EFFECT OF SURFACE ROUGHNESS ON THE BOND PERFORMANCE BETWEEN ULTRA-HIGH PERFORMANCE CONCRETE AND PRECAST CONCRETE IN BRIDGE DECK CONNECTIONS

Transcription

1 0 EFFECT OF SURFACE ROUGHNESS ON THE BOND PERFORMANCE BETWEEN ULTRA-HIGH PERFORMANCE CONCRETE AND PRECAST CONCRETE IN BRIDGE DECK CONNECTIONS Zhengqi Li Graduate Student Glenn Department of Civil Engineering Clemson University Lowry Hall, 0 South Palmetto Blvd, Clemson, SC zhengql@clemson.edu Tel: ()-; Prasada Rao Rangaraju, Corresponding Author Associate Professor Glenn Department of Civil Engineering Clemson University Lowry Hall, 0 South Palmetto Blvd, Clemson, SC prangar@clemson.edu Tel: ()-- Word count: (text) + Tables/Figures x 0 words (each) = words Revised Manuscript Submission date: //0

2 0 ABSTRACT This study investigated the bond performance between Ultra-High Performance Concrete (UHPC) and precast concrete using three bond test methods slant shear test, third-point flexural bond test and pull-off test methods. The influence of degree of surface roughness, prepared by sandblasting, on the bond between UHPC and precast was also studied. The test results showed that the third-point flexural bond test was a convenient and a more realistic bond test to conduct. The increase in the duration of sandblasting increased the roughness of the precast concrete surface which was quantified by the sand-spread test. Duration of sandblasting as short as seconds was enough to achieve adequate bond between UHPC and precast concrete at the age of days after casting UHPC, which was indicated by a failure in the precast concrete part. The ultimate flexural load of those specimens was about the same as the ultimate flexural load of monolithic precast concrete specimen. Specimens without any surface roughening (i.e. sandblasting) failed at the bond between UHPC and precast concrete, regardless of the type of precast concrete surface sawed surface or molded surface. The ultimate load carried by these specimens was lower than that of monolithic precast concrete specimens. Moreover, the bond strength between UHPC and precast concrete cast on un-roughened sawed surface was higher than that cast on un-roughened molded surface. Keyword: UHPC, Precast Concrete, Bond, Roughness, Flexural Bond Test

3 INTRODUCTION In recent years Ultra-High Performance Concrete (UHPC) has been increasingly adopted in Accelerated Bridge Construction (ABC) projects employing precast bridge elements. Ultra-high performance concrete (UHPC) refers to cementitious mixtures with superior workability, compressive strength (over 0 MPa), pre-and post-cracking tensile strengths (above MPa) and enhanced durability (-). Generally, UHPC is produced with very low water-cementitious materials ratio (i.e. w/c < 0.), high cementitious materials content (>00 kg/m ), fine aggregate, high-range water reducing admixture (HRWRA), and reinforcing fibers (-). Coarse aggregate is typically not incorporated in the UHPC formulations. Because of its unique properties, UHPC lends itself to be an appropriate material for the construction of shear keys in precast bridge construction. Strong and durable shear key is important for the integral performance of the bridge during its service life. Of particular importance in the performance of shear key in precast concrete bridges is the bond between the UHPC and the precast concrete. The bond strength depends not only on the inherent characteristics of the two adjoining concrete mixtures and but also on the surface conditions of the substrate concrete such as its surface texture, cleanliness (i.e. free of any surface coatings such as dust and/or laitance) and its moisture content. While the importance of bond strength is well recognized, there is no broad agreement in the industry on the specific bond test method that is most reliable or representative for any given situation. Typical test methods used to determine the bond strength between UHPC and precast concrete include slant-shear test ASTM C which evaluates the bond performance under shear (), and pull-off test ASTM C which evaluates the bond performance under direct tension (). Splitting tensile test has also been used to evaluate bond performance between two concrete mixtures under indirect tension (; ). Several studies have been conducted in the past to evaluate the test methods and the surface roughness on the bond strength. In one study, the influence of surface roughness on the bond performance between UHPC and normal strength substrate concrete was evaluated with three different bond test methods: splitting tensile test, slant-shear test and pull-off test (). The substrate concrete surface conditions included sawed, brushed, chipped, sandblasted and grooved surface (). Macro-texture depth test according to ASTM E was conducted to evaluate the degree of roughness of textured substrate concrete surface (; ). The results of this study showed that the roughness of the substrate concrete surface was not a critical factor among the different textures considered to obtain a good bond when the substrate surface was in saturated surface dry condition (). This was attributed to an assumption that a saturated substrate concrete surface helped to generate hydration products and create a high enough cohesion between UHPC and substrate concrete, as a considerable amount of un-hydrated cement presented in UHPC (). However, when the substrate surface was ambient dry, de-bonding failure mode likely occurred when the substrate surface was not roughened sufficiently (). In another study, the bond performance between UHPC and substrate concrete was studied using two different bond test methods: slant-shear test and splitting tensile test (). The conditions of the substrate concrete surface included as cast without roughening, sand blasted, wire brushed, drilled holes and grooved surface (). The results showed that the highest bond strength was achieved by using sand blasted surface, and the failure always occurred at the substrate concrete in both the bond test methods (). The other four surface conditions did not provide adequate bond as some of the specimens exhibited de-bonding failure in either of the two bond test methods, even when the substrate concrete surface was in saturated surface dry condition (). However, one of the shortcomings of this study was that it did not provide quantitative surface roughness measurement on the different

