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Effect of joint interface conditions on shear transfer behavior of recycled aggregate concrete
Construction and Building Materials 105 (2016) 343–355
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of joint interface conditions on shear transfer behavior of recycled aggregate concrete Jianzhuang Xiao a,⇑, Chang Sun a, David A. Lange b a b
Department of Structural Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, PR China Department of Civil and Environmental Engineering, University of Illinois, 2129B NCEL, USA
h i g h l i g h t s Effect of joint interface conditions on shear transfer behavior of RAC was tested. RCA replacement ratio negatively affects the aggregate interlock action of RAC. The shear stress in RAC with uncracked or cold-joint interfaces was predicted.
a r t i c l e
i n f o
Article history: Received 15 February 2015 Received in revised form 11 September 2015 Accepted 5 December 2015 Available online 18 December 2015 Keywords: Recycled aggregate concrete (RAC) Joint interface Shear transfer behavior Aggregate interlock Recycled coarse aggregate (RCA) replacement ratio
a b s t r a c t In this study, 38 uncracked and cold-joint push-off specimens were tested to explore how joint interface characteristics affect shear transfer behavior of recycled aggregate concrete (RAC). The major test variables included the recycled coarse aggregate (RCA) replacement ratio and joint interfaces. The results indicate that the shear transfer behavior of RAC with different joint interfaces is similar to that of conventional concrete. It was shown that the RCA replacement ratio has some effect on the ultimate shear transfer strength. The joint interface has great influence on the shear transfer behavior of RAC. Aggregate interlock, bond, and doweling action are aspects of the joint surface condition that effect shear transfer. The RCA replacement ratio negatively affects the aggregate interlock action of RAC. This paper demonstrates that design equations in ACI and PCI codes are conservative and effective for predicting the shear stress in RAC specimens with uncracked or cold-joint interfaces. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recycled aggregate concrete (RAC) is an effective strategy for improving the sustainability of civil infrastructure. RAC is implemented by using recycled aggregates instead of natural aggregates [1]. In recent years, a large amount of waste concrete has been produced from demolition of old buildings and natural disasters such as the 2008 Wenchuan earthquake and the 2010 Yushu earthquake in China [2]. To solve environment and resource problems and to achieve sustainable development of the construction industry, the Chinese government actively supports the development of RAC. Previous investigations have shown that recycled aggregates have low apparent density, high water absorption and high porosity [3,4] due to old cement mortar attached to RCA surfaces and micro-cracks produced in the crushing process. Consequently, the mechanical properties of RAC are typically different from conven⇑ Corresponding author. E-mail address: [email protected] (J. Xiao). http://dx.doi.org/10.1016/j.conbuildmat.2015.12.015 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
tional concrete made with the same mixture proportions. RAC properties such as compressive strength and elastic modulus are lower than those of conventional concrete with same water-tocement ratio [5–7]. And many studies have shown that the RCA replacement ratio has a large effect on mechanical properties [8]. In the design of reinforced concrete structures, shear transfer across a certain plane should be considered for deep beams, corbels, shear walls, and concrete construction joints. Shear forces are transferred by mechanisms corresponding to cracking (e.g. bond action, aggregate interlock, dowel action) [9]. Many scholars have conducted tests using push-off specimens to investigate the interfacial shear transfer capacities. The concrete strength, reinforcement ratio and the joint surface condition (uncracked, precracked and cold-joint) are the main factors considered in the tests. Anderson [10] and Hanson [11] initially conducted tests on coldjoint push-off specimens, and proposed the equations to predict the ultimate shear transfer stress of cold-joint specimens. Mattock and Hawkins [12] carried out shear transfer tests on pre-cracked and uncracked push-off specimens, and provided an equation
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Nomenclature fcu1 0 fc
qv fy
cube compressive strength of concrete (test day) cylinder compressive strength of concrete (test day), 0 f c = 0.79 fcu reinforcement ratio normal to the shear plane (qv = Av/Ac, Av = 402 mm2, Ac = 36,000 mm2) yield strength of horizontal restraint steel
which related shear transfer strength to the concrete strength, reinforcement ratio and clamping force. Kahn and Mitchell [13] tested high-strength concrete push-off specimens with three types of connections, and proposed a new equation to predict the shear transfer strength of cold-joint and uncracked specimens. Xiao et al. [14] tested the shear transfer behavior of high-strength concrete at elevated temperatures. Crack width, aggregate size and Table 1 Physical properties of RCA and NCA. Coarse aggregate
Crush index (%)
Bulk density (kg m3)
Apparent density (kg m3)
Water absorption (%)
Silt content (%)
RCA NCA
10.0 3.5
1320 1465
2500 2810
5.6 0.6
3.5 0.9
Table 2 Mechanical properties of steel. Steel bars
Yield strength fy (MPa)
Modulus of elasticity E (105 MPa)
HPB235 HRB335
325 549.4
2.109 1.96
Pu
ultimate shear load ultimate shear stress (su = Pu/Ac) crack width at the ultimate shear load shear slip at the ultimate shear load
su
wu Du
temperature effect are among the factors that influence the shear transfer capacity of conventional concrete [15–17]. Shear friction is a concept which is applicable to design provisions for shear transferred across a certain plane. Birkeland and Birkeland [18] in 1966 were first to describe the shear force transfer mechanism at a concrete-to-concrete interface. Guided by the concept, formulas have been proposed to calculate the shear transfer stress for conventional concrete, high-strength concrete, lightweight concrete and so on [19–22]. Walraven and Reinhardt [23] proposed a simplified physical model to account for the aggregate interlock in concrete by treating concrete as a two-phase material consisting of coarse aggregates and cement mortar. The authors [24] conducted experiments on pre-cracked specimens of RAC, and analyzed the influence of RAC strength, RCA replacement ratio and reinforcement ratio. However, the effect of joint surface conditions and the aggregate interlock action of RAC were not considered at that time. In this paper, pre-cracked, uncracked and cold-joint specimens were tested to study the shear transfer behavior of RAC with different joint interface details.
2. Research significance Shear transfer mechanisms and aggregate interlock action with different joint interfaces are of great importance to the shear
Table 3 Mix proportions of concrete (kg/m3). RCA Replacement ratio
W/C
C (kg)
S (kg)
NCA (kg)
RCA (kg)
Mixing water (kg)
Additional water (kg)
M1 M2 M3 M4 M5 M6 M7
0 30 50 70 100 100 100
0.488 0.481 0.474 0.457 0.43 0.488 0.388
373 385 390 405 430 373 430
730 723 744 730 700 730 700
1120 742 510 300 0 0 0
0 318 510 700 950 1120 1000
182 185 185 185 185 182 165
0 12.72 20.4 28 38 44.8 40
A
40
165
120
150 80
4 14
110 100 110 400
section A-A 30
A
600
165
110
φ8@70mm
165
φ8@70mm
4 14
210
65
30
Mix
135 60 400
Fig. 1. The dimensions and reinforcements of specimens (unit: mm).
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
345
Fig. 3. The photo of test specimens and setup.
Fig. 2. Cold-joint specimen prior to casting.
The reinforced steel stirrups were HPB235 round steel bar with a diameter of 8 mm and the reinforcement perpendicular to stirrups was HRB335 ribbed steel bar with a diameter of 14 mm. The dimensions and mechanical properties are listed in Table 2.
Table 4 Basic information of push-off specimens. Specimen
RCA replacement ratio (%)
Number of stirrups
Mix
Piece
NC-1-U RC-2-U RC-3-U RC-4-U RC-5-U RC-6-U RC-7-U NC-1-C RC-2-C RC-3-C RC-4-C RC-5-C RC-6-C RC-7-C
0 30 50 70 100 100 100 0 30 50 70 100 100 100
4 4 4 4 4 4 4 4 4 4 4 4 4 4
M1 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 M7
5 1 5 1 5 1 1 5 1 5 1 5 1 1
design of RAC structures. The uncracked state and pre-cracked state represent the normal working situation, and the case with an existing crack represents the worst-case working situation. The cold-joint state can be used to simulate an interface between concretes cast at different time. Previous research has shown that the shear stress transfer capability and the mechanism of aggregate interlock across cracks in RAC will be influenced by the joint surface conditions. However, the effect of this factor has not been well understood. This paper considers the influence of different joint surface conditions (such as uncracked, pre-cracked and cold-joint) on the shear transfer behavior and shear transfer mechanism. This study will further confirm the effect of RCA replacement ratios on both ultimate shear transfer stress and aggregate interlock, and prove the adequacy of current code provisions for RAC structures. 3. Experimental descriptions
3.2. Mix proportions Seven different concrete mixture proportions, summarized in Table 3, were designed using the method presented by Xiao et al. [1]. The water is divided into net water consumption and additional water consumption. The mixing water in Table 3 refers to the net water consumption. The additional water consumption refers to the effective water absorption of RCAs, which is the 80% of the water absorption. The measured effective water absorption was about 4%. M1–M5 were designed as one series of concrete with similar concrete strength but different RCA replacement ratios. M5–M7 presented another series with a 100% RCA replacement ratio but different concrete strength.
3.3. Specimen design The dimensions and reinforcements of the push-off specimens are shown in Fig. 1. Two v-slots on the top and bottom of the specimen ensured that the initial crack appeared and developed in the shear plane. The length of v-slot was 300 mm and the depth was 15 mm. The shear plane surface area was 36,000 mm2. Horizontal restraint steel stirrups were placed across the shear plane vertically, the numbers of stirrups in the test were four closed stirrups of HPB235. The stirrup near the shear plane was wrapped by soft plastic tube that was 40 mm long, shown in Fig. 2. This measure was intended to weaken the steel bars’ dowel action at the shear plane, which have been proved by Walraven and Reinhardt [23]. Basic information for the push-off specimens is presented in Table 4. The specimen name is explained as follows: the first letters before the first hyphen ‘-’ means the aggregate type (NC for natural aggregate, RC for recycled aggregate); the numeral after the first hyphen represents the mix type (M1–M7 in Table 3); the letter after the second hyphen expresses ‘U’ for the uncracked interface condition or ‘C’ for the cold-joint interface; the last letter indicates replicate specimens. For each concrete mixture, the push-off specimens were cast along with six 100 100 100 mm cubes and 300 100 100 mm prisms. After 48 h, all specimens, cubes and prisms were demolded and moved in the curing room for curing. After 28 days, all of them were removed into the laboratory. All the uncracked specimens were cast monolithically. For the cold-joint specimens shown in Fig. 2, the right half of specimens were cast first, and two days later the left halves were then cast. The cold-joint surfaces were cast as smooth surfaces.
