*"Structural Pavement Design" financed by the Transport and Communications Research Board (KFB). The projectstarted in September 1993 and comes to an end with the publication of this report.
The project group consisted of Jorgen Svensson (project leader), Hans G Johansson and Bo G. Karlsson.
VTI MEDDELANDE 8 09A
Development and description of the VTI prototype moisture measuring equipment were carried out by Bo G. Karlsson.
Text and diagrams were edited by Anita Carlsson and Janete de Castro. Special thanks go to Lotta Andersson, at the Department of Water and Environmental studies, Linkoping University for valuable comments and for the loan of moisture measuring equipment for the initial experiments.
Contents
SUINIMNATY nees 9
1 BACKQTOUNG rere reese sre rere eee ese ece sere esen esse ese eres errr reer eres ece e ees 11
2 PUIDOSE esse ees 12
3 MEtROG serre ree reer reese reese seee reena es 13
3.1 Definition of the concept Of MOIStUTLE esse sess sre reese reer rere reer essere seee een es 13
« PTODIEM esses ress serves reese reer esse sess enses enn es 14
35 WAteL 11 the esses r errs sere ress eres seee seee seres rere reran essen nece neenee cen e enne ee eee neenee ee 15 6 Water and its 1MpPOIt@NCE 1N the FOAG seres eres cesses errr sec seee neenee eee ees 18 6.1 Degradation and change in CONGItiON Of the FOAG srs sss sss sss errr seers serre reese esse ener revers 19 6.2 Studies Of Water CONtENt 1N OAQ esses ress seres evere verre errr reese eee se cece rere rere snene eee es 21 7 MEASUIING MEtHOGS ss sss eres esse serre rr esses essere ene cscs cc err rere ener neenee es 24 7.1 Dif@Ct MEtROGS seee sess sree seve sss sees seer esse seee eres seee ees 24 7.2 INGifECt MEtROGS rere reese ree reer esse sees errs re rere rere erence seers eee eee ree eae es 24 8 Assessment of various measuring MethOGdS @NG errr eevee verses srs serre reese ese reer eee. 31
8.1 TDR seee 31
PARE OF]EToTenteAnte e 32
ae iteCecen- e 33
8.4 NMR eee ccs cece ene neenee cece eee eee es 33
8.5 GPR esse seen nee ences scc eee een es 33
8.6 RESIStANCE@ esses sere reve eevee veers reese ece cess ece rere ere seee nees 33 8.7 Othe MEASUIING MEtROGS sess sess reese sere verre errs rere ess ees seee ee eens esse anes ceara neben enne ee nen eee eee ee eee 34 8.8 Conclusions prior to further development of the VTI @QUIPMENt. eee eve eevee eve ev ener rere ee errr ee 34 9 Development Of SIMpPIG@ eee ses esses eee ee sec renee errr reer rere rere neer ern 35 9.1 INItI@l EXPETIME@NUS sees ese essere seers ece ece errr ere reer errr eae ees 35 9.2 PreliMiN@tYy T@SUItS ANG EXPENMIEMCE. reese seres esses ese rere seres reese rere neer ense sneer eee eee reer rae e ees 37 9.3 DeveloOpMeNt Of the VTI EQUIPMENt sess creer reer errr ere eee rere rere ek 40 9.4 D1SCUSS10N Of d@VEIOPMENt CAITI@QU OU. esse eee eee eevee eee eee serre eee ene rere seee ener een nene enne eee ees 42 10 Need for further research @NAQ G@V@IOPMENt. eres sess eres sree cece recs errr sree errr reece rere rere errr ree 43 11 LItt@IAtUIE severs esses esse evere errr sess esses sess cece errr rere ene nees f
11.1 esses esses serre eevee reer crece eee cece nene es cc.
11.2 Othe esses sss reve seers sss esses ress sees esse esse cscs cers errr errr renee ere es 45
Moisture Content in Road Pavements
State of the art and development of a simple moisture measurement equipment By Jorgen Svensson, Swedish National Road and Transport Research Institute (VTI) S$-581 95 LINKOPING, Sweden
Summary
"Moisture Content in Road Pavements. State of the Art and development of a simple measurement equipment" is a subsidiary project under the theme "Structural pavement design" financed by the Swedish Transport and Communications Research Board.
