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IMPORTANCE OF THE INITIAL SMOOTHNESS

(IRI) AND UNIFORM DENSITY IN THE

CONSTRUCTION OF HOT MIX ASPHALT

PAVEMENTS

Eng. Paul Lavaud

International Director for Latin America ROADTEC, INC.

plavaud@roadtec.com plavaud351@gmail.com

www.roadtec.com 2011

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IMPORTANCE OF THE INITIAL SMOOTHNESS (IRI) AND UNIFORM DENSITY IN

THE CONSTRUCTION OF HOT MIX ASPHALT PAVEMENTS

1. INTRODUCTION

As per last economical studies, one way to increase wealth of the people out of the big

cities is through new or better infrastructure like roads and airports, so that they can

integrate economically by taking out their products to the market and receiving tourists.

On the other hand, the big cities with more number of population and vehicles require

better roads to arrive faster to their work or study centers.

Asphalt pavements are an economical and sustainable solution for the construction of

roads, since traffic can be open subject to the daily job advance and they can be

recycled permanently. Roads are usually designed for a useful life of 10 to 20 years.

However, many of them are failing prematurely showing cracking, potholes and rutting,

mainly due to construction methods. These premature failures make us spend

unnecessarily millions of dollars in maintenance each year.

The success on constructing a quality asphalt pavement is not only given by a good

Hot Mix Asphalt (HMA) design and the preparation of HMA with good materials, but

also by a correct placing and compaction of the mixture, which is very important and

what will finally give us initial smoothness, currently measured in International

Roughness Index (IRI).

It is very important that the HMA design engineers, asphalt plants operators, paving

equipment (Material Transfer Vehicles, Paver, Cold Planers and Rollers) operators

and Job Site supervisors understand the relevant variables during the construction of

asphalt pavements that will have repercussions on their performance (useful life and

costs of maintenance) and comfort of the users.

The two more important variables on the quality during the building of an asphalt

pavement are the initial smoothness, currently known as IRI, and the uniform density .

As per well known studies, in the last 50 years the smoothness obtained during a

pavement construction has influence on:

- The Roughness and the Service Index (PSI) during the useful life of the pavement.

- The road maintenance cost, maintenance costs are reduced considerably on

pavements with an initial smoothness less than 1,5 mt/km (IRI).

- The useful life of the road. Reducing the initial smoothness in 50%, life of the

pavement increases 27%.

- The vibrations perceived by drivers and passengers which affect the users comfort

perception as per pavement conditions.

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- The noise level inside and outside the vehicles.

- Traffic accidents, when the IRI goes from 0-100 in/mi (1,59 mt/km) to 101 in/mi (1,60 mt/km)-200 (3,18 mt/km), the crash frequency would be increased by 1.649 (e0.5). - Vehicle Rolling Resistance increases with roughness at a rate of 3 to 6 percent per

unit of IRI.

- Fuel consumption. Prior to a scheduled rehabilitation at Westrack the averaging

trucks fuel economy was 4,2 miles/gallon. After rehabilitation of the track the

measured roughness was reduced by 10% and the fuel economy was increased by

4,5%

The yearly roughness (IRI) measurement of a road is the result of the initial

smoothness (IRI) which grows when appears rutting and potholes. The non uniform

density affects prematurely the useful life of the pavements appearing rutting and

potholes.

The HMA has a 94% of crush stones of different sizes and a 6% of asphalt. In each

design, a percentage of aggregate is established as per its size in the mix, the smaller

aggregates fill the vacuums that remain among the bigger ones, the asphalt covers the

aggregates and stick them.

This definition of HMA is not new among people of the asphalt industry, however,

during the transportation of the HMA from the asphalt plant to the job site, due to the

truck movements, the big stones roll to the side of the bed truck, separating from the

small ones producing a segregation of aggregates during the paving. The big

aggregates that get together without the presence of smaller stones will have more

vacuums and the smaller aggregates will have a major percentage of asphalt, in both

cases the structural properties of the mixture will be reduced.

On the other hand, the HMA gets cold during transport by the sides around the

perimeter and the top of the truck bed. The spots with cold mix are more difficult to

compact. For the same quantity of hits during the compaction at different

temperatures it is obtained different percentages of vacuums and therefore useful life.

Physical and thermal segregation do not allow obtaining uniform densities. Currently

in the U.S.A., specifications do not require a minimum density anymore; a range of

compaction is required, out of this there are penalties.

It is very important to encourage and specify correct construction procedures in order

to obtain pavements with initial smoothness less than 1,5mt/km and without

segregation. The correct placing of the HMA is a critical issue to obtain a quality

pavement.

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2. IMPORTANCE TO CONSTRUCT SMOOTH ROADS

2.1 Definition of Service Index (PSI) and Roughness

The Road Test conducted by the AASHO of 1962 allowed the creation of the Present

Serviceability Rating (PSR), being a function of comfort when driving on a pavement

with different conditions and is based on individual observations. See chart 1.

PSR CONDITION

5-4 Very Good

4-3 Good

3-2 Fair

2-1 Poor

1-0 Very Poor

Chart 1. PSR (Present Serviciability Rating)

The PSR was replaced by the PSI, Present Service Index, consists on measuring

different pavement conditions as cracking, rutting and roughness (slope variance) and

through an equation obtained a score similar to the PSR to determine the condition of

a road.

PSI equation:

PSI = 5.03-log(1 + SV)-1.38(RD)2 -0.01(C + P)1/2

Where, PSI = the present serviceability index which is a statistical estimate of the mean of the present serviceability ratings given by the panel, SV = Slope variance over section from CHLOE profilometer (slope variance was an early roughness measurement) RD = mean rut depth (in.), C = cracking (ft / 1000 ft2) (flexible), P = patching (ft2 / 1000 ft2)

Currently, the most representative variable to determine the condition of a road is the

roughness. In some countries the word roughness is synonym of friction, while in

others it is Superficial Regularity. In the present work the word roughness is referred

to longitudinal waves determined in the tire treads with respect to a line of reference.

The type of longitudinal waves is determined by the wavelength. See Figure 1.

The Wave Number is the number of cycles per meter (1m/wavelength). The Hz

frequency is number of cycles per second ( It consider the wavelength and car speed).

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Figure 1 . Wavelenght and Amplitude

The waves in the wheel tracks on a pavement surface are divided in:

Surface Frecuency Range

Wavelenght Wave Number (cycle/m )

Microtexture < 0,5 mm > 2000

Macrotexture 0,5 – 50 mm 20 - 2000

Megatextura 50 – 500 mm 2 - 20

Roughness 0,5 – 50 m 0,02 - 2

Chart 2. Frequency range specifications as per surface characteristics, according to

PIARC 1990.

Source: Road surface characteristics and conditions: effects on road users. ARRB

Transport Research Australia. ARR Report 314 (Year 1998)

Currently there are inertial profilers with laser and ultrasound sensors that allow to take

readings for macrotextures, our analysis will be focused on the waves larger than 50

mm, say megatexture and roughness, since they produce vibrations on the vehicles,

due that they affect the tires and suspension performance.

Microtexture and macrotexture are more connected to friction studies.

It is important to analyze the sections that produce problems with driving comfort;

microtextures and slopes are not so important for the roughness analysis, this

information should be filtered when obtaining the IRI.

A constant megatexture is usually produced during construction, for instance in truck

exchanges and change of the paver operation speed. In isolated areas the cause is

cracking, joints and patching mainly.

Wavelenght Wavelenght

Amplitude Amplitude

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As we will see, the wavelength type and the vehicles speed will be related to the

vibrations and noise perceived by drivers and passengers when using roads with

irregular surfaces.

2.2 International Roughness Index (IRI)

Currently, a pavement roughness is measured by the International Roughness Index,

well known as IRI.

The IRI was developed as a universal unit to measure the roughness of a pavement in

m/km or in/mi. This system works through a mathematical model which interprets a

vehicle performance according to the longitudinal profile of a road. This study was

conducted by the World Bank in 1982. This system makes it possible that different

measurement equipment can give the same values through correlations and

calibrations.

Before explaining the interpretation of the model “Quarter Car”, it is recommended to

review the suspension system of a vehicle. See Figure 2.

Through the tires the road surface effects (input) are transmitted to the suspension.

The dampers are installed over the same axles where wheels are placed. All the

additional weight (mass) of the vehicle is supported by the suspension system formed

by dampers and coil springs. What we feel (output) are the vibrations transmitted by

the seats and steer wheel.

These effects of the vehicle speed and the wavelength of the irregularities will be

transmitted to the passengers as a vertical acceleration.

Figure 2: Suspension system formed by dampers and coil springs.