3.1. Materials 3.4. Test facility, instrumentation and test procedure The coarse aggregates used in the tests were natural coarse aggregates (NCAs) and recycled coarse aggregates (RCAs). The RCAs were obtained by crushing abandoned concrete. Both NCAs and RCAs were 5–26.5 mm continuous gradation. The physical properties of RCAs and NCAs are given in Table 1. The fine aggregate used was river sand. Mixing water was tap water, and the cement was PO42.5 Portland cement.
Tests of the push-off specimens were conducted using the computer controlled electro-hydraulic servo testing machine with the capacity of 3000 kN at Tongji University. Fig. 3 is the uncracked specimens’ testing device schematic. The load was applied through a knife edge loading head and a roller. The loading was applied at 0.05 mm/min in displacement control.
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
Steel bar
220
Glass sheet
220
346
160
80
80
Welding point Glass sheet
220
220
Fixed point Fixed point 110
180
Steel bar 110
(a) LVDTs measuring crack width
110
180
110
(b) LVDTs measuring slip
Fig. 4. The photo of displacement transducer arrangement.
Fig. 5. Typical uncracked specimens after failure.
Fig. 6. Typical cold-joint specimens after failure.
Linear variable displacement transducers (LVDTs) were used to measure the displacement as shown in Fig. 4. On one side of the specimen, the crack width was measured by three LVDTs. On the other side, two LVDTs were arranged vertically to measure slip. Electrical resistance strain gauges were applied to monitor the strain in the horizontal restraint stirrup near the shear plane. In order to mount the LVDTs, eight holes were drilled at the corresponding positions and anchors were glued into the holes with epoxy resin two days before testing.
4. Experimental results 4.1. Failure modes For the uncracked specimens, the initial cracks appeared on the two sides of specimen near the loading point. As the load increased to 50% ultimate shear load, some visible crack appeared along the
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(a) uncracked specimens (b) pre-cracked specimens[24]
(c) cold-joint specimens
Fig. 7. Typical specimens for three joint surfaces after failure.
Table 5 Mechanical properties and test results of push-off specimens. Specimen No.
fcu1 (MPa)
f c (MPa)
qvfy (MPa)
Pu (kN)
P u (kN)
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E RC-2-U RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E RC-4-U RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E RC-6-U RC-7-U NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E RC-2-C RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E RC-4-C RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E RC-6-C RC-7-C
39.16
30.94
3.63
308.22
39.76 32.46
31.41 25.64
3.63 3.63
38.05 38.94
30.06 30.76
3.63 3.63
29.66 41.81 39.16
23.43 33.03 30.94
3.63 3.63 3.63
39.76 32.46
31.41 25.64
3.63 3.63
38.05 38.94
30.06 30.76
3.63 3.63
29.66 41.81
23.43 33.03
3.63 3.63
304.30 325.70 311.30 288.30 311.50 282.40 283.00 283.10 269.90 278.90 281.40 314.20 267.50 285.10 259.30 267.40 295.60 292.20 287.80 107.6 160.1 113.8 92.7 117.7 146.3 111.7 148.7 117.6 164.8 81.8 129.7 158.4 212.7 172.5 145.6 102.1 191.4 206.7
0
shear plane. The crack width and shear slip increased along with the increasing of shear loading. When reaching the ultimate shear load, the crack width and shear slip increased rapidly. Eventually, the test ended when the crack width and shear slip became large resulting in shear plane crushing. Typical specimens after failure are shown in Fig. 5. The main shear cracks opened along the shear plane, and appeared approximately linear and slightly serrated. For the cold-joint specimens, the initial cracks along the shear plane were observed at 80% of the ultimate shear load. Small cracks started to appear on the two sides of specimens and the loading
282.40 279.26
314.20 274.98
292.20 287.80 118.38
146.3 124.92
129.7 158.26
191.4 206.7
su (MPa)
wu (mm)
Du (mm)
8.45 9.05 8.65 8.01 8.65 7.84 7.86 7.86 7.50 7.75 7.82 8.73 7.43 7.92 7.20 7.43 8.21 8.12 7.99 2.99 4.45 3.16 2.58 3.27 4.06 3.10 4.13 3.27 4.58 2.27 3.60 4.40 5.91 4.79 4.04 2.84 5.32 5.74
0.64 0.83 0.78 0.53 0.62 1.16 0.97 0.71 0.80 1.37 1.18 0.54 0.61 0.81 0.99 0.53 0.49 0.74 0.53 0.04 0.1 0.06 0.11 0.08 0.09 0.12 0.11 0.13 0.09 0.04 0.12 0.13 0.2 0.17 0.2 0.09 0.25 0.19
0.08 0.22 0.29 0.29 0.25 0.39 0.29 0.09 0.34 0.56 0.60 0.14 0.31 0.31 0.11 0.09 0.30 0.53 0.18 0.02 0.17 0.002 0.11 0.001 0.04 0.18 0.002 0.001 0.05 0.1 0.14 0.002 0.05 0.17 0.25 0.08 0.03 0.003
point, and developed slowly. The crack width remained very small until failure. When reaching the ultimate shear load, the crack width along the shear plane increased slowly and the shear slip increased rapidly. The failure pictures of cold-joint specimens are shown in Fig. 6. The main shear cracks appeared linear, and appeared more fine and straight than those of uncracked specimens. Typical failed specimens for the three joint surfaces are shown in Fig. 7. The shape of the main shear crack for pre-cracked and uncracked specimens are similar to each other; both appeared
top middle bottom
0.5
1.0
1.5
2.0
2.5
top middle bottom
0.0
crack width (mm)
0.5
9 8 7 6 5 4 3 2 1 0
top middle bottom
0.5
1.0
1.5
1.5
2.0
2.5
top middle bottom
0.0
2.0
2.5
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
2.0
2.5
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
crack width (mm)
(f) RC-5-U-C
shear stress (MPa)
1.5
9 8 7 6 5 4 3 2 1 0
crack width (mm)
(e) RC-4-U
1.0
(c) RC-3-U-A
9 8 7 6 5 4 3 2 1 0
crack width (mm)
0.5
crack width (mm)
(b) RC-2-U
shear stress (MPa)
shear stress (MPa)
(a) NC-1-U-D
0.0
1.0
9 8 7 6 5 4 3 2 1 0
crack width (mm)
shear stress (MPa)
0.0
9 8 7 6 5 4 3 2 1 0
shear stress (MPa)
9 8 7 6 5 4 3 2 1 0
shear stress (MPa)
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
shear stress (MPa)
348
(g) RC-6-U
9 8 7 6 5 4 3 2 1 0
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
crack width (mm)
(h) RC-7-U Fig. 8. Typical shear stress – crack width curves for uncracked specimens.
approximately linear and slightly serrated. The main shear crack for the cold-joint specimen is more fine and straight because there is no aggregate interlock.
4.2. Test results The results of push-off specimen tests are presented in Table 5. The ultimate shear stress su of uncracked specimens is 7–9 MPa and the distribution is concentrated. The shear slips of uncracked specimens at the ultimate shear load is in the range of 0–0.6 mm. The ultimate shear stress su of cold-joint specimens is 2–6 MPa, and the values are more dispersed than for uncracked specimens. The shear slips of cold-joint specimens at the ultimate shear load is in the range of 0–0.30 mm, and the average values are lower than that of uncracked specimens. For the uncracked specimens, no obvious difference has been observed comparing the test results (the ultimate shear strength, crack width and shear slip) of conventional concrete to those of RAC, similar phenomenon was also observed for the cold-joint specimens. So in the case of uncracked and cold-joint interfaces, the shear capacity of RAC is similar to that of conventional concrete.
4.3. Shear stress- crack width curves The uncracked specimens’ shear stress – crack width curves are shown in Fig. 8, and all curves are similar in shape. The response may be divided into three stages. Stage I: linear elastic behavior is observed before cracking occurs along the shear plane; Stage II: after about 50% of the ultimate shear stress, initial cracks begin to appear near the shear surface, the crack width grows rapidly with the increasing load, until the ultimate shear load; Stage III: the curve decreases at a reduced slope, eventually to a horizontal line. At this stage, the steel dowel action is remaining mechanism resisting shearing load. From Fig. 9, it can be seen that the cold-joint specimens’ curves shape are slightly different for the various concrete mixtures. All of them, however, can be divided into four stages. Stage I: linear elastic behavior is observed before cracking until about 80% of the ultimate shear stress is reached; Stage II: this phase is very short and characterized by a convex curve as the crack width grows until ultimate shear stress; Stage III: the shear stress drops to 70% of ultimate shear stress quickly, and bond along the shear plane fails; Stage IV: a horizontal section of the curve occurs when shear load is borne by the steel which plastically deforms.
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3 2 1 0
4 3 2 1 0
0.2
0.4
0.6
0.8
1.0
4 3 2 1
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
crack width (mm)
crack width (mm)
crack width (mm)
(a) NC-1-C-B
(b) RC-2-C
(c) RC-3-C-B
top middle bottom
6 5 4 3 2 1
top middle bottom
6
0
5 4 3 2 1 0
0.0
5
0 0.0
shear stress (MPa)
0.0
shear stress (MPa)
5
shear stress (MPa)
4
top middle bottom
6
0.2
0.4
0.6
0.8
1.0
5 4 3 2 1 0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
crack width (mm)
crack width (mm)
crack width (mm)
(e) RC-4-C
(f) RC-5-C-D
(g) RC-6-C
1.0
top middle bottom
6
shear stress (MPa)
1.0
top middle bottom
6
shear stress (MPa)
shear stress (MPa)
5
top middle bottom
6
shear stress (MPa)
top middle bottom
6
5 4 3 2 1 0 0.0
0.2
0.4
0.6
0.8
1.0
crack width (mm)
(h) RC-7-C Fig. 9. Typical shear stress – crack width curves for cold-joint specimens.
4.4. Shear stress-shear slip curves Typical uncracked specimens’ shear stress – shear slip curves are displayed in Fig. 10. Curves for all concrete mixtures show similar trends. The curves can be divided into three stages. Stage I: linear elastic behavior is seen up to 80% of ultimate shear stress; Stage II: the curves deviate from linear elastic before peak load as the shear slip extends above 80% of the ultimate shear stress; Stage III: the curve decreases at a reduced slope, eventually to a horizontal line. The shear slip (Fig. 10) develops later than crack widths (Fig. 8), because the mechanism of bond connection between left and right concrete parts will contribute in the shear transfer process of uncracked specimens. At the beginning of loading, the concrete is uncracked. With the increase of the shear load, some tension cracks occur along the shear plane, and the crack width develops. When the maximum load capacity is reached, debonding occurs at the concrete-to-concrete interface, then the slip occurs [25]. The shear stress – shear slip curves for cold-joint specimens are displayed in Fig. 11. While there are some differences seen for different concrete mixtures, the overall trends are roughly the same.
