Introduction

Italy is the European country with the highest number of roadway tunnels with length over 500 meters: 526

tunnels are managed by the principal Italian motorway management company, with a total length of 292

kilometers which represent 64% of the European roadway tunnels. As well Switzerland is an alpine state with

a high density of tunnels with respect to its surface. Some of the most serious accidents recently reported in

European news papers occurred in roadway tunnels like Mont Blanc Tunnel (March 1999) or Gotthard

tunnel (October 2001) in which serious human losses were counted. These accidents, which are

characterized by a low occurrence probability, had shown as tunnel conditions can involve very severe

damage level on structure, plant and people. Moreover in urban areas resting on soft soil such phenomena

can even arise the partial collapse of the lining giving rise to significant settlement, real dangerous for

building at ground level.

Actually the design of tunnels in soft soil is based on semi-empirical formulation that lead to approximate

solutions of the soil-structure interaction problems in both the transverse and the longitudinal plane of the

tunnel and rarely take into account the stress induced by dig. The stresses in the tunnel ring can significantly

vary along longitudinal axes due to the variation of geotechnical and hydraulic parameters, to the loading

area and, above all, to the localized damage caused by fire and blast. Moreover design of tunnel built with a

Tunnel Boring Machine (TBM) has to take into account the compressive actions caused by the advancing of

the TBM, in fact the use of TBM technique in tunnel construction leads to very high concentrated loads on

the tunnel lining when the TBM hydraulic jacks push over the concrete structure in order to advance the

excavation front.

It is worth noting that fire and blast are exceptional loads never taken into account in lining design. As a

matter of fact we need to introduce such kind of exceptional load in a design process in order to increase the

safety level of segment tunnel lining and prevent catastrophic structural collapse.

In the beginning of the 2009 a project named ACCIDENT is started, its main aim is design of prefabricated

tunnel segments, which will be used in a Tunnel Boring Machine technology, by taking into account

exceptional loads like fire and blast. ACCIDENT is the acronym of Advanced Cementitious Composites In

DEsign and coNstruction of safe Tunnel, it is a special cooperation between Italy and Switzerland funded

by INTERREG and supported by the European Commission and the Swiss Confederation. The two project

leaders are the Politecnico di Milano – Polo Territoriale di Lecco (Lecco-Italy) and the University of Applied

Sciences of Southern Switzerland (Lugano – Switzerland); nine industrial and institutional partners are also

involved in the research project. The project is addressed to develop new classes of fibre reinforced

structural materials which will be used by industrial partners to manufacture new products for tunnel

segments able to reduce the mortality rate in accidents where fire and blast take place, by causing

permanent damage or even preventing the first aid of fire brigades due to partial collapses. In particular, the

project is planned to investigate the problem at three different levels: the material level, the meso structural

level and the macro structural level.

The Material level

New types of cementitious composites reinforced by fibre randomly distributed and/or textile are developed

and characterized both in a static and dynamic field considering fire condition, blast condition and their

interaction.

The behaviour of concrete at high temperatures is a very important issue in designing tunnel linings. A

comprehensive experimental investigation at room and high temperature was carried out on HPFRC (High

Performance Fibre Reinforced Concrete), SFRC (Steel Fibre Reinforced Concrete) and TRC (Textile

reinforced concrete); furthermore different matrix material (i.e. mortar), fibre types (i.e. glass, polypropylene,

textile) and contents has also been investigated. The use of such materials was planned for precast tunnel

linings characterized by a multi-layer structure.

With reference to dynamic characterization an experimental investigation is in progress by using a Modified

Hopkinson Bar (MHB) available at the DynaMat Laboratory of the University of Applied Sciences of Southern

Switzerland of Lugano and a closed loop electromechenical press for statical tests available at the

Politecnico di Milano – Polo Territoriale di Lecco. From these experimental campaigns a proper constitutive

model could be defined by taking into account thermal damage and strain rate effects.

The Meso-structural level

Tests on slabs, interacting with soil, thermally damaged and subjected to a plane shock wave are in

progress.

A research group in the Department of Structural Engineering at the Politecnico di Milano designed a new

type of shock tube which will be used to carry out tests on slabs. A double diaphragm shock tube is

considered; it is composed by stainless steel tube with two chambers separated by means of buffer section

limited by two diaphragms.