4 roughening methods (). Another shortcoming of the previous studies on evaluation of the bond strength was related to the test methods employed in the investigation. Test methods such as slant shear test, split tensile and pull-off tests represented shear or indirect or direct tensile stress conditions, which did not reflect flexural tensile stress conditions that were expected in shear key connections or in any other flexural elements typically encountered on bridge decks. The present study investigates the effect of surface roughness of the precast concrete on the bond performance between UHPC and precast concrete, using different test methods to elicit the relative importance of both degree of surface roughness and the test method employed to determine the bond behavior. In this investigation a new flexural bond strength test the third point flexural bond strength test is introduced. The experimental study was carried out in two phases. In the first phase, the bond performance between UHPC and precast concrete was studied using three different test methods: slant shear test, third-point flexural bond test and pull-off test. Third-point flexural bond test had not been extensively used for studying the bond performance between UHPC and precast concrete in previous literature. In the second phase, third-point flexural bond test was used to study the effect of surface roughness on the bond performance between UHPC and precast concrete. Sandblasting of the precast concrete unit, applied for different durations, was used to achieve different surface roughness of the precast concrete. The original surface conditions of precast concrete before sandblasting included sawed face and as cast/molded face. In this study, the degree of surface roughness was quantified using a sand-spread test. The UHPC formulation used in this study was developed using local materials in previous research studies (-). The precast concrete used in this study had -day compressive strength of MPa which was similar to the substrate concrete used in local precast bridge structures and was similar to substrate precast concretes employed in other studies discussed above (; ). EXPERIMENTAL PROGRAM Materials For preparing UHPC mixture, a Type III portland cement conforming to ASTM C0 specification was used (). The principal oxide composition of cement was as follows: CaO -.%, SiO - 0.%, Al O - %, Fe O -.%, Na O eq - 0.% and SO -.%. The specific surface area of cement was 0m /kg. A low loss-on-ignition (LOI) silica fume (SF) was used as a supplementary cementitious material. The LOI value of the SF was 0.% and the SiO content was %. The specific surface area of SF was 0,000 m /kg as determined by Brunauer Emmett Teller (BET) method using nitrogen adsorption. The steel micro fiber (SMF) used in this study was approximately mm in length and 0. mm in diameter. The specific gravity and ultimate tensile strength of SMF were. and 000 MPa, respectively. Fine aggregate used in this study was a sub-rounded natural siliceous sand meeting ASTM C gradation specification (). The percent passing values through each of the standard sieves for the fine aggregate used in this study are as follows:.-mm sieve 0%,.-mm sieve.%,.-mm sieve.%,.-mm sieve %, 00-µm sieve.%, 00-µm sieve.0%, 0-µm sieve 0.% and -µm sieve 0.%. The specific gravity, water absorption, and fineness modulus of the fine aggregate were., 0.0%, and. respectively. No coarse aggregate was used in preparing the UHPC mixture. A polycarboxylic ester based high-range water-reducing admixture (HRWRA) in a powder form was used to improve workability.