The project aims both at increasing knowledge of moisture measurement in roads and designing a simple and reliable measuring equipment for installation in the pavement structure. Moisture measurement in roads aims primarily at determining/assessing the water content in the unsaturated zone of the soil, which includes the pavement structure.
In a pavement structure, water can behave as free, capillary and adsorptive bound. Water occurs in the liquid phase, solid form and vapour. The interaction between soil, water and climate is of decisive importance for the ability of the road to withstand traffic loads. Unfavourable combinations of soil and water in conjunction with spring thaw period or heavy precipitation may lead to reduced bearing capacity and damage, primarily on minor and nonconstructed roads. This leads in turn to limited accessibility. The water content also influences the intensity of the deterioration of road construction materials and subgrade through weathering and leaching.
During 1980s, there was increasing interest in measuring the water content in pavement structures. In the USA, the SHRP (Strategic Highway Research Program) was started by the U.S. Department of Transportation, Federal Highway Administration (FHWA). The activities included a study of the change in condition of selected roadsections. To clarify how variations in factors such as temperature and water content influence the pavement structure, a program were also initiated for long-term seasonal observation designated LTPP (Long Term Pavement Performance). Similar experiments have been completed or are in progress in Australia, Northern Ireland and the Netherlands, among other countries. In Sweden, continuously moisture measurement has not so far been used in studying long term changes of road conditions.
VTI MEDDELANDE 8 09 A
Methods of determining the water content of the soil may be divided into direct and indirect types. The direct method, gravimetric, entails excavating and drying a specimen, and provides the most reliable result if the procedure is carried out carefully. However, it is limited since further measurement at the same sampling point not will give comparable results owing to disturbance in the soil.
Indirect methods, on the other hand, use a radiation source or measuring probe in the soil. The sensibility of these equipments are influenced by the water content of the soil. Indirect methods may in turn be classified according to measuring principle. In one case, the surrounding soil constitutes part of the measuring circuit. Radiation or electromagnetic pulses are influenced by variations in the water content of the soil. In the other case, the measuring body adjusts to the water conditions in the surrounding soil. Changes in the measuring body produce variations in pressure or resistance, for example. None of the methods and equipment currently in use dominates moisture measurement in soil. The purpose of moisture measurement is decisive for the choice of equipment. In investigations of the water content in road structures, TDR (Time Domain Reflectometry) is the method most widely used. TDR is an indirect method in which the measuring probe senses changes according to the permittivity (dielectric constant) of the surrounding soil. Permittivity is related to the water content of the soil.
Parallel to the state of the art, a simple and inexpensive measuring equipment has been developed. The measuring principle is based on the surrounding soil constituting part of the measuring circuit. Variations in the water content of the soil causing measurable changes. A prototype has been built and laboratory measurements have been made to study its accuracy and reliability in unbound materials. Further on the equipment will be tested in situ in a pavement structure.
1 Background
"Moisture Content in Road Pavements, state of the art and development of a simple moisture measurement equipment " forms a subsidiary project under the theme "Structural pavement design", financed by the Swedish Transport and Communications Research Board, KFB. In a road structure, many factors must interact in the best possible way if the road is to meet the requirements imposed on it. The composition and technical properties of the materials used must also comply with standards and requirements. Road design is studied by road management authorities and others in order to monitor changes in condition, i.e. degradation. Bearing capacity, rutting and evenness are examples of the factors investigated. Analyses of the measurements aim at optimising current methods and techniques in maintenance work. Investigations are also a part of development work forming a basis for new methods and techniques. The experience and knowledge gained are successively incorporated in directives/manuals for road construction.
The particle size distribution and water content of the soil are important parameters influencing its properties. There are several methods and types of equipment for determining soil properties. In the laboratory, the strength properties of soil and road building materials can be determined by using apparatus such as triaxial equipment to simulate the actual conditions in the road.
