Construction and Building Materials 48 (2013) 830–839

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Construction and Building Materials

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Flexural behavior of PVC stay-in-place formed RC walls

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.07.073

⇑ Corresponding author. Tel.: +1 5197290889.E-mail addresses: nwahab@uwaterloo.ca (N. Wahab), soudki@uwaterloo.ca

(K.A. Soudki).1 On leave from Department of Civil Engineering, Cairo University, Egypt.

Noran Wahab ⇑,1, Khaled A. SoudkiDepartment of Civil and Environmental Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada

h i g h l i g h t s

� PVC stay-in-place forming system enhanced the flexural behavior of RC walls.� Effect of the PVC on the yield load decreased as the reinforcement ratio increased.� PVC increase to ultimate loads depends on concrete thickness and/or reinforcement ratio.� PVC enhanced the ductility of the wall system with an increase in ultimate deflection.� A model was proposed to predict the capacity of the PVC encased walls.

a r t i c l e i n f o

Article history:Received 4 March 2013Received in revised form 16 June 2013Accepted 21 July 2013Available online 24 August 2013

Keywords:Concrete wallsStay-in-place formworkFlexureCracksBendingPVCRuptureCompressionStatic loadConnectors

a b s t r a c t

The study experimentally investigates the flexural behavior of concrete wall strips encased with PVCstay-in-place (SIP) concrete forming system. The system consists of interconnected Polyvinyl chloride(PVC) elements; panels, connectors and bracings. Thirty PVC encased wall specimens were cast withdimensions: 457 mm wide by 200 mm or 250 mm deep by 3050 mm long. The variables studied were:the concrete core thickness (200 mm or 250 mm), the reinforcement ratio, and the connector type (mid-dle and braced connector). The connectors used were middle connectors with flat panels or connectorswith inclined bracing on the compression side and insulation (foam) on the tension side. All the speci-mens were tested under four point bending with a shear span of 1150 mm. The test results showed thatthe PVC stay-in-place formwork system was effective in enhancing the flexural behavior of the encasedwalls. For a given reinforcement, the PVC encased specimens cracked and failed in the same mode,regardless of the connector type or the concrete core’s thickness. The PVC encased walls exhibited higheryield and ultimate loads in comparison to control walls. The increase in load for the PVC encased wallsdecreased with higher reinforcement ratios. The contribution of the PVC stay-in-place system to the ulti-mate load increased as the concrete core thickness decreased and/or as the reinforcement ratiodecreased. A model based on strain compatibility was developed to predict the flexural capacity ofPVC stay-in-place encased walls. Predictions were in close agreement with measured results.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Stay-in-place (SIP) formwork is being utilized in current con-crete construction as an alternative to the conventional woodformwork. In contrast to conventional formwork, stay-in-placeformwork is left in place permanently with the structure. Applica-tions of the system include agricultural, industrial and residentialbuildings as; foundation walls, retaining walls, water and wastetreatment tanks, noise abatement walls, and in swimming pools.Advantages of such systems include increased structural strength,improved durability and protection from corrosion [1–4,6,7,9–11].

Below is a review of some of the previous research work on SIPsystems.

Chahrour et al. [2] investigated experimentally a polymer-basedstay-in-place formwork (Royal Building System) for concrete walls.The program consisted of testing a total of 15 wall specimens inflexure under four point bending. The test variables were: the wallthickness (100, 150 and 200 mm) and the reinforcement ratio(plain and reinforced concrete walls). The specimens were simplysupported with a clear span of 2000 mm and a shear span of500 mm. The walls failed by rupture of the polymer flange in ten-sion. Control walls were not tested in this program. They reportedthat polymer stay-in-place encased concrete walls tested in flexureexhibited a ductile response that depends on the specimen thick-ness and steel reinforcement ratio. As the wall thickness increasedfrom 100 mm to 150 mm, the toughness increased by 10%. For thesame wall thickness (150 mm), the toughness of the reinforced

N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839 831

specimens was 57% more than the toughness of the un-reinforcedspecimens.

Rteil et al. [11] tested eight PVC stay-in-place encased wallsmonotonically under four point bending. All specimens were305 mm wide and 2500 mm long. The specimens were reinforcedwith two 10M rebars. The variables were the depth of the speci-men (150 mm or 200 mm) and the connector configuration (flatin the middle or inclined at the corner). Test results showed thatthe PVC stay-in-place formwork system increased the crackingload, yield load and ultimate load by 36%, 78% and 36% on average,respectively, compared to the control specimens. In addition, theductility index for the PVC stay-in-place encased specimens in-creased by 25%. The type of the connectors had no effect on thebehavior of the PVC SIP formwork specimens.

Kuder et al. [8] investigated the flexural performance of thePVC-encased systems by testing reinforced concrete beams withand without the PVC components. The beams were 152.4 mm deepby 152.4 mm wide by 609.6 mm long. The beams were longitudi-nally reinforced with a #3 rebar (9.5 mm diameter). The main testvariable was the wall configuration. Four different configurationswere used where the main configuration was the panels with thestandard connectors. The other three configurations were formedby adding middle connector or inclined bracing on tension andcompression faces or inclined bracing on tension face only. Theywere tested in three-point bending with a span of 508 mm. ThePVC encased beams showed an increase in the peak load by 39–66% depending on the PVC configuration where the PVC configura-tion influence the extent of the increase in peak load andtoughness.

