The uniqueness of Ultra-High Performance Fiber Concrete (UHPFC) is its extremely low porosity gives its low permeability and high durability, making it potentially suitable for rehabilitation and retrofitting reinforced concrete structures or for use as a new construction material. This experimental study was performed to assess the bond strength between UHPFC as a repair material and Normal Concrete (NC) substrate as an old material; split tensile strength and slant shear tests were performed to quantify the bond strength in indirect tension and shear respectively, also the correlation between split tensile strength and slant shear were studied. The result showed that UHPFC has been cured by steam, gives high bond strength at the early age of the repair process, and interacts well with the surface of NC, as a result the failure occurred mostly in the NC substrate. A good correlation between the slant shear test results and the split tensile test results has been observed

Abu Bakar, Badorul H

Johari, MA Megat

Tayeh, Bassam A.

English

Institutional Repository of the Islamic University of Gaza

1035Concrete Repair, Rehabilitation and Retrofi tting III – Alexander et al. (eds)© 2012 Taylor & Francis Group, London, ISBN 978-0-415-89952-9Mechanical properties of old concrete—UHPFC interfaceBassam A. TayehSchool of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, Pulau Pinang, MalaysiaEngineering Division, Islamic University of Gaza, Gaza, PalestineB.H. Abu Bakar & M.A. Megat JohariSchool of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, Pulau Pinang, MalaysiaABSTRACT: This investigation was aimed at assessing the bond strength between Ultra-High Perform-ance Fiber Concrete (UHPFC) as a repair material and Normal Concrete (NC) substrate as a mature concrete in existing structures. Splitting tensile strength and slant shear tests were performed to quantify the bond strength in indirect tension and shear, respectively. The correlation between splitting tensile strength and slant shear was assessed. It is found that UHPFC gives high bond strength at the early age of the repair process, and interacts strongly with the surface of the substrate; as a result, the failure occurred mostly in the substrate. A good correlation was obtained between the slant shear strength and the splitting tensile strength.In the field of rehabilitation and strengthening, the bond strength between the new repair material and old concrete generally presents a weak link in the repaired structures (Momayez, Ehsani, Ram-ezanianpour, & Rajaie, 2005). Good bond is one of the main requirements for successful repair (Gorst & Clark, 2003). UHPFC could be used as a repair material as it has strong mechanical bond, which is formed between the UHPFC as an overlay mate-rial and the substrate material (Sarkar, 2010) and (Harris, Sarkar, & Ahlborn, 2011).The main purpose of this paper to evaluate the bond strength between UHPFC as the repair mate-rial and normal concrete (NC) substrate as old concrete using the splitting tensile test and slant shear test to quantify the bond strength in indirect tension and shear, respectively, also to examine if  there is a correlation between the splitting tensile strength and slant shear strength.2 EXPERIMENTAL PROGRAMME2.1 Normal concrete substrate and UHPFC propertiesThe mix design of the NC used in this study ensures an average characteristic compressive strength of 45 MPa is achieved at 28 days. The NC used contains Type-I ordinary Portland cement, river sand with fineness modulus of 2.4, coarse aggre-gate (granite) with a maximum size of 12.5 mm, a 1 INTRODUCTIONIn this modern era, a highly developed infrastruc-ture is vital for economic growth and prosperity of mankind. In many nations throughout the world, a lot of structures that constitute the whole infra-structures, especially those made of reinforced concrete, have suffered severe degradation which for some cases commenced shortly after construc-tion due to the combined effects of aggressive envi-ronments such as freeze–thaw cycles, deicing salts, marine exposure and significantly increasing live loads. Hence, some of the most important chal-lenges faced by today’s civil engineers are to save, retrofit and maintain these deteriorated struc-tures. Implementation and development of new, efficient, as well cost-effective rehabilitation and repair methods are required to extend the useful service life of the deteriorated structures (Green, Bisby, Beaudoin, & Labossière, 2000).The extremely low permeability of  UHPFC which concurs with their outstanding mechanical properties supports the main idea of  using UHPFC to rehabilitate and strengthen the zones where the concrete structures are exposed to high mechanical loading and severe environmental exposures. All other parts of  the structures remain in normal structural concrete as these parts are subjected to relatively