Unfavourable combinations of soil and water, mainly on minor and non-constructed roads, can lead to problems and damage in connection with thawing or
VTI MEDDELANDE 8 09A
periods with large amounts of precipitation. In Sweden, use is made of established methods such as open ground water pipes or pore pressure gauges in order to follow the fluctuations in the saturated zone (ground water level). However, no moisture measurements are currently being made in Swedish road structures for studying variations in the unsaturated zone above the ground water level resulting from weather and seasonal variations.
Measurement of water content in situ in the road structure and subgrade above the saturated zone is an urgent need. This information can provide a basis for a more detailed interpretation and understanding of bearing capacity measurements, condition assessments and changes in material properties resulting from processes such as weathering and leaching. Research into moisture measurement in roads and the development of a simple moisture measuring equipment are also aimed at increasing knowledge of soil and the technical properties of unbound road construction materials, in addition to methods by which these can be evaluated and used in the structural design of roads.
The state of the art has been concentrated on moisture measurement in road structures. At the same time, there is a similar need for moisture measurement in railway embankments and on airfields. Measurements in these contexts are most interesting with regard to unbound road building materials owing to the influence from traffic loads, and also with regard to the unsaturated zone of the subgrade in order to study possible capillary water transport above the saturated zone.
2 Purpose
The purpose is to produce a state of the art and increased competence concerning moisture measurement in soil and unbound road construction materials, mainly in the unsaturated zone of the ground. This will in turn generate increased knowledge and understanding of the
12
interaction between soil, water, climate and road condition. A further objective is to locate or develop simple equipment together with a method for in situ moisture measurement in road structures.
3 Method
The state of the art is based partly on a report (Nieber and Baker 1989), describing various methods and equipment for moisture measurement in soil, and partly on a survey of specialist periodicals, conference proceedings and research reports in the areas of bearing capacity on roads and soil mechanics. Literature reviews and knowledge acquisition have focused on both national and international publications.
Contacts with national and international educational and research institutions working in moisture measurement in the natural soil or in connection with road construction have also contributed to knowledge acquisition.
A study has also been made of specialist literature, training manuals and handbooks.
Development work began with an inventory and survey of simple, easily available measuring equipment.
VTI MEDDELANDE 8 09 A
Based on the results of initial tests and experience from the literature review, a VTI prototype has been designed and developed. The decision to manufacture independent equipment was based on utilising the Institute's competence in measuring technology and thereby producing equipment that could be adapted to our requirements for installation in road structures.
3.1 Definition of the concept of moisture measurement
In this report, the concept of moisture measurement in roads is defined as methods and equipment for assessing/ determining the water content in unbound road construction materials and soils, mainly above the zone of saturation in the ground, see Chapter 5.
4 Problem
The investigation has been based on the following three issues:
1. What are the reasons for measuring water content continuously?
14
#A
3.
What methods and equipment are available for moisture measurement? Which is most suitable? Can VTI develop a simple suitable equipment specially adapted for moisture measurement in roads?
5 Water in the ground
Soil can be defined as consisting of a fixed and a variable portion. The fixed portion consists of mineral particles and in some cases also organic material. The variable portion consists of water and air. Water can appear in solid, liquid or vapour phase. In principle, water occurs in the soil as free, capillary, adsorptive and chemically bound. Water occurs in the pores, or voids, between the particles and is more or less tightly bound to the mineral particles in the soil, Figure 1.
The water content of the soil is mainly determined by precipitation and other climatic factors.
Mineral and rock particles
Water
Movements ofwater are determined by the relation between drainage and retaining forces, which in turn are dependent
on the water content.
The ground can be classified as follows with regard to the presence of water (Knutsson and Morfeldt 1993), Figure 2.
1. Root zone
2. Zone of gravitational water 3. Zone of capillary water 4. Ground water zone
LNCol}, Organic matter
Figure 1 Solid substance, organic matter, water and air in soil.
Ground surface Root zone Zone of gravitational water Upper capillary limit Lower capillary limit Zone of capillary water
Ground water table L1 Ground water zone
Water content, schematic
Unsaturated zone
Saturated zone
Figure 2 Schematic division into zones ofwater beneath the ground surface. (After Holmstrand and Wedel 1976).
The root zone is limited upwards by the ground surface and downwards by the lower roots of the vegetation. The type of soil and vegetation influence the thickness of the root zone. The amount of water in the root zone varies widely during the year owing to seasonal variations in water uptake by the vegetation.
