RELIABLE SHRINKAGE AND CRACK DESIGN: CEOS.FR FRENCH NATIONAL RESEARCH PROGRAM, EXPERIMENTAL ASPECTS.

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Demilecamps

and al.

3
rd
fib

International Congress
-

2010





RELIABLE SHRINKAGE AND CRACK DESIGN: CEOS
.
FR

FRENCH NATIONAL
RESEARCH PROGRAM, EXPERIMENTAL ASPECTS.


Louis Demilecamps,
Vinci Construction, Nanterre

(Paris)
, France

on b
e
half of CEOS.
FR

Program Partners (www.ceosfr.org)



ABSTRACT


Numerous concrete c
onstructions have to ensure various functions, aside from their
structural resistance, many of which are linked to reinforced concrete cracking.

Crack design and forecast has to be made under ultimate or service states, or after
such stress.


The aim of CE
O
S
.
FR

project

is to provide the designers at last with reliable crack
design codes, the validity of which extends to massive and strongly reinforced
structures.
This implies to provide the codes designers with experimental data that
could help to better tu
ne the codes formulas
, or allow to propose new ones.


That is
the reason
why CEOS
.
FR

project has made the choice to develop a virtual
testing approach in a four steps program:

-

Design and scientific research calculation models are benchmarked with
available

experimental data.

-

Structurally simple but thoroughly instrumented massive experimental bodies are
to be specifi
cally designed, cast and tested
. The experimental plan will permit to
identify the influence of “second order” parameters. External and limit c
onditions
will be fully recorded. Materials will be fully characterised. Shrinkage and cracks
will be tracked down and registered using simultaneously various and
complementary sensors. About 25 pieces, some of them weighing up to 20 metric
tons, will thus

be tested und
er
static
monoton
ous
,
THM and cyclic loadings.

-

For each test body, code and scientific calculations will be performed before and
after testing, and the data will be matched with the observation. Collected data
shall allow
improving and certif
ying

the accuracy of scientific calculation codes.

-

These validated scientific codes will then be used to perform virtual
experimentation on complex mas
sive structures and allow
readjusting

the
calculation rules.



Keywords:

Shrinkage,

Cra
cking,
Reinforced
Concrete
,

Experimentation
,

Simulation
,

CEOS.
FR



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INTRODUCTION


Numerous concrete constructions have to ensure various functions and feature specific
performances aside from their structural resistance, many of which being linked to reinforced
concrete cra
cking or better low
-

or
no
-
cracking; these properties are durability, water or air
tightness, safety (for nuclear vessels fro example). To the current practice and as regards
crack control, structural design is based either on formulas like in design codes

CEB model
Code 78,
fib

CEB model Code 90, EC 2
-
1
-
1, SIA or ACI 318
-
15, or
on
detailing procedures.
Experience has shown that this approach is not accurate and sometime
s

wrong, for thick
slabs, walls and other structural massive elements and special concre
te structure.


Therefore, the French concrete construction community has initiated the
CEOS.FR

national
research project which gathered more than 50 first rank organizations and companies,
including infrastructure owners, construction companies, engineerin
g companies, private or
public research centers, with the support of French Construction Ministry (MEEDDM).
Granted with a total budget
of 7 million Euros, the project is
intended to last four years,

starting from 2008
.


The aim of
CEOS.FR

is to improve th
e knowledge of the cracking phenomenon and
at last
to
provide the designers with reliable crack design codes, able to give a reasonable appreciation
of crack width and spacing, applicable to a greater number of structures.


The project has been organized o
n a cross theme approach. Three types of situations
involving cracking have been identified:

-

static and monotonous loading

-

thermo
-
hydro
-
mechanic loading

-

cyclic or seismic loading.


And three ways to investigate these loading types have been retained:

-

stru
ctural analysis and design

-

scientific
modelling

-

experimental testing.


From
a structural analysis and design
point of view, a first benchmark between
above
mentioned
calculation codes was led by different engineers from already available
experiment results
. It has confirmed that design cracks spacing and width estimation could
range
from
a 1 to 5 factor. This point is more thoroughly approached in another
fib

paper by
P. Bisch, D. Chauvel and J. Cortade.


The main object of this paper is to present the expe
rimental aspect and program of
CEOS.FR
.


The first
point
that must be emphasized is that the aim of experimental program is
not

to
provide new data on unknown
phenomen
a

or specific cracking situations, but full sets of
coherent data that

shall allow scient
ific research to finely and accurately adjust and qualify
their models to be then able to run virtual tests in complex situations. From these virtual
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(numerous) experimentations, it shall be possible to produce enough data to initiate the
expected code rev
ision.



