- Email: [email protected]
A new composite for external reinforcement: Steel cord reinforced polymer
Available online at www.sciencedirect.com
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1929–1938
www.elsevier.com/locate/conbuildmat
A new composite for external reinforcement: Steel cord reinforced polymer W. Figeys a
a,*
, L. Schueremans a, D. Van Gemert a, K. Brosens
b,1
K.U. Leuven, Department of Civil Engineering, Building Materials and Building Technology Division, Kasteelpark Arenberg 40, B-3001 Heverlee, Belgium b Triconsult NV, Lindekensveld 5 bus 3.2, B-3560 Lummen, Belgium Received 8 September 2006; received in revised form 13 July 2007; accepted 15 July 2007 Available online 27 August 2007
Abstract Today, mostly steel plates and carbon fibre reinforced polymers (CFRP) are used as external reinforcement. This paper deals with a new material that can be used as external reinforcement: steel cord reinforced polymer (SCRP). It consists of thin high-strength steel fibres embedded in a polymer matrix. This innovative material combines the advantages of steel and carbon fibres. The material cost of SCRP is relatively low and the laminate preserves the flexibility. The applicability of the new material as external reinforcement is investigated. 2007 Elsevier Ltd. All rights reserved. Keywords: External reinforcement; Steel cord reinforced polymer (SCRP); Shear; Slip
1. Introduction Structural strengthening is an important aspect in the preservation and restoration of the monumental heritage. Often, a construction has to be strengthened to meet the current standards and modern comfort requirements, especially when a new function is assigned to the building. Sometimes a civil construction can be damaged during its service life by fire, explosion, earthquakes or other accidents. Other reasons for strengthening need are bad maintenance which accelerates degradation of the construction, e.g. unsatisfactory protection to corrosion, and mistakes during design or construction, e.g. incorrectly placed or missing reinforcement. Steel plates and CFRP are commonly used as epoxy bonded external reinforcement. Advantages of steel plates are the high stiffness and the low material cost compared to CFRP. On the other hand, CFRP is more flexible and *
1
Corresponding author. Tel.: +32 16 32 86 70; fax: +32 16 32 19 76. E-mail address: [email protected] (W. Figeys). Tel.: +32 13 52 36 61; fax: +32 13 52 36 64.
0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.07.006
easily applicable, unlike steel plates. CFRP is much stronger than standard steel for a comparable stiffness, and it can be applied at longer lengths. Drawbacks are the high material cost and its brittleness. Therefore, large safety factors are required. Disadvantages of steel plates are the high density which makes it difficult to apply and mostly supports are required during hardening of the adhesive. As steel plates cannot be cut easily, they are brought on site in the required length. The length of steel plates is practically limited to 6 m [1–3]. Both materials have their specific application areas, e.g. steel plates are often used for deformation and deflection problems because of the large section that can be added at reasonable cost. CFRP is advantageous for strengthening of concrete plates, because their strength can be better exploited in these cases. A growing interest concerns the development of steel cord reinforced polymer (SCRP) [4–6]. SCRP is a new material that can be used as external reinforcement. The new material is developed in a cooperation with the Belgian company Bekaert N.V. and the Laboratorium Reyntjens of KULeuven. It consists of thin high-strength steel fibres,
1930
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
bundled into cords. These cords are woven into unidirectional sheets with a synthetic textile. This textile can be used in a wet lay up system or in a pre-cured system. 2. Why SCRP? SCRP combines the advantages of steel and CFRP, [7– 9]. The Young’s modulus is high, as well as the strength which is comparable to CFRP . The material cost is low and the laminate remains quite flexible. Another advantage is the ductile behaviour of the steel composite. Therefore, lower material safety factors can be applied in design. The steel fibres have a protective layer of zinc or brass to protect the fibres from corrosion, Fig. 1. The new composite enables the same applications as steel plates and CFRP sheets and laminates, but also new application challenges can be tackled such as shear strengthening of complex shapes, wrapping of rectangular beams and columns without the need of rounding of corners and improved use of pre-stressing. The first prototype used in the experiments of this paper consists of 65 steel cords (width: 95 mm). Nineteen filaments are twisted in a cord. The central filament has a diameter of 0.25 mm, the 18 others have a diameter of 0.22 mm, Fig. 2. 3. Requirements to SCRP
Fig. 2. Section, prototype 1:65 · [1 · 0.25 mm + 18 · 0.22 mm].
Sufficient impregnation. Glue has to surround all fibres, to ensure a continuous force transfer to the concrete. Together with a good impregnation, SCRP needs to stick to the concrete without extra auxiliary means. Bond strength. Failure has to take place in the concrete. Concrete should be the weak link in the connection. Evaluation of the material properties, such as tensile strength and Young’s modulus. These material properties are design parameters. Bond behaviour. The use of SCRP as externally bonded reinforcement requires the design of the anchorage length and the transferable force. Non-linear fracture mechanics is applied to model and describe the shear– slip behaviour of the bonded connection. In this paper the model parameters, specific for the new material, are experimentally determined from direct shear tests.
4. Flexibility
To use SCRP as external reinforcement, different requirements have to be fulfilled. These requirements are not only related to the tensile strength and stiffness. Practical experience adds additional requirements for an optimal material design. The overall requirements are: Flexibility. SCRP seeks an easy application. Therefore, SCRP will be delivered on a small roll on site. If used as external shear reinforcement, even a higher foldability is necessary to wrap rectangular beams without any preparation and kept in place without special arrangements.
The flexible CFRP sheets are easy to apply, which is an important advantage in practice. When wrapping CFRP, the reinforcement sticks to the beam or column without extra auxiliary actions. Also SCRP seeks an easy application. The stiffness of SCRP can be checked by calculating the cross-sectional moment of inertia [10]. Without taking into account the torsion of the filaments, the moment of inertia equals 4.05 mm4, for the type of SCRP studied in this paper and presented in Fig. 2. The moment of inertia is calculated with Expression 2 I cord ¼
k X
ðI filamenti þ Ai y 2i Þ
ð1Þ
i¼1
with Ai is the section of filament i; yi is the distance of filament i to the neutral axis. The stiffness of one layer of CFRP sheet with the same width of 95 mm can be calculated, assuming that the layer is about 0.3 mm for an equivalent thickness of 0.167 mm. I CFRP ¼
Fig. 1. Steel cord reinforced 1:65 · [1 · 0.25 mm + 18 · 0.22 mm].
polymer,
topview,
prototype
0:33 0:167 ¼ 0:12 mm4 :95: 0:3 12
ð2Þ
This means that this studied type of SCRP is more than 40 times stiffer than CFRP. A further decrease in stiffness can be reached if fewer filaments belong to a cord, or if the filaments are used single. From these conclusions, new types of SCRP are developed with improved properties, types 2, 3 and 4, see
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1931
Fig. 3. Different prototypes of steel cord reinforced polymer.