The zone of gravitational water can in general be said to comprise the whole zone between ground surface and ground water level, but in particular the zone between the root- and capillary water zone. Precipitation and melt water drain further through the zone of gravitational water when field capacity of the soil are exceeded. The field capacity index is the amount of water the soil can retain with the aid of surface tension and adsorptive forces. Excess water, or free water, drains or sinks to the ground water.
The capillary water zone contains water which, as a result of surface tension in the soil and rock, is retained in the smaller voids. The capillary water is sucked up from the ground water zone. Depending on the particle size distribution and density index of the soil, which influence porosity, capillary water may rise and be retained at different levels above the ground water level, see Table 1. It is difficult to state the capillarity for tills owing to the heterogeneous composition of the soil and the fact that capillary determination is always performed on sieved material with a fraction of <2 mm. As a rule, an increasing content of fines with a fraction of <0.06 mm leads to higher capillarity.
Apart from particle size distribution and density index, a decisive factor for the extent of the capillary zone is whether the capillary level results from rising or draining water. The capillary rise reaches a lower level when water is sucked up in a dry soil material than if the same material is saturated and exposed to draining. Up to the ground water level, the ground is saturated, i.e. all pores and voids are full of water. Above the ground water level, the ground is unsaturated. At the ground water level, the water pressure is equal to atmospheric pressure. Below ground water level, the water pressure increases with depth. Water in the soil above the ground water level is retained by adsorptive and capillary forces. The adsorptive force is of an electrostatic nature. Adsorptive bound water surrounds the mineral particles as a thin membrane. The closer to the surface of the mineral particles, the more tightly bound is the water. Part of the adsorptive bound water is termed "hygroscopic water" and is very tightly bound to the particle surfaces; pF >4.8, see below. Capillary water is taken up and retained in the smaller voids as a result of surface tension.
16
The amount of adsorptive, and hygroscopic water is governed by the particle size distribution of the soil/ material, the mineralogical composition and air humidity from which the hygroscopic water is adsorbed. The adsorptive and capillary forces are influenced by draining (gravitation) and drying-out (suction).
In the unsaturated zone, the water has a lower pressure than the surrounding air. Water in a porous material is in equilibrium when the matric potential corresponds to the pressure difference between water and air. The water's binding to the mineral particles, matric potential, is the collective effect of the capillary and adsorptive forces.
The matric potential, or pore water negative pressure (tension, suction) of the soil, which is the relation between water content and pressure difference, can be illustrated in a pF diagram, see Figure 3. In the pF diagram, the water content is expressed as a function of the logarithm of the negative pressure, expressed in cm water column. Higher pF values indicate that the water is more closely bound to the mineral particles. A pF value of 2 corresponds to an negative pressure of
100 cm water column.
A hydrological manual (Falkenmark and Forsman 1974) also describes the classification of water below the ground surface. In this context, the unsaturated zone is also referred to as the zone of aeration. The designation "zone of aeration" is more suitable when comparing the occurrence of water in a road structure.
Table 1 Approximate values of capillarity for a number of selected soil types.
Soil Type Capillarity
Coarse sand 4-5 cm
Medium sand 12-50 cm
Fine sand 40-120 cm
Silt 1.5-10 m
Clay >10 m
100 000 m.w.c 100 1000 10
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Figu re 3 Example of a pF diiagram sh owing th erelatition between volumetriric water content and matritric potentiential fo r a
6 Water and its importance in the road
In comparison with the above, a road structure can largely be regarded as a soil air zone. The lower part of the road structure, the interface to the subgrade, can in many cases be considered to belong to the saturated ground water zone or lower part of the capillary water zone, see Figure 2 above. The position of the road in the terrain, the profile level, the soil composition of the subgrade and the hydrological conditions are of decisive importance. A road built deep in a cutting or on a low level in the terrain will be influenced more by proximity to ground water and surface water than a road in a higher position.
