Interfacial bond properties between normal strength concrete substrate and ultra-high performance concrete as a repair material

Construction and Building Materials 235 (2020) 117431

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Interfacial bond properties between normal strength concrete substrate and ultra-high performance concrete as a repair material Yang Zhang a, Ping Zhu a,b,⇑, Zhaoqian Liao a, Lianhua Wang a a b

Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, China

h i g h l i g h t s
The interfacial bond performance of the UHPC-NSC is sufficient for rehabilitation of concrete structures.
Proper surface treatment and moisture degree are the major factor for the bond capacity of a UHPC-to-NSC interface.
The bond strength of the UHPC-NSC interface can be evaluated by using the cohesion and friction coefficient.

a r t i c l e

i n f o

Article history: Received 18 February 2019 Received in revised form 28 October 2019 Accepted 31 October 2019

Keywords: Ultra-high performance concrete (UHPC) Normal strength concrete (NSC) Slant shear test Splitting tensile test Direct tensile test Bond strength Bonding enhancement mechanism Friction coefficient

a b s t r a c t Ultra-high performance concrete (UHPC) is suitable for the durable rehabilitation and strengthening of deteriorated normal strength concrete (NSC) structures due to its excellent characteristics, such as superior compressive strength, high tensile capacity, and extremely low permeability. However, a successful repair depends on whether the UHPC-NSC interface can provide good bonding performance under various applied loads and working conditions throughout its service life. In this study, the bond characteristics between the NSC substrate and UHPC layer were investigated by applying slant shear, splitting tensile, and direct tensile tests, and the interfacial bond strength and corresponding failure modes were observed. Seven studied factors, the roughness of substrate surface, UHPC age, moisture degree of substrate, curing condition of UHPC, strength of NSC substrate, bonding agent and expansive agent were included to explore their influences on the bond strengths, and the UHPC-NSC interfacial bonding enhancement mechanism. Results indicated that the UHPC overlay achieved superior interfacial bond performance for the rehabilitation of concrete structures, with the appropriate surface roughness and substrate wetness. The UHPC-NSC composite samples met the minimum requirements of the interfacial bond strengths specified in ACI 546-06 and achieved an ‘‘excellent” bonding quality. Furthermore, the friction coefficient for calculating interfacial bond strength was back-calculated according to the results obtained from the corresponding direct tensile and slant shear tests. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Early and severe damage of reinforced concrete structures under severe environmental conditions and high mechanical loading is a problem worldwide, which could affect public safety and increase the repair cost. Therefore, implementing efficient, durable, and cost-effective repairs is a challenge to civil engineers in extending the useful service life of damaged concrete structures [1]. To this end, some novel repair materials for concrete structures have been developed and applied, such as ultra-high performance ⇑ Corresponding author at: Key Laboratory for Wind and Bridge Engineering of Hunan Province, College of Civil Engineering, Hunan University, Changsha, China. E-mail address: [email protected] (P. Zhu). https://doi.org/10.1016/j.conbuildmat.2019.117431 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

concrete (UHPC), geopolymer and alkali-activated binder, etc [2– 4]. UHPC, a recently discovered advanced cementitious composite material, is usually made up of Portland cement, ground quartz, fine sand, water reducer, steel fibers, and accelerating admixtures [2], and offers superior material properties, such as high strength, significant tensile toughness, exceptional durability, and minimal long-term creep or shrinkage [2,5]. These excellent mechanical properties enable UHPC to be particularly suitable as an overlay for the rehabilitation of reinforced concrete structures [6]. An adequately strong and durable bonding at the interface is important for ensuring the monolithic behavior of UHPC-NSC composite members. Thus, the accurate assessments of bonding performance and interaction between UHPC and NSC substrate during mechanical loading are necessary for an effective and successful repair [7].

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Several research efforts have been devoted to evaluating the bond capability of the UHPC-NSC interface. Carbonell et al. [8] applied slant shear, splitting tensile, and put-off tests to explore the bond characteristics between the UHPC overlay and NSC substrate. Some studied parameters, such as surface roughness, moisture degree of the concrete substrate, UHPC age, and freeze–thaw cycles, were simply included in their research. It was concluded that the interfacial bond performances between the two materials are sufficiently strong for concrete repair using UHPC, regardless of the different studied parameters and applied loads. Harris et al. [9] investigated the bond performance between UHPC overlay and NSC substrate by experimental and numerical analyses, and they identified the challenges in characterizing the bond properties, including characterization of substrate surface roughness, premature failure of a sample, mismatch of material strength, and consistency of applied test methods. Tayeh et al. [7,10,11] investigated the bond performances and permeability properties of the UHPCNSC interface. The interfacial bond strengths were studied by using slant shear and splitting tensile tests, and the permeability properties were evaluated by water, gas, and rapid chloride permeability tests. The test results indicated that the UHPC layer produced high bonding strength with the NSC substrate. The permeability characteristics of the UHPC-NSC interface were good, thereby significantly improving the impermeability of the concrete substrate. Hussein et al. [12] conducted a direct tensile test according to the ASTM protocols to determine the cohesion between UHPC and NSC with different roughness degrees of the substrate surface. The friction coefficients between UHPC and NSC substrate, as specified in the American Association of State Highway and Transportation Officials (AASHTO) [13], were back-calculated by using the cohesion values obtained from the direct tensile test, concomitant with the results from the slant shear tests in the previous study. Although these studies have focused on the interfacial bond properties between UHPC overlay and NSC substrate, experimental information on and assessment methods for the bonding behavior of UHPC-NSC composite samples with different studied parameters remains insufficient, in particular, the investigations for the effects and the enhancement mechanism on bonding capacities of the interface between UHPC and NSC. The primary objective of this research is to systematically and comprehensively estimate the interfacial bonding performances between UHPC as repair material and NSC substrate by using the slant shear, splitting tensile, and direct tensile tests. Special attention and discussion are given to the influences of the roughness degree of substrate surface, the age of UHPC, the moisture degree of substrate, the curing condition of UHPC, and the strength of

NSC substrate, bonding agent and expansive agent on the bond strengths between the two materials, and the bonding enhancement mechanism of UNPC-NSC interface was also discussed. The cohesion and friction coefficients for calculating the minimum bonding resistance of UHPC-NSC interfaces are presented by using the test results obtained from the direct tensile and slant shear tests. The research results should be of interest to design and construction engineers involved in evaluating the bond strength between NSC substrate and UHPC repair material. 2. Experimental program 2.1. Mix proportion of material In this experimental program, UHPC overlay was used as a repair material for the NSC substrates. The NSC used for testing was designed according to the typical concrete of Grade-50, Grade-40 and Grade-30 using Portland cement (42.5R), natural river sands with fineness moduli of 2.3–2.6, crushed stone coarse aggregates with particle sizes of 5 mm to 20 mm, and water. The UHPC applied for the test was prepared by mixing ultra-high performance premix, high-strength steel fiber, water reducer, and water. The UHPC premix consisted mainly of Portland cement, quartz sands with particle sizes of 450 lm to 900 lm and a unit weight of 2.65 g/cm3, silica fume containing 93.8% SiO2 with particle sizes of 2 nm to 280 nm and a specific surface area of 18.52 m2/g, fly ash containing 45.2% SiO2, and quartz powder with an average particle size of 50.2 lm. The round straight steel fibers with a minimum tensile strength of 2000 MPa were 0.12 and 0.2 mm in diameter and 8.0 and 13.0 mm in length. The commercially available polycarboxylate high-range water reducer with a water reduction rate greater than 35% was used. The mix proportions of NSC and UHPC are presented in Table 1. For comparison purposes, two control UHPC mixtures with the addition of the expansive agent and accelerator, respectively, were developed based on the UHPC mix proportion specified in Table 1. The material properties of NSC and UHPC were achieved by the test methods specified in GB/T50081-2002 [14] and GB/T313872015 [15], respectively. The specimens used for the material characteristic tests were cube samples of 150  150  150 mm3 and 100  100  100 mm3 for their compressive strengths, prism samples of 150  150  300 mm3 and 100  100  300 mm3 for the elastic moduli, and prism samples of 100  100  400 mm3 for the flexural strengths. The NSC samples were maintained at a normal temperature for more than 28 days before the tests. The UHPC samples prepared were classified into four groups according