Fig. 3. Laminates 2 and 3 are much more flexible because of the smaller diameter. The cord of type two exists of seven twists of four filaments with a diameter of only 0.12 mm. In type 3, a different geometry is chosen: three twists of three filaments. Each filament has a diameter of 0.18 mm. Type 4 is composed of single filaments. The filaments have a larger diameter of 0.30 mm. These new types are much more flexible, but with a loss of steel section, Table 1. Type 2 has the highest flexibility, however with a low section. Therefore, prototype 5 is developed from the second type of SCRP with an increased density, Fig. 4. The sheet with 65 cords has a width of 65 mm, compared to 100 mm for type 2. This prototype can easily be delivered on a roll with a diameter of only 150 mm, Fig. 4.
5. Sufficient impregnation and bond strength It is important that the external reinforcement can be bonded, without causing a weak link in the connection. As the reinforcement is often applied above the head of the worker, a viscous adhesive has to be used. When using this kind of adhesive, it has to be checked that all steel fibres are surrounded by the adhesive. If not, the glue is the weakest link in the connection and can cause premature failure. The impregnation of SCRP is tested by means of pull-off tests. Several pieces of SCRP are glued on concrete with Epicol U [11], a viscous adhesive. It is necessary that the external reinforcement is squeezed into the glue, which is
Table 1 Calculated moments of inertia Type
Cords
A (mm)
w (mm)
I (mm4)
CFRP Type 1
300 · 0.167 65 · [1 · 0.25 + 18 · 0.22 mm] 65 · [7 · 4 · 0.12 mm] 58 · 2 · [3 · 3 · 0.18 mm] 65 · 2 · [0.30 mm] 65 · [7 · 4 · 0.12 mm]
50.10 47.67
95 95
0.12 4.9
20.58 26.57 9.189 20.58
95 75 45 65
0.58 0.89 0.052 0.58
Type Type Type Type
2 3 4 5
not convenient in practice. Therefore, new adhesives have to be developed. Pull-off tests were carried out on the SCRP-laminates. For that purpose, a cylindrical element is glued on the SCRP. After hardening of the adhesive, a cylindrical saw cut is made to define the failure area. Afterwards, the cylindrical element is pulled off. In Fig. 5, test samples are presented after the pull-off test. The impregnation of the cords fullfills the requirements. All test samples failed in the concrete, none in the adhesive nor in the connection between cylinder and SCRP or between SCRP and concrete. The results are obtained in laboratory circumstances and with a careful application to ensure an optimal impregnation. Therefore, it seems that enhanced impregnation is necessary for on site practice. This is possible by developing new adhesives or with pre-impregnated laminates. The latter are factory produced and thus can have a very good impregnation not dependent of the on site conditions. 6. Evaluation of the material properties To determine the strength and Young’s modulus, different tensile tests are carried out on the material. The individual cords, the laminate itself and an impregnated laminate are tested. The tests are displacement controlled, Fig. 6. From the test results, the tensile strength and the Young’s modulus are calculated, Table 2. Test results are presented in Figs. 7 and 8. The tensile strength is independent of the kind of test. The lowest measured tensile strength is 2100 MPa which belongs to SCRP type 3. The three other types have a mean tensile strength higher than 2500 MPa. The calculated Young’s moduli are also presented in Fig. 8 and Table 2. A correlation can be found between the Young’s modulus and the kind of test set-up [12]. For all types of SCRP, a lower Young’s modulus is measured for non-impregnated SCRP. Possibly, the different cords are not perfectly parallel which causes a different stress level in the cords. At failure, the cords break one after the other. Cords within a pre-impregnated SCRP are subjected to a more uniformly distributed stress level. When reaching the tensile capacity, all cords break simultaneously. A ductile behaviour is observed, the high-strength
1932
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Fig. 4. Type 5 of SCRP.
Fig. 5. Section after a pull-off test.
Fig. 6. Tensile test on steel cord reinforced polymer on a single cord, laminate and impregnated laminate.
steel cords yield before failure. Because of this ductile behaviour, lower safety factors can be used in the design.
the KULeuven, model parameters are determined in a test program by means of direct shear tests [1]. 7.1. Pure shear model
7. Bond behaviour When using SCRP as external reinforcement, the required anchorage length and transferable anchorage force must be known. Therefore, non linear fracture mechanics is applied describe the shear–slip behaviour of the bonded connection. At the Reyntjens Laboratory of
Volkersen [13] derived the following differential equation which describes the shear stresses as a function of the slip of the external reinforcement: d2 sl ðxÞ 1 þ ml cl sl ðxÞ ¼ 0 dx2 E l hl
ð3Þ
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1933
Table 2 Measured strength and Young’s modulus Single cord
Laminate fm std (N/mm2)
Em std (kN/mm2)
fm std (N/mm2)
Em std (kN/mm2)
Type 1: 65 · [1 · 0.25 + 18 · 0.22 mm] 2970 178 183 17
2829 –
136 –
2621 74
148 7
Type 2: 65 · [7 · 4 · 0.12 mm] 2540 166 151 8
2609 –
137 –
2543 136
203 4
Type 3: 58 · 2 · [3 · 3 · 0.18 mm] 2010 166 291 12
2197 –
116 –
2155 89
167 16
Type 4: 65 · 2 · [0.30 mm] 2595 190 26 9
3215 –
172 –
2930 183
200 16
fm std (N/mm2)
Em std (kN/mm2)
Impregnated laminate
sl ðxÞ ¼ A sinhðxxÞ þ B coshðxxÞ
ð4Þ
slm 1þml cl . slm El hl
with x2 ¼ Three model parameters slm, slm and sl0 are introduced in the bilinear shear–slip relationship. The shear peak stress slm depends only on the strength properties of the concrete because failure will occur in the concrete. Applying a linear Mohr–Coulomb failure criterion, Eq. (5) can be derived from Mohr’s circle for pure shear and a tangential intrinsic curve [1]. slm ¼
fctm fcm fctm þ fcm
ð5Þ
with fcm = compressive strength of concrete, fctm = tensile strength of concrete. Fig. 7. Results of tensile test on SCRP type 3, measured stress in function of elongation for SCRP type 3.