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The IRI is calculated in 4 steps:

Step 1. The road profile is converted into a slope

Step 2. Then a mobile average is applied for a length of 250 mm (similar to the wheel

track on the pavement)

Step 3. A simulation of the “Quart Car” is carried out.

Step 4. A rectified average value is accumulated. IRI is estimated for wavelengths that

will be called interval sections that “smooth” the IRI. Usually 160 mts (0,1 mile)

Step 5. Mean IRI

The “Quart Car” model represents one corner of the vehicle, which predicts a wheel

and suspension system response to a profile with the weight supported by the

suspension. See Figure 3. The IRI is more sensitive to wavelengths of 1 to 30 mts.

with top points at 2,4 and 15,40 mts. .

Figure 3. Quarter-Car Model

Parameters for the Quarter-Car Model

V = 80 km/h

m/M = 0.15

kt/M = 653 1/sec2

ks/M = 63.3 1/sec2

cs/M = 6 1/sec

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Figure 4. IRI Roughness Scale

It is important to consider that the number of the IRI roughness scale mentioned

in figure 4 were determined in 1982. In the last 20 years there are several

studies about the importance to reduce roughness to increase the life of the

roads and reduce vehicles operation costs. For higher traffic and vehicle speed

it is important to consider lower levels of IRI due to the impact of the

maintenance costs of roads and vehicles. Also the IRI has a direct impact

on fuel consumption.

Figure 5. Recommendations of the Transportation Research Board (TRB) to

select the maximum IRI values with respect to annual average daily traffic,

abbreviated AADT

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It is important to mention that in most Latin American countries it is requested an initial

IRI of 2,0 m/km for new jobs and 3,5 m/km as acceptable service IRI. As we will

demonstrate in this study the trend is to decrease the requested IRI levels for the

reception of works as well as for the serviceability IRI with the purpose of offering more

comfort to users, increasing roads useful life, and decreasing roads maintenance

costs, as well as decreasing vehicles operation costs (fuel, tires and time traveled).

There is a well known correlation that relates the IRI to the PSR where:

-0,26 (IRI)

PSR=5e

This correlation was reported in a 1992 Illinois funded study performed by Al-Omari and Darter.

where: PSR = present serviceability rating

IRI = international roughness index

Regarding a study effected by Carey and Irick in 1960, 50% of drivers said that a PSR

of 2,0 was not acceptable and that a PSR of 3,0 was acceptable. For the ASSHO

(1962) a road must be between a PSI of 2 to 3 to be accepted.

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0,40 m/k

2,00 m/k

3,50 m/k

2,68 m/k ( FHWA Concept)

Figure 6. Acceptable PSI range as per 1960 study.

As we will see in the next chart, in 1998 the FWHA determined that the acceptable IRI

for pavements was an IRI ≤ 2,68 m/km.

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Condition Term

PSR Rating

IRI NHS Ride Quality

Very Good ≥ 4.0 < 60 in/mi

(< 0.95 m/km)

Acceptable ≤ 2,68 m/km

Good 3.5 - 3.9 60 – 94 in/mi

(0.95 – 1.48 m/km

Fair 3.1 - 3.4 95 – 119 in/mi

(1.50 – 1.88 m/km)

Poor 2.6 - 3.0 120 – 170 in/mi

(1.89 – 2.68 m/km)

Very Poor ≤ 2.5 > 170 in/mi

(> 2.68 m/km)

No Acceptable

> 2.68 m/km

Chart 3. FHWA Pavement Roughness Thresholds for Interstate Facilities

When initial IRI is lower the useful life of the pavements is extended before reaching

the service IRI.

Following is the 1993 AASHTO equation for flexible pavement where the differential of

the initial and final Serviceability Index (PSI) is considered.

where: W18 = predicted number of 80 kN (18,000 lb.) ESALs

ZR = standard normal deviate

So = combined standard error of the traffic prediction and

performance prediction

SN = Structural Number (an index that is indicative of the total

pavement thickness required)

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PSI = difference between the initial design serviceability index,

po, and the design terminal serviceability index, pt

MR = subgrade resilient modulus (in psi)

In the 1993 AASHTO design guide specifications, pavements are usually designed

with an initial Serviceability Index PSI of 4,2 to 4,5, being the final PSI of 2,0 to 2,5.

For highways it is recommended a final PSI of 2,5 or IRI of 2,68 m/km. As per the

previously mentioned correlation between PSI and IRI, for an initial PSI of 4,2

corresponds an IRI of 0,65 m/km. However, many projects designed as per 1993

AASHTO use an initial PSI of 4,2 while the initial IRI requested in the constructions

specifications is 2 m/km, when it should be 0,65 m/km.

PSI IRI (m/km)

Initial Service Index 4,5 0,40

4,2 0,65

Terminal Service Index 2,5 2,68

2,0 3,38

Chart 4. Relation PSI and IRI

2.3 How IRI influences users comfort and security

The study effected by AASHO in 1960 allowed us to know the interpretation of a panel

of observers regarding the different roads conditions. Currently, users qualifications

on road conditions can be explained as follows:

2.3.1 Vehicles passengers perceive road roughness as vibrations

The engineers that construct cars measure the acceleration of the seats movements

to evaluate the performance of the suspension.

The vehicle mass is separated from the wheels by the vehicle´s suspension. This

design is necessary to isolate the passengers from the vehicle´s vibration produced by

the imperfections of the road surface.

The pavement roughness produces vertical elevations which are detected by

passengers as vibrations.

Cars, depending on their speed, read the roughness (wavelength and amplitude) from

the road surface as a frequency. See Figure 1.

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It is known that for frequencies near 0, the suspension transmits the reading of the

tires to the vehicle mass, for example large waves as ups and downs. Near 1 Hz the

vehicle amplifies the vertical acceleration. This can be detected in roads with short

waves and then the cars go at low speeds. At high frequencies the acceleration

transmitted to the vehicle is reduced.

Figure 7. Wavelenght vs Gain for Profile Slope

This response is very similar to the IRI. The highest point of the response to IRI is for

wavelength of 15,40 mts, which corresponds to a frequency of 1,4 Hz. at a speed of 80

kph, it is known as the “Boudy bonce”. Another high point is at a wavelength of 2,40

mts, which corresponds to a frequency of 9,3 Hz. at a speed of 80 kph it is known as

the “Axle Hop”.

The human body has a minimum tolerance to the vertical vibration at 5Hz due to the

abdominal cavity resonance. The tolerance to horizontal vibrations is of 1 Hz.

Vehicles are designed to minimize transmission at frequencies of 1 to 10 Hz.

The wavelength affects the sensibility of a car passenger. The major sensitivity is for

wavelengths of 2,40 and 15,40 mts. The vertical frequencies produced by the

pavement between 10 and 15 Hz are absorbed by the tires, reducing the transmission

to the driver.

For frequencies between 1 to 2 Hz driver practically moves equal or more than the car

movement, while between 10 to 12 Hz driver feels a jump of the car.

It can also be determined as per studies effected by the Michigan Department of

Transport (MDOT) that the effects produced by the trucks dynamic cargo that travel at

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speeds of 100 km/hr on highways at frequencies between 1,5 to 4 Hz and 8 to 15 Hz,

are bigger than in other frequencies. The wavelengths of these frequencies are of 6,7

to 17,9 mts and of 1,8 to 3,3 mts. respectively.

2.3.2 The Noise produced by the car over the pavement

There are several specifications that regulate the permitted noises as per place and

range of hours (day or night). Following there is a chart regarding the weight of the

vehicles.

Vehicle Gross

Weight (kg)

<3000 3.000 – 10.000 > 10.000

Maximum

Permisible limit in

dB (A)

79 81 84

Chart 5. Noises permitted at 15mts. As per vehicle weight. Source: United Nations

Environmental Program

The pain threshold is considered 140 dB. Currently, there are economical noise

measurement instruments where noise can be verified as per the pavement

conditions. The noise fatigues drivers and upsets passengers. The noise will depend

on the cars speed and the wavelength of the roughness.

The road surface irregularity causes noise inside the car as well as around it. The

more the wavelength is similar to the tire contact length measurement over the

surface, the bigger the noise.

Currently, tire rubber is being used as polymer in the asphalt mixtures. One of the

advantages encouraged for this application is the noise reduction. It is very important

to place mixtures with a low IRI to avail the advantages in the use of the rubber in the

mixtures.

2.3.3. Number of Accidents as per the Road IRI

When a road is in bad conditions with high roughness and potholes it is difficult to

drive due to changes of speed and brusque movements to avoid potholes, which could

cause accidents.