Helium with different pressures is stored in the chambers: when the pressure difference reaches fixed

threshold, diaphragms fail and a shock wave propagates from high pressure chamber to the lower pressure

one, striking a slab specimen located at the end of the tube. Two main differences characterize the new

shock tube with respect to the conventional one described above. The first modification consists in a section

placed behind the specimen tested, which is filled by soft soil in order to investigate soil-structure interaction

induced by impulsive waves. Finally, in order to study a coupled problem as impulsive waves – high

temperature a furnace is used to create a temperature gradient inside the specimen such as in fire condition.

The Macro-structural level

At this level two different tests, fire and blasting test, has been carried out on cylindrical plain concrete tube

(diameter = 1m; thickness = 0.08m; length = 25m), embedded in a soil at a depth of 2.30 meters, already

existing at the training campus of the Lombardia Fire-brigade in Bovisio Masciago (Milan).

Fire tests will consist in a reduced fire load, thus avoiding the collapse of the structure’s section; in order to

validate the reliability of the numerical approaches used in fire design of tunnels. The explosion tests were

performed with different quantities of one kind of explosive in order to investigate the behaviour of an

embedded concrete tube and its interaction with the soil; this research step allowed us to understand the

behaviour of the soil-structure interaction system under internal explosions of different intensity.

This volume collects papers published before the Protect Worshop in order to allow people who attended the

Conference to have a more comprehensive idea of the research activities carried out up to now in the

framework of the Project Accident.

The papers are divided in three section according to the same project levels discussed before. Also some

paper related to this Project can be found in the Protect 2011 Proceedings and are listed below:

• A. Caverzan, E. Cadoni and M. di Prisco, “Dynamic tensile behaviour of Self Compacting Steel Fibre

Reinforced Concrete”.

• I. Colombo, M. Colombo, A. Magri, G. Zani and M. di Prisco, “Textile Reinforced Mortar at High

Temperatures”.

• M. Colombo and P. Martinelli, “SDOF models for RC and FRC circular plates under blast loads”.

• P. Bonalumi, M. Colombo and M. di Prisco, “Internal Explosions in Embedded Concrete Pipes”.

• P. Bonalumi, M. Colombo, C. Comina, M. di Prisco, S. Foti, G. Galli, “Characterization of blast

effects on surrounding soil: internal detonations in underground pipes”.

The chairmen of project Accident

Ezio Cadoni – SUPSI

Marco di Prisco – Politecnico di Milano

Researchers who give, in different way, their contribution to Accident Project:

• Bonalumi Pamela – Research Assistant – Politecnico di Milano

• Bamonte Patrick – Assitant Professor – Politecnico di Milano

• Caverzan Alessio – Research Assistant – Politecnico di Milano

• Colombo Isabella – Research Assistant – Politecnico di Milano

• Colombo Matteo – Assistant Professor – Politecnico di Milano

• Dotta Matteo – Research Assistant – SUPSI

• Ferrara Liberato – Assistant Professor – Politecnico di Milano

• Forni Daniele – Research Assistant – SUPSI

• Galli Andrea – Assistant Professor – Politecnico di Milano

• Magri Anna – Ph.D Student – Politecnico di Milano

• Martinelli Paolo – Research Assistant – Politecnico di Milano

• Zambelli Clara – Research Assistant – Politecnico di Milano

• Zani Giulio – Ph.D. Student – Politecnico di Milano

Partner of the Project:

• Dynalab di Carlo Alberini – Impact Testing Technology

• Gavazzi Tessuti Tecnici S.p.a.

• Geniobeton SA

• Lombardi SA Ingegneri Consulenti

• Mako-Shark S.r.l.

• Provincia di Lecco

• TGM Prefabbricati SA

• Studio Tecnico Associato di Ing. Giorgetti e Ing. Riganti

List of Publications

Material Level

• Cadoni E., Caverzan A. and di Prisco M. (2009) “Dynamic Characterization of Advanced

Cementitious Composites in Design and Construction of Safe Tunnel”, “Proceedings of 2nd

workshop PROTECT 2009, Hayama, Japan.

• Caverzan A., Cadoni E. and di Prisco M., (2009) “Behaviour of advance cementitious composites

under dynamic loading and fire”, “Proceedings of the 1st International workshop on structures

response to impact and blast (IWSRIB’09)”,Haifa, Israel.

• di Prisco, M., Lamperti, M.G.L., Lapolla, S. (2010) “Double-Edge Wedge Splitting Test: Preliminary

Results”, Proceedings of FraMCoS-7, 7th International Conference on Fracture Mechanics of

Concrete and Concrete Structures, Jeju, Korea May 23-28.

