Causes of unbound flexible pavement distress Essay Example

causes of unbound flexible pavement distress

Infiltration Rate of Hume Freeway near Wallan

Introduction

The rate of infiltration through a pavement structure depends on the hydraulic conductivity, porosity and size of particles used. These parameters may vary over the length of the freeway. In some areas with high rainfall regimes, infiltration into the pavement — specifically lateral infiltration through defected wearing surfaces, through joints, or through unsealed shoulders – can have a very big influence on the conditions of moisture in the layers of the pavement (Department of Transport and Main Roads, 2013). This needs a specific action to be taken to guard the pavement against water infiltration. However, it should also be noted that reducing the rate of infiltration through a pavement reduces the rate of water percolation into the soil, resulting in increased volumes of storm water runoff. To prevent flooding, a proper drainage should be taken into consideration to ensure that there will be no excessive flooding during heavy rainfall.

Effects of Infiltration on flexible pavement

Infiltration into the layers of a pavement structure, especially in the aggregate base layer and the sub-base can weaken the materials used in these layers by building pore pressure and reducing their resistance to shear. As the storm runoff propagates into pavement layers, it can result in debonding of interfaces of the layers within the pavement structure and weakening of subgrade and granular layers. In addition, infiltration increases the level of moisture that may cause differential heaving in the layers of a pavement (Austroads, 2012). This will cause the pavement structure to heave up as the lower layers expand. Increase in moisture content in the flexible pavement structure also enhances distress and pavement deterioration

Determination of a suitable rate of infiltration into the pavement structure is therefore, very important such that there will be minimal infiltration into the pavement layers and the subgrade. This will ensure that the performance and service life of the structure is not compromised by infiltration from storm runoff.

Determining the infiltration rate of Hume freeway

The infiltration rate for a flexible pavement depends on a number of infiltration factors, such as nature of soil, types and particles size of construction materials used, slope, and rainfall characteristics, such as mean annual rainfall and mean rainfall intensity. According to Country Roads Board of Victoria, Technical Bulletin No 32;

Infiltration rate = Infiltration factor × mean intensity of a two-year, 1-hour rainfall

The table below shows different infiltration factors that determine the amount of water expected to flow through a flexible pavement structure based on the material on the wearing surface course. We can use this values to predict the rate of infiltration expected on Hume freeway near Wallan town.

Table 1: The standardinfiltration factors expected in a flexible pavement (Source: Country Roads Board of Victoria, Technical Bulletin No 32).

Aggregate of Surface Type

Infiltration Factor

Sprayed Seal

0.2-0.25

Cement concrete

Unsealed shoulders

It is mainly the storm water that propagates through the surface course affects the moisture content of the pavement structure, even though sometimes water in the subgrade soil may also rise into the sub-base and base layers through capillary. However, infiltration occurs through the binding/sealing surface layer and through sealed shoulders, and for this reason, they play an important role in controlling the rate of infiltration (Vicroads, 2013). In this particular case, the binding element used in Hume freeway is asphalt in two layers of 50 mm asphalt. The most appropriate infiltration factor to be used in this case is 0.2-0.4.

Determining the rate of infiltration using time of concentration of a storm (Tc)

The rate of infiltration can be obtained using Tc, which is determined by the following relation:

Tc =
causes of unbound flexible pavement distress 1

causes of unbound flexible pavement distress 2— Non-dimensional time parameter that can be obtained from the Green and Ampt graph (attached at the end of the document).

causes of unbound flexible pavement distress 3 – Fillable pore space

D – Thickness of shoulder layer (mm)

Kr – Permeability (m/s)

Rainfall depth (Rd) is obtained from the relation:

Rd =
causes of unbound flexible pavement distress 4

The actual rainfall intensity rate (Rs) that will provide this amount of water (Rd) in a time Tc is given by:

causes of unbound flexible pavement distress 5

Rs is then entered in the relevant rainfall intensity diagram after selecting a recurrence period. These diagrams are provided by the Bureau of Meteorology. When the rain duration TD (hrs.) has been determined, for which Rs will fall over the period of pavement design, TD is compared with Tc.