Table 1 Mix proportions of UHPC and NSC. Materials

Constituents

Amount (kg/m3)

Portland cement (42.5R) Silica fume (18.5 m2/g) Fly ash Quartz sand (<900 lm) Quartz powder (50.2 lm) Polycarboxylate water reducer Water Steel fibers (2% Vol.) W/P Ratio

771.2 154.2 77.1 848.4 154.2 20.1 180.5 157.5 0.18 G50 470 1060 710 155 0.33

UHPC

NSC Portland cement (42.5R) Coarse aggregate (max. 20 mm) River sand (F.M. = 2.45) Water W/P ratio Note: W/P ratio means the weight ratio of water to paste material.

G40 420 1273 573 185 0.44

G30 360 1167 665 190 0.53

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Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

to the curing conditions and the addition of the expansive agent and accelerator, and their notations are given in the note below Table 2 which shows the results of the material characteristic tests for NSC and UHPC used in the present study.

spherical hinges were set up in the two ends of the specimen, and two equal axial tension forces were loaded at a steady load rate of 0.1 kN/s. The direct-tensile bond strength (f t ) (in MPa) of the UHPC-NSC interface was estimated as,

2.2. Test specimens and preparation

ft ¼

2.2.1. Test specimens According to the ASTM-C882 [16], the specimens of the slant shear test were cast to form the composite prism samples (100  100  300 mm3), shown in Fig. 1(a). Two equal concentrated loads were applied to the slant shear specimens at a constant load rate of 5 kN/s. The slant-shear bond strength (f n ) (in MPa) can be expressed as,

where P = the failure load (in N); At = the cross-sectional area of the test specimen (in mm2), taken as 10,000 mm2.

fn ¼

P ; An

rn ¼ f n sin a; sn ¼ f n cos a

ð1Þ

where rn , and sn are normal compressive stress and shear stress that occurred at the interface at failure (in MPa); P = the failure load (in N); An = the area of the slant bonding plane (in mm2), taken as 20000 mm2; a = the inclined vertically angle of the slant surface, taken as 30°. According to the ASTM C496 [17], each specimen for the splitting tensile test is a composite cylinder of 150 mm diameter  300 mm high (see Fig. 1(b)). The splitting tensile specimens were loaded with compressive forces at the UHPC-NSC interface along the longitudinal direction of the cylinder, and the adopted load rate was 0.05 MPa/s to 0.08 MPa/s. Eq. (2) was applied to calculate the splitting-tensile bond strength (f sp ) (in MPa) between UHPC and NSC.

f sp ¼

2P

ð2Þ

pAsp

where P = the failure load (in N); Asp = the area of the bonding plane (in mm2), taken as 45,000 mm2. The direct tensile test can offer the most conservative measured bond strength because of the absence of influence caused by friction or other stresses in the other test methods [18]. As shown in Fig. 1(c), the geometry defined for the direct tensile specimens was 100  100  300 mm3 with the interface line at middle height. To avoid the excessive axial tension eccentricity, the two-way

P At

ð3Þ

2.2.2. Specimen preparation For each considered studied parameter, i.e., surface roughness, age of UHPC, moisture degree of substrate, curing condition, NSC strength, bonding agent and expansive agent, 90 UHPC-NSC composite specimens were cast, in which 3 specimens with the same studied parameters were classified as a test group. The NSC substrates were first cast into the respective molds, and then were demolded and cured for 2 days in water. Afterwards, the NSC surfaces were roughened according to the six surface treatments listed in Table 3 respectively. The NSC substrates were kept in a moist environment until the age of 28 days at a normal temperature, and were left to dry for another 30 days before casting the UHPC overlay. Before the UHPC overlay was cast, NSC substrates were wetted according to the three different moisture degrees shown in Table 3, respectively. Then, UHPC was cast to the interface facing upward of NSC substrates. The fresh UHPC bonded to NSC was covered with plastic films, placed for 48 h in the laboratory, and cured by sprinkling water at a normal temperature for additional 1 day to 180 days or steam-cured at 60 °C for 72 h or at 90 °C for extra 48 h. The corresponding NSC-NSC samples roughened to macrotexture depths of 4.0 mm to 5.0 mm at the interface, and the monolithic cast-in-place concrete samples for each type of tests were also cast for comparisons. 2.3. Studied parameters Table 3 lists the studied parameters that affect the bond performance of UHPC-NSC interface in the present study. To evaluate their influence on the interfacial bond performance, three different moisture degrees of NSC substrates were considered in the tests [19]. The first category of the moisture degree is defined as the

Table 2 Mechanical characteristics of UHPC and NSC. Material

f c (MPa) E(GPa) f t (MPa)

NSC

ES-UHPC

E-UHPC

N-UHPC

HC-UHPC

G-50

G-40

G-30

1d

2d

3d

28d

3d

7d

28d

180d

60 °C

90 °C

53.0 33.0 6.53

42.2 32.2 5.68

31.9 31.9 4.59

80.1 36.9 20.4

92.8 39.5 21.2

63.5 38.1 15.4

120.4 44.8 22.6

76.6 40.1 21.6

100.3 41.3 22.4

123.6 45.7 22.5

126.2 46.0 25.0

133.3 46.3 25.6

162.1 48.7 26.5

Note: ES-UHPC is the UHPC with the accelerator for increasing the gain rate of early-aged strength subjected to a curing at a normal temperature; E-UHPC and N-UHPC, respectively, denote the UHPC incorporating and not incorporating the expansive agent under the same curing condition as the ES-UHPC; HC-UHPC is the heat-cured UHPC under a 60 °C steam-curing for 3 days or 90 °C steam-curing for 2 days; f c , E, and f t represent the compression strength, elastic modulus, and flexural strength, respectively.

Fig. 1. Test specimens: (a) Slant shear test; (b) Splitting tensile test; (c) Direct tensile test.