There is a study carry out in 2008 by the Southeastern Transportation Center

University of Tennessee, in the report “Effects of Asphalt Pavement Conditions on

Traffic Accidents in Tennessee Utilizing Pavement Management System (PMS)”,

where some correlations were found between the number of accidents and the

roughness and PSI conditions.

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The study was based on the accidents statistics of a journey of 110 miles (176 kms) in

the cities of Knoxville, Memphis, Nashville and Chattanooga in 2006. The IRI, PSI,

AADT were analyzed on sections of 0,1 mile (160 mts). The number and the

conditions of how the accidents were produced were analyzed.

The conclusion was when the IRI was increased from 0-100 in/mi (1,59 mt/km) to 101

in/mi (1,60 mt/km)-200 (3,18 mt/km), the crash frequency would be increased by 1.649

(e0.5).

2.4. SMOOTH ROADS LAST LONGER AND COST LESS

At the end of 1988, Michael S. Janoff studied the effect of initial smoothness over the

pavement performance in the long term. Mr. Janoff presented the results of his

findings at the annual meeting of NAPA, held in January 1990, after collecting data

from 400 sections of roads and their performance in 10 years, in his publication

entitled "The Effect of Increased Pavement Smoothness on Long Term Pavement

Performance & Annual Pavement Maintenance Cost”.

The results of Mr. Janoff’s studies were as follows:

1) Pavements with increased initial smoothness have lower roughness levels in the 10 years following construction. 2) Pavements with increased initial smoothness have lower cracking levels in the 10 years following construction. 3) Pavements with increased initial smoothness have lower average annual maintenance costs in the 10 years following construction.

Initial Smoothness Long Term Roughness

Long Term

Cracking

Annual Average

Cost

METRO MAYS IRI* METRO MAYS IRI* Lane

mm/km m /km m/km m /km mm/km US$ / Km

553 1,41 647 1,50 410 590

474 1,34 553 1,41 268 416

395 1,26 474 1,34 150 73

316 1,19 379 1,25 63 162

237 1,11 300 1,17 16 81

158 1,04 205 1,08 0 32

Chart 6. Initial Smoothness vs Long Term Roughness and Average Annual Maintenance Cost (* Correlation) The studies were conducted by the State of Arizona using the Metro Mays to determine the roughness during various periods. To convert Metro Mays readings displayed on Janoff’s test in a profile reading of about 7.6 meters, -method used to record the roughness profile index - PI (Profile Index) – divide Metro Mays reading by

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4. In addition, there is a relation between the PI 5mm and the IRI, which is IRI (mm): 3,78601*PI 5mm + 887,51. The units should be in mm. In 1989 one km of lane of 3,7 mts and 2” width cost US$ 11,875. The benefit of paving with a lower IRI is that the rehabilitation or maintenance period is extended. If we only estimate that the useful life when starting with a smoothness of 158 mm/km instead of 553 mm/km is extended to 12,7 years instead of ten, we have a 27% more of useful life. The saving for more useful life will be 27% of US$ 11,875, which means US$3,206. When decreasing initial roughness from 553 to 158 mm/km, there are less crackings and therefore pavement needs less repairs; based on a study the annual savings are US$ 590 less US$ 32, this means US$ 558. In 12,7 years the savings are equal to US$ 7,087. Therefore, the total savings for paving with an initial roughness of 158 mm/km are of US$ 3,206 + US$ 7,087 = US$ 10,293. If we consider that the pavement initial cost was US$ 11,875, we talk about a saving of 87%. If the entities in charge of establishing the technical specifications offered a bonus between 5 to 10% for reaching an initial smoothness of 158 mm/km (IRI of 1,04 m/km) instead of 553 mm/km (IRI of 1,41 m/km), they would have a great benefit. The National Cooperative Highway Research Program (NCHRP) has conducted a study which confirms that a lower initial roughness increases the life of pavements.

Reduction in Initial Roughness

Average % Increase in Service Life

Asphalt Concrete

10% 5 7

25% 13 18

50% 27 36

Chart 7: Results of sensitivity according to the initial roughness (NCHRP1-31 Smoothness specifications for Pavements) Source: www.tfhrc.gov/pubrds/septoct00/smooth.htm In another study titled “Impacts of Smoothness on Hot Mix Asphalt Pavement

Performance”, effected by the Department of Transport of the State of Washington

between 1999 and 2002, it was determined that roughness of pavements built with low

IRI are smoother in time. This can be proved in the following figure, where it is

determined a correlation of IRI in the third year taking into account the IRI of the

pavement in the first year.

.

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Figure 8. Impacts of Smoothness on Hot Mix Asphalt Pavement Performance In

Washington State 2004.

The IRI increase in two years is 0,2 when the IRI of the first year is 1 m/km, while it

increases 0,45 when the IRI of the first year is 2,0 m/km.

2.5 TRENDS OF IRI

2.5.1 ACCEPTABLE LEVEL OF SERVICE

In 1996 the FHWA carried out a national poll which results demonstrated that users

asked for better driving conditions over security or less traffic congestion.

This caused that in 1998 the Congress launched a National Strategic Highway Plan

where it is established the term of “Acceptable Ride Quality” to the highways of the

National Highway System with an IRI less or equal to 2,68 m/km.

In the U.S.A. the FHWA considered important to increase for the year 2008 the

kilometers of ways of the NHS (National Highway System) traveled by vehicles with a

“good ride quality” (IRI less or equal to 1,49 mts/km) to 58,5% and increase the

percentage of kilometers of ways traveled by vehicles which transit over pavements

with an “acceptable ride quality” (IRI less or equal to 2,68 mts/km) to 95%. The latest

report of 2004 for good ride quality was 51,8% and acceptable 90,6%.

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PSI IRI m/km Ride Quality

3,90 ≤ 1,49 Good

> 1,49 to ≤ 2,68 Acceptable

2,50 > 2,68 No Acceptable

Chart 8: FHWA Smoothness Source: http://www.fhwa.dot.gov/pavement/smoothness/index.cfm

Figure 9. Percentage of Interstate ways considered as “Good Ride Quality”, which

means with an IRI ≤ 1,49 m/km

In a study titled “A Statistical Analysis of Factors associated with Driver-Perceived

Road Roughness on Urban Highways” conducted by the Washington State

Transportation Center in 2002, it was determined that 70% of users defined as

“Acceptable” the pavements with an IRI ≤ 2,68 m/km, while for an IRI ≤ 3,5 m/km the

users define Ride Quality as Acceptable and Not Acceptable in 50% respectively. See

figure 10.

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Figure 10. A Statistical Analysis of Factors associated with driver-perceived road

roughness on urban highway

Research Project 1803 Task 28 June 2002. Washington State Transportation Center

2.5.2 Specifications related to Initial Roughness

Since the 60s the American Transport Agencies have admitted the importance of

controlling roughness and started developing and implementing initial smoothness

specifications. In 1988 the AASHTO proposed the specifications mentioned in Chart

9, which were measured with a California profilograph as per the standard 526 of 1978

Profile Index. 0,1 mile section IRI* Contract Unit Price Adjustment

% of Pavement pulg/milla mm/km m/km

3 or lesss 47,6 or less 1,068 o menos 105

Over 3 ty 4 Over 47,4 to 63,5 Over 1,068 to 1,128 104

Over 4 to 5 Over 63,5 to 79,4 Over 1,128 to 1,188 103

Over 5 to 6 Over 79,4 to 95,3 Over 1,188 to 1,248 102

Over 6 to 7 Over 95,3 to 111,5 Over 1,246 to 1,308 101

Over 7 to 10 Over 111,1 to 158,8 Over 1,308 to 1,489 100

Over 10 to 11 Over 158,8 to 174,6 Over 1,489 to 1,549 98

Over 11 to 12 Over 174,6 to 190,5 Over 1,549 to 1,609 96

Over 12 to 13 Over 190,5 to 206,4 Over 1,609 to 1,669 94

Over 13 to 14 Over 206,4 to 222,3 Over 1,669 to 1,729 92

Over 14 to 15 Over 222,3 to 238,1 Over 1,729 to 1,789 90

Over 15 Over238,1 Over 1,789 Corrective work require

Chart 9. Guide Specifications for Highway Construction. AASHTO 1988 for Asphalt paving

(* Correlation)

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Considering all the studies mentioned before, the Departments of Transport (DOT) of

the different states in the U.S.A. have changed the initial smoothness specifications.

Charts 10 and 11 show some of them. In U.S.A. each state DOT has its own

specifications.

IRI pulg/milla IRI m / km Percent Adjustment

<51 < 0,80 +10%

51-60 0.80 – 0.95 +5%

61-80 0.96 – 1.26 0

81-100 1.27 – 1.58 -5%

101-110 1.59 – 1.74 -10%

111-120 1.75 – 1.89 - 25

> 120 > 1.89 Replacement required

Chart 10. Road Construction Incentives based on IRI, Arizona, U.S.A.