in which ml ¼ EEcl and cl ¼ AAcl , sl(x) slip of the external reinforcement at x, sl(x) shear stresses in the adhesive at x, El Young’s modulus of the external reinforcement, Ec Young’s modulus of the concrete, hl thickness of the external reinforcement, Al section of the external reinforcement, Ac concrete section. The best results are given by the assumption of a bilinear shear–slip relationship, Fig. 9, as shown in [14]. Initially, the shear stress increases, until the maximum shear stress slm is reached. Afterwards, cracks appear in the concrete and consequently the shear stress decreases. When the ultimate slip sl0 is reached, no forces are transferred anymore and the connection fails. With this assumption, the solution of Eq. (3) becomes
However, additional parameters must be introduced to account for the relationship between laboratory circumstances and reality, Eq. (6). fctm fcm slm ¼ k b1 k b2 k c ð6Þ fctm þ fcm kc is the concrete influence factor, varying between 0.65 and 1 depending on the workmanship: kc is 1 in case of good workmanship, falling to 0.65 [1] in case of bad workmanship. kb1 describes the size effect for brittle materials and is given by Eq. (7) [14]. It indicates that mechanical strength increases when the test sample becomes smaller. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k k b1 ¼ ð7Þ 1 þ bl ðkþ1Þ href where bl is the width of the external reinforcement and href is an empirical factor. href is the depth of the concrete that is influenced by the shear stresses in the adhesive. Holzenka¨mpfer [14] proposed a value for href of 2.5–3 times the size of the biggest gravel stones. k is an empirical factor
1934
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Tensile strength
Young smodulus cord 250 notimpr. sheet impr. sheet
Young smodulus [kN/mm²]
3500
σ [N/mm²]
3000
2500
2000
1
2
3
4
type
200
150
100
1
2
3
4
type
Fig. 8. Results of tensile test for the different types of steel cord reinforced polymer.
the slip in the different layers: the concrete (height href), the adhesive and the external reinforcement: k X hi href hg hl slm ¼ ¼ slm 2:4 þ 2:5 þ 2ð1 þ ml Þ ð9Þ Gi Ec Eg El i¼1 with thickness of the external reinforcement, hl hg thickness of the adhesive layer, Ei Gi ¼ 2ð1þm shear modulus of the ith layer, iÞ m Poisson’s ratio (e.g. concrete: mc = 0.2; adhesive: mg = 0.25; steel cord: ml = 0.3), Ac concrete section.
Fig. 9. Bilinear s–sl relationship.
F external reinforcement
concrete
Fig. 10. Spreading out of forces.
which takes into consideration the multi-axial stress situation. This parameter has to be determined from experiments. kb2 introduces a second width effect: it accounts for the spreading out of the forces in the concrete, Fig. 10. sffiffiffiffiffiffiffiffiffiffiffiffiffi bl ð8Þ k b2 ¼ 2 bc in which bc is the width of the concrete. The second model parameter slm is the value of the slip at peak shear stress. The slip is determined as the sum of
As slm depends on slm, it also depends on the parameters kb1, kb2, kc. The last model parameter is the slip sl0 at which no more forces can be transferred. Its value is based on the total fracture energy Gf. This fracture energy is the energy per unit area, needed to bring a connection into complete failure. It is given by the area under the shear–slip curve, Fig. 9. Eq. (10) gives the fracture energy for a bilinear shear–slip relationship. Z 1 slm sl0 Gf ¼ ð10Þ sðsl Þdsl ¼ 2 0 If the fracture energy is known, the slip sl0 can easily be calculated. To determine the fracture energy, expression (11), is proposed by Holzenka¨mpfer: Gf ¼ k 2b1 k 2b2 k 2c C f fctm
ð11Þ
Gf depends on the tensile strength of the concrete, fctm. Again, the parameters kb1, kb2, kc are included. A new empirical factor is introduced. Cf is a parameter that fits the experiments to the model.
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1935
When the three model parameters – slm, slm and sl0 – are known, the differential equation of Volkersen can be solved. The shear stresses can be calculated at every location as a function of the external loading. Also the anchorage length and the corresponding maximum transferable load can be derived [1]. 7.2. Experiments Eight direct shear tests were executed at the Reyntjens Laboratory. On four of them, a single SCRP is tested. A pre-impregnated double layer SCRP (marked with I) is used in the four others. The test set-up is shown in Fig. 11. Two concrete prisms are bonded together with SCRP on two opposite sides which are grit blasted on beforehand. Between the two prisms, there is a small gap of 18 mm. Bonding length is 150 mm or 200 mm. On the two other sides, steel plates are glued. These plates transfer the load from the testing machine to the test samples. Commercially available adhesive Epicol U is used. This adhesive is a glue paste, with Young’s modulus of 7000 MPa. After hardening, a saw-cut is made alongside the SCRP, to prevent spreading out of forces. Therefore, the parameter kb2 equals 1. Since the test specimens are carefully prepared under laboratory conditions, kc is also 1. The test is displacement controlled. The change of the gap between the two concrete blocks is monitored by means of linear voltage differential transformers (LVDT) at two opposite sides of the block, Fig. 11. The gap increases at a constant average rate of 0.001 mm/s. By comparing the two individual signals, one can check whether or not the tensile force acts centrically. If this is not the case, the test results are not taken into consideration. One test was oblique and therefore left out of the results. In all test specimens, failure was due to failure of concrete. This means that the concrete is the weakest link in the connection. Test specimen I150a (the first test with pre-impregnated laminate and 150 mm anchorage length) after failure is shown in Fig. 12.
Fig. 12. Failure of test sample I150a.
Brosens [1] determined empirically the values of the parameters for CFRP: k = 1.47 mm and Cf = 0.40. With these parameters, a first estimation of the transferable load can be made, Table 3. The model parameters are calculated with the data from Table 3. In Fig. 13, the measured shear force and slip are compared with the model using the CFRP model parameters. In the test with single SCRP a small deviation is observed. At the end of the test, the measured curve diverges more from the model and failure took place later than predicted. The deviation on the force is about 12% and the test results are always underestimated. The SCRP seems to behave stronger and stiffer compared to elements strengthened with CFRP. The model parameters for CFRP predict the results relatively good, but are a rather conservative approach. The test results on pre-impregnated double layer SCRP are presented in Fig. 13. Smaller slip is measured, compared to the elements strengthened with single SCRP because of the higher amount of reinforcement. The measured curves diverge more from the CFRP-model (average deviation for the transferred forces is +20%) [15]. For the same cross-sectional properties SCRP seems to present higher stiffness. This might be due to slip between carbon fibres, twisting of fibres in the cords prevent such slip.
Fig. 11. Pure shear tests on concrete reinforced with SCRP, type 1.
