INSTITUTO SUPERIOR TÉCNICO

Universidade Técnica de Lisboa

Design of Block Pavements

Paulo Roberto da Silva Morgado

Extended Abstract to obtaining the Master Degree in

Civil Engineering

December 2008

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1. Introduction

Pavement is the part of the road or street, consisting of various materials and that are placed on

natural ground or in landfills, in order to directly support traffic. [BRANCO; 2006]

One of the first forms of paving in Portugal were the Roman roads, great work of engineering, which

enabled several sections to endure for centuries, being still in operation.

Nowadays one of the coatings used in the design of pavements are precast concrete blocks for

pavements – CB.

The application of pavements which use precast CB´s generally associated to sidewalks and

accesses to residential areas. No less common is their use in gasoline service stations, parking lots, and

sometimes bus stop areas.

With a structural behavior similar to that of flexible pavements, the concrete blocks pavement –

CBP’s – allow repairs without leaving a trace. This is a great alternative, both from a technical and

economic point of view.

Given the importance of the CB’s in paving, and still with a very limited study on this issue in our

country, this work has three main objectives:

Summary knowledge of the design and construction of CBP’s.

To analyze aspects related to the contribution to a better structural knowledge of the

surface course consisting of CB, including though the conduction of load tests on an

experimental pavement;

Present a catalog that brings together several classes of traffic and subgrade, appropriate

to the national reality.

2. Block Pavements

2.1 Scope

There is a vast array of application possibilities in which we can use CBP. When CBP were used for

the first time, architectural applications predominate, but afterwards, according to the increasing

knowledge on the subject, these applications extend to areas exposed to light and even heavy road traffic.

Here’s a wide range of applications for this kind of pavements:

Pavements on buildings.

Pavements in pedestrian areas.

Pavements with decorative purposes.

Urban pavements exposed to road traffic.

Pavements exposed to heavy road traffic (industrial, seaport and airport pavements).

Pavements for special zones (hydraulic applications, mining zones, agricultural zones).

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2.2 Functions and Materials in use on the Various Layers

We will now study the materials in use in the various layers of a CBP and their respective functions.

On Fig. 2.1 a schematic cut of the standard structure of a CBP is presented, with respective

designations given to the several layer of the pavement and subgrade.

Generally, these pavements are composed by surface courses (concrete block course and laying

course), foundation courses (base and sub-base courses) and by the subgrade.

Fig. 2.1 – Type Structure of a CBP

The subgrade of the area to be paved shall be inspected to know if it is formed by local natural soil

or soil from another place. This soil should be non expansive.

The base course is purpose to receive distributed stress by the surface course. It shall whit stand

and distribute tensions to the sub-base course, in case it is present, or to the subgrade, thus avoiding

permanent deformations and consequent deterioration of the pavement [MÜLLER; 2005]. The materials to

use in the layers should be preferably tout venant or cement-soil.

The laying course is to be formed by sand, with thicknesses ranging 3 to 5 cm. The main purpose

of the pavement’s laying course is to serve as support for the settlement of CB. The thickness and quality

of the sand directly influences on the pavement’s end performance. The structural behavior of the CB’s is

directly related to the thickness of the layer, granulometry and the aggregate’s shape index. [MÜLLER;

2005]

The surface course formed by the CB’s ensures comfort to the final user, pavement durability and

determinately contributes to the structural function of the pavement (stress distribution), trough its features

of interlock, besides providing support to shear surface stresses caused by vehicle wheels.

2.3 Quality Control

The quality control scheme used, which is duly adjusted by function of the greater qualification and

reliability with which the production and placement on field takes place, includes:

Collection of samples for the control of aggregate granulometry, fineness module and

moisture content in sands, etc.

Requirements and test methods for the precast concrete blocks for CBP according to

European standard EN1338:2003.

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The EN 1338:2003 standard stands out from other international standards mainly for incorporating

a system in the manufacture process of the CB’s, which allows the producer to guarantee an adequate

quality system of its products and shipping to its clients, in conformity with specific rules of the producing

countries and those of the CEN.

For the CB’s to be incorporated or applied on a permanent basis on the field they must have CE

marking, which means they have to be evaluated against a set of minimum requirements as set on annex

ZA of standard EN 1338:2003.

2.4 Construction Overview

The most important steps to follow in building a CBP are: preparing the subgrade, spreading and

compacting the sub-base course, spreading and compaction of the base course, completion of the edge of

the kerb, spreading and leveling of the laying course, placing the blocks, pavement vibration and correct

interlock of the CB’s and sealing of joints with sand.

2.5 Solicitations

Requests operating in CBP are dependent on the type of traffic or loads applied to the pavement.

