Concrete Prestressed with Textile Fabric

Transcription

1 Journal of Advanced Concrete Technology Vol. 1, No. 3, November 3 / Copyright 3 Japan Concrete Institute 31 Invited Paper Concrete Prestressed with Textile Fabric Hans W. Reinhardt 1, Markus Krüger and Christian U. Große Received 18 November, revised April 3 Abstract Textile reinforcement is standard meanwhile since there is large experience with continuous and chopped fibers. However, the prestressing of continuous fibers opens more advantages since the initial strain is anticipated and larger stiffness is obtained. The paper shows that this theoretical prediction has been validated. 1. Motive Concrete and steel are a very successful couple: steel has high tensile strength and ductility and concrete is strong in compression, but rather brittle. Reinforced concrete is the typical example of a composite which consists of a brittle matrix and a ductile fiber. Concrete protects steel against corrosion as long as it is not carbonated and/or chloride contaminated. That steel can corrode after a certain time is a drawback of this composite especially when the structural elements are thin-walled and the concrete cover is small. In these circumstances, non-corroding materials as a reinforcement have advantages. To mention are glass, aramid, carbon as strong materials or polyvinyl alcohol (PVA) or polyethylene (PE) as flexible material. They are stable in carbonated concrete and in chloride environment. These materials have proven their versatility (Proceedings Sapporo 1997; Taerwe 1995; Peled et al. ). Instead of using fibers and/or one-dimensional reinforcement one can also make use of plane or even three-dimensional fabrics (Sonderforschungsbereich 53; Sonderforschungsbereich 58). Modern weaving machines allow a large variety of weaving, knitting, stitching and brading techniques. The fabrics can be made to purpose to fulfill special requirements. This will be shown later. There is another aspect cited in the title of the paper: prestressing. In order to make profit of the very high strength of steel or other material on the one side and to reduce cracking of concrete on the other side prestressing is used. It will be shown that prestressing is even more advantageous in case of high strength fabrics. The following chapters deal with a research project on prestressed textiles in concrete, discussing results and pointing on the applicability of this new method in practical engineering.. Fabrics In this research, AR glass and carbon fabrics have been used. Figure 1 shows some typical examples of glass fabrics. On the left hand side there are two fabrics with rectangular meshes. The upper one has a mesh size of 1 mm, the lower one of mm. Both fabrics have interwoven yarns in the intersection points. The right hand side shows two examples of layered fabrics when the yarns run into three directions. The reinforcement can be adjusted according to the necessity of the client. It should be noticed that a fabric is never straight in a neutral state of stress but it is undulated of crimpled. Figure shows two different fabrics of carbon fibers. The left hand side is a rectangular fabric with two interwoven yarns whereas the right hand side is a typical fabric with anisotropic properties. One thread is larger than the other. The threads are interconnected by a stitch yarn. The magnification of the lower picture of the right hand side shows that a secondary yarn is running around the main yarn which is important in connection with the bond between fabric and matrix. The properties of a synthetic fabric and steel are very 1 Professor, Institute of Construction Materials University of Stuttgart, Germany. reinhardt@iwb.uni-stuttgart.de Institute of Construction Materials, University of Stuttgart, Germany. Fig. 1 Glass fabrics.