IRI (m/km) Percent Adjustment

< 0,79 10

< 0, 789 – 0,947 63,29 ( 0,947 – IRI)

0,948 – 1,262 0

1,263 – 1,893 39,68 (1,263 – IRI)

> 1,893 -50

Chart 11. Road Construction Incentives based on IRI, Connecticut

This criterion of the Connecticut DOT is applicable to the two superficial layers of the

pavement. The total project is divided in sections of 160 mts. and to each section it

corresponds an average IRI value; each section will be classified as per the scale of 5

payments of the chart. Each factor will be multiplied by the length of each section and

the addition will be divided by the total length of all sections.

Initial roughness index specifications of the different Departments of Transports in the

U.S.A. can be found in the following web page:

Pavement Smoothness Index Relationships, Final Report. Publication No-FHWA-RD-

02-057,2002

http://www.tfhrc.gov/pavement/ltpp/reports/02057/02057.htm

Since 1980, the FHWA receives annually the interstate highways conditions through

the Highway Performance Monitory System. Since 1980 to 1989 the conditions of the

roads were requested in PSI, from 1990 the road conditions are evaluated through the

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IRI. You can see in Figure 11 the average IRI of ways controlled by the FHWA, in

1994 the IRI was 1,72 mts/km, very similar to the one reached in 1980 of 1,75 mts/km.

From 2000, the IRI decreased to an average of 1,5 mts/km and in 2006 to 1,35

mts/km. In the U.S.A. each year there are smoother, long lasting and more safety

roads due that the construction process has been improved. Figure 11 also shows

important studies mentioned in this Technical Paper that have a relation with the

smoothness improvements in the U.S.A.

1989 MTV

1988 AASHTO Bonus & Penalty

1990Michael Janoff Study

1982IRI

1998FHWA : IRI ≤ 2,69 m/km“Aceptabloe Ride Quality Concept”

DOT WashingtonTemperature Segregation

1993SuperpaveLTTP 1996

Napa Manual

1,79 m/km

1,59 m/km

1,43 m/km

1,27 m/km

1,21 m/km

Figure 11. Interstate Highway IRI

For the FHWA to improve the smoothness of the national highway network, state highway agencies must not only rebuild existing rough pavements, but they must perform timely, effective maintenance on their portion of the network that currently meets the smoothness goals. This is not an easy task. An effective pavement-smoothness program requires a comprehensive approach with smoothness as a goal from the very start. To accomplish this objective, a smoothness program should have the following components:

• Processes that identify the best projects to maintain and improve pavement smoothness on the total highway network.

• A method for specifying pavement smoothness during initial construction.

• A method for measuring pavement smoothness during initial construction.

• Tools for contractors to build smooth pavements.

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2.6 RELATIONSHIP WITH RESPECT TO FUEL

There are several studies that found that roads with lower roughness can improve the

fuel efficiency of trucks. Vehicle Rolling Resistance increases with roughness at a rate

of 3 to 6 percent per unit of IRI.

In a study called WesTrack it was verified that by reducing the roughness on 10% fuel

consumed by trucks was reduced 4.5%. Figure 12 shows a similar study by the

National Center for Asphalt Technology (NCAT)

Figure 12. Fuel economy and roughness vs. Time. Source NCAT It is of utmost importance to recognize the sections where there is more AADT to

reduce the roughness in order to achieve important fuel savings. In the following chart

you can see the savings of a road of 50km with 5000 AADT.

AADT

Vehicle

Type

%

Quantity Travel

Fuel

Comsuption

Fuel

Price

Gal Total Savings 4,5%

Uni.

Km /

day

Km/

Gal

Gal/

día US$ US$/día

US$/day

Uni

USS /

day

US$

/Year

5.000

Sedan 85% 4.250 50 30 1,67 5,77 9,6 0,43 1.839 671.214

Bus 5% 250 50 26 1,92 4,69 9,0 0,41 101 37.025

Truck 10% 500 50 12 4,17 4,69 19,5 0,88 440 160.440

Total 2.380 868.678

10 years

8.686.780

Chart 12. 4,5% Fuel Savings for each 50 km and AADT of 5000 vehicles.

Fuel Economy and Roughness vs Time

4.14.24.34.44.54.64.74.84.9

55.1

1-Oct-00 20-Nov-00 9-Jan-01 28-Feb-01 19-Apr-01 8-Jun-01 28-Jul-01 16-Sep-01

Time

mp

g

1.001.021.041.061.081.101.121.141.161.181.20

IRI

(m/k

m)

mpg IRI

21

2. 7 IRI MEASUREMENT

The IRI measurement is used for:

- During construction, it is recommended to measure the IRI daily to verify results

of construction procedures and make the necessary corrections.

- Check annually the road net conditions

- Evaluate vehicles operation costs as per road conditions

- Diagnose road conditions to determine repair strategy

- Study specific places for evaluation

- Study trucks dynamic loads in critical zones

The IRI measurements must be done over longitudinal traces of vehicles over each

lane. Both traces must be measured (right and left).

2.7.1 Measurement during construction:

For measurements during a pavement construction it is recommended to use the floor

profiler or a walking profiler; although they don’t have a high performance on daily

basis, they are economical.

The floor profiler stands on two support legs. The operator simply walks the dipstick

along a survey line alternately pivoting the instrument about each leg. Two digital

displays show the elevation difference between the dipstick’s support two legs. Audible

and visual signals alert the operator when each elevation difference is reading and

automatically recorded.

The Walking Profiler is a precise measurement instrument for collecting and

presenting continuous paved surface information.

The dipstick and walking profiler meets World Bank Class 1 Profilometry requirements

and produces outputs in International Roughness Index (IRI).

Figure 13. Floor profiler Figure 14 Walking profiler

22

The term lightweight profiler is used to refer to devices in which a profiling system has

been installed in a light vehicle, such as a golf cart or an all terrain vehicle. Lightweight

profilers have gained popularity during the last several years, with many State highway

agencies as well as contractors purchasing these systems. The profiling system in a

lightweight profiler is similar to the profiling system of an inertial high-speed profiler

and consists of height sensor(s), accelerometer(s), and a distance measuring system.

The profile recorded by these devices can be used to generate a roughness index

such as IRI, or to use the profile data to simulate a profilograph and get the Profile

Index (PI).

Figure 15 Lightweight profiler

2.7.2. Control Measurements

It is recommended to use the inertial profiler, which are installed in vehicles to travel at

speeds of 80 km/hr and process the information as per the model “Quart Car”.

Although these systems are expensive, they are very efficient.

A schematic diagram of an inertial profiler is shown in figure 16. The principal

components of an inertial profiler are height sensors, accelerometers, distance

measuring system, and computer hardware and software for computation of the road

profile. The height sensors record the height to the pavement surface from the vehicle.

The accelerometers that are located on top of the height sensors record the vertical

acceleration of the vehicle. Data from the accelerometers are used to determine the

height of the vehicle relative to an inertial reference frame. The distance measuring

system keeps track of the distance with respect to a reference starting point. Using the

data recorded by the distance measuring system, height sensor and the

accelerometer, a computer program computes the profile of the pavement surface.

The non-contact height sensor types that are used in profilers today are either laser,

23

ultrasonic, optical or infrared. Ultrasonic sensors were the most common type of

sensors used in the 1980s. However, because of problems with this type of sensors,

their use has declined over the past several years. Currently laser sensors are the

most commonly used height sensors used in profilers.

Figure 16. Inertial Profiler

2.7.3. Important considerations during IRI measurements

- Equipment operators must be duly trained.

- IRI must be measured over the left and right traces of the vehicles.

- The equipment to measure roughness must be certified and calibrated as per

standard procedures .AASHTO PP 49-03 “Certification of Inertial Profiling systems”

AASHTO PP50-03 Operating Inertial Profilers and Evaluating Pavement Profiles.

- At high speeds over 100 km/hr it is difficult to follow the trace

- It is recommended to do at least one measurement a year

- It is recommended to do daily measurements during a pavement construction to

correct any imperfections in the constructive procedures.

- IRI must be measured on the base surfaces or milled surfaces, due that the IRI

of the rolling layer will depend on the conditions of the surface where the asphalt mix

will be placed.

- As long as the measurement intervals have less length, it will be easier to

detect and correct imperfections.