5 0 0 0 Mixture Proportions Ultra-high Performance Concrete (UHPC) Two UHPC mixtures were prepared for this study. The first mixture (UHPC ) was a non-fiber reinforced mortar with relative mass proportions of component materials as follows: w/cm at 0.0, SF/c at 0.0, s/cm at. and HRWRA/cm at The second mixture (UHPC ) was a fiber reinforced mortar prepared by adding SMF to the first mixture UHPC and re-proportioned for unit volume. The SMF content in the mixture was % by volume of the total mixture. The HRWRA/cm was 0.0 by mass. The quantities of materials used for m of UHPC mixture are presented in Table. TABLE Quantities of Materials Used for m UHPC Mixture (kg/m ) Mixture ID Constituent Cement SF Water Sand SMF HRWRA UHPC 0. UHPC Precast Concrete The precast concrete was proportioned based on typical mixture proportions employed by a local precaster for use in construction of standard highway girders. The mixture proportions for m of fresh concrete are given as follows: Type I cement - kg, fly ash 0 kg, coarse aggregate kg, fine aggregate kg, water kg and HRWRA. kg. The fly ash used in the precast concrete is a Class F fly ash with the sum of silica, alumina and iron oxide contents of 0% and lime content of.%. Specimens Preparation Preparation of Fresh UHPC Mixture In the first phase of the study, UHPC mixtures (UHPC and UHPC ) were prepared by mixing the ingredients in a 0. m Whiteman mortar mixer. A sequential mixing procedure was followed to avoid overloading the mixer at the initial stage when the UHPC mixture was very viscous. As a first step, half of the dry material including cement, SF, fine aggregate and HRWRA were mixed for to minutes. This was followed by adding half of the mixing water. As far as the mixtures had enough ability to flow, the rest of the dry materials and water were gradually introduced into the mixer. SMF was added as a last step after ensuring that the non-fiber mortar was able to flow sufficiently as observed by visual evidence in the mixer. The total process of mixing took to 0 minutes from the time of initial mixing operation. In the second phase of the study, the fresh mixture UHPC was prepared in a 0.0 m UNIVEX M0 planetary mixer. At first, the dry materials were mixed for to min at low speed (0 RPM). Then the mixing water was added to the dry mixture. The mixing continued at low speed for to min before the mixture became flowable. After the mixture became flowable, mixing continued for another to min at medium speed (00 RPM). SMF were added at the final step, and the mixing continued for min. The entire mixing process lasted for to min. Preparation of Specimens for Mechanical Properties of UHPC and Precast Concrete Specimens for evaluating the mechanical properties of UHPC and precast concrete were cast in the lab. External vibration to the molds was applied to remove any unintended entrapped air. Specimens were kept in a moist room conforming to ASTM C specification after

