OPTIMIZATION OF TENSILE STRAIN-HARDENING
CEMENTITIOUS COMPOSITES FOR TENSILE STRAIN CAPACITY
R. Shionaga (1), W . Pansuk (2), S. Grünevvald (3), J . A . den Uijl (3) and J . C . Walraven(3)
(1) I H I Corporation, Research Laboratoiy, Japan (2) Chulalongkorn University, Bangkok, Thailand
(3) D e l f t University o f Technology, D e l f t , the Netherlands
Abstract
The synergistic action o f a cementitious matrix and fibres can result i n strain hardening i n tension. The accompanied tensile strain capacity can be an important design parameter f o r strain-hardening cementitious composites i n order to prevent the localization i n a single crack and to assure that the stress is at least the tensile strength o f the unreinforced matrix. Such a behaviour is desired f o r example for bridge decks or joints where the strain-hardening material has to f o l l o w the deformations o f either the substructure or that o f the total structure.
A n experimental study was executed i n order to determine the tensile strain capacity o f cementitious composites. As a start o f the study, a reference mixture developed at the University o f Michigan was chosen. Adjusted mixtures i n three different compressive strength classes and straight fibres o f two different lengths were applied. The six mixtures were tested on characteristics i n the fresh and i n the hardened state. This paper discusses experimental results and observed failure pattems. The tensile strain capacity was the highest f o r the lowest compressive strength and w i t h longer P V A-fibres applied.
1. I N T R O D U C T I O N
H i g h Performance Fibre Reinforced Cement Composites (HPFRCC) according to the Japanese guideline [1] are composites comprising a cement-based matrix and short reinforcing fibres exhibiting multiple fine cracks and pseudo strain-hardening characteristics under uniaxial stress. The average ultimate tensile strain o f HPFRCC has to be at least 0.5%. Engineered Cementitious Composites (ECC) have been developed having a strain capacity up to 7% applying a plasma treatment process o f the fibres [ 2 ] . Oiling o f fibres also relatively increases the complementary energy and the ultimate strain capacity compared to fibres without surface treatment [ 3 ] . The mechanical interaction o f the matrix and fibres and their interface characteristics are taken into account by micromechanical calculations that l i m i t the
fi-ictional and chemical bond o f the fibres [ 4 ] . A common fibre dosage at w h i c h strain-hardening behaviour is obtained f o r Polyvinyl A l c o h o l (PVA)-fibres is 2 V o l . - % [ 5 ] .
2. E X P E R I M E N T A L SET--UP
Six mixtures (Table 1) were designed i n three different strength classes to study the ultimate strain capacity i n tension. T w o types o f P o l y v i n y l A l c o h o l (PVA)-fibres were applied 1) 1^6 m m , df=26 | i m (lf/df=231) and 2) lf=8 m m , df=40 \xm (lf/df=200). The volume o f fibres was fixed to 2.0 V o l . - % . The fibres had an oiling content o f 1.0 % .
Table 1: M i x t u r e composition o f six mixtures containing either 6 or 8 m m P V A-fibres
Mixture component M l M2 M3 GEM 1 52.5 R [kg/m'] 404 .526 627 Fly ash [kg/m^] 752 663 545 Quartz sand (<0.16) [kg/m^] 338 351 363 Fine sand (0.125-0.25) [kg/m'] 122 117 173 Water [kg/m^] 305 302 298 Superplasticizer Glenium 51 [kg/m^] 34.9 33.9 30.0 PYA fibre (6/0.026 mm) or PVA-fibre (8/0.040 mm) [kg/m'] 26 26 26
W/C-ratio [-] 0.83 0.63 0.52
W/B-ratio [6] [-] 0.74 0.56 0.46
Figure 1 shows the dimensions o f the applied test specimens. The compressive and flexural strengths were determined at an age o f 7 and 28 days; direct tensile testing was executed 28 days after casting.
40 40 160 m m
u
40 <-^< > 30 50 < ><-^ 50 30 20 20 60 40 50 40f-120 1 2 0 . LVDT's 50 M U 120 cfz
i-4—
30 30 180Figure 1: Test specimens and experimental set-up f o r a, left) flexural and compressive tests and b, right) direct tensile test
A set o f three specimens was produced to determine the mechanical characteristics at different ages. For each mixture 3.5 litres were mixed w i t h a small Hobart mixer (maximum capacity: 5.0 litres). The m i x i n g procedure consisted o f adding cement, f l y ash and sand, w h i c h were mixed f o r 10 s. Then, 90% o f the water was put into the mixer and mixed for additional 60 s, after w h i c h the superplasticizer was added w i t h the remaining water ( m i x i n g duration: 180 s). The fibres were added at the end o f the m i x i n g process and distributed by m i x i n g during 120 s. Tests i n the hardened state were executed on prisms (flexural and compressive strength tests (load-controlled tests); the compressive tests were executed on remaining parts o f specimens tested i n bending) and dog bone-shaped specimens (direct tensile test, deformation-controlled test). The rate o f displacement f o r the direct tensile tests was 0.28 m m / m i n .