24

3. THE PREMATURELY INCREASE OF THE YEARLY ROUGHNESS DUE TO POTHOLES AND RUTTING IS CAUSED BY NO UNIFORM DENSITY OF PAVEMENT DURING CONSTRUCTION. PHYSICAL AND THERMAL SEGREGATION DO NOT PERMIT TO OBTAIN UNIFORM DENSITY. Using an infrared camera to observe the transport of asphalt mixtures from the asphalt plant until the discharge into the paver, it became clear that the temperature differential in the asphalt mat was significantly higher than expected. The detrimental effects of low-temperature compaction or segregation of aggregates have been documented in the last 12 years. It is recommended reviewing the Traffic Management Study (DOT) Washington State, Research Report 476.1: Construction-Related Asphalt Concrete Pavement Temperature Differentials and the Corresponding Density Differentials (http://www.wsdot.wa.gov/research/reports/ fullreports/476.1.pdf).

The Traffic Division of Washington (WSDOT) in 1996 studied the effects of low-density areas that appear in cycles using the Nuclear Densitometer. In 1998, through an infrared camera, these areas were located. It concluded that in cold areas there was a lower density than in the rest of the mat. See Figure 17 .

Figure 17. Areas of cold asphalt mix below 79 º C are relatively rigid and resistant to

compaction, which results in lower densities than warmer areas, and therefore areas

subject to premature failure. Observe in the photo the areas of low temperature below

106.1 º C and compare with dark spots on the picture of the road after a year of

service, showing premature failures.

25

Figure 18. In 1999 the WSDOT together with the University of Washington (UW)

studied the relationship between temperature differentials before compaction and

density after compaction. To colder pavement sections correspond lower densities

after the compaction. For example, in the infrared photo at a density of 87.4% it

corresponds an asphalt mix reading of 74.5 º C.

In the study conducted by the WSDOT in 2000, it was verified that temperature

differentials greater than 14 º C were presented in the asphalt mix placed by the paver.

Field data showed that for every 1% increase in the percentage of voids on a threshold

of 7%, there is a reduction of approximately 10% in pavement life. On this basis, a 2%

increase in voids can shorten pavement life by 20%, reducing the useful life by 3 years

of a 15-year projected road. The temperature differentials based on current

construction methods are often greater than 14 º C, which will continue presenting

premature failure of pavement with a consequent increase in roughness.

Then, the high percentage of voids that occur in these areas will enable the infiltration

of water into the mixture, which will freeze in the winter and break the pavement to

produce a bump. It is important that the phenomenon described above will act exactly

like a segregated point with concentrated coarse particles, resulting in the formation of

a pothole. However, in this case, instead of occurring particle segregation, the main

cause is the temperature segregation. In examining this phenomenon and realize the

causes, it is apparent that the asphalt pavement contractor does not control many of

the causes of temperature segregation.

In an attempt to determine the severity of damage caused by cold spots, Mr. Ronald

Collins of PTI, through a PTI vibratory compactor and an asphalt pavement analyzer

(APA), compacted a typical Georgia mixture at 300 º F (149 ° C), 280 º F (138 ° C),

260 º F (127 ° C), 240 º F (116 ° C), 220 º F (104 ° C) to 200 º F (93 ° C). A vibratory

compactor was used to compact the mixture at 149 ° C to obtain a 7% voids.

26

The time required to compact (approximately 17 seconds), amplitude and frequency

of vibration were kept constant.

Figure 19. Fatigue resistance under different compaction temperatures.

Figure 19 shows the effect on the percentage of voids when the temperature

decreases. As can be seen, the percentage of voids increases from 6.8% when

compacted at 149 ° C (300 º F) to 9.3% when compacted at 93 ° C (200 º F). Each of

the beams produced in this study was placed on the asphalt pavement analyzer (APA)

and conducted a fatigue test to failure the beams. As shown in Figure 19, the cycles

required for failure decreased significantly while the percentage of voids in the

pavement increased. The mixture compacted at 104 ° C (220 º F) would have

approximately 10 to 12% of the life of the mixture compacted at 149 ° C (300 º F).

It also presents the physical segregation during transport and placement of the

mixture, where larger aggregates are separated without smaller aggregate will present

a high percentage of voids: the other side where a spot has higher percentage of fines

will present a high amount of asphalt. In both cases the structural capacity is reduced

dramatically.

The difference in density is a major concern in the preparation and placement of hot

asphalt mixtures. Density is important to prevent the entering of water to the lower

layers, asphalt oxidation, an increase in density under traffic and to provide adequate

shear resistance. In chapter 8 the Arkansas DOT temperature specifications are

described. There already are sensors installed behind the screed that record the

temperature of the mixture placed behind the paver.

It is necessary to be more careful in the placement of HMA micro pavements and the

use of polymers in the mixtures because the loss of temperature is faster than in

conventional mixtures.

3/8" HMA APA Fatigue Results

180200220

240260280300

320340360

0 10000 20000 30000 40000 50000

APA Cycles to Failure

Co

mp

acti

on

Tem

pera

ture

(oF)

9.3

8.4

8.4

7.8

7.3

6.8Air Voids

27

It is also important to use pavers that do not produce central segregation due to the

design of the paver. This not only produces physical segregation, as it is shown in the

figure 20, but also produces temperature segregation. In figure 20 the temperature in

the center of the pavement was less than 80◦C, it is not recommended to compact a

HMA below 80◦C. This pavement will have a premature failure in the center.

Figure 20. Physical and Thermal segregation produced by the paver.

4. OBTAINING QUALITY PAVEMENT

To obtain a mat with roughness less than 1.5 m / km. and uniform density, it is vital to

consider the following:

Material level: Industry recommends to keep the level of material always in the middle

height of the augers of the paver. There are devices, such as ultrasonic sensors, that

automate the control of material level. When the control is done manually, each time

the material is over the augers, the mixture will push the screed and rise, otherwise,

when the material is below the auger, the screed will come down by its own weight.

These movements will increase the initial IRI. For this reason, it is recommended to

use automatic level sensors and put the augers and tunnels extensions (12-foot

sections) at a distance of 30 to 45 cm from the edge of the screed. This also will help

to avoid segregation at the end of the screed where is built the longitudinal joint.

Paving speed: In the paving process, the screed is in a floating position. Changes in

the paver speed will affect the amount of material in the auger, resulting in vertical

movements in the screed and increased IRI. When increasing the speed of the paver,

28

the angle of attack decreases, reducing the thickness of the asphalt mat. When the

paving speed decreases, the angle of attack increases as the thickness of the

pavement. It is important that the operator maintains a constant speed of the paver.

Screed Forces

Speed

( P )

Material Level

( M )

Screed Weight

( W )

M

P W

Direction of Paving

Figure 21: Screed forces

Gradient Sensing: It is important that the paver counts on automatic level sensors.

There are contact and without contact types (with ultrasonic sensors).

Importance to place a pavement over a smooth surface: It is advisable to have flat

bases, as they become the reference of the level sensors. For repaves it is recommend to mill before placing a new mat and fill in the dips spots that remain after milling. It is important to measure the roughness of the mill surface, the maximum variations should be less than 5 mm for a 3 meter (10 foot) straightedge. See figure 22

Figure 22. Surface milled that require fill dips

29

.

Freshly Placed MatFreshly Placed Mat-75%

15% Compaction From Roller

Mat Profile After CompactionMat Profile After Compaction

Approximately 75% Better thanApproximately 50% Better than

Original Base ProfileOriginal Base Profile

Differential CompactionDifferential Compaction

Original Uneven BaseOriginal Uneven Base

15 m (50 ft)

Leveling Course

Profiled/Planed Off

Figure 23: Differential Compaction

A single lift of uncompacted asphalt will improve the IRI by approximately 75% on the base reading, after compacted it is estimated that the improvement will be only 50%. It is suggested to fill in the dips that remain after milling, as well as putting two layers instead of one, it will depend on the asphalt thickness. See Figure 23 The explanation of the importance to place a pavement over a smooth surface, either

a new base or a milled pavement, is described in the following chart. If we consider

that a hot mix asphalt in a truck has a density of 1,76 kg/mt3, equivalent to 76%, and

when being compacted at 96% reaches a density of 2,24 kg/mt3, it is required that the

pavement is compacted in 25% of the original thickness.