1936
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Table 3 Calculated parameters and forces Type (–)
Single layer
Pre-impregnated double layer
2
fctm (N/mm ) fcm (N/mm2) Eg (N/mm2) hg (mm)
2.65 35.0 7000 0.98
Gf (N/mm) slm (N/mm2) slm (mm) sl0 (mm) Name (–) Bonding length (mm) FCFRP (kN) Ftest (kN) F test F CFRP (–)
150b 150 25.9 29.5 1.14
200b 200 31.0 35.1 1.13
I150a 150 27.7 33.4 1.21
lt = 150 mm, single laminate
45 40
40
35
35
30
30
25
25
20
20
15
15
10
10
5 0
5
model experiment 0
0.1
0.2
0.3
0.4
0
0.5
model experiment 0
0.1
sl (x = l) (mm)
0.3
0.4
0.5
l = 200 mm, pre–impreg. double layer
lt = 150 mm, pre–impreg. double layer
45 40
40
35
35 30
30
25
25
F (kN)
F (kN)
0.2
sl (x = l) (mm)
45
20
20
15
15
10
10
5 0
I200b 200 35.0 41.7 1.19
lt = 200 mm, single laminate
45
F (kN)
F (kN)
0.736 2.06 0.0067 0.7134 200a 200 31.0 34.4 1.10
2.65 35.0 7000 0.98 (concrete-SCRP) 0.65 (SCRP-SCRP) 0.736 2.06 (concrete-SCRP) 0.0084 0.7134 I150b I200a 150 200 27.7 35.0 35.1 39.5 1.27 1.13
5
model experiment 0
0.1
0.2
0.3
0.4
0.5
0
model experiment 0
0.1
sl (x = l) (mm)
0.2
0.3
sl (x = l) (mm)
Fig. 13. Measured force in function of slip.
0.4
0.5
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938 Table 4 Calculated forces with adapted model parameters Type (–)
Bonding length (mm)
FSCRP (kN)
Ftest (kN)
FSCRP/ Ftest (–)
Single layer Single layer
150 200
28.1 33.6
Pre-impregnated double layer Pre-impregnated double layer
150
33.2
200
41.9
29.5 34.4 35.1 33.4 35.1 39.5 41.7
1.05 1.02 1.04 1.01 1.06 0.94 1.00
A sensitivity analysis on the transferable forces is performed. Based on this analysis, it is concluded that a deviation on the material characteristics cannot explain a deviation of 12% and 20% from the model. Also the sensitivity of the model parameters href, k, Cf is examined. Changes on href only cause small changes in the transferable forces. Parameter Cf is a factor, needed to calculate the fracture energy. Its effect at small bonding lengths is almost negligible. The influence of Cf is only observed for long bonding length, especially longer than the theoretical anchorage length. The anchorage length can be calculated as the length required to take 97% of the maximum transferable load, [1]: with the CFRP parameters it becomes 285 mm for the single SCRP and 399 mm for pre-impregnated SCRP with two layers. Finally, the parameter k takes into consideration the multi-axial stress situation. When changing this parameter, it has an influence on all bonding lengths. Using the parameters experimentally determined for CFRP, all test results are systematically underestimated. Therefore, adapted model parameters are proposed. The tests were made on test specimens with a small bonding length. As concluded from the sensitivity analysis, only the parameter k will be varied. The authors propose to decrease this parameter k: for single SCRP, k is to be taken 1.2 mm. For pre-impregnated double SCRP, calculations with k = 1.0 mm give improved results. In Table 4, the transferable forces, calculated with the adapted model parameters for SCRP (FSCRP), are listed. To evaluate the parameter Cf, additional experiments are needed for confirmation, especially tests with a longer bonding length. These tests are running and will be investigated later. 8. Conclusion SCRP is a new material that combines the advantages of steel plates and CFRP. It combines a relatively low material cost with an ease of application. A first prototype has been developed in a collaboration between Bekaert and KULeuven. For this prototype, different requirements are investigated such as flexibility, impregnation, bond strength, material properties and bond behaviour. It has been demonstrated that the initial prototype needed a higher flexibility. Therefore, three new types of SCRP were
1937
developed and will be further investigated. The impregnation is sufficient if accurately applied. However, the development of a new adhesive is necessary to improve impregnation for on site application. Also, pre-impregnated laminates are an interesting option. Also the bond behaviour is studied. Non-linear fracture mechanics is applied to model and describe the shear–slip behaviour of the bonded connection. Experiments are presented which demonstrate that the new material behaves stronger and stiffer than elements strengthened with CFRP. Adapted material dependent parameters for SCRP are proposed. Therefore, the design equations set up for CFRP can be used with slightly altered material parameters. As a result, an unbiased non-conservative design is obtained also in the case of SCRP. Further research concerns the optimization of SCRP to improve the impregnation. Also the durability of SCRP and to further ease wrapping. Acknowledgements The authors express their thanks to: the Flemish Institute for Promotion of Scientific and Technological Research in the Industry (IWT–Vlaams Instituut voor de Bevordering van WetenschappelijkTechnologisch Onderzoek in de Industrie) for their financial support. NV Bekaert SA, represented by dr. ir. W. Dekeyser and ir. J. Gallens, for the collaboration and technical support. References [1] Brosens K. Anchorage of externally bonded steel plates and CFRP laminates for strengthening of concrete element. PhD thesis. Leuven: Katholieke Universiteit Leuven; May 2001. [2] Matthys S. Structural behaviour and design of concrete members strengthened with externally bonded FRP reinforcement. PhD thesis. Ghent: Universiteit Gent; September 2000. [3] Triantafillou T. General concepts and design aspect – materials and techniques, fib Bulletin 35. Retrofitting of concrete structures by externally bonded FRPs 2006:1–27. April. [4]. [5] Casadei P, Nanni A, et al. Performance of Double-T prestressed concrete beams strengthened with steel reinforced polymer. In: Proceedings of the seventh international symposium of fibre reinforced polymer reinforcement of concrete structures, vol. 1, (Kansas City), FRPRCS 7, Novembre 2005, p. 763–78. [6] Barton B, Wobbe E, Dharani L, Silva P, et al. Characterization of reinforced concrete beams strengthened by steel reinforced polymer and grout (SRP and SRG) composites. Mater Sci Eng A 2005:129–36. [7] Casadei P, Nanni A. Steel reinforced polymer: an innovative and promising material for strengthening the infrastructures. In: Proceedings of concrete engineering international, vol. 9, (United Kingdom), Concrete engineering international, Novembre 2005, p. 54–8. [8] Huang X, Birman V, Nanni A, Tunis G. Properties and potential for application of steel reinforced polymer and steel reinforced grout composites. Compo Eng 2005;Part B(36):73–82.
1938
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
[9] Prota A, Tan KY, et al. Performance of shallow RC beams with externally bonded steel reinforced polymer. Struct J 2006;103(2): 163–70. [10] Figeys W, Schueremans L, Brosens K, Gemert DV. Steel wire reinforced polymer for strengthening of construction. In: Proceedings of the third international convergence on composites in construction (Lyon), CCC 2005, July 2005, p. 831–9. [11] R.N.V. Product catalogus betonherstelling 2003. [12] Figeys W, Schueremans L, Brosens K, et al. Steel cord reinforced polymer (SCRP) for externally bonded reinforcement. In: Proceedings of the 11th international conference of structural faults and repair (Edinburgh), Structural faults and repair 2006, June 2006.