• Ferrara, L., di Prisco, M., Lamperti, M.G.L. (2010) “Identification of the stress-crack opening behavior

of HPFRCC: the role of flow-induced fiber orientation”, Proceedings of FraMCoS-7, 7th International

Conference on Fracture Mechanics of Concrete and Concrete Structures, Jeju, Korea May 23-28.

• Cadoni E., Caverzan A. and di Prisco M., (2010) “On influence of high temperature on the dynamic

behaviour of high performance fibre reinforced cementitious composites”, “Proceedings of the 6th

International conference on concrete under severe conditions, environmental and loading

(CONSEC’10)”,Merida, Mexico.

• Cadoni E., Caverzan A., and di Prisco M., (2010) “Behaviour of High Performance Fibre Reinforced

Cementitious Composites under high dynamic loading and fire for safe tunnels” Urban Habitat

Constructions under Catastrophic Events, M. Mazzolani, F. (ed.), Taylor & Francis Group, 933-938

• Caverzan A., Cadoni E., di Prisco M. (2011) “Dynamic behavior of HPFRCC at high strain rate: the

fiber role” High Performance Fiber Reinforced Cement Composites (HPFRCC 6), Ann Arbor, USA

• Colombo, I., Colombo, M., Magri, A., Zani, G., di Prisco, M. (2011) “Tensile behaviour of Textile:

influence of multilayer reinforcement” High Performance Fiber Reinforced Cement Composites

(HPFRCC 6), Ann Arbor, USA

• Cadoni E., Bragov A. M., Caverzan A., di Prisco M., Konstantinov, (2011) “Mechanical Response of

HPFRCC in Tension and Compression at High Strain Rate and High Temperature” Engineering

Transactions, Polish Academy of Sciences, Institute of Fundamental Technological Research, 58, 3

- 4, pag. 95 – 107.

Meso-structural Level

• Colombo, M., di Prisco, M., Martinelli, P. (2011) “A New Shock Tube Facility for Tunnel Safety”

Experimental Mechanics, 51, 7, DOI 10.1007/s11340-010-9430-7.

Macro-structural Level

• Bonalumi, P., Colombo, M., di Prisco, M. (2010) “Numerical investigation of internal explosions in

steel pipe”, Urban Habitat Constructions under Catastrophic Events, M. Mazzolani, F. (ed.), Taylor &

Francis Group.

• Bonalumi, P., Colombo, M., di Prisco, M.,Zambelli C. (2010) “Repeatibility of small charge detonation

in pipes”, Proceedings of MABS 21 International Symposium on Military Aspects of Blast and Shock,

Jerusalem, Israel.

Material level

Dynamic Characterization of Advanced Cementitious Composites in

Design and Construction of Safe Tunnel

E. Cadoni1, A. Caverzan2 and M. di Prisco2

1 University of Applied Sciences of Southern Switzerland, Switzerland 2 Politecnico di Milano, Italy

Abstract Fibres in High Performance Cementitious Composites are often used to improve impact and

blast resistance due to their ability in energy absorption. In order to appreciate the real

effectiveness in uniaxial tension behaviour at increasing strain rates, an experimental

investigation is in progress, aimed to compare dynamic with static behaviour by using a

modified Hopkinson bar for dynamic loadings and a conventional closed loop electro-

mechanical press for static tests. Due to limitations imposed by dynamic loadings, small

notched cylindrical specimens are considered. The specimens were cored by prismatic

specimens previously subjected to third point bending tests. The material investigated was a

steel fibre reinforced mortar used for the production of thin prefabricated roof elements. The

main object of the research was to highlight the role of thermal damage in uniaxial tension at

low and high strain rate loadings. Straight low carbon steel micro-fibres were used. The fibre

content was 1.25 % and the mix design guaranteed a self compacting mixture. The results show

that, despite the relative low fibre amount, the material is characterized by a hardening

behaviour in quasi static tests at room conditions. Moreover, in low strain rate tests after high

temperature exposure up to 600°C, the thermal damage progressively reduces the toughness

and weakly increases the strength, while for high strain rate tests the peak strength is

significantly increased, but such increase is accompanied by an abrupt post-peak softening

branch.

Keywords: self compacting mortar, high strength, steel fibres, uniaxial tension behaviour,

fracture, thermal damage, residual behaviour, high strain rates.