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Table 3 Studied parameters used in the tests. Parameter

Detailed situation

Moisture degree of NSC substrate Age of UHPC Treatment of NSC surface Curing condition

Air surface dry (ASD), air surface wet (ASW), and saturated surface dry conditions (SSD) 0.5, 1, 2, 3, 7, 28, 90, 180 days Sm, WB, LR, HR, RD and RG

Strength of NSC Adhesive agent Expansive agent

Normal-temperature curing, steam curing at 60 °C and 90 °C Grade-50, Grade-40 and Grade-30 With and without With and without

air surface dry (ASD) condition, in which the NSC substrate was left to dry in laboratory for more than 7 days before UHPC overlay was placed. The second category of the moisture degree is the air surface wet (ASW) condition, in which the surface of NSC substrate was wetted by sprinkling water for approximately 20 min and wiped dry before casting UHPC. The third category of the moisture degree is defined as the saturated surface dry (SSD) condition, i.e., the NSC substrate was kept in water for at least 24 h, and then removed from water, and the interface of the NSC substrate was dried with compressed air before placing UHPC overlay. The surface roughness of NSC substrate, previously identified as the one that best correlates with the bond strength of a UHPC-NSC interface [7,8,20], was selected as a major studied parameter in this test. The roughness of NSC surface was evaluated by means of the average macrotexture depth (Rt ) measured by the sand-pour method [21,22], and then the classification of the surface roughness was determined according to fib Model Code for Concrete Structures 2010 [23], i.e., Rt < 1:5 mm for a smooth surface, Rt  1:5 mm for a moderately rough surface and Rt  3:0 mm for a rough surface. As shown in Fig. 2, the six different surface treatments of NSC substrate in this study were defined as follows: (i) smooth (Sm), i.e., the flat cut surface without roughness measurement; (ii) wire brushed (WB), obtained using a steel wire brush to clean up the slurry cement on the substrate surface without exposing the aggregates, and Rt ¼ 0:6  1:2 mm; (iii) low roughness (LR), exposing the partial coarse aggregates, and Rt ¼ 1:0  3:0 mm; (iv) high roughness (HR), exposing the most coarse aggregate, and Rt ¼ 4:0  5:0 mm; (v) rough + drilled hole (RD), the substrate surface was roughened to HR combined with some drilled holes with a diameter of 30 mm and a depth of 30 mm; (vi) rough + groove (RG), the substrate surface was roughened to HR associate with grooves that are 20 mm in width and 10 mm in depth. In addition, two ages of 1 and 2 days for the high early strength UHPC with the accelerator and five ages of 3, 7, 28, 90, and 180 days for the other UHPC overlays, were considered with the aim of exploring the effect of curing time of UHPC under a normal temperature on the bond strength. Three curing conditions, i.e., the

composite specimens were cured at a normal temperature of laboratory environment and by high-temperature steam at 60 °C for 72 h and at 90 °C for 48 h were also introduced in this study. 3. Result analysis According to the ACI 546-06 Concrete Repair Guide [24], the minimum acceptable slant-shear and direct-tensile bond strengths between the concrete substrate and the repair material were specified, as shown in Table 4. The evaluation criterion of the bond quality was proposed by Sprinkel and Ozyildirim [25] in Table 5. An appropriate repair material for rehabilitating deteriorated concrete structures can thus be selected based on the requirement for interfacial bond quality specified in the guide and the evaluation criterion. 3.1. Failure modes As shown in Fig. 3, according to the characteristics and location of the failure plane, the typical failure modes of the composite specimens observed in the slant shear, splitting tensile and direct tensile tests, can generally be categorized into the following three types: (i) Pure interface failure (B) (Fig. 3(a)): the failure occurred at the bond line, the surfaces of two materials remained smooth, and no cracking and fracturing could be found at both NSC substrate and UHPC overlay. (ii) Partial interface failure (B/C) (Fig. 3 (b)): the failure, combined with partial NSC substrate failure, occurred in the transition zone, and UHPC surface still attached the partial thin layer of NSC substrate material at failure. (iii) Complete NSC substrate failure (C) (Fig. 3(c)): the failure occurred in NSC substrate near the interface, which was crushed or fractured, and the UHPC-NSC interface was in an intact state. In addition, if the UHPC attached a significant amount of NSC substrate at failure, i.e., the transition zone with 90% or more of the NSC thin layer area attached to UHPC surface, it is also identified as the failure mode C. Usually, the failure strength obtained from the failure mode B or B/C is the actual bond strength of UHPC-NSC interface, but it is the lower limit value of bond strength for the failure mode C. 3.2. Test results Table 6 summarizes the results of the slant shear, splitting tensile and direct tensile tests, including the mean bond strengths (f n , rn , sn , f sp and f t ), the typical failure mode and the coefficient of variation (Cov) of each test group. The ratios of the bond strengths of a UHPC-NSC composite sample to those of the corresponding

Table 4 Minimum acceptable slant-shear and direct-tensile bond strengths (ACI) [24]. Description

Slant shear Direct tensile

Bond Strength (MPa) At 1st day

At 7th day

At 28th day

2.8–6.9 0.5–1

6.9–12 1–1.7

14–21 1.7–2.1

Table 5 Quantitative bond quality in terms of tensile bond strength [25].

Fig. 2. Different surface treatments for the NSC substrates.

Bond quality

Bond Strength (MPa)

Excellent Very good Good Fair Poor

2.1 1.7–2.1 1.4–1.7 0.7–1.4 0–0.7

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

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Fig. 3. Typical failure modes: (a) B: Pure interface failure (bond); (b) B/C: Partial interface failure (bond-concrete); (c) C: NSC substrate failure (concrete).

NSC-NSC sample and a monolithic cast-in-place concrete sample (continuous NSC sample) were also calculated for comparisons. For a continuous NSC sample, no interface exists. Hence, the failure strength of a continuous NSC sample obtained by each test was defined as the corresponding equivalent interfacial bond strength for comparisons with the composite specimens. In Table 6, each group of specimens was identified with the following notations: substrate moisture degree (D = ASD, W = ASW, or S = SSD), surface treatment (Sm, WB, LR, HR, RD, or RG), age of UHPC overlay (1, 2, 3, 7, 28, 90, or 180 days), and curing condition (N = normaltemperature curing, and 60 and 90 = steam curing at 60 °C and 90 °C). The studied parameters of the specimens can be clearly defined according to the notation. For example, the sample with ‘‘S-LR-N-7” expresses that the composite specimen with LR substrate surface and SSD substrate has been cured at a normaltemperature condition for 7 days. It should be noted that the substrate of a UHPC-NSC composite sample is made of the concrete of Grade-50, unless stated as G40 or G30 which stands for the concrete of Grade-40 and Grade-30, respectively. Additionally, the notation followed by a symbol of ES E, or A represents the composite specimen having the high early strength UHPC overlay with the addition of the accelerator, expansive agent or having the interface with the epoxy-based agent. f n , rn , sn , f sp and f t are the average bond strengths calculated by using Eqs. (1) through (3) according to the corresponding test results, and Cov is the coefficient of variation. It is clear according to Table 6 that, the bond performances of UHPC-NSC interfaces obtained by the slant shear, splitting tensile and direct tensile tests were generally excellent and most of the composite samples failed mainly after the NSC substrates experienced a certain degree of damages. The observed failure modes indicate that the composite specimens with the failure modes of B, B/C, and C (refer to Fig. 3) accounted for 13.6%, 51.5%, and 34.9% of the total samples on average, respectively. As can be seen, the composite specimens with an Sm substrate surface or an ASD substrate, under the 90 °C steam curing, or at the early age of UHPC (say 1 to 3 days), might have relatively low bond strengths associated with the failure mode B. Nevertheless, the failure mode C was observed in those specimens with the surfaces roughened to the macrotexture depths greater than 1.5 mm, including LR, HR, HD, and HG (refer to Fig. 2), or the SSD substrate, and under the normal temperature curing, which signifies that the UHPC-NSC interfaces could have resisted the stresses larger than the substrate strength before failure. The bond capacities thus obtained do not represent the actual interface bonding performance but are considered as the lower limits of the actual bond strength value.