One of the differences between a natural ground surface and a road surface is that the pavement, particularly on newer roads, may act as a more or less close fitting cover. This prevents water from penetrating the road and also seals against evaporation. Often, the pavement is not completely sealed which allows water to penetrate it in greater or lesser extent. Vertical transport of water vapour due to the temperature gradient beneath the pavement has proved to be negligible (van Schelt et al. 1994).
Precipitation and melt water collect on the surface and penetrate into underlying layers. The composition, thickness and cracking in the wearing course and other bound layers are very important for the quantity of water
tftA
that penetrates the road. Pavement joints and road surface inclination in both longitudinal and transverse directions also affect water penetration. The particle size distribution and layer thicknesses of the road building materials determine how much of the penetrating water is retained.
Water may also originate from the surrounding terrain. Owing to deficiently designed or maintained drainage of the road structure, water may penetrate it from the side, see Figure 4. Grading and clearing of the ditches sometimes leads to fine-grained material being deposited in a layer on the inner slope, which may lead to water being trapped within the road structure. Water may also penetrate the formation, often in a cutting, owing to high pressure ground water flows from higher level terrain nearby.
Water may be transported by capillarity upwards in the road structure from an underlying ground water level. This often occurs in non-constructed roads, 1.e. roads not designed with a sub-base and/or roadbase, for example, and on roads where unsuitable materials have been used in construction. If the road is built of standard materials, no capillary suction will occur.
In the complicated and complex energy exchange between road surface and atmosphere, small amounts of water may also occur due to condensation.
Figure 4 Water liable to penetrate a road structure from the side owing to inadequate drainage. (Photo VTI).
New roads built as part of the public road network are designed according to the requirements of the National Road Administration set out in the VAG 94, general technical description for road structures. The directives on unbound material include the requirement that the water must be able to drain away. There are also requirements on formation and ditch design with regard to geometry, material, proximity to groundwater, draining and freezing index. Here, the term "freezing index" refers to the sum of the average daily temperatures, both positive and negative, during a winter season, and provides a measure of how cold a winter season has been.
In older roads, the materials used often have a large variation in composition and function depending on where, when and how they were built. In newer roads built in accordance with the relevant standards, the water retention capacity of the unbound materials may increase due to small changes in particle size distribution. The increase in the quantity of fines, material fraction <0.06 mm, may in these cases be due to poorer rock quality and/or unfavourable effects of construction traffic on the roadbase. This may lead to accelerated degradation and damage to the road structure.
6.1 Degradation and change in condition of the road
Degradation and change in the material properties of the road in standard and non-constructed roads are often mainly due to material migration, in addition to mechanical and chemical weathering processes. The load
on the road structure, material composition, temperature, time and presence of water in the various material layers of the road, including the subgrade, are decisive factors for degradation and thereby the change in condition.
In Sweden, the thawing period and rainy autumns have been considered to most critical for road conditions, particularly in the case of older, smaller and non-constructed roads. It is primarily these types of road where it is important to have control over the water content. In new roads built according to standards, the thawing periods or periods with heavy precipitation normally do not lead to any problems regarding road condition.
During ground freezing, water is transported through suction from unfrozen soil to the freezing front in the soil, see Figure 5. This often results in a water surplus that in thawing can be drained only to a small extent through the unfrozen, air filled pores in the underlying frozen soil. The frozen subgrade can therefore be regarded as more or less sealed.
In a soil volume, equilibrium exists between the amount of soil particles, the pore volume and the amount of water when the soil returns to its original form after being loaded by a vehicle, for example. The elastic properties of the soil, apart from those of the soil particles themselves, are due to the presence of water between the particles, mainly capillary water. In this context, the adsorptive and hygroscopically bound water has less influence on the mechanical properties of the soil.
Air temperature below freezing point
Frozen pavement <- O _-C- <-> Frozen i ll subgrade <- <--> <-> o <C> Unfrozen
T
T
T
subgrade
I
1
T
|-
Wearing course
Road base
Sub base
Ice lenses in
fine-grained sediments
Freezing front
Capillary water sucked
T T towards the freezing front
Figure 5 Schematic diagram of the groundfreeze process.