According NEVES (2001) the main characteristics of requests applied to the pavement caused by the

passage of vehicles are:

The intensity of the load that acts on each wheel of the vehicle.

The conditions involved on the application of that cargo, usually characterized by geometry

and tension between the tire’s contact surface and the pavement.

3. Design Methodologies for CBP

SHACKEL (1990) and HALLAC (1998) describe that the methods of scaling and design of CBP

can be divided into four categories:

Based on field experience, or local experience.

Based on empirical data.

Based on modifications of existing methods for flexible pavements, through modeling the

equivalence of materials.

Based on computer models.

The design of CBP has developed in a context of various types of applications in the past 25

years, since the exclusive use on sidewalks up to the special applications in areas with large

concentrations of static loads, such as seaport and airport areas.

HALLAC (1998) says that the greatest challenge on the design of CBP is determining the value of

the modulus of elasticity of the abrasive layer composed by the CB’s + laying course. The figures obtained

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in studies on traffic simulators or in situ measurements show a wide dispersion. In addition, simulation to

determine these values in conjunction with various types of base (stabilized with cement, stabilized with

bituminous materials, sand and granular materials) is complex, revealing the need, therefore, for the

establishment of procedures for obtaining results from laboratory tests, retro analysis from deflection

measurements provided by FWD in experimental areas or through the observation and monitoring of

pavements in service.

4. Study and Modeling of an Experimental Pavement

4.1 Pavement description

The pavement under study is located at the Air Force Academy’s Military Hospital’s parking lot,

more precisely in Azinhaga dos Ulmeiros, Lisbon, with the following structure (Fig. 4.1):

Fig. 4.1 – Schematics of the pavement under study

4.2 Pavement load tests

In order to determine the mechanical characteristics of the various layers on the experimental

pavement, non-destructive load testing with an impact deflectometer was used. This equipment was

designed to study the deflections of pavements subject to dynamic loads that would reflected the

movement of vehicles in terms of speed.

In each test, after the initial impact of the first drop height, whose aim is to adjust the plate to the

surface of the pavement, 3 impacts were carried out based on 3 levels of load data in time, in order of

increasing impact loads. The drop heights were set to match the approximate values of impact forces as

listed next: Impact of the 2nd drop height to match the peak load of 40 kN; impact of the 3rd drop height to

match the peak load of 100 kN; impact of the 4th drop height to match the peak load of 150 kN. [NEVES,

2001].

The pavement deflections induced by the impact load were measured at various points through

geophones supported on the surface of the pavement.

Deflection values for normalized load values of 40kN, 100kN and 150kN are graphically

represented in Fig. 4.2. Please note that the colored lines are the average values for each peak load.

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Fig. 4.3 - FWD test (for standard values of 40, 100 and 150 kN)

By analysis of the results of measured deflections obtained through FWD on the abrasive layer we

can conclude that there is a good homogeneity of the structural behavior in each layer.

4.3 Behavior Modeling in Load Tests

Determination of mechanical characteristics of the materials that constitute the different layers of

pavement was made by resource to the program of automatic calculation BISAR. Through several

iterations, varying the module of deformability of the layers, dislocations were obtained until the calculated

vertical dislocations of the pavement would converge with the experimental dislocations determined

through FWD load tests. The materials mechanical parameters are fixed when the average of the

convergence error is inferior to ±15 %.

Table 4.1 presents the values of the modules of deformability estimated for the pavement’s various

layers under study, so that the deflection was calculated as close as possible to that measured in situ. For

a better analysis, and with the aim of reducing the average error, two layers of subgrade were added.

Table 4.1 - Deformability modules estimated for the various layers

INPUT

OUTPUT

Layer E

(MPa) Distance

(cm)

Deflections for 40 kN

(µm) Deflections (µm) Error

CB (5,5 cm) 1000 0,30

-30 1,912E+02 1,914E+02 0,12%

Fine Crushed aggregate (20 cm) 200 0,25

0 5,413E+02 5,649E+02 4,36%

Crushed aggregate sub-base (20 cm) 400 0,35

30 1,285E+02 1,914E+02 48,94%

Subgrade (100 cm) 200 0,35

45 8,448E+01 8,282E+01 1,96%

Subgrade (100 cm) 1000 0,35

60 5,281E+01 4,981E+01 5,68%

Subgrade 2000 0,35

90 2,053E+01 2,399E+01 16,84%

120 9,237E+00 1,188E+01 28,61%

150 5,132E+00 6,104E+00 18,94%

180 3,692E+00 3,397E+00 7,98%

Average 14,82%

0

500

1000

1500

2000

-30 20 70 120 170

Ave

rage

De

fle

ctio

ns

(μm

)

Distance to axis (cm)

F=40 kN

F=100 kN

F=150 kN

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5. Pavement catalog

5.1 Traffic classification

For the elaboration of the CBP catalog traffic shall be classified in 3 different classes based on a

classification by the Spanish Association of CBP [ADAH; 2004]: Low, Average and High.