reasonable exposure (Brühwiler & Denarié, 2008; Brühwiler, Denarié, & Habel, 2005).ICCRRR3_III.indb   1035 7/20/2012   3:33:07 PM1036water-to-cement ratio of 0.5 and a slump value of between 150–180 mm. The mix proportions of the NC substrate are presented in Table 1. The control specimens used consists of (i) 100 mm diameter by 200 mm high cylinder for the splitting indirect tensile strength test and (ii) 100 mm × 100 mm × 300 mm tall prism for uniaxial compressive strength test. The NC was tested for 28 days strength and experi-mental results showed that the NC has an average splitting tensile strength and compressive strength of 2.75 MPa and 38 MPa, respectively.The mix proportions of the UHPFC which was used as a repair material are given in Table 2. The UHPFC contains Type-I Portland cement, densi-fied silica fume, well graded sieved and dried mining sand, high strength micro-steel fiber and polycar-boxylate ether based (PCE) superplasticizer. The steel fiber used has a fiber length and fiber diameter of 10 mm and 0.2 mm, respectively, and the steel fiber has ultimate tensile strength of 2500 MPa. The UHPFC achieved an average 28 days cube compres-sive strength, fcu =170 MPa.2.2 Specimens preparationEach of the tested specimens comprised of two dif-ferent materials, being the NC as a substrate and UHPFC as a repair material. The fresh NC was covered and left to set in the respective moulds for 24 hours after casting. After 24 hours the NC spec-imens were demoulded and were cleaned and cured for another two days in a water curing tank. At the age of three days, the NC substrate specimens were taken out from the water tank for surface prepara-tion. In this study, the experimental parameter is the surface texture of the substrate. Five different types of surfaces were prepared, which are (i) as cast, without roughening (AC), (ii) sand blasted (SB), (iii) wire brushed (WB), (iv) drilled holes (DR) and (v) grooved (GR).Figure 1 shows the different surfaces of the NC substrate specimens after undergoing surface prep-aration. These specimens represent the first-half  of the substrate for the slant shear test. Prior to cast-ing the UHPFC onto this roughened NC surface, the NC specimens were further cured in a water tank until the age of 28 days since the casting date. At the age of 28 days, the NC substrate specimens were left to dry for two months.Before casting the UHPFC, the surfaces of the NC substrate specimens were moistened for 10 minutes and wiped dry with a damped cloth. The NC substrate specimens were then placed into steel moulds with the slant side facing upward. Mixing of the UHPFC was carrying out using a pan type mixer. The moulds were then filled with the UHPFC.Figure 2 shows the complete composite speci-mens for the split cylinder strength test and slant shear strength test. The composite specimens were steam cured for 48 hours at a temperature of 90°C. After the steam curing, it was cured in a water tank. The splitting tensile and slant shear tests were performed on the 3rd and 7th day after cast-ing the composite specimens.2.3 Splitting tensile testThe splitting tensile test, as an indirect tension test, was conducted to determine the bond strength between the NC substrate and UHPFC, according to ASTM C496. In this test procedure, UHPFC was cast and bonded to the NC substrate speci-mens to form a composite cylinder (100 mm diam-eter and 200 mm height) as shown in Figure 2a.Table 1. NC substrate mix design.Item Mass (kg/m3) RemarkOPC 400 Type ICoarse Aggregate 930 Max 12.5 mmFine Aggregate 873Water 200Superplasticizer 4Table 2. UHPFC mixture proportions.Item Mass (kg/m3)OPC 768Silica Fume 192Sieved Sand 1140Micro-Steel Fiber 157Superplasticizer 40Free Water 144 Figure 1. Slant shear test specimens with the different surface textures.ICCRRR3_III.indb   1036 7/20/2012   3:33:07 PM1037where T is the splitting tensile strength (in MPa); P is the maximum experimental force (in kN); and A is the total area of the bonded plane (in mm2) which is taken as 200 mm × 100 mm = 20,000 mm2.The splitting tensile strength test results are summarized in Table 3. As shown in Figure 3, the failure modes generally can be categorized into three types; Type A is a pure interfacial failure; Type B is an interfacial failure with partial sub-strate failure and Type C is substratum failure. The test results show that the bond strength result from UHPFC generally was very good, since most fail-ure occurred in the NC substrate.According to Sprinkel & Ozyildirim ( 2000), the interfacial bond strength test results may be classi-fied as follows (Table 3):As shown in Table 4, all the surfaced prepared specimens fall in the category of “excellent bond,” where the average splitting tensile strength (Tav) was the highest for the samples with sand-blasted surface preparation. The samples with as cast (AC) surface exhibit the lowest bond strength among the