[13] Volkersen O. Die Nietkra¨ftverteilung in zugbeanspruchten Nietverbindungen mit konstanten Laschenquerschnitten. 15, Luftfahrtforschung 1938. p. 41–7. [14] Holzenka¨mpfer P. Ingenieurmodelle des Verbunds geklebter Bewehrung fu¨r Betonbauteile. PhD thesis, T.U. Braunschweig: Braunschweig; 1994. p. 5–86. [15] Figeys W, Schueremans L, Brosens K, Gemert DV. Strengthening of concrete structures using steel wire reinforced polymer. In: Proceedings of the seventh international symposium of fibre reinforced polymer reinforcement of concrete structures, vol. 1. Kansas City, FRPRCS 7, Novembre 2005. p. 743–62.
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1929–1938
www.elsevier.com/locate/conbuildmat
A new composite for external reinforcement: Steel cord reinforced polymer W. Figeys a
a,*
, L. Schueremans a, D. Van Gemert a, K. Brosens
b,1
K.U. Leuven, Department of Civil Engineering, Building Materials and Building Technology Division, Kasteelpark Arenberg 40, B-3001 Heverlee, Belgium b Triconsult NV, Lindekensveld 5 bus 3.2, B-3560 Lummen, Belgium Received 8 September 2006; received in revised form 13 July 2007; accepted 15 July 2007 Available online 27 August 2007
Abstract Today, mostly steel plates and carbon fibre reinforced polymers (CFRP) are used as external reinforcement. This paper deals with a new material that can be used as external reinforcement: steel cord reinforced polymer (SCRP). It consists of thin high-strength steel fibres embedded in a polymer matrix. This innovative material combines the advantages of steel and carbon fibres. The material cost of SCRP is relatively low and the laminate preserves the flexibility. The applicability of the new material as external reinforcement is investigated. 2007 Elsevier Ltd. All rights reserved. Keywords: External reinforcement; Steel cord reinforced polymer (SCRP); Shear; Slip
1. Introduction Structural strengthening is an important aspect in the preservation and restoration of the monumental heritage. Often, a construction has to be strengthened to meet the current standards and modern comfort requirements, especially when a new function is assigned to the building. Sometimes a civil construction can be damaged during its service life by fire, explosion, earthquakes or other accidents. Other reasons for strengthening need are bad maintenance which accelerates degradation of the construction, e.g. unsatisfactory protection to corrosion, and mistakes during design or construction, e.g. incorrectly placed or missing reinforcement. Steel plates and CFRP are commonly used as epoxy bonded external reinforcement. Advantages of steel plates are the high stiffness and the low material cost compared to CFRP. On the other hand, CFRP is more flexible and *
1
Corresponding author. Tel.: +32 16 32 86 70; fax: +32 16 32 19 76. E-mail address: [email protected] (W. Figeys). Tel.: +32 13 52 36 61; fax: +32 13 52 36 64.
0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.07.006
easily applicable, unlike steel plates. CFRP is much stronger than standard steel for a comparable stiffness, and it can be applied at longer lengths. Drawbacks are the high material cost and its brittleness. Therefore, large safety factors are required. Disadvantages of steel plates are the high density which makes it difficult to apply and mostly supports are required during hardening of the adhesive. As steel plates cannot be cut easily, they are brought on site in the required length. The length of steel plates is practically limited to 6 m [1–3]. Both materials have their specific application areas, e.g. steel plates are often used for deformation and deflection problems because of the large section that can be added at reasonable cost. CFRP is advantageous for strengthening of concrete plates, because their strength can be better exploited in these cases. A growing interest concerns the development of steel cord reinforced polymer (SCRP) [4–6]. SCRP is a new material that can be used as external reinforcement. The new material is developed in a cooperation with the Belgian company Bekaert N.V. and the Laboratorium Reyntjens of KULeuven. It consists of thin high-strength steel fibres,
1930
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
bundled into cords. These cords are woven into unidirectional sheets with a synthetic textile. This textile can be used in a wet lay up system or in a pre-cured system. 2. Why SCRP? SCRP combines the advantages of steel and CFRP, [7– 9]. The Young’s modulus is high, as well as the strength which is comparable to CFRP . The material cost is low and the laminate remains quite flexible. Another advantage is the ductile behaviour of the steel composite. Therefore, lower material safety factors can be applied in design. The steel fibres have a protective layer of zinc or brass to protect the fibres from corrosion, Fig. 1. The new composite enables the same applications as steel plates and CFRP sheets and laminates, but also new application challenges can be tackled such as shear strengthening of complex shapes, wrapping of rectangular beams and columns without the need of rounding of corners and improved use of pre-stressing. The first prototype used in the experiments of this paper consists of 65 steel cords (width: 95 mm). Nineteen filaments are twisted in a cord. The central filament has a diameter of 0.25 mm, the 18 others have a diameter of 0.22 mm, Fig. 2. 3. Requirements to SCRP
Fig. 2. Section, prototype 1:65 · [1 · 0.25 mm + 18 · 0.22 mm].
Sufficient impregnation. Glue has to surround all fibres, to ensure a continuous force transfer to the concrete. Together with a good impregnation, SCRP needs to stick to the concrete without extra auxiliary means. Bond strength. Failure has to take place in the concrete. Concrete should be the weak link in the connection. Evaluation of the material properties, such as tensile strength and Young’s modulus. These material properties are design parameters. Bond behaviour. The use of SCRP as externally bonded reinforcement requires the design of the anchorage length and the transferable force. Non-linear fracture mechanics is applied to model and describe the shear– slip behaviour of the bonded connection. In this paper the model parameters, specific for the new material, are experimentally determined from direct shear tests.
4. Flexibility
To use SCRP as external reinforcement, different requirements have to be fulfilled. These requirements are not only related to the tensile strength and stiffness. Practical experience adds additional requirements for an optimal material design. The overall requirements are: Flexibility. SCRP seeks an easy application. Therefore, SCRP will be delivered on a small roll on site. If used as external shear reinforcement, even a higher foldability is necessary to wrap rectangular beams without any preparation and kept in place without special arrangements.
The flexible CFRP sheets are easy to apply, which is an important advantage in practice. When wrapping CFRP, the reinforcement sticks to the beam or column without extra auxiliary actions. Also SCRP seeks an easy application. The stiffness of SCRP can be checked by calculating the cross-sectional moment of inertia [10]. Without taking into account the torsion of the filaments, the moment of inertia equals 4.05 mm4, for the type of SCRP studied in this paper and presented in Fig. 2. The moment of inertia is calculated with Expression 2 I cord ¼
k X
ðI filamenti þ Ai y 2i Þ
ð1Þ
i¼1
with Ai is the section of filament i; yi is the distance of filament i to the neutral axis. The stiffness of one layer of CFRP sheet with the same width of 95 mm can be calculated, assuming that the layer is about 0.3 mm for an equivalent thickness of 0.167 mm. I CFRP ¼
Fig. 1. Steel cord reinforced 1:65 · [1 · 0.25 mm + 18 · 0.22 mm].
polymer,
topview,
prototype
0:33 0:167 ¼ 0:12 mm4 :95: 0:3 12
ð2Þ
This means that this studied type of SCRP is more than 40 times stiffer than CFRP. A further decrease in stiffness can be reached if fewer filaments belong to a cord, or if the filaments are used single. From these conclusions, new types of SCRP are developed with improved properties, types 2, 3 and 4, see
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1931
Fig. 3. Different prototypes of steel cord reinforced polymer.
