PRESTRESSED CONCRETE PLATES WITH HIGH STRENGTH FABRIC

Transcription

1 PRESTRESSED CONCRETE PLATES WITH HIGH STRENGTH FABRIC H.W. Reinhardt, M. Krueger Constructions Materials Institute, University of Stuttgart, Germany Abstract Tests on fine grain concrete plates with textile reinforcement have been carried out. The textiles used were biaxial warp knitted fabric of alkaline resistant glass, carbon, and aramid. Load displacement curves are presented which show a considerable increase of the bearing capacity due to prestressing. The plates failed either to diagonal crushing or punching. Keywords: textile reinforcement, fine grain concrete, plate, prestressing, load displacement behaviour, fabric 1. Introduction Concrete reinforced with textiles seems to be a versatile material. It combines the high compressive strength of concrete with high strength reinforcing polymer materials. Several authors have shown that construction elements of various shapes can be produced [1-5]. However, a certain drawback is that the reinforcing polymer material has a lower elastic modulus than steel and, as soon as a crack appears, large displacement of a bending element occurs. That is why prestressing may be a remedy for this situation. A positive effect of prestressing has been demonstrated on linear elements []. The present paper will show that prestressing is also an advantageous solution for plates in biaxial bending.. Testing programme The testing programme consisted of six prestressed plates of the same concrete composition and six non-prestressed plates. The size of the plates 1 m x 1 m x mm. The plates were loaded by a concentrated load in the middle, they were simply supported 7

2 at the edges but clamped in the corners. Load and center displacement were measured during the tests. The production of the plates was as follows. A mold was placed in the middle of a prestressing frame. The textile was laid on the mold and clamped at the edges with ten specially designed clamps [7]. Ten hydraulic pistons at each edge were connected to the clamps. The pistons have a maximum load of 3.5 kn. The non-prestressed plates were concreted in two layers. First, a 5 mm thick layer was put on the mold and vibrated by a linear vibrator. After that the textile was put on the concrete and the second layer was concreted on top of it. The prestressed plates were concreted in one layer and vibrated and smoothed. The plates were covered by a plastic sheet for hours. Thereafter, the plates were unmolded and put under water for 7 days. Then, the plates were stored at C and 5% RH for about another year until testing. The plates were loaded by a concentrated circular load of 15 mm diameter. The deflection velocity was controlled at. mm/min. At deflections of about 3 and 5 mm the plates were unloaded and loaded again. Cracks were monitored during the tests and photographed at the end of the test. Two plates were only prestressed in one direction. The prestress amounted to 3. MPa either in one direction or two directions. Fig. 1 shows the prestressing frame as prepared for a uniaxial prestressing of fabric. The clamps and the hydraulic pistons can be seen in the frame with a total size of about.5 m x.5 m. Fig. 1. Prestressing frame

3 3. Materials used 3.1 Concrete A fine grain concrete (essentially a mortar) was used with maximum aggregate size of 1. mm. The composition of the fine grain concrete is shown in Table 1. Table 1: Concrete mix Component kg/m³ CEM I.5 R Flyash 15 Silica fume 1 Sand. mm Sand. 1. mm 9 Water 11 Superplasticizer 17. Density of fresh concrete Water-cement ratio. Water-binder ratio 1.31 Binder-aggregate ratio.9 1 with % counting of flyash and silica fume The cement used was a rapid hardening Portland cement according to European standards. Flyash from hard coal and silica fume was added so that a total binder content of 75 kg/m 3 was received. Sand fractions of to. and. to 1. mm were added. A superplasticizer was used for a flowable workability. The water-cement ratio amounted to. and the water-binder ratio to.31. The concrete had a very high binder-aggregate ratio of.9. The properties of the hardened concrete were determined on the prisms of x x mm 3. The prisms were stored at C and 95% RH for one day and thereafter at C and 5% RH. Table shows the compressive strength, the bending strength, and the shrinkage strain. Table : Hardened concrete properties Hardening time [d] Compressive strength [MPa] Bending strength [MPa] Shrinkage strain [mm/m]