Based on the above, few studies are reported in the literature onthe effectiveness of PVC stay-in-place formwork on the structuralbehavior of concrete walls. The main objectives of this study wereto investigate the flexural behavior of concrete walls encased withPVC stay-in-place formwork system and to develop a simple modelto predict the flexural capacity of such walls.

2. Test program

The test program consisted of testing thirty specimens as givenin Table 1. Six specimens were cast without a stay-in-place form-ing system to act as control walls. The remaining 24 specimenswere cast in a stay-in-place (SIP) forming system.

The SIP system used in this study is manufactured by Octaformand consists of interconnected polyvinyl chloride (PVC) elements;

Table 1Test matrix.

Group Thickness Concrete core Connector type

C-8-NA-10 200 mm (8 in.) 200 mm (8 in.) NAC-8-NA-15C-8-NA-20O-8-M-10 200 mm (8 in.) 200 mm (8 in.) MiddleO-8-M-15O-8-M-20O-8-B-10 250 mm (10 in.) 200 mm (8 in.) BracedO-8-B-15O-8-B-20C-10-NA-10 250 mm (10 in.) 250 mm (10 in.) NAC-10-NA-15C-10-NA-20O-10-M-10 250 mm (10 in.) 250 mm (10 in.) MiddleO-10-M-15O-10-M-20O-10-B-10 305 mm (12 in.) 250 mm (10 in.) BracedO-10-B-15O-10-B-20

a RFT stand for reinforcement.

panels, connectors and bracings. The PVC elements have a seriesof openings in the interconnecting elements for steel placementand lateral flow of concrete. The elements are shipped separatelyto the site where they are assembled together forming the shell(formwork) for a concrete wall. The panels are used to erect thetwo faces of the wall which are connected by the hollowconnectors.

The test variables studied were: (a) the concrete core thickness(200 mm or 250 mm (8 in. or 10 in.)), (b) the amount of steel rein-forcement (three 10M rebars, three 15M rebars and three 20M re-bars), and (c) the connector type. Two configurations for theconnectors were used; middle connectors with flat panel or con-nectors with inclined bracing (45�) on the compression side andinsulation (foam) on the tension side (Fig. 1). Some of these vari-ables were previously studied in the literature [11] and Kuderet al. [8]). However, the work presented here provides a widerrange of values for these variables than the work presented before.

The specimen notation is as follows: the first letter stands forthe specimen type; control (C) and Octaform PVC encased walls(O). The second number stands for the concrete core thickness ininches (8 in. or 10 in.). The third letter represents the type of theconnector, middle (M) and braced (B). The last number representsthe diameter of the reinforcement. For example: O-10–M-20stands for a 10 in. thick encased wall with middle connector rein-forced with 3–20M.

2.1. Test specimen

The specimen had a rectangular cross section. All specimenswere 457 mm wide with variable concrete core thickness(200 mm or 250 mm) and were 3050 mm long. Specimens withmiddle connectors were 200 mm (8 in.) or 250 mm (10 in.) thick.Specimens with inclined bracing and insulation were 250 mm(10 in.) or 305 mm (12 in. thick). The insulation was 50 mm thick(2 in.). Therefore, the concrete core thickness for all specimenswas 200 mm (8 in.) or 250 mm (10 in.).

Each specimen consisted of 3 bottom and 3 top panels. Speci-mens with middle connectors had 7 connectors at 76 mm. Speci-mens with braced connectors had 4 connectors at 152 mm,inclined bracing on the compression side and insulation (2 in. thickfoam) on the tension side. All specimens were reinforced in the lon-gitudinal direction (3050 mm) with 3 (10M or 15M or 20M) rebars.They were also reinforced in the transverse direction (457 mm)with 5–10M rebars to simulate the transverse reinforcement used

RFTa RFTa ratio (%) Panel type No. of specimens

3–10M 0.43 – 13–15M 0.86 13–20M 1.29 13–10M 0.43 Flat 23–15M 0.86 23–20M 1.29 23–10M 0.43 Insulated 23–15M 0.86 23–20M 1.29 23–10M 0.32 – 13–15M 0.63 13–20M 0.95 13–10M 0.32 Flat 23–15M 0.63 23–20M 0.95 23–10M 0.32 Insulated 23–15M 0.63 23–20M 0.95 2

(a) Assembling a wall withmiddle connector

(b) Cross section of the wall withinclined bracing after assembly

Connector

Panel

Inclinedbracing

Fig. 1. The Octaform walls.

0

5

10

15

20

25

30

35

40

45

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stre

ss (M

Pa)

Strain (με)

Fig. 2. Stress strain behavior for the PVC.

832 N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839

in practice. The longitudinal and transverse steel were tied togetherusing spiral ties.

