Concrete International - Learning Through Hands On Beam Experiments 2015.Pdf

40 JULY 2015 | Ci | www concreteinternational com Learning through Hands On Reinforced Concrete Beam Experiments Students in senior level design class gain valuable insights into beam behavior by Ian[.]

Learning through Hands-On Reinforced Concrete

Beam Experiments

Students in senior-level design class gain valuable insights into beam behavior

by Ian N Robertson and Gaur P Johnson

Reinforced concrete design can be a relatively dry topic

for instructors and their students While the introduction

of outstanding reinforced concrete construction

projects and current laboratory research projects can pique

students’ interest, the basic theory and design requirements for

reinforced concrete beams, for example, can be difficult to

convert to an exciting topic Possibly more importantly,

students seldom gain an appreciation for the anticipated

performance of reinforced concrete members when loaded to

failure It is difficult for them to appreciate the variability of

actual performance or the intrinsic margin of safety built into

the ACI 318-111 design requirements

In an attempt to augment classroom instruction, a laboratory

section was added to the senior-level reinforced concrete

design class at the University of Hawaii (UH) at Manoa

Students work in groups of six or seven, and each group

fabricates and tests a large-scale reinforced concrete beam by

loading it to failure Each beam is designed with a particular

objective in mind, with the intent of highlighting a specific

failure mode

During the Fall 2014 semester, 88 students participated in

this course and laboratory section, constructing 13 beams with

various types and amounts of flexural and shear reinforcement

In addition to conventional steel deformed reinforcing bars

used in most of the beams, two beams were fabricated with

glass fiber-reinforced polymer (GFRP) bars as flexural

reinforcement to demonstrate the differences in design and

behavior All beams were cast using the same concrete

mixture, except that polypropylene (PP) fibers were added to

the concrete prior to placement of two beams All beams were

tested to failure in three-point bending, either with a relatively

long span to induce flexural failure or a short span to induce

shear failure

Beam Details

All beams had rectangular, 10 in wide and 18 in deep (254 x 457 mm) cross sections Cover to the reinforcing steel was set at 1 in (25 mm) on all sides Longitudinal and transverse reinforcement varied per Table 1 Each student group was responsible for cutting and bending the longitudinal reinforcement required for their beam Because the amount of shear reinforcement varied significantly between beams, each group was assigned an equal portion of the total stirrup fabrication required for the 13 beams

Beams 1 through 6 were designed to demonstrate various flexural limit states Beams 1, 2, and 5 were designed to demonstrate flexural response of beams with less than minimum reinforcement, tension-controlled, and transition zone flexural reinforcement, respectively Beam 3 was designed per ACI 440.1R-062 recommendations, and the reinforcement was selected to provide the same design flexural strength as Beam 2 with steel bars (to demonstrate that varying the modulus of elasticity of the reinforcement affects the flexural behavior of beams) Beam 3F was identical

to Beam 3 except that PP fibers were added to the concrete (to evaluate the effects of fibers on beam flexural behavior) Beam 4 was identical to Beam 2, except that two of the three tension bars were spliced at midspan using splice lengths that did not comply with ACI 318-11 Code requirements (to demonstrate the importance of meeting splice requirements) Finally, Beam 6 had the same tension reinforcement as Beam 5, but it also included significant compression reinforcement (to demonstrate that compression reinforce-ment can modify the beam response from transition zone to tension-controlled behavior)

Beams 7 through 12 were designed to demonstrate shear limit states Beams 7 and 8 had no shear reinforcement, but

the former comprised plain concrete and

the latter comprised concrete with PP

fibers (to evaluate the effects of fibers

on shear strength) Beams 9, 10, and 11

had increasing amounts of shear

reinforcement—from slightly more than

A v,min (per ACI 318-11) in Beam 9 to

more than A v,max in Beam 11 (the steel

contribution exceeded the maximum of

8√f c′(b w d)) While all of the other beams

had hooks at both ends of all tension

reinforcement, Beam 12 had straight

bars with no hooks (to demonstrate the

necessity for well-anchored tension

reinforcement to achieve the beam

shear capacity)

The groups assembled their own

reinforcing cages and placed them in the

beam forms with appropriate cover

blocks and lifting hoops (Fig 1) Each

group was also responsible for placing,

vibrating, screeding, and curing the

concrete in their beam The concrete

required for all beams was delivered in a

single mixer truck After the as-delivered concrete had been placed in 11 of the beams, PP fibers and a high-range water-reducing admixture were added to the remaining concrete, and the truck was run at mixing speed for 5 minutes