test where its average splitting tensile strength was 1.67 and 1.82 MPa at 3rd and 7th days, respectively.3.2 Slant shear testWhere the interface is subjected to the shear stress or the combination of shear stress and compression forces, the slant shear test is the most appropriate test for such bond assessments. Over the year, this test method has become the most widely accepted method and has been adopted by a number of inter-national codes. The bond strength based on the slant shear test was calculated by dividing the maximum load by the bond area which can be expressed as:S PAL=  (2)Where S is the bond strength (in MPa); P is the maximum force recorded (in kN), and AL is the area of the slant surface (in mm2). In this case, the slant surface area is taken as 100 × 100 /sin 30° = 20,000 mm2.Figure 2. (a) Split cylinder specimen and (b) Slant shear test specimen.Table 3. Classification of bond strength.Bond QualityBond Strength(MPa)Excellent ≥ 2.1Very Good 1.7–2.1Good 1.4–1.7Fair 0.7–1.4Poor 0–0.72.4 Slant shear testThe slant shear test was used in this study to determine the bond strength between the NC substrate and the UHPFC, according to ASTM C 882. Following this test procedure, UHPFC was cast and bonded to the NC substrate speci-mens on a slant plane inclined vertically at a 30° angle to form composite prism specimens (100 mm × 100 mm × 300 mm) (Figure 2a.).3 DISCUSSION OF RESULTS3.1 Split tensile testThe splitting tensile test provides a measure of the indirect tensile capacity of the composite interface. The splitting tensile strength was calculated using the following equation:T PA=2π  (1)ICCRRR3_III.indb   1037 7/20/2012   3:33:12 PM1038The experimental slant shear strength test results are presented in Table 5. Figure 4 shows the failure modes for the slant shear specimens which can be categorized into four types, that Type A is the inter-facial bond failure; Type B is the interfacial failure and substrate cracks or small parts broken; Type C is the interfacial failure and substrate fracture, and Type D is the substratum failure. As shown in Table 5, the average slant shear test was the highest in samples with sand blasted substrate Sav = 17.18 and 17.17 MPa at 3rd and 7th days, respectively.The ACI Concrete Repair Guide specifies the acceptable bond strength for repair work shall be within the ranges of 6.9–12 MPa for slant shear strength at test ages 7 days (Chynoweth et al., 1996). This guideline is particularly useful for the selection of appropriate repair material. The test results surpass the required bond strength at 7 days, specified by this ACI guideline due to the higher bond between the NC and UHPFC which has been fully cured via accelerating steam cur-ing method. Comparison shows that most of the slant shear strengths at 3rd and 7th days are within or greater the minimum range stipulated the ACI requirement at 7th day.3.3 Correlation between the slant shear strength and splitting tensile strengthFrom Figure 5 and Figure 6, a good correlation between the slant shear test results and the split-ting tensile test results could be observed, since the relationship between the slant shear strength and the splitting tensile strength is strongly correlated Table 4. Split tensile strength and failure type of composite NC and UHPFC specimens.Surface preparationSampleno.Max. Force TAverage Tav S.D. Failure ModeMax.Force TAverageTav S.D. Failure Mode(kN) (MPa) (MPa) (MPa) (kN) (MPa) (MPa) (MPa) Test results at 3 days Test results at 7 daysAs cast surfaceAC1 56.95 1.81 A 58.756 1.87 AAC2 48.714 1.55 1.67 0.13 A 61.28 1.95 1.82 0.16 BAC3 52.057 1.66 [7.88] A 51.796 1.65 [8.57] AGrooved surfaceG1 96.43 3.07 C 98.67 3.14 CG2 85.70 2.73 3.03 0.29 C 90.074 2.87 3.14 0.26 CG3 103.75 3.30 [9.53] C 106.61 3.40 [8.40] CDrill holes surfaceDH1 78.13 2.49 B 73.875 2.35 CDH2 88.44 2.82 2.46 0.37 B 60.906 1.94 2.28 0.31 BDH3 65.05 2.07 [15.18] B 79.875 2.54 [13.55] CSandblast surface SB1 105.28 3.35 C 103.88 3.31 C SB2 120.59 3.84 3.55 0.26 C 130.5 4.16 3.53 0.55 C SB3 108.49 3.46 [7.25] C 98.371 3.13 [15.49] CWire brush surfaceWB1 70.60 2.25 C 62.5 1.99 BWB2 66.02 2.10 2.32 0.27 B 81.324 2.59 2.33 0.31 CWB3 82.251 2.62 [11.47] C 76.029 2.42 [13.25] CT = Splitting tensile strength; S.D. = standard deviation; [ ] = Coefficient of variationFigure 3. Types of split tensile failure.ICCRRR3_III.indb   1038 7/20/2012   3:33:16 PM1039by a linear relationship at all test ages, where R2 = 0.9742 and 0.9625 for testing ages of 3 and 7 days, respectively.4 CONCLUSIONSThis paper reports the experimental results of the bond behaviour between normal concrete (NC) which is the substrate and UHPFC as a repair material using the slant shear test and splitting test to quantify the bond strength in shear and indirect tension, respectively. In the study, the NC and UHPFC used achieved 28 day compressive strength of 45 MPa and 170 MPa, respectively. The