Effect of vertical reinforcing bars on formwork pressure of SCC containing recycled aggregates

Journal of Building Engineering 13 (2017) 159–168

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Effect of vertical reinforcing bars on formwork pressure of SCC containing recycled aggregates Pierre Matara, Joseph J. Assaadb, a b

MARK

⁎

Dept. of Civil Engineering, Faculty of Engineering, Lebanese University, Roumieh, Lebanon Notre Dame University, PO Box 72, Zouk Mikayel, Lebanon

A R T I C L E I N F O

A B S T R A C T

Keywords: Self-consolidating concrete Formwork pressure Vertical reinforcement Recycled aggregates Thixotropy

The feasibility of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) was demonstrated in structural civil engineering applications. Yet, limited information exists regarding the effect of RCA additions, especially in presence of steel reinforcement, on lateral pressure exerted on formworks. This paper reports experimental data obtained from twenty-one SCC mixtures cast in 200 × 400 × 1600-mm formwork containing up to 4.71% vertical steel (the spacing between transverse steel was set to 450 mm). Test results have shown that mixtures incorporating RCA exhibited reduced initial pressure, which was mostly attributed to higher aggregate surface roughness that increases internal friction. The decrease in pressure was accentuated with the increase in vertical steel density, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. Special emphasis was placed to develop conservative reinforcement indices for appropriate prediction of SCC pressure, as well as propose modifications for existing models to account for the effect of RCA additions and presence of vertical steel bars.

1. Introduction The feasibility of recycled concrete aggregate (RCA) in reinforced concrete structural members was demonstrated over the past years [1–6]. Fonteboa and Abella [2] found negligible differences in shear strength of concrete beams made with 50% replacement of natural coarse aggregate (NCA) by RCA. The concrete had 40-MPa compressive strength, and tested beams contained 2.4% flexural steel together with 0–0.22% shear reinforcement. Etxeberria et al. [3] reported that the code-predicted ultimate shear strength of concrete made with 100% RCA could be fully achieved, provided the quantity of steel required by relevant standards is included. Ajdukiewicz and Kliszczewicz [4] reported that the difference in load-bearing capacity of real-scale beam and column members produced with 100% fine and coarse RCA could be neglected in practice. Yet, the difference in deformation should be considered when assessing deflection of beams and shortening of columns [4]. Highly flowable self-consolidating concrete (SCC) containing recycled aggregates is suitable for casting congested reinforced members, while enhancing sustainability practices. However, RCA-modified SCC requires proper understanding of dynamic/static stability including the developed lateral pressure on vertical formworks. In fact, recycled aggregates are composed of NCA with about 30–40% of adhered mortar

⁎

Corresponding author. E-mail addresses: [email protected] (P. Matar), [email protected] (J.J. Assaad).

http://dx.doi.org/10.1016/j.jobe.2017.08.003 Received 17 July 2017; Received in revised form 2 August 2017; Accepted 4 August 2017 Available online 05 August 2017 2352-7102/ © 2017 Elsevier Ltd. All rights reserved.

that gives the granular phase particular characteristics such as rougher surface texture, greater angularity, and higher water absorption [1,7–10]. Earlier studies have shown that SCC free deformability is worsened with RCA additions due to increased internal friction and plastic viscosity, leading to reduced rate of flow and passing ability among various obstacles and reinforcing bars [11,12]. Although reduced workability can be overcome by incorporating high-range water reducer (HRWR), several researchers found that the rate of slump loss over time remarkably increased with RCA additions [9,13,14]. This was explained by the fact that recycled aggregates (whether soaked or not in water prior to batching) can still absorb moisture after concrete mixing, leading to increased slump loss over time. After placement and until onset of hardening, several researchers found that RCA materials lead to improved static stability including resistance to bleeding and aggregate segregation [6,10,12,13]. This was attributed to a combination of phenomena including increased amounts of fine particles that possibly improve hydraulic activity of the cementitious phase as well as higher RCA water absorption that reduces the detrimental effects associated with free mixing water in fresh concrete. The current knowledge on formwork pressure developed by virginaggregate SCC cannot be applicable if part or all the granular phase is replaced by RCA, especially knowing that this phase constitutes more than 60% of the total concrete volume [15–17]. Recently, Assaad and