2 3 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 Table 1 Properties of textile materials. Modulus of Ultimate strain elasticity [GPa] [%] Type of reinforcement Tensile strength [GPa] Coefficient of thermal expansion [1-6 K -1 ] AR glass Carbon HT Caron HM axial: -.1 to -1.3 radial: 18 Steel S355JO Steel St18/ Concrete with fine aggregate different. Table 1 shows a few properties. Only alkali resistant glass (AR glass) can be used with a tensile strength of 3. to 3.5 GPa (i. e. 3 to 35 MPa) and an elastic modulus of 71 to 7 GPa. The ultimate strain is limited to. to.3 %, i. e. it is a brittle material. Structural measures have to be taken in order to make a structure ductile. The coefficient of thermal expansion is about half of concrete. There are two variations of carbon, i. e. the high tenacity (HT) mode and the high modulus (HM) one. Both have a very high strength between and 5 GPa and both have a modulus of elasticity which is in the vicinity of steel or higher. The ultimate strain is taken 1.75 and 1.9%, i. e. also this material is brittle. The coefficient of thermal expansion is negative in the longitudinal direction and positive in the transverse one. For the sake of comparison numbers are also given for steel (mild steel and prestressing steel) and concrete. Of course, all numbers are only indications which may vary considerably. Figures 1 and show the geometry of fabrics. There are many variations of the cross-section and the mesh size. In this research, four types of fabrics have been used as shown in Table. There are two carbon fabrics and one AR-glass fabric. All of them have a biaxial structure with rovings running at and 9 degrees. The cross-section of a thread is.9 mm and depending on the fact whether it is plain or impregnated there is a large difference in tensile strength. The plain fabric is suffering from damages at the surface and also from testing effects like clamping stresses. The impregnated fabric is much less sensitive for that. The stress-strain relation of the material used depends also strongly on the type of fabric. Figure 3 shows the theoretical stress-strain curves and the measured ones. Looking to carbon one can see that the stress-strain diagram is straight up to failure at about 5 MPa. The impregnated yarn reaches about 3 MPa and follows the straight line. However, the plain one yields only 15 MPa or less depending on the strain rate. A lower strain rate during testing leads to a lower modulus of elasticity and also to a lower strength. AR glass behaves similarly, i. e. the impregnated glass follows about exactly the theoretical line whereas the plain glass has rather inferior properties. A woven fabric shows a completely different stress-strain behavior from a straight yarn. Due to the geometry the warp is almost straight whereas the weft is undulated around the warp. When the threads are loaded they stretch and that makes that the rigidity is rather low. Especially when a weft yarn is stretched the first % Table Properties of textiles. Property, unit Type C CE AR 1 mm Fig. Carbon fabrics. Material Carbon Carbon AR glass Structure of fabric Biaxial Epoxy imp. biaxial Biaxial /9 /9 /9 Weight, g/m Roving, tex Area of one roving, mm Mesh size, square, mm Maximum tensile force per roving, N Tensile strength of roving, MPa

3 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 33 Tensile stress, MPa 3 Carbon,, %/min Carbon, 1 %/min Carbon, epoxy, %/min Carbon, epoxy 1 %/min Carbon, theor. calc. AR glass, 1 %/min AR glass,, %/min AR glass, theor. calc. AR glass, epoxy 1 %/min N Nu Ncr Nd O Reinforced composite Prestressed composite Np Advantages of prestressing: - Activation of reinforcement already in uncracked state - Increase of cracking force - Serviceability state extended crack 1 O' l Fig. 5 Principle of prestressing Strain, % Fig. 3 Stress-strain diagramme of various rovings. strain do not lead to a comprehensible resistance. After a loading to about kn/m and reversed loading the stiffness is highly increased and remains almost constant during subsequent loading cycles. Referring to prestressing this means that prestressing eliminates the initial strain and leads to the real stiff behavior of the yarns. 3. Effect of prestressing Prestressing means to extend the uncracked state of a brittle-matrix composite. Figure 5 illustrates the situation. Point D is the origin of a reinforced composite. At Ncr the matrix cracks and the reinforcement takes the forces. Depending on the stiffness of the reinforcement and the reinforcement ratio the curve is more or less steep after cracking. Especially with low reinforcement ratio and low stiffness there is a large displacement after the first crack. When the element is prestressed the origin is shifted to O', i.e. the elastic non-cracked state is extended considerably depending on the degree of prestressing. In the case of prestressing Nd means the state of decompression when the material is stress-free and tensile stresses develop. Ncr means that the matrix cracks and the same situation arises as in the reinforced case. The great advantage of prestressing is that the useable state of serviceability is greatly extended.. Testing programme.1 Concrete The "concrete" used is a fine grain material which has high compressive strength and is almost self-leveling. The composition is shown in Table 3. A Portland cement CEM I.5 R according to the European Standard is used which is a rapid hardening cement. Fly ash is added for better workability and silica fume is used for high early strength. Two sand fractions are applied up to maximum grain size of 1. mm. A superplasticizer was necessary in order to obtain good fluidity. The water-cement ratio amounts to., but the water-binder ratio is more rational which is.31. The compressive strength should be high especially at early ages since the prestressing is released after one day. The values are 5 MPa after one day, 6 MPa after 7 days and 75 MPa after 8 days. The flexural strength amounts to 5, 9 and 11.5 MPa, resp. Shrinkage has been measured after 7 and 8 days which is.5 and.6 mm/m. The values will not increase much since autogenous shrinkage is the main contribution.. Prestressing and concreting procedure A rigid steel frame has been designed comprising ten hydraulic pistons at each side. Each piston has a capac- [kn/m] Cycles 1,,3,,1 Table 3 Concrete mix and properties. Component kg/m 3 Cement CEM I.5R 8 Fly ash 15 Silica fume (dry) 1 Sand -.6 mm 6 Sand.6-1. mm 9 Superplasticizer 17 Total water warp weft Fig. Stress-strain diagramme of a woven fabric (Reinhardt, 1976). Property after 1d 7d 8d Compressive strength, MPa Flexural strength, MPa Shrinkage, mm/m -.5.6

