1

Ekkehard Fehling Prof. Dr.-Ing. Universität Kassel Kassel, Germany Kai Bunje Dipl.-Ing. Fehling - Jungmann GmbH Kassel, Germany Torsten Leutbecher Dipl.-Ing. Universität Kassel Kassel, Germany

Design relevant properties of hardened Ultra High Performance Concrete

Summary

Ultra High Performance Concrete (UHPC) offers a great spectrum of favourable material proper-ties. The structural behaviour of UHPC differs from normal- or high-strength concrete, particu-larly for a supplement of fibres. In most countries valid design rules for this new material do not yet exit. In Germany a work group of the German Commission on Reinforced Concrete (Deutsche Ausschuss für Stahlbeton DAfStb) has presented a state of the art report on UHPC [1]. Chapter 6 of this report describes the knowledge about the properties of hardened UHPC. This paper summarises some of the main aspects dealt with there. Keywords: material properties, shrinkage, creep, fatigue behaviour, bond, fire resistance

1 Introduction

The properties of hardened Ultra High Performance Concrete (UHPC) [2] are determined by the very dense structure of this material. The microstructure of UHPC differs significantly from nor-mal- and high-strength concrete. With respect to the mechanical behaviour, UHPC with fibres shows, depending on the type and quantity of fibres contained in the mix, ductile behaviour un-der compression as well as in tension. In contrast to this, UHPC without fibres behaves brittle, if no additional measure such as confinement is chosen. Since the pre-peak behaviour does not show significant differences, the elastic properties of UHPC with and without fibres can be de-scribed in common whereas the influence of fibres has to be described separately. As one consequence of the dense structure of UHPC, the porosity of UHPC is much lower than for normal- and even for high-strength concrete. Another consequence is the improvement of

2

the contact zones between the cement matrix and the aggregates as well as the fibre rein-forcement which allows a short length of fibres. This is illustrated by Figure 1 showing the good bond between the different components (a and b). Figure 1c shows a situation which can often be found in normal-strength concrete. The dense structure is also responsible for the very low permeability for gases and fluids and hence superior durability of this material. The low permeability has an adverse effect with respect to fire resistance. This, however, can be compensated as shown in chapter 8.

a) good bond between cement matrix and quartz sand particles (left) b) exceptional bond between matrix and steel fibre c) less satisfactory bond between matrix and quartz sand particle (right)

Figure 1: Contact zones in UHPC between matrix and aggregates or fibres

2 Behaviour under Compression

The typical compressive strength of UHPC is in the range of 150 to 220 MPa. Until about 70 to 80 % of the compressive strength, UHPC shows a linear elastic behaviour (Figure 2). According to experimental evidence as obtained until now, this holds true for UHPC regardless of the maximum aggregate size. The failure of UHPC without fibres is of explosive nature. No de-scending branch in the stress-strain-diagram does exist. This, however, can also be observed for HSC with fc > 90 MPa.

Stress-Strain-Diagram of UHPC without fibres

Concrete strain

Com

pres

sive

stre

ss

fc

~0,70 - 0,80 fc

εc1

1

E � 45 - 55 GPa

Figure 2: Stress-strain-diagram of UHPC without fibres

3

Due to the dense structure, the elastic modulus of UHPC is higher than for normal- and high- strength concrete when using identical aggregate types. Figure 3 shows typical results as ob-tained at Leipzig University [1].

Figure 3: Elastic modulus versus compressive strength

The Poisson ratio in general has been determined to be about 0.2 in the linear elastic range.The strain at peak stress for UHPC with fine aggregates amounts to approximately 4.4 ‰. For basal-tic aggregates or e. g. DENSIT-UHPC using bauxite, lower values have to be expected. For UHPC with fibres (UHPFRC), a pronounced descending branch can be developed by the effect of the fibres (Figure 4). The slope of the descending branch depends on

- fibre content, - fibre geometry (length, diameter), - fibre length in relation to maximum aggregate size, - fibre stiffness (in case of fibre cocktails) and - fibre orientation.

Although in general the influence of fibres on the compression strength is low. Due to 2.5 vol.-% of fibres, an increase of the compressive strength of about 15 % has been noted [3]. For UHPC, the geometry of test specimens seems to have less influence on the compressive strength. However, contradictory results from different sources exist with respect to this ques-tion. Heat treatment can speed up the development of the compressive strength. At 250 °C, a signifi-cant increase of the strength can be obtained, since the high temperature does not only accel-erate the chemical reaction but also leads to an improvement of the microstructure. According to [1 ,3], even at 90°C heat treatment lasting 48 hours will enable higher compressive strength values than for the case of curing 28 days in water.