2.2. Specimen fabrication

Wooden boxes were fabricated for pouring the control walls.For the stay-in-place- formwork encased walls, all the specimenswere assembled horizontally. The walls were then flipped verti-cally to the casting position using the crane. The walls were castin gang form. They were stacked against one another forming arow in a box. After the row was completed a sheet of ply woodwas placed vertically to separate the rows from one another. Oncethe box was completed, it was girdled with studs (2 in. � 4 in.)spaced at 600 mm (centerline to centerline).

The concrete was supplied by a local ready mix plant. The con-crete was poured in 3 lifts using a bucket until the walls were com-pletely filled. After each lift, the specimens were vibrated using ahand vibrator that was 3 m long. Cylinders (100 mm � 200 mm)were taken from the concrete mix at the beginning and the middleof the casting.

The walls were cured for 5 days by spraying them with watertwice daily and keeping them covered with wet burlap and plastic.After 5 days, concrete cylinders were tested to ensure that thecompressive strength is more than 20 MPa and therefore the wallscan be stripped, handled and rotated horizontally without crackingthe concrete.

2.3. Material properties

The same concrete mix was used for the two batches. Batch 1was used for the control walls and the PVC encased walls withmiddle connectors. Batch 2 was used for the SIP encased walls withbraced connectors. The concrete had 10 mm aggregate size. Superplasticizers and retarders were used to provide a workable con-crete. The slump for all the mixes ranged from 180 mm to200 mm. Compressive tests were conducted on the concrete cylin-ders taken from the batches at 21 days and 28 days. The walls weretested at 21 days and hence the strength of the concrete was deter-mined at this age. At the desired age, eight concrete cylinders weretested. The highest and lowest compressive strengths were elimi-nated. Thus, the compressive strength value represents the averageof six tested cylinders. The target concrete strength was 35 MPa.However, the actual concrete strength at 21 days was 48 MPaand 40 MPa for batch 1 and 2, respectively. The concrete strengthat 28 days was 53 MPa and 43 MPa for batch 1 and 2, respectively.

Steel rebars 10M, 15M and 20M were used. The reinforcing steelbars had a yield strength of 490 MPa and an ultimate strength of630 MPa according to the manufacturer’s data sheet. The polyvinylchloride (PVC) had a tensile strength of 45.9 MPa and a tensilemodulus of 2,896 MPa, as provided by the manufacturer (Fig. 2).

2.4. Instrumentation and test procedure

For the walls without foam, two strain gauges were mounted ontwo steel rebars at mid span. For the walls with foam, one straingauge was mounted on one steel rebar at mid span. For all thewalls, two gauges were mounted on the middle connector at midconcrete core height and three quarters the concrete core height(measured from the compression side). Five (5) mm long gaugeswere used for the steel reinforcement and the PVC connectors.The strain gauges were waxed and coated with V–M tape (3 mmthick) to protect the gauge during casting. After casting the speci-mens and before load testing, two strain gauges were mounted onthe PVC panels. One gauge was mounted on the top panel and theother gauge was placed at the bottom panel. Also, a gauge wasmounted on the concrete on the compression face inside a cutmade in the PVC panel. Sixty (60) mm long gauges were used formeasuring the strain in the concrete.

The beams were tested at age of 21 days in four-point bendingusing a servo-hydraulic actuator controlled by a MTS 407 control-ler. The shear span was 1150 mm and the constant moment regionwas 600 mm. The loading rate was 2.5 mm/min. The duration ofthe test varied between 60 and 120 min. The test set up is shownin Fig. 3. The beam had a hinge support at one end and a roller sup-port at the other. The hinge support was a half cylinder resting on acurved plate. The roller support was a steel cylinder between twocurved plates. The load was measured using a 333 kN load cell. Thedeflection of the top panel (compression side of the wall) was mea-sured using the internal LVDT. An external LVDT was mounted atmid span to measure the deflection of the soffit of the wall (tension

External LVDTRoller

support

1150 mm

Loading points

Fig. 3. Test set up.

N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839 833

side). The test was stopped when the load dropped by more than20% of the peak load or if the deflection exceeded 200 mm.

2.5. Sectional analysis

A sectional analysis was carried out to compute the capacity ofthe PVC encased walls at concrete crushing or PVC rupture. Fig. 4shows a schematic of the PVC encased wall used in the analysis.The tension forces are resisted by the steel, the solid part of theconnector and the PVC panel where the contribution of the webof the connector are neglected. The compression forces are resistedby the concrete and the PVC panel where the contribution of theconnector above the neutral axis is neglected.