Prior to testing any of the beams, each student group was required to submit a draft report including the ACI 318-11 Code nominal strength for

Table 1:

Beam span, reinforcement, and parameters of interest

* Two of the No 6 bottom bars were spliced with 10 in laps at midspan

† No hook at end of No 8 bottom bars

Note: 1 in = 25.4 mm

Fig 1: Beam fabrication: (a) closing forms with reinforcing cages assembled by students; and (b) concrete placement and finishing

all beams except those with GFRP reinforcing bars and those with PP fibers They were also required to predict the failure load for each of these beams, taking into account the level of conservatism inherent in ACI Code predictions The group with the closest predictions was promised prizes: a $25 music gift card for each group member (courtesy of the instructor)

Material Properties

Concrete

Concrete was ordered from a local ready mixed concrete

supplier, with a specified compressive strength of 4000 psi

(27.6 MPa) During the concrete placement, a number of

standard 6 x 12 in (150 x 300 mm) cylinders were made from

samples of the as-delivered concrete and the mixture modified

with PP fibers One of the cylinders from the latter mixture

was immediately washed out to determine the actual fiber

content With 1.133 oz (32 g) of fiber collected from the

cylinder, the fiber content in the concrete was computed to be

9.74 lb/yd3 (5.78 kg/m3) Three cylinders were tested at 28 days

to determine the compressive strengths per ASTM C39/C39M,

“Standard Test Method for Compressive Strength of Cylindrical

Concrete Specimens”; see Table 2 Four 6 x 6 x 24 in

(150 x 150 x 600 mm) beams were fabricated (two from

each mixture type) to evaluate modulus of rupture per

ASTM C78/C78M, “Standard Test Method for Flexural

Strength of Concrete (Using Simple Beam with Third-Point

Loading).” It is noteworthy that the presence of fibers

increased the compressive strength of the concrete slightly,

but also appeared to have a detrimental effect on bending

strength (Table 2) A larger test program would be required to

determine if this is a valid result

Reinforcing steel

Coupons of the various sizes of reinforcing steel were

tested in tension to determine yield and tensile strengths

Table 2:

Concrete material properties

Concrete mixture

Unit weight,*

Compressive strength*

(f c′), psi Modulus of rupture† (f r), psi

* Average of three 6 x 12 in (150 x 300 mm) cylinder tests

† Average of two 6 x 6 x 24 in (150 x 150 x 600 mm) beam tests

Note: 1 lb/ft 3 = 16 kg/m 3 ; 1 psi = 0.00689 MPa

Table 3:

Reinforcing steel properties

Bar size

Yield strength

Tensile

ksi

* Exceeded capacity of test frame Note: 1 ksi = 6.89 MPa

70 in 140 in

45 in 90 in

78 in

39 in

14 in

14 in

P

18 in

d´

d2 d1

10 in

18 in

d´

d2 d1

128 in

64 in

(a)

(b)

70 in 140 in

45 in 90 in

78 in

39 in

14 in

14 in

P

18 in

d´

d2 d1

10 in

18 in

d´

d2 d1

128 in

64 in

(a)

(b)

(Table 3) Because of limitations of the test equipment, the

No 6 bar tests did not reach the tensile strength, and the No 8 bars could not be tested The yield strength of the No 8 bars was assumed to be the same as the No 6 bars for purposes of calculation of beam capacity per ACI 318-11 The GFRP bars were not tested, so the elastic modulus and tensile strength and strain reported by the manufacturer were used in determining the GFRP beam capacities

Test Setup

The beams were tested in three-point bending over a 128 in

(3250 mm) span for the long beams with anticipated flexural failure and over a 78 in (1980 mm) span for the short beams with anticipated shear failure (Fig 2) The load was applied through a 14 in (355 mm) wide bearing plate by a 300 kip (1335 kN) MTS actuator Linear potentiometer displacement transducers were placed over the supports and adjacent to the loading plate to measure midspan deflection Load was applied under displacement control, and the load was periodi-cally held to allow for crack identification (Fig 3)

Flexural Limit State Test Results

Figure 4 shows midspan load-deflection curves for the seven long beams with various flexural failure mechanisms, while Fig 5 shows each beam after testing As expected, flexural strength increased with flexural reinforcement (from Beam 1 to Beam 2 to Beam 5) However, the relatively low deflection exhibited by Beam 5 (Fig 4) and the compression

Fig 2: Test beam dimensions and loading: (a) for long beams; and (b) for short beams (Note: 1 in = 25.4 mm)

20

40

60

80

100

120

140

160

Midspan Deflection, in.