6 0 0 casting (). After demolding at the age of hours, specimens were stored in the moist room until testing. Preparation of Specimen for Bond Strength between UHPC and Precast Concrete For the first phase of the study, the specimens for the slant shear test, third-point flexural bond test and pull-off test were prepared. The slant shear specimen was cylindrical specimen with diameter of mm and length of 0 mm. The precast half-slant shear substrate concrete specimen was cast days before applying UHPC. Plastic inserts were placed in the molds during casting of substrate concrete to create an inclined surface that is 0 o measured from the longitudinal axis of the specimen. Subsequently, the slant surface of the precast concrete was roughened with a sand blaster for about min on the day before casting the complementary half of UHPC mixtures. The specimen for third-point flexural bond test was a composite prismatic specimen with dimensions of mm mm mm, wherein half of the specimen comprised of precast substrate concrete and the complementary half consisted of the UHPC cast against the surface of the precast section. For this purpose, precast concrete specimens of the same dimensions as the composite specimens were cast days before fabricating the composite prism using UHPC. To prepare the composite prism for the third-point flexure test, the original precast concrete prism was sawed in the middle into two half-prisms (i.e. x x.mm). The face on the half-prism to be in contact with UHPC was perpendicular to the longitudinal axis of the specimen, and it was roughened with a sand blaster for about min one day before casting UHPC. The specimen for pull-off test was prepared by placing a mm thick layer of fresh UHPC mixture on top of a precast concrete slab. The top of the precast concrete slab was roughened by sandblasting for minute. For the second phase of the study, composite prismatic beam test specimens for investigating the influence of surface roughness on the bond between UHPC and precast concrete under flexure were prepared. The precast concrete portions of the composite flexure beams were prepared by sawing longer precast prisms. Six different surface conditions on the precast concrete portion of the flexure beam were used in this study. Three out of the six surface conditions were prepared by sandblasting the sawn surface for periods of s, 0 s and 0 s. One additional control precast specimen was used in which no sandblasting was done on the sawn surface. In addition, two other surface preparations were used, where a molded surface of the precast prism (i.e. cast surface against the prismatic mold) was used in preparing the composite prismatic specimen with no sandblasting and seconds of sandblasting to roughen the surface. UHPC was cast against these surface textures to study the influence of surface roughness on the flexural bond strength of the prismatic section. The finished specimens for bond performance evaluation are shown in Figure. (a) Slant shear specimen

7 (b) Flexural bond specimen FIGURE (c) Pull-off specimen Specimens for bond tests. (Darker half of the specimen in slant-shear and flexural bond specimens is UHPC). Before casting UHPC, the bond face on the precast concrete was moistened to saturated surface-dry state. In preparing the composite specimens, UHPC was cast with gentle external vibration to remove any trapped air-voids. At hour after casting UHPC, the specimens were de-molded. The slant shear specimen and flexural bond specimens were stored in the moisture room. The pull-off specimens were stored in the lab under ambient temperature ( o C). Tap water was sprayed on the UHPC layer of the pull-off test slab periodically to keep the surface wet and cured. Test Methods Material Properties of UHPC and Precast Concrete The workability of UHPC was measured immediately after mixing using the flow method described in ASTM C (). The compressive strength, third-point flexural strength and modulus of elasticity (MOE) of UHPC were measured following methods described in ASTM C, ASTM C and ASTM C (-0), respectively. The compressive strength, third-point flexural strength and modulus of elasticity (MOE) of precast concrete were measured following methods described in ASTM C, ASTM C and ASTM C (; 0; ), respectively. Three duplicate specimens were used for evaluating each of the mechanical

8 properties at each of the selected ages. Roughness of the Sandblasted Precast Concrete Surface Sand-spread test was used to quantify the roughness of the bond face on the precast concrete half-prism. In this test, a sample of fine sand weighing gram with a particle size distribution of 0 percent passing 0-micron sieve and 0 percent retained on -micron sieve was used. The sand-spread test method is shown in Figure. To conduct the test, the sample of sand was first filled into a plastic tube with diameter of cm resting on the surface of which the texture was to be quantified. After gently lifting the plastic tube, the sand was spread evenly under circular motion of a flat-tipped steel rod until no noticeable rim of excess sand remained on the outer edges of the patch was observed. The steel rod used to spread the sand had a diameter of cm. The logic behind this test method was that a surface with a rough texture had enough hills and valleys such that a given mass of fine sand could not be spread much beyond a certain diameter during spreading. However, the same quantity of sand would spread to a much larger diameter on a smooth-textured surface. The surface roughness was quantified as a ratio expressed in percentage between the final diameter of the spread and the initial diameter (i.e. inside diameter of the plastic tube) before spreading the sand on the surface, and this value is referred to as sand-spread value. (a) Initial diameter (b) Final diameter 0 0 FIGURE Sand-spread test. Bond Strength between UHPC and Precast Concrete Slant Sear Test: To prepare slant shear specimens for testing, the composite cylinders consisting of part precast concrete and part UHPC were cut with a water saw to achieve two smooth and parallel loading surfaces. The cylinders were placed in direct contact with the steel loading plate of the tester. Polymer pads were not used during test. Three companion specimens were tested at selected ages after casting UHPC, following the procedures described in ASTM C (). Third-point Flexural Test: Three specimens of same combination of precast concrete and UHPC were loaded at selected ages after casting UHPC, following the loading procedures described in ASTM C (0). Pull-off Test: This test was conducted at selected ages after casting UHPC, following ASTM C (). Three shallow cores with diameter of mm were drilled into the slab. The

