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Behavior of Reinforced Concrete Beams with RecycledConcrete Coarse AggregatesAdam M. Knaack1 and Yahya C. Kurama, M.ASCE2
Abstract: This paper investigates the flexural and shear behavior of reinforced concrete beams that use recycled concrete aggregates (RCA)as replacement for coarse natural aggregates (e.g., crushed stone, gravel). The experimental results from 12 twin pairs of normal strengthconcrete beam specimens are presented and compared with predictions from existing code methods and analytical models for conventionalconcrete. Each pair of beams is saw-cut from a single, longer member to investigate the inherent variability in the results, specifically focusingon locally available recycled materials with minimal processing and construction methods that are consistent with currentU.S. practice. It is found that the use of RCA does not cause an observable change in the progression of nonlinear behavior and failure.The effect of RCA on the flexural and shear strength of the beams is also small; however, there is a considerable reduction in the initialstiffness and an increase in the ultimate flexural deflections as the amount of RCA is increased. In general, the predicted results are reasonablyclose to the measured trends, indicating that existing analytical models and code-based procedures for conventional concrete can also beapplied to RCA concrete beams. DOI: 10.1061/(ASCE)ST.1943-541X.0001118. © 2014 American Society of Civil Engineers.
Author keywords: Recycled concrete aggregates (RCA); Reinforced concrete beams; Concrete and masonry structures.
Introduction
This paper describes an experimental and analytical investigationon the behavior of reinforced concrete beams that use recycledconcrete aggregates (RCA) as replacement for coarse natural aggre-gates (e.g., crushed stone, gravel). The U.S. state of practice per-taining to the sustainable use of structural concrete has mostlyfocused on the partial replacement of cement with industrial by-products (e.g., fly ash, silica fume). In comparison, conservationof natural coarse aggregates has been largely ignored even thoughthese materials make about 40 to 50% of the concrete mix byvolume while cement makes only about 10%. By recycling oldconcrete to replace natural aggregates, it may be possible to sub-stantially reduce the need for new aggregates, thus helping with thepreservation of forested areas and riverbeds.
To date, the use of RCA in the U.S. has been limited to nonstruc-tural applications such as roadway subbase even though the qualityof the RCA is generally significantly higher than is required in theseapplications and the material is widely available in many recyclingplants across the country. With a vision for the increased use of RCAin structural applications, the primary goal of this paper is to inves-tigate the flexural and shear behavior of reinforced concrete beamsmade using locally available recycled materials with minimalprocessing, and constructed in a manner that is consistent with cur-rent U.S. practice. A unique aspect of the study is the prequalificationof the RCA to achieve target performance objectives with respect toconcrete strength and stiffness. The paper presents the experimental
results from 12 twin pairs of normal strength concrete beams andcompares the measured results with predictions from existing codemethods and analytical models for conventional concrete.
Background
Variability in RCA Concrete Properties
Since the quality of RCA can vary considerably, Knaack and Kurama(2013a) investigated performance-based qualification specificationsusing materials from 16 recycling plants across the Midwestern U.S.For each RCA source, concrete mixes were designed for varyingcoarse aggregate replacement ratios, R (where R ¼ 0% means noRCA, and R ¼ 100% means no virgin natural aggregate, NA inthe concrete mix) and using three different methods as1. Direct weight replacement (DWR) method: In this method, the
total weight of the RCA plus virgin natural aggregate is keptconstant for any R ratio.
2. Equivalent mortar replacement (EMR) method (Abbas et al.2008; Fathifazl et al. 2009a): This method accounts for theresidual mortar in the RCA so that the concrete mix hasthe same volume of total mortar (i.e., residual mortar plusfresh mortar) and the same volume of total natural aggregate(i.e., recycled plus virgin natural aggregate) for any R ratio.
3. Direct volume replacement (DVR) method: In this method, thetotal volume of the RCA plus virgin natural aggregate is keptconstant for any R ratio.
As a major drawback, the EMR method results in significantlyreduced workability because of a decrease in the fresh mortar tocoarse material ratio (Knaack and Kurama 2013a). In comparison,the DVR method provides similar workability as conventional con-crete and is also a much simpler method consistent with conven-tional volume-based mixes from ACI 211 [American ConcreteInstitute (ACI) 1991]. Multiple least squares regression analyseswere conducted to determine predictive design relationships forthe RCA concrete compressive strength, f 0
c, and stiffness, Ec,(including 238 average data points utilizing all 16 RCA sources),
1Design Engineer, Schaefer Inc., 10411 Medallion Dr., Cincinnati, OH45241; formerly, Graduate Student, Civil and Environmental Engineeringand Earth Sciences, Univ. of Notre Dame, Notre Dame, IN 46556.
2Professor, Civil and Environmental Engineering and Earth Sciences,Univ. of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556(corresponding author). E-mail: [email protected]
Note. This manuscript was submitted on January 8, 2013; approved onMay 23, 2014; published online on July 8, 2014. Discussion period openuntil December 8, 2014; separate discussions must be submitted for indi-vidual papers. This paper is part of the Journal of Structural Engineering,© ASCE, ISSN 0733-9445/B4014009(12)/$25.00.
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which can be obtained from (Knaack and Kurama 2012) as well asfrom Knaack and Kurama (2013b). The most important factors af-fecting fcm and Esec were found as the aggregate replacement ratio,R, aggregate water absorption, Arca, [measured as % by weightabsorption according to ASTM C127 (2007)], and deleteriousmaterial content, Drca [measured as % by weight of deleteriousmaterials, such as brick, asphalt, and wood, similar to ITM 206[Indiana Department of Transportation (INDOT) 2012].
