Fire-Damage or Freeze-Thaw of Strengthening Concrete Using Ultra High Performance Concrete

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1 Fire-Damage or Freeze-Thaw of Strengthening Concrete Using Ultra High Performance Concrete Ming-Gin Lee 1,a, Yi-Shuo Huang 1,b 1 Department of Construction Engineering, Chaoyang University of Technology,Taichung County, 41349, Taiwan, R.O.C. a mglee@mail.cyut.edu.tw, b yishuo@cyut.edu.tw Keywords: UHPC, fire-damage, freeze-thaw, strengthening, CFRP Abstract. There are some reinforced concrete structures exposed to severe environmental conditions might require maintenance or strengthening. Many of these severe circumstances are the result of extreme climate conditions such as low temperature, freeze thaw action, fire attack, and exposure to deicing salts. Because of this, the environmental durability of both the repair materials and methods used in rehabilitation applications are of utmost importance. A small fire can reach 250, while a common blaze can easily produce temperatures of around 800. In major conflagrations the temperature can even reach At this level, the heat affects most materials, provoking the spontaneous combustion of some of them and affecting the resistance of others. However, very little research has been performed in evaluating the environmental durability of strengthening materials for concrete members. Very little work has been done on the effects of freeze thaw cycling on bonding and repair materials. In this study, ultra high performance concrete(uhpc)was used to investigate the effect of strengthening concrete members by fire-damage test or freeze-thaw test. The results show that the mechanical properties of UHPC possess high strength, toughness, and freeze-thaw resistance. The CFRP (carbon fiber reinforced plates) wrapping specimens exposed at 300 showed totally failure with the deterioration of the adhesive. The UHPC with bonding 10 mm thickness specimens exposed at 400 and duration of 1 hour still in good shape. The UHPC with 1-cm or 2-cm thickness on strengthening concrete members could be obtained specific retrofit effects. The performance of UHPC specimens is better than those of CFRP wrapping specimens during high temperature exposure. The results of slant shear tests show that the bond strength of PC/PC, UHPC/PC and UHPC/UHPC decreased significantly after 600 freeze thaw cycles or high temperature exposure. Introduction Some reinforced concrete structures exposed to severe environmental conditions which might require maintenance or strengthening. Many of these severe circumstances are the result of extreme climate conditions such as low temperature, freeze thaw action, fire attack, and exposure to deicing salts. Because of this, the environmental durability of both the repair materials and methods used in rehabilitation applications are of utmost importance [1]. However, only little research has been performed in evaluating the environmental durability of strengthening materials for concrete members. Very little work has been done on the effects of freeze thaw cycling on bonding and repair materials [2]. Green et al conducted durability tests on the bond between FRP and concrete, and found that the bond was not significantly damaged up to 300 freeze thaw cycles. However, Pang et al conducted hygrothermal and UV radiation attacks on the FRP strengthened RC beams, and found that the reinforcing efficiency is greatly reduced [3]. Experiments The Portland cement concrete (PC), which was used as reference in this study, has a compressive strength of 30 Mpa contains no admixture and yields a slump of 150 mm. The mix design of the PC is