According to Table 4, the measured interfacial bond strengths of most of the UHPC-NSC composite samples in the slant shear and direct tensile tests meet the minimum acceptable bond strengths specified by the ACI 546–06 [24], except for the specimens associated with some unfavorable conditions, such as the Sm surface, ASD substrate and the steam curing at 90 °C. It is noteworthy that the composite specimens with LR surface and SSD substrate obtained a better bond capacity at 3rd day than the requirement given in Table 4 at 28th day. Therefore, an appropriate surface treatment and moisture degree of substrate are required to ensure the minimum acceptable interfacial bond strength for a UHPC-NSC specimen. Furthermore, the measured interfacial bond strength level of most of the composite specimens in the splitting tensile and direct tensile tests fell into an ‘‘Excellent’’ bonding category proposed by Sprinkel and Ozyildirim [25] as shown in Table 5, while the composite samples with an ASD substrate and WB surface, or under a 90 °C steam curing attained strength values that could be classified as ‘‘very good”. In comparisons with the NSC-NSC sample, the measured bond strengths of all the UHPC-NSC composite specimens were much higher by an average ratio of 147.9%, and even some of them were twice that in case of the favorable treatments of the NSC substrate adopted. In addition, the average bond strengths of the UHPC-NSC composites samples reached approximately 76.8% of the corresponding monolithic NSC sample. Some UHPC-NSC composite specimens associated with the favorable studied parameters, i.e., LR, HR, RD, or RG surface, the SSD substrate moisture and the normal-temperature curing, had bond strengths close to or even slightly higher than that of the monolithic samples, which indicates that rehabilitated concrete structures by the appropriate use of UHPC as the repair material could achieve mechanical performance as much as the undamaged ones. By comprehensively analyzing the test results obtained by the slant shear, splitting tensile and direct tensile tests, it can be seen that the slant shear tests resulted in the highest bond strengths for all the specimens because of the high interlock and friction forces caused by the compression. The splitting tensile tests exhibited slightly higher interfacial bond strengths (indirect tensile strength) than those of the direct tensile tests, but significantly lower than the slant shear tests. Thus, the bond strengths obtained by the slant shear tests should not be adopted when the UHPC-NSC interface is in a tension state without applying appropriate correction factors. However, the applications of the splitting and direct tensile bond strengths for a UHPC-NSC interface subjected to pure shear or

6

Table 6 Experimental results in the slant shear, splitting tensile and direct tensile tests. Variable

Samples

Slant shear test

Splitting tensile test

fn (MPa)

rn

sn

(MPa)

duc=cc

duc=nc

(MPa)

Cov (%)

Failure mode

Scu (%)

Direct tensile test

f sp (MPa)

Cov (%)

duc=cc

duc=nc

Failure mode

Scu (%)

ft (MPa)

Cov (%)

duc=cc

duc=nc

Failure mode

Scu (%)

S-WB-N-3 S-WB-N-7 S-WB-N-28 S-LR-N-1(ES) S-LR-N-2(ES) S-LR-N-3 S-LR-N-7 S-LR-N-28 S-LR-N-90 S-LR-N-180

16.57 18.00 18.58 21.90 23.35 21.60 22.92 24.64 24.96 23.88

8.29 9.00 9.29 10.95 11.68 10.80 11.46 12.32 12.48 11.94

14.35 15.59 16.09 18.97 20.22 18.71 19.85 21.34 21.62 20.68

2.3 3.4 4.0 4.6 6.3 4.7 4.8 3.4 6.9 6.0

1.23 1.34 1.38 1.63 1.73 1.60 1.70 1.83 1.85 1.77

0.66 0.72 0.74 0.87 0.93 0.86 0.91 0.98 1.00 0.95

B B/C B/C B/C B/C B/C B/C C C C

0 12 44 33 42 51 82 100 100 100

2.69 2.98 3.11 3.83 4.08 3.38 3.82 3.85 3.72 3.80

4.4 5.8 5.9 6.4 4.8 3.6 5.5 8.1 8.2 10.4

0.97 1.08 1.13 1.39 1.48 1.22 1.38 1.39 1.35 1.38

0.66 0.73 0.76 0.93 1.0 0.82 0.93 0.94 0.91 0.93

B B/C B/C C C B/C C C C C

0 17 28 92 90 28 100 100 100 100

2.11 2.44 2.50 2.38 2.65 2.45 2.75 2.92 2.85 2.81

6.1 3.6 3.3 5.2 7.6 6.7 3.0 3.3 6.3 5.6

1.56 1.81 1.85 1.76 1.96 1.81 2.04 2.16 2.11 2.08

0.63 0.73 0.75 0.71 0.79 0.73 0.82 0.87 0.85 0.84

B/C B/C B/C B/C C B/C C C C C

40 47 48 88 93 84 97 100 100 100

NSC surface treatment

S-Sm-N-28 S-HR-N-28 S-RD-N-28 S-RG-N-28

13.93 24.59 25.10 25.39

6.97 12.30 12.55 12.70

12.06 21.29 21.74 21.99

2.9 3.9 5.5 4.3

1.03 1.83 1.86 1.89

0.56 0.98 1.00 1.01

B C C C

0 100 100 100

2.77 3.7 3.73 3.65

8.7 5.7 4.4 7.4

1.00 1.34 1.35 1.32

0.68 0.90 0.91 0.89

B/C C B/C C

18 100 85 100

2.18 2.76 2.45 2.54

6.2 8.0 4.8 7.6

1.61 2.04 1.81 1.88

0.65 0.82 0.73 0.76

B/C C B/C C

20 100 89 100

Moisture degree of substrate

W-WB-N-28 D-WB-N-28 W-LR-N-28 D-LR-N-28

16.95 11.72 23.54 19.21

8.48 5.86 11.77 9.61

14.68 10.15 20.39 16.64

6.3 8.6 3.2 5.5

1.26 0.87 1.75 1.43

0.68 0.47 0.94 0.77

B/C B/C B/C B/C

19 8 51 24

2.74 2.20 3.44 3.02

4.3 9.0 3.7 5.0

0.99 0.80 1.25 1.09

0.67 0.54 0.84 0.74

B/C B B/C B/C

17 0 65 25

2.28 1.70 2.49 2.26

8.2 2.7 9.5 2.9

1.69 1.26 1.84 1.67

0.68 0.51 0.74 0.67

B/C B B/C B/C

27 0 89 68

Curing condition

S-Sm-60-28 S-Sm-90-28 S-WB-60-28 S-WB-90-28 S-LR-60-28 S-LR-90-28

13.26 12.66 17.98 15.25 25.18 19.49

6.63 6.33 8.99 7.63 12.59 9.75

11.48 10.96 15.57 13.21 21.81 16.88

10.4 9.4 3.4 5.5 7.4 6.0

0.99 0.94 1.34 1.13 1.87 1.45

0.53 0.51 0.72 0.61 1.00 0.78

B/C B B/C B/C C B/C

12 0 14 15 100 70

2.94 2.24 3.19 2.37 3.92 3.22

6.0 7.6 5.0 10.9 2.8 6.4

1.07 0.81 1.16 0.86 1.42 1.17

0.72 0.55 0.78 0.58 0.96 0.79

B/C B B/C B/C C C

25 0 28 9 100 91

2.13 1.75 2.56 1.90 2.98 2.03

7.3 8.2 6.3 9.0 4.4 2.6

1.58 1.30 1.90 1.41 2.21 1.50

0.64 0.52 0.76 0.57 0.89 0.61

B/C B B/C B C C

17 0 34 0 100 92

Strength of NSC

S-LR-N-28(G40) S-LR-N-28(G30)