Behavior of RCA Concrete Beams
To the best of the authors’ knowledge, most of the previous re-search on the behavior of RCA concrete beams was conducted out-side the U.S. The results have shown that the flexural capacity ofreinforced concrete beams is not significantly affected by the use ofRCA even at full replacement (i.e., R ¼ 100%) (Ajdukiewicz andKliszczewicz 2007; Fathifazl et al. 2009b; Li 2009; Maruyamaet al. 2004; Mukai and Kikuchi 1988; Nishiura et al. 2000).As such, existing building codes were adequate in predicting thestrength of beams failing in flexure.
Considerable increases have been observed in the immediatedeflections of RCA concrete beams (Li 2009; Sato et al. 2007;Maruyama et al. 2004; Malesev et al. 2010). One exception isthe study by Fathifazl et al. (2009b), which reported that the de-flection increase was significantly smaller when using the EMRmix design method. Note however that large amounts of aggregatereplacement (i.e., large R) may not be possible due to the decreasedworkability of EMR concrete even with large amounts of waterreducer (Knaack and Kurama 2013a).
The previous research has reported that RCA concrete beamswithout shear reinforcement tend to have decreased shear strength(Etxeberria et al. 2007; Maruyama et al. 2004), whereas beams withshear reinforcement can achieve similar strength as conventionalconcrete (Han et al. 2001). Fathifazl et al. (2009c, 2010) showedthat the shear strength of EMR concrete beams with and withoutshear reinforcement was similar to that of beams using naturalaggregate. The majority of the research found that the initiationof diagonal cracking for RCA concrete occurred at lower loads thanfor conventional concrete. Etxeberria et al. (2007) suggested thatthis might indicate decreased aggregate interlock for RCA.
Experimental Program
A total of 24 reinforced concrete beams were tested. INDOT (2012)No. 23 concrete sand was used as fine aggregate and crushed
limestone (NA-CL) from a local ready-mix concrete plant was usedas natural coarse aggregate, which are both typical materials in theMidwestern U.S. The RCAwas obtained from a local constructionand demolition recycling plant. While this facility receives demo-lition debris from many sources, the RCA used in the project(RCA-S) was made from the foundation of a late 1920s manufac-turing plant that was being demolished in South Bend, Indiana.Both the coarse natural aggregate and the RCA satisfied INDOTNo. 8 (2012) gradation requirements with a nominal maximumaggregate size of 19.0 mm (3=4 in:). All aggregates, includingthe RCA, were available in this gradation with no additionalprocessing because of the common use of INDOT No. 8 aggregatein Indiana.
Table 1 shows the properties of the aggregates. The specificgravity and water absorption were determined using ASTMC127 and C128 (2009) for coarse and fine aggregates, respectively.The residual mortar content, RMrca, which is a measure of the per-cent (% by weight) of residual mortar in the RCA, was determinedas described in Abbas et al. (2008). The deleterious material con-tent, Drca, provides the percent (% by weight) of the deleterioussubstances in the RCA. Because RCA-S came from a demolition,there was invariably some brick, asphalt, and wood in the material.These substances were identified (but not removed) by visual in-spection of the RCA retained on a 9.50 mm (3=8 in:) sieve duringwashing as well as after oven drying (which resulted in colorvariations of the bituminous materials). Finally, the L.A. abrasionloss, which measures the change in the aggregate mass due tomechanical degradation through tumbling and falling, was deter-mined according to ASTM C131 (2009).
Concrete Mix Design and Preparation
One target natural aggregate concrete (NAC) mix design and twoRCA concrete mix designs utilizing the DVR method were used.The DVR method simply replaces an equivalent volume of coarseNA with RCA according to Eq. (1)
R ¼ 1 − VDVRna
VNACna
ð1Þ
where VDVRna = volume of NA in DVR mix; and VNAC
na = volumeof NA in NAC mix. Thus, the volume proportions of the totalcoarse aggregate (RCA plus NA), fine aggregate, cement, andwater remain constant between the different mix designs.
Table 2 shows the dry-weight proportions of the three mixdesigns corresponding to R ¼ 0% (target NAC), 50%, and 100%.
Table 2. Dry-Weight Proportions of Concrete Mix Designs
Replacement,R (%)
Watera
(kg=m3)Cement(kg=m3)
NA(kg=m3)
RCA(kg=m3)
FA(kg=m3)
HRWR(mL=m3)
AEA(mL=m3)
0 (NAC) 150 341 1137 — 676 1,874 31450 (RCA-50) 150 341 568 458 676 1,874 314100 (RCA-100) 150 341 — 917 676 1,874 314aAmount of mix water required beyond saturated surface dry condition of coarse and fine aggregates.
Table 1. Natural Aggregate and Recycled Concrete Aggregate Properties
Aggregateidentifier
Type/sourcelocation
Specific gravity, SGrca Water absorption,Ana or Arca
(% by weight)
Residual mortarcontent, RMrca(% by weight)
Deleterious materialcontent, Drca(% by weight) Gradation
L.A. abrasionloss, LArca
(% by weight)Bulk dry SSD Apparent
FA Concrete sand 2.59 2.63 2.69 1.39 — — INDOT #23 —NA-CL Crushed limestone 2.71 2.73 2.76 0.74 — — INDOT #8 21.0RCA-S South bend, IN 2.18 2.32 2.52 6.06 30.4 5.68 INDOT #8 37.8
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The target NAC mix design was proportioned with a water-to-cement (w=c) ratio of 0.44 for an approximate target 28-daystrength of 40 MPa (6,000 psi), slump of 125� 25 mm(5� 1 in:), and air content of 5.0� 1.5%. The cement used wasASTM C150 (2009) Type I portland cement, and an air-entrainingagent (AEA) and high range water reducer (HRWR) satisfying therequirements of ASTM C260 (2009) and ASTM C494 (2009)(Types A and F), respectively, were also used in each mix.