4 3 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 7 Carbon, no prestressing Carbon, prestressed.5 MPa Fig. 6 Prestressing frame and concrete placing. ity of kn. The fabric is clamped with ten individual steel clamps allowing the necessary transverse flexibility in order not to restrain the Poisson effect. The total loading amounts to kn on a square sheet of 1 m, thus kn/m. The concrete is placed on top of the fabric which is situated in the center plane of the plate and slightly vibrated in order to rinse through the meshes and to compact completely. The 1 mm thick plates were covered with a plastic sheet. The piston was released after one day. Thereafter, the plates were kept wet during 5 days and then stored at C/65 % RH. Stripes of 1 mm width were sawn for the four point bending tests. A similar procedure was applied to the flat pull-out specimens. 5. Tests with plane fabric 5.1 Bending tests Four-point bending tests have been carried out on 1 mm wide strips. The length of the specimen was 3 mm, the span 5 mm. The loading points were located at the third points. The displacement rate was kept constant at mm/min. The following graph shows the force vs. the midpoint deflection. Figure 7 shows the load-deflection curves of a AR glass, no prestressing AR glass, prestressed 1.5 MPa Deflection [mm] Fig. 7 Load vs. deflection of a AR-glass reinforced and prestressed strip with 1 mm thickness Deflection [mm] Fig. 8 Load vs. deflection of a carbon reinforced and prestressed strip. prestressed and a non-prestressed strip. After the first crack the reinforced strip shows the typical drop of load and subsequent stages of cracking until the stable crack pattern has been reached at about 7 mm deflection. The material hardens in deflection until about 35 N when the modulus of rupture is reached. The prestressed material does not exhibit the large increase of displacement but it develops immediately after cracking the hardening state. It fails at about 5 N with a displacement which is smaller than in the case of the reinforced material. The same procedure has been followed with the carbon reinforced material. Figure 8 shows that the cracking load is almost the same as in Fig. 7 but the subsequent behaviour is different. The reinforced material takes immediately more load and sustains a maximum load of about 7 N whereas the prestressed one shows a large increasing deflection and fails at 35 N. The reason for this unexpected behavior will be explained later. Deflection and crack width have been measured. Table shows that the prestressed AR-glass specimen has a large deflection in the reinforced configuration whereas the prestressed one shows much less deformation and especially the crack width is much reduced. Cracks of.1 mm are hardly visible with the naked eye. The carbon reinforced strip behaves very well in the reinforced case with a crack width of. mm only at failure but it is worse in the prestressed case with.6 mm crack width. As the serviceability is concerned a crack of.6 mm would not be acceptable. 5. Bond tests with acoustic emission analysis Since the result of the tests were very unexpected the reason of this behavior had to be investigated. It was supposed that the bond properties determined the overall behavior. To this end, specimens have been prepared according to Fig. 9. A roving has been placed in the center of a 8 x 8 x mm 3 concrete specimen. On the surface of the specimen seven transducers have been placed which pick up acoustic emission signals. The appropriate software allows to determine the

5 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 35 Table Deflection and crack width of plates tested. Reinforcement Crack width Deflection mm mm AR glass Reinforced (failure) Prestressed, 1.5 MPa <.1 9 (failure) Carbon Reinforced (failure) Prestressed,.5 MPa (failure) location of events inside the specimen (Grosse et al. 1995). Figure 1 is a concise illustration of the pull-out behavior of AR glass. On the left hand side, the upper graph shows the acoustic events which develop right after the beginning of the test. The middle graph shows the location of the events. It is obvious that the events came from the lower 15 mm of the specimen which indicates a good bond between AR glass and concrete. The lower graph confirms that there is little slip until the maximum load is reached. The upper right hand graph is a 3 D illustration of the event while the lower graph is a representative of the pull-out force vs. displacement behavior indicating fiber failure outside the specimen. Summarizing, AR glass has very good bond properties. An analogous plot shows the behavior of a carbon roving in concrete. Figure 11 illustrates that the acoustic events are distributed almost uniformly over the total height of the specimen. Further, the picture on the left hand side shows that slip occurs immediately after the beginning of loading. This means that the bond between plain carbon and concrete is rather inferior. 5.3 Discussion When a multifilament roving is considered one has to distinguish between the external and internal filaments. The external filaments are bonded in the cement matrix whereas the internal filaments are not or, depending on the penetration of cement paste into the roving, bonded only to a smaller extent. This makes that the interior filaments slip when a differential force occurs such as at a crack or an anchorage. load cell specimen 8 x 8 x mm 3 roving LVDT strain gage clamp Fig. 9 Pull-out specimen, loading configuration and AE receivers.