The assumptions of the analysis were as follows:

� Linear strain distribution along the cross section of the wall.� Tension forces are resisted by the steel rebar, the solid parts of

the connector and the PVC panel. The contribution of the con-nector web was neglected.� Compression forces are resisted by the concrete and the PVC

panel.� The contribution of the connector in compression was

neglected.� The steel stress strain curve is modeled as bilinear with stress in

steel rebar taken as:

Fs ¼ es � Es if es 6 ey

Fs ¼ Fy þ 0:01� Es � ðes � eyÞ � As if es > ey ð1Þ

where Es is the Young’ modulus of the steel reinforcement, equal to200 GPa, es is the strain in the steel from the sectional analysis, ey isthe yield strain, AS is the cross sectional area of the steel and Fy isthe yield stress.� PVC stress strain behavior is nonlinear as shown in Fig. 2. The

relation between the stress (rpvc) and the stain (epvc) in thePVC is represented by

rpvc ¼ �71518� e2pvc þ 3412:1� epvc ð2Þ

� Concrete crushing strain is taken equal to 0.0025. This strainvalue is in agreement with the measured experimental crushingstrain and with the value recommended by CSA A23.3-04 forhigh concrete strength. The actual concrete compressive stresscan be represented by an equivalent rectangular stress blockusing coefficients a and b [5]:

a� b ¼ ec

eco� 1

3ec

eco

� �2

ð3Þ

a ¼4� ec

eco

h i

6� 2ececo

h i ð4Þ

The depth of the compression block (c) is calculated by straincompatibility and equilibrium of forces as follows (Fig. 4):

a� b� c � b� fc þ rpvcpc � Apvcp

¼ rpvcpt � Apvcp þ rpvcc � Apvcc þ es � Es � As ð5Þ

The moment capacity of the wall is calculated using the follow-ing equation:

Mapp ¼ rpvctp � Apvcp � h� a� b� c2

� �þ rpvcc � Apvcc

� dpvcc � a� b� c2

� �es � Es � As � ds � a� b� c

2

� �

� rpvcpc � Apvcp � a� b� c2

� �ð6Þ

The peak load is calculated from moment capacity using statics(Fig. 3):

Pu ¼2�Mapp

a; a ¼ 1100 mm

where

a

shear span Apvcp area of the PVC panel Apvcc area of the PVC connector As area of steel reinforcement b width of the cross section c depth of the compression block dpvcc depth of the centroid of the PVC connector measured

from the compression face

ds depth of the steel reinforcement measured from the

concrete compression face

Es Young’s modulus of steel reinforcement fc compressive strength of the concrete h height of the cross section Mapp externally applied bending moment Pu ultimate load a ratio of the average stress in the compression block to

the concrete strength

b ratio of the depth of the compression stress block to

the depth of the neutral axis

ec strain at extreme top fibre of concrete for a given load

level

eco strain at maximum concrete compressive strength epvc strain in the PVC es strain of the steel reinforcement rpvc stress in the PVC rpvcpt stress of the PVC panel on the tension side rpvcpc stress of the PVC panel on the compression side rpvcc stress of the PVC connector

The input parameters for the analysis were the material properties,

the concrete crushing strain and rupture strain for the PVC. By com-bining the linear strain distribution with the force and momentequilibrium equations, the peak load for the PVC encased wall atconcrete crushing or PVC rupture was computed. The calculatedloads will be presented and compared to the experimental loadsin the following sub sections.

834 N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839

2.6. Test results

Table 2 gives a summary of the cracking, yield and ultimate loadand deflection for all the control specimens. As the rebar diameterand/or the concrete core thickness (depth of the steel) increases,the yield and ultimate load increase. Tables 3 and 4 give a sum-mary of the cracking, yield and ultimate load and deflection forall the PVC encased specimens with middle and braced connectors,respectively.

2.7. Modes of failure

For a given reinforcement, the PVC encased specimens crackedand failed in the same mode, regardless of the connector type orthe wall thickness. Cracks first appeared on the tension side atmid span in the constant moment region. As the load increased,cracks occurred underneath and close to the loading point. As theloading continued, cracks propagated through the depth of thespecimen until the steel reached yielding. After yielding, thebehavior of the specimen was slightly different depending on steelreinforcement ratio. The PVC encased specimens reinforced with3–10M failed by yielding of the steel rebar followed by ruptureof the PVC panel on the tension side as shown in Fig. 5a. The PVC

Table 2Loads and deflection for control specimens.

Specimen RFT f’c (MPa)(21 days)

Cr.a

load (kN)Yieldload (kN)

Peakload (kN)

Dcra

(mm)Dyie

(mm

Thickness = 200 mm (8 in.)C-8-NA-10 3–

10M48 8 26.7 39.5 0.8 11.4

C-8-NA-15 3–15M

8.1 54.6 74.7 0.5 18.4

C-8-NA-20 3–20M

10.2 85 103.8 0.7 20

Thickness = 250 mm (10 in.)C-10-NA-

103–10M

48 17.68 41.2 62.7 0.54 8.3

C-10-NA-15

3–15M

17.43 81.5 107.8 0.52 11.9

C-10-NA-20

3–20M

11 128 150.7 0.72 16.3

a Cr = cracking.

Table 3Test results for Octaform encased walls with middle connectors.