Beam 1

ρ = 0.14%

Beam 5

ρ = 2.46%

Beam 6

ρ = 2.46%; ρ ' = 1.48%

Beam 3F

ρ = 0.96% GFRP + PP fiber Beam 3

ρ = 0.96% GFRP

Beam 2

ρ = 0.83%

Beam 4

ρ = 0.83% with 10 in splice

Beam 1

Beam 2

Beam 3

Beam 3F

Beam 4

Beam 5

Beam 6

Fig 3: Students highlight cracks forming during beam testing

Fig 4: Load-deflection response for long beams (Note: 1 kip = 4.45 kN;

1 in = 25.4 mm)

Fig 5: Long beams after testing to failure

failure in the concrete (Fig 5) demonstrates the low ductility

associated with transition zone flexural members The

response of Beam 6 (Fig 4) and the lack of a crushing failure

in the concrete (Fig 5) shows how the addition of compression

reinforcement can convert a non-ductile transition zone beam

to a tension-controlled beam The poor performance of Beam 4,

with identical reinforcement to Beam 2, demonstrated the

consequence of an inadequate splice at the location of

maximum moment demand Bond failure along the short

splice resulted in a large tension crack developing at one end

of the splice along with significant splitting of the cover

within the length of the splice as the bars pulled through the

concrete (Fig 5)

While Beam 3, which had GFRP tension reinforcement,

exhibited strength comparable to the equivalent steel-reinforced

Beam 2, it also had considerably lower post-cracking stiffness

and overall ductility (Fig 4) Beam 3F also had GFRP

reinforcement, but included 2.25 in (57 mm) long PP fibers

at a dosage of 9.74 lb/yd3 (5.78 kg/m3) While it exhibited a

slightly higher cracking strength than Beam 3, it had essentially

the same post-cracking stiffness, strength, and ductility In

both Beam 3 and Beam 3F, the GFRP bars remained intact and within the elastic range up to crushing of the compression zone concrete The beams therefore exhibited almost complete recovery of deflection after the test in contrast to the permanent deformation due to yielding of the reinforcement, as seen in Beam 2 (Fig 5)

Shear Limit State Test Results

Figure 6 shows midspan load-deflection curves for the six short beams, while Fig 7 shows each beam after testing Beam 7, with no shear reinforcement, provides a measure of the concrete shear capacity The effect of adding PP fibers at 9.74 lb/yd3 (5.78 kg/m3) is evident in the increased shear capacity of Beam 8 As expected, the addition of increasing amounts of shear reinforcement in Beams 9, 10, and 11 is associated with increases in shear capacity Beams 11 and 12 had more shear reinforcement than the maximum specified by ACI 318-11, resulting in a compression zone failure (Fig 7) Although intended to demonstrate the effect of inadequate anchorage at the ends of the tension reinforcement, Beam 12 performed almost identically to Beam 11 This is possibly because the embedment required to develop a No 8 bar that’s partially confined by a bearing plate is less than the develop-ment length calculated per ACI 318

Comparisons with Predicted Strengths

Table 4 provides a comparison between the beam test

results and the predicted strengths Predicted strengths were

based on the ACI 318-11 Code and ACI 440.1R-06 for beams

with steel and GFRP flexural reinforcement, respectively

The nominal capacities are based on the specified concrete

compressive strength of 4000 psi (27.6 MPa), steel

reinforce-ment yield strength of 60 ksi (414 MPa), and the manufacturer’s

specified properties for the GFRP bars As neither ACI 318-11

nor ACI 440.1R-06 include adjustments for the addition of

PP fibers, calculations for Beam 3F and Beam 8 include

concrete, steel, and GFRP properties only

The ratio between the test results and the nominal capacities

predicted per the ACI documents shows that for all beams

with flexural limit states (Beams 1 to 6), the test result

consistently exceeds the predicted nominal capacity by 38 to

115% This provides a considerable margin of safety, which is

obviously increased by inclusion of the appropriate strength

reduction factor This is particularly evident for the

transition-zone Beam 5, which requires a smaller reduction factor than

the tension-controlled beams, and the GFRP-reinforced

Beams 3 and 3F, which require an even smaller reduction

factor than the beams with steel reinforcement Including the

strength reduction factor, the test results exceed the ACI

prediction by 55 to 158% This relatively large margin of

safety in the performance of beams subjected to flexural limit

states was considerably greater than students had anticipated

in their pre-test estimates

For short beams (Beams 7 to 12), the test results were

again significantly higher than the nominal capacities predicted

per ACI documents While the nominal strength of both

Beam 11 and Beam 12 were relatively close to the test results,

it’s important to note that the nominal shear strength provided

by shear reinforcement in these beams exceeded the code

specified maximum of 8√f c′(b w d) Beams 7 and 8 had no

stirrups and thus had low shear capacities However, the test

results exceeded the nominal capacities by 49% and 108%,

respectively Beam 8, which contained PP fibers, attained a

Fig 6: Load-deflection response for short beams (Note: 1 kip = 4.45 kN;

1 in = 25.4 mm)

Fig 7: Short beams after testing to failure

Beam 7

Beam 8

Beam 9

Beam 10

Beam 11

Beam 12

Midspan Deflection, in.