Using the measured absorption, Arca and deleterious materialcontent, Drca for RCA-S, and the predictive linear regression rela-tionships based on the data described in Knaack and Kurama(2013a), the use of RCA was expected to result in a 7 and 17%reduction in the 28-day concrete compressive strength, f 0
c atR ¼ 50% and R ¼ 100%, respectively, and in an 18 and 40%reduction in the modulus of elasticity, Ec, at R ¼ 50% andR ¼ 100%, respectively. An important objective in this prequalifi-cation is that while the determination of Arca and Drca is necessaryfor structural RCA applications, costly modifications to thematerial (for example, removal of the deleterious materials) maynot be needed as long as certain acceptance criteria are satisfiedto achieve the target mix design performance objectives.
All coarse RCA and NAwas washed and then oven-dried so thatthe material would be weighed consistently. The sand was notwashed but it also was oven-dried and weighed. At least 19 h beforecasting, all aggregates (including sand) were saturated with water.Following this period, the excess water was decanted and theaggregates were weighed again. Using the absorption values inTable 1, the amount of water in excess of the saturated surfacedry (SSD) condition was determined and subtracted from the totalmix water in Table 2 to determine the additional amount of waterneeded to achieve the desired w=c ratio. Note that the aggregatedrying and presoaking process above is not intended for commer-cial applications but rather it was used in this research to consis-tently control the amount of free water in each mix.
Design of Test Beams
The test beams were slender specimens that were 2.0 m (6.5 ft)long, 150 mm (6.0 in.) wide, and 230 mm (9.0 in.) deep. As listedin Table 3, each specimen was designed with either flexure-criticalor shear-critical details. Figs. 1(a and b) show the flexure-criticaland shear-critical cross sections, respectively, and Fig. 1(c) showsthe span view of a typical flexure-critical beam. Two No. 16(U.S. No. 5) Grade 420 (U.S. Grade 60) bars were used as the mainbottom (tension) longitudinal reinforcement. The flexure-criticalbeams also had two No. 10 (U.S. No. 3) Grade 420 bars as top
longitudinal reinforcement and No. 10 Grade 420 rectangular stir-rups as transverse reinforcement at 95 mm (3.75 in.) spacing oncenter. The shear-critical beams had no transverse stirrups or toplongitudinal reinforcement.
Casting and Curing of Test Beams
The beams were cast in their as-tested configuration (i.e., with themain longitudinal tension reinforcement located at the bottom of thebeam during casting). One 4.0 m (13 ft) long beam and a number ofcompanion cylinders and modulus of rupture beams to characterizethe concrete strength and stiffness were cast from each batch of con-crete mixed in a 0.34 m3 (12 ft3) rotating drummixer. All of the mixwater was first added to the mixer along with the liquid admixtures.With the mixer turning, the coarse and fine aggregates were addedsimultaneously and then the cement was added slowly to ensureproper mixing. Once all the material was in the mixer, the concretewas mixed for 3 min, allowed to rest for 3 min, and mixed again for2 more minutes. The beams were cured under plastic sheeting for2 days, and then they were removed from their wooden formworkand kept inside the laboratory. Following a period of approximately5 months under service loads (Knaack and Kurama 2013b), eachbeam was saw-cut into two equal lengths. The saw-cutting processresulted in a pair of specimens with identical as-cast properties foreach configuration.
Table 3. Concrete Material Properties
Beam identifier R (%) Failure mode
28-day concrete properties Beam test-day concrete properties
Ec (GPa) f 0c (MPa) f 0
t (MPa) Ec (GPa) f 0c (MPa) f 0
t (MPa)
F0-1a, F0-1b 0 Flexure 30.9 38.6 4.50 32.3 37.4 4.43F0-2a, F0-2b 0 Flexure 39.1 46.5 4.20 36.1 44.9 4.10F50-1a, F50-1b 50 Flexure 28.8 40.0 3.51 26.0 36.7 3.36F50-2a, F50-2b 50 Flexure 28.3 39.3 4.11 24.6 37.6 4.02F100-1a, F100-1b 100 Flexure 23.3 43.8 3.43 21.9 42.0 3.35F100-2a, F100-2b 100 Flexure 21.6 38.5 2.80 22.5 37.5 2.80S0-1a, S0-1b 0 Shear 29.4 32.6 4.19 27.7 31.2 4.10S0-2a, S0-2b 0 Shear 34.4 50.3 5.23 33.2 46.4 5.02S50-1a, S50-1b 50 Shear 29.4 43.6 4.41 27.6 41.8 4.32S50-2a, S50-2b 50 Shear 31.0 40.2 4.30 25.2 37.4 4.15S100-1a, S100-1b 100 Shear 23.3 41.4 3.85 20.9 39.1 3.75S100-2a, S100-2b 100 Shear 22.3 35.7 3.52 22.7 39.2 3.69
Note: Ec = modulus of elasticity, f 0c = compressive strength, f 0
t = tension strength (modulus of rupture).