Specimen RFT f’c (MPa)(21 days)

Crackingload (kN)

Yieldload (kN)

Peakload (kN)

Dcr.(mm)

Thickness = 200 mm (8 in.)O-8-M-10 3–10M 48 23.7 33.9 54.8 3.1Average increase over

control (%)– 27% 38.75% –

O-8-M-15 3–15M 23.5 65.9 84.95 2.2Average increase over

control (%)– 20.6% 13.7% –

O-8-M-20 3–20M 22.4 104.9 123.4 2.3Average increase over

control (%)– 23.4% 18.9% –

Thickness = 250 mm (10 in.)O-10-M-10 3–10M 48 37 50.4 75.6 1.4Average increase over

control (%)– 22.3% 20.6% –

O-10-M-15 3–15M 38.5 92.5 130.2 1.4Average increase over

control (%)– 13.5% 20.7% –

O-10-M-20 3–20M 40 151.8 183.2 2.4Average increase over

control (%)– 18.6% 21.8% –

encased specimens reinforced with 3–15M or 3–20M failed bycrushing of the concrete followed by buckling of the PVC panelon the compression side as shown in Fig. 5b. This mode of failureis similar to the failure reported by Rteil et al. [11]. In contrast,all the control specimens, except the control walls that are250 mm (10 in.) thick and reinforced with 3–10M or 3–15M, failedby steel yielding at mid span followed by concrete crushing at midspan as shown in Fig. 6a. The two control walls that are 250 mm(10 in.) thick and reinforced with 3–10M or 3–15M (C-10-NA-10and C-10-NA-15) failed by steel yielding at mid span followed bya flexural-shear crack growing in the shear span causing a shearfailure as shown in Fig. 6-b.

2.8. Flexural behavior

2.8.1. Load – deflection behaviorThe load–deflection behavior for all specimens was similar.

Fig. 7 shows the load versus deflection for PVC encased specimenwith middle and braced connectors versus the control specimen.The vertical axis represents the load (kN) and the horizontal axisrepresents the deflection (mm). At the beginning, the load in-creased with minimum deflection (less than 3 mm) until the con-crete cracks at mid span. Past the cracking load, the deflection

ld

)Dpeakload

(mm)Dult

(mm)Ductility index Mode of failure

160 160 14 Steel yielding at mid span followedby concrete crushing

111 111 6

85 85 4.3

153 155 18.7 Steel yielding at mid span followedby a flexural-shear crack growing inthe shear span causing a shear failure80 87 7.3

52 54 3.3 Steel yielding at mid span followedby concrete crushing

Dyield

(mm)Dult

(mm)Ductility index Mode of failure

10.4 193 18.75 Yielding of the steel rebar followedby rupture of the PVC panel on thetension side

– 20.9% 46.5%

17.8 149 8.4 Crushing of the concrete followed bybuckling of the PVC panel on thecompression side

– 33.7% 43.5%

21.6 91.5 4.2– 7.6% 14.9%

9.4 157 16.7 Yielding of the steel rebar followed byrupture of the PVC panel on the tension side– 2.6% Zero

11.9 187 15.7 Crushing of the concrete followed bybuckling of the PVC panel on thecompression side

– 115% 114%

16.3 86.5 5.3– 60.2% 66%

Fig. 4. Schematic showing the stress and strain distribution in the cross section.

Table 4Test results for Octaform encased walls with braced connectors.

Specimen Reinforcement f’c (MPa)(21 days)

Crackingload (kN)

Yieldload (kN)

Peakload (kN)

Dcr.(mm)

Dyield

(mm)Dult

(mm)Ductility index Mode of failure

10 in. thick specimens (8 in. concrete core)O-8-B-10 3–10M 40 21.2 31.3 54 1.7 10.4 168.1 16.2 Yielding of the steel rebar followed by

rupture of the PVC panel on the tension sideAverage increase overcontrol (%)

– 17% 36.6% – – 5% 26.4%

O-8-B-15 3–15M 24.6 65.3 89.5 1.8 15.8 136.4 8.6 Crushing of the concrete followed bybuckling of the PVC panel on thecompression side

Average increase overcontrol (%)

– 19.6% 19.8% – – 22.9% 49.8%

O-8-B-20 3–20M 27.2 106.1 121.6 1.6 19.7 74.5 3.8Average increase over

control (%)– 24.8% 17.2% – – Zero Zero

12 in. thick specimens (10 in. concrete core)O-10-B-

103–10M 40 31.5 42 77.8 1.7 8.3 181.5 21.9 Yielding of the steel rebar followed

by rupture of the PVC panel onthe tension sideAverage increase over

control (%)– 2% 23.3% – – 18.6% 17%

O-10-B-15

3–15M 32 94.4 126.1 1.5 13.2 164.6 12.5 Crushing of the concrete followedby buckling of the PVC panel on thecompression sideAverage increase over

control (%)– 15.8% 17% – – 89% 71.7%

O-10-B-20

3–20M 36.7 141.3 164 1.7 16 126.5 7.9

Average increase overcontrol (%)

– 10.4% 8.7% – – 134% 145%

(a) Failure by steel yielding followedby rupture of PVC panel (O-8-M-10)

(b) Failure by concrete crushing followedby PVC buckling

O-10-M-20

Front view

Bottom view

O-8-M-10

Fig. 5. Modes of failure for Octaform encased walls.

N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839 835

increased as the load increased until the steel yields. Past yielding,the slope of the load versus deflection was less steep than beforeyielding until failure was attained. The load versus deflectionbehavior was not affected by the type of the connector. For a givenconcrete core thickness and reinforcement, the load versus deflec-tion curves for all the specimens were almost identical (see Fig. 7).