EXPERIMENTAL PROGRAM


The experimental program is divided in three parts, according to the three loading cases:
cyclic, static
monotonous

and thermo
-
hydro
-
mechanic, leading to three experimental
arrangements. In each of them, some common principl
e
s have been retained:

-

cast concrete testing bodies as large as allowed by the testing machines or
installations

-

design testing bodies as geometrically simple as possible in order to allow a
very accurate transfer to
modelling
,

-

start from one reference test
ing body, and then just change one "second
order" parameter, to allow to check if the calculation model is able to
represent reliably even slight detail changes likely to influence cracking
generation,

-

as it is essentially not possible to foresee where and

how the cracks will
appear, it has been decided to use (over)numerous sensors and moreover
based
o
n different physical principles. Therefore we expect to be able to
pick
-
up the relevant information from at least one set of filed data,

-

material relevant ch
aracteristics will be determined thoroughly (not less
than 15 parameters for concrete) and from early age for concrete strength
and
modul
us,

-

as it was fanciful to think that we could ensure stable external conditions, a
complete set of additional sensors (
wind ins
ula
tion, temperatures,
hygrometry
)

was placed
close to the testing bodies fabrication and testing
areas. It will provide a full set of limit conditions data which will
then
be
introduced within the
modelling

phase.



ALTERNATE LOADING TE
ST ON 1/3 S
CALE CONCRETE WALLS


The aim of this sub
-
program is to get data on cracking mechanism and pattern when a wall is
submitted to alternate loading applied within its symmetry plane. The overall dimensions
selected for the testing sam
p
les were 4.20

m in length

by 1.50

m in height. Due to the level of
forces to be applied to reach
the
ultimate shear load

and to the affordable jacks and testing
bench
,
the
thickness of the wall was limited to 0.15

m, more or less representing a 1/3 scale
d

massive
and highly reinfo
rced
wall
as it can
be found in industrial buildings.


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Fig.
1

Design Principle


Design specification
s

w
e
re as follows:

-

wall behaviour must be tested over ULS

-

testing bodies behaviour should

be similar to the one that cou
l
d

be obser
ved
at real scale, which implies to
reduce
concrete
thickness
and reinforcement
diameter,

-

reinforcement ratio should
largely
be over non
-
fragility minima

-

stress state and cracking pattern due to cyclic loading have to be perfectly
isolated.


This
led to th
e following choices:

-

scale factor : 1/3

-

minimum reasonable height/length ratio (about 1/3), to avoid flexion to
interfere with shear

-

concrete category: C40, with one variant of C25

-

reinforcement ratio: 0.5% to 1% in two layers

-

limited reinforcement resista
nce to accommodate the bench loading
capacity: 4 MN.


Table
1



Static
reference

test

Dynamic

reference

test

Dynamic test
with

concrete
change

Dynamic test
with
reinforcement
change

Concrete

C40

C40

C25

C40

Cracking limi
t

(MN)

1.
47

1.47

1.09

1.47

Reinforcement



,


x㄰⁣m


1〬

㄰1㄰⁣m


1〬

㄰1㄰⁣m




㡸㠠8m

牳ⱥ晦

〮〱〵

〮〱〵

〮〱〵

〮〰㠴

獳s

⡍偡(

㌵〮3

㌵〮3

㈶〮2

㐳㐮4

ULS

⡍()

㌮㌰

㌮㌰

㌮㌰

㈮㘴

睫
嘩

⡭洩




〮㐹

〮㔴

〮㐶

S牭

⡭洩

ㄷ1

ㄷ1

ㄷ1

ㄷ1

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Fig.
2

Testing Body General Dimensions



Fig.
3

General Reinforcement Arrangement


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Fig.
4

Loading bench arrangement, principle


INSTRUMENTATION AND
DATA COLLECTION


Apart fr
o
m measurement of

external alternate efforts applied, each testing body will be
thoroughly equipped with various and numerous sensors:

-

17 "long base" optical extensometers will be placed externally on one side
of the wall giving information on global deformations,

-

4 temper
ature optical sensors based on Bragg nets, inside,

-

19 Bragg optical local deformation sensors placed on reinforcing bars.
Accuracy of such sensors is over 1

µm/m,

-

40 electrical strain gauges place
d

on reinforcement bars, plus one directly
within concrete.



Fig.
5

External
Extensometers


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Fig.
6

Internal Bragg Optical Fiber
Sensors


According
to
these more or less "local" deformation measurements,
a full area def
ormation
field will be recorded

using a new meas
urement techni
que
: image correlation (see detailed
description hereafter).