If Tc
causes of unbound flexible pavement distress 6 TD, the shoulder is impermeable. If Tc
causes of unbound flexible pavement distress 7 TD, the standard infiltration profiles are used to determine the rate of infiltration by applying the relevant profile. The moisture infiltration profiles (attached at the end of the document) indicate the extent of wetting up for different multiples of Tcauses of unbound flexible pavement distress 8and enable the determination of the pavement moisture condition at all distances from the pavement seal edge. Tc represents the shortest rainfall duration in which saturation of the pavement shoulder can occur. Saturation may also be experienced over longer durations of lesser rainfall intensity or from multiple rainfall events that are closely spaced.

Determining of Infiltration rates through different layers of the pavement structure by Different methods and formulas:

  • Using Country Roads Board of Victoria, Technical Bulletin No 32
    to Determine Permeability through the pavement layers;

  • For Asphalt layer

According to Australian Bureau of Meteorology, Wallan receives an average annual rainfall of about 544 mm. The maximum mean rainfall intensity is about 65 mm in a duration of one hour. Using an infiltration factor of 0.2;

Infiltration rate through the asphalt layer = 0.2 x 65 mm/hr. = 13 mm/hr = 1.3 cm/hr

causes of unbound flexible pavement distress 9Infiltration rate through the asphalt layer =
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  • For
    Base Layer (Labelled Layer C) – Crushed basalt, nominal size 20mm, using an infiltration factor of 0.3

Infiltration rate through the base layer = 0.3 x 65 mm/hr = 19.5 mm/hr = 1.95 cm/hr

causes of unbound flexible pavement distress 11Infiltration rate through the base layer =
causes of unbound flexible pavement distress 12

  • For
    SubBase Layer (Labelled Layer D and E) – Siltstone nominal size 40mm, using an infiltration factor of 0.4

Infiltration rate through the sub-base layer = 0.4 x 65 mm/hr = 26 mm/hr = 2.6 cm/hr

causes of unbound flexible pavement distress 13Infiltration rate through the sub-base layer =
causes of unbound flexible pavement distress 14c

However, a special ARRB Report No. 35 suggested that infiltration rates obtained by using mean 2-year, 1- hour rainfall intensity is likely to overestimate the rate of infiltration through a pavement.

So the team adopted a more accurate way to estimate the permeability by using Austroads Technical Report No. AP-T53/06 and Berg’s model, taking the asphalt surface layer to be permeable with substantial amount of water entering the pavement under traffic. The range of permeability coefficient will range from 10-100 x 10-5 cm/s (category C) as indicated in Austroads Technical Report. The upper permeability limit can be takes as,

Permeability (k) of asphalt layer = 100 x 10-5 cm/sec.

Insert table of Austroads

  • Using Berg’s Model to Determine Permeability through the pavement layers

  • For Base Layer (Labelled Layer C) – Crushed basalt, nominal size 20mm

Berg’s model relates geometrical properties of particles such as size, shape and sorting to permeability. The expression relating all these parameters is shown below:

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causes of unbound flexible pavement distress 16= Permeability (in darcies)

causes of unbound flexible pavement distress 17 – Percentage of porosity

causes of unbound flexible pavement distress 18 – mean diameter of the particles

causes of unbound flexible pavement distress 19 – a sorting term

A sorting term is a value dependent on the mean of particle sizes. In this case, the particles used in a single layer are of the same size, therefore, the value of
causes of unbound flexible pavement distress 20 = 1 (no variation in particle sizes).

According to Texas Department of transportation, porosity in the pavement structure should not be less than 3% or exceed 8%. However, according to Argonne National Laboratory, the porosity of crushed rocks ranges between 3% as minimum value up to 35% as a maximum porosity value.