______________________________________

Ezio Cadoni

University of Applied Sciences of Southern Switzerland – DynaMat Lab

Campus SUPSI - Trevano

6952 Canobbio

Switzerland

Email: ezio.cadoni@supsi.ch

Tel: +41-58-666-6377

1.0 Introduction

The mechanical behaviour of fibre-reinforced cementitious composites when subjected to extreme

temperatures, impact or blast has still many aspects open to investigation [1,2,3,4], with specific

reference to large and socially-sensitive structures, as tunnels, sheltering structures, high-rise

buildings, bridges, off-shore platforms, pipelines, gasification reactors and secondary containment

shells for nuclear power plants [5,6]. As a matter of fact, the scanty information provided so far by

such specialized equipments as the Hopkinson bar for very high strain rates (as in explosions)

shows significant increases in mechanical properties, but these increases still need to be related to

the main engineering parameters [7,8,9,10]. Explosions and fires in tunnels, potential hazards from

highly-energetic materials stored in tanks and reservoirs and – last but not least – terroristic attacks

are increasingly becoming safety issues. For such reasons, the mechanical response of concrete

structures exposed to high temperature and impact loading can only be predicted – and controlled

– by formulating proper materials models for cementitious composites [11,12], including strain-

rate effects and thermal damage. In this frame is inserted the project ACCIDENT (Advanced

Cementitious Composites In DEsign and coNstruction of safe Tunnel) funded by INTERREG, a

special cohesion program between Italy and Switzerland supported by the European Commission

and the Swiss Confederation. The two project leaders are the Politecnico di Milano- Polo

Regionale di Lecco (Lecco-Italy) and the University of Applied Sciences of Southern Switzerland

(Lugano-Switzerland); the other subjects are nine industrial and institutional partners (3 Swiss and

6 Italian). The project is addressed in particular to develop new tunnel segments, designed by

using advanced structural materials as HPFRCC [13,14,15]. The constitutive laws represent the

basic knowledge for the structure design, oriented to the manufacturing of new products for

tunnels, trying to increase safety in accidents where fire and blast cause permanent damage or

obstacle the first aid, due to partial collapse.

2.0 Materials: mix design and manufacturing

The mix design of the HPFRCC material is specified in Table 1. Steel fibres are high carbon

straight fibres, 13 mm long, with a 0.16 mm diameter; their content is equal to 100 kg/m³ [11].

Manufacturing process was composed by more phases. First of all a 30 mm thick slab 1.6 m x 0.60

m in plane was cast. The casting was carried out by applying a unidirectional flow in order to

guarantee a certain fibre orientation. Twelve prismatic samples, 40 mm wide and 600 mm long,

were sawed from the slab taking the larger side of beams parallel to the casting flow direction.

Three specimens were tested in bending at room temperature [16]. Nine beam specimens, with the

same geometry, were used to investigate the degradation of post-cracking residual strengths in

bending after exposure to high temperatures [16]. From the bent specimens, several small

cylinders, objects of the present work, were cored in the direction of tensile stresses to be tested in

uniaxial tension at different loading rates.

The thermal treatment of the samples was carried out in a furnace by performing thermal cycles up

to three different maximum temperatures: 200, 400 and 600 °C. A heating rate equal to 50°C/h

was imposed up to the maximum thresholds, and then two hours of stabilization were guaranteed

in order to assure a homogeneous temperature distribution within the sample volume. The

temperature was after reduced with a rate of 25°C/h down to 100°C and then a cooling process at

room temperature was carried out (see Fig. 1). For each cycle, three nominally identical samples

were introduced into the furnace.

Table 1: Mix composition

Dosage (kg/m3)

Cement type I 52.5 600

Slag 500

Water 200

Super plasticizer 33 (l/m3)

Sand 0-2 mm 983

Fibres (lf =13mm; df = 0.16mm) 100

3.0 Static tests

Uniaxial tension tests were carried out on notched 20 mm high cylinders with a 20 mm diameter

(notch depth = 1.5 mm), glued to the press platens by means of an epoxy resin. Two aluminium

cylinders connected to the press by means a knuckle joint (Figure 2) were used as press platens. In

both cylinders a 5 mm deep cylindrical cavity with a 22 mm diameter was made in order to

increase the glued sample surface. The tests were carried out by means of a closed loop electro-

mechanical press INSTRON 5867 with the maximum load capacity equal to 30 kN. Stroke was

considered as feedback parameter during the tests. The displacement rate imposed during the tests

was equal to 5.0x10-5 mm/s up to 1.5 mm and after progressively increased to 10-3mm/s. For each

temperature (room condition, 200, 400 and 600°C) two samples were tested.