3. R E S U L T S A N D D I S C U S S I O N
Figure 2 shows pictures o f f i v e o f the six mixtures after execution o f the mortar f l o w test (dimensions o f cone: upper/lower diameter: 70/100 m m , height: 60 m m ) . M i x M 2 was also tested without fibres; the flow spread o f this mixture was 428 m m . The flowability o f mixtures containing 6 mm-fibres was lower. The fibres have a pronounced effect on the flowability by counteracting the flow due to the formation o f a network o f fibres. The network is weak and concrete containing short synthetic fibres can be easily compacted. M i x M 3 ( w i t h 8 m m fibres) was less stable compared to the other mixtures and paste separated at the border o f the concrete (Figure 2e). The fibre distribution o f all mixtures appeared to be homogenous; no larger clusters o f fibres were observed. Moderate and short ( < 10 s) vibration after the f i l l i n g o f the moulds was applied to allow f o r better surface finishing o f the test specimens.
c) M l , 8 m m , FS: 247 m m d) M 2 , 8 m m , 258 m m e) M 3 , 8 m m , 226 m m
Figure 2 a-e: Pictures o f the mixtures ( w i t h exception o f M i x M 2 , 6 mm-fibres) after execution o f the fiow spread test
The highest compressive strength was obtained w i t h M i x M 2 (8 mm-fibres), w h i c h was 76.2 MPa. The flexural strength o f this mixture was 23.2 MPa, which is about the same as was obtained w i t h M i x M 2 containing 6 mm-fibres. Table 2 summarizes the results o f the tests on the flexural and compressive strengths f o r 7 and 28 days after casting as an average o f thi-ee measurements. M i x M 3 had a lower w/c-ratio compared to M i x M l and a higher compressive strength was expected. However, this change i n mixture composition had little effect on the compressive strength f o r mixtures containing 6 m m PVA-fibres (range o f compressive strengths w i t h 6 mm-fibres: 56.8-60.6 MPa). W i t h 8 mm-fibres, significantly higher compressive strengths were obtained w i t h M i x M 2 and M i x M 3 compared to M i x M l .
Table 2: Flexural and compressive strengths o f the six mixtures at different ages
Mixture Fibre type Flexural strength (MPa) Compressive strength (MPa) Mixture Fibre type
7 days 28 days 7 days 28 days
M l PVA6 - 16.4 - 60.6 M2 PVA6 23.0 23.3 53.1 56.8 M3 PVA6 - 16.7 - 57.3 M l PVA8 - 18.2 - 51.2 M2 PVA8 22.1 23.2 56.5 76.5 M3 PVA8 15.2 22.0 62.2 74.6
N o clear relation was obtained between the compressive and flexural strengths (Figure 3). W i t h 6 mm-fibres the flexural strength decreased at increasing compressive strength (results: 28 days). A decrease o f the flexural strength at increasing compressive strength was observed w i t h 8 mm-fibres at 7 days, whereas the flexural strength was higher at increasing compressive strength at 28 days.
A 7 days,; 6 m m • 7 days,^ 8 m m A28 days, 6 m m O 28 days. 8 m m | 40 50 60 70 80
Compressive strength [MPa]
Figure 3: Flexural and compressive strengths at different ages
Figure 4 summarises the results o f the direct tensile tests. The stress i n the specimens was obtained by dividing force w i t h the area o f the thinnest cross-section o f the dog-bone specimen. The results o f mixtures containing 6 mm-fibres are shown i n the left column. Only t w o valid test results were obtained for M i x M l (8 m m ) and M i x M 2 (6 m m ) . The measuring length was 120 m m , w h i c h was selected w i t h the motivation that cracks might appear i n a cross-section different f r o m the thinnest cross-section. I n the tests, cracking also was observed i n the area o f increasing specimen w i d t h (Fig. 5). A displacement o f 1.2 m m equals a 1 % i n
strain capacity f o r the case o f a constant cross-section. However, a localisation o f strain i n the thinner part o f the dog-bone can be assumed. Instead o f 'strain capacity' the parameter
'displacement capacity' was used f o r the presentation o f the results. A ductile behaviour was obtained f o r only seven out o f sixteen specimens as Figure 4 indicates.