Similar to the cold-joint specimens’ shear stress – crack width curves, the curve can be divided into same four stages. Stages II and III are noteworthy because they are very short compared with the shear stress – crack width curves for the same materials. This occurs because the cold-joint surface is smooth without any benefit of aggregate interlock action. When the crack width is small, the shear slip is very limited. As the load increased, due to slippage, the shear reinforcement will be subjected to shear, the resistance of the bars usually referred to as dowel action. After the bond fails, the shear load is mainly carried by the dowel action [12].
5. Experimental analysis 5.1. Effect of concrete strength Research has shown that, for conventional concrete, an increase in concrete strength will not cause a proportionate increase in ultimate shear stress [26,27]. In this paper, M5–M7 were designed to study the relationship between concrete strength and ultimate shear stress for RAC concrete materials. Fig. 12a shows the ulti-
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shear stress (MPa)
shear stress (MPa)
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E
10 9 8 7 6 5 4 3 2 1 0
RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E
10 9 8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
slip (mm)
shear stress (MPa)
(a) RCA replacement ratio 0% (NC-1-U)
(b) RCA replacement ratio 50% (RC-3-U)
RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E
10 9 8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
(c) RCA replacement ratio 100% (RC-5-U) Fig. 10. Typical uncracked specimens’ shear stress – shear slip curves.
NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E
shear stress (MPa)
6 5
RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E
7 6
shear stress (MPa)
7
4 3 2 1 0
5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
slip (mm)
(a) RCA replacement ratio 0%(NC-1-C)
(b) RCA replacement ratio 50%(RC-3-C) RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E
7
shear stress (MPa)
6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
(c) RCA replacement ratio 100%(RC-5-C) Fig. 11. Typical cold-joint specimens’ shear stress – shear slip curves.
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RC-6-U RC-5-U RC-7-U
350 300
RC-6-C RC-5-C RC-7-C
250
Shear load (kN)
Shear load (kN)
200 250 200 150 100
150
100
50 50 0
0 20
25
30
35
40
20
25
Concrete strength (MPa)
30
35
40
Concrete strength (MPa)
(a) Uncracked specimens
(b) Cold-joint specimens
Fig. 12. Effects of concrete strength on shear load.
Shear load (kN)
350
300
250
200
NC-1-C RC-2-C RC-3-C RC-4-C RC-5-C
300
Shear load (kN)
NC-1-U RC-2-U RC-3-U RC-4-U RC-5-U
400
240
180
120
60 0
50
100
0
50
100
Recycled aggregate replacement (%)
Recycled aggregate replacement (%)
(a) Uncracked specimens
(b) Cold-joint specimens
Fig. 13. Ultimate shear load for different joint interfaces with different RCA replacement ratio.
Table 6 The coefficient of variation of uncracked and cold-joint push-off specimens. Specimen
RCA replacement ratio (%)
CV (%)
NC-1-U RC-3-U RC-5-U NC-1-C RC-3-C RC-5-C
0 50 100 0 50 100
4.4 1.9 5.4 21.3 26.1 25.4
mate shear load of uncracked specimens for those three concrete mixtures. From Reference [23], it can be seen when the difference among concrete strength is small, concrete strength has no significant effect on the ultimate shear load. By comparing specimens with compressive strengths of 19.9 MPa, 31.4 MPa and 56.1 MPa, the conclusion that fc is one of the influencing parameter in shear friction capacity can be made. Also, from the section ‘‘5.3 Dispersion analysis” and Ref. [23], the dispersion of the shear stress of both conventional concrete and RAC are large. There are no replicate specimens in M6 and M7 mixes. The results of RC-6 and RC7 may not represent the shear stress of M6 and M7 accurately. The results in this paper show that RAC strength has no significant effect on the ultimate shear load of RAC uncracked push-off specimens with concrete cylinder compressive strength ranging from 23.43 to 33.03 MPa.
The ultimate shear load of cold-joint specimens with M5–M7 are shown in Fig. 12b. Again, RAC strength has no significant effect on ultimate shear load. It can be observed that the concrete strength has little effect on the ultimate shear load of RAC with concrete strength ranging from 23.43 to 33.03 MPa. 5.2. Effect of recycled aggregate replacement ratio In addition to the effect of concrete strength, this paper considers the effect of RCA replacement ratio on the shear transfer properties of RAC. The results of M1–M5 for uncracked specimens are shown in Fig. 13a. NC-1-U, RC-3-U and RC-5-U are uncracked specimens with the same concrete strength and different RCA replacement ratio. Each group consists of five replicate specimens. The average values of ultimate shear load for those three groups are connected by line. The compressive strength of M3 is a little different (25.65 MPa) from the others (about 30 MPa). In this paper, it is found that when the difference among concrete strength is small, the concrete compressive strength has no significant effect on the shear transfer properties. So this difference has little influence on these experimental results. It can be seen in Fig. 13a, on the whole, ultimate shear load decreases with the increasing RCA replacement ratio. The results for M1–M5 cold-joint specimens are illustrated in Fig. 13b. With the increasing RCA replacement ratio, the ultimate
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J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
10 9
NC-1-U-E NC-1-C-A N-14b
8 7
shear stress (MPa)
shear stress (MPa)
10 9
6 5 4 3 2 1 0
RC-3-U-A RC-3-C-A R50-74b
8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
slip (mm)
slip (mm)
(a) RCA replacement ratio 0% (M1)
(b) RCA replacement ratio 50% (M3)
10
RC-5-U-A RC-5-C-A R-44b
9
shear stress (MPa)
8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
slip (mm)
(c) RCA replacement ratio 100% (M5) Fig. 14. Shear stress – shear slip curves for three joint surfaces.
(a) uncracked specimens
(b) cold-joint specimens
Fig. 15. Crack surface structure.
shear load increases. For cold-joint specimens, the joint interface is smooth, under shear load there is no aggregate interlock action. The bond at the interface is the primary factor that controls ultimate shear load. From Table 3, it can be observed that with the increasing of the RCA replacement ratio, the amount of cement used in M1–M5 increases. The bond connection between concrete parts will be enhanced when the concrete mix uses more cement. So for cold-joint specimens in this investigation, the water–cement ratio is more influential than the RCA replacement ratio. 5.3. Dispersion analysis Defects in RCAs cause large dispersion of the basic mechanical properties of RAC. Specimen sets NC-1-U, RC-3-U, and RC-5-U were designed to study the variation coefficient of push-off specimens.
The Coefficient of Variances(CVs) are listed in Table 6. The CVs of three groups are in the range of 1.9–5.4%. The ultimate shear load values and the RCA replacement ratio values are independent, which suggests the dispersion in results for uncracked conventional concrete specimens is similar to the dispersion for uncracked RAC specimens. The CVs for NC-1-C, RC-3-C and RC-5-C are calculated and listed in Table 6. The CVs of three groups are in the range of 21.3–26.1% which is much higher than that of uncracked specimens. One explanation is that it is difficult to control the cast quality of a cold-joint interface, and so greater variability is introduced. While the differences among those three groups is small, it can be seen that the RCA replacement ratio has little effect on the CVs for both uncracked specimens and cold-joint specimens.
353
M1 M3 M5
0.35
45
0.3
180 40 160 35 140 30
120
0.25
τu / f'c
200
Standard deviation (%)
Value of aggregate interlock action (kN)
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
0.2
Mattock and Hawkins [12] Loov and Patnaik [19]
0.15
Mansur et al. [20] 0.1
Test data(0%)
0.05
Test data(50%) Test data(100%)
0 0 100
0.05
0.1
0.15
0.2
50
0.3
0.35
0.4
0.45
ρv fy / f'c
25 0
0.25
100
(a) Comparison between test results and model equations for uncracked specimens
Recycled aggregate replacement (%) Fig. 16. Effects of recycled aggregate replacement ratio on aggregate interlock action.
0.4 0.35 0.3
5.4. Effect of joint interface
5.5. Aggregate interlock action Walraven and Reinhardt [23] proposed a model to account for the aggregate interlock in concrete. In this model, concrete was treated as a two-phase material consisting of coarse aggregates and cement mortar. In general, the strength and stiffness of the aggregate must be greater than hardened cement mortar. However, the interface of two materials in the transition zone is the weakest part of the system. Therefore, cracks usually propagate through the mortar and around the surface of coarse aggregate. The aggregates can be modeled as spheres with a probability distribution used to determine the depth that aggregate embeds into the
τu / f'c
0.2 ACI [21] PCI [22] Test data(0%) Test data(50%) Test data(100%)
0.15 0.1 0.05 0 0
0.1
0.2
0.3
0.4
0.5
ρv fy / f'c
(b) Comparison between test results and code values for uncracked specimens 0.3 0.25 0.2
τu / f'c
The joint interface condition is a very important factor for shear transfer mechanism of RAC. The uncracked specimens were designed to analyze the reinforced concrete member under shear load, and the pre-cracked specimens are designed to analyze the member working with cracks. The shear transfer mechanism of pre-cracked specimens is similar to that of uncracked specimens with the primary difference being the lack of the bond action. Research on the cold-joint specimens shows that the shear transfer strength of cold-joint specimens would be expected to be lowest of these specimen types because there is no aggregate interlock action in the shear transfer mechanism of cold-joint specimens (smooth interface). It is well known that the shear transfer capability across a certain plane in concrete, through mechanisms such as concrete bond action, aggregate interlock action and dowel action, contributes significantly to the shear strength of concrete structures. In order to simplify the analysis, the ultimate shear load for uncracked specimens will be divided into three main parts: V1, means aggregate interlock action; V2, means bond action; V3, means dowel action. By the same token, the ultimate shear load for precracked specimens will be simplified as the sum of two parts: V1 and V3. The ultimate shear load for cold-joint specimens will be simplified as the sum of V2 and V3. Typical uncracked and coldjoint specimens are chosen from group M1, M3 and M5 to analyze the ultimate shear transfer load. A comparison of the behavior of those specimens with pre-cracked specimens tested previously by Xiao et al. [24] is shown in Fig. 14. Fig. 14 shows that the ultimate shear stress of uncracked specimens is the highest and the values for cold-joint specimens are the lowest.