6 36 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 1,3 Energy per event [-] 8 6,5,,15,1,5, : : : :6 :8 Σ energy [-] Z-Axis [cm] 3 acoustic event 1 Z-Axis [mm] 3 1 X-Axis [cm] Y-Axis [cm] : : : :6 : Fiber failure outside specimen Pull-out [mm] : : : :6 :8 Time after start [min],,5,5,75 1, Pull-out [m m ] Fig. 1 Pull-out force, displacement and AE events of AR glass. The slip is more pronounced when the friction between the filaments is small and when the pore fluid does not penetrate into the interior of a roving. Comparing Figs. 1 and 11 the conclusion can be drawn that carbon has the worst interfilament friction. In the case of prestressing, the carbon filaments displace to each other and the prestress is not effectuated. The situation is even worse compared to the reinforced situation because the filaments are straightened during prestressing and the available space between the filaments is blocked. Obviously, AR glass behaves better due to better adhesion between the mineral surface of glass and concrete. Although this is not yet proven there may to some interaction between the alkaline pore solution and the glass which improves the bond properties. 6. Tests with impregnated fabric 6.1 Bending tests In order to improve the performance of the fabrics they have been impregnated with epoxy resin. This measure should guarantee a better bond between the roving and the concrete. Especially the interfilament bond should be straightened. The same procedure has been followed as in Chapter 5.1. For the sake of available space only the results of carbon reinforced strips are given in the following graph. The lower curve exhibits a big decrease of stress when the first crack occurs. It drops down from about 1 MPa to about MPa which is due to the initial strain in a fabric. The stress recovers to 1 MPa but jumps again to 3 MPa after the subsequent crack. This feature Table 5 Results of bending tests with carbon fabric. LOP MOR Reinforcement Stress Deflection Stress Deflection (MPa) (mm) (MPa) (mm) Carbon, plain Reinforced Prestressed.5 MPa Carbon, epoxy impregnated Reinforced Prestressed 1.5 MPa Prestressed.5 MPa Prestressed 3. MPa LOP = Limit of proportionality MOR = Modulus of rupture

7 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 37,8 Bond slip Energy per event [-] 3 1,6,,, : : : :6 :8 :1 Σ energy [-] Z-Axis [cm] 3 1 Z-Axis [mm] 3 1 acoustic event Pull-out X-Axis [cm] Y-Axis [cm] Pull-out [mm] : : : :6 :8 :1 1,,8,6,,, : : : :6 :8 :1 Time after start [min] ,,5,5,75 1, P u ll-o u t [m m ] Fig. 11 Pull-out force, displacement and AE events of carbon. is repeated until to the end of the test. With increasing prestressing, the cracking load is increased to 17 MPa and the subsequent cracking intervals are reduced. The maximum stress before failure is also increased considerably. Table 5 summarizes the results of the carbon fabric tests. The limit of proportionality (LOP) and the modulus of rupture (MOR) are given for plain carbon and impregnated carbon reinforcement. It can be seen that both limits are considerably increased for impregnated carbon with increasing prestressing. This means compared to plain carbon a large improvement. The same is true for the cracking behavior which can be depicted in Table 6. Whereas bond failure occurs with plain carbon there is only fiber failure in case of impregnated rovings. The larger the prestressing the smaller the cracks are and the more likely is the failure at small crack sizes. 6. Pull-out tests In order to confirm the beneficial feature of impregnation pull-out tests have been carried out. The specimen shape and the loading configuration have been changed compared to Fig. 9. The specimens are a 1 mm long, 7 mm wide and 1 mm thick plate which is notched until a depth of a single roving of a fabric. The upper part of the specimen is mm long whereas the lower part is 1 mm. Figure 1 illustrates the situation. Concrete Internal filaments External filaments (TU Dresden) Fig. 1 Bonding mechanism in a multifilament roving (Sonderforschungsbereich 58).