150 mm
230 mm200 mm
2 No. 16 Bars
No. 10Stirrup
1980 mm
21 No. 10 Stirrups 95 mm O.C.
2 No. 10 Bars
150 mm
230 mm200 mm
2 No. 16 Bars
(a) (b)
(c)
Fig. 1. Beam specimens: (a) shear-critical section; (b) flexure-criticalsection; (c) flexure-critical span
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Concrete and Reinforcing Steel Properties
A minimum of three specimens were tested to determine thematerial properties for each concrete mix and steel bar size. Aver-age results can be found in Tables 3 and 4 for concrete and steel,respectively. The concrete strength, f 0
c, and modulus of elasticity,Ec, were obtained by testing 75 × 150 mm (3 × 6 in:) cylindersaccording to ASTM C39 (2009). The modulus of elasticity wasdetermined as the secant modulus between two points on the mea-sured stress-strain curve [using an Epsilon (Jackson, Wyoming)3542RA extensometer with 50 mm (2 in.) gauge length], withthe first point at a strain of 0.00005 and the second point at a stressof 0.40f 0
c. The tension strength, f 0t , was measured from modulus of
rupture tests according to ASTM C293 (2009).The measured trends from the concrete material tests can be seen
in Fig. 2. The data are plotted as average results from all ofthe specimens at each R ratio, with the ranges of the results fromthe different specimens also indicated. Note that because of the smallnumber of data points, the lines connecting these points do not nec-essarily suggest a relationship, but rather help visually distinguishbetween the different sets of data. The mechanical properties ofthe concrete cylinders were generally lower on the day that the beamspecimens were tested than the earlier, 28-day properties of the con-crete. This may be because air curing of the concrete cylinders (underthe same ambient conditions as the corresponding beam specimens)following removal from their molds two days after casting couldhave limited the hydration of the concrete. It can be seen from Fig. 2that increased R resulted in a considerable decrease in the concrete
stiffness, Ec, and tension strength, f 0t ; however, the effect on the
compression strength, f 0c, was small. The linear regression models
based on the data in Knaack and Kurama (2013a) provided reason-ably conservative predictions for the effect of RCA on the 28-day f 0
cand Ec results and trends from the cylinder tests.
All of the reinforcement was ASTMA615 steel and all bars of thesame size came from the same manufacturing heat. ASTM A370(2009) standards were used to determine the stress-strain behaviorsof the No. 16 and No. 10 bars in tension [Figs. 3(a and b)]. The steelstrains were measured using an MTS Model 634.25E-24 (MTS,Eden Prairie, MN) extensometer with 50 mm (2 in.) gauge length.To prevent damage to the sensor, the extensometer was removed aftera 0.5% drop in stress beyond the ultimate (peak) strength of the steel.The incremental strains following extensometer removal were calcu-lated from the differential position of the test machine crossheadsassuming that nonspecimen displacements were negligible afterthe attainment of the ultimate strength of the steel. All of the barshad a distinct yield point, which was used to determine the yieldstrength, fy, and yield strain, ϵy. The modulus of elasticity, Es,was determined from the experimental stress-strain curve as the se-cant modulus between steel stresses of 34.5 and 68.9 MPa (5.0 and10.0 ksi). The “fracture” strain, ϵf , was taken as the strain at 20%drop from the ultimate strength, fu (i.e., at a stress of ff ¼ 0.80fu).
Since the No. 10 bars were used as longitudinal reinforcement inthe compression region (i.e., top) of each flexure-critical beam, thecompressive stress-strain behavior of the bars was also measured.These tests were conducted using the set-up in Fig. 3(c), whereeach bar specimen was placed between two Jacobs chuck fixturesand loaded in compression. The free length of the bars between thechucks was the same as the center-to-center spacing of the No. 10stirrups in the flexure-critical beams. It can be seen that buckingof the bars initiated at a stress very close to the yield strength,fy, of the steel and dominated the behavior of the bars.
Test Setup
As shown in Fig. 4, each 2.0 m long (6.5 ft) beam specimen wastested monotonically to failure using a four-point loading setup.
Table 4. Reinforcing Steel Material Properties in Tension
Bar sizeEs
(GPa)fy
(MPa) ϵy fu (MPa) ϵu ϵf
No. 10 198 443 0.00290 732 0.0971 0.128No. 16 197 572 0.00360 706 0.0882 0.131
Note: Es = modulus of elasticity, fy = yield strength, ϵy = yield strain, fu =ultimate (peak) strength, ϵu = strain at ultimate strength, ϵf = “fracture”strain at stress, ff ¼ 0.80fu.
Replacement, R (%)
(G
Pa)
10000.0
45.0
28-day mean28-day rangeTest-day meanTest-day range
Knaack& Kurama Model
Replacement, R (%)1000
0.0
50.0
(M
Pa)
28-day mean28-day rangeTest-day meanTest-day range
Knaack& Kurama Model
Replacement, R (%)1000
0.00
5.50
(M
Pa)
28-day mean28-day rangeTest-day meanTest-day range
(a) (b)
(c)
Fig. 2. Effect of RCA on concrete behavior: (a) modulus of elasticity, Ec; (b) compression strength, f 0c; (c) tension strength, f 0
t
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The clear beam span between simple supports was 1.7 m (5.5 ft)and the load was applied symmetrically at 7.5 cm (3.0 in.) oneither side of the midspan. The load was applied using a servo-controlled hydraulic actuator at a displacement-controlled rate of2.5 mm=min (0.10 in:=min). A load cell was used to measure thetotal applied force and the midspan deflection of each beam wasmeasured using two string potentiometers connected to concreteinserts cast on both sides of the beam web. The average displace-ment from the two string potentiometers was taken as the midspandeflection.