T e s t 1 -- T e s t 2 _ - T e s t s „ T e s t 1 -- T e s t 2 _ - T e s t s „ T e s t 1 -- T e s t 2 _ - T e s t s „ s. "h 1 2 3 Displacement [mm] a) M i x M l , P V A 6 m m Test 1 Test 1 -- T e s t 2 '
f
1 \ • 1 2 3 Displacement [mm] c) M i x M 2 , P V A 6 m m 7 CJ 6 5 str e 4 (D 3 en s 2 H — Test 1 T e s t 2 — Test 1 T e s t 2 — Test 1 T e s t 2-^ -^ -^ -^
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7 | 4 ^ 3 1 0 1 2 3 Displacement [mm] b) M i x M l , P V A 8 m m — Test 1 -- T e s t 2 _ — Test 3wm
— Test 1 -- T e s t 2 _ — Test 3\
— Test 1 -- T e s t 2 _ — Test 3\\
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1 2 3 Displacement [mm] d) M i x M 2 , P V A 8 mm i — Test 2 _ — Test 3 _ — Test 2 _ — Test 3 _\\
V *< "Vj 1 2 3 Displacement [mm] e) M i x M 3 , P V A 6 m m ^5 | 4 — Test 1 -- T e s t 2 _ - T e s t 31
— Test 1 -- T e s t 2 _ - T e s t 3I
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— Test 1 -- T e s t 2 _ - T e s t 3f v\
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1 2 3 Displacement [mm] f ) M i x M 3 , P V A 8 m mTable 3 summarises the direct tensile tests w i t h respect to the stress at first cracking (tensile yield strength), the m a x i m u m tensile stress (highest stress obtained in the test), the ratio o f both stress values and the displacement capacity. The 'displacement capacity' is the displacement at w h i c h the stress deviated f r o m at least a horizontal plateau, w i t h a continuous decrease (softening) o f the tensile stress as a result. Multiple cracking was obtained w i t h M i x M l (all test specimens and both fibre types) and Mixture M 2 (first t w o specimens and w i t h 8 m m fibres). The observed cracks i n the specimens w i t h multiple cracking are shown by F i g . 5.
Table 3: Characteristics o f sixteen specimens tested i n uniaxial tension
Fibre M i x t u r e Tensile yield Tensile strength Ratio Displacement
type code strength (ftyO (f«) fri / ftvi capacity
PVA, If [MPal [MPal
r-l
[mm]6 mm M l 3.64/3.30/3.09 3.64/4.53/4.95 1.00/1.37/1.60 0.88/1.19/1.34 M2 4.39/3.63 5.92/6.11 1.35/1.68 0.37/0.34 M3 5.86/5.04/5.84 6.08/5.61/5.84 1.04/1.11/1.00 0.44/0.22/0.37 8 mm M l 4.01/4.15 4.81/4.64 1.20/1.12 1.78/2.12 M2 7.20/5.10/6.79 7.38/7.23/6.79 1.03/1.42/1.00 1.25/1.20/0.34 M3 6.81/5.55/5.05 6.83/6.13/6.28 1.00/1.10/1.24 0.29/0.34/0.42 Mixture M l , 6 mm: Specimen 1
V V
Specimen 2V Y
Specimen 3V V
Mixture M l , 8 mm: Specimen 1 Specimen 2 Specimen 3
V V
^ V V
Mixture M2, 8 mm: Specimen 1
V V
Specimen 2
J
Specimen 3 (location of cracldng)
V V
Figure 5: Crack pattern o f mixtures w i t h multiple cracking ( M l - 6 m m . M l - 8 m m and M 2 - 8 m m ) ; multiple cracking was not obtained f o r Specimen 3 o f M i x M 2 (8 m m )
The cracks o f other specimens were located more or less i n a single crack ( w i t h small cracks i n its close vicinity; similar crack pattern like M i x M 2 , 8 m m , Specimen 3; F i g . 5). Figure 6 relates the compressive strength w i t h the tensile yield strength (Figure 6a) and the highest obtained tensile strength (Figure 6b). The tensile strength increased at increasing compressive strength f o r mixtures containing 8 mm-fibres. The scatter o f the tensile strength was higher for mixtures containing 6 mm-fibres. Fibres can decrease the strength o f concrete due to the entrapment o f air and/or a non-homogenous distribution o f fibres.