0.25
0.15 0.1
ACI [21] PCI [22] Test data(0%) Test data(50%) Test data(100%)
0.05 0 0
0.1
0.2
0.3
0.4
0.5
ρv fy / f'c
(c) Comparison between test results and code values for cold-joint specimens Fig. 17. Comparisons between test results and available equations with different joint interfaces.
crack surface. Models for different joint surface conditions are shown in Fig. 15. Through this kind of simplification, the value of aggregate interlock action can be calculated by Eqs. (1) and (2) as follows:
r ¼ rpu ðAx lAy Þ
ð1Þ
s ¼ rpu ðAy þ lAx Þ
ð2Þ
where l is the coefficient of friction, rpu is the matrix yielding stress, Ax and Ay are the average surface area of the projection in the Y and X direction under the unit crack area.
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J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
M1–M5 are five concrete mixtures with the same concrete strength but different RCA replacement ratios. Groups M1, M3 and M5 all have five replicate specimens. The results for M1, M3 and M5 are listed in Fig. 16. The values shown in the table are average values. From Fig. 16, it can be seen that with the increasing RCA replacement ratio, the value of aggregate interlock action decreases and the standard deviation increases. The decreasing of aggregate interlock action may be caused by the poor properties of RCA. The properties of RAC are mainly affected by the old cement mortar attached to RCA surface. Due to the initial damage and degradation of old mortar, cracks of recycled concrete are mainly through the old mortar and old interfacial transition zone. So the diameter of aggregate participating in aggregate interlock action is reduced. The aggregate interlock action will be weaken. 6. Evaluation of available equations 6.1. Evaluation for uncracked specimens Prior to the 1960s, many experiments had been conducted to study the shear transfer behavior of concrete. With the development of this research area, many modified formulas suitable for uncracked specimens have been proposed. In 1972, Mattock and Hawkins [12] provided a generalized expression which included the effect of external stresses on the shear plane, as follows:
su 0
fc
¼
qv f y þ rn c 0 þl 0 fc fc
!
0:3
ð3Þ
where c = 2.8 MPa and l = 0.8, and this equation is only suitable for
qvfy P 1.4 MPa.
Loov and Patnaik [19] put forward the single curve formulation, as follows:
su 0 fc
qv f y
¼ 0:573
!0:45 0:3
0 fc
ð4Þ
Compared with large amount of push-off tests, a new trilinear formulation was proposed by Mansur et al. [20]:
8 q f qv f y > < 0:075 2:5 fv0 y > f 0c > c > < su q f q f v y 0:56 0:075 fv0 y 0:27 0:385 þ 0:55 0 ¼ f 0c ðf 0c Þ c fc > > > > qv f y : 0:3 > 0:27 f0
ð5Þ
c
The shear transfer stress was given in Eq. (6) in ACI [21], Eq. (7) in PCI [22].
su 0 fc
su 0
fc
q fy 5:5 ¼ 1:4 v0 0:2 0 fc fc ¼
8 < : 1:4
1:4
2:07 qv f y
qv f y f 0c
þ 0:5
f 0c
Specimen
Test (MPa)
ACI (MPa)
Test/ACI
PCI (MPa)
Test/PCI
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E
8.453 9.047 8.647 8.008 8.653
5.082 5.082 5.082 5.082 5.082
1.663 1.780 1.702 1.576 1.703
5.082 5.082 5.082 5.082 5.082
1.663 1.780 1.702 1.576 1.703
RC-2-U RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E
7.844 7.861 7.864 7.497 7.747 7.817
5.082 5.082 5.082 5.082 5.082 5.082
1.544 1.547 1.547 1.475 1.524 1.538
5.082 5.082 5.082 5.082 5.082 5.082
1.544 1.547 1.547 1.475 1.524 1.538
RC-4-U RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E
8.728 7.431 7.919 7.203 7.428 8.211
5.082 5.082 5.082 5.082 5.082 5.082
1.717 1.462 1.558 1.417 1.462 1.616
5.082 5.082 5.082 5.082 5.082 5.082
1.717 1.462 1.558 1.417 1.462 1.616
RC-6-U
8.117
4.686
1.732
5.082
1.597
RC-7-U
7.994
5.082
1.573
5.082
1.573
Table 8 The shear strength su from the test and codes (cold-joint specimens). Specimen
Test (MPa)
ACI (MPa)
Test/ACI
PCI (MPa)
Test/PCI
NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E
2.989 4.447 3.161 2.575 3.269
2.178 2.178 2.178 2.178 2.178
1.372 2.042 1.451 1.182 1.501
2.178 2.178 2.178 2.178 2.178
1.372 2.042 1.451 1.182 1.501
RC-2-C
4.064
2.178
1.866
2.178
1.866
RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E
3.103 4.131 3.267 4.578 2.272
2.178 2.178 2.178 2.178 2.178
1.425 1.896 1.500 2.102 1.043
2.178 2.178 2.178 2.178 2.178
1.425 1.896 1.500 2.102 1.043
RC-4-C
3.603
2.178
1.654
2.178
1.654
RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E
4.400 5.908 4.792 4.044 2.836
2.178 2.178 2.178 2.178 2.178
2.020 2.713 2.200 1.857 1.302
2.178 2.178 2.178 2.178 2.178
2.020 2.713 2.200 1.857 1.302
RC-6-C
5.317
2.178
2.441
2.178
2.441
RC-7-C
5.742
2.178
2.636
2.178
2.636
ð6Þ 6.2. Evaluation for cold-joint specimens
qv f y < 4:14 qv f y
Table 7 The shear strength su from the test and codes (uncracked specimens).
qv f y 4:14
ð7Þ
Various equations have been proposed to predict the shear transfer strength in conventional concrete. Fig. 17a applies several such equations to the results of this study. The test data are mostly scattered above the curve calculated by Eqs. (3)–(5). Those equations can be used to calculate the shear transfer strength of uncracked RAC specimens. The predicted su from Eqs. (6) and (7) for the uncracked push-off specimens are compared with the test values in Table 7. From Fig. 17b, it can be seen that ACI and PCI code equations are conservative for shear transfer strength of uncracked RAC specimens with concrete strength from 23.43 to 33.03 MPa.
The ACI and PCI code equations, when l = 0.6, can be used to calculate the shear stress for a cold-joint at a smooth concrete interface, calculated results are listed in Table 8. Fig. 17c shows the ACI and PCI code provisions with the test data, and it is seen that the code equations are conservative for prediction of the shear transfer strength of cold-joint RAC specimens.
7. Conclusion The tests results and analysis of 38 uncracked and cold-joint push-off specimens, along with results for pre-cracked specimens previously published by the authors, support the following conclusions:
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
1. The joint interface condition has a great effect on the shear transfer mechanism and the shear transfer behavior of RAC. The ultimate shear transfer strength of uncracked specimens is slightly greater than that of pre-cracked specimens, but much greater than that of cold-joint specimens. 2. The RCA replacement ratio has a significance influence on the aggregate interlock action of RAC. With the increasing of the RCA replacement ratio, the value of aggregate interlock action decreases. 3. Overall, for uncracked specimens, the failure mode of RAC and that of conventional concrete are similar to each other. The shape of the shear stress – crack width curves of RAC is consistent with those of conventional concrete, and the same trends are seen in the shear stress-shear slip curve. A similar observation applies to the cold-joint specimens. 4. The strength of the RAC has no obvious effect on the ultimate shear strength of uncracked specimens with concrete cylinder compressive strength ranging from 23.43 to 33.03 MPa. With the increase of the RCA replacement ratio, the ultimate shear stress of uncracked specimens decreases. 5. Some empirical equations can be used to calculate the shear transfer strength of RAC. ACI and PCI code equations are conservative for the prediction of the shear transfer performance of uncracked and cold-joint RAC specimens with concrete cylinder compressive strength from 23.43 to 33.03 MPa.
Acknowledgements The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Project No: 51325802). The authors also acknowledge the China Scholarship Council (CSC) for support of Chang Sun’s visit to the University of Illinois. References [1] J.Z. Xiao, J.B. Li, C.H. Zhang, Mechanical properties of RAC under uniaxial loading, Cem. Concr. Res. 35 (6) (2005) 1187–1194. [2] J.Z. Xiao, W.G. Li, Y.H. Fan, X. Huang, An overview of study on RAC in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383. [3] E. Fumiya, Elasto-plastic behavior of recyclable R/C columns, Trans. Jpn. Concr. Inst. 38 (6) (1998) 301–308.
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[4] Z.X. Xing, Y.N. Zhou, Basic properties of RAC, J. N China I Water Conserv. Hydroelectr. Power 19 (2) (1998) 30–32 (in Chinese). [5] I.B. Topçu, N.F. Gunean, Using waste concrete as aggregate, Cem. Concr. Res. 25 (7) (1995) 1385–1390. [6] I.B. Topçu, S. Sengel, Properties of concretes produced with waste concrete aggregate, Cem. Concr. Res. 34 (8) (2004) 1307–1312. [7] C.S. Poon, Z.H. Shui, L. Lam, Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates, Constr. Build. Mater. 18 (6) (2004) 461–468. [8] S.C. Kou, C.S. Poon, C. Dixon, Influence of fly ash as cement replacement on the properties of RAC, J. Mater. Civ. Eng. 19 (9) (2007) 709–717. [9] R. Parker, T. Paulay, Reinforced Concrete Structures, John Wiley & Sons, 1975. [10] A.R. Anderson, Composite designs in precast and cast-in-place concrete, Prog. Architect. 41 (9) (1960) 172–179. [11] N.W. Hanson, Precast–Prestressed Concrete Bridges 2: Horizontal Shear Connections, Portland Cement Association, Research and Development Laboratories, 1960. [12] A.H. Mattock, N.M. Hawkins, Shear transfer in reinforced concrete—recent research, PCI J. 17 (2) (1972) 55–75. [13] L.F. Kahn, A.D. Mitchell, Shear friction tests with high-strength concrete, ACI Struct. J. 99 (1) (2002) 98–103. [14] J.Z. Xiao, Z.W. Li, J.B. Li, Shear transfer across a crack in high-strength concrete after elevated temperatures, Constr. Build. Mater. 71 (30) (2014) 472–483. [15] T. Paulay, P.J. Loeber, Shear Transfer by Aggregate Interlock. ACI Special Publication SP42-1, 1974, pp. 1–15. [16] N. Saenz, C.P. Pantelides, Shear friction capacity of concrete with external carbon FRP strips, J. Struct. Eng. 131 (12) (2005) 1911–1919. [17] R. Ince, E. Yalcin, A. Arslan, Size-dependent response of dowel action in RC members, Eng. Struct. 29 (6) (2007) 955–961. [18] P.W. Birkeland, H.W. Birkeland, Connections in precast concrete construction, ACI 63 (3) (1966) 345–368. [19] R.E. Loov, A.K. Patnaik, Horizontal shear strength of composite concrete beams with a rough interface, PCI J. 38 (1) (1994) 48–69. [20] M.A. Mansur, T. Vinayagam, K.H. Tan, Shear transfer across a crack in reinforced high-strength concrete, J. Mater. Civ. Eng. 20 (4) (2008) 294–302. [21] American Concrete Institute, ACI. Building Code Requirements for Structural Concrete, No. 318-2008, Committee 318, Farmington, Hills, Mich, 2008. [22] Prestressed Concrete Institute, PCI Design Handbook, sixth ed., PCI, Chicago, 2004. [23] J.C. Walraven, H.W. Reinhardt, Theory and experiments on mechanical behaviour of cracks in plain and reinforced concrete subjected to shear loading, Heron 26 (1) (1981) 5–68. [24] J.Z. Xiao, H. Xie, Z.J. Yang, Shear transfer across a crack in RAC, Cem. Concr. Res. 42 (2012) 700–709. [25] K. Zilch, R. Reinecke, Capacity of shear joints between high-strength precast elements and normal-strength cast-in-place decks, in: fib International Symposium on High Performance Concrete, Orlando, USA, September 2000. [26] J. Walraven, J. Frenay, A. Pruijssers, Influence of concrete strength and load history on the shear friction capacity of concrete members, PCI J. 32 (1) (1987) 66–84. [27] A.H. Mattock, Influence of concrete strength and load history on the shear friction capacity of concrete members – comments, PCI 33 (1) (1988) 165–166.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of joint interface conditions on shear transfer behavior of recycled aggregate concrete Jianzhuang Xiao a,⇑, Chang Sun a, David A. Lange b a b
Department of Structural Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, PR China Department of Civil and Environmental Engineering, University of Illinois, 2129B NCEL, USA
h i g h l i g h t s Effect of joint interface conditions on shear transfer behavior of RAC was tested. RCA replacement ratio negatively affects the aggregate interlock action of RAC. The shear stress in RAC with uncracked or cold-joint interfaces was predicted.