Based on those facts, and for the purpose this study, a conservative approach will be taken in obtaining the porosity values, therefore the taken values are within this range are 5% or 6%. So that,

For 3% porosity

causes of unbound flexible pavement distress 21=
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causes of unbound flexible pavement distress 25= 0.14causes of unbound flexible pavement distress 26

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For 4% porosity

causes of unbound flexible pavement distress 28=
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causes of unbound flexible pavement distress 32= 0.6causes of unbound flexible pavement distress 33

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For 5% porosity

causes of unbound flexible pavement distress 35=
causes of unbound flexible pavement distress 36

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causes of unbound flexible pavement distress 39= 1.87causes of unbound flexible pavement distress 40

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  • For Sub-Base Layer (Labelled Layer D and E) – Crushed siltstone, nominal size 40mm

The sub-base layer is made of larger particles. The allowable porosity should not exceed 8%, according to Argonne National Laboratory, the effective porosity of Siltstone ranges between 1% as minimum value up to 39% as a maximum effective porosity value. Based on those facts and for the purpose this study, a conservative approach will be taken in obtaining the porosity values, therefore the taken values are within this range 7% or 8%. So that,

For 3.5% porosity

causes of unbound flexible pavement distress 42=
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causes of unbound flexible pavement distress 45

causes of unbound flexible pavement distress 46=causes of unbound flexible pavement distress 47 =causes of unbound flexible pavement distress 48

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For 4.5% porosity

causes of unbound flexible pavement distress 50=
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causes of unbound flexible pavement distress 54=
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For 7% porosity

causes of unbound flexible pavement distress 58=
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causes of unbound flexible pavement distress 62=
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The sub-grade (Labelled Layer F) – Fill Material

This layer is made of natural filler materials as specified in the code of practice shown in the table below.

Table 2: Subsurface filler requirements (Vic Roads – RC 500.22)

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We have the sub-grade of the freeway made of silty or sandy clays and stratified clays with moderate to low permeability (Vicroads, 2013). The permeability of this layer, according to table 2 would range from 10 -9 to 10-5 m/second. Taking an average limit of;

Permeability (causes of unbound flexible pavement distress 68) =
causes of unbound flexible pavement distress 69 =
causes of unbound flexible pavement distress 70/sec

During a heavy rainfall intensity, the pavement reaches a saturation level when infiltration is expected to be maximum. At the saturation level, the rate of infiltration will be equal to the permeability (k) and moisture saturation,
causes of unbound flexible pavement distress 71= 100%. If a unit area is considered, we can express the rate of infiltration in cm3/cm2/sec as shown in table 3. We know that saturation level is the main factor in strength reduction as in the relation:

Mr = 45.2 – 0.428Sr, where unit is in Ksi

1 Ksi = 6.895 Mpa

This implies that at 100% saturation level, when the rates of infiltration will be as determined, the strength reduction in the pavement structure will be maximum.

Effect of Traffic Load on the rate of infiltration

According to the data supplied by VicRoads on characteristics of weight in motion (WIM) at Wallan site in Victoria in 2010 and 2015, the following information is available.

Insert table from 2015 wim data

Heavy vehicles count in 2015 = 3100 Hv per day

Growth percentage = 4.1%, Fluctuation =
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Axle groups per heavy vehicle (HVAG) = 3.29

Predicting the heavy vehicle count for 2047 and taking highest growth rate of 4.1 + 0.7 = 4.8%;

causes of unbound flexible pavement distress 73=
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If only 25% of heavy vehicles are expected to pass on the pavement during a 1-hour rainfall that is falling in the peak hour of the day (with mean intensity) and exert pressure on the pavement, the number of heavy vehicles passing at this period of time will be:

(0.25 x 13260.55) = 3315.137 HVs

Insert table page 3 of Andrew-Papacostas-witness-statement

The standard axle load applied on the pavement from the above axle group is 181 kN. This implies that in one hour of rainfall, 3315 heavy vehicles with a standard axle load of 181 kN pass through a single lane of the flexible pavement. Apart from gravity, this is the additional force expected to push storm water on the pavement through the asphalt layer into the underlying layers of the pavement structure. The water trapped between the wheels and the pavement surface is forced into the pavement structure. Increased number of axles causes further increase in the pore pressure in the pavement structure, especially if the heavy vehicles drive close to one another at the same wheel path. The increased pore pressure causes a decrease in the stiffness and increase in deformations of pavement materials. Under consecutive loading repetitions from heavy vehicles, the rate of water infiltration increases as the pavement structure will not have time to recover before another axle load through the same wheel path. This will definitely increase the infiltration rate which will significantly deteriorate the pavement.