For design purposes of the CBP, only the effect of the annual daily average traffic of heavy vehicles

will be considered, per direction of circulation, on the route most sought by these vehicles (TMDA)p and

this value should be obtained from a traffic study.

The design of a pavement is to ensure appropriate conditions for traffic movement over a given

period, which is known as period of design, minimizing the need for conservation works in this period. For

CBP’s the usual design period is 20 years.

Since there are no specific studies for the CBP, a growth rate of 1% for low and average classes of

traffic and 3% for the high class traffic will be considered.

To express the effect of a given number of passages by heavy vehicles with diversified

characteristics, a conversion into passages of an equivalent standard axle is done, taking into account

factors of aggression, whose values are defined according to the manual of pavements design for the

national road network [JAE, 1995].

Trough the evaluation of the adequacy of the proposed CBP structures, in terms of their respective

load capacity, an acceptable value for the total number of standard axles of 80 kN was defined.

The accumulated traffic of standard axles during the period of design, corresponding to three

classes of traffic is given by:

N80dim

= 365 x (TMDA)p x C x α x p 5.1

With: tp

1p

t)(1C 5.2

In which:

C is the growth factor for road traffic; t - average annual growth rate of heavy traffic (%);α -

damage factor of traffic (α); p – design period (years); (TMDA)p – annual daily average

traffic of heavy vehicles, for direction of circulation, on the route most sought by these

vehicles, on the opening year.

Table 5.1 indicates the 3 traffic classes considered for the annual daily average traffic of heavy

vehicles, for direction of circulation, on the route most sought by these vehicles, on the opening year.

(TMDA)p, the average growth rate, aggressiveness factor and N80dim

(20 years).

Table 5.1 – Traffic related data

Class (TMDA)p Average

Growth Rate (%)

CBP

Damage Factor (α)

N80 dim

(20 years)

Low T3

Parking lots for light vehicles; pedestrian areas; commercial streets with width <6 m;

< 15 1 2 2,4*105

Average T2

Main or structural arteries with a width > 6 m, which do not cross roads with traffic superior to 49 trucks per day;

15 – 49 1 2 7,9*105

High T1

Streets or main arteries with high traffic, Bus Stops; Service stations etc.., not exceeding 150 heavy vehicles per day;

49 – 150 3 2 2,9*106

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When considering values for the average daily traffic, the growth rate, the factor of

aggressiveness or the design period different from those adopted (Table 5.1), or when carrying out a

phased construction, the values of accumulated traffic will be calculated in accordance with the adopted

methodology.

5.2 Characteristics of the Materials

Three classes of pavement foundation will be considered, as shown in Table 5.2. For each class a range

of values is admitted for the modules of deformability for the pavement’s foundation.

Table 5.2 – Foundation Classes [JAE; 1995]

Foundation Class Foundation Modulus (MPa)

Traffic Class Range Calculating Value

F1 30 a 50 30 T3; T2; T1

F2 51 a 80 60 T3; T2; T1

F3 > 80 100 T3; T2; T1

For design purposes of typical CBP structures, approximate mechanical characteristics are

considered for the natural materials, granular base or treated with cement, used in the base and sub-base

courses, as shown in Table 5.3.

Table 5.3 – Mechanical characteristics considered for materials [JAE; 1995]

Symbol Designation Deformability Modulus (E)

Poisson ratio (ν)

BG tout-venant applied on the base course ≈ 2 x E inferior

layer 0,35

SbG tout-venant applied on the sub-base course

≈ 2 x E inferior layer

0,35

AGEC Lean concrete 15000 0,25

Sc Cement-soil 2000 0,35

Along with the laying course the CB’s are considered to act as a single layer, with constant

deformability modulus and Poisson coefficient, with deformability modulus of 1000 MPa and a Poisson's

ratio of 0.30. These values were obtained using a case study discussed in the previous chapter.

The CB’s to be used on roads and streets should have a thickness of 80 mm, being this a

common practice in Portugal.

5.3 Criteria for design

The pavement was modeled, using the program BISAR, as a multi-layered structure, with a

standard axle of 80 kN, a load on each wheel of 20 kN and 500 kPa of pressure contact as a model basis.

It is assumed that each layer is homogeneous, isotropic and elastic and is characterized by E - module of

deformability - and ν - Poisson ratio.

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For the CBP’s there are two main limit states of ruin present, the shearing by fatigue (horizontal

traction tensile) on the basis of mixtures of materials with hydraulic binders and permanently deformed

(vertical compression strain) on the top of the subgrade.