a r t i c l e
i n f o
Article history: Received 15 February 2015 Received in revised form 11 September 2015 Accepted 5 December 2015 Available online 18 December 2015 Keywords: Recycled aggregate concrete (RAC) Joint interface Shear transfer behavior Aggregate interlock Recycled coarse aggregate (RCA) replacement ratio
a b s t r a c t In this study, 38 uncracked and cold-joint push-off specimens were tested to explore how joint interface characteristics affect shear transfer behavior of recycled aggregate concrete (RAC). The major test variables included the recycled coarse aggregate (RCA) replacement ratio and joint interfaces. The results indicate that the shear transfer behavior of RAC with different joint interfaces is similar to that of conventional concrete. It was shown that the RCA replacement ratio has some effect on the ultimate shear transfer strength. The joint interface has great influence on the shear transfer behavior of RAC. Aggregate interlock, bond, and doweling action are aspects of the joint surface condition that effect shear transfer. The RCA replacement ratio negatively affects the aggregate interlock action of RAC. This paper demonstrates that design equations in ACI and PCI codes are conservative and effective for predicting the shear stress in RAC specimens with uncracked or cold-joint interfaces. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recycled aggregate concrete (RAC) is an effective strategy for improving the sustainability of civil infrastructure. RAC is implemented by using recycled aggregates instead of natural aggregates [1]. In recent years, a large amount of waste concrete has been produced from demolition of old buildings and natural disasters such as the 2008 Wenchuan earthquake and the 2010 Yushu earthquake in China [2]. To solve environment and resource problems and to achieve sustainable development of the construction industry, the Chinese government actively supports the development of RAC. Previous investigations have shown that recycled aggregates have low apparent density, high water absorption and high porosity [3,4] due to old cement mortar attached to RCA surfaces and micro-cracks produced in the crushing process. Consequently, the mechanical properties of RAC are typically different from conven⇑ Corresponding author. E-mail address: [email protected] (J. Xiao). http://dx.doi.org/10.1016/j.conbuildmat.2015.12.015 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
tional concrete made with the same mixture proportions. RAC properties such as compressive strength and elastic modulus are lower than those of conventional concrete with same water-tocement ratio [5–7]. And many studies have shown that the RCA replacement ratio has a large effect on mechanical properties [8]. In the design of reinforced concrete structures, shear transfer across a certain plane should be considered for deep beams, corbels, shear walls, and concrete construction joints. Shear forces are transferred by mechanisms corresponding to cracking (e.g. bond action, aggregate interlock, dowel action) [9]. Many scholars have conducted tests using push-off specimens to investigate the interfacial shear transfer capacities. The concrete strength, reinforcement ratio and the joint surface condition (uncracked, precracked and cold-joint) are the main factors considered in the tests. Anderson [10] and Hanson [11] initially conducted tests on coldjoint push-off specimens, and proposed the equations to predict the ultimate shear transfer stress of cold-joint specimens. Mattock and Hawkins [12] carried out shear transfer tests on pre-cracked and uncracked push-off specimens, and provided an equation
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J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
Nomenclature fcu1 0 fc
qv fy
cube compressive strength of concrete (test day) cylinder compressive strength of concrete (test day), 0 f c = 0.79 fcu reinforcement ratio normal to the shear plane (qv = Av/Ac, Av = 402 mm2, Ac = 36,000 mm2) yield strength of horizontal restraint steel
which related shear transfer strength to the concrete strength, reinforcement ratio and clamping force. Kahn and Mitchell [13] tested high-strength concrete push-off specimens with three types of connections, and proposed a new equation to predict the shear transfer strength of cold-joint and uncracked specimens. Xiao et al. [14] tested the shear transfer behavior of high-strength concrete at elevated temperatures. Crack width, aggregate size and Table 1 Physical properties of RCA and NCA. Coarse aggregate
Crush index (%)
Bulk density (kg m3)
Apparent density (kg m3)
Water absorption (%)
Silt content (%)
RCA NCA
10.0 3.5
1320 1465
2500 2810
5.6 0.6
3.5 0.9
Table 2 Mechanical properties of steel. Steel bars
Yield strength fy (MPa)
Modulus of elasticity E (105 MPa)
HPB235 HRB335
325 549.4
2.109 1.96
Pu
ultimate shear load ultimate shear stress (su = Pu/Ac) crack width at the ultimate shear load shear slip at the ultimate shear load
su
wu Du
temperature effect are among the factors that influence the shear transfer capacity of conventional concrete [15–17]. Shear friction is a concept which is applicable to design provisions for shear transferred across a certain plane. Birkeland and Birkeland [18] in 1966 were first to describe the shear force transfer mechanism at a concrete-to-concrete interface. Guided by the concept, formulas have been proposed to calculate the shear transfer stress for conventional concrete, high-strength concrete, lightweight concrete and so on [19–22]. Walraven and Reinhardt [23] proposed a simplified physical model to account for the aggregate interlock in concrete by treating concrete as a two-phase material consisting of coarse aggregates and cement mortar. The authors [24] conducted experiments on pre-cracked specimens of RAC, and analyzed the influence of RAC strength, RCA replacement ratio and reinforcement ratio. However, the effect of joint surface conditions and the aggregate interlock action of RAC were not considered at that time. In this paper, pre-cracked, uncracked and cold-joint specimens were tested to study the shear transfer behavior of RAC with different joint interface details.
2. Research significance Shear transfer mechanisms and aggregate interlock action with different joint interfaces are of great importance to the shear
Table 3 Mix proportions of concrete (kg/m3). RCA Replacement ratio
W/C
C (kg)
S (kg)
NCA (kg)
RCA (kg)
Mixing water (kg)
Additional water (kg)
M1 M2 M3 M4 M5 M6 M7
0 30 50 70 100 100 100
0.488 0.481 0.474 0.457 0.43 0.488 0.388
373 385 390 405 430 373 430
730 723 744 730 700 730 700
1120 742 510 300 0 0 0
0 318 510 700 950 1120 1000
182 185 185 185 185 182 165
0 12.72 20.4 28 38 44.8 40
A
40
165
120
150 80
4 14
110 100 110 400
section A-A 30
A
600
165
110
φ8@70mm
165
φ8@70mm
4 14
210
65
30
Mix
135 60 400
Fig. 1. The dimensions and reinforcements of specimens (unit: mm).
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
345
Fig. 3. The photo of test specimens and setup.
Fig. 2. Cold-joint specimen prior to casting.
The reinforced steel stirrups were HPB235 round steel bar with a diameter of 8 mm and the reinforcement perpendicular to stirrups was HRB335 ribbed steel bar with a diameter of 14 mm. The dimensions and mechanical properties are listed in Table 2.
Table 4 Basic information of push-off specimens. Specimen
RCA replacement ratio (%)
Number of stirrups
Mix
Piece
NC-1-U RC-2-U RC-3-U RC-4-U RC-5-U RC-6-U RC-7-U NC-1-C RC-2-C RC-3-C RC-4-C RC-5-C RC-6-C RC-7-C
0 30 50 70 100 100 100 0 30 50 70 100 100 100
4 4 4 4 4 4 4 4 4 4 4 4 4 4
M1 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 M7
5 1 5 1 5 1 1 5 1 5 1 5 1 1
design of RAC structures. The uncracked state and pre-cracked state represent the normal working situation, and the case with an existing crack represents the worst-case working situation. The cold-joint state can be used to simulate an interface between concretes cast at different time. Previous research has shown that the shear stress transfer capability and the mechanism of aggregate interlock across cracks in RAC will be influenced by the joint surface conditions. However, the effect of this factor has not been well understood. This paper considers the influence of different joint surface conditions (such as uncracked, pre-cracked and cold-joint) on the shear transfer behavior and shear transfer mechanism. This study will further confirm the effect of RCA replacement ratios on both ultimate shear transfer stress and aggregate interlock, and prove the adequacy of current code provisions for RAC structures. 3. Experimental descriptions
3.2. Mix proportions Seven different concrete mixture proportions, summarized in Table 3, were designed using the method presented by Xiao et al. [1]. The water is divided into net water consumption and additional water consumption. The mixing water in Table 3 refers to the net water consumption. The additional water consumption refers to the effective water absorption of RCAs, which is the 80% of the water absorption. The measured effective water absorption was about 4%. M1–M5 were designed as one series of concrete with similar concrete strength but different RCA replacement ratios. M5–M7 presented another series with a 100% RCA replacement ratio but different concrete strength.