Measured Behavior of Beam Specimens
Fig. 5 shows the applied midspan moment, M (neglecting beamself weight) versus average midspan deflection, Δ behavior of the
flexure-critical beams, with key results listed in Table 5. The M-Δbehavior of the beams demonstrated the following states ofresponse: (1) flexural cracking at moment, Mc and deflection,Δc (○ markers in Fig. 5); (2) yield moment, My and deflection,Δy (▵ markers); (3) a small reduction in moment due to cover con-crete crushing; (4) subsequent strain hardening up to an ultimatemoment, Mu, and deflection, Δu (▿ markers); and (5) flexuralfailure due to concrete crushing at moment, Mf , and deflection,Δf (× markers). The cracking moment, Mc, was taken whenthe secant stiffness started to deviate from the initial stiffness.The yield point, My, was determined as the point where a largedrop in stiffness occurred associated with the yielding of the lon-gitudinal bars in tension. The failure deflection, Δf, was taken at20% drop from the ultimate moment, Mu, (i.e., at Mf ¼ 0.80Mu).Note that as described in Knaack and Kurama (2013b), approxi-mately one side of the midspan of each 2.0 m (6.5 ft) long flexure-critical beam was precracked from the prior application of serviceloads on the corresponding full-length 4.0 m (13 ft) long beam.Thus, the deviation in secant stiffness at Mc during the ultimateload tests of the 2.0 m (6.5 ft) long beams likely occurred whencracking initiated on the uncracked side of the midspan.
Similarly, Fig. 6 shows the applied shear force, V, versus aver-age midspan deflection, Δ, behavior of the shear-critical beams,with key results listed in Table 6. The shear force, V, (neglectingbeam self-weight) was taken at the critical location adjacentto the load application point and was normalized into a shear forcefactor, v ¼ V=ðbd
ffiffiffiffiffif 0c
pÞ, where b is the beam width (150 mm) and
d is the effective depth to the flexural reinforcement (200 mm).Since the beam specimens were slender, cracking initiated as flexu-ral cracks at moment, Mc, and deflection, Δc (○ markers). Thesubsequent failure of the beams occurred suddenly when the shearstrength was reached at Vs (▿ markers) upon the formation ofdiagonal cracking (i.e., flexure-shear cracks). The initial stiffness,
Strain, (mm/mm)St
ress
, (
MPa
)
0.150.000
800
Tension: No. 16 Bars Measured DRAIN
Yield ( , )Ultimate ( , )Fracture ( , )
Strain, (mm/mm)
Stre
ss,
(M
Pa)
0.150.000
800
Yield ( , )Ultimate ( , )Fracture ( , )
Tension: No. 10 Bars Measured
Machine Crosshead
Machine Crosshead
Jacobs Chuck
Bar Specimen
50 mm Extensometer
Strain, (mm/mm)St
ress
, (
MPa
)
0.120.000
450
Compression: No. 10 Bars Measured DRAIN
Initiation of Buckling
(a) (b)
(c)
Fig. 3. Behavior of reinforcing steel: (a) No. 16 bars in tension; (b) No. 10 bars in tension; (c) No. 10 bars in compression
Steel Rods
Roller Support
Strong Floor
SATEC ®ReactionCrossheadServo-ControlledHydraulic Actuator
Load Cell
String Potentiometer
Beam Specimen
1680 mm
150 mm
Fig. 4. Beam test setup schematic
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Ki, for both the flexure-critical and shear-critical beams was deter-mined as the total applied load at Mc divided by Δc.
Although not visually observed, it is possible that the shear-critical beams were also precracked from the prior service loadingor shrinkage of the full-length beams. Thus, similar to the flexure-critical beams, the initial stiffness during the ultimate load testingof the shear-critical beams was expected to be lower than theuncracked stiffness. For Beams S50-2a and S50-2b [Fig. 6(d)],additional and more significant cracking occurred unintentionallyduring the transportation of the full-length beam prior to cutting. Asa result, the measured behavior of Beams S50-2a and S50-2b couldnot be used reliably, with the exception of possibly the shearstrength, Vs. Note also that during the ultimate load testing of BeamS50-1a, the hydraulic pump was not exerting the proper pressure
and the test was paused. When the test was resumed, the pumpcaused a sudden load on the beam, resulting in shear failure.The unintentional fast rate of loading may have resulted in anapparent increase in the shear strength of Beam S50-1a.
It can be seen from Figs. 5 and 6 that even though the twin spec-imens in each pair of beams were cut from a single, longer member,there were considerable differences in their load-deflection behav-ior. For the flexure-critical beams, the largest differences occurredin the ultimate and failure deflections, Δu and Δf , respectively,with relatively small differences in strength. For the shear-criticalbeams, there were significant discrepancies in the shear strengths,Vs, within each pair. These differences demonstrate the inherentvariability in the behavior of reinforced concrete structures. No sig-nificant differences occurred between the failure mechanisms of the
Mom
ent,
M (
kN-m
)
0.0
50.0
Deflection, ∆ (mm)800
R = 0%Measured: F0-1a, F0-1bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu) M
omen
t, M
(kN
-m)
0.0
50.0
Deflection, ∆ (mm)800
R = 0%Measured: F0-2a, F0-2bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu)
Mom
ent,
M (
kN-m
)
0.0
50.0
Deflection, ∆ (mm)800
R = 50%Measured: F50-1a, F50-1bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu) M
omen
t, M
(kN
-m)
0.0
50.0
Deflection, ∆ (mm)800
R = 50%Measured: F50-2a, F50-2bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu)
Mom
ent,
M (
kN-m
)
0.0
50.0
Deflection, ∆ (mm)800
R = 100%Measured: F100-1a, F100-1bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu) M
omen
t, M
(kN
-m)
0.0
50.0
Deflection, ∆ (mm)800
R = 100%Measured: F100-2a, F100-2bDRAIN
Cracking (∆c, Mc)Yield (∆y, My)Ultimate (∆u, Mu)Flex. Fail. (∆f, 0.8Mu)
(a) (b)
(c) (d)
(e) (f)
Fig. 5. Flexure-critical beam behavior: (a) R ¼ 0%, F0-1a and F0-1b; (b) R ¼ 0%, F0-2a and F0-2b; (c) R ¼ 50%, F50-1a and F50-1b; (d) R ¼ 50%,F50-2a and F50-2b; (e) R ¼ 100%, F100-1a and F100-1b; (f) R ¼ 100%, F100-2a and F100-2b
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twin specimens in each pair of beams. Furthermore, the use ofRCA did not cause an observable change in the progression ofnonlinear behavior and failure. Representative photos depictingtwo flexure-critical beams (for R ¼ 0 and 100%) at Δf and twoshear-critical beams immediately after shear failure (i.e., afterthe attainment of Vs) are shown in Fig. 7.