Fig.
7

Image
Field Arrangement


Principle



MONOTON
OUS LOADING ON

LARGE SCALE TESTING
BODIES, "FREE
SHRINKAGE" OR "RESTR
AINED SHRINKAGE"


The aim of these

two sub
-
programs is to collect extensive data from large dimensions testing
bodies. One
series

of prismatic testing bodies will be cast,
freely
matured during about four
weeks, then brought to a seriously cracked stage by simple flexion on a specially mad
e
testing bench. The second
series

intends to explore the field of restrained shrinkage. Similar
sized testing bodies will be cast and appropriate arrangements taken to prevent
almost every
possibility of shrinkage. After one month of maturation, they will

also be submitted to a
flexion test on the same bench.


FREE SHRINKAGE TESTI
NG PROGRAM


Large
ly

dimension
ed

testing bod
i
es
(6.10

mx1.60

mx0.80

m)
have been designed to explore
the influence of different "second order" parameters.

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After being cast, they w
ill be let to maturation freely, slightly protected from major weather
influence, for about four weeks? Then they will be placed on a flexion bench, tightened to it
by prestressing bars and brought to limit flexion stage by two
rows

of four 100

T jacks.



Fig.
8

Free
Shrinkage Testing Bodies (Scheme
)



Fig.
9

First
Free
Shrinkage Testing Body


Different situations to be tested are as follows:

-

RL1:
Reference
: reference

grade
concrete, reference reinforcement
(p
ercentage, diameter)

-

RL2:
Reference concrete reduced percentage, same bar diameter

-

RL3:
Reference concrete, same percentage, reduced bar diameter

-

RL4:
Reference concrete, same reinforcement but increased concrete cover

-

RL5:
Same reinforcement, reduced conc
rete grade

-

RL6:
Variability test: reference block

-

RL7:
Reference block with small defaults: Ø27

mm

plastic tubes

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Fig.
10

Scale 1
Prismatic Body. General Layout



Fig.
11

Typical
Reinforcement


Instrumentati
on


Apart fr
o
m external parameters, all blocks will be fully instrume
nted, externally and
internally:

-

12 points for internal temperature measurement

-

16 vibrating cord sensors for local internal deformation measurement

-

16 vibrating cord sensors for external

deformation measurement

-

15 displacement sensors during the flexion loading phase

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-

3 internal optical fiber sensor

-

6 external optical fiber sensor

-

12 electrical strain gauges placed on reinforcement bars



Fig.
12

Instrumentation


Mo
reover two complementary technique
s will be used
.


Acoustic
s
urvey
:

A set of acoustic sensors is placed on lateral faces of the testing body during the flexion test
phase. Each new cracking event or extension generates acoustic signal that is recorded in
t
ime and in frequency by the lateral sensors. Numerical treatment allows

localizing

the event
even when
it
is not
yet

observable externally.



Fig.
13

Acoustic
Sensors Arrangement


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Fig.
14

Cracking
Events Loc
alization


Image
c
orrelation
:

Different pictures of lateral face of the testing body are taken during the flexion test phase,
and then numerically correlated. The comparison allows
making

a full area displacement
field analysis with a precision of about 0.
1

mm.



Fig.
15

Paint
Pattern Preparation


The technique implies a preparation of the recorded face by projection of a special painting
pattern.



Fig.
16

Paint
Pattern Preparation (Detail
)

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RESTRAINED SHRINK
AGE


Restrained shrinkage testing bodies are
I
-
shaped, with a central part (the one that has to be
tested) which has a cross
-
section of 0.80

mx0.50

m. Two largely dimensioned steel members
placed laterally between the two transverse heads will prevent almo
st any shrinkage (but they
will be equipped with strain gauges to allow any correction if necessary).


Three
THM (Thermal/Hygro/Mechanical)
testing
bodies
will be realized:

-

THM8:
a reference one

-

THM9:
reduced reinforcement

-

THM10:
increased reinforcement



Fig.
17

I
-
Shaped Restrained Shrinkage Testing Body



Fig.
18

THM8 Testing Body



Reinforcement


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During the first days of maturation, formworks will be thermally insulated to maximize
thermal and shrinkage phe
nomen
a
.


After a 4
-
week maturation, testing bodies will be placed on the testing bench and submitted
to a flexion test, like for prismatic testing bodies.


General instrumentation layout is similar to the one for prismatic testing bodies.



Fig.
19

THM8 Testing Body


Cross Section


TESTING BENCH AND AR
EA


As the size of
the
testing bodies and the associated testing forces were not available within
the
CEOS.FR

partners facilities, a specific testing bench was designed and cast for th
e
project.


With overall dimensions of 6.20

mx2.00

mx1.40

m, it has allowed to include some of the
sensors intended to be used for the testing bodies and to qualify the general procedures.


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Fig.
20

Testing Bench



Fig.
21

Overall View of the Testing Area


FIRST TESTS PERFORMA
NCE AND RESULTS


By mid
-
April 2010, the experimentation program is now active. RL1, RL2 and RG8 have
been cast and submitted to bending tests after a 28
-
day maturation. The first wall has

been

cast and will be submitted to alternative loading test within a few days.