8 38 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, 3 5 Carbon, epoxy impregnated no prestressing prestress 1.5 MPa prestress.5 MPa prestress 3. MPa Load cell Flexural stress [MPa] mm mm Clamp Deflection [mm] Fig. 13 Flexural stress vs. deflection of carbon reinforced strips. 1 mm LVDT Test specimen Failure occurs always by pulling-out of the upper part of the fabric. Consequently the following graph shows the pull-out vs. slip mainly of the upper part whereas the lower part is much better fixed in the large lower part of the specimen. The results of these pull-out tests are given in Fig. 15 as pull-out force per unit length vs. slip. The maximum bond stress reaches 6 N/mm for all impregnated fabrics whereas the maximum is 3 N/mm for plain carbon. This illustrates clearly that the bond has been improved by impregnation. It is also striking how large the bond resistance is at a displacement of 1 mm, i. e. the frictional bond is considerable. Table 6 Crack width in bending tests of carbon reinforced strips. Reinforcement Crack width Deflection mm mm Carbon, plain Carbon, epoxy impregnated (1) fibre failure () bond failure < () () - < (1) (1) (1) (1) Fig. 1 Pull-out tests for impregnated fabric. 6.3 Discussion It has been demonstrated that the bond strength is greatly increased when the fabric is impregnated with epoxy. The main reason for that is the interfilament adhesion of the roving and the second is that the epoxy has a better bond and friction resistance at the surface of the roving. Both together lead to an excellent behavior in concrete. As has been anticipated by theoretical arguments the cracking stress increases with increased prestressing. Also the deflection hardly increases with increasing prestressing which is due to the larger compressive zone with increasing prestressing. These effects make that the prestressed concrete element is greatly superior to reinforced one. In order to maintain the prestressing over a long time the prestress losses have to be taken into account, i. e. losses due to creep and shrinkage of concrete and due to relaxation of the prestressing material. In the investigated case, only short time tests have been performed. But the prestrain of the material was such that about % of the prestress would vanish. This effect deserves more attention. P/(c- s*), N/mm Carbon, plane fabric no prestressing prestress 15 N/roving prestress 5 N/roving Carbon, epoxy impreg. no prestressing prestress 375 N/roving prestress 65 N/roving 1 3 slip s*, mm Fig. 15 Bond stress vs. slip of carbon in concrete.

9 H. W. Reinhardt, M. Krüger and C. U. Grosse / Journal of Advanced Concrete Technology Vol. 1, No. 3, 31-39, Conclusions A research programme has been carried out establishing the feasibilty of prestressed textile reinforcement. The following conclusions can be drawn. - Plain AR glass is suitable for reinforced and prestressed concrete elements - Plain carbon rovings are suitable as a reinforcing material, but they fail when prestressed - Impregnated carbon is very suitable for prestressing - The effect of higher limit of proportionality has been demonstrated with higher prestressing - The modulus of rupture increases also considerably with prestressing. - Crack width of prestressed elements with plain AR glass and impregnated carbon roving are very small and not visible with naked eye. - The largest effect of prestressing is that the initial strain of a fabric is anticipated and that deflection and crack width after first cracking is minimized. Acknowledgement The financial support by the Gips-Schüle Foundation and by the German Research Society (DFG) is greatly acknowledged. References Grosse, C., Reinhardt, H. W. and Dahm, T. (1995). Localization and classification of fracture types in concrete with quantitative acoustic emission measurement techniques. in: Proceedings Intern. Symp. Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin, Peled, A., Shah, S., and Banthia, N. (). High-Performance Fiber-Reinforced Concrete Thin Sheet Products. in: ACI SP-19. Proceedings of the third International Symposium, Non-metallic (FRP) Reinforcement for Concrete Structures (1997). Sapporo, Japan 1-16 October 1997, Vol.. Tokyo, ISBN Reinhardt, H.-W. (1976). On the biaxial testing and strength of coated fabrics. Experimental Mechanics 16 (), Sonderforschungsbereich 58: Textile Bewehrungen zur bautechnischen Verstärkung und Instandsetzung, Dresden. Sonderforschungsbereich 53: Textilbewehrter Beton - Grundlagen für die Entwicklung einer neuartigen Technologie, Aachen. Taerwe, L. (Ed.) (1995) Non-Metallic (FRP) Reinforcement for Concrete Structures. E & FN SPON, London.