9 0 drilling bit was controlled to penetrate into the composite slab to a depth of 0 mm, such that the core specimen consisted of mm thick UHPC on the top and mm thick precast concrete at the bottom. A high strength epoxy was used to glue the aluminum disc on the top of core specimen to apply the tensile load. RESULTS AND DISCUSSIONS Bond Performance under Different Test Methods The mechanical properties of the precast concrete and UHPCs at selected ages are shown in Table. TABLE Precast UHPC UHPC Mechanical Properties of the Precast Concrete and UHPC at Selected Ages -day -day -day Properties Average Cov Average Cov Average Cov (MPa) (%) (MPa) (%) (MPa) (%) Compressive strength Flexural strength MOE Compressive strength Flexural strength MOE Compressive strength Flexural strength MOE As shown in Table, the compressive strength of precast concrete at the ages of days and days were very close at. MPa and. MPa, respectively. The average compressive strength at the ages of days was considered the ultimate compressive strength of precast concrete. Similarly, the average flexural strength of precast concrete at the ages of days and days were. MPa and. MPa, respectively. UHPC exhibited very high compressive strength and post-crack flexural strength of. MPa and. MPa, respectively. By comparing the mechanical properties of UHPC and UHPC, it was recognized that the use of SMF significantly improved the compressive strength and post-crack flexural strength of UHPC mixtures at the age of days. This was attributed to the crack arresting effect of SMF in the hardened UHPC mixture. However, the use of SMF did not have significant effect on the MOE of UHPC. It should be noted that both of the UHPC mixtures exhibited flow value of 0% which was the highest flow value that could be captured by the test method described in ASTM C (), indicating good workability. The test results of the three bond test methods are shown in Table.

10 0 0 TABLE Ultimate Load and Failure Mode of Bond Tests at Selected Ages after Casting UHPC Test Mixture -day -day method ID Failure mode a Failure mode a Slant shear Flexural bond Pull-off Average (kn) COV (%) Average (kn) COV (%) UHPC.0. Precast.. Precast+UHPC UHPC.. De-bonding or precast.. Precast UHPC Precast UHPC Precast UHPC.. Precast.. Precast UHPC.. Precast.. Precast Note: a Precast failure in precast concrete; De-bonding or precast de-bonding failure or failure in precast concrete; Precast+UHPC failure in precast or both precast concrete and UHPC As shown in Table, the basic failure modes observed during the slant shear test were classified as (i) failure in precast concrete, (ii) failure at the bond and (iii) failure in both precast concrete and UHPC, based on the location of the main cracks. The failure modes are shown in Figure. The dashed line indicates the location of the interface between UHPC and precast concrete. The most frequent failure mode that occurred was the failure in precast concrete. In this failure mode, the main cracks occurred in the precast concrete portion, although some minor cracks at the bond between precast concrete and UHPC and propagating into UHPC portion were observed. However the UHPC portion and precast concrete portion still had strong bond after testing was completed. Another failure mode that occurred was de-bonding at the interface between precast concrete and UHPC. In this failure mode, minor cracks or no cracks occurred in either the precast or the UHPC portion of the specimen. The last failure mode was the failure in precast concrete or both precast concrete and UHPC. In this failure mode, the cracks initiated first in the precast concrete portion and then penetrated into the UHPC portion when the specimen failed. No cracks causing de-bonding of the UHPC and precast concrete were observed. All the specimens exhibited adequate bond between precast concrete and UHPC under shear stress, except one out of the three specimens made with UHPC failed at the bond at days after casting UHPC. Moreover, failure in the UHPC part was also observed for specimens made with UHPC when failure happened at the age of days, though the UHPC and precast still maintained good bond. As discussed earlier, the interface in the slant shear test is under a combined influence of compressive stress and shear stress (; ). The observed phenomenon in this study also indicated that the stress condition on the interface of slant shear specimen was complicated, which would cause inconsistent failure modes in the test results.