2 shown in Table 1. The UHPC to be used as a prospective repair material, contains a type II cement, silica fume, silica sand, quartz powder, steel fiber and a superplasticizer. The mix design of the UPC repair material is also shown in Table 1. The CFRP sheet commonly used as a prospective repair and retrofit material, their elasticity modulus and tensile strength of the CFRP sheet are 230,000 Mpa and 3,400 Mpa respectively. Fire-damage or Freeze-thaw Tests. Freeze thaw cycles were applied to the blocks at a rate of one cycle per 185 minutes, in accordance with ASTM C666 (1997), with 90 minutes of freezing in cold air at 18 C followed by 90 minutes of thawing in cool air at +4.4 C, for more detail see Lee et al 5. Specimens that were not subjected to freeze thaw cycling were stored in the curing chamber for 24 hours prior to testing. The specimens were subjected to 0, 200, 400, or 600 freeze thaw cycles, with two specimens for each of these cycles. Before and after freeze thaw cycling, the samples were tested for their flexural strength and slant shear strength. The high temperature exposure test was selected for the evaluation of the performance of repairing concrete member using CFRP or UHPC after fire attack. Muffle furnace of big furnace chamber ( mm) and maximum temperature of 1250 could be used. Specimens were placed unloaded in the cooled furnace chamber and the temperature was increased to reach certain degrees with ratting of 10 /min. After two hours, the muffle furnace turned off and left tills to cool, then the samples were ready to test. The specimens were subjected to 200, 300, or 400, with two specimens for each of these temperatures. After different high temperature exposures, the samples were tested for their strength. Test Specimens. The strengthening specimens are flexural beam concretes, with two specimens for flexural tests. The flexural beams had dimensions of 100 mm 100 mm 300 mm with bonding repair material (CFRP or UHPC) and the repair system UHPC/1, UHPC/2, CFRP/1, CFRP/2, and CFRP/3 with different repair layer and thickness as explained in Table 2. The cylinder specimens, dimensions of 50 mm in diameter and 100 mm in height, were made according to ASTM C882-99, Test Method for Bond Strength of Epoxy-Resin Systems Used with Concrete by Slant Shear. The experimental program was conducted to investigate the efects of freeze thaw cycling and high temperature exposure on the performance of strengthening concrete member using UHPC [2]. Results and Discussion Strengthening Effects with Repair Materials. The flexural strength of the normal concrete PC and the repair specimens by different repairing systems UHPC/1, UHPC/2, CFRP/1, CFRP/2, and CFRP/3 with different repair layer and thickness are shown in Table 2. The effects of flexural strengthening with bonding UHPC of 10 mm and 20 mm thickness are about 154 % and 210 % respectively more than those of normal concrete. The results show multiple layers of the CFRP wrapping specimens are still better than one layer of the CFRP wrapped ones. The amounts of the flexural strength ratio ranged from 167% to 260% by using various layers of the CFRP sheets as compared with those of normal concrete. The flexural specimens with bonding CFRP material increase strength but also have interface cracking after the flexural strength test. Freeze-thaw Cycle Test. The changes of flexural strength of the concrete beam specimens by different repairing systems UHPC/1, UHPC/2, CFRP/1, CFRP/2, and CFRP/3 with different repair layer and thickness subjected to freeze thaw cycling up to 600 cycles are given in Table 2. A steady small decrease in flexural strength was found after freezing and thawing. All strengthening beam specimens had not surface cracking until 600 freeze-thaw cycles. For the UHPC/1, and UHPC/2 specimens, the average values of the ratio of flexural strength at 200, 400 and 600 cycles were 97, 93 and 91 percent, respectively, compared with the corresponding values of 97, 95 and 92percent, respectively, for CFRP/1, CFRP/2, and CFRP/3. Although the flexural strength of the CFRP wrapping specimens are slightly reduced by subjected to freeze-thaw cycle attacks, multiple layers of the CFRP wrapping specimens are still better than one layer of the CFRP wrapped ones. It