20.70 15.89

10.35 7.95

17.93 13.76

7.6 9.8

1.54 1.18

0.83 0.63

C C

100 96

3.51 2.60

4.0 5.8

1.27 0.94

0.86 0.63

C C

94 96

2.28 1.75

2.9 7.1

1.69 1.30

0.68 0.52

C C

100 100

Adhesive agent

S-Sm-N-28(A) S-LR-N-28(A)

16.65 19.45

8.33 9.73

14.42 16.85

4.2 6.1

1.24 1.45

0.66 0.78

B/C C

30 100

3.38 3.78

9.6 6.7

1.22 1.37

0.82 0.92

B/C C

52 94

2.20 2.82

6.4 5.2

1.63 2.09

0.66 0.84

B/C C

40 100

Expansive agent

S-LR-N-3(E) S-LR-N-28(E)

16.03 23.98

8.02 11.99

13.88 20.77

3.2 7.9

1.19 1.78

0.64 0.96

B C

4 100

2.88 3.94

3.0 7.6

1.04 1.43

0.70 0.96

B/C C

19 91

1.90 2.86

8.9 2.8

1.41 2.12

0.57 0.85

B/C C

35 100

13.46 25.06

6.73 10.28

11.65 17.80

9.1 5.5

— —

— —

B —

0 —

2.76 4.10

9.8 4.0

— —

— —

B —

7 —

1.35 3.35

8.2 7.3

— —

— —

B —

0 —

NSC-NSC samples Monolithic concrete sample

Note: duc=cc , and duc=nc are the ratios of the bond strengths of the UHPC-NSC composite sample to those of the corresponding NSC-NSC sample, and a monolithic cast-in-place concrete sample, respectively; Scu is the area ratio of NSC substrate attached to UHPC to the entire interface at failure; B, B/C and C represent the typical failure modes shown in Fig. 3.

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

Age of UHPC

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

7

Fig. 4. SEM/EDS of UHPC-NSC and NSC-NSC interfaces: (a) SEM images of NSC-NSC interface; (b) SEM images of UHPC-NSC interface; (c) EDS of UHPC-NSC interface.

combined shear / compression loading would underestimate the actual bond strengths. In addition, the coefficient of variations (Cov) of the bond strengths of UHPC-NSC interface in all the tests varied from 2.3% to 10.9% with an average of 5.7%, which are similar to the Covs obtained by other researchers, such as Hussein H.H. (2016) [12], Carbonell et al. (2014) [8], and Tayeh Bassam A. (2012) [7]. The low Covs in the present tests can be considered within a reasonable range, which justifies the reliability of the tests. 3.3. Discussions on bonding enhancement of UHPC-NSC interface As shown in the above analyses, a UHPC-NSC interface could generally achieve a good bonding performance compared with an NSC-NSC interface. According to the bonding mechanism of NSCNSC interfaces and the SEM micrographs of the interfacial transition zones at the NSC-NSC and UHPC-NSC interfaces shown in Fig. 4, the possible reasons for the enhanced bond performance of the UHPC-NSC interfaces are as follows. (i) For an NSC-NSC interface, on account of the strong hydrophilicity of an NSC substrate, a ‘‘transition layer” with a high water-cement ratio can be formed around the interface as the new concrete is poured [26]. In the transition layer, the crystals of calcium hydroxide and ettringite produced by the hydration products of new concrete, have the large size and quantity. The crystal structures of ordered preferred orientations are formed with the crystals arranged around some special orientations, which leads to the increasing of porosity and loose structure as shown in Fig. 4(a), and thereby to the strength reduction of the transition layer at the NSC-NSC interface [26]. As shown in Fig. 4(b) for a UHPC-NSC interface, the water-cement ratio (say 0.16–0.18) of the UHPC overlay is much lower than that (say 0.3–0.5) of the NSC substrate, and the transition layer certainly has a relatively low water-cement ratio, which leads to a possible reduction of the amount of the large crystals at the interface. Thus, the porosity of the transition layer is small and the density correspondingly increases, so the bonding strength of the UHPC-NSC interface increases. Additionally, it is easy to produce bleeding phenomenon due to vibration during the pouring process of new concrete with a high water-cement ratio, i.e., the coarse aggregate sinking and the water rising. The bleeding bubbles gather around the NSC-NSC interface, which results in a larger number of microcracks and pores (Fig. 4(a)), and thus more vulnerable damage at the interface [27]. On the contrary, no bleeding phenomenon was observed when pouring UHPC for the UHPC-NSC specimens, and only a few micro cracks and pores existed in the transition zone of the interface (Fig. 4(b)). Therefore, the interfacial bonding strength of the UHPC–NSC specimens can be improved.

(ii) The coarse aggregates in the new concrete may sink and accumulate on the surface of an NSC substrate by vibrating in the pouring process. The ‘‘point contact” forms easily between the coarse aggregates with the roughened surface of the NSC substrate, which can prevent the viscous cement slurry from entering the pores and the concave parts, and infiltrating fully the aggregates and cement stones on the substrate surface. The NSC-NSC interface is thus in a state of insufficient slurry that leads to the abundant pores and crackings in the interfacial transition zone (Fig. 4(a)), and it weakens the interfacial bonding of the NSC-NSC specimens [28]. For the UHPC-NSC ones, due to the self-compacting UHPC and no coarse aggregates used, the so-called point contact would not form at their surface and only a few tiny cracks were observed in the transition zone (Fig. 4(b)), which makes the relatively better bonding properties of the UHPC-NSC interface. (iii) From the viewpoints of the chemical reaction and physical effect, the formation of the hydration product (C-S-H) by the silicon dioxide of silica fume in the UHPC mixture and the Ca (OH)2 in the NC substrate [11], and micro filling of silica fume (see Fig. 4(c)) are mainly responsible for the strength improvement of the transition zone. Strong and durable C-S-H has few air voids and other vacant spaces, and makes the zone tough, dense, and uniform [29,30]. Moreover, a secondary reaction of the Ca(OH)2 and pozzolana with time can further improve the microstructure of the transition zone and the bond strength of the UHPCNSC interface [31]. Generally speaking, Fig. 4 shows that the interfacial transition zone between the NSC substrate and UHPC overlay is denser, stronger, and more uniform compared with the NSC-NSC one, which demonstrates that UHPC as a repair material can create a conducive environment for the micro-filler effect and pozzolanic reaction of silica fumes, and thus contribute to the superior interfacial bond strength. 4. Effects of studied parameters The primary purpose of this section is to discuss the effects of the moisture degree and strength of the NSC substrate, age of UHPC overlay, treatment of substrate surface, curing condition, and the epoxy-based agent and expansive agent on the bond performance of the UHPC-NSC interface. The detailed variations in the interfacial bond strength caused by the different studied parameters are presented in the following subsections. 4.1. Age of UHPC overlay The UHPC-NSC composite specimens associated with the WB or LR substrate surface, the SSD substrate and normal temperature curing, were selected to investigate the influences of different ages