Chart 13. Thickness of the asphalt mat vs.density. ASTEC T-123S bulletin

Density HMA Mat Thickness pulg ( mm)

96% FINAL 1,00 (25,4) 2,00 (50,8) 3,00 (76,2) 4,00 (101,6) 5,00 (127,0)

94% 1,02 (25,9) 2,04 (51,8) 3,06 (77,7) 4,08 (103,6) 5,10 (129,5)

92% 1,04 (26,4) 2,08 (52,8) 3,13 (79,5) 4,14 (105,2) 5,22 (132,6)

90% 1,07 (27,2) 2,13 (54,1) 3,20 (81,3) 4,27 (108,5) 5,33 (135,4)

88% 1,09 (27,7) 2,18 (55,4) 3,26 (82,8) 4,36 (110,7) 5,45 (138,4)

86% 1,11 (28,2) 2,23 (56,6) 3,35 (85,1) 4,46 (113,3) 5,58 (141,7)

84% 1,14 (29,0) 2,28 (57,9) 3,43 (87,1) 4,57 (116,1) 5,71 (145,0)

82% 1,17 (29,7) 2,34 (59,4) 3,51 (89,2) 4,68 (118,9) 5,84 (148,3)

80% 1,20 (30,5) 2,40 (61,0) 3,60 (91,4) 4,80 (121,9) 6,00 (152,4)

78% 1,23 (31,2) 2,46 (62,5) 3,69 (93,7) 4,92 (125,0) 6,15 (156,2)

76% INICIAL 1,26 (32,0) 2,53 (64,3) 3,79 (96,3) 5,05 (128,3) 6,31 (160,3)

30

For instance, if we want to place a mat of 2 inch. (50mm) thick, we must place the

paver screed over a table of 2,53 inch (64 mm) thick so that during compaction

process the mat is reduced to 2 inches. If we have an irregular surface with bumps

and dips at the moment of compacting with rollers, different densities will be obtained

affecting the pavement useful life. At lower density more vacuums, which causes

erosion due to the water that comes thought the pavement and a lower resistance of

the mix to the traffic.

Prepare for paving:

(1) Check wear components as screed plates and auger. Check chains adjustment

as per factory recommendations.

(2) Check if the auger and tunnel extensions are installed, depending of the paving

width.

(3) Check if the auger feed sensors are installed correctly.

(4) Check that pre strike off is 1” above the bottom of the screed plate.

(5) Set auger height (HMA surface level + 2”).

(6) Lower screed onto shims (25% taller than paving depth compacted).

(7) Set slope on extensions to 0

(8) Set lead crown of the screed to 1/16” positive crown.

(9) Null the depth crank and then do a complete turn to the depth crank to obtain

and angle of attack between of 3,2 to 6, 4 mm

(10) In case is used a screed with frontal extensions, set extensions angle of

attack at least in 3/16” using the screed bolts.

(11) Set tow point in order to have a horizontal line of pull.

(12) Set grade & slope controls**

(13) Keep paver speed constant

** It takes approximately 25 to 50 feet ( 8 – 15 mts), generally three to five tow arms

lengths) for all the resultant changes to appear in the mat.

Continuous paving: Do not allow trucks to bump with the paver. Although screed

floats and can withstand some movement of the paver without damage, sudden

movements will produce bumpy surfaces.

Therefore, whenever the paver stops, changes speed, or bumps with trucks, there are

bumps that increase the roughness of the paved surface. Continuing Paving

eliminates the causes of roughness mentioned above. This is accomplished through

the use of Material Transfer Vehicle. See Figure 24. If for any reason the paver has to

stop, it is recommendable to leave material covering all the tunnel height in order to

keep material below the screed, avoiding the screed to move down.

A smooth pavement is a quality pavement. To obtain a smooth pavement the paver

should be operated continuously. 90% of all problems are eliminated if the paver runs

31

at a constant speed.

Figure 24 Paving continuous with the use of Material Transfer Vehicle

This self-propelled equipment allows a truck to discharge very quickly (3 minutes), has

a storage bin of up to 25 ton. capacity where the material is re-mixed by an anti-

segregation three-stage auger, homogenizing temperature (without heating) and

significantly reducing the segregation of temperature and aggregates, thus obtaining a

uniform mixture so that a same equipment eliminates the problem of aggregate and

temperature segregation while obtaining a smooth road by the paving continuous.

Figure 25. Triple-pitch used in Roadtec Shuttle Buggy MTV’s

The homogenized hot mix asphalt is then transported to the paver, where it is possible

to insert a 10 ton hopper capacity, increasing the amount of material on the train in 35

tons of asphalt and avoiding disturbing material spills. See Figure 26.

By reducing discharge and waiting times, due to the use of the Material Transfer

Vehicle, the number of trucks is reduced between 3 to 4 units. Often the US$/ton.

32

savings are equal to half or full cost of acquisition and operation of a Material Transfer

Vehicle.

Figure 26. Material Transfer Vehicle, Germany

The Material Transfer Vehicle is an equipment that permits to reduce the problems

produced during paving. Many times the truck drivers are inexperienced and bump

the paver, or even difficult to coordinate the discharge operation by pressing the

brakes, when the paver has to push the truck. The 10 ton capacity insert bin that is

installed in the paver hopper will help prevent spillage during the truck discharge in

front of the paver hopper. The consequence of spillage is that rise the screed when it

pass over it increasing the roughness. It is very common that the spilled material is

thrown on the mat just placed. This material is cold and will difficult compaction.

Each time the paver is stopped, the HMA placed close to the screed is not

compacted, while the temperature descends. This will difficult the compaction to

obtain a uniform density.

When the paver starts moving the screed will move vertically producing an irregular

surface, for this reason, it is important to reduce the paver stops.

Longitudinal joints: The second pass (HMA) of the paver should overlap ¾” the first

pass (rolled mat). The roller should compact the longitudinal joint overlapping the

rolled mat in 6 inches. Then each pass of the roller should overlap the previous pass

in 6 inches.

33

5. HOW TO RECOVER A DAMAGED PAVEMENT ECONOMICALLY

We can find deteriorated asphalt pavements, where the pavement structure (base and

sub-base) is in good condition in over 90% of the length of the road. Currently there

are recycling procedures for the bituminous pavement that are efficient and economic.

Figure 27. Cold Recycling in place using emulsions

In Figure 27 we can see a train of recycling, where a 950 HP cold planer cuts 4"

thickness of a damaged pavement and 3.8 m width in one pass. Then the material is

transported to a mobile recycler pulled by the mill. The recycling asphalt product

(RAP) is classified by a screen, the larger material (eg 1.25 ") is transported to an

impact crusher and the processed material is again transported to the top of the

screen through a conveyor circuit. The classified material that pass through the

screed is weighed by a belt’s electronic scale, accurate to +/- 1%, and then mixed with

emulsions (2 to 3% of the RAP weight) that are injected into a 2.4 meter long pugmill

mixer. The emulsion rate is controlled by a computer. The cold planer pushes the

additive tanker containing the emulsions. The processed material is placed by a

conventional paver (see Figure 28) and finally compacted by two roller compactors.

34

Figure 28. The processed material is placed by a conventional paver.

Before the cold planer cutter drum cuts the mat in some cases it is necessary to add

an additive like cement, usually 1% of the RAP weight. Finally it is necessary to place

a cold or hot road surface mat due that the recycled pavement will have 14% of voids.

The main advantages of this process are the following:

-Permits recycle 100% of damaged pavement.

-Permits recycle up to 2.4 km per day.

-Production capacity of 500 ton / hr.

-High quality recycled product due that the material size is classified and the good mix

in the 2.4-meter pugmill mixer.

-This recycling process reduces in 50% the cost of conventional road repairs.

-This recycling process has an estimated life of 6 to 8 years using a cold micro

pavement and 7 to 15 years using a hot mix asphalt pavement over the recycling mat.

-The recycling obtained in this procedure could have up to 80% of the strength

capacity of a new hot mix asphalt pavement.

35

6. COMPACTION

After placing the bituminous pavement correctly, that means over a smooth surface

base without bumps and dips, a homogeneous hot mix asphalt without segregation

and with constructive procedures that permits to place the pavement as smoother as

possible, it is of utmost importance to finish with a good compaction.

The table below recommended minimum lay down temperature, based on mat

thickness and base temperature. The numbers at the bottom of the table indicate the

time available to roll the mix after laying.

Chart 14. Minimum Laydown Temperature for Various Thickness, Shell Bitumen

Handbook. ( NAPA, Roller Operations for Quality)

Also, we must take into account that a 2” thick pavement placed at a temperature of

135°C has approximately 15 minutes before the temperature decreases to 80°C.

With a thermometer it is possible to prove how the temperature decrease in the first 15

minutes. It also depends of the wind and type of mix. In the next table it is possible

to see the temperature of a mix after the first 20 minutes of laying.