Fig. 3. Laminates 2 and 3 are much more flexible because of the smaller diameter. The cord of type two exists of seven twists of four filaments with a diameter of only 0.12 mm. In type 3, a different geometry is chosen: three twists of three filaments. Each filament has a diameter of 0.18 mm. Type 4 is composed of single filaments. The filaments have a larger diameter of 0.30 mm. These new types are much more flexible, but with a loss of steel section, Table 1. Type 2 has the highest flexibility, however with a low section. Therefore, prototype 5 is developed from the second type of SCRP with an increased density, Fig. 4. The sheet with 65 cords has a width of 65 mm, compared to 100 mm for type 2. This prototype can easily be delivered on a roll with a diameter of only 150 mm, Fig. 4.
5. Sufficient impregnation and bond strength It is important that the external reinforcement can be bonded, without causing a weak link in the connection. As the reinforcement is often applied above the head of the worker, a viscous adhesive has to be used. When using this kind of adhesive, it has to be checked that all steel fibres are surrounded by the adhesive. If not, the glue is the weakest link in the connection and can cause premature failure. The impregnation of SCRP is tested by means of pull-off tests. Several pieces of SCRP are glued on concrete with Epicol U [11], a viscous adhesive. It is necessary that the external reinforcement is squeezed into the glue, which is
Table 1 Calculated moments of inertia Type
Cords
A (mm)
w (mm)
I (mm4)
CFRP Type 1
300 · 0.167 65 · [1 · 0.25 + 18 · 0.22 mm] 65 · [7 · 4 · 0.12 mm] 58 · 2 · [3 · 3 · 0.18 mm] 65 · 2 · [0.30 mm] 65 · [7 · 4 · 0.12 mm]
50.10 47.67
95 95
0.12 4.9
20.58 26.57 9.189 20.58
95 75 45 65
0.58 0.89 0.052 0.58
Type Type Type Type
2 3 4 5
not convenient in practice. Therefore, new adhesives have to be developed. Pull-off tests were carried out on the SCRP-laminates. For that purpose, a cylindrical element is glued on the SCRP. After hardening of the adhesive, a cylindrical saw cut is made to define the failure area. Afterwards, the cylindrical element is pulled off. In Fig. 5, test samples are presented after the pull-off test. The impregnation of the cords fullfills the requirements. All test samples failed in the concrete, none in the adhesive nor in the connection between cylinder and SCRP or between SCRP and concrete. The results are obtained in laboratory circumstances and with a careful application to ensure an optimal impregnation. Therefore, it seems that enhanced impregnation is necessary for on site practice. This is possible by developing new adhesives or with pre-impregnated laminates. The latter are factory produced and thus can have a very good impregnation not dependent of the on site conditions. 6. Evaluation of the material properties To determine the strength and Young’s modulus, different tensile tests are carried out on the material. The individual cords, the laminate itself and an impregnated laminate are tested. The tests are displacement controlled, Fig. 6. From the test results, the tensile strength and the Young’s modulus are calculated, Table 2. Test results are presented in Figs. 7 and 8. The tensile strength is independent of the kind of test. The lowest measured tensile strength is 2100 MPa which belongs to SCRP type 3. The three other types have a mean tensile strength higher than 2500 MPa. The calculated Young’s moduli are also presented in Fig. 8 and Table 2. A correlation can be found between the Young’s modulus and the kind of test set-up [12]. For all types of SCRP, a lower Young’s modulus is measured for non-impregnated SCRP. Possibly, the different cords are not perfectly parallel which causes a different stress level in the cords. At failure, the cords break one after the other. Cords within a pre-impregnated SCRP are subjected to a more uniformly distributed stress level. When reaching the tensile capacity, all cords break simultaneously. A ductile behaviour is observed, the high-strength
1932
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Fig. 4. Type 5 of SCRP.
Fig. 5. Section after a pull-off test.
Fig. 6. Tensile test on steel cord reinforced polymer on a single cord, laminate and impregnated laminate.
steel cords yield before failure. Because of this ductile behaviour, lower safety factors can be used in the design.
the KULeuven, model parameters are determined in a test program by means of direct shear tests [1]. 7.1. Pure shear model
7. Bond behaviour When using SCRP as external reinforcement, the required anchorage length and transferable anchorage force must be known. Therefore, non linear fracture mechanics is applied describe the shear–slip behaviour of the bonded connection. At the Reyntjens Laboratory of
Volkersen [13] derived the following differential equation which describes the shear stresses as a function of the slip of the external reinforcement: d2 sl ðxÞ 1 þ ml cl sl ðxÞ ¼ 0 dx2 E l hl
ð3Þ
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1933
Table 2 Measured strength and Young’s modulus Single cord
Laminate fm std (N/mm2)
Em std (kN/mm2)
fm std (N/mm2)
Em std (kN/mm2)
Type 1: 65 · [1 · 0.25 + 18 · 0.22 mm] 2970 178 183 17
2829 –
136 –
2621 74
148 7
Type 2: 65 · [7 · 4 · 0.12 mm] 2540 166 151 8
2609 –
137 –
2543 136
203 4
Type 3: 58 · 2 · [3 · 3 · 0.18 mm] 2010 166 291 12
2197 –
116 –
2155 89
167 16
Type 4: 65 · 2 · [0.30 mm] 2595 190 26 9
3215 –
172 –
2930 183
200 16
fm std (N/mm2)
Em std (kN/mm2)
Impregnated laminate
sl ðxÞ ¼ A sinhðxxÞ þ B coshðxxÞ
ð4Þ
slm 1þml cl . slm El hl
with x2 ¼ Three model parameters slm, slm and sl0 are introduced in the bilinear shear–slip relationship. The shear peak stress slm depends only on the strength properties of the concrete because failure will occur in the concrete. Applying a linear Mohr–Coulomb failure criterion, Eq. (5) can be derived from Mohr’s circle for pure shear and a tangential intrinsic curve [1]. slm ¼
fctm fcm fctm þ fcm
ð5Þ
with fcm = compressive strength of concrete, fctm = tensile strength of concrete. Fig. 7. Results of tensile test on SCRP type 3, measured stress in function of elongation for SCRP type 3.