0

50

100

150

200

250

Beam 7: V s = 0

Beam 8: V Beam 9: V s = 0 with PP fiberss = 1.2 f c ´

Beam 10: V s = 3.5 f c ´

Beam 11: V s = 9.4 f c with hooked bottom bars ´

Beam 12: V s = 9.4 f c´ with straight bottom bars

peak load that was 40% greater than the peak load reached

by Beam 7, which comprised plain concrete As with flexural test results, the high margins of safety evident in the ACI 318-11 shear design requirements was greater than the students had anticipated

Summary and Conclusions

The hands-on experience of building and testing large-scale reinforced concrete beams gave students in the senior-level reinforced concrete design course at UH Manoa valuable exposure to design and construction aspects of reinforced concrete Perhaps more importantly, the tests physically demonstrated behaviors that would normally be covered only

in the abstract Primary conclusions drawn from the beam test results include the following:

•Predictions of nominal flexural and shear capacity made per ACI documents provide significant margins of safety when compared with test results For flexural members, the margin of safety ranged from 38 to 115%, while for shear members meeting ACI 318-11 Code requirements, the margin of safety ranged from 49 to 75%;

Table 4:

Comparison between test results and predicted strengths per ACI 318-11 and ACI 440.1R-06

* Bending strength of Beams 3 and 3F (with GFRP bars) is based on recommendations in ACI 440.1R-06

† The predicted strengths include no considerations for the PP fibers

Note: 1 kip = 4.45 kN

ACI member Ian N Robertson is a Professor

of structural engineering at the University

of Hawaii at Manoa, HI, where he teaches senior- and graduate-level courses on structural design A licensed structural engineer in Hawaii, he received his MS and PhD in civil engineering from Rice University, Houston, TX He has 30 years

of research and design experience in the performance of structures during extreme events including earthquakes, hurricanes, and tsunamis, and has participated in a number of post-disaster reconnaissance surveys His research interests include response of structures to extreme loading, durability of concrete materials, and the long-term performance of reinforced concrete structures.

ACI member Gaur P Johnson is an

Assistant Professor at the University of Hawaii, Honolulu, HI He received his BS,

MS, and PhD in civil engineering from the University of Hawaii, and is a licensed structural engineer in Hawaii He is a member of ACI Committee 437, Strength Evaluation of Existing Structures, and ACI Subcommittees 562-B, Loads, and 562-E, Education His research interests include performance evaluation of deteriorating reinforced concrete structures and field instrumentation for seismic and long-term monitoring

•Adding PP fibers at 9.74 lb/yd3 (5.78 kg/m3) to the concrete

mixture increased the concrete shear capacity by 40%, but

had only a marginal impact on the flexural response; and

•Replacement of steel tension reinforcement with GFRP

bars required a 40% increase in reinforcement

cross-sectional area to provide equivalent flexural design

strength The lower stiffness of the GFRP bars also resulted

in larger deflections at the service load condition

Acknowledgments

The authors appreciate the significant assistance of laboratory technicians

Mitchell Pinkerton and Kim Hertzog as well as their student assistants

during the various stages of beam fabrication and testing The authors

also acknowledge the assistance of Michael Bell, who served as teaching

assistant for this course, and Benjie Batangan, who helped in the design

of the beams Finally, the contributions of Kimo Scott of OK Hardware,

Inc., who generously donated the Aslan 100 GFRP bars and Forta-Ferro

PP fibers for the test program, are gratefully acknowledged

References

1 ACI Committee 318, “Building Code Requirements for Structural

Concrete (ACI 318-11) and Commentary,” American Concrete Institute,

Farmington Hills, MI, 2011, 503 pp.

2 ACI Committee 440, “Guide for the Design and Construction

of Structural Concrete Reinforced with FRP Bars (ACI 440.1R-06),”

American Concrete Institute, Farmington Hills, MI, 2006, 44 pp.

Note: Additional information on the ASTM standards discussed in this

article can be found at www.astm.org.

Selected for reader interest by the editors.

Foundati ons

37 2015 CFA Awards

Floors &