The trends from the key results in Tables 5 and 6 are plotted inFigs. 8 and 9 for the flexure-critical and shear-critical beams, re-spectively. Similar to the concrete properties in Fig. 2, the data inFigs. 8 and 9 are plotted as average results from all of the specimensat each R ratio, with the range of the results from the different spec-imens indicated using gray shaded bands. To show the effect ofRCA, the data is normalized with respect to the average behaviorof the beams with R ¼ 0% (i.e., with no RCA). It can be seen thatincreased amounts of RCA (increased R) resulted in a reductionin the initial stiffness, Ki, but the effect on the flexural or shearstrength of the beams was relatively small. For the flexure-criticalbeams, the deflection, Δu, at ultimate moment increased with R.The trend for the increased beam deflections is consistent withthe smaller stiffness of RCA concrete as compared with conven-tional concrete. As shown in Fig. 2 and investigated in Knaackand Kurama (2013a), the effect of RCA on the compressionstrength of concrete is much smaller than the effect on the concretestiffness, which is also consistent with the results on the flexuralstrength of the tested beams.
Predicted Behavior of Beam Specimens
The beam specimens were modeled using a modified version ofthe DRAIN-2DX program (Prakash et al. 1993) as well as conven-tional analysis based on ACI 318 (ACI 2011). The flexure-criticalbeams were analyzed using the axial-flexural fiber element (re-ferred to as Element 15) in DRAIN-2DX. For example, Fig. 10(a)shows a typical model elevation. Within each element, parallelfibers represent the beam cross-section (or slice) at the mid-lengthof the element where the behavior of the structure is monitored. Alarger number of fiber elements and fibers are typically used wherethe nonlinear behavior is expected to concentrate.
As depicted in Fig. 10(a), each fiber slice consists of concreteand longitudinal steel fibers. Fig. 10(b) shows typical uniaxialstress-strain models for the concrete in compression, which weredetermined for each beam based on the measured material datain Table 3. The steel models, shown by the dashed lines in Figs. 3(a and c), were also based on the measured stress-strain behaviors.The concrete pre-peak stress-strain behavior was modeled using therelationship proposed by Popovics (1973), while the post-peakbehavior was determined according to Kent and Park (1971), in-cluding the confining effect of the transverse stirrups on the coreconcrete. As a small variation of the original model from Kent andPark (1971), the descending branches of the unconfined and con-fined concrete stress-strain behaviors were extended to a residualstress of 0.15f 0
c. Since about one-half the span length of all of theflexure-critical beams was precracked from service-load testing(Knaack and Kurama 2013b), the concrete over the entire beamlength was modeled with no strength in tension.
The shear-critical beams were analyzed using similar con-cepts, but with a microplane based fiber element (referred to asElement 16) that incorporates coupled axial-flexural-shear interac-tion in the nonlinear range (Jiang and Kurama 2010). All of theconcrete in these beams was modeled as unconfined. As describedin Jiang and Kurama (2010), the concrete stress-strain properties inElement 16 were input as microplane stresses and strains, whichwere calibrated so that the resulting fiber stresses and strainsT
able
5.Flexure-Critical
Beam
Test
Results
Beam
identifier
R(%
)
Initial
stiffnessK
i,(kN=m
m)
Flexural
cracking
mom
ent,M
c(kN-m
)Yield
mom
ent,
My(kN-m
)Ultimatemom
ent,M
u(kN-m
)Ultimatedeflectio
n,Δ
u(m
m)
Flexural
failu
redeflectio
n,Δ
f(m
m)
Measured
ACI-318
DRAIN
Measured
ACI-318
Measured
DRAIN
Measured
ACI-318
DRAIN
Measured
DRAIN
Measured
DRAIN
F0-1a
028.0
43.8
15.2
7.8
6.8
42.6
40.0
42.6
41.6
40.3
37.6
33.8
73.4
61.2
F0-1b
30.7
7.5
41.9
43.1
33.5
77.5
F0-2a
052.1
46.4
15.9
8.3
6.1
42.2
40.6
43.8
45.2
40.9
37.6
45.2
77.0
73.7
F0-2b
37.4
8.0
41.6
43.8
44.5
66.5
F50-1a
5020.8
33.9
14.6
8.8
6.4
41.3
40.0
41.8
42.1
40.3
35.1
35.6
60.2
62.2
F50-1b
18.5
8.6
42.2
43.1
42.9
74.4
F50-2a
5018.0
31.6
14.6
8.0
5.3
41.0
40.0
41.3
41.9
40.2
36.8
34.0
59.2
60.7
F50-2b
19.5
8.0
41.0
41.3
43.4
66.8
F100-1a
100
20.6
28.6
14.2
7.1
5.5
40.5
39.8
41.7
43.8
40.7
39.6
38.6
71.9
70.1
F100-1b
19.6
8.3
40.5
41.7
51.1
81.0
F100-2a
100
17.7
27.2
14.2
9.5
4.6
41.8
39.6
44.1
43.6
40.3
50.8
33.5
67.1
61.7
F100-2b
15.1
8.5
40.0
42.5
44.2
59.4
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matched the measured behavior of the concrete. The behavior ofthe tensile longitudinal reinforcing bars was also based on the mea-sured steel stress-strain relationships. Since there was no observ-able service-load cracking of the shear-critical beams (Knaack andKurama 2013b), the concrete was modeled as uncracked using thetension stress-strain behavior in Fig. 10(c).