The results of tensile tests are shown in Figures 4a, 4b, 5a and 5b respectively for temperature 20,

200, 400 and 600 °C in terms of nominal stress ( N) versus crack opening displacement (COD),

while Figure 6 shows the comparison of average curves. Besides in Figures 4 and 5 a detail of the

peaks zone are plotted in order to highlight the first linear elastic and post peak behaviour close to

the peak. Peak strengths and corresponding crack opening displacements are listed in Table 2; the

values of peak strains reported in Table 2 were calculated as it follows:

peak= wpeak / L* (1)

where L* (=10 mm) represents the equivalent specimen length. According to this assumption the

cavity depth of the platens (5 mm for each side, where samples were glued, Figure 3) can be

approximately considered as a rigid zone.

It is interesting to observe how steel fibres are able to favour stable propagation at room

temperature up to a strain, computed according to Equation (1), of about 4%. The pre-peak

behaviour is well described by a parabola-rectangular model, where the plateau is very close to a

value of about 2%. By increasing the temperature, the peak strength does not significantly change

up to a maximum temperature of 400°C and this is very interesting. The elastic modulus decreases

with temperature growth in the pre-peak region, where a multi-crack propagation takes place. The

constant stress plateau becomes weakly softening at 200°C; even though up to 400°C the peak

strength remains very close to 2%. At 600°C the behaviour is significantly changed by a peak

strength reduction of about 40% a significant elastic modulus decrease, but the material can exhibit

a stress plateau up to about 2% of strain.

Figure 1. Thermal cycles.

0 20 40 60 80

T im e [h ]

0

200

400

600

T [ C ]600 C

400 C

200 C

Cooling at room conditions

Figure 2. Test set-up Figure 3. Press platens and glued specimen

a) b)

Figure 4. Nominal stress vs COD: (a) room temperature tests; (b) tests on samples exposed to a

thermal cycle up to 200 °C.

a) b)

Figure 5. Nominal stress vs COD: tests on samples exposed to a thermal cycle up to:

(a) 400 °C; (b) 600 °C.

averagesample 2sample 1

COD [mm]

σN

[MP

a]

543210

12.5

10

7.5

5

2.5

0

0.40.30.20.10

12.5

10

7.5

5

2.5

0

averagesample 2sample 1

COD [mm]

σN

[MP

a]

543210

12.5

10

7.5

5

2.5

0

0.40.30.20.10

12.5

10

7.5

5

2.5

0

averagesample 2sample 1

COD [mm]

σN

[MP

a]

543210

12.5

10

7.5

5

2.5

0

0.40.30.20.10

12.5

10

7.5

5

2.5

0

averagesample 2sample 1

COD [mm]

σN

[MP

a]

1.51.2510.750.50.250

12.5

10

7.5

5

2.5

0

0.40.30.20.10

12.5

10

7.5

5

2.5

0

L*

A A

Cavity

Section A-A

Press platen

Temp.

[°C]n°

wpeak

[mm]peak

[MPa]peak =

wpeak/L*

20 1 0.22 9.71 2.20

20 2 0.23 7.83 2.22E 02

200 1 0.32 9.50 3.18E

200 2 0.24 10.45 2.40E

400 1 0.17 11.88 1.67E

400 2 0.21 10.37 2.09E

600 1 0.14 5.95 1.45E

600 2 0.16 6.08 1.58E

Figure 6. Nominal stress vs. COD: average curves Table 2: Peak strengths and crack widths

The mechanical strengths of the specimens tested are listed in Table 3 and the meaning of each

parameter is explained as it follows: fIf is the first cracking strength, representing the matrix

flexural tensile strength, a reference for the onset of the multi-cracking behaviour. According to

Italian Guidelines [17] it is assumed as the maximum strength in the COD range 0 - 0.05 mm. feq1

represents the average nominal strength in COD range 3wI - 5wI and can be generally considered

as a serviceability strength reference. Finally, feq2 is the average nominal strength in COD range

0.8wu - 1.2wu, when the material behaviour is usually governed by pull-out mechanism. wI is the

crack opening displacement when first crack propagates. As it is suggested in the Italian

Recommendation (CNR-DT204) [17] wI is computed as the value corresponding to the highest

load in a COD range 0 - 0.05 mm and, in the cases investigated, it always remains equal to 0.05

mm. wu is the ultimate crack opening displacement (it is equal to 1.5 mm as suggested in [17]).