2.8.1.1. Cracking stage. Tables 3 and 4 show that the PVC encasedspecimens with concrete cores 200 mm (8 in.) and 250 mm(10 in.) had an increase in the cracking load by at least 270% and216%, respectively over the control specimens. The cracking deflec-tion for most of the PVC encased walls varied between 1.5 mm and3 mm. Meanwhile, the cracking deflection for the control

(a) Failure by steel yielding followed byconcrete crushing (Specimen C-8-NA-15)

(b) Failure by flexural-shear crack inthe shear span (Specimen C-10-NA-15)

Concrete crushing

Shear span

Fig. 6. Failure modes for the control specimens.

(a) Octaform walls 8 incPh thickreinforced with 3-10M

(b) Octaform walls 10 inch thickreinforced with 3-10M

(e) Octaform walls 8 inch thickreinforced with 3-20M

(f) Octaform walls 10 inchthick reinforced with 3-20M

O-8-M-10O-8-B-10C-8-NA-10

020406080

100120140160180200

Load

(kN

)

Deflection (mm)

C-10-NA-10O-10-M-10 O-10-B-10

020406080

100120140160180200

Deflection (mm)

Load

(kN

)

C-10-NA-10O-10-M-10O-10-B-10

020406080

100120140160180200

Load

(kN

)

O-10-M-15

O-10-B-15

C-10-NA-15

020406080

100120140160180200

Load

(kN

)

O-10-M-15O-10-B-15C-10-NA-15

C-8-NA-20

O-8-M-20O-8-B-20

020406080

100120140160180200

(c) Octaform walls 8 inchthick reinforced with 3-15M

(d) Octaform walls 10 inchthick reinforced with 3-15M

0 50 100 150 200

Deflection (mm)0 50 100 150 200

Deflection (mm)0 50 100 150 200

Deflection (mm)0 50 100 150 200

0 50 100 150 200 250

Deflection (mm)0 50 100 150 200 250

Load

(kN

)

C-8-NA-20

O-8-M-20

O-8-B-20

O-10-M-20

C-10-NA-20

O-10-B-20

020406080

100120140160180200

Load

(kN

)

Fig. 7. Load versus deflection for the Octaform encased walls.

836 N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839

Table 5Comparison of toughness of the specimens.

Specimen Toughness (kN mm) Increase over control Specimen Toughness (kN mm) Increase over control

C-8-NA-10 5400 NA C-10-NA-10 8426 NAO-8-B-10 8160 51% O-10-B-10 9817 17%O-8-M-10 9213 71% O-10-M-10 13,841 64%C-8-NA-15 6813 NA C-10-NA-15 9484 NAO-8-B-15 10,797 59% O-10-B-15 21,074 122%O-8-M-15 12,004 76% O-10-M-15 26,273 177%C-8-NA-20 7357 NA C-10-NA-20 6309 NAO-8-B-20 7393 4.8% O-10-M-20 17,420 176%O-8-M-20 9000 22% O-10-B-20 17,662 180%

120

140

N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839 837

specimens was less than 1 mm. That difference in the crackingdeflection is expected because of the higher cracking load of thePVC encased specimens compared to the control specimens.

steel

PVC panel

0

20

40

60

80

100

0 5000 10000 15000 20000

Load

(kN

)

Strain (με)

Fig. 8. Load versus tensile strain for the Octaform encased wall (O-8-M-20).

2.8.1.2. Yield stage. For a given wall thickness and reinforcement,the average yield load was almost the same for most of the PVC en-cased specimens with different connector types as shown in Tables3 and 4. The PVC encased walls showed a higher yield load com-pared to the yield load of the control specimens. The average yieldload for the 200 mm (8 in. thick) PVC encased walls reinforced with3–10M, 3–15M and 3–20M rebar increased by 27%, 20.6% and23.4%, respectively, over the control specimens. The average yieldload for the 250 mm (10 in. thick) PVC encased walls reinforcedwith 3–10M, 3–15M and 3–20M rebar increased by 22.3%, 13.5%and 18.6%, respectively, over the control specimens. However,the yield deflection for the PVC encased walls was almost the sameas the control walls. The presence of the PVC did not affect theyield deflection.

2.8.1.3. Ultimate stage. The PVC encased walls showed an increasein the ultimate load over the control specimens as shown in Tables3 and 4. For a given reinforcement and concrete core thickness, theincrease in the ultimate load for the PVC encased walls with mid-dle or braced connector was of same order of magnitude except forthe walls with 250 mm (10 in.) concrete core and reinforced with3–20M rebar. The maximum increase in the ultimate capacitywas for the walls with 8 in. thick concrete cores reinforced with3–10M, regardless of the connector type. For the same concretecore thickness, as the reinforcement increased from 3–10M to 3–15M or 3–20M, the increase in the ultimate capacity (contributionof PVC panels) decreased from an average of 37.7% to 17.4%. Also,for the PVC encased walls with middle or braced connectors rein-forced with 3–10M, as the concrete core thickness increased from8 to 10 in., the contribution of the PVC panels to the ultimate loaddecreased from 37.7% to 22%. For the PVC encased walls 250 mm(10 in.) thick concrete core with middle or braced connectors, asthe reinforcement increased from 3–10M to 3–15M or 3–20M,the contribution of PVC panels (to the increase in ultimate capacityof the wall) decreased but not as much as the 8 in. concrete cores.The contribution of the PVC panels to the increase in ultimatecapacity of the wall decreased from an average of 22% to 17%.Therefore, the contribution of the PVC stay-in-place forming sys-tem to the ultimate load increases as the concrete core thicknessdecreases and/or as the reinforcement ratio decreases.