The numerous sensors, including acoustic sensors, are delivering a large quantity of data, the
analysis of which is in progress. First analysis on RL1 data (tested in November

2009) shows
that:

-

cracks appear at the previously forecast stress level (close to SLS),

-

electrical gauges function in traction on the upper side, in compression on the bottom side,
as normally forecast,

-

optical fiber sensors function as well,

-

acoustic sen
sors have given signals which should allow a 3D analysis.




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Fig.
22

RL1: Strain gauges signals


SN4060
SN4059
SN4057
SN4051
SN4053
SN4067
SN4068
SN4069
SN4145
12/14, 12h
12/14, 14h
12/14, 16h
12/14, 18h
12/14, 20h
12/14, 22h
12/15, 0h
2009
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
mm
15ème palier - Allongement maximum
Resserrage

Fig.
23

RL2: Optical fiber sensors signals


All these data will be now submitted to a preliminary analysis.


An international benchmark, based on a part of these results, has been organized and
launched at the beginning of 2010.

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In the meantime, the scientific teams associated to t
he project are presently modeliz
ing the
first tests and matching calculation resul
ts with test observations.


CONCLUSION


The experimental part of CEOS.
FR

program should be over by the end of 2010
.

N
umerous
and interesting results
will be
available by the time of
fib

congress
.

T
hen
they
shall
be
released to the concrete international sc
ientific community.


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REFERENCES


1.

Benboudjema F., Meftah F., Torrenti J.
-
M., "Interaction between drying, shrinkage,
creep and cracking phenomena in concrete",
Engineering Structures
, 2005, 27,
p
p. 239

250.

2.

Benboudjema F., Torrenti J.
-
M., "Early age behavi
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containments",
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,

September 11
-
13, Quebec, Canada,
CD
-
ROM.

3.

Bisch P
., "Ouvertures de fissures en cas de séisme".
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de Février
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CQN 01067 GC


O. E. n°10
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Bisch P. & Viardin C. "Interprétation des essais «

SAFE

» effectués à Ispra".
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Séchaud et Metz

de Décembre 2000. Contrat EDF SEPTEN N°

CQN 0752030 GC.

5.

Bisch P., Coin A., "The CAMUS 2000 Research",
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6.

Buffo
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Lacarrière L., Sellier A., Escadeillas G., Turatsinze A., "Multiphasic finite
element modeling of concrete hydration",
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, Volume 37, Issue
2 , February 2007,
pp.

131
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138.

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Delaplace,A.,
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,
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De
laplace

A., Ibrahimbegovic A.,
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Desmorat R., Ragueneau

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"Numerical Modelling of
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Kotronis P., Raguenau F. and Mazars J., "A Simplified Mod
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Lacarriere L., Sellier A., Escadeillas G., Turatzinse A., "Modelling of hydration of
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based composed binders: phenomenological approach",
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Mazars J., Pijaudier
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Mivelaz P., "Etanchéité des structures en béton armé fuite au travers d’un élément
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2010

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Torrenti J.M., Benboudjema F., "Mechanical threshold of concrete at an early age",
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Eurocodes

:


1.

EN1992
-
1
-
1. Eurocode

2: Design of concrete structures
-

Part 1
-
1: General rules and
rules for buildings (pour le calcul de l’ouverture des fissures)

2.

EN1992
-
1
-
2 Eurocode 2: Design of concrete structures
-

Part 1
-
2: General rules
-

Structural fire design (pour les caractéristiq
ues thermiques du béton vis
-
à
-
vis du feu)

3.

EN1992
-
3 Eurocode 2
-

Design of concrete structures
-

Part 3: Liquid retaining and
containment structures (pour l’estimation des déformations gênées),

4.

1992
-
2 Eurocode 2
-

Design of concrete structures
-

Concrete
bridges
-

Design and
detailing rules (pour l’estimation des déformations de retrait et fluage).

5.

Supporting documents for section 7 of EN1992
-
1
-
1 by Hugo Corres Peiretti


Model codes

:


1.

Code modèle FIP
-
CEB 1978

2.

Code modèle FIP
-
CEB 1990

3.

Bulleti
n 235 CEB Ser
viceability Models,
April 1997

4.

BAEL 91 révisées 99

: "Règles techniques de conception et de calcul des ouvrages et
constructions en béton armé suivant la méthode des états lim
ites",
Bulletin officiel
,

Avril
1999.

5.

prEN

1992
-
1
-
1



EUROCODE 2: "Calcul des str
uctures en béton

; partie 1
-
1:

règles
générales et r
ègles pour les bâtiments",
CEN
,
Décembre 2003.

6.

CEB
-
F
IP MODEL CODE 1990,

Final draft
,
July 1991.