8

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

Fig. 5. Effects of UHPC age on the interfacial bond strength.

of the UHPC overlays on the bond performance. Their interfacial bond strengths versus the UHPC age are illustrated in Fig. 5. From Fig. 5, the interfacial bond strengths between UHPC and NSC in the three tests gradually increase with the growth of the UHPC age. The measured interfacial strengths of almost all specimens reached their peak values at the age of 28 days. However, the interfacial bond strengths achieved at the long-term ages of 90 and 180 days were slightly lower than those of 28 days, with average decreases of 1.5% and 2.7%, respectively. These results demonstrate that an increase in long-term shrinkage of UHPC has a certain negative effect, and thereby weakens the bond capacities of UHPC-NSC interface. Interestingly, the bond strengths of the UHPC-NSC interface obtained by the three tests developed faster in the early age of UHPC. For the composite specimens with the high early strength UHPC overlay, the slant shear strength, splitting tensile strength and direct tensile strength of the interfaces at the ages of 1 day and 2 days reach 89.9% and 94.7%, 99.5% and 106%, as well as 81.5% and 90.8% of those for the specimens at the age of 28 days. The results indicate that use of a high early strength UHPC makes the UHPC-NSC interface obtain a sufficiently high interfacing bonding even one day after pouring the UHPC overlay. With the conventional UHPC overlay at the age of 3 days, however, the average interfacial bond strength obtained in the slant shear, splitting tensile and direct tensile tests, also reached to a high level, around 86.6% of those of the specimens at 28th day, while the average strength corresponding to the age of UHPC of 7 days accounted for most of the bonding capacity of the specimens at 28th day, say an average ratio of 96.1%. Thus, the UHPC with or without the accelerator, as a repair material of NSC substrate, could yield high interfacial bond strengths at a very early age of UHPC overlay, and the strengths only developed slightly during the further curing period. This advantage could ensure the repaired structures to establish a sufficient strength in a short period of time after the rehabilitation. Listed in Table 6, it can be seen that most of the composite specimens exhibited a complete NSC substrate failure (C) or partial interface failure (B/C) shown in Fig. 3. Since the strength of the

UHPC is much higher than the NSC, it can be considered that the bond strength of the UHPC-NSC interface depends largely on the strength of the NSC. According to Table 2, the strength of UHPC develops more rapidly at its early age (1 day to 7 days) than at its later age (28 days to 180 days). The strengths of the ES-UHPC, and N-UHPC at 1st day, and 3rd day have reached 80.1 MPa and 76.63 MPa, respectively, which far exceed the NSC strength, so that the bond strength of the UHPC-NSC interface developed rapidly in the early age. 4.2. Treatment of substrate surface The roughness preparation of an NSC substrate surface is a major factor on the interfacial bond performance between the concrete substrate and its repair material [32–34]. Fig. 6 exhibits the changes in the interfacial bond strengths obtained in the three tests with the different treatments of the SSD substrate surface, under normal maintenance for 28 days. Fig. 6 shows that the bond strengths obtained in the slant shear, splitting tensile, and direct tensile tests are sensitive to the roughness of the NSC substrate surface. For a single roughness preparation of Sm, WB, LR, and HR, the interfacial bond strength increased in the proper order of the Sm\WB\HR\LR surface. Compared with the Sm surface, the rough surfaces resulted in much improvement in the interfacial bond strength, with the average increases of 33.4%, 76.9%, and 76.5% for the WB, LR, and HR surfaces in the slant shear test, of 12.3%, 39.0%, and 33.6% in the splitting tensile test, and of 14.7%, 33.9%, and 26.6% in the direct tensile test, respectively. The bond strengths of the Sm and WB interface were relatively low, but for the LR substrate surface it achieved the highest bond strength due to the superior cohesion and excellent interlocking action with the UHPC overlay, which were promoted by the roughened NSC matrix and the exposed aggregates. Although the HR surfaces were roughened to a certain macrotexture depth, their recorded bond strengths were slightly lower than those of LR surfaces with an average decrease of approximately 3%. The possible reason of the negative effect on the interfacial perfor-

Fig. 6. Effects of treatment of substrate surface on the interfacial bond strength.

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

9

Fig. 7. Effects of different moisture degrees on the interfacial bond strength.

mance might be the seriously disturbed aggregates and the textured hardened NSC matrix around the substrate surface caused by drilling during the HR surface preparation. On the whole, the bonding capacities of the Sm, WB, LR, and HR interfaces are provided largely by the mechanical interlock and the adhesive bonding between the UHPC overlay and NSC substrate. The rougher the surface of an NSC substrate is, the greater the mechanical interlock force and the larger the bonding area are, and so the higher the bond strength of the UHPC-NSC interface is. For the specimens with the combined roughness preparations of RD and RG, the bond strengths obtained by the slant shear test were slightly higher than those specimens with the LR surface by a slight increase of approximately 2.4%. The enhancement of the slant-shear bond strength could generally be attributed to the additional mechanical interlocking by the UHPC filling in the holes or the grooves of the NSC substrate. However, for the splitting and direct tensile tests, the recorded bond strengths of the RD and RG composite specimens exhibited the decreases by 2.8% and 14.6%, respectively, compared with the LR specimens. The possible reason is that the existence of Sm surface in the holes or grooves leads to a reduction in the rough surface area and decrease in the indirect or direct tensile performance of UHPC-NSC interface. In sum, the adequate roughening treatments of the substrate surface, such as LR and HR, are the most effective techniques for improving the interfacial bond capacity and reliable performance of repaired NSC structures. 4.3. Moisture degree of substrate Three different moisture degrees specified in Table 3, i.e., ASD, ASW and SSD, were selected to evaluate their influences on the bond strength of the UHPC-NSC interface. Fig. 7 shows the differences in the interfacial bond strengths of the WB and LR composite specimens which were treated by the three moisture degrees on the substrates, and cured at the normal temperature condition for 28 days.

As can be seen from Fig. 7, the excellent bond performance of the UHPC-NSC interface was achieved for the specimens with the SSD substrate, then followed by the specimens with the ASW and ASD substrates, i.e., the bond strengths obtained from all the three tests being in the ascending order of ASD \ ASW \ SSD. Compared with the ASD reference samples, the substrate moisture degrees of ASW and SSD provided the samples having a WB surface with the relative percentage increases in bond strength by 34.4% and 49.0%, respectively, and by 15.5% and 28.3%, respectively, for the specimens having an LR surface. Hence, the wetting condition of the substrate is actually an effective parameter on the interfacial bond strength of UHPC-NSC interface, and the adequately wetting the NSC substrates can significantly improve the interface bond strength. During the pouring of UHPC and its subsequent hydration, the strong hydrophilicity of the NSC substrate would lead water to transfer to the UHPC overlay easily, which likely results in further lowering the water content in the UHPC that itself has a low water cement ratio and thus causes the incomplete hydration reaction in the UHPC and weakens bond strength at the UHPC-NSC interface. 4.4. Curing condition of UHPC After a 48-h steam treatment at the temperatures of approximately 80 °C to 100 °C, the HC-UHPC will acquire more outstanding mechanical properties than the N-UHPC with normal-temperature curing, such as higher tensile strength and toughness, better durability, and minimal long-term shrinkage and creep [35,36]. Therefore, the influence of the curing condition on the mechanical bonding characteristics of the UHPC-NSC interface needs to be discussed. The different curing conditions of UHPC, including normal temperature curing and steam curing at 60 °C and 90 °C, were considered in this study. Fig. 8 presents the variation in bond strength of the Sm, WB, and LR specimens with the SSD substrate corresponding to different curing conditions.