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influence on pressure drop over time, since the decay in pressure is mostly governed by concrete thixotropy and cement hydration. The tests conducted by Santilli et al. [21] were performed on 300 × 300 mm square columns having 1200 mm height and containing vertical reinforcement ratio (ρsv) varying from 0% to 1.8%. Omran and Khayat tests were carried out using 250 × 500 mm columns having 1400 mm height and reinforced with 0% to 4% vertical steel that are attached together with rectangular ties 5 Ø8 mm/m. The authors concluded that heavily 4% reinforced section with 25-mm concrete cover can significantly reduce σmax/hyd by about 35% [22]; the reduction in initial maximum pressure decreases with the decrease in concrete thixotropy, reduction in reinforcement density, and increase in cover depth [22]. Omran and Khayat extended their originally developed model (Eq. (4)) to account for the effect of concrete cover and reinforcing bars.

Harb [18] tested the SCC pressure resulting from the substitution of NCA by 25–100% RCA; the mixtures had 700 ± 20 mm slump flow and water-to-binder ratio (w/b) varying from 0.5 to 0.38. The concrete pressure was measured using strain gages mounted at different elevations on plastic tube measuring 1600 mm height and 150 mm diameter. Regardless of SCC composition, test results have shown that mixtures prepared with increased RCA replacement levels led to reduced initial pressure after placement, as well as accelerated rates of pressure drop over time. The former phenomenon was related to higher RCA surface roughness that increases internal friction, while the later was attributed to the combined effects of thixotropy (i.e., structural material build-up at rest) and higher RCA water absorption that improves concrete stability [18]. It is important to note that the results obtained are quite conservative, given that SCC lateral pressure is not solely affected by material intrinsic properties, but concurrently by external factors such as placement conditions and formwork characteristics (i.e., presence of reinforcing bars). The literature is not abundant with studies treating the effect of reinforcing bars on SCC formwork pressure, albeit it is reasonable to presume that such reinforcement would carry part of the concrete load and contribute in reducing lateral pressure. In 2006, Perrot et al. [19] evaluated the differences in SCC pressure following the vertical positioning of one 25-mm diameter steel bar in the middle of cylindrical column measuring 1300 mm high and 100 mm diameter. The columns were vibrated right after filling so that all structuration built during casting process is destroyed and pressure becomes hydrostatic (i.e., providing repeatable reference state). The authors found significant acceleration in pressure decay over time resulting from the presence of reinforcing bar; this varied from 0.27 to 0.17 kPa/min for the test performed with or without steel bar, respectively [19]. Nevertheless, it should be pointed out that the bar positioning in the middle of experimental column does not reflect practical situations, whereby reinforcing bars are normally arranged along the formwork perimeter with certain concrete cover. Extending the model developed by Ovarlez and Roussel [20] (Eq. (1)) to predict the relative SCC pressure (σmax/hyd) at bottom of formwork, Perrot et al. [19] proposed a theoretical model given in Eq. (2) that includes the effect of reinforcing steel bars.

σmax/hyd,% = 1−

HAThix ρg e R

ϕ +2Sb ⎞ HAThix σmax/hyd,% = 1−⎛⎜ b ⎟ (e ⎝ −Sb)ϕ b ⎠ ρg R

σmax /hyd, % = 95.9–3.84 H+ 0.71 R + 4.1 Dmin –0.29 PVτ0rest (t)

where Dmin and PVτ0rest(t) refer to minimum formwork dimension (m) and evolution of static yield stress with resting time (i.e., similar to AThix), respectively. Hence, this later model was multiplied by a statistically determined factor to account for concrete cover and ρsv, as follows:

σmax/hyd,% = (95.9 − 3.84H + 0.71R + 4.1Dmin 106.3 ⎞ ⎞ − 0.29PVτ0rest (t)) ⎛1−ρsv ⎛4.63+ cover ⎠ ⎠ ⎝ ⎝