Ultrahochfester Feinkornbeton:Ecm = 19000 (fc/10)1/3

R2 = 0,8878

25000

30000

35000

40000

45000

50000

55000

60000

1,7 1,9 2,1 2,3 2,5 2,7 2,9

Ecm

(N/m

m²)

HPC mit Sand-Kies-Zuschlag: Ecm=20500 (fc/10)1/3

49 19712269 93 156 244fc,zyl100*200

(fc/10)1/3

Ultra High Strength Concrete with fine aggregates Ecm = 19000 (fc/10)1/3 R2 = 0,8878

HPC with sand/gravel aggregates: Ecm = 20500 (fc/10)1/3

4

UHPC with fibres, 1,0 Vol.% - 2,5 Vol.%stess-strain diagram

Concrete strain

Com

pres

sive

stre

ssfc2

fc1

εc1 εc2

Range of possible descending depending on fibre amountand fibre orientation

E � 45 - 55 GPa

1

Figure 4: Typical stress-strain-diagrams of UHPFRC

3 Behaviour in Tension

The tensile strength can be determined experimentally using prismatic or cylindrical specimens. It may be advantageous to use probes sawn out of plates. In principle, it is possible to use specimens with or without notches. Direct tension tests on UHPC without fibres have delivered tensile strength values between 7 and 10 MPa. According to results obtained at the Universities of Kassel and Leipzig, there are only small differences between UHPC with fine or coarse aggregates. The failure is rather brit-tle, hence without a significant descending branch. Depending on the amount, type and orientation of fibres, the tensile strength of UHPFRC can be increased beyond the matrix strength. Values in the range between 7 and 15 MPa [3] have been recorded. Due to the effect of fibres, the behaviour becomes ductile. After onset of crack-ing, the material may be characterised by the stress-crack-opening-diagram. The typical behav-iour is depicted in Figure 5. It should be noted, that the slope of the descending branch can be very different, depending on the fibre orientation and the content and type of fibres.

Tensile strength - UHPC with fibres

Crack opening

Tens

ile s

tress

fc,t

2

fc,t

1

Range of possible descending depending on fibre amountand fibre orientation

good fibre orientationwithout fibres

Figure 5: Typical stress-crack-opening-curve for UHPC

5

In [1], a formula for the estimation of the splitting tensile strength in dependency on the com-pressive strength is given. Furthermore, the axial tensile strength may be derived from the bending tensile strength considering the size effect.

Figure 6: Standard bending test according to DAfStb-Guideline on Steel-Fibre Concrete [4]

In order to simplify the testing procedure for tensile properties, the DAfStb-Guideline on Steel-Fibre Concrete [4] defines a test setup which allows the determination of the tensile strength as well as a stress-crack-opening-relationship. This is possible, since in general only one crack opens significantly so that the bending deformation is concentrated there and the crack opening is almost proportional to the deflection. Furthermore, a method to transform the stress-crack-opening-relationship into a stress-strain-relationship is presented in these guidelines. Figure 7 shows the typical result as obtained from bending tests, see [1].

Bending tensile strength - UHPC with fibres

Crack opening

Ben

ding

tens

ile s

tress fc,t

2

fc,t

1

Range of possible descending depending on fibre amount and fibre orientation

good fibre direction

bad fibre direction

Figure 7: Typical results from bending tensile tests

The influence of the fibre orientation has been studied by Bernier und Behloul [1], see Figure 8. The influence of the fibre orientation is also picted in Table 1, where different types of speci-mens with vertical and horizontal pouring direction are compared.