11 (a) Failure in precast concrete (b) De-bonding (c) Failure in both precast concrete and UHPC FIGURE Failure modes of slant shear test. The only failure mode observed during third-point flexural bond test was failure in precast concrete, which indicated that both UHPC and UHPC had adequate bond with the precast concrete under flexure, with failure loads of. MPa and. MPa for UHPC and UHPC, respectively. This is about the same as the -day flexural strength of monolithic precast concrete. Similar to the third-point flexural bond test, the only failure mode observed during the pull-off test was failure in precast concrete, which indicated that both UHPC and UHPC had adequate bond with the precast concrete under direct tension. However, it was observed that although the failure surface was within the precast concrete, the pull-off failure load was

12 0 0 0 higher in specimens bonded to UHPC than in specimens bonded to UHPC at both and -days. The reason for this trend was not clear and needed further investigation. Considering the consistence and the ease of test, third-point flexural bond test was used to study the influence of surface roughness on the bond behavior between mixture UHPC and precast concrete. Mixture proportion of UHPC was used for the second part of this study, as UHPC exhibited -day compressive strength of. MPa which fell into the typical range of compressive strength of UHPC. Influence of Surface Roughness on the Bond Performance The third-point flexural bond tests were conducted at the age of days after casting UHPC against the precast portions of the specimens that were subjected to different surface treatments. The compressive strength and post-crack flexural strength of UHPC were. MPa and.0 MPa, respectively. The flexural strength of precast concrete was. MPa. The test results of the influence of substrate surface roughness on the flexural bond behavior between UHPC and precast concrete are shown in Table. TABLE Influence of Surface Roughness on the Bond Performance between UHPC and Precast Concrete Original SandBlasting Sand-spread Ultimate Ultimate Cov Failure surface duration (s) (%) load (kn) stress (MPa) (%) mode Bond Sawed... Precast Precast Precast Molded 0... Bond 0... Precast As shown in Table, the sand-spread value decreased with the increase in the roughening duration, which indicated an increase in the surface roughness. For sawed surface, the sand-spread value of surface roughened for s, 0 s and 0 s were %, % and % lower than that of sand-spread values of surface without roughening, respectively. For molded surface, the sand-spread value of surface roughened for s was % lower than that of surface without roughening. Two failure modes were observed in the third-point flexure test. Failure in the precast concrete and failure at the bond/interface between the precast and UHPC sections (see Figure ). Failure at bond only occurred when no sandblasting was applied, regardless of sawed surface or molded surface. The failure stress when de-bonding occurred was lower than the flexural strength of monolithic precast concrete. It was also noted that the bond strength obtained from using molded face of the precast concrete specimens was weaker than that obtained when sawed face precast specimens were used. The possible reason for higher bond strength of UHPC with the sawed surface of the precast concrete was due to the bond of UHPC with the exposed aggregate particles along with the cementitious mortar in the cross-section. However, in the case of bond of UHPC with the molded surface of the precast concrete, the bond was entirely between the UHPC and the cementitious mortar of the precast concrete at the interface. As a result, it was likely that the sawed surface yielded better bond strength than the molded surface with the UHPC, when no surface roughening was provided. However, all the flexural bond specimens with surface roughening (i.e. sandblasting) on either sawed or molded specimens failed in the precast concrete part. The failure stress when failure occurred was about the same as the flexural strength of monolithic precast