(5)

The growing use of sustainable RCA-modified SCC in structural applications invoked the need for better understanding of casting processes including the effect of reinforcing bars on formwork pressure development. Twenty-one SCC mixtures possessing relatively low to high stability levels and incorporating various RCA replacement rates are investigated. Formwork pressure was tested using 200 × 400 × 1600 mm column containing ρsv ranging from 0% to 4.71%. Special emphasis was placed to validate Eqs. (1)–(5) and propose suitable modifications that account for the effect of RCA additions and presence of vertical reinforcing bars. The combined effect of vertical bars as well as transverse bars (i.e., ties) on formwork pressure is treated in followup paper. Such data can be of interest to researchers, concrete technologists, and contractors to better understand the effect of RCA additions and formwork characteristics on lateral pressure exerted by highly flowable concrete.

(1)

(2)

2. Materials and mix proportions

where H, e, ρ, R, g, and AThix refer to concrete head in formwork (m), width of form (m), material density, casting rate, gravity acceleration, and structuration rate, respectively. The Sb and Øb in Eq. (2) refer to horizontal steel section per linear meter of width and average diameter of vertical reinforcing bars, respectively. Perrot et al. [19] estimated that the presence of steel had twice the contribution of thixotropic behavior in reducing the formwork pressure. It is to be noted that Assaad and Harb [18] findings have shown that formwork pressure of RCA-modified SCC cannot be predicted using Eq. (1), given the under-estimation of internal friction resulting from the presence of recycled aggregates. The authors proposed introducing, through iterative regression analysis, the relative water absorption factor (Qw) of the granular phase to improve the measured-to-predicted σmax/hyd values (the Qw factor is defined later in text). Hence, mixtures containing increased RCA replacement rates would be characterized by higher Qw, leading to reduced initial pressure, as per Eq. (3) below.

1 HAThix ⎞ σmax/hyd,% = ⎜⎛ 0.05 ⎟⎞ ⎜⎛1− ⎟ ⎝ Qw ⎠ ⎝ ρg e R ⎠

(4)

2.1. Materials for SCC production Ternary binder composed of 70% portland cement, 25% blast furnace slag, and 5% silica fume conforming to ASTM C150 Type I, C989 Grade 100, and C1204, respectively, was used. The cement had C3S, C3A, and Na2Oeq of 61.4%, 6.1%, and 0.7%, respectively. The binder Blaine surface area, median particle size, and specific gravity were 3750 cm2/g, 21.2 µm, and 3.03, respectively. The natural fine aggregate consisted of well-graded siliceous sand complying to ASTM C33 specification; its bulk specific gravity, fineness modulus, and absorption rate were 2.65%, 2.53%, and 0.95%, respectively. The physical coarse aggregate properties are summarized in Table 1; these were checked several times during the execution period of experimental program. The NCA consisted of crushed limestone rocks, while RCA was obtained by crushing returned concrete from readymixed batching plant (the returned hard concrete had about 25–35 MPa compressive strength). The RCA adhered mortar portion determined by the freeze-thaw test [23] and aggregate crushing value (ACV) determined as per BS 812–110 test method [24] were 39.5% ± 3% and 23.6% ± 2.1%, respectively. The NCA and RCA water absorption rates were 0.6% ± 0.08% and 7.2% ± 1.14%, respectively. The aggregate

(3)

Contrary to Perrot et al. [19] concluding remarks, Santilli et al. [21] and Omran and Khayat [22] reported that reinforcing bars have no 160

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Table 1 Physical properties of NCA and RCA materials.