T ⁰C

139 132 125 117 115 111 104 103 102 99 96 95 83 81 81 81 80 79 75 73 71

T ⁰F

282 270 257 243 239 232 219 217 216 210 205 203 181 178 178 178 176 174 167 163 160

t min

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Chart 15. HMA Cooling Temperature vs Time (minutes)

Base Temp. C (F) Degrees

Recommended Minimum Laydown Temperatures, ⁰ C (⁰F)

13 m (0,5 in)

19 mm (0,75 in)

25 mm ( 1 in)

38 mm (1,5 in)

50 mm ( 2 in)

75 mm (> 3 in)

-7 to 0 (20 -32)

- -

- -

- -

- -

- -

141* (285)

1 – 4 (33 – 40)

- - -

- -

152 (305)

146 (295)

138 (280)

5 – 10 (41 – 50)

- -

- -

154 (310)

149 (300)

141 (285)

135 (275)

11 – 16 ( 51 – 60)

- -

154 (310)

149 (300)

146 (295)

138 (280)

132 (270)

17 – 21 (61 – 70)

154 (310)

149 (300)

143 (290)

141 (285)

135 (275)

129 (265)

22 – 27 ( 71 – 80)

149 (300)

143 (290)

138 (280)

138 (280)

132 (270)

129 (265)

28 – 32 ( 81 – 90)

143 (290)

138 (280)

135 (275)

132 (270)

129 (265)

127 (260)

> 32 (> 90)

138 (280)

135 (275)

132 (270)

129 (265)

127 (260)

124 (265)

Rolling Time

4 6 8 12 15 15+

36

Sometimes the rollers continuous compacting after the mix reaches 175 ⁰F (80⁰C)

when it is no possible to compact more due the increase of viscosity.

The plant temperature for a normal mix is around 300⁰F (149⁰C), while for a SMA mix

is around 340⁰F (170⁰C). Polymers mixtures should generally be completely

compacted before they cool below 280⁰F (138⁰C).

If we estimate that a paver normally operates at a speed of 5mpm, compaction must

be ended in a section of 75 meters behind the paver.

It is recommended that rollers give at least 33 impacts per minute. For instance, for a

speed of 50 mpm and with a frequency of 1800 vpm, it will have 36 impacts per

minute. It is very important to make a test strip of 150 mts and measure density at

each roller passing to establish a compaction pattern.

We have to be careful not to over compact, due that it can produce exudation or

fracture the aggregate.

Density differential is one of the main concerns on hot mix asphalt. Quality control

tests must be encouraged to obtain uniform density mats. Density is important to

prevent filter of water onto lower layers, reduce asphalt oxidation, an increased of

density when the traffic is open and to provide an appropriate cutting resistance.

Some Departments of Transport of the U.S.A. are using penalties when the pavement

density is out of the parameters required, like the Pennsylvania Department of

Transport. Payment factors are assigned to gradation, density at work and asphalt

contents. Payment factors are determined as per specifications limits deviations. The

following Chart shows the payment factors regarding asphalt content, gradation and

density:

Asphalt content Test Value Percente Adjustment

±0.07% 100

±0.8-1.0% 75

> ±1.0% *

Sieve #200 ±3.1-4.0% 75

±3.0% 100

> ±4.0% *

Density ≥ 92% or < 97% of DMT 100

90-91% or 97-99% of DMT 98

≤ 89% or > 99% of DMT *

Chart 16. Payment adjustment. DOT Pensilvania. DMT: Density Max. Theorical

37

7. SAVINGS OBTAINED WHEN PLACING HOT MIX ASPHALT WITH IRI LESS

THAN 1,5 M/KM AND WITHOUT SEGREGATION:

7.1 Transport costs analysis when using Material Transfer Vehicles

Information on transport and placing of HMA:

- Trucks of 28 ton capacity.

- Pavement speed of 5m/min.

- Lane of 3,7 m width and 2” thick compacted at 96% of density (2,2 ton/m3)

- Truck discharge time with Material Transfer Vehicle 3 min. This is due that the

Material Transfer Vehicle slat conveyor capacity is 1000 ton/hr and counts on a

storage capacity of 25 ton.

- Asphalt Plant capacity of 100 ton/hr

With the mentioned information the discharge time of a truck with asphalt mix can be

calculated:

- Production: 5m/mi x 3,7 m x 0,0508 m = 0,94 m3/min x 2,2 ton/m3 = 2,1

ton/min

Discharge time of a 28-ton truck = 28 ton / 2,1 ton/min = 14 min

If we consider that to plan a paving we have 6 truck at the job site, we can define in

the following chart the average waiting times for each truck :

MTV no used 14 min MTV used 3 min

28 ton truck Delay at Job (min) Dump time (min) Delay at Job (min) Dump time (min)

1 0 14 0 3

2 14 14 3 3

3 28 14 6 3

4 42 14 9 3

5 56 14 12 3

6 70 14 15 3

Total 210 84 45 18

Average Time 42 8

Chart 17. Average Delay Time at Job

38

Figure 29. Hot Mix Asphalt trucking. Roadtec Shuttle Buggy SB2500D in Brazil

In the case of the discharge directly to the paver the average waiting time of each

truck is 42 min.; while with the Material Transfer Vehicle it is 8 min.

If we consider an 8-hour working day with an efficiency of 50% due to weather

conditions, material delay, equipment failure, among other reasons, we can estimate a

production and placement of 400 ton per day.

In the following chart we can see the trucks savings regarding the discharge

procedure, either directly or using the Material Transfer Vehicle (MTV).

Using the Material Transfer Vehicle we can save 3 trucks, being the saving cost per

ton of US$ 1,68.

The operation cost of a Material Transfer Vehicle flows between 1 to 3 US$ per ton

depending on the annual hours of use. However, for a use of 1500 hours per year and

considering a plant capacity of 100 ton/h, the operation cost and the acquisition of a

Material Transfer Vehicle is 1,70 US$/ton., very similar to transport savings.

The use of the Material Transfer Vehicle enables to increase the efficiency of the

trucks from 65 to 87% as we can see in chart 16, this allows to reduce the holding

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transport costs in case the transport per ton is paid, due that trucks can make more

trips.

TRUCK CYCLE

Using MTV No using MTV

Plant Production Tons/ Hr 100 100

Production Tons/ Day 400 400

Tons per truck 28 28

Truck Cost/Hr $60 $60

Hrs /Día (Efficiency 50%) 4 4

Minuts Minuts

Delay at plant 0 0

Loading time

1 1

Ticket, Tarp % Sampling 5 5

Haul to Job 60 60

Delay at job 42 8

Truck excchange 3 1

Dump to paver 14 3

Return to Plant 60 60

Total minutes 185 138

# Cycles 1,3 1,7

# of Cycles/Truck 14 14

# of Trucks 11 8

Daily truck cost $2.642,86 $1.971,43

Cost per ton $6,61 $4,93

Truck efficiency 65% 87%

Chart 18. Trucking cost.

7.2 IRI reduction and uniform density savings

Considering the previous information, it can be calculated the savings quantity per ton

of placed mixture by larger useful life and lesser maintenance costs:

First, we calculate the daily advance for a lane of 3,7 mts width:

Advance: 5 m/min x 60 min/h x 4h/d = 1200 m of advance per lane per day

If we consider that the penalty in the Connecticut DOT is 31,5% for obtaining an IRI of

1,863 instead of 1,263 m/km, due to all benefits shown, like larger useful life of the

pavement and less maintenance costs and considering the penalty in the

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Pennsylvania DOT of 2% for being out of density ranges (considering that densities

less than 89% and more than 98% are not acceptable), due to mixture loss of

resistance because of segregation, we have a saving of 33,3% per each kilometer or

ton placed.

Saving per each 1200 m place in one day:

1200 m/d x 3,7 m x 0,0508 m = 225,6 m3 x 2,2 ton/m3m = 496,2 ton/day

If we consider an average cost of the prepared and placed mixture of 100 US$/ton, we

can calculate the saving by larger useful life and less maintenance cost of a paver:

Asphalt daily cost: 496,2 ton x 100 US$ / ton = 49.632 US$ / day

Savings for larger useful life and less maintenance cost per 1,2 Km lane: 49.621,99

US$ * 33,5% = US$ 16.623

The saving for larger useful life and less maintenance cost per each km is of US$

13.852 and for a road of 50 km with two lanes, the saving is of US$ 1.385.253

It is recommended to invest in specialized equipment for hot mix asphalt placement

like the material transfer vehicles and improve construction procedures to place

pavements with IRI less than 1,5 km and with a uniform density, instead of spend

prematurely on maintenance, with the consequent economical and environmental

impact.