3.3. Specimen design The dimensions and reinforcements of the push-off specimens are shown in Fig. 1. Two v-slots on the top and bottom of the specimen ensured that the initial crack appeared and developed in the shear plane. The length of v-slot was 300 mm and the depth was 15 mm. The shear plane surface area was 36,000 mm2. Horizontal restraint steel stirrups were placed across the shear plane vertically, the numbers of stirrups in the test were four closed stirrups of HPB235. The stirrup near the shear plane was wrapped by soft plastic tube that was 40 mm long, shown in Fig. 2. This measure was intended to weaken the steel bars’ dowel action at the shear plane, which have been proved by Walraven and Reinhardt [23]. Basic information for the push-off specimens is presented in Table 4. The specimen name is explained as follows: the first letters before the first hyphen ‘-’ means the aggregate type (NC for natural aggregate, RC for recycled aggregate); the numeral after the first hyphen represents the mix type (M1–M7 in Table 3); the letter after the second hyphen expresses ‘U’ for the uncracked interface condition or ‘C’ for the cold-joint interface; the last letter indicates replicate specimens. For each concrete mixture, the push-off specimens were cast along with six 100 100 100 mm cubes and 300 100 100 mm prisms. After 48 h, all specimens, cubes and prisms were demolded and moved in the curing room for curing. After 28 days, all of them were removed into the laboratory. All the uncracked specimens were cast monolithically. For the cold-joint specimens shown in Fig. 2, the right half of specimens were cast first, and two days later the left halves were then cast. The cold-joint surfaces were cast as smooth surfaces.
3.1. Materials 3.4. Test facility, instrumentation and test procedure The coarse aggregates used in the tests were natural coarse aggregates (NCAs) and recycled coarse aggregates (RCAs). The RCAs were obtained by crushing abandoned concrete. Both NCAs and RCAs were 5–26.5 mm continuous gradation. The physical properties of RCAs and NCAs are given in Table 1. The fine aggregate used was river sand. Mixing water was tap water, and the cement was PO42.5 Portland cement.
Tests of the push-off specimens were conducted using the computer controlled electro-hydraulic servo testing machine with the capacity of 3000 kN at Tongji University. Fig. 3 is the uncracked specimens’ testing device schematic. The load was applied through a knife edge loading head and a roller. The loading was applied at 0.05 mm/min in displacement control.
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
Steel bar
220
Glass sheet
220
346
160
80
80
Welding point Glass sheet
220
220
Fixed point Fixed point 110
180
Steel bar 110
(a) LVDTs measuring crack width
110
180
110
(b) LVDTs measuring slip
Fig. 4. The photo of displacement transducer arrangement.
Fig. 5. Typical uncracked specimens after failure.
Fig. 6. Typical cold-joint specimens after failure.
Linear variable displacement transducers (LVDTs) were used to measure the displacement as shown in Fig. 4. On one side of the specimen, the crack width was measured by three LVDTs. On the other side, two LVDTs were arranged vertically to measure slip. Electrical resistance strain gauges were applied to monitor the strain in the horizontal restraint stirrup near the shear plane. In order to mount the LVDTs, eight holes were drilled at the corresponding positions and anchors were glued into the holes with epoxy resin two days before testing.
4. Experimental results 4.1. Failure modes For the uncracked specimens, the initial cracks appeared on the two sides of specimen near the loading point. As the load increased to 50% ultimate shear load, some visible crack appeared along the
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(a) uncracked specimens (b) pre-cracked specimens[24]
(c) cold-joint specimens
Fig. 7. Typical specimens for three joint surfaces after failure.
Table 5 Mechanical properties and test results of push-off specimens. Specimen No.
fcu1 (MPa)
f c (MPa)
qvfy (MPa)
Pu (kN)
P u (kN)
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E RC-2-U RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E RC-4-U RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E RC-6-U RC-7-U NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E RC-2-C RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E RC-4-C RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E RC-6-C RC-7-C
39.16
30.94
3.63
308.22
39.76 32.46
31.41 25.64
3.63 3.63
38.05 38.94
30.06 30.76
3.63 3.63
29.66 41.81 39.16
23.43 33.03 30.94
3.63 3.63 3.63
39.76 32.46
31.41 25.64
3.63 3.63
38.05 38.94
30.06 30.76
3.63 3.63
29.66 41.81
23.43 33.03
3.63 3.63
304.30 325.70 311.30 288.30 311.50 282.40 283.00 283.10 269.90 278.90 281.40 314.20 267.50 285.10 259.30 267.40 295.60 292.20 287.80 107.6 160.1 113.8 92.7 117.7 146.3 111.7 148.7 117.6 164.8 81.8 129.7 158.4 212.7 172.5 145.6 102.1 191.4 206.7
0
shear plane. The crack width and shear slip increased along with the increasing of shear loading. When reaching the ultimate shear load, the crack width and shear slip increased rapidly. Eventually, the test ended when the crack width and shear slip became large resulting in shear plane crushing. Typical specimens after failure are shown in Fig. 5. The main shear cracks opened along the shear plane, and appeared approximately linear and slightly serrated. For the cold-joint specimens, the initial cracks along the shear plane were observed at 80% of the ultimate shear load. Small cracks started to appear on the two sides of specimens and the loading
282.40 279.26
314.20 274.98
292.20 287.80 118.38
146.3 124.92
129.7 158.26
191.4 206.7
su (MPa)
wu (mm)
Du (mm)
8.45 9.05 8.65 8.01 8.65 7.84 7.86 7.86 7.50 7.75 7.82 8.73 7.43 7.92 7.20 7.43 8.21 8.12 7.99 2.99 4.45 3.16 2.58 3.27 4.06 3.10 4.13 3.27 4.58 2.27 3.60 4.40 5.91 4.79 4.04 2.84 5.32 5.74
0.64 0.83 0.78 0.53 0.62 1.16 0.97 0.71 0.80 1.37 1.18 0.54 0.61 0.81 0.99 0.53 0.49 0.74 0.53 0.04 0.1 0.06 0.11 0.08 0.09 0.12 0.11 0.13 0.09 0.04 0.12 0.13 0.2 0.17 0.2 0.09 0.25 0.19
0.08 0.22 0.29 0.29 0.25 0.39 0.29 0.09 0.34 0.56 0.60 0.14 0.31 0.31 0.11 0.09 0.30 0.53 0.18 0.02 0.17 0.002 0.11 0.001 0.04 0.18 0.002 0.001 0.05 0.1 0.14 0.002 0.05 0.17 0.25 0.08 0.03 0.003
point, and developed slowly. The crack width remained very small until failure. When reaching the ultimate shear load, the crack width along the shear plane increased slowly and the shear slip increased rapidly. The failure pictures of cold-joint specimens are shown in Fig. 6. The main shear cracks appeared linear, and appeared more fine and straight than those of uncracked specimens. Typical failed specimens for the three joint surfaces are shown in Fig. 7. The shape of the main shear crack for pre-cracked and uncracked specimens are similar to each other; both appeared
top middle bottom
0.5
1.0
1.5
2.0
2.5
top middle bottom
0.0
crack width (mm)
0.5
9 8 7 6 5 4 3 2 1 0
top middle bottom
0.5
1.0
1.5
1.5
2.0
2.5
top middle bottom
0.0
2.0
2.5
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
2.0
2.5
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
crack width (mm)
(f) RC-5-U-C
shear stress (MPa)
1.5
9 8 7 6 5 4 3 2 1 0
crack width (mm)
(e) RC-4-U
1.0
(c) RC-3-U-A
9 8 7 6 5 4 3 2 1 0
crack width (mm)
0.5
crack width (mm)
(b) RC-2-U
shear stress (MPa)
shear stress (MPa)
(a) NC-1-U-D
0.0
1.0
9 8 7 6 5 4 3 2 1 0
crack width (mm)
shear stress (MPa)
0.0
9 8 7 6 5 4 3 2 1 0
shear stress (MPa)
9 8 7 6 5 4 3 2 1 0
shear stress (MPa)
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
shear stress (MPa)
348
(g) RC-6-U
9 8 7 6 5 4 3 2 1 0
top middle bottom
0.0
0.5
1.0
1.5
2.0
2.5
crack width (mm)
(h) RC-7-U Fig. 8. Typical shear stress – crack width curves for uncracked specimens.
approximately linear and slightly serrated. The main shear crack for the cold-joint specimen is more fine and straight because there is no aggregate interlock.
4.2. Test results The results of push-off specimen tests are presented in Table 5. The ultimate shear stress su of uncracked specimens is 7–9 MPa and the distribution is concentrated. The shear slips of uncracked specimens at the ultimate shear load is in the range of 0–0.6 mm. The ultimate shear stress su of cold-joint specimens is 2–6 MPa, and the values are more dispersed than for uncracked specimens. The shear slips of cold-joint specimens at the ultimate shear load is in the range of 0–0.30 mm, and the average values are lower than that of uncracked specimens. For the uncracked specimens, no obvious difference has been observed comparing the test results (the ultimate shear strength, crack width and shear slip) of conventional concrete to those of RAC, similar phenomenon was also observed for the cold-joint specimens. So in the case of uncracked and cold-joint interfaces, the shear capacity of RAC is similar to that of conventional concrete.
4.3. Shear stress- crack width curves The uncracked specimens’ shear stress – crack width curves are shown in Fig. 8, and all curves are similar in shape. The response may be divided into three stages. Stage I: linear elastic behavior is observed before cracking occurs along the shear plane; Stage II: after about 50% of the ultimate shear stress, initial cracks begin to appear near the shear surface, the crack width grows rapidly with the increasing load, until the ultimate shear load; Stage III: the curve decreases at a reduced slope, eventually to a horizontal line. At this stage, the steel dowel action is remaining mechanism resisting shearing load. From Fig. 9, it can be seen that the cold-joint specimens’ curves shape are slightly different for the various concrete mixtures. All of them, however, can be divided into four stages. Stage I: linear elastic behavior is observed before cracking until about 80% of the ultimate shear stress is reached; Stage II: this phase is very short and characterized by a convex curve as the crack width grows until ultimate shear stress; Stage III: the shear stress drops to 70% of ultimate shear stress quickly, and bond along the shear plane fails; Stage IV: a horizontal section of the curve occurs when shear load is borne by the steel which plastically deforms.