Each specimen was also analyzed using ACI 318 (ACI 2011). Forthe ultimate moment, Mu, of the flexure-critical beams, the strainhardening of the tension steel was ignored and the Whitney stressblock was used to evaluate this common assumption for RCA con-crete. For the shear-critical beams, the shear strength, Vs, was deter-mined based on Equation 11-5 of ACI 318. The cracking moment,Mc, for the flexure-critical and shear-critical beams was calculated
using the uncracked transformed section. The corresponding deflec-tion,Δc, for the shear-critical beams was also based on the uncrackedtransformed section, but for the flexure-critical beams, an effectivemoment of inertia was used for the precracked half of each beam con-sidering the service-load testing of the corresponding full-length beam.
Looking at Figs. 5 and 6, the DRAIN-2DX models providedgood overall estimates of the flexure-critical and shear-criticalbeam behavior. These results indicate that existing analytical ap-proaches can be used to predict the flexural and shear behavior ofRCA concrete beams within similar variability as for conventionalconcrete. The DRAIN-2DX as well as ACI 318 predictions arealso given in Tables 5 and 6 and superimposed with the measuredresults in Figs. 8 and 9. The models for the flexure-critical beams
0.00
(a) (b)
(c) (d)
(e) (f)
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 0%Measured: S0-1a, S0-1bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
0.00
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 0%Measured: S0-2a, S0-2bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
0.00
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 50%Measured: S50-1a, S50-1bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
S50-1a
0.00
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 50%
Measured: S50-2a, S50-2bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
0.00
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 100%Measured: S100-1a, S100-1bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
0.00
0.25
Deflection, ∆ (mm)100
Shea
r Fo
rce
Fact
or,
R = 100%Measured: S100-2a, S100-2bDRAIN
Cracking (∆c, Mc)Ultimate (∆s, Vs)
Fig. 6. Shear-critical beam behavior: (a) R ¼ 0%, S0-1a and S0-1b; (b) R ¼ 0%, S0-2a and S0-2b; (c) R ¼ 50%, S50-1a and S50-1b; (d) R ¼ 50%,S50-2a and S50-2b; (e) R ¼ 100%, S100-1a and S100-1b; (f) R ¼ 100%, S100-2a and S100-2b
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generally predicted the reduction in Ki with increased R; however,the amount of reduction was underestimated, especially usingDRAIN-2DX. This is likely because the predictions assumed thesame amount of precracking for each R, whereas as discussedin Knaack and Kurama (2013b), there was an increase in theamount of observed cracking with an increase in R. It can also beseen from Table 5 that the fully cracked beam assumption forDRAIN-2DX resulted in a significant underestimation of Ki, whilethe overall stiffness up to My was predicted reasonably well asshown in Fig. 5. The models were consistent with the measuredresults regarding the relatively small effect of RCA on the flexuralstrength of the beams; however, the DRAIN-2DX model was notable to capture the increased deflection, Δu, with increased R.
For the shear-critical beams (Table 6), ACI 318 typically gaveconservative estimates of the shear strength, Vs while DRAIN-2DXprovided more accurate estimates. The predicted Ki based on theuncracked beam assumption for the shear-critical beams signifi-cantly overestimated the measured Ki, which could have occurred
due to a small (unobservable) amount of precracking from theservice-load tests of the beams as stated previously.
Future Research Needs
Several additional studies should be conducted to further theknowledge and use of RCA in reinforced concrete building struc-tures as follows: (1) additional concrete mix designs, natural andrecycled aggregates (especially from different regions of the U.S.),as well as higher-strength concrete; (2) development length ofreinforcement; (3) effects of aggregate surface conditions, angular-ity, and interfacial transition zone; (4) durability of RCA concrete;(5) long-term creep and shrinkage deformations; (6) curing, envi-ronmental, and age effects (e.g., strength gain of concrete); (7)prediction and prequalification of RCA concrete properties; and(8) service-load and ultimate-load behavior of RCA concretecolumns, beam-column joints, frames, and walls.