H 0
c8
0 O&
0 o g
• 6 mm O 8 mm c0
8
o
® 6 imn O 8 mm 40 50 60 70 80 Compressive strength [MPa]90 40 50 60 70 80
Compressive strength [MPa] 90
Figure 6: Compressive strength versus a) tensile yield strength and b) maximum tensile strength (concrete age: 28 days)
Figure 7 compares the ratio o f maximum and first-cracking (tensile yield) strengths and the parameter 'displacement capacity' o f a l l specimens. Since the cross-section o f a dog-bone specimen is not constant, the stress was not constant along the measurement length; the measured parameter 'displacement' is selected f o r the presentation o f the results. The best resuhs concerning tensile strain capacity were obtained w i t h M i x M l ; the strain capacity improved w i t h 8 mm-fibres compared to 6 mm-fibres. The displacement capacity is not correlated w i t h the ratio o f m a x i m u m and tensile yield strengths. The highest ratio o f tensile strengths was obtained f o r M i x 2 (6 mm-fibres), w h i c h was 1.68. A value higher than 1 indicates that the concrete is able to maintain the stress after first cracking. The highest displacement capacity (Figure 7; 2.12 m m ) was obtained f o r M i x 1 (8 mm-fibres), w h i c h is also the mixture w i t h the lowest compressive strength o f all mixtures.
^ ® 6 i n m ; O 8 mm
f ;
O
0.8 1.0 1.2 1.4 1.6 1.8 2.0 Ratio tensile / tensile yield strength [MPa] Figure 7: Ratio o f m a x i m u m and tensile y i e l d strengths compared w i t h the displacement capacity f o r all mixtures
e
2.0 1.5 1.0 0.5W i t h 8 mm-fibres a higher displacement capacity was obtained for lower compressive and flexural strengths. This was not the case w i t h 6 mm-fibres; the displacement capacity was i n a wide range at a compressive strength o f about 60 MPa. W i t h a high flexural strength the strain capacity was relatively l o w (i.e. M i x M 2 , 6 mm-fibres).
2? 3.0 2.5 2.0 1.5 1.0 0.5 0.0 © 6 mm O O 8 mm O
8
0
0
3
\ 93
3.0 2.5 2.0 1.5 1.0 0.5 0.0 • 6 mm O 8 mm O i O8
0
o
0
Qo
40 50 60 70 80 90Compressive strength [MPa]
Figure 8: Displacement capacity relative to a) the flexural and b) the compressive strengths 10 15 20
Flexural strength [MPa] 25
4. C O N C L U S I O N S
A n experimental study was executed on the effect o f the mixture composition and fibre type on the strain-hardening capacity o f cementitious composites. Based on the results the f o l l o w i n g conclusions can be drawn:
- The PVA-fibres (fibre dosage: 2 V o l . - % ) significantly decreased the flowability o f the fine mortar. Mechanical characteristics o f mixtures w i t h 6 mm-fibres showed a l o w sensitivity to a change i n w/c-ratio, w h i c h might be the result o f the fibre network on the homogeneity o f the mortar.
- A higher strain capacity was obtained w i t h 8 mm-fibres; the selection o f this fibre type does not guarantee strain-hardening behaviour i n tension. The strain capacity increased at decreasing compressive strength f o r 8 mm-fibres.
R E F E R E N C E S
[1] Japan Society of Civil Engineers, Recommendations for Design and Construction of High Performance Fiber Reinforced Cement Composites with Muhiple Fine Cracks, (2008).
[2] L i , V.C., Wu, H.C., and Chan, Y.W., Effect of plasma treatment of polyethylene fibres on interface and cementitious composite properties, J. of Amer. Ceramics Soc, 79(3) (1996) 700¬ 704.
[3] Yang, E.-H., Garcez, E.O. and L i , V.C., Pigmentable Engineered Cementitious Composites, 2"^" Int. RILEM conference on Strain Hardening Cementitious Composites (SHCC2-Rio), Eds.: Toledo Filho et al., Rio De Janeiro, (2011), 107-112.
[4] L i , V.C., Engineered Cementitious Composites - Tailored composites tiirough micromechanical modeling, in: Fiber Reinforced Concrete: Present and the Futore, Eds.: N . Banthia et al., CSCE, Montreal, (1998) 64-97.
[5] Wittmann, F.H., Wang, P., Zhang, P., Zhao, T.-J. and Beltzung, F., Capillary absorption and chloride penetration into neat and water repellent SHCC under imposed strain, 2""^ Int. RILEM conference on Strain Hardening Cementitious Composites (SHCC2-Rio), Eds.: Toledo Filho et al., Rio De Janeiro, (2011), 165-172.