Conclusions and Recommendations

The layers of the pavement structure will have different rates of infiltration rates, depending on the material used, size of particles, and pore size. Larger particles exhibit increased pore size and subsequently, increased rate of infiltration. The recommended infiltration rates for each layer are summarized below:

Table 3: Infiltration rates

Pavement Layer

Infiltration rate (cm/sec)

Asphalt surface layer

Crushed Basalt

5.78 x 10-4

Siltstone

1.17 x 10-3

Sub-grade

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The infiltration rate is lowest at the top layer and highest at the sub-base layers. The idea behind reducing permeability of the surface course and the sub-grade is to prevent entry of water into the pavement structure, to reduce deformations on loading.

Storm durations of 2, 3 and 4 days.

For low emission scenario, the design storm peak is estimated to take place at 2039, with a 2 days storm duration.

The difference between 2039 and 2016 (current year) = 23 years.

The estimated traffic volume AADT at 2039 =
causes of unbound flexible pavement distress 76= 9,114 HV/day

If the estimated storm duration is 2 days, therefore the estimated number of heavy vehicles expected to pass over the investigated pavement is 9,114 x 2 = 18,228 HV

If the vehicle (HVAG) = 3.29, therefore the total number of axles expected to pass on the investigated pavement over the estimated storm duration is:

18,228 x 3.29 = 59,971 = 60,000 axles.

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Assuming that the pavement is frequently maintained, category B Asphalt’s permeability and aproximately7.5% air voids is adopted = 7 x 10-5 cm/s

Theoretically, this value represents permeability of water in the asphalt under gravity alone, therefore the assumption is made that, under the load of each axle, which is moving in an average speed of 100km/hr, which is equal to 2.7m/s. Therefore, the assumption can be made that each will take 1 second to pass over the point of interest, and with produce a force that will push the drop of rainfall into the asphalt by 5 x 10-5 cm

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Knowing that the existing asphalt layer’s thickness is 5cm, therefore, calculating how much rainfall water will be pushed into the asphalt:

60,000 x 7 x 10-5 = 4.2 cm

Therefore, only 60% of the asphalt layer will be saturated by the end of the predicted 2 days storm.

For medium emission scenario, the design storm peak is estimated to take place at 2036, with a 3 days storm duration.

The difference between 2036 and 2016 (current year) = 20 years

The estimated traffic volume AADT at 2039 =
causes of unbound flexible pavement distress 79= 7,918 HV/day

If the estimated storm duration is 3 days, therefore the estimated number of heavy vehicles expected to pass over the investigated pavement is 7,918 x 3 = 27,754 HV

If the vehicle (HVAG) = 3.29, therefore the total number of axles expected to pass on the investigated pavement over the estimated storm duration is:

27,754 x 3.29 = 78,151 axles.

Knowing that the existing asphalt layer’s thickness is 5cm, therefore, calculating how much rainfall water will be pushed into the asphalt:

78,151 x 7 x 10-5 = 5.5 cm

Therefore, the asphalt layer will get completely saturated.

Estimating the time it will take for the asphalt layer to get saturated:

X x 7 x 10-5 = 5 cm

X = 71,429 axles

There are 78,151 axles passing in 3 days, therefore 71,429 axles will pass in 2.7 days.

The remaining axles will continue to push water through the asphalt into the base layer with a 10cm thickness. The estimated permeability of the basalt crushed rock 20mm base layer is 5.78 x 10-4.

Finding whether the remaining axles will saturate the base layer:

(78,151 – 71,429) x 5.78 x 10-4

6,722 x 5.78 x 10-4 = 3.9 cm = 4 cm

Therefore, only 40% of the base layer will be get saturated by the end of the predicted 3 days storm duration by 2036.

For Severe emission scenario, the design storm peak is estimated to take place at 2046, with a 4 days storm duration.