6

Figure 8: Influence of the fibre orientation on the bending behaviour

Table 1: Influence of casting direction on tensile strength and fracture energy

Axial tension Bending tension Test specimens A

ge

Ø80

R10

R90

5050

100

5050

5020

050

300

35 80 35150

Prism 160 * 40 *40

Beam 700 * 150 * 150

Concrete M1Q B3Q M1Q M1Q B3Q Curring 90° 90° 90° WL 90° WL 90°

Pouring direction

hori-

zontal vertical horizontal

hori-zontal

vertical horizontal vertical horizontal

Fracture energy

GF,10%[N/m]

7d 28d 28d*

16757 14555 17014

9993 - -

- 12932

-

20100 18052 19820

15097 - -

- - -

20355 19892

-

14543 - -

- - -

- - -

Tensile strength

fct [N/mm²]

7d 28d 56d

14,2 13,3 17,7

7,9 - -

- 7,0 -

34,0 35,7 36,3

22,5 - -

11,1 13,3 16,2

22,1 22,2 22,1

17,6

- -

18,3 20,4 24,2

18,0 17,9 18,1

Tests at TU Delft show, that especially the flow direction during the casting process affects the fibre orientation highly and thus, the tensile strength and ductility properties. The fracture energy of UHPC without fibres has been investigated e. g. at Leipzig University. Results are shown in Table 2. Figure 9 shows the decrease of the characteristic length with the compressive strength for NSC, HSC and UHPC.

F

F

783

78

160

40

55

40

bending tensile strength [N/mm²]

deflection [mm]

matrix without fibres

7

Table 2: Fracture parameters of UHPC for different mix designs

Mortar 1 Mortar 2 Mortar 3

selfcompacting

fine-grained

concrete

compacted fine-

grained concrete

UHPC with

basalt grain

Cylinder compressive strength

[N/mm²] 40 81,2 106,6 149,1 196,3 145,0

Fracture energy GF [(N/m] 53,7 65,1 66,5 62,8 54,7 95,0

Tensile strength [N/mm²], fct=0,9fct,sp 3,2 6,1 8,0 9,4 11,9 8,3

Characteristic length lch [mm] 133,5 61,3 44,7 32,6 20,1 80,6

Softening function bilinear bilinear Linear linear linear bilinear

Limit crack width [µm] 79,2 65,6 15,1 13,2 9,8 127,2

Figure 9: Characteristic length lch versus compressive strength

0

50

100

150

200

250

300

350

400

450

10 20 30 40 50 60 70 80 90 100 110 120 130 140 145 149 196

Druckfestigkeit (N/mm²)

char

akte

ristis

che

Läng

e (m

m)

UH

FB m

it B

asal

tspl

itt

Fein

korn

beto

n

Fein

korn

beto

n

Compressive Strength [N/mm²]

Cha

ract

eris

tic L

engt

h l ch

[mm

]

UH

PC

with

bas

altic

spl

it

fine-

aggr

egat

ed c

oncr

ete

fine-

aggr

egat

ed c

oncr

ete

8

4 Time dependant Properties

4.1 Shrinkage

According to data from Kassel and Leipzig [1, 3], the total shrinkage of sealed UHPC with fine aggregates amounts to 0.7 mm/m under isothermal conditions in the first seven days after pour-ing. Until an age of 28 days, the total shrinkage increases to about 0.9 mm/m. The influence of steel fibres on the autogenous shrinkage is of minor importance. Figure 10a and b give exam-ples for the development of shrinkage versus time.

Figure 10: Development of shrinkage versus time

The development of drying shrinkage of UHPC is similar as of HPC. Due to the high density of the matrix structure, however, the amount of drying shrinkage is reduced in comparison to HPC. For heat treated UHPC, drying shrinkage can practically be neglected after the end of the heat treatment.

-600

-500

-400

-300

-200

-100

0

0 100 200 300 400 500 600 700

Zeit ab höchster Betontemperatur (Std.)

auto

gene

s S

chw

inde

n ab

Tm

ax (

µm/m

)

UHFB mit Basaltsplitt (CEM I 42,5R-HS)Feinkorn-UHFB (CEM I 42,5R)

Probekörper: 150*150*700 mm

UHPC with basaltic split (CEM I 42,5R-HS)

fine-aggregated UHPC (CEM I 42,5R) test specimes: 150*150*700 mm

time, starting at highest concrete temperature [h]

auto

geno

us s

hrin

kage

, st

artin

g at

Tm

ax [µ

m/m

]

b)

UHPC with basaltic split (CEM I 42,5R-HS) fine-aggregated UHPC (CEM I 42,5R)

test specimens: 150*150*700 mm

UHPC with basaltic split (CEM I 42,5R-HS)

fine-aggregated UHPC (CEM I 42,5R)

-1200

-1000

-800

-600

-400

-200

0

0 100 200 300 400 500 600 700

Zeit nach Betonage (Std.)