13 concrete. This indicated adequate bond between the UHPC and precast concrete. It was determined that seconds of roughening (sand-spread value of % for sawed surface and 0% for molded surface) was good enough to achieve adequate bond strength between UHPC and precast concrete. It should also be noted that with increasing duration of roughening there was an increase in the COV of the ultimate stress at failure load. This trend suggested limiting the roughening to a minimal time possible, as extended duration of roughening might induce micro-cracking in the precast concrete due to the sandblasting operation. (a) Failure in precast (b) Failure at bond-sawed without sandblasting (c) Failure at bond-molded without sandblasting FIGURE Failure mode of flexural bond test. DISCUSSION OF THE BOND TEST METHODS AND RESULTS The widely used test methods for evaluating bond performance between UHPC and precast concrete include slant-shear test, splitting tensile test and pull-off test, and these tests evaluate

14 the bond performance between the two materials under shear, indirect tension and direct tension, respectively (; ). In this study, slant-shear test, pull-off test and third-point flexural test were used to evaluate the bond performance between the two materials. Splitting tensile test was not used. Third-point flexural test evaluates the bond performance under flexural tension. This test method is new, as it has not been used for studying the bond performance between UHPC and precast concrete in previous literature. One of the problems with the slant-shear test is that the interface between UHPC and precast concrete is subjected to a combined influence of compressive stress and shear stress (; ). The complicated stress condition causes inconsistency in the test results. As observed in this study, even within the same group of specimens, different failure modes have been observed in the slant shear test. Specifically, two failure modes de-bonding failure and failure in precast concrete - were observed between UHPC and precast concrete at the age of days, and two failure modes - failure in UHPC and failure in precast concrete - were observed between UHPC and precast concrete at the age of days. Such inconsistence in the test results gives confusing information. The problem with the pull-off test is the difficulty of conducting the test including: the need for a large scale precast concrete slab to be cast; the effect of drilling on formation of any micro-cracking; strong bonding materials (usually ultra-strong epoxy) is needed to provide strong bond between the metal disc and the concrete specimen (see Figure c), and such bonding materials are costly; and all the preparation work needs to be done one day before the test as the bonding materials require certain curing time. Using splitting tensile strength test to determine the bond strength has been employed in some of the past research studies (; ). However, this test method is highly sensitive to operator-induced error and having a perfect orientation of the interface with the loading direction is very essential to obtain the true bond strength between the UHPC and precast concrete. In contrast, the third-point flexural bond strength test is much more resilient due to the fact that the region of the beam which contains the interface is subjected to a constant moment and therefore the failure mode is not dependent on the exact location of the interface within this constant moment region. Third-point flexural bond test is a convenient and reliable test method to investigate the effect of substrate surface roughness on the bond performance between UHPC and precast concrete. The test setup is easy. No special and costly device or materials are needed. The test results show that this method can give consistent results. As seen in Table, only one failure mode is observed within the same group of specimens. Also, the influence of surface roughness on the bond strength can be easily studied using the flexural bond test. (; ; ) CONCLUSIONS In this study slant shear test, third-point flexural bond test and pull-off test were used in evaluating the bond behavior between UHPC and precast concrete. The influence of the roughness of the surface on the bond behavior between UHPC and precast concrete was also investigated using third-point flexural bond test. Based on the materials and test method used, the following conclusions are drawn: Third-point flexural bond test was an easy test to conduct and determine the bond behavior between UHPC and precast concrete. This test method yielded results that were in agreement with other test methods. The increase in the sandblasting duration resulted in an increase in the roughness of the surface of precast concrete, and the roughness was easily quantified by using the sand-spread test. All the flexural bond specimens with the surface roughened by sandblasting failed in