4 During testing of the plates the concrete had hardened about 1 year, i.e. the 9 days values were at least appropriate. It can be seen that the compressive strength amounted to 5 MPa, the bending strength to 11.5 MPa and the shrinkage strain was.5 mm/m. 3. Fabrics Six different fabrics have been used. Since preliminary tests have shown that plain fabrics have a very minor bond strength in the concrete it was decided to use only epoxy impregnated fabrics and epoxy impregnated and sand coated fabrics. Table 3 shows the properties of the fabrics. There was an alkali resistant glass (AR-EP and AR-EPS), a carbon (C-EP and C-EPS) and an aramid (A-EP and A-EPS) fabric. All fabrics had a biaxial warp knitted structure /9 with mm mesh size. The materials were rather different as the weight per unit area is concerned, the strength and the elastic modulus. However, the diameters of the rovings used are similar. The strongest material was carbon, the second aramid and the third AR-glass. The elastic modulus of carbon is the highest, aramid is the second and AR-glass has the lowest. and sand coating improved the bond strength considerably as previously shown in [, ].. Testing results.1 Bearing capacity Table shows the bearing capacity of the plates and the maximum center displacement. As the bearing capacity is concerned the highest capacity is shown by the carbon epoxy and carbon epoxy sand reinforced plates. The second are the aramid and the third the glass reinforced plates. Table : Bearing capacity and maximum center displacement Bearing AR- Prestress capacity C-EP C-EPS AR-EP A-EP A-EPS EPS displacement - kn mm uniaxial mm kn V3, kn mm So, the plates show the same order of bearing capacity as were the strength properties of the fabrics. The uniaxially prestressed plates are in the same order of magnitude. The biaxially prestressed plates show all a higher bearing capacity than the non-prestressed plates. The difference goes up to about %. As the displacement at maximum load is concerned all values were rather similar, about to 7 times the plate thickness. 19

5 Table 3: Properties of fabrics used Textile designation AR-EP AR-EPS C-EP C-EPS ARA-EP ARA-EPS Material AR-Glas AR-Glas Carbon Carbon Aramid Aramid (NEG) (Toray T k) (Twaron ) Weight per unit area, g/m (rovings) Weight per unit area, g/m (total) ca. ca Titer of single roving 5 tex 5 tex 17 tex 17 tex x 1 tex x 1 tex Cross section of single roving ca..93 mm² ca..93 mm² ca..95 mm² ca..95 mm² ca..9 mm² ca..9 mm² Mesh size mm mm mm mm mm mm Portion of size.5%.5%.7%.7%. %. % Usable strength in concrete N/mm² N/mm² 5 N/mm² 7 N/mm² N/mm² N/mm² Modulus of elasticity of roving Remarks 7, N/mm² 7, N/mm², N/mm², N/mm² 15, N/mm² 15, N/mm² and sand coating and sand coating and sand coating 191

6 . Load displacement behaviour The following figures show the load displacement diagrams. Figs. and 3 belong to the glass reinforced plates, Figs. and 5 to the carbon reinforced plates and Figs. and 7 to the aramid reinforced plates. Fig. shows the load displacement curve for the plates which are reinforced and / or uniaxial and biaxially prestressed. It can be seen that prestressing in one direction increases the maximum load and also the stiffness of the plate to certain extent whereas the biaxially prestressed plate has a maximum load and maximum stiffness. This figure and all others which follow show a similar behaviour. There is a linear increase of the load with displacement to a certain value which is about 1 kn for the reinforced plates and about 1. kn for the prestressed plates. The stiffness of this portion amounts to N/mm. After this first linear increase a second stage is arrived which is characterized by a curved linear shape. This part is the most important contribution to the load and the displacement. The unloading cycles show a considerable irreversible displacement. Also the stiffness has decreased considerably compared to the initial stiffness of the plate. AR-EP V3, uniaxial AR-EPS Fig. and 3. Load displacement curves for AR-glass reinforced plates Remark: = non-prestressed, V3, uniaxial = uniaxially prestressed with 3. MPa, = biaxially prestressed with 3. MPa Fig. 3 shows the results of the sand coated glass fabric. There the same characteristic behaviour can be seen although the difference between the reinforced and prestressed plate is larger. The next two figures and 5 show the results of carbon reinforced and prestressed plates. The same features can be recognized. The plates have a little larger stiffness than the glass reinforced plates and the irreversible displacements at unloading are smaller. It is good to see that after reloading the load displacement curve continues almost exactly at the unloading point. That means not much damage could have occurred at this instant. 19

7 1 C-EP C-EPS Fig. and 5. Load displacement curves for carbon reinforced plates Figs. and 7 belong to the aramid reinforced plates. As the load displacement curves in Fig. are concerned there is not much difference between the reinforced and prestressed plates although the sequence is as expected. On this time it is obvious that the reinforced and prestressed plates show in the beginning almost the same behaviour and differ only in the second stage. As Fig. 7 with the sand coated aramid fabric is concerned there is a rather large improvement by the sand coating. Not only the initial stiffness is increased and the difference between reinforcing biaxial prestressing is evident but also the irreversible displacement at unloading is decreased. That means that the stiffness of the material has gained much due to sand coating. The load displacement curves can be interpreted as follows. The first part of the curve belongs to the linear elastic uncracked behaviour of the plates. After this stage a decrease of the stiffness occurs which is due to multiple cracking in the plate. Then, the stiffness increases again which is due to membrane action in the plate. A-EP V3, uniaxial A-EPS Fig. and 7. Load displacement curves for aramid reinforced plates 193