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3 2 1 0
4 3 2 1 0
0.2
0.4
0.6
0.8
1.0
4 3 2 1
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
crack width (mm)
crack width (mm)
crack width (mm)
(a) NC-1-C-B
(b) RC-2-C
(c) RC-3-C-B
top middle bottom
6 5 4 3 2 1
top middle bottom
6
0
5 4 3 2 1 0
0.0
5
0 0.0
shear stress (MPa)
0.0
shear stress (MPa)
5
shear stress (MPa)
4
top middle bottom
6
0.2
0.4
0.6
0.8
1.0
5 4 3 2 1 0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
crack width (mm)
crack width (mm)
crack width (mm)
(e) RC-4-C
(f) RC-5-C-D
(g) RC-6-C
1.0
top middle bottom
6
shear stress (MPa)
1.0
top middle bottom
6
shear stress (MPa)
shear stress (MPa)
5
top middle bottom
6
shear stress (MPa)
top middle bottom
6
5 4 3 2 1 0 0.0
0.2
0.4
0.6
0.8
1.0
crack width (mm)
(h) RC-7-C Fig. 9. Typical shear stress – crack width curves for cold-joint specimens.
4.4. Shear stress-shear slip curves Typical uncracked specimens’ shear stress – shear slip curves are displayed in Fig. 10. Curves for all concrete mixtures show similar trends. The curves can be divided into three stages. Stage I: linear elastic behavior is seen up to 80% of ultimate shear stress; Stage II: the curves deviate from linear elastic before peak load as the shear slip extends above 80% of the ultimate shear stress; Stage III: the curve decreases at a reduced slope, eventually to a horizontal line. The shear slip (Fig. 10) develops later than crack widths (Fig. 8), because the mechanism of bond connection between left and right concrete parts will contribute in the shear transfer process of uncracked specimens. At the beginning of loading, the concrete is uncracked. With the increase of the shear load, some tension cracks occur along the shear plane, and the crack width develops. When the maximum load capacity is reached, debonding occurs at the concrete-to-concrete interface, then the slip occurs [25]. The shear stress – shear slip curves for cold-joint specimens are displayed in Fig. 11. While there are some differences seen for different concrete mixtures, the overall trends are roughly the same.
Similar to the cold-joint specimens’ shear stress – crack width curves, the curve can be divided into same four stages. Stages II and III are noteworthy because they are very short compared with the shear stress – crack width curves for the same materials. This occurs because the cold-joint surface is smooth without any benefit of aggregate interlock action. When the crack width is small, the shear slip is very limited. As the load increased, due to slippage, the shear reinforcement will be subjected to shear, the resistance of the bars usually referred to as dowel action. After the bond fails, the shear load is mainly carried by the dowel action [12].
5. Experimental analysis 5.1. Effect of concrete strength Research has shown that, for conventional concrete, an increase in concrete strength will not cause a proportionate increase in ultimate shear stress [26,27]. In this paper, M5–M7 were designed to study the relationship between concrete strength and ultimate shear stress for RAC concrete materials. Fig. 12a shows the ulti-
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shear stress (MPa)
shear stress (MPa)
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E
10 9 8 7 6 5 4 3 2 1 0
RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E
10 9 8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
slip (mm)
shear stress (MPa)
(a) RCA replacement ratio 0% (NC-1-U)
(b) RCA replacement ratio 50% (RC-3-U)
RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E
10 9 8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
(c) RCA replacement ratio 100% (RC-5-U) Fig. 10. Typical uncracked specimens’ shear stress – shear slip curves.
NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E
shear stress (MPa)
6 5
RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E
7 6
shear stress (MPa)
7
4 3 2 1 0
5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
slip (mm)
(a) RCA replacement ratio 0%(NC-1-C)
(b) RCA replacement ratio 50%(RC-3-C) RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E
7
shear stress (MPa)
6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
slip (mm)
(c) RCA replacement ratio 100%(RC-5-C) Fig. 11. Typical cold-joint specimens’ shear stress – shear slip curves.
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RC-6-U RC-5-U RC-7-U
350 300
RC-6-C RC-5-C RC-7-C
250
Shear load (kN)
Shear load (kN)
200 250 200 150 100
150
100
50 50 0
0 20
25
30
35
40
20
25
Concrete strength (MPa)
30
35
40
Concrete strength (MPa)
(a) Uncracked specimens
(b) Cold-joint specimens
Fig. 12. Effects of concrete strength on shear load.
Shear load (kN)
350
300
250
200
NC-1-C RC-2-C RC-3-C RC-4-C RC-5-C
300
Shear load (kN)
NC-1-U RC-2-U RC-3-U RC-4-U RC-5-U
400
240
180
120
60 0
50
100
0
50
100
Recycled aggregate replacement (%)
Recycled aggregate replacement (%)
(a) Uncracked specimens
(b) Cold-joint specimens
Fig. 13. Ultimate shear load for different joint interfaces with different RCA replacement ratio.
Table 6 The coefficient of variation of uncracked and cold-joint push-off specimens. Specimen
RCA replacement ratio (%)
CV (%)
NC-1-U RC-3-U RC-5-U NC-1-C RC-3-C RC-5-C
0 50 100 0 50 100
4.4 1.9 5.4 21.3 26.1 25.4
mate shear load of uncracked specimens for those three concrete mixtures. From Reference [23], it can be seen when the difference among concrete strength is small, concrete strength has no significant effect on the ultimate shear load. By comparing specimens with compressive strengths of 19.9 MPa, 31.4 MPa and 56.1 MPa, the conclusion that fc is one of the influencing parameter in shear friction capacity can be made. Also, from the section ‘‘5.3 Dispersion analysis” and Ref. [23], the dispersion of the shear stress of both conventional concrete and RAC are large. There are no replicate specimens in M6 and M7 mixes. The results of RC-6 and RC7 may not represent the shear stress of M6 and M7 accurately. The results in this paper show that RAC strength has no significant effect on the ultimate shear load of RAC uncracked push-off specimens with concrete cylinder compressive strength ranging from 23.43 to 33.03 MPa.
The ultimate shear load of cold-joint specimens with M5–M7 are shown in Fig. 12b. Again, RAC strength has no significant effect on ultimate shear load. It can be observed that the concrete strength has little effect on the ultimate shear load of RAC with concrete strength ranging from 23.43 to 33.03 MPa. 5.2. Effect of recycled aggregate replacement ratio In addition to the effect of concrete strength, this paper considers the effect of RCA replacement ratio on the shear transfer properties of RAC. The results of M1–M5 for uncracked specimens are shown in Fig. 13a. NC-1-U, RC-3-U and RC-5-U are uncracked specimens with the same concrete strength and different RCA replacement ratio. Each group consists of five replicate specimens. The average values of ultimate shear load for those three groups are connected by line. The compressive strength of M3 is a little different (25.65 MPa) from the others (about 30 MPa). In this paper, it is found that when the difference among concrete strength is small, the concrete compressive strength has no significant effect on the shear transfer properties. So this difference has little influence on these experimental results. It can be seen in Fig. 13a, on the whole, ultimate shear load decreases with the increasing RCA replacement ratio. The results for M1–M5 cold-joint specimens are illustrated in Fig. 13b. With the increasing RCA replacement ratio, the ultimate
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10 9
NC-1-U-E NC-1-C-A N-14b
8 7
shear stress (MPa)
shear stress (MPa)
10 9
6 5 4 3 2 1 0
RC-3-U-A RC-3-C-A R50-74b
8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
slip (mm)
slip (mm)
(a) RCA replacement ratio 0% (M1)
(b) RCA replacement ratio 50% (M3)
10
RC-5-U-A RC-5-C-A R-44b
9
shear stress (MPa)
8 7 6 5 4 3 2 1 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
slip (mm)
(c) RCA replacement ratio 100% (M5) Fig. 14. Shear stress – shear slip curves for three joint surfaces.
(a) uncracked specimens
(b) cold-joint specimens
Fig. 15. Crack surface structure.
shear load increases. For cold-joint specimens, the joint interface is smooth, under shear load there is no aggregate interlock action. The bond at the interface is the primary factor that controls ultimate shear load. From Table 3, it can be observed that with the increasing of the RCA replacement ratio, the amount of cement used in M1–M5 increases. The bond connection between concrete parts will be enhanced when the concrete mix uses more cement. So for cold-joint specimens in this investigation, the water–cement ratio is more influential than the RCA replacement ratio. 5.3. Dispersion analysis Defects in RCAs cause large dispersion of the basic mechanical properties of RAC. Specimen sets NC-1-U, RC-3-U, and RC-5-U were designed to study the variation coefficient of push-off specimens.
The Coefficient of Variances(CVs) are listed in Table 6. The CVs of three groups are in the range of 1.9–5.4%. The ultimate shear load values and the RCA replacement ratio values are independent, which suggests the dispersion in results for uncracked conventional concrete specimens is similar to the dispersion for uncracked RAC specimens. The CVs for NC-1-C, RC-3-C and RC-5-C are calculated and listed in Table 6. The CVs of three groups are in the range of 21.3–26.1% which is much higher than that of uncracked specimens. One explanation is that it is difficult to control the cast quality of a cold-joint interface, and so greater variability is introduced. While the differences among those three groups is small, it can be seen that the RCA replacement ratio has little effect on the CVs for both uncracked specimens and cold-joint specimens.
353
M1 M3 M5
0.35
45
0.3
180 40 160 35 140 30
120
0.25
τu / f'c
200
Standard deviation (%)
Value of aggregate interlock action (kN)
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
0.2
Mattock and Hawkins [12] Loov and Patnaik [19]
0.15
Mansur et al. [20] 0.1
Test data(0%)
0.05
Test data(50%) Test data(100%)
0 0 100
0.05
0.1
0.15
0.2
50
0.3
0.35
0.4
0.45
ρv fy / f'c
25 0
0.25
100
(a) Comparison between test results and model equations for uncracked specimens
Recycled aggregate replacement (%) Fig. 16. Effects of recycled aggregate replacement ratio on aggregate interlock action.