3 can be concluded that the ratio of flexural strength of strengthening CFRP specimens decreased by almost same amount with freeze thaw attack as compared with that of UHPC ones after freeze-thaw cycle attacks. Slant Shear Strength and Failure Type. The changes of slant shear strength for the different combinations of concretes after freeze-thaw cycling up to 600 cycles or at high temperature attack are given in Table 3. The results show that an evident decrease in slant shear strength was found after freezing and thawing. For the PC/PC specimens, the average values of bond strength at 0 and 600 cycles were 11.5 and 9.9 MPa, respectively, compared with the corresponding values of 50.7 and 39.2 MPa, respectively, for UHPC/UHPC. The results also show that a high decrease in slant shear strength was found after high temperature exposure. For the PC/PC specimens, the average values of bond strength at 25 and 300 were 11.5 and 5.0 MPa, respectively, compared with the corresponding values of 50.7 and 37.4 MPa, respectively, for UHPC/UHPC. There were three different failure modes in this slant shear test. The failure type for PC/PC specimens was interface failure. The failure modes for UHPC/UHPC specimens included repair material failure and substratum failure. The failure modes for UHPC/PC and UHPC/PC specimens were interface failure or substratum failure. High Temperature Exposure Test. The changes of flexural strength of the concrete beam specimens by different repairing systems UHPC/1, UHPC/2, CFRP/1, CFRP/2, and CFRP/3 with different repair layer and thickness subjected to high temperature exposure from 200 to 400 are given in Table 4. The results indicated that the performance loss of the CFRP wrapping specimens grows with temperature, mainly due to the volatilization of the epoxy resin. A gradual decrease on the maximum load supported in bending was observed on the CFRP wrapping specimens, compared with the control specimens. A total loss of retrofit effect of CFRP composite was noticed on the 240 threshold proximately. The CFRP wrapping specimens exposed at 300 showed totally failure with the deterioration of the adhesive [3]. The performance of UHPC specimens is better than those of CFRP wrapping specimens during high temperature exposure. The UHPC/1 specimen exposed at 400 still in good shape, while the UHPC/2 specimens exposed at 400 showed totally failure. Summary The main findings from this study are summarized as follows: (1) The loss percentage of flexural strength of strengthening UHPC specimens has the almost same as compared with those of CFRP ones after 600 freeze-thaw cycles. (2) A total loss of retrofit effect of CFRP composite was noticed on the 240 threshold proximately. The CFRP wrapping specimens exposed at 300 showed totally failure with the deterioration of the adhesive. (3) The performance of UHPC specimens is better than those of CFRP wrapping specimens during high temperature exposure. The UHPC with bonding 10 mm thickness specimens exposed at 400 still in good shape. The results of slant shear tests show that the bond strength of PC/PC, UHPC/PC and UHPC/UHPC decreased significantly after 600 freeze thaw cycles or high temperature exposure. References [1] Peter H. Emmons, Concrete Repair and Maintenance Illustrated: Problem Analysis, Repair Strategy and Techniques, R. S. Means Company, Inc., Kingston, MA, (1993). [2] Lee, M. G., Chiu, C. T., and Y. C. Wang, The study for bond strength and bond durability of reactive powder concrete, ASTM International Journal, Vol. 2, No.7, (2005).

4 [3] Lee, M. G., and Cho-Liang Tsai, Fire-Damage and Freeze-Thaw of Strengthening Concrete Members Using Reactive Powder Concrete, Proceedings of the 2nd International Conference (2006), p. 97. Table 1. Mix Design of PC and UHPC (kg/m 3 ) [3] Mix Cement Stone Sand Silica Silica Crushed Steel SP Water Sand Fume Quartz Fiber PC UHPC Note: Sp means Super-plasticizer. Table 2. Comparison of Flexural Strength with its Repair Materials in Freeze-thaw (MPa) Repair Types 0 Cycle PC (control) UHPC/1 a UHPC/2 b CFRP/1 c CFRP/2 c CFRP/3 c a The dimension of specimens is 100 mm 100 mm 350 mm with bonding repair material of 10 mm thickness. b The dimension of specimens is 100 mm 100 mm 350 mm with bonding repair material of 20 mm thickness. c The dimension of specimens is 100 mm 100 mm 350 mm with bonding CFRP sheet of 1 ~ 3 layers. Table 3. Slant Shear Strength and Failure Type of UHPC in Comparison with PC in Freeze-thaw or at High Temperature Attack (MPa) Freeze-thaw High Temperature Repair Types 0 Cycle Slant Shear Strength PC/ PC Failure Type Interface Failure Slant Shear Strength Failure UHPC/PC Failure Type Substratum Failure Slant Shear Strength Failure UHPC/UHPC Failure Type Repair Material and Substratum Failure Table 4. Comparison of Flexural Strength with its Repair Materials at High Temperature (MPa) Repair Types PC (control) UHPC/1 a UHPC/2 b Failure CFRP/1 c Failure - CFRP/2 c Failure - CFRP/3 c Failure - Note: a, b and c are the same as Table 2.

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