The estimated traffic volume AADT at 2046 =
causes of unbound flexible pavement distress 80= 12,654 HV/day

If the estimated storm duration is 4 days, therefore the estimated number of heavy vehicles expected to pass over the investigated pavement is 12,654 x 4 = 50,616 HV

If the vehicle (HVAG) = 3.29, therefore the total number of axles expected to pass on the investigated pavement over the estimated storm duration is:

50,616 x 3.29 = 166,527 axles.

Knowing that the existing asphalt layer’s thickness is 5cm, therefore, calculating how much rainfall water will be pushed into the asphalt:

166,527 x 7 x 10-5 = 11.6 cm

Therefore, the asphalt layer will get completely saturated.

Estimating the time it will take for the asphalt layer to get saturated:

X x 7 x 10-5 = 5 cm

X = 71,429 axles

There are 166,527 axles passing in 4 days, therefore 71,429 axles will pass in 1.72 days.

The remaining axles will continue to push water through the asphalt into the base layer with a 10cm thickness. The estimated permeability of the basalt crushed rock 20mm base layer is 5.78 x 10-4.

Finding whether the remaining axles will saturate the base layer:

(166,527 – 71,429) x 5.78 x 10-4

95,098 x 5.78 x 10-4 = 55 cm

Therefore, the base layer will be completely saturated.

Estimating the time it will take for the base layer to get saturated:

X x 5.78 x 10-4 = 10 cm

X = 17,302

There are 166,527 axles passing in 4 days, therefore 17,302 axles will pass in 0.42 days.

The remaining axles will continue to push water through the asphalt into the base layer with a 20cm thickness. The estimated permeability of the siltstone crushed rock 40mm nominal size Sub-base layer is 1.17 x 10-3.

Finding whether the remaining axles will saturate the base layer:

(95,098 — 17,302) x 1.17 x 10-3

77,796 x 1.17 x 10-3 = 91 cm

Therefore, the Sub-base layer will be completely saturated.

Hence, the whole pavement will get completely saturated in a predicted 4 days duration storm in 2046 due to severe emission scenario.

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References

Austroads, 2006. AUSTROADS TECHNICAL REPORT — Asphalt Permeability (AP-T53/06), Sydney: Austroads.

Austroads, 2012. GUIDE TO PAVEMENT TECHNOLOGY. Part 2: Pavement Structural Design, Sidney: Austroads.

Department of Transport and Main Roads, 2013. Pavement Design Supplement: Supplement to ‘Part 2: Pavement Structural Design’ of the Austroads Guide to Pavement Technology, Queensland: State of Queensland (Department of Transport and Main Roads).

MAIN ROADS Western Australia — Materials Engineering Branch, 2012. Engineering Road Note 9: PROCEDURE FOR THE DESIGN OF ROAD PAVEMENTS, Western Australia: MAIN ROADS Western Australia.

Rada, G. R., 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: Transportation Research Board.

Vicroads, 2013. Selection and Design of Pavements and Surfacings (RC 500.22), Victoria: Vicroads.

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САUSЕS ОF UNBОUND FLЕХIBLЕ РАVЕMЕNT DISTRЕSS

Unbound granular material characteristics are of great significance in the performance of pavement structures. Ideally, pavements are designed to be as much drained as possible, but there is usually presence of moisture even in the pavement systems with good drainage. The unbound layers become partially saturated in periods of excess atmosphere. Moisture content can cause detrimental effects on the behavior of unbound materials. For example, in course-grained unbound granular materials, high moisture content often result in reduced resilient modulus because of reduced particle interlock strength and contact friction. In fine-grained granular materials and sub-grade materials, pore water normally lowers the effective stress of the structure as a result of decreased pore suction which leads to reduction in material stiffness. In addition, presence of moistures causes a lubrication effect. Pore pressure build-up in materials that exhibit low permeability and are frequently exposed to traffic stress loads can experience a considerable loss in bearing capacity.

Possible causes of HMA pavement distress (Table 2.4)

Poor subgrade Compaction – Compaction can be defined as the process of increasing materials’ density using mechanical means. Inadequate compaction of the subgrade as a result of poor construction will result in a compromised material density and structural strength. Excessive presence of moisture will make a poorly drained subgrade that may not compact well to form a flat and proper grade (Rada, 2013). In addition, poor compaction allows infiltration of moisture that can result in inadequate or loss of structural support due to reduced shear strength and settling under repeated loading.