Most of the PVC encased walls showed an increased ultimatedeflection and ductility index as shown in Tables 3 and 4. The in-crease in ultimate deflection for the PVC encased walls over thecontrol walls varied between 2.5% and 134%. The ductility indexfor the PVC encased walls was higher than the ductility index forthe control walls. The increase in ductility index for the PVC en-cased walls varied between 14.5% and 145% over the control spec-imens. In addition, flexural toughness can be determined as the

area under the load versus deflection curve as shown in Table 5.It is clear that the Octaform encased specimens showed highertoughness values than the control specimens.

2.8.2. Load – tensile strain behaviorThe tension forces were resisted mainly by the steel reinforce-

ment and the PVC panel on the tension side in addition to part ofthe connector below the neutral axis. Fig. 8 shows a typical loadversus strain in the steel and the PVC panel. The vertical axis rep-resents the load (kN) and the horizontal axis represents the strain(le From Fig. 8, it is evident that the strain in the steel and PVC wassmall (less than 200 le) until a load of 26 kN where the concreteon the tension side cracked. Once the concrete cracked, the tensileforces were transmitted to the steel and the PVC panel. A suddenincrease in tension strain was recorded as seen in Fig. 8. Past crack-ing, the load–strain behavior was bi-linear similar to the load–deflection curves. As the load increased, the strain in the steeland the PVC panel increased almost linearly until the steel yieldedat about 108 kN. Past the yield load, the strains in the PVC in-creased at a higher rate until complete failure. The PVC strain atfailure ranged from 10,000 to 42,000 le.

2.8.3. c- Load- compressive strain behaviorThe compression forces were resisted mainly by the concrete

and the PVC panel on the compression side in addition to part ofthe connector above the neutral axis. Fig. 9 shows a typical loadversus compressive strain behavior for the concrete and the PVCpanel. It is clear that the strain in the concrete and the PVC panelincreased as the load increased. Strain readings for both the con-crete and the PVC were almost the same until reaching a load of156 kN. Past 156 kN, the concrete strain was slightly higher thanthe PVC strain. At 178 kN, the concrete started to crush. The con-crete strain dropped and the forces were transmitted to the PVC

concretePVC

0

20

40

60

80

100

120

140

160

180

200

-10000-8000-6000-4000-20000

Load

(kN

)

Strain (με)

Fig. 9. Load versus compressive strain for the Octaform encased wall (O-10-M-20).

(a) walls with middle connectors

(b) walls with bracedconnectors

Strain gauges

Fig. 10. Strain gauge location along the connector’s height.

838 N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839

panel. The PVC strain reading continued to increase until reachinga load of 185 kN. At a load of 185 kN, the PVC started to buckle andthe load dropped. It is worth noting that for the walls that failed by

(a) 200mm (8inch) thick wall with middleconnectors reinforced with 3-15M

80%

0

50

100

150

200

250

300

350

Hei

ght (

mm

)

20%40%60%80%100%Steel @ 60%Steel @ 80%

-5000 0 5000 10000 15000

Hei

ght (

mm

)

Strain (με)

Fig. 11. Strain distribution along the connectors

buckling of PVC, the maximum PVC strain reading at failure initia-tion varied from 3233 le to 9200 le with an average of 6500 ledepending on the buckling location with respect to the straingauge location.

2.8.4. d- Strain distribution along the connectorsFig. 10 shows the strain gauges location along the connector’s

height. Fig. 11 shows a typical strain distribution along the heightof the connector for the PVC encased walls with middle and bracedconnectors. The horizontal axis represents the strain in the PVCand the vertical axis represents the gauge location. The strain dis-tribution was plotted at different percentages of the peak load levelas indicated by the legend. In addition, the steel strain reading at agiven load level was superimposed on the plot. At a given load levelthe steel strain reading was given the same symbol as that for thePVC strain. The PVC strain readings are connected with straightlines, but the steel strain is represented by a single point. It is clearfrom Fig. 11 that as the load increases the strain at all locations in-creases. The strains in the steel and the connector were consistentin the walls with middle connectors (Fig. 11a). The PVC strain dis-tribution versus wall height was linear up to 60% of the peak loadlevel and bi-linear for 80% and 100% of the peak load level. In thewalls with braced connectors (Fig. 11b), the strain gauges on thesteel rebar and the PVC connector were not at the same location.The strain in the steel rebar at a given percentage of the peak loadwas almost equal to the strain on the PVC panel on the tensionside. Also, the strains in the web of the connector were low. Thismeans that the contribution of the web in resisting the applied mo-ment can be ignored.