0.4 0.35 0.3
5.4. Effect of joint interface
5.5. Aggregate interlock action Walraven and Reinhardt [23] proposed a model to account for the aggregate interlock in concrete. In this model, concrete was treated as a two-phase material consisting of coarse aggregates and cement mortar. In general, the strength and stiffness of the aggregate must be greater than hardened cement mortar. However, the interface of two materials in the transition zone is the weakest part of the system. Therefore, cracks usually propagate through the mortar and around the surface of coarse aggregate. The aggregates can be modeled as spheres with a probability distribution used to determine the depth that aggregate embeds into the
τu / f'c
0.2 ACI [21] PCI [22] Test data(0%) Test data(50%) Test data(100%)
0.15 0.1 0.05 0 0
0.1
0.2
0.3
0.4
0.5
ρv fy / f'c
(b) Comparison between test results and code values for uncracked specimens 0.3 0.25 0.2
τu / f'c
The joint interface condition is a very important factor for shear transfer mechanism of RAC. The uncracked specimens were designed to analyze the reinforced concrete member under shear load, and the pre-cracked specimens are designed to analyze the member working with cracks. The shear transfer mechanism of pre-cracked specimens is similar to that of uncracked specimens with the primary difference being the lack of the bond action. Research on the cold-joint specimens shows that the shear transfer strength of cold-joint specimens would be expected to be lowest of these specimen types because there is no aggregate interlock action in the shear transfer mechanism of cold-joint specimens (smooth interface). It is well known that the shear transfer capability across a certain plane in concrete, through mechanisms such as concrete bond action, aggregate interlock action and dowel action, contributes significantly to the shear strength of concrete structures. In order to simplify the analysis, the ultimate shear load for uncracked specimens will be divided into three main parts: V1, means aggregate interlock action; V2, means bond action; V3, means dowel action. By the same token, the ultimate shear load for precracked specimens will be simplified as the sum of two parts: V1 and V3. The ultimate shear load for cold-joint specimens will be simplified as the sum of V2 and V3. Typical uncracked and coldjoint specimens are chosen from group M1, M3 and M5 to analyze the ultimate shear transfer load. A comparison of the behavior of those specimens with pre-cracked specimens tested previously by Xiao et al. [24] is shown in Fig. 14. Fig. 14 shows that the ultimate shear stress of uncracked specimens is the highest and the values for cold-joint specimens are the lowest.
0.25
0.15 0.1
ACI [21] PCI [22] Test data(0%) Test data(50%) Test data(100%)
0.05 0 0
0.1
0.2
0.3
0.4
0.5
ρv fy / f'c
(c) Comparison between test results and code values for cold-joint specimens Fig. 17. Comparisons between test results and available equations with different joint interfaces.
crack surface. Models for different joint surface conditions are shown in Fig. 15. Through this kind of simplification, the value of aggregate interlock action can be calculated by Eqs. (1) and (2) as follows:
r ¼ rpu ðAx lAy Þ
ð1Þ
s ¼ rpu ðAy þ lAx Þ
ð2Þ
where l is the coefficient of friction, rpu is the matrix yielding stress, Ax and Ay are the average surface area of the projection in the Y and X direction under the unit crack area.
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J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
M1–M5 are five concrete mixtures with the same concrete strength but different RCA replacement ratios. Groups M1, M3 and M5 all have five replicate specimens. The results for M1, M3 and M5 are listed in Fig. 16. The values shown in the table are average values. From Fig. 16, it can be seen that with the increasing RCA replacement ratio, the value of aggregate interlock action decreases and the standard deviation increases. The decreasing of aggregate interlock action may be caused by the poor properties of RCA. The properties of RAC are mainly affected by the old cement mortar attached to RCA surface. Due to the initial damage and degradation of old mortar, cracks of recycled concrete are mainly through the old mortar and old interfacial transition zone. So the diameter of aggregate participating in aggregate interlock action is reduced. The aggregate interlock action will be weaken. 6. Evaluation of available equations 6.1. Evaluation for uncracked specimens Prior to the 1960s, many experiments had been conducted to study the shear transfer behavior of concrete. With the development of this research area, many modified formulas suitable for uncracked specimens have been proposed. In 1972, Mattock and Hawkins [12] provided a generalized expression which included the effect of external stresses on the shear plane, as follows:
su 0
fc
¼
qv f y þ rn c 0 þl 0 fc fc
!
0:3
ð3Þ
where c = 2.8 MPa and l = 0.8, and this equation is only suitable for
qvfy P 1.4 MPa.
Loov and Patnaik [19] put forward the single curve formulation, as follows:
su 0 fc
qv f y
¼ 0:573
!0:45 0:3
0 fc
ð4Þ
Compared with large amount of push-off tests, a new trilinear formulation was proposed by Mansur et al. [20]:
8 q f qv f y > < 0:075 2:5 fv0 y > f 0c > c > < su q f q f v y 0:56 0:075 fv0 y 0:27 0:385 þ 0:55 0 ¼ f 0c ðf 0c Þ c fc > > > > qv f y : 0:3 > 0:27 f0
ð5Þ
c
The shear transfer stress was given in Eq. (6) in ACI [21], Eq. (7) in PCI [22].
su 0 fc
su 0
fc
q fy 5:5 ¼ 1:4 v0 0:2 0 fc fc ¼
8 < : 1:4
1:4
2:07 qv f y
qv f y f 0c
þ 0:5
f 0c
Specimen
Test (MPa)
ACI (MPa)
Test/ACI
PCI (MPa)
Test/PCI
NC-1-U-A NC-1-U-B NC-1-U-C NC-1-U-D NC-1-U-E
8.453 9.047 8.647 8.008 8.653
5.082 5.082 5.082 5.082 5.082
1.663 1.780 1.702 1.576 1.703
5.082 5.082 5.082 5.082 5.082
1.663 1.780 1.702 1.576 1.703
RC-2-U RC-3-U-A RC-3-U-B RC-3-U-C RC-3-U-D RC-3-U-E
7.844 7.861 7.864 7.497 7.747 7.817
5.082 5.082 5.082 5.082 5.082 5.082
1.544 1.547 1.547 1.475 1.524 1.538
5.082 5.082 5.082 5.082 5.082 5.082
1.544 1.547 1.547 1.475 1.524 1.538
RC-4-U RC-5-U-A RC-5-U-B RC-5-U-C RC-5-U-D RC-5-U-E
8.728 7.431 7.919 7.203 7.428 8.211
5.082 5.082 5.082 5.082 5.082 5.082
1.717 1.462 1.558 1.417 1.462 1.616
5.082 5.082 5.082 5.082 5.082 5.082
1.717 1.462 1.558 1.417 1.462 1.616
RC-6-U
8.117
4.686
1.732
5.082
1.597
RC-7-U
7.994
5.082
1.573
5.082
1.573
Table 8 The shear strength su from the test and codes (cold-joint specimens). Specimen
Test (MPa)
ACI (MPa)
Test/ACI
PCI (MPa)
Test/PCI
NC-1-C-A NC-1-C-B NC-1-C-C NC-1-C-D NC-1-C-E
2.989 4.447 3.161 2.575 3.269
2.178 2.178 2.178 2.178 2.178
1.372 2.042 1.451 1.182 1.501
2.178 2.178 2.178 2.178 2.178
1.372 2.042 1.451 1.182 1.501
RC-2-C
4.064
2.178
1.866
2.178
1.866
RC-3-C-A RC-3-C-B RC-3-C-C RC-3-C-D RC-3-C-E
3.103 4.131 3.267 4.578 2.272
2.178 2.178 2.178 2.178 2.178
1.425 1.896 1.500 2.102 1.043
2.178 2.178 2.178 2.178 2.178
1.425 1.896 1.500 2.102 1.043
RC-4-C
3.603
2.178
1.654
2.178
1.654
RC-5-C-A RC-5-C-B RC-5-C-C RC-5-C-D RC-5-C-E
4.400 5.908 4.792 4.044 2.836
2.178 2.178 2.178 2.178 2.178
2.020 2.713 2.200 1.857 1.302
2.178 2.178 2.178 2.178 2.178
2.020 2.713 2.200 1.857 1.302
RC-6-C
5.317
2.178
2.441
2.178
2.441
RC-7-C
5.742
2.178
2.636
2.178
2.636
ð6Þ 6.2. Evaluation for cold-joint specimens
qv f y < 4:14 qv f y
Table 7 The shear strength su from the test and codes (uncracked specimens).
qv f y 4:14
ð7Þ
Various equations have been proposed to predict the shear transfer strength in conventional concrete. Fig. 17a applies several such equations to the results of this study. The test data are mostly scattered above the curve calculated by Eqs. (3)–(5). Those equations can be used to calculate the shear transfer strength of uncracked RAC specimens. The predicted su from Eqs. (6) and (7) for the uncracked push-off specimens are compared with the test values in Table 7. From Fig. 17b, it can be seen that ACI and PCI code equations are conservative for shear transfer strength of uncracked RAC specimens with concrete strength from 23.43 to 33.03 MPa.
The ACI and PCI code equations, when l = 0.6, can be used to calculate the shear stress for a cold-joint at a smooth concrete interface, calculated results are listed in Table 8. Fig. 17c shows the ACI and PCI code provisions with the test data, and it is seen that the code equations are conservative for prediction of the shear transfer strength of cold-joint RAC specimens.
7. Conclusion The tests results and analysis of 38 uncracked and cold-joint push-off specimens, along with results for pre-cracked specimens previously published by the authors, support the following conclusions:
J. Xiao et al. / Construction and Building Materials 105 (2016) 343–355
1. The joint interface condition has a great effect on the shear transfer mechanism and the shear transfer behavior of RAC. The ultimate shear transfer strength of uncracked specimens is slightly greater than that of pre-cracked specimens, but much greater than that of cold-joint specimens. 2. The RCA replacement ratio has a significance influence on the aggregate interlock action of RAC. With the increasing of the RCA replacement ratio, the value of aggregate interlock action decreases. 3. Overall, for uncracked specimens, the failure mode of RAC and that of conventional concrete are similar to each other. The shape of the shear stress – crack width curves of RAC is consistent with those of conventional concrete, and the same trends are seen in the shear stress-shear slip curve. A similar observation applies to the cold-joint specimens. 4. The strength of the RAC has no obvious effect on the ultimate shear strength of uncracked specimens with concrete cylinder compressive strength ranging from 23.43 to 33.03 MPa. With the increase of the RCA replacement ratio, the ultimate shear stress of uncracked specimens decreases. 5. Some empirical equations can be used to calculate the shear transfer strength of RAC. ACI and PCI code equations are conservative for the prediction of the shear transfer performance of uncracked and cold-joint RAC specimens with concrete cylinder compressive strength from 23.43 to 33.03 MPa.
Acknowledgements The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Project No: 51325802). The authors also acknowledge the China Scholarship Council (CSC) for support of Chang Sun’s visit to the University of Illinois. References [1] J.Z. Xiao, J.B. Li, C.H. Zhang, Mechanical properties of RAC under uniaxial loading, Cem. Concr. Res. 35 (6) (2005) 1187–1194. [2] J.Z. Xiao, W.G. Li, Y.H. Fan, X. Huang, An overview of study on RAC in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383. [3] E. Fumiya, Elasto-plastic behavior of recyclable R/C columns, Trans. Jpn. Concr. Inst. 38 (6) (1998) 301–308.
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