Läng

enän

deru

ng (

µm/m

)

UHFB mit Basaltsplitt (CEM I 42,5R-HS)Feinkorn-UHFB (CEM I 42,5R)

Zeitpunkt der Tmax

Probekörper: 150*150*700 mmtest specimes: 150*150*700 mm

time Tmax

time after concreting [h]

elon

gatio

n [µ

m/m

]

a)

UHPC with basaltic split (CEM I 42,5R-HS) fine-aggregated UHPC (CEM I 42,5R)

test specimens: 150*150*700 mm

time Tmax

9

4.2 Creep

Creep of UHPC is generally less than for concrete with lower strength. Table 3 shows the re-sults according to [5] and [1]. A strong dependency on the concrete age at the start of loading becomes evident.

Table 3: Results of investigations on creep of fine-aggregated-UHPC [6]

Age at Start of Loading [days] Amount of Creep (10-6/MPa) Creep-Number φ (-) 1 46,9 2,27

4 37,2 1,80

7 32,5 1,57

28 22,2 1,08

(Age at Start of Loading = 7 d, Loading Duration = 135 d)

Degree of Utilisation Amount of Creep (10-6/MPa) Creep-Number φ (-) 45%, sealed 37,2 1,613

45%, free 42,1 1,787

53%, sealed 34,0 1,378

5 Coefficient of thermal expansion

As for NSC, the coefficient of thermal expansion is age-related. For UHPC with fine aggregates, 12 µm/mK have been recorded in [7]. This value is in the same range as for NSC (about 11,0 µm/mK). For very young UHPC (less than 3 days), further research is necessary.

6 Fatigue Resistance

For fatigue loading under compression, tests performed at the University of Kassel for UHPFRC have shown a rather good-natured behaviour. S-N-curves for UHPC and NSC are compared in Figure 11. The (relative) stress range of UHPFRC for a large number of load reversals (> 2 mil-lion) is similar high as for NSC, while the absolute stress level is much higher than for NSC. Thus, it can be said, that in contrast to other high strength materials, the high strength of UHPC with fibres does not lead to disadvantages with regard to fatigue. Currently, fatigue tests in bending are conducted at Delft University of Technology. Since a pa-per on this topic is given to the conference, the subject is not to be discussed here.

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Figure 11: S-N-curves for UHPC in comparison to NSC

7 Bond of Reinforcement

Due to the high compressive strength and the high density, UHPC enables very high bond stresses. For smooth fibres (l = 13 mm, Ø = 0.15/0.2 mm), Behloul [1] reports a value of fb = 11.5 MPa for BPR (DUCTAL). For prestressing wires and strands, the maximum bond stress depends on the concrete cover (see Figure 12)

Figure 12: Bond strength for prestressing strands (∅ 12,5 mm) depending on contact length to diameter according to [Cheyrezy, Roux, Behloul, Ressicaud and Demonte (1998)]

max. bond stress [N/mm²] max. bond stress [N/mm²]

bond length L/d concrete cover c/d

max. bond stress [N/mm²] max. bond stress [N/mm²]

bond length L/d with c/d = 4.8 concrete cover c/d with L/d = 5.6

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09 1E+10 1E+11 1E+12

stress reversals [-]

non

- dim

ensi

onal

rang

e 2�

a/f c

[-]

UHPC �u = (0,06 bis 0,075) fc = const.

UHPC �u = (0,21bis0,25) fc = const.

NC �u = 0,05 fc = const. [5]

NC �u = 0,20 fc = const. [5]NC �u = 0,05 fc = const. [6]

11

For ribbed reinforcing bars, test results are available from Weiße in Leipzig and Greiner/Reineck (University of Stuttgart, Figure 13). Very high bond stresses in the range of 40 to 70 MPa have been reported. In tests on rebars with 10 mm diameter, Weiße observed splitting failure in the concrete cover for a cover less than 25 mm. Due to the high bond stresses, the bond length in

the standard RILEM pull-out specimen has to be reduced to 2 ∅ instead of 5 ∅ (see Figure 14).

Otherwise, no pull-out would be feasible before the yielding of steel. Weiße used 1.5 ∅ for his tests.