VTI MEDDELANDE 8 09 A
Ground water table
Schematic diagram of falling weight deflectometer ! Deflection [~- Sensors K
|
__T
- O p J
''''''''''''''''' nnnnnnnnnnnnnnnnnnnuuuuuu nnnnnnnuuuuuuu1111111llllll---nnnnnn nnnnnnn---nnnnnnnnnnnn---nnnnnnnnnnnnnnnnnn ------El] Falling weight deflectometer II I Computer
!
|
Wearing course
| Road base Sub base Sub grade!
!
!
!
!|
Figure 6 Principle offalling weightdeflectometer. (AfterHakan Carlsson, VTI).
When there is too much water in the pores of the
soil, as in thawing orafterextensiveperiods with heavy
precipitation, the pore pressure may increase
considerably in the soil with repeated loadings. The
capillarywaterisforcedoutofthefilledvoids, orpores,
and the soil material becomes highly unstable owing to
the friction between the mineral particles decreasing
drastically or vanishing altogether. This may result in
migration, such as frost boil, of the material, whereby
its original function more or less ceases.
Another effect of freezing may be uneven frost
heave, which in many cases leads to cracking of the
pavement. Water penetrates the cracks, which in turn
accelerates degradation of the road.
During the winter season, when thewaterinthe soil
freezes and thaws, the water may penetrate cracks or
small voids between the mineral particles, which freeze
anddisintegrateintoeven smallerparticles.Thisisaform
of mechanical weathering, frost action, which affects
road-buildingmaterials.Weatheringoftheparticles may
also be of a chemical nature, as in leaching of certain
minerals orbasic elements. Degradation attributable to
weathering and leaching includes processes resulting
from access to water, cold and heat.
Frost susceptibility and water retention capacity,
capillarity, are intimately linked with the particle size
distributionoftheunboundroadmaterials and subgrade
20
soils. Reduced bearing capacity, in addition to damage
leading to permanent deformation, is often the resultof
unfavourable combinations of soil, water, climate and
loading.
The bearing capacity can be determined by falling
weight deflectometer, which is often performed within
the critical period ofthawing or late autumn. In falling
weightdeflectometermeasurements, Figure6, aweight
withknownmassandsizeisreleasedfromagivenheight
onto the road surface. The retardation ofthe weight on
impact and the depression in the road surface are
recorded. The depression is recorded both at the centre
and at predetermined distances from the centre. The
values obtained are used to calculate the elasticity
modulus (E-modulus) for the respective layer in the
road. Thedecreaseinbearingcapacitythatmaybefound
in the form of a lower E-modulus may be attributed to
changes inproperties ofthe road structure, embankment
and/or subgrade. The E-modulus of a material is a
measure of its stiffness or deformation resistance.
Experience has shown thatroad sections with more
or less clear signs of damage depending on deficient
bearing capacity are in many cases located in low parts
of the terrain and in cuttings with defective drainage.
Damagesoftenappearsintheformofrutting, longitudinal
cracks and alligator cracking.
In summary, the water content influences the unbound road construction materials and the technical properties of the subgrade soils influenced through:
e reduced bearing capacity at an increased water content, owing to lower internal friction between the material particles,
e the frost process,
e migration of material in connection with thawing, e weathering and leaching.
The processes described above are traditionally the dominating factors in the context of influence on road condition. Since the mid-eighties, the question has been raised of the way in which temperature and water content in the road structure vary in daily and seasonal cycles, apart from the thawing period and autumns with large amounts of precipitation. Researchers occupied with unbound road construction materials are generally agreed that variations occur due to climate, which in some way affect the layers in the road structure.
Continuous measurements of water content in constructed and non-constructed roads are important for studying the size of the variations and their importance in the relations between soil, water, temperature and bearing capacity. Measurements of water content are particularly important in unconventional materials, such as slag, with regard to leaching and environmental effects. Weather data such as temperature and precipitation must be monitored continuously, together with the groundwater levels in the saturated ground zone, when measuring the water content in the unsaturated zone of the subgrade and road structure. The investigations are aimed at increasing understanding and knowledge so that future roads can be designed with closer attention to variations in temperature and water content.