The study of ruts, which evolve over time with the passage of traffic and contribute to the

formation of rutting, was based on the validation of the strain criteria:

z,apl < z,adm 5.3

In which z,apl is determined with the assistance of the BISAR software and z,adm is obtained

through the Shell equation:

z,adm = a N b

5.4

In which:

z,apl – strain of application of vertical compression at the top of the subgrade; z,adm –

admissible strain of vertical compression at the top of the subgrade; N - acceptable number

of passages of the standard axle (80 kN); a and b - characteristic parameters of the

materials, where a = .2,1 or 1.8 (for a reliability of 85 and 95% respectively) and b =- 0.25.

For the design of layers with hydraulic binders, it was admitted that the fatigue of the mixes

subject to bending by the wheels of heavy vehicles could relate to the maximum tensile stress (σt), and

must check the following relation:

apl < adm

5.5

To calculate the maximum admissible tensile stress comes to use the expression of the JAE

presented in the manual for design of pavements for the national road network:

dim

80

r

t Nlogx a1σ

σ 5.6 [JAE; 1995]

In which:

σt – maximum tensile stress induced by the standard axle; σr – resistance to traction under

bending (Rbending); N80dim

– admissible number of passages by the standard axle (80 kN); a –

constant, which depends on the composition and properties of the mix, admitting values

ranging -0,06 to -0,1 (-0,08 was adopted).

Should the resistance to traction be evaluated from direct compression tests, it is admissible to use

a factor of 1.5 to convert the value obtained to the one expected to be obtained from a bending test:

Rflexão ≈ 1,5 x Rcd 5.7 [JAE; 1995]

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In which:

Rf – resistance to traction under bending stress; Rcd – resistance to traction under diametral

compression, where Rcd assumes the value of 1 MPa in case its AGEC and 0,3 MPa in

case its Sc.

5.4 Proposal for type structures for CBP

This sub-chapter proposes, in the form of a catalog, a standard set of structures to be adopted

during the phase of previous study for CBP.

The structure of this catalog is based following principles:

Definition of type structures for CP, based on the combination of different types of

materials for the constituent layers.

Consideration of the worst extreme conditions of the classes of traffic and foundation, to

determine the proposed thicknesses, so that when facing actual conditions set in the

implementation stage of design, proposals should be adjusted.

Change in thicknesses of the sub-base and base course according to the traffic class and

foundation class (T1, T2, T3, F1, F2 and F3).

Fig. 5.1 shows, in the form of a catalog, type sections proposed for the CBP.

Figura 5.1 – Proposed type sections

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6. Conclusions

Below is an overview of the main conclusions, considered as the most important of the work that

was done:

We can conclude that most methods of scaling of the CBP stems from adjustments to the

methodology of calculation for flexible pavements, where the concept of equivalent layers is used.

Through numerical modeling of experimental pavement we can apprehend that the module of

deformability of the CB’s figure on the order of 1000MPa, a figure that was used in preparing the

catalog of structures in CBP.

The worst conditions of traffic and foundation were considered when building the catalog. The

resulting catalog does not apply to the design stage of implementation, but rather serves to

support and guide the design of structures for CBP to be taken into consideration in the

construction of new national road infrastructures.

Bibliographic references

[ADAH; 2004] Asociación y Desarrollo del Adoquín de Hormigón. Manual Euroadoquín. Madrid :

Publicaciones Euroadoquín, 2004.

[BRANCO; 2006] BRANCO, Fernando, PEREIRA, Paulo e SANTOS, Luís Picado. Pavimentos

Rodoviários. Coimbra : Almedina, 2006.

[HALLAC; 1998] HALLHAC, A. Dimensionamento de Pavimentos com Revestimento de Peças Pré-

Moldadas de Concreto para Áreas Portuárias e Industriais. São Paulo : Escola Politécnica, 1998.

[JAE; 1995] JAE. Manual de concepção de pavimentos para a rede rodoviária nacional. Almada, 1995.

[MÜLLER; 2005] MÜLLER, Rodrigo Menegaz. Avaliação de transmissão de Esforços em Pavimentos

Intertravados. Rio de Janeiro : Universidade Federal do Rio de Janeiro, Junho de 2005.

[NEVES; 2001] NEVES, José Manuel Coelho das. Contribuição para a modelação do comportamento

estrutural de pavimentos rodoviários flexíveis. Lisboa : Tese de Doutoramento apresentada ao Instituto

Superior Técnico, 2001.

[SHACKEL; 1990] SHACKEL, Brian. Design and Construction of Interlocking Concrete Block. New York

and London : First Edition and Reprinted 1991, 1990.

[ORIGINAL BLOCOS; 2008] http://www.originalblocos.com.br/ (Consultation to 08 May 2008).