8. HOT ASPHALT PAVEMENT CONSTRUCTION SPECIFICATIONS

8.1 ARKANSAS STATE HIGHWAY AND TRANSPORTATION DEPARTMENT

SPECIAL PROVISION

Division 400 of the Standard Specifications for Highway Construction, Edition of 1996

is hereby amended as follows:

The following is hereby added to Subsection 409.04, Mechanical Spreading and

Finishing Equipment:

Materials Transfer Device (MTD) / Materials Transfer Vehicle (MTV), A Materials

Transfer Device or Materials Transfer Vehicle (MTD/MTV) shall be used on all State,

US, and Interstate highways for the placement of all ACHM courses. ACHM quantities

exempt from this requirement are projects or phases of work with less than 1,000 tons

of hot mix, temporary pavements (such as detours, crossovers, driveways and

turnouts), and ACHM placement in trench widening areas less than 3.3 m (11’) in

width. The ACHM mixture shall be transferred mechanically to the paver by means of

a MTD/MTV. The material shall be continuously remixed or reblended either internally

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in the transfer vehicles, in a paver hopper insert, or in the paver’s hopper.

Remixing/reblending shall be accomplished by the use of remixing augers, paddles or

screens capable of continuously blending the hot mix asphalt.

The MTD/MTV, haul units, and paver shall work together to provide a continuous

uniform, segregation free flow of material. The number of haul units, speed of the

paver, plant production rate, and speed of the MTD/MTV shall be coordinated to avoid

stop and go operations. The wings of the paver receiving hopper shall not be raised

(dumped) at any time during the paving operation.

If a MTD/MTV or remixing/reblending unit malfunctions during lay-down operations,

the Contractor may continue hot mix lay-down operations until any hot mix asphalt in

transit or stored in a silo has been laid and until such time as there is sufficient hot mix

placed to maintain traffic in a safe manner. Lay-down operations shall cease

thereafter, until such time as equipment is operational.

The Engineer will evaluate the performance of the MTD/MTV and remixing/reblending

equipment by measuring the temperature profile of the mat immediately behind the

screed of the paver during the placement of the rolling pattern test strip. The ACHM to

be placed for temperature profile test shall be held in the haul truck(s) for at least 45

minutes, measured from the time of loading to the time of discharging into the

MTD/MTV. If the bed of the haul truck is covered, the cover will be removed once

arriving at the test strip location. The temperature profile measurements shall be

taken of the surface of the mat at six 13 m (50 ft) intervals during test strip construction

using a non-contact thermometer. Each temperature profile shall consist of three

surface temperature measurements taken transversely across the mat in a straight line

at a distance of 0.3m to 1m (1 foot to 3 feet) from the screed while the paver is

operating. The three temperature measurements in each profile shall be taken

approximately 0.3 m (one foot) from each edge and one in the middle of the mat. The

difference between the maximum and minimum temperature of each individual profile

shall not be more than 6° C (10°F).

Additional surface temperature profile measurements may be taken transversely

across the mat at any time during the project to determine if the MTD/MTV and

remixing/reblending equipment are working properly. During this verification testing, if

two consecutive temperature measurement profiles do not comply with the 6° C (10°

F) temperature differential requirement, the paving operation shall be halted and

adjustments made to the MTD/MTV or remising/reblending equipment to ensure that

the hot-mix placed by the paver is within the above temperature requirements.

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8.2 PENNSYLVANIA DEPARTMENT OF TRANSPORT

Use of the Transfer Vehicle of bituminous materials for pavements projects of the

National Highways System.

The Department has been reviewing the quality of the pavement projects with

bituminous mats in the main highways. Additionally, there is information about

equipment and processes that can improve works quality and durability.

Therefore, this is an effort to minimize the temperature and aggregates segregation

together with the improvement of ride quality, the Material Transfer Vehicles will be

required in all bituminous paving projects of the National way system where 5,000 ton

or more of material are placed in the contract.

It is important that we provide our users with the greater value for their taxes and the

Material Transfer Vehicle will improve our road investments.

Section 4.10 review

Provide Material Transfer Vehicles (MTV) to be used on an intermediate way and with

a self-propelled unit between the trucks and the asphalt pavers as follows:

- Provide covers of appropriate size to protect the material in the MTV.

- It must be capable of transferring the material from the trucks to the paver

hopper at a uniform and continuous range to permit the continuous movement of the

paver.

- Equipped with mixing augers to remix the bituminous concrete before

transferring it to the paver.

- Free of petroleum oils, solvents, o other materials that affect bituminous

concretes.

Additionally, paver must be equipped with a hopper that can provide a material flow

directly over the paver slat conveyors.

8.3 Spanish Standard, Works Ministry

The current articles on hot bituminous mixes of the road and bridges specifications

(PG-3) were published by order FOM/891/2004, on March 1st, though which most of

the articles were updated.

It is mandatory to use in from of the paver a Material Transfer Vehicle for different

categories of high traffic or road surfaces over 70,000 mt2.

These specifications were used from 2008.

.

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Tabla 542.14 (PG-3) Indice de Regularidad Internacional (IRI) (m(km) para

pavimentos de nueva construcción

Percent of Kilometers Mat type

Road Surface or Intermediate Other bitumen

mats Road type

Highways Other roads

50 < 1,5 < 1,5 < 2,0

80 < 1,8 < 2,0 < 2,5

100 < 2,0 < 2,5 < 3,0

IRI average 1,69 1,85 2,35

Typical

Deviation 0,2022 0,3905 0,3905

Chart 19. Spain IRI specifications

8.5 Nova Scotia, Canada

Material Transfer Device – Optional

Contractor will be paid an additional of $1.50 per ton when applying asphalt concrete

without segregation using the Material Transfer Vehicle (MTV). The MTV is a self-

propelled equipment designed to re-mix and transfer hot asphalt mixes from a truck

into the paver hopper, without direct contact with the paver. The areas subject to

repairs will not be chosen for the prize of $1.50 per ton.

8.6 New Brunswick, Canada

The attached specifications apply when the Shuttle Buggy is required by contract. In

provincial road projects where the Shuttle Buggy is not requested in the contract,

contractors have the option to use a Shuttle Buggy and charge an additional prize of

$2.0 per ton.

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9.CONCLUSIONS

It is possible to build roads with initial roughness index less than 1,5 m / km under the

IRI terms, as described in this technical paper. It is important to encourage new

specifications and procedures such as the use of infrared camera to control and record

the temperature differentials, the Material Transfer Vehicle to obtain smooth surface

pavements with uniform density, and the use of modern equipment to measure

roughness. The Governmental offices and Private road concessions responsible for

the construction, maintenance and control of the roads may apply incentives or bonus

when smooth roads (less than 1,5 mts/km) are built. It is more economical to

maintain smooth pavements so they last longer. This helps reduce the maintenance

costs of vehicles and trucks, and roads will be safer and less noisy.

The service index can be maintained higher and the roughness lower when smooth

roads are built using correct procedures and equipment that permit to place pavement

at a uniform temperature and within the design parameters, so that they can last and

do not present premature rutting and potholes, which will cause an IRI increase and a

low serviceability, increasing vehicles maintenance costs and fuel consumption.

The use of asphalt in road construction is sustainable because it allows 100%

recycling forever. It is very important to correctly assess traffic conditions, climate,

drainage, pavement structure, materials analysis, choose the appropriate additives

and plan an adequate quality control before starting a job of recycling.

In Annex 1 it was held a Thermal Study during the placement of hot mix asphalt in the

conventional way (direct discharge of truck to the paver), and the use of Material

Transfer Vehicle 'Shuttle Buggy' during construction of the motorway A1 EX in the

month of March 2005 in Spain.

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Annex 1- Thermal study

PROJECT : Autovía EX A1

PLACE : Malpartida de Plasencia, Spain

CONSTRUCTOR : Sacyr

DATE : March 31, 2005

STUDY : Thermal study using infrared camera

REALIZED BY : SACYR SPAIN -TEMAC SPAIN - ROADTEC EE.UU.

Behind the paver and truck Behind the paver using a MTV.

Temperature differential Uniform temperature

46

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1. Pavement Smoothing Technical Bulletin T-123 by J. Don Brock, PhD., E.g. and

Jim Hedderich ASTEC Industries, Inc..

2. Technical Bulletin Temperature Segregation by J. T-134 Don Brock, PhD., E.g.

Herb Jacob ASTEC Industries, Inc..

3. Operation Quality paving machines, NAPA

4. Operation Quality roller compactor, NAPA

5. Centerline Volume III, Follow 1, Spring 1.998

News from the Flexible Pavement Council of West Virginia

6. Pavement Smoothness Index Relationships, Final Report, FHWA 2002

7. Pavement smoothness by Ronald Collins (PTI), 2001

8. Washington State Department of Transportation.

9. 2005 AASHTO Provisional Standards Guide

10. World Bank Technical Paper Number 46, 1986

Guidelines for Conducting and Calibrating Road Roughness Measurements

Michael W. Sayers, Thomas D. Gillespie, and William D.O. Paterson

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Technical Report UMTRI-2005-24, Steven M. Karamihas

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Transport. Research Australia. ARR Report 314 (Año 1998)

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