According to Italian Guidelines, at room temperature the material can be recognized as a

hardening material because the first cracking strength is close to 3 MPa, while the peak strength is

close to 10 MPa. This means that even if a notch depth ratio equal to 0.15 was used, the material

can distribute over the whole length the cracking process, even if the geometrical defect introduced

by the notch prevents a correct evaluation of the effective ductility measured in terms of strain of

the peak plateau. New tests without any notch are in progress.

Table 3: Static tensile tests: first crack and residual strengths

Temperature

[°C]

Sample

number

fIf

[MPa]

fIf,av[MPa]

(std)

feq1

[MPa]

feq1,av[MPa]

(std)

feq2

[MPa]

feq2,av[MPa]

(std)

20 1 3.16 2.81

(0.49)

9.25 8.38

(1.23)

5.95 5.37

(0.82) 20 2 2.46 7.51 4.78

200 1 1.68 2.32

(0.90)

6.72 8.23

(2.14)

3.13 3.26

(0.19) 200 2 2.96 9.74 3.39

400 1 4.06 3.37

(0.98)

9.82 9.78

(0.05)

0.99 1.49

(0.71) 400 2 2.67 9.74 1.99

600 1 2.11 2.03

(0.11)

3.99 4.15

(0.23)

--

600 2 1.95 4.32 0.04

600 ° C400 ° C200 ° C20 ° C

COD [mm]

σN

[MP

a]

543210

12.5

10

7.5

5

2.5

0

4.0 Set-up for high strain rate test

In the research program, different Modified Hopkinson Bars (MHB) have been used. These MHBs

are installed in the DynaMat laboratory of the University of Applied Sciences of Southern

Switzerland (SUPSI) of Lugano (Fig. 7). In particular the results described in this paper have been

obtained with a MHB for specimen of 20mm in diameter.

Figure 7. DynaMat laboratory: set-up for the high strain-rate tensile tests on specimen Ø=20mm

The MHB consists of two circular aluminum bars, called input and output bars, having a length of

3 m and 6m, respectively with a diameter of 20 mm to which the HPFRCC specimen is glued

using a bi-component epoxy resin. The input bar is connected with a high strength steel pretension

bar used as pulse generator. A test with the MHB is performed as follows:

a) first a hydraulic actuator, of maximum loading capacity of 600 kN, is pulling the pretension

high strength steel bar having a length of 6 m and a diameter of 12 mm; the pretension stored in

this bar is resisted by the blocking device (see Figure 7)

b) second operation is the rupture of the fragile bolt in the blocking device which gives rise to a

tensile mechanical pulse of 2.4 ms duration with linear loading rate during the rise time,

propagating along the input and output bars bringing to fracture the specimen. The pulse

propagates along the input bar with the velocity C0 of the elastic wave with its shape remaining

constant. When the incident pulse ( I) reaches the HPFRCC specimen, part of it ( R) is reflected by

the specimen whereas another part ( T) passes through the specimen propagating into the output

bar as shown in Figure 8. The relative amplitudes of the incident, reflected and transmitted pulses,

depend on the mechanical properties of the specimen. Strain-gauges glued on the input and output

0

0.0001

0.0002

0.0003

0.0004

0.0005

0 0.0005 0.001 0.0015 0.002

raw signals

inputoutput

str

ain

in

in

pu

t a

nd

ou

tpu

t b

ars

[m

/m]

time [s]

incid

en

t p

uls

e

reflecte

d p

uls

etr

ansm

itte

d p

uls

e

0

5

10

15

20

25

30

0

50

100

150

200

250

300

0 0.0002 0.0004 0.0006 0.0008 0.001

stress strain-rate

str

es

s [

MP

a]

stra

in ra

te [s

-1]

time [s]

Figure 8. Signals measured on the input and output bar

versus time curves of HPFRCC.

Figure 9. Stress and strain-rate vs. time

curves of HPFRCC (strain-rate=138 s-1).

bars of the device are used for the measurement of the elastic deformation (as a function of time)

created on both half-bars by the incident/reflected and transmitted pulses, respectively.