Table 6. Shear-Critical Beam Test Results
Beamidentifier
R(%)
Initial stiffness,Ki (kN=mm)
Flexural crackingmoment, Mc (kN-m) Shear strength, Vs (kN)
Shear strength factor,vs ¼ Vs=ðbd
ffiffiffiffiffif 0c
pÞ
Measured ACI-318 Measured ACI-318 MeasuredACI-318Eq. (11-5) DRAIN Measured
ACI-318Eq. (11-5) DRAIN
S0-1a 0 27.6 47.7 7.65 6.28 31.1 28.8 37.6 0.18 0.17 0.22S0-1b 32.9 8.01 36.9 0.21S0-2a 0 36.4 56.2 7.65 7.49 40.4 34.7 43.2 0.20 0.17 0.21S0-2b 45.4 7.83 42.3 0.21S50-1a 50 28.2 47.6 8.15 6.62 44.0 33.0 38.9 0.22 0.17 0.20S50-1b 30.1 8.15 39.1 0.20S50-2a 50 — 43.8 — 6.46 43.7 31.3 37.9 0.24 0.17 0.20S50-2b — — 41.2 0.22S100-1a 100 22.4 37.1 7.46 6.02 36.4 32.0 35.3 0.19 0.17 0.19S100-1b 23.2 8.01 38.0 0.20S100-2a 100 27.8 40.0 6.96 5.85 39.9 32.1 35.1 0.21 0.17 0.19S100-2b 24.3 7.33 36.1 0.19
F0-1b
F100-1b
S0-2aS100-1a
(a)
(b)
Fig. 7. Typical beam behavior: (a) flexure-critical beams at Δf; (b) shear-critical beams immediately after failure
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Replacement, R (%)K
i(R
)/K
i(R
=0%
)1000
0.00
1.60 ACI 318 meanACI 318 rangeDRAIN meanDRAIN range
Measured meanMeasured range
Replacement, R (%)
Mc(
R)/
Mc(
R=
0%)
10000.00
1.60
ACI 318 meanACI 318 rangeMeasured meanMeasured range
Replacement, R (%)
My(
R)/
My(
R=
0%)
10000.00
1.60
Measured meanMeasured rangeDRAIN meanDRAIN range
Replacement, R (%)
Mu(
R)/
Mu(
R=
0%)
10000.00
1.60
ACI 318 meanACI 318 rangeDRAIN meanDRAIN range
Measured meanMeasured range
Replacement, R (%)
∆ u(R
)/∆ u
(R=
0%)
10000.00
1.60
Measured meanMeasured rangeDRAIN meanDRAIN range
Replacement, R (%)
∆ f (
R)/
∆ f (
R=
0%)
10000.00
1.60
Measured meanMeasured rangeDRAIN meanDRAIN range
(a) (b)
(c) (d)
(e) (f)
Fig. 8. Effect of RCA on flexure-critical beam behavior: (a) initial stiffness,Ki; (b) flexural cracking moment,Mc; (c) yield moment,My; (d) ultimatemoment, Mu; (e) ultimate deflection, Δu; (f) flexural failure deflection, Δf
Replacement, R (%)
Ki(
R)/
Ki(
R=
0%)
10000.00
(a) (b)
(c)
1.60
ACI 318 meanACI 318 rangeMeasured meanMeasured range
DRAIN meanDRAIN range
Replacement, R (%)
Mc(
R)/
Mc(
R=
0%)
10000.00
1.60
ACI 318 meanACI 318 rangeMeasured meanMeasured range
Replacement, R (%)
v s(R
)/v s
(R=
0%)
10000.00
1.60
ACI 318 meanMeasured meanMeasured rangeDRAIN meanDRAIN range
Fig. 9. Effect of RCA on shear-critical beam behavior: (a) initial stiffness, Ki; (b) flexural cracking moment, Mc; (c) shear strength factor, vs
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Summary and Conclusions
The experimental results from 12 twin pairs of normal strengthconcrete beam specimens (with different reinforcement details andRCA amounts) are presented and compared with predictions fromexisting analytical models and code-based procedures for conven-tional concrete. The important findings from the investigation aresummarized below. Note that these findings may be limited to thematerials and specimens tested.1. RCA does not cause an observable change in the progression
of nonlinear behavior and failure in flexure-critical and shear-critical reinforced concrete beams.
2. Increased amounts of RCA (increased R) result in a reductionin the initial stiffness, Ki, but the effect on the flexural or shearstrength of the beams is relatively small.
3. For flexure-critical beams, deflection, Δu, at ultimate momentincreases as R increases.
4. The predicted results are reasonably close to the measuredtrends, indicating that existing analytical models and code-based procedures for conventional reinforced concrete beamscan also be applied to RCA concrete beams.
5. While further research is certainly needed, a preliminary ex-pectation from this paper is that locally available RCA withprequalified absorption and deleterious material content maybe suitable for use in reinforced concrete beams constructedin a manner that is consistent with current U.S. practice forconventional concrete applications.
Acknowledgments
The authors thank Danny Atkinson of Concrete Recycling Center,South Bend, Indiana, and Mark Zeltwanger of American MobileAggregate Crushing, Nappanee, Indiana, for their help in acquiring
the RCA. Additional materials were provided by Buzzi Unicem,Sika Corporation, and Transit Mix South Bend. The authors alsoacknowledge Dave Schelling of the LaPorte district INDOT officefor his help in conducting the L.A. Abrasion testing. Any opinions,findings, conclusions, and/or recommendations in the paper arethose of the authors and do not necessarily represent the viewsof the individuals or organizations acknowledged.
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0.50
0.15
Confined
Unconfined
(a)
(b) (c)0.006
~0
Beam Outline 15 EvenlySpaced Nodes
DisplacementControl Node
AppliedLoads
Support Node Support Node
305 mm 305mm
20-UC Fibers
34-UC Fibers34-CC Fibers
1-UC Fiber1-CC Fiber
34-UC Fibers34-CC Fibers
Steel Fiber
Steel Fiber6-CC Fibers
20-UC Fibers6-CC Fibers
5.0 mm
25.5 mm
87.0 mm
87.0 mm
25.5 mm
ConfinedConcrete
(CC)
UnconfinedConcrete
(UC)
}
}
}
}
}Elevation Cross-Section
Fig. 10. DRAIN-2DX modeling: (a) flexure-critical beam model elevation and cross section; (b) concrete fiber in compression (not to scale);(c) concrete fiber in tension used to model shear-critical beams (not to scale)
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