Poor base/sub-base compaction – High density of base/sub-base layer is required in order to develop sufficient cohesion with HMA layer. Like Poor subgrade compaction, poor compaction of these layers can allow permeability and absorption of moisture, and lower the shear strength. These leads to a weak base/sub-base support. Types of distresses that can occur as a result of poor compaction include fatigue or alligator cracking, edge cracks, settlement or grade depressions and upheaval/swell.

Poor base/sub-base gradation – The base/sub-base provides drainage for the pavement structure. Well graded materials provide a uniform support. Performance of pavement structures built with granular base/sub-base is highly influenced by gradation of the materials used. Pavement distress evidenced by traverse and restraint cracks can be attributed to use of poorly graded base/sub-base materials.

High exposure to moisture – Unbound materials in flexible pavements with HMA layer contribute significantly to the structural behavior of the pavement. Moisture content and its variation in these materials affect the stiffness as well as permanent deformation of characteristics of these layers, which can in turn lead different types of distresses such as upheaval and fatigue cracking.

Inadequate pavement structure – Proper understanding of conditions under which a pavement structure will function well requires the designer to have information on surface drainage, pavement geometrics, material properties and the climate (Rada, 2013). Distress types such as subgrade rutting are usually caused by permanent deformation in the pavement layers due to consolidation of materials which all result from inadequate pavement structure.

Poor material selection – This refers to the use ofmaterials that do not meet structural requirements. Poor selection of materialscan cause pavement distresses such asbleeding, segregation and checking.

Poor drainage – Drainage is one of the greatest concern in pavement drainage. Use and placement of materials that allow transmission of water or drain, with the combined effect of traffic loading often causes pavement distress due to the moisture present in the pavement structure. Water can cause significant loss of strength of the pavement.

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Portland cement concrete (PCC) Distress types and possible primary causes (Table 2.5)

Pumping – This is the ejection or seepage of water and fines from underneath the pavement through cracks due to water pressure. As the PCC slab deflects under traffic loading, water accumulated beneath it builds pressure which may make water to move about under the PCC slab, move to the pavement surface, or move to adjacent slab. The overall result is the slow removal of base/sub-base or subgrade material, leading to reduced structural support. Movement from one slab to adjacent slab causes faulting.

Faulting – Slab pumping is the most common mechanism of faulting. Other causes include slab settlement, warping and curling. It is usually noticed by a difference of about 2.5 mm in elevation across a crack or joint, with the approach slab at a higher level than the pavement section.

D-cracking – This refers to cracking of concrete pavements due to the freeze-thaw deterioration of aggregates in the concrete. The D-crack formations are parallel to longitudinal and transverse joints that later proliferate outwards from the joints to center of the pavement system. Accumulation of water in the base and sub-base layers, the aggregate becomes saturated. Through a series of freeze-thawing cycles, the concrete starts to crack near the layer of saturated aggregates to the wearing surface.

Alkali-Silica Reactivity (ASR) – This is a reaction that occurs in concrete between cement paste (highly alkaline) and the reactive non-crystalline silica found in aggregates in the presence of sufficient moisture. This results in expansion of the aggregates by forming a swelling gel (calcium silicate hydrate) which increases in volume with increasing water content. The overall result is that pressure is exerted in the pavement, causing spalling, and reduction in strength and subsequently lead to failure. In serious cases, cracking of a pavement structure may occur.

Freeze-thaw damage – Freezing and thawing occurs when soil moisture freezes in the pavement structure which may cause an upward movement of the sub-grade as the layers expand. Thaw weakening results when the ice-saturated soil melts. These process can cause detrimental effects on the pavement.

Heave/Swell – This is an upward movement of a pavement system due to swelling that occurs in the sub-grade. This is caused by expansive soils that swell as a result of frost heave or moisture.

Conclusion

Moisture has a great effect on the structural stability and performance of a pavement structure. With all other factors remaining constant, increase in moisture content can result in accelerated rate of pavement distress.

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