In Figure 8 the raw signals measured on the input and output bars are shown. It can be observed

the clean resolution of incident, reflected and transmitted pulses, the sharp rise time of the incident

pulse of the order of 30 s, and the nearby constant amplitude of the incident pulse. Moreover

during the fracture process the specimen is subjected to the load equilibrium, because the signals

( I+ R) and T are equal. In fact, naming F1 the load in the specimen-input bar interface and F2 the

load in the specimen-output interface they are defined as: F1(t) = A0 E0 ( I+ R) and F2(t) = A0 E0 T

By using the theory of the elastic wave propagation in bars, and the well substantiated assumption

of specimen equilibrium attainment, the stress and strain in the specimen as well as the history of

the crack opening displacement (COD) and the strain-rate can be calculated [7, 8, 9, 13, 14]:

tA

AEt

T

0

0 (2)

dttL

2t

t

0R

0C (3)

dtt2tCODt

0R0C (4)

tL

2t R

0C (5)

where: E0 is the elastic modulus of the bars; A0 their cross-sectional area; A is the specimen cross

section area; L is the specimen gauge length; C0 is the sound velocity of the bar material; t is time.

In the case of these HPFRCCs strain-rate can be calculated as for metallic or ductile materials,

where the plastic zone is well defined, the stain-rate is obtained as an average of the strain-rate

value during the abovementioned zone. Using the equation (4) and observing Figure 9 the strain

rate can be easily obtained: in the case it is 138 s-1.

5.0 Preliminary results at high strain rate

The high strain rate results obtained on HPFRCC exposed to 20, 200, 400 and 600°C are reported

in Table 4.

Table 4: High strain rate results on HPFRCC

Sample identification Thermally

exposed at

ft

[MPa]

ft,av

[MPa]

CODu

[mm]

CODu,av

[mm]

Gf

[J/m²]

Gf,av

[J/m²]

SFRCC_20_3_n2 20.46 0.038 34017

SFRCC_20_3_n3 20°C 22.28 24.96±2.3 0.032 0.031±

0.00731123

28861±

4911

SFRCC_20_3_n4 24.96 0.024 24442

SFRCC_200_3_n2 26.65 0.022 8148

SFRCC_200_3_n3 200°C 26.72 26.08±1.0 0.035 0.026±

0.0087700

10348±

4205

SFRCC_200_3_n7 24.88 0.022 15197

SFRCC_400_2_n1 27.86 0.057 2206

SFRCC_400_2_n2 400°C 27.22 25.28±3.9 0.019 0.033±

0.0201342

1308±

914

SFRCC_400_2_n4 20.77 0.025 378

SFRCC_600_3_n1 31.13 0.017 1002

SFRCC_600_3_n2 600°C 32.24 32.49±1.5 0.026 0.018±

0.008718

736±

257

SFRCC_600_3_n5 34.09 0.011 489

In Figure 10 are shown the stress versus time curves of the HPFRCC specimen tested with the

same high strain rate. It can be observed as the behavior change in function of the exposition to

high temperature. Analyzing the data Table 4 two facts can put in evidence: at this velocity the

peak increases with the exposition to high temperature as well as the post-peak strength decrease

till to disappear for higher temperature.

By observing the fracture surface of the specimen it is evident as the failure has changed. In Figure

10, in fact the comparison between the reference specimen cured at room temperature and that

exposed to 600°C is represented. In the first, the hole in the specimen that demonstrates the pullout

process of the fibers is while in the second one all the fibers are collapsed. This means that, during

the exposition, fibres became more brittle due to thermal damage.

6.0 Conclusions

The results experimentally obtained in uniaxial tension allow us to evidence the following

remarks:

- at low strain rates, the material is strain hardening at room temperature; high temperature

exposure up to 400°C does not decrease the peak strength, weakly decreases the Elastic

modulus and the homogeneous ductility before the localization in a single crack and the

post-peak fracture energy progressively decreases; at 600°C a peak strength

- at high strain rate, the dynamic factor related to peak strength is increased by high

temperature exposure, passing from about 2.5 up to 400°C to more than 5 at 600°C at a

strain rate close 140s-1. On the contrary, the toughness is progressively reduced.

- The use of notched specimens could have significantly affected the ductility of the tests

for both high and low strain rates. New tests on un-notched specimens made of the same

material are in progress.

0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006 0.0008 0.001

N [

MP

a]

time [s]

HPFRCC as received (20°C)

0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006 0.0008 0.001

N [

MP

a]

time [s]

HPFRCC exposed to 200°C

0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006 0.0008 0.001

N [

MP

a]

time [s]

HPFRCC exposed to 400°C

0

5

10

15

20

25

30

35

0 0.0002 0.0004 0.0006 0.0008 0.001

N [

MP

a]

time [s]

HPFRCC exposed to 600°C

Figure 10. Stress versus time curves of HPFRCC with different thermal damage.