8 The displacement is so large that tensile forces occur in the center of the plate whereas compressive forces counteract at the edges of the plate..3 Fracture modes All plates show circular cracks around the concentrated load at the upper side of the plate. Also cracks at the lower side of the plates occurred. With increasing load the cracks at the upper side close again whereas the cracks at the lower side opened more and more. With increasing load cracks in the diagonals appeared and finally concrete failed due to crushing in the diagonals. This was true for the carbon and aramid reinforced plates. At the glass reinforced plates the concentrated load punched through the plate. Figs. and 9 show the crack patterns of the non-prestressed glass reinforced plates at the upper and lower side. At the upper side the concentrated cracks are clearly visible. In the center of the plate meshes of the fabric are visible which are due to punching of the concentrated load through the plate. At the lower side circular cracks can be seen as well as diagonal cracks in all corners. The failure of the plate was due to punching. Figs. and 11 show the crack patterns of the biaxially prestressed carbon reinforced plates. On the upper side one can see multiple concentric cracking. This crack pattern can only be explained by assuming that there is a large membrane action. The lower side exhibits also concentric cracking and diagonal cracking. Compared to Figs. and 9 there are much more circular cracks reaching even the edges of the plate. Final failure was due to crushing of the diagonals which can be clearly seen on the upper right corner where the meshes of the fabric are exposed. Fig. and 9. Crack patterns of non-prestressed glass reinforced plates at the upper (left) and lower (right) side (AR-EP ) 19

9 Fig. and 11. Crack patterns of biaxially prestressed carbon reinforced plates at the upper (left) and lower (right) side (C-EP V3,) The analysis of the plates is rather complicated. Analytical methods fail due to the large displacement and the cracking of the concrete. Furtheron the bond between the fabric and the concrete has to be considered. Finite element computations are underway which will be reported later. 5. Conclusion The tests on reinforced and prestressed plates have shown the following: - alkali resistant glass, carbon, and aramid fabrics with epoxy can be used for reinforcement and prestressing of fine grain concrete plates - prestressed plates show higher bearing capacity at about the same maximum displacement as reinforced plates - the most efficient is carbon with epoxy and sand coating - the displacement at maximum load is about six times the plate thickness, i.e. membrane action is a considerable contribution to the load bearing behaviour - the failure is due to either diagonal crushing or punching of the concentrated load through the plate. The analysis of the plates can only be performed by advanced finite element computations which are underway. Acknowledgement The considerable financial support of the Gips-Schüle Foundation is gratefully acknowledged. 195

10 References 1. RWTH Aachen SFB 53: 'Textilbewehrter Beton - Grundlagen für die Entwicklung einer neuartigen Technologie', Arbeits- und Ergebnisbericht TU Dresden SFB 5: 'Textile Bewehrungen zur bautechnischen Verstärkung und Instandsetzung', Arbeits- und Ergebnisbericht Peled, A., Bentur, A., 'Cement impregnated fabrics for repair and retrofit of structural concrete', In Naaman, A.E., Reinhardt, H.W. (Eds.) High Performance Fiber Reinforced Cement Composites (HPFRCC), RILEM PRO 3 (3) Krueger, M., Ozbolt, J., Reinhardt, H.W., 'A new 3D discrete bond model to study the influence of bond on the structural performance of thin reinforced and prestressed concrete plates', In A.E. Naaman and H.W. Reinhardt (Eds.) High Performance Fiber Reinforced Cement Composites (HPFRCC), RILEM PRO 3 (3) Reinhardt, H.-W., Krueger, M., Grosse, C.U., 'Thin plates prestressed with textile reinforcement', In Balaguru, P., Naaman, A., Weiss, W. (Eds.) Concrete: Material science to application, Farmington Hills, ACI () (ACI SP -), Reinhardt, H.W., Krueger, M., Grosse, C.U., 'Concrete prestressed with textile fabric', Journal of Advanced Concrete Technology 1 (3) (3) Reinhardt, H.-W., Krueger, M., 'Fine grain concrete panels prestressed with a textile fabric', In 'Ductile Fiber Reinforced Cementitious Composites (DFRCC) - Application and Evaluation', Proceedings of the JCI Intern. Workshop, Takayama, Japan, (Japan Concrete Institute, Tokyo, ) 3-3. Krueger, M., Reinhardt, H.-W., Fichtlscherer, M., 'Bond behaviour of textile reinforcement in reinforced and prestressed concrete', Otto Graf Journal 1 (1) pp