Fig. 8. Effects of different curing conditions on the interfacial bond strength.

10

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

Fig. 9. Effects of Strength of NSC on the interfacial bond strength.

Fig. 10. Effects of epoxy-based adhesive agent on the interfacial bond strength.

The specimens cured under the normal temperature condition can basically obtain the highest bond strengths after 28 days, and the interfacial bond strengths of the specimens under the steam curing at 60 °C for 72 h were slightly lower than or basically the same as those with normal temperature curing. However, the interfacial bond strengths of the specimens cured at 90 °C steam for 48 h decreased significantly, although the high-temperature steam curing could enhance the mechanical properties of UHPC. The percentage decreases were on the average of 16.0% for the Sm specimens, 21.9% for the WB specimens, and 22.6% for the LR specimens, respectively. The possible reason is that UHPC develops its full potential material properties and shrinkage rapidly in a short period of time and its time-dependent deformation is practically negligible after the high-temperature steam curing [2,35], which implies a rapid increase in the differential shrinkage between the NSC substrate and UHPC overlay, and thus results in the rapid increase in the interfacial shear stress caused by shrinkage before the interfacial bond strength reaches its maximum. For this reason, the excessive curing temperature could exert a significant negative effect on the interfacial bond strength, and is unsuitable for actual in situ repairing of NSC substrate with UHPC overlay.

4.5. Strength of NSC substrates The differences in the interfacial bond strengths for the composite specimens with the NSC substrates of Grade-30, Grade-40 and Grade-50 were shown in Fig. 9. It can be seen that the bond strengths of the UHPC-NSC interfaces treated by the same way increase significantly with the increase of the NSC strength, which indicates that the NSC strength has a great influence on the interfacial bond property. Fig. 3 and Table 6 show that the majorities of the failure modes of the composite specimens were either the pure or partial shear failure (C or B/C) of the NSC substrates adjacent to the interface. Therefore, it can be considered that the bond

strength of a UHPC-NSC interface largely depends on the NSC strength. 4.6. Interfacial bonding agent (epoxy-based adhesive agent) At the NSC-NSC interface, the application of the interfacial bonding agent between a concrete substrate and its overlay can enhance the interfacial bond strength [33]. In this study, two groups of the composite specimens (S-Sm-N-28, S-LR-N-28), respectively, with and without applying the epoxy-based adhesive agent on the NSC substrate surfaces were considered. Fig. 10 presents the comparisons of the interfacial bond strengths of these two groups of specimens. The results show that the bond strength of the group of S-Sm-N-28(A) is higher than that of the control group of S-Sm-N-28, but the opposite seems to hold true for the group of S-LR-N-28(A) and its control group. The effectiveness of the adhesive agent for improving the interfacial bond strength can be directly demonstrated by the composite specimens with the relatively weaker interface, say the Sm interface, since their failure modes involved the interfaces. For the composite samples with the rough LR interfaces, their failure modes that all were type C did not change when applying the adhesive agent. However, the decrease in the measured strengths of the specimens of S-LR-N-28 (A) does not indicate that the adhesive agent would undermine the interface bonding performance. 4.7. Expansive agent UHPC will usually undergo large post-curing shrinkage deformation when adopting the natural curing conditions [37]. Adding the expansive agent to the UHPC can effectively compensate its shrinkage [38], and it is expected to reduce the additional shrinkage stress caused by the interfacial shrinkage difference between the UHPC overlay and NSC substrate. The test results of two groups of the composite specimens with and without the expansive agent

11

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

Fig. 11. Effects of Expansive agent on the interfacial bond strength.

Table 7 Influence of studied parameters on the bond strength of UHPC-NSC interface. Parameter

Moisture degree

UHPC Age

Surface treatment

Curing

NSC strength

Adhesive agent

Expansive agent

Sensitivity

Significant

Moderate

Significant

Moderate

Significant

Slight

Slight

were compared, as shown in Fig. 11, to explore the effect of the expansive agent on the bond properties of the UHPC-NSC interfaces at different ages of the UHPC. It can be seen from Fig. 11 that the bond strength between the UHPC overlay incorporating the expansion agent and the NSC substrate at the age of 3 days is significantly lower than the UHPC-NSC sample without the expansion agent, with an average decrease of about 20%. At the age of 28 days for the UHPC overlay, the discrepancy in the interfacial bond strength between the two groups of the composite specimens became almost negligible. According to Table 2, the compressive and flexural strengths were observed to have a similar relative difference to the interfacial bond strength between the two groups of specimens at both the early and later ages of their UHPC overlays. This indicates that the interfacial bond strength is highly dependent on the development of the material properties of the UHPC overlay. Therefore, the addition of the expansion agent into the UHPC overlay would delay its full development of the material properties and thus weaken the early age interfacial bond strength of the composite UHPC-NSC specimens. For better understanding the influence of the studied parameters on the interfacial bonding strength of a UHPC-NSC interface, the sensitivity evaluation of the studied parameters was established and tabulated in Table 7. It can be inferred that the moisture degree, the surface treatment of an NSC substrate, and the NSC strength play significant roles on the bond strength, and the UHPC age and the curing condition are relatively less important for the interfacial bonding performance. Additionally, the influences of

the adhesive agent and expansive agent on the interfacial bond strength seem trivial. 5. Cohesion and friction coefficient The evaluation method on the bond capacity of an NSC-to-NSC interface can be used for reference to quantitatively identify the interfacial properties between the UHPC overlay and NSC substrate [12]. The primary factors that affect bond performance, including cohesion, friction, and dowel action [39], are included in some codes, such as AASHTO LRFD (2010) [13] and Fib Model Code for Concrete Structures (2010) [23], to specify the bond strength of an NSC-to-NSC interface. If no shear reinforcement considered at the interface, the calculation methods for the interfacial bond capacity in the two specifications are identical and can be simplified as

su ¼ c þ lrn

ð4Þ

where su = shear bond strength of interface (in MPa); c = cohesion, which is expressed as sa in the Fib Code [23] (in MPa);l = friction coefficient; and rn = compressive stress normal to the shear interface (in MPa). AASHTO provides the values of c and l on the NSCNSC interface with different roughnesses of substrate surface, i.e., 1.65 MPa and 1.0 for the rough surface, and 0.5 MPa and 0.6 for the Sm surface, respectively. When the interface is in a tension state, rn is zero, and Eq. (4) gives the tensile bond strength of the failure plane. For the UHPC-NSC interface, the cohesion (c) between

Table 8 Cohesion and friction coefficients obtained by the slant shear and direct tensile tests. Sources

Roughness

Slant shear test

Direct tensile or pull-off test

Friction coefficient

sn (MPa)

rn (MPa)

c (MPa)

This study

Smooth Wire brushed Low rough

12.06 16.09 21.34

6.97 9.29 12.32

2.18 2.50 2.92

1.42 1.46 1.50

Husam et al. (2016) [12]

Smooth Mid-rough Rough

4.10 13.50 33.10

2.30 7.80 19.10

3.02 5.01 5.63

0.47 1.09 1.44

Tayeh et al. (2012, 2013) [7,10]