in which ml ¼ EEcl and cl ¼ AAcl , sl(x) slip of the external reinforcement at x, sl(x) shear stresses in the adhesive at x, El Young’s modulus of the external reinforcement, Ec Young’s modulus of the concrete, hl thickness of the external reinforcement, Al section of the external reinforcement, Ac concrete section. The best results are given by the assumption of a bilinear shear–slip relationship, Fig. 9, as shown in [14]. Initially, the shear stress increases, until the maximum shear stress slm is reached. Afterwards, cracks appear in the concrete and consequently the shear stress decreases. When the ultimate slip sl0 is reached, no forces are transferred anymore and the connection fails. With this assumption, the solution of Eq. (3) becomes
However, additional parameters must be introduced to account for the relationship between laboratory circumstances and reality, Eq. (6). fctm fcm slm ¼ k b1 k b2 k c ð6Þ fctm þ fcm kc is the concrete influence factor, varying between 0.65 and 1 depending on the workmanship: kc is 1 in case of good workmanship, falling to 0.65 [1] in case of bad workmanship. kb1 describes the size effect for brittle materials and is given by Eq. (7) [14]. It indicates that mechanical strength increases when the test sample becomes smaller. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k k b1 ¼ ð7Þ 1 þ bl ðkþ1Þ href where bl is the width of the external reinforcement and href is an empirical factor. href is the depth of the concrete that is influenced by the shear stresses in the adhesive. Holzenka¨mpfer [14] proposed a value for href of 2.5–3 times the size of the biggest gravel stones. k is an empirical factor
1934
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Tensile strength
Young smodulus cord 250 notimpr. sheet impr. sheet
Young smodulus [kN/mm²]
3500
σ [N/mm²]
3000
2500
2000
1
2
3
4
type
200
150
100
1
2
3
4
type
Fig. 8. Results of tensile test for the different types of steel cord reinforced polymer.
the slip in the different layers: the concrete (height href), the adhesive and the external reinforcement: k X hi href hg hl slm ¼ ¼ slm 2:4 þ 2:5 þ 2ð1 þ ml Þ ð9Þ Gi Ec Eg El i¼1 with thickness of the external reinforcement, hl hg thickness of the adhesive layer, Ei Gi ¼ 2ð1þm shear modulus of the ith layer, iÞ m Poisson’s ratio (e.g. concrete: mc = 0.2; adhesive: mg = 0.25; steel cord: ml = 0.3), Ac concrete section.
Fig. 9. Bilinear s–sl relationship.
F external reinforcement
concrete
Fig. 10. Spreading out of forces.
which takes into consideration the multi-axial stress situation. This parameter has to be determined from experiments. kb2 introduces a second width effect: it accounts for the spreading out of the forces in the concrete, Fig. 10. sffiffiffiffiffiffiffiffiffiffiffiffiffi bl ð8Þ k b2 ¼ 2 bc in which bc is the width of the concrete. The second model parameter slm is the value of the slip at peak shear stress. The slip is determined as the sum of
As slm depends on slm, it also depends on the parameters kb1, kb2, kc. The last model parameter is the slip sl0 at which no more forces can be transferred. Its value is based on the total fracture energy Gf. This fracture energy is the energy per unit area, needed to bring a connection into complete failure. It is given by the area under the shear–slip curve, Fig. 9. Eq. (10) gives the fracture energy for a bilinear shear–slip relationship. Z 1 slm sl0 Gf ¼ ð10Þ sðsl Þdsl ¼ 2 0 If the fracture energy is known, the slip sl0 can easily be calculated. To determine the fracture energy, expression (11), is proposed by Holzenka¨mpfer: Gf ¼ k 2b1 k 2b2 k 2c C f fctm
ð11Þ
Gf depends on the tensile strength of the concrete, fctm. Again, the parameters kb1, kb2, kc are included. A new empirical factor is introduced. Cf is a parameter that fits the experiments to the model.
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
1935
When the three model parameters – slm, slm and sl0 – are known, the differential equation of Volkersen can be solved. The shear stresses can be calculated at every location as a function of the external loading. Also the anchorage length and the corresponding maximum transferable load can be derived [1]. 7.2. Experiments Eight direct shear tests were executed at the Reyntjens Laboratory. On four of them, a single SCRP is tested. A pre-impregnated double layer SCRP (marked with I) is used in the four others. The test set-up is shown in Fig. 11. Two concrete prisms are bonded together with SCRP on two opposite sides which are grit blasted on beforehand. Between the two prisms, there is a small gap of 18 mm. Bonding length is 150 mm or 200 mm. On the two other sides, steel plates are glued. These plates transfer the load from the testing machine to the test samples. Commercially available adhesive Epicol U is used. This adhesive is a glue paste, with Young’s modulus of 7000 MPa. After hardening, a saw-cut is made alongside the SCRP, to prevent spreading out of forces. Therefore, the parameter kb2 equals 1. Since the test specimens are carefully prepared under laboratory conditions, kc is also 1. The test is displacement controlled. The change of the gap between the two concrete blocks is monitored by means of linear voltage differential transformers (LVDT) at two opposite sides of the block, Fig. 11. The gap increases at a constant average rate of 0.001 mm/s. By comparing the two individual signals, one can check whether or not the tensile force acts centrically. If this is not the case, the test results are not taken into consideration. One test was oblique and therefore left out of the results. In all test specimens, failure was due to failure of concrete. This means that the concrete is the weakest link in the connection. Test specimen I150a (the first test with pre-impregnated laminate and 150 mm anchorage length) after failure is shown in Fig. 12.
Fig. 12. Failure of test sample I150a.
Brosens [1] determined empirically the values of the parameters for CFRP: k = 1.47 mm and Cf = 0.40. With these parameters, a first estimation of the transferable load can be made, Table 3. The model parameters are calculated with the data from Table 3. In Fig. 13, the measured shear force and slip are compared with the model using the CFRP model parameters. In the test with single SCRP a small deviation is observed. At the end of the test, the measured curve diverges more from the model and failure took place later than predicted. The deviation on the force is about 12% and the test results are always underestimated. The SCRP seems to behave stronger and stiffer compared to elements strengthened with CFRP. The model parameters for CFRP predict the results relatively good, but are a rather conservative approach. The test results on pre-impregnated double layer SCRP are presented in Fig. 13. Smaller slip is measured, compared to the elements strengthened with single SCRP because of the higher amount of reinforcement. The measured curves diverge more from the CFRP-model (average deviation for the transferred forces is +20%) [15]. For the same cross-sectional properties SCRP seems to present higher stiffness. This might be due to slip between carbon fibres, twisting of fibres in the cords prevent such slip.
Fig. 11. Pure shear tests on concrete reinforced with SCRP, type 1.