experimental parameter was the surface texture of the substrate. Five different types of surface prep-arations were used which are; (i) as cast without roughening, (ii) sand blasted, (iii) wire brushed, drilled holes and grooved surfaces. The following conclusions are drawn the experimental study.Table 5. Slant shear strength and failure type of composite NC and UHPFC specimens.Surface treatmentSample no.Max. Force(S) Shear sterssAverage shear stress S.D.Failure ModeMax.Force(S) Shear sterss)Average shear stress S.D.Failure Mode(kN) (MPa) (MPa) (MPa) (kN) (MPa (MPa) (MPa) Test results at 3 days Test results at 7 daysAs cast surfaceAC1AC2AC3193.6157.1140.279.687.867.018.18 1.36[16.66]BBA144.1170.3193.77.218.529.698.47 1.24[14.65]ABBGrooved surfaceG1G2G3284.91294.06253.3014.2514.7012.6713.87 1.07[7.71]CDC296.69292.22244.5214.8314.6112.2313.89 1.45[10.41]CCCDrill holes surfaceDH1DH2DH3235.68194.38231.8011.789.7211.5911.03 1.14[10.34]CBC241.94202.34221.8312.1010.1211.0911.10 0.99[8.92]CBBSandblast surfaceSB1SB2SB3335.91351.13236.9116.8017.5611.8517.18 0.54[3.13]DDD322.66370.06337.6116.1318.5016.8817.17 1.21[7.06]DDDWire brushsurfaceWB1WB2WB3225.94246.38213.8711.3012.3210.6911.44 0.82[7.18]CCB209.72263.34225.9110.4913.1711.3011.65 1.38[11.81]BCCS.D. = Standard deviation, [ ] = Coefficient of variation, A = Interface failure, B = Interface failure and substrate cracks,C = Interface failure and substrate fracture, D = Substratum failureFigure 4. Types of slant shear failure.ICCRRR3_III.indb   1039 7/20/2012   3:33:20 PM1040i. The result of the split cylinder test shows that the UHPFC overlays have “excellent bond quality” with the NC substrates as per the quantitative requirement. Since most of the failure modes in the split cylinder test wre through the NC sub-strate specimen, this indicates the bond strength between UHPFC and NC substrate is stronger than the cracking strength of the NC substrate.ii. The results of the slant shear test show that the bond strength was very high since the interface failure occurred after the NC substrate exhibited damage. In some cases, the failure occurred only in the NC substrate and no separation between the NC substrate and the UHPFC which indicates the superior bond behaviour of the UHPFC.iii. The results showed that there is a good correla-tion between the slant shear test results and the splitting tensile test results.iv. Without sufficient statistical supporting evi-dence on the bond behaviour and types of surface preparation of the NC substrate, it is recommended (for the time being) that all the NC substrate surfaces shall be sand blasted prior to overlaying the UHPFC as a repair material.ACKNOWLEDGMENTSThe authors gratefully acknowledge the Universiti Sains Malaysia for providing the financial support for this study.REFERENCESBrühwiler, E., Denarié, E. & Habel, K. (2005). Ultra High Performance Fibre Reinforced Concrete for Advanced Rehabilitation of Bridges.Brühwiler, E., & Denarié, E. (2008). Rehabilitation of concrete structures using Ultra-High Performance Fibre Reinforced Concrete. The Second International Symposium on Ultra High Performance Concrete, Kas-sel, Germany.Chynoweth, G., Stankie, R.R., Allen, W.L., Anderson, R.R., Babcock, W.N. & Barlow, P., et al. (1996). Con-crete Repair Guide. ACI Committee, Concrete Repair Manual, 546, 287–327.Gorst, N. & Clark, L. (2003). Effects of thaumasite on bond strength of reinforcement in concrete. Cement and Concrete Composites, 25(8), 1089–1094.Green, M.F., Bisby, L.A., Beaudoin, Y. & Labossière, P. (2000). Effect of freeze-thaw cycles on the bond dura-bility between fibre reinforced polymer plate rein-forcement and concrete. Canadian Journal of Civil Engineering, 27(5), 949–959.Harris, D.K., Sarkar, J. & Ahlborn, T.M. (2011). Interface Bond Characterization of Ultra-High-Performance Concrete Overlays. The Transportation Research Board 90th Annual Meeting.Momayez, A., Ehsani, M., Ramezanianpour, A. & Rajaie, H. (2005). Comparison of methods for evalu-ating bond strength between concrete substrate and repair materials. Cement and Concrete Research, 35(4), 748–757.Sarkar, J. (2010). Characterization of the bond strength between ultra high performance concrete bridge deck overlays and concrete substrates: Master of Science in Civil Engineering, Michigan Technological University.Sprinkel, M.M. & Ozyildirim, C. (2000). Evaluation of high performance concrete overlays placed on Route 60 over Lynnhaven Inlet in Virginia.Figure 5. Correlation between slant shear test and split tensile test results at 3 days.Figure 6. Correlation between slant shear test and split tensile test results at 7 days.ICCRRR3_III.indb   1040 7/20/2012   3:33:27 PM

Mechanical properties of old concrete—UHPFC interface

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Mechanical properties of old concrete—UHPFC interface

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