Smooth Wire brushed Sandblasted

7.51 11.04 15.42

4.34 6.38 8.90

2.30 2.32 2.34

1.20 1.37 1.47

Carbonell et al. (2014) [8]

Wire brushed Sandblasted

16.10 21.70

9.30 12.30

2.20 2.30

1.49 1.58

l (MPa)

12

Y. Zhang et al. / Construction and Building Materials 235 (2020) 117431

the UHPC overlay and NSC substrate with different roughness degrees should be determined by the results of the direct tensile test, and sn and rn could be obtained from the slant shear test (Table 6). Therefore, the friction coefficient (l) can be backcalculated according to Eq. (4). The interfacial shear strength (su ) of the UHPC-NSC interface could then be calculated by Eq. (4) according to the cohesion (c) and friction coefficient (lÞ determined by the tests. As shown in Table 8, the cohesion (c) and the back-calculated friction coefficients (l) of the UHPC-NSC interface obtained by the correlative test results of the composite specimens with the Sm, WB, and LR substrate surfaces in this study and the past similar research [7,8,10,12], are presented for further investigation. The results in Table 8 reveal that the cohesion (c) and friction coefficient (l) for the UHPC-NSC interface are significantly higher than the typical values for the NSC-NSC interface specified in the AASHTO [13]. Except for the friction coefficients for the UHPCHSC (high strength concrete) interface from Husam et al. (2016) [12], the values of c and l for the UHPC-NSC interface are 4 and 2 times those for an Sm surface, and 1.5 and 1.4 times those for WB, and LR surfaces, respectively. According to the minimum results in Table 8, the recommended values of c and l for the UHPC-NSC interface can be determined as 2.2 MPa and 1.37 for the rough surface, and 2.18 MPa and 1.2 for the Sm surface, respectively. It is noteworthy that in spite of the limited samples in the present study, the results are still beneficial to establishing a determination method of the cohesion (c) and friction coefficient (l) to calculate the interfacial bond strength according to AASHTO. However, for different factors, such as the roughness of a substrate surface, the wetness of a substrate, the curing condition, the strength of concrete, and the stress state of interface, the bond performances of the UHPC-NSC interfaces vary greatly. Therefore, the authors suggest that the extensive experimental studies for the UHPC-NSC interfaces with different parameters need to be conducted to verify the generality of c and l. 6. Conclusions The interfacial bond performances between UHPC and NSC were studied by the slant shear, splitting tensile, and direct tensile tests in this study. From the discussions of test results, some conclusions can be drawn as follows: 1) Due to the ultra-fine aggregate and relatively low watercement ratio (w/b) in UHPC, the bond strengths of UHPCNSC interfaces are generally good and sufficiently strong, and exceed the minimum requirements specified in ACI 546-06 or achieved an ‘‘Excellent” bonding category. 2) Regardless of which test method is adopted, the interfacial bond strengths of the UHPC-NSC composite samples were apparently higher than that of the NSC-NSC sample, and close to or even slightly higher than that of the monolithic NSC specimen in case of the favorable studied parameters adopted. 3) The surface roughness and moisture degree of the NSC substrate, and NSC strength are the major factors for the interfacial bonding capacity between the NSC substrate and UHPC overlay. The LR surface of the substrate provides the most superior mechanical bond performance but the Sm surface the worst. The adequate wetting of an NSC substrate (SSD) can significantly enhance the interfacial bond strength that also increases significantly with the increase of NSC strength. The appropriate roughness preparation and fully moistness for the NSC substrate before casting UHPC are necessary to ensure the reliable bond performance.

4) The bond strengths of the UHPC-NSC interfaces develop rapidly at the early age, which could reach almost their peaks at the age of 28 days of the UHPC overlays, and increase only slightly in the later age. 5) The excessive curing temperature (say 90 °C) leads to decreasing to a certain extent the interfacial bond properties due to the rapid development of the UHPC shrinkage. The composite specimens cured at normal temperature for 28 days possess a higher interfacial bond strength. 6) The use of the epoxy-based interfacial-bonding agent enhances the bonding capacity for the Sm interface, but weakens the bond strength for the rough LR interface. Moreover, the addition of an expansion agent in the UHPC leads to slow development of the early-age bond strength of the UHPC-NSC interface, and its effect on the interfacial bond strength keeps getting smaller with the increase of UHPC age. 7) According to AASHTO (2010), the bond strength of the UHPC-NSC interface can be calculated by using the cohesion (c) obtained from the direct test and the friction coefficient (l) back-calculated by the cohesion, along with the results from the slant shear test.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research is sponsored by the National Natural Science Foundation of China (Grant Nos. 51578226, 51778221) the China Postdoctoral Science Foundation (Grant No. M602411), Major Research Project of Industrial Technology of Guangzhou, China (Grant No. 201902010019), and Programs for Science and Technology Innovation of Department of Transportation of Hunan Province, China (Grant No. 201714). These supports are gratefully acknowledged. References [1] M.F. Green, L.A. Bisby, Y. Beaudoin, P. Labossière, Effect of freeze-thaw cycles on the bond durability between fibre reinforced polymer plate reinforcement and concrete, Can. J. Civ. Eng. 27 (5) (2000) 949–959. [2] B. Graybeal, Material property characterization of ultra-high performance concrete. Rep. No. FHWA-HRT-06-103, Federal Highway Administration, Washington, DC, 2006. [3] G.F. Huseien, J. Mirza, M. Ismail, S.K. Ghoshal, A.A. Hussein, Geopolymer mortars as sustainable repair material: a comprehensive review, Renewable Sustainable Energy Rev. 80 (2017) 54–74. [4] M.A. Moeini, M. Bagheri, A. Joshaghani, A.A. Ramezanianpour, F. Moodi, Feasibility of alkali-activated slag paste as injection material for rehabilitation of concrete structures, J. Mater. Civ. Eng. 30 (10) (2018). 04018252-1-15. [5] Association Française de Génie Civil (AFGC). Ultra-high performance fiberreinforced concrete interim recommendations. Paris, 2013. [6] E. Brühwiler, E. Denarié, Rehabilitation and strengthening of concrete structures using ultra-high performance fibre reinforced concrete, Struct. Eng. Int. 23 (2013) 450–457. [7] B.A. Tayeh, B.H.A. Bakar, M.A.M. Johari, C.Y. Lei, Mechanical and permeability properties of the interface between normal concrete substrate and ultra high performance fiber concrete overlay, Constr. Build. Mater. 36 (2012) 538–548. [8] M.A.C. Muñoz, D.K. Harris, T.M. Ahlborn, D.C. Froster, Bond performance between ultra-high performance concrete and normal-strength concrete, J. Mater. Civ. Eng. 26 (8) (2014). 04014031-1-9. [9] D.K. Harris, M.A. Carbonell, A. Gheitasi, T.M. Ahlborn, S.V. Rush, The Challenges Related to Interface Bond Characterization of Ultra-high-performance Concrete with Implications for Bridge Rehabilitation Practices, ASTM, West Conshohocken, PA, 2014. [10] B.A. Tayeh, B.H.A. Bakar, M.A.M. Johari, Evaluation of bond strength between normal concrete substrate and ultra high performance fiber concrete as a repair material, Proc. Eng. 54 (2013) 554–563.

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