1936
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
Table 3 Calculated parameters and forces Type (–)
Single layer
Pre-impregnated double layer
2
fctm (N/mm ) fcm (N/mm2) Eg (N/mm2) hg (mm)
2.65 35.0 7000 0.98
Gf (N/mm) slm (N/mm2) slm (mm) sl0 (mm) Name (–) Bonding length (mm) FCFRP (kN) Ftest (kN) F test F CFRP (–)
150b 150 25.9 29.5 1.14
200b 200 31.0 35.1 1.13
I150a 150 27.7 33.4 1.21
lt = 150 mm, single laminate
45 40
40
35
35
30
30
25
25
20
20
15
15
10
10
5 0
5
model experiment 0
0.1
0.2
0.3
0.4
0
0.5
model experiment 0
0.1
sl (x = l) (mm)
0.3
0.4
0.5
l = 200 mm, pre–impreg. double layer
lt = 150 mm, pre–impreg. double layer
45 40
40
35
35 30
30
25
25
F (kN)
F (kN)
0.2
sl (x = l) (mm)
45
20
20
15
15
10
10
5 0
I200b 200 35.0 41.7 1.19
lt = 200 mm, single laminate
45
F (kN)
F (kN)
0.736 2.06 0.0067 0.7134 200a 200 31.0 34.4 1.10
2.65 35.0 7000 0.98 (concrete-SCRP) 0.65 (SCRP-SCRP) 0.736 2.06 (concrete-SCRP) 0.0084 0.7134 I150b I200a 150 200 27.7 35.0 35.1 39.5 1.27 1.13
5
model experiment 0
0.1
0.2
0.3
0.4
0.5
0
model experiment 0
0.1
sl (x = l) (mm)
0.2
0.3
sl (x = l) (mm)
Fig. 13. Measured force in function of slip.
0.4
0.5
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938 Table 4 Calculated forces with adapted model parameters Type (–)
Bonding length (mm)
FSCRP (kN)
Ftest (kN)
FSCRP/ Ftest (–)
Single layer Single layer
150 200
28.1 33.6
Pre-impregnated double layer Pre-impregnated double layer
150
33.2
200
41.9
29.5 34.4 35.1 33.4 35.1 39.5 41.7
1.05 1.02 1.04 1.01 1.06 0.94 1.00
A sensitivity analysis on the transferable forces is performed. Based on this analysis, it is concluded that a deviation on the material characteristics cannot explain a deviation of 12% and 20% from the model. Also the sensitivity of the model parameters href, k, Cf is examined. Changes on href only cause small changes in the transferable forces. Parameter Cf is a factor, needed to calculate the fracture energy. Its effect at small bonding lengths is almost negligible. The influence of Cf is only observed for long bonding length, especially longer than the theoretical anchorage length. The anchorage length can be calculated as the length required to take 97% of the maximum transferable load, [1]: with the CFRP parameters it becomes 285 mm for the single SCRP and 399 mm for pre-impregnated SCRP with two layers. Finally, the parameter k takes into consideration the multi-axial stress situation. When changing this parameter, it has an influence on all bonding lengths. Using the parameters experimentally determined for CFRP, all test results are systematically underestimated. Therefore, adapted model parameters are proposed. The tests were made on test specimens with a small bonding length. As concluded from the sensitivity analysis, only the parameter k will be varied. The authors propose to decrease this parameter k: for single SCRP, k is to be taken 1.2 mm. For pre-impregnated double SCRP, calculations with k = 1.0 mm give improved results. In Table 4, the transferable forces, calculated with the adapted model parameters for SCRP (FSCRP), are listed. To evaluate the parameter Cf, additional experiments are needed for confirmation, especially tests with a longer bonding length. These tests are running and will be investigated later. 8. Conclusion SCRP is a new material that combines the advantages of steel plates and CFRP. It combines a relatively low material cost with an ease of application. A first prototype has been developed in a collaboration between Bekaert and KULeuven. For this prototype, different requirements are investigated such as flexibility, impregnation, bond strength, material properties and bond behaviour. It has been demonstrated that the initial prototype needed a higher flexibility. Therefore, three new types of SCRP were
1937
developed and will be further investigated. The impregnation is sufficient if accurately applied. However, the development of a new adhesive is necessary to improve impregnation for on site application. Also, pre-impregnated laminates are an interesting option. Also the bond behaviour is studied. Non-linear fracture mechanics is applied to model and describe the shear–slip behaviour of the bonded connection. Experiments are presented which demonstrate that the new material behaves stronger and stiffer than elements strengthened with CFRP. Adapted material dependent parameters for SCRP are proposed. Therefore, the design equations set up for CFRP can be used with slightly altered material parameters. As a result, an unbiased non-conservative design is obtained also in the case of SCRP. Further research concerns the optimization of SCRP to improve the impregnation. Also the durability of SCRP and to further ease wrapping. Acknowledgements The authors express their thanks to: the Flemish Institute for Promotion of Scientific and Technological Research in the Industry (IWT–Vlaams Instituut voor de Bevordering van WetenschappelijkTechnologisch Onderzoek in de Industrie) for their financial support. NV Bekaert SA, represented by dr. ir. W. Dekeyser and ir. J. Gallens, for the collaboration and technical support. References [1] Brosens K. Anchorage of externally bonded steel plates and CFRP laminates for strengthening of concrete element. PhD thesis. Leuven: Katholieke Universiteit Leuven; May 2001. [2] Matthys S. Structural behaviour and design of concrete members strengthened with externally bonded FRP reinforcement. PhD thesis. Ghent: Universiteit Gent; September 2000. [3] Triantafillou T. General concepts and design aspect – materials and techniques, fib Bulletin 35. Retrofitting of concrete structures by externally bonded FRPs 2006:1–27. April. [4]
1938
W. Figeys et al. / Construction and Building Materials 22 (2008) 1929–1938
[9] Prota A, Tan KY, et al. Performance of shallow RC beams with externally bonded steel reinforced polymer. Struct J 2006;103(2): 163–70. [10] Figeys W, Schueremans L, Brosens K, Gemert DV. Steel wire reinforced polymer for strengthening of construction. In: Proceedings of the third international convergence on composites in construction (Lyon), CCC 2005, July 2005, p. 831–9. [11] R.N.V. Product catalogus betonherstelling 2003. [12] Figeys W, Schueremans L, Brosens K, et al. Steel cord reinforced polymer (SCRP) for externally bonded reinforcement. In: Proceedings of the 11th international conference of structural faults and repair (Edinburgh), Structural faults and repair 2006, June 2006.
[13] Volkersen O. Die Nietkra¨ftverteilung in zugbeanspruchten Nietverbindungen mit konstanten Laschenquerschnitten. 15, Luftfahrtforschung 1938. p. 41–7. [14] Holzenka¨mpfer P. Ingenieurmodelle des Verbunds geklebter Bewehrung fu¨r Betonbauteile. PhD thesis, T.U. Braunschweig: Braunschweig; 1994. p. 5–86. [15] Figeys W, Schueremans L, Brosens K, Gemert DV. Strengthening of concrete structures using steel wire reinforced polymer. In: Proceedings of the seventh international symposium of fibre reinforced polymer reinforcement of concrete structures, vol. 1. Kansas City, FRPRCS 7, Novembre 2005. p. 743–62.