7.0 Acknowledgements

The Authors are grateful to Ing. Matteo Dotta, Ing. Daniele Forni and Mr. Samuel Antonietti of the

SUPSI University for their precious collaboration in the execution of the laboratory tests. The

research was financially supported by the INTERREG IVA Program (Cross-Border Territorial

Cooperation Program for Italy and Switzerland): project ACCIDENT.

8.0 References

1. Ahmad, A., di Prisco, M., Meyer, C., Plizzari, G.A. and Shah, S. (Eds.), “Fiber Reinforced

Concrete: From theory to practice” Starrylink Editrice, 2004.

2. Bindiganavile, V. and Banthia, N.: “Impact and blast resistance of fiber reinforced concrete,”

Draft report circulated in ACI 544 Technical Commission, 2007.

3. Bindiganavile, V.; Banthia, N. and Aarup, B. 2002.” Impact response of ultra-high strength fiber

reinforced cement composite,” ACI Materials Journal, 99 (6): 543-548.

4. Pacios, A., Ouyang, C.and Shah, S. P. 1995. “Rate effect on interfacial response between fibers

and matrix,” Materials and Structures, RILEM, 28: 83-91.

5. McVay, M. K.: “Spall damage of concrete structures,” Technical Report SL-88-22,Waterways

Experiment, Corps of Eng., Vicksbury, MS, 1998.

6. di Prisco, M., Felicetti, R., Lamperti, M., Menotti, G., “On size effect in tension of SFRC thin

plates,” Fracture Mechanics of Concrete Structures, V.C. Li C.K.Y. Leung, K.J. Willam, S.L.

Billington, (Eds.), 2, B.L.Schmick and A.D.Pollington, USA, 2004, pp. 1075-1082.

7. Albertini, C. and Montagnani, M., “Testing techniques based on the split Hopkinson bar,” Int.

Conf. on “The mechanical properties at high strain-rates”, Oxford University, Institute of Physics

Conference Series, 21, 1974,pp. 22-32.

8. Albertini, C., Cadoni, E. and Labibes, K.. 1999. “Study of the mechanical properties of plain

concrete under dynamic loading,” Experimental Mechanics, 39 (2): 137-141.

9. Cadoni, E., Solomos, G. and Albertini, C. 2009. “Mechanical characterization of concrete in

tension and compression at high strain-rate using a modified Hopkinson bar”, Magazine of

Concrete Research, 61(3): 221-230.

10. Asprone, D., Cadoni, E. and Prota, A. 2009. “Experimental Analysis on Tensile Dynamic

Behavior of Existing Concrete under High Strain Rates”, ACI Structural Journal, 106(1): 106-113.

11. Cadoni, E., Caverzan, A. and di Prisco, M. “Dynamic behaviour of HPFR cementitious

composites” in Ultra High Performance Concrete, eds. Fehling, Schmidt and Stürwald, Kassel,

Heft 10, Structural Materials and Engineering Series, Kassel University Press, 2008, , pp. 743-750.

12. Ferrara L., Nilufer O. and di Prisco M. 2009. “High mechanical performance of fiber

reinforced cementitious composites: the role of “casting-flow induced” fiber orientation”

submitted to Materials and Structures.

13. Cadoni, E., Meda, A. and Plizzari, G.A., “Rate sensitivity of HPFRCC under tensile loading:

from quasi-static to high strain rates”, Fibre Reinforced Concrete Design and Applications, Editor

R. Gettu, Rilem Pro60, Chennai, India, 2008,pp.441-450.

14. Cadoni, E., Meda, A. and Plizzari, G.A. “Tensile behavior of FRC under high strain-rate”, in

print Materials and Structures.

15. Cadoni, E., Dotta, M., Meda, A. and Plizzari, G.A. “Analysis of Fiber Reinforced Concrete

under impact loading,” Proeedings of 1st workshop PROTECT 2007, Whistler, Canada, 2007.

16. Caverzan, A., Colombo, M. and di Prisco, M., “High Performance Steel Fibre Reinforced

Concrete: Residual Behaviour at high Temperature, “ Proceedings of 2nd workshop PROTECT

2009, Hayama, Japan, 2009, on press.

17. CNR-DT 204: Instruction for design, execution and control of fibre reinforced concrete

structures, Italian Standards, 2006.