Figure 13: Bond-stress-slip-relationship of UHPC according to Reineck and Greiner [1]

Figure 14: Modified RILEM pull-out test

0

10

20

30

40

50

0,0 0,5 1,0 1,5 2,0 2,5 3,0

Verschiebung am lastfreien Ende [mm]

Ver

bund

span

nung

[N/m

m²]

Würfel 40 mm aus Ductal, 2 Vol-% Fasern,gerippter Stabstahl Ø = 4 mm, Verbundlänge 2Ø = 8 mm

disp

lace

men

t at t

he u

nloa

ded

end

[mm

]

cube 40 mm of Ductal, 2 vol.-% fibres, ribbed reinforcement bar ∅ = 4 mm, bond length 2 ∅ = 8 mm

displacement at the unloaded end [mm]

load direction

located in the centre of the specimen

12

8 Fire Resistance

Due to the extremely high density of UHPC, high water pressure can arise when UHPC is ex-hibited to fire. This can lead to deterioration of the concrete structure. The problem can be over-come by the use of fibres, e. g. polypropylene fibres. One effect of the fibres is that they create capillary pores due to melting and burning. Furthermore, around the fibres transition zones to the cement matrix are formed. By this, the existing transition zones between aggregates and matrix are interlinked so that the permeability increases and the steam pressure is reduced. Experiments have shown the effectivity of adding polypropylene fibres [8, 9, 10, 11]. Another problem is associated with the anomaly of quartzitic compounds with respect to the

volumetric expansion occurring at 573 °C due to the change of crystal phases (α-quartz to β-quartz). Good results could be obtained by replacing quartz by basalt [12].

9 References

[1] Sachstandsbericht Ultrahochfester Beton - Betontechnik und Bemessung - in preparation, Deutscher Ausschuss für Stahlbeton im DIN Deutsches Institut für Normung e. V., 2004.

[2] Bornemann, R.; Schmidt, M.; Fehling, E.; Middendorf, B.: Ultra-Hochleistungsbeton UHPC – Herstel-lung, Eigenschaften und Anwendungsmöglichkeiten. Beton- und Stahlbetonbau 96, No. 7, p. 458-467, 2001.

[3] Fehling, E.; Schmidt, M.; Teichmann, T.; Bunje, K.: Entwicklung, Dauerhaftigkeit und Berechnung Ultra-Hochfester Beton (UHPC), Forschungsbericht an die DFG, Universität Kassel, 2003.

[4] DAfStb-Richtlinie Stahlfaserbeton (10. Entwurf), Ergänzungen zu DIN 1045-1, Teile 1 bis 4, March 2003, Deutscher Ausschuss für Stahlbeton im DIN Deutsches Institut für Normung e. V.

[5] AFGC Groupe de travail BFUP: wissenschaftliche und technische Berichte über Ultra High Perfor-mance Fibre-Reinforced Concretes, Zwischenbericht, Frankreich, January 2002.

[6] Ma, J; Schneider, H.: Creep of ultra-high performance concrete under compressive stresses, Leipzig Annual Civil Engineering Report, No. 8, 2003.

[7] Ma, J.; Dehn, F.; König G.: Autogenous shrinkage of self-compacting ultra-high perfomance con-crete, in Proceeding “International Conference on Advances in Concrete and Structures”, May 2004, Xuzhou, P.R. China.

[8] Diederichs, U.: Hochtemperatur- und Brandverhalten von hochfestem Stahlfaserbeton. In: Betonbau Forschung, Entwicklung und Anwendung, No. 142, p. 67-76, TU Braunschweig 1999.

[9] Schneider, U.; Horvath, J.; Dehn, F.: Abplatzverhalten von ultrahochfestem Beton (UHPC) unter Brandbeanspruchung, Leipzig Annual Civil Engineering Report (LACER), No. 6, Universität Leipzig 2001.

[10] Schneider, U.; Horvath, J.; Dehn, F.: Faserbewehrte ultrahochfeste Betone, in: König, G.; Holsche-macher, K.; Dehn, F. (Eds.): Faserbeton - Innovationen im Bauwesen, Bauwerk-Verlag, Berlin 2002.

[11] Dehn, F.; König, G.: Fire Resistance of different Fibre Reinforced High-Performance Concretes, In: Naaman, A; Reinhardt, H.-W.: Workshop HPFRCC 4, Ann Arbor (USA), June 2003.

[12] Bornemann, R.; Schmidt, M.; Vellmer, C.: Feuerwiderstand ultra-hochfester Betone, Beton 52, No. 9, p. 418-422, 2002.