6.2 Studies of water content in road structures The SHRP (Strategic Highway Research Program) was started in the mid-eighties in the US Department of Transportation, Federal Highways Administration (FHWA). The wide variety of activities included studying a large number of road sections with regard to change in condition. To clarify how the variations in temperature and water content influence a road structure, a special programme for studying seasonal variations was started as part of the long-term follow-up LTPP (Long Term Pavement Performance).
VTI MEDDELANDE 809A
The programme (Rada et el. 1994 and Hadley et al. 1994) consisted of choosing 64 out of about 3,000 test sections for continuous measurements and investigations of:
e Variations in water content and temperature beneath the road surface,
e Frost penetration and thawing, e Fluctuations in the ground water level, e Air temperature and precipitation.
The test sections represented a variety of road structures, subgrade conditions and climate situations in North America.
In addition to these investigations, continuous measurements are made. Determinations of bearing capacity and observations of pavement joints are made at least once a month. Levelling of the road surface to determine frost heave or the effects of swelling soils is performed at least once every season. During each season, measurements are also made of the longitudinal and transverse evenness of the test section.
The illustration of instrumentation in Figure 7 shows where and how various measuring equipment is located. The road structure consists of wearing course, roadbase, sub-base and subgrade. Measurement of the water content in the soil is performed with TDR probes (see Chapter 7), frost penetration and thawing with a resistivity probe and temperature with a thermistor probe. Air temperature and precipitation are recorded. Fluctuations in the groundwater levels are measured with a piezometer.
At the CRREL (Cold Regions Research and Engineering Laboratory) in the US Army
Corps of
Engineers, research is conducted into seasonal
variations in temperature and water content in road
structures and airfields. Questions concerning the way
in which variations in temperature and water content
influence embankment structures on airfields have also
interested the FAA (Federal Aviation Administration). The
FAA has funded research in this area with the aim of
improving embankment structures and making better
assessments of how these are influenced by climate
factors.
Examples include investigations performed by the
CRREL on an LTPP test section in Vermont, Berlin Town
Highway 27, the new international airport at Denver, and
the CRREL test area in Hanover, New Hampshire (Janoo
et al. 1994). Initial results show that it is possible to
follow seasonal variations in water content in the various
road structures.
TOP VIEW
SIDE VIEW
TDR Probes -__ {rotated 10° to keep cables in
... one layer along
-_' 33531"s side of bore hole) f Rain Gauge
2ainl k Resistivity Probe jg _.--- Temperature
<] sates. ~_|
g
Pavement Surtace
|
{mmm
o.g. 55mm (2 in) - 523551?
direction from hole:
' _\
LThonnlstor Probe
Figure 7 Instrumentation of LTPP section (Rada et al. 1994).
Investigations aimed at studying temperature and water content in different road structures have been performed or are in progress at several Universities and Institutes of Technology in the USA. Collaboration between the University of Minnesota, Civil and Mineral Engineering Department and the Minnesota Department of Transportation, Physical Research Section, has resulted in the Mn/Road project. One of the aims of the project is to study seasonal changes with regard to temperature and water content in road building materials. Since the properties of road building materials change in relation to climate variations, it is considered important to make measurements on materials and conditions in an actual road.
Along Interstate Highway 94 outside St Paul/ Minneapolis, test sections have been built with various designs and instrumentation. Some of the sections have been laid parallel to the existing highway. Here, investigations are being performed over 5 and 10 years respectively, starting in 1994. The advantage of this part of the installation is that normal highway traffic can be diverted in order to load the test sections. Before
22
measurements and studies are made on the test sections, the traffic is routed back to the normal highway. The other part of the installation consists of a separate road section resembling a road with low traffic volumes, which in turn is divided into a number of test sections. One of the many operations on these test sections consists of measuring and studying water content in the pavement layers and subgrade of various road structures. Water content measurements are performed with TDR,
resistance measurement and neutron probe, see
Chapter 7.
Apart from the USA, several projects have been performed in Australia with the aim of studying water content both in cultivated land and in road structures. In a collaborative project between the Australian Road Research Board and Queensland Transport, a full-scale test with the ALF (Accelerated Loading Facility) was performed. Ten test sections were instrumented with moisture measuring equipment to record the water content in the road structure in connection with the full scale test (Baran 1994). The project also aimed at studying the reliability of the moisture measuring