Table 6 shows a comparison between the experimental and cal-culated peak loads for all the PVC encased walls. The predictedpeak load using the proposed analysis correlated well with themeasured loads with reasonable errors. The average error is 5%and the maximum error is 10%. These results agree well with Rteilet al. [11]. They used a linear strain distribution in their model.They concluded that the difference between the predicted andexperimental moment capacity values ranged between 12% and16%. Ref. [8] used limit-state analysis to calculate the capacity ofthe PVC encased specimen. Their analysis provided a lower boundto their experimental results.

3. Conclusions

Based on the experimental results, the use of the PVC stay-in-place forming system enhanced the flexural behavior of a reinforced

(b) 305mm (12inch) thick wall with bracedconnectors reinforced with 3-20M

0

50

100

150

200

250

300

350-4000 -3000 -2000 -1000 0 1000 2000 3000

Strain (με)

20%40%60%80%100%Steel @40%Steel @ 60%Steel @ 80%

height at different percentages of peak load.

Table 6Comparison between the predicted and experimental peak loads.

Wall type Wallthickness

RFT Experimentalload (kN)

Calculatedload (kN)

Error(%)

Wallswithoutinsulation

8 in. 3–10M 54.8 50.5 7.93–15M 87.5 83.9 4.13–20M 114.3 114.3 0.0

10 in. 3–10M 75.6 69.05 8.73–15M 124.2 117.2 5.63–20M 178.6 161.8 9.4

Walls withinsulation

10 in. 3–10M 54 51.6 4.53–15M 82.2 84.7 �3.13–20M 120 114.3 4.8

12 in. 3–10M 77.77 70 9.63–15M 115.8 117.5 �1.53–20M 159.8 161.5 �1.1

N. Wahab, K.A. Soudki / Construction and Building Materials 48 (2013) 830–839 839

concrete wall. The main conclusions of the study can be summarizedas follows:

1. The PVC encased walls failed in a similar manner to the controlspecimens: yielding of the steel reinforcement with rupture ofthe PVC panel on the tension side and concrete crushing withbuckling of the PVC panel on the compression side in somecases. The wall thickness and connector type did not affectthe mode of failure.

2. The PVC encased specimens showed an increase in the crackingload that ranged from 216% to 270% over the control specimens.

3. The effect of the PVC on the yield load decreased as the rein-forcement ratio increased and the type of the connector didnot affect the yield load. The increase in yield load ranged from15% to 22% above the control walls.

4. The contribution of the PVC system to the ultimate loadincreased as the concrete core thickness decreased and/or asthe reinforcement ratio decreased. The increase in ultimate loaddue to the PVC stay-in-place system ranged from 17.4% to 37.7%over the control.

5. The PVC stay-in-place system contributed to enhancing theductility of the wall system with an increase in ultimate deflec-tion between 2.5% and 134% over the control walls.

6. A model based on strain compatibility and force equilibriumwas proposed to predict the capacity of the PVC encased walls.The predicted loads correlated well with the measured loadswith a 5% error on average.

References

[1] ACI committee 318. Building Code Requirements for Structural Concrete (ACI318–05). Farmington Hills, (MI): American Concrete Institute; 2005.

[2] Chahrour A, Soudki K, Straube J. RBS Polymer encased concrete wall. Part I:experimental study and theoretical provisions for flexure and shear. ConstrBuild Mater 2005;19(7):550–63.

[3] Canadian Standards Association. Design of Concrete Structures. CSA A23.3;2004.

[4] Chahrour A, Soudki K. RBS Polymer encased concrete wall part II: experimentalstudy and theoretical provisions for combined axial compression and flexure.Constr Build Mater 2006;20(10):1016–27.

[5] Collins M, Mitchell D. Prestressed concrete basics. Ottawa, Ontario,Canada: Canadian Prestressed Concrete Institute (CPCI); 1987.

[6] Fam AZ, Flisak B, Rizkalla S. Experimental and analytical modeling of concrete-filled fiber reinforced polymer tubes subjected to combined bending and axialloads. ACI Struc J 2003;100(4):499–509.

[7] Fam AZ, Rizkalla SH. Flexural behavior of concrete-filled fiber-reinforcedpolymer circular tubes. ASCE J Compos Construct 2002;6(2):123–32.

[8] Kuder K, Rishi G, Harris-Jones C, Hawksworth R, Henderson S, Hitney J. Effect ofPVC stay-in-place formwork on mechanical performance of concrete ASCE. JMater Civil Eng 2009;21(7):309–15.

[9] Li G, Torres S, Alaywan W, Abadie C. Experimental study of FRP tube-encasedconcrete columns. J Compos Mater 2005;9(13):1131–45.

[10] Octaform. General Guide: version 2 revision 1. Vancouver, (BC): OctaformSystems Inc.; 2004. 87 p.

[11] Rteil A, Soudki K, Richardson D. Flexural Behavior of Octaform™ FormingSystem, vol. 257(9). ACI SP; 2008. p. 133-148.