15 the precast concrete part at the age of days after casting UHPC. The ultimate load of those specimens was about the same as the ultimate load of monolithic precast concrete specimen. This indicated that sandblasting was effective to achieve adequate bond behavior between UHPC and precast concrete. In this study, it was observed that even with s of roughening resulted in adequate roughness (i.e. spread value of % for sawed surface and 0% for molded surface) that produced adequate bond between UHPC and precast concrete. Flexural bond specimens with no surface roughening failed at the bond between UHPC and precast concrete with bond strength of. MPa for sawed face of precast concrete and bond strength of. MPa for molded face of precast concrete, at the age of days after casting UHPC. The ultimate load of those specimens was significantly lower than that of monolithic precast concrete specimen, indicating the failure was of the bond between the UHPC and the precast concrete. It was also determined that the bond strength based on un-roughened molded face of precast concrete was weaker than that based on un-roughened sawed face of precast concrete. REFERENCES. Russell, H. G., and B. A. Graybeal. Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community. FHWA-HRT--00, 0.. Wille, K., A. E. Naaman, and G. J. Parra-Montesinos. Ultra-High Performance Concrete with Compressive Strength Exceeding 0 MPa ( ksi): A Simpler Way. ACI Materials Journal, Vol., No., 0, pp. -.. Graybeal, B. A. Material property characterization of ultra-high performance concrete. Report FHWA-HRT-0-, FHWA, U.S. Department of Transportation, 00.. Wille, K., and C. Boisvert-Cotulio. Development of Non-Proprietary Ultra-High Performance Concrete for Use in the Highway Bridge Sector. FHWA-HRT--0, 0.. Wille, K., A. E. Naaman, S. El-Tawil, and G. J. Parra-Montesinos. Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing. Materials and Structures, Vol., No., 0, pp ASTM. C Standard Test Method for Bond Strength of Epoxy-Resin Systems Used With Concrete By Slant Shear, ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method).In, ASTM International, West Conshohocken, PA, 0.. Munoz, M. A. C., D. K. Harris, T. M. Ahlborn, and D. C. Froster. Bond Performance between Ultrahigh-Performance Concrete and Normal-Strength Concrete. Journal of Materials in Civil Engineering, Vol., No., 0.. Tayeh, B. A., B. H. Abu Bakar, M. A. M. Johari, and Y. L. Voo. Evaluation of Bond Strength between Normal Concrete Substrate and Ultra High Performance Fiber Concrete as a Repair Material. nd International Conference on Rehabilitation and Maintenance in Civil Engineering (ICRMCE), Vol., 0, pp. -.. ASTM. E Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique., ASTM International, West Conshohocken, PA, 0.. Li, Z., H. Kizhakommudom, P. R. Rangaraju, and S. D. Schiff. Development of ultra-high performance concrete for shear-keys in precast bridges using locally available materials in South Carolina. 0 0th Anniversary Convention and National Bridge Conference, Precast/Prestressed Concrete Institute, Washington D.C, 0.. Rangaraju, P. R., H. Kizhakommudom, Z. Li, and S. D. Schiff. Development of High-Strength/High Performance Concrete/Grout Mixtures for Application in Shear Keys in Precast Bridges. FHWA-SC--0a, FHWA, U.S. Department of Transportation, 0.

16 0. Li, Z., and P. R. Rangaraju. Effect of Sand Content on the Properties of Self-Consolidating High Performance Cementitious Mortar. Transportation Research Record: Journal of the Transportation Research Board, Vol. 0, 0, p... ASTM. C0 Standard Specification for Portland Cement. ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard Specification for Concrete Aggregates. ASTM International West Conshohocken, PA, 0.. ASTM. C Standard Specification for Mixing Rooms, Moist Cabinets, Moist Rooms, and Water Storage Tanks Used in the Testing of Hydraulic Cements and Concretes. ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard Test Method for Flow of Hydraulic Cement Mortar. ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard Test Method for Static Modulus of Elasticity and Poisson s Ratio of Concrete in Compression. ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard test method for compressive strength of hydraulic cement mortars (Using -in. or [0-mm] cube specimens).in, ASTM International, West Conshohocken, PA, ASTM. C Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading).In, ASTM International, West Conshohocken, PA, 0.. ASTM. C Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. ASTM International, West Conshohocken, PA, 0.. Wall, J. S., and N. G. Shrive. Factors Affecting Bond between New and Old Concrete. ACI Materials Journal, Vol., No.,, pp. -.. Momayez, A., M. R. Ehsani, A. A. Ramezanianpour, and H. Rajaie. Comparison of methods for evaluating bond strength between concrete substrate and repair materials. Cement and Concrete Research, Vol., No., 00, pp. -.