NCA RCA

Specific gravity

Oven-dry rodded bulk density, kg/m3

Absorption rate, %

Material finer than 75-μm, %

Fineness modulus

Adhered mortar content, %

ACV, %

2.72 2.42

1760 1515

0.6 7.2

0.42 0.88

6.71 6.75

n/a 39.5

17.2 23.6

unit volumes (in L/m3), respectively. As shown in Table 2, Qw varied from 0.761% for control mixtures to 2.65%, 3.5%, and 4.34% with 50%, 75%, and 100% RCA, respectively.

grain size distributions were within ASTM C33 (Size no. 7) limitations, with nominal maximum particle sizes of 12.5 mm. Naphthalene-based HRWR with specific gravity of 1.18 and solid content of 36% was used. This admixture complies with ASTM C494 Type F; it can be used up to 3.5% of cement mass. Liquid hydroxyethyl cellulose ether viscosity-modifying admixture (VMA) with specific gravity and solid content of 1.04% and 15%, respectively, was used. This VMA is compatible with naphthalene-based HRWR, and is typically employed to regulate cohesiveness and stability of SCC mixtures [25,26].

Qw =

aQ NCA + bQ NFA + cQRCA a+b+c

(6)

All coarse aggregates (i.e., RCA and NCA) were pre-soaked for 24 ± 3 h in water to assure full saturation prior to use. Then, 1-h prior to batching, the materials were distributed over a large absorbent dry mat until all visible water films are completely eliminated; the concrete batch proportions were adjusted for aggregate surface moisture to maintain constant w/b. The mixtures were prepared in open-pan mixer of 250-L capacity. The mixing sequence consisted of homogenizing the sand, aggregate, and about 50% mixing water before introducing the cementitious binder. After one minute of mixing, the other 45% water was added, followed by HRWR, and then VMA diluted in the remaining 5% of water. The concrete was mixed for two additional minutes. The ambient temperature and relative humidity during mixing and sampling hovered 26 ± 3 °C and 45% ± 10%, respectively.

2.2. Concrete proportioning and mixing Seven (i.e., 3 control and 4 RCA-modified) SCC mixtures selected from previous study [18] to cover a wide range of AThix levels are considered in this paper (Table 2). The control mixtures were prepared with 375, 410, or 450 kg/m3 binder, while w/b varied from 0.5 to 0.44 and 0.38, respectively. The HRWR was adjusted at 1.6%, 2.2%, and 2.35% of binder mass, respectively, to achieve slump flow of 700 ± 25 mm; while stability secured by regulating the VMA dosage to achieve Visual Stability Index (VSI) in order of 1 ± 0.5 (i.e., this value reflects slump flow with no mortar halo, but with some slight bleeding of concrete in mixer drum and/or wheelbarrow) [27]. Hence, the VMA decreased from 0.4% to 0.2% and 0.1% of binder mass for mixtures made with 0.5, 0.44, and 0.38 w/b, respectively. The sand-to-total aggregate ratio remained fixed at 0.45. The NCA was substituted by 50% and 100% RCA using the direct volumetric method in SCC mixtures made with 0.5-w/b, while the replacement level was set at 75% for those prepared with 0.44 and 0.38 w/b (Table 2). The VMA concentration remained fixed as in control mixtures, while HRWR adjusted to counter-balance the increase in internal friction due to RCA and secure the targeted slump flow of 700 ± 25 mm [7,11,12]. For example, an additional HRWR of 0.35% (i.e., from 1.6% to 1.95%) was needed when 100% RCA replacement level was used in 0.5-w/b mix. The relative water absorption of granular phase (Qw) for tested mixtures was determined following Eq. (6) [9,12]. The QNCA, QNFA, and QRCA refer to water absorption of NCA, natural fine aggregate (or sand), and RCA, respectively; while a, b, and c are the corresponding mixed

2.3. Fresh concrete properties 2.3.1. Stability of tested mixtures Right after concrete mixing, the slump flow, passing ability, and segregation resistance were determined as per the European Guidelines for SCC [28] (Table 2). Three 12-mm diameter smooth bars were used in the L-box to determine passing ability. The segregation resistance was obtained after pouring the fresh SCC into a sieve with 5-mm square apertures, then weighing the material that has passed through the sieve after 2-min rest interval. The bleeding of tested mixtures was determined as per ASTM C232 Test Method [29], which consists of measuring the relative quantity of mixing water that bled from the fresh material placed in given container. Generally speaking, the addition of RCA led to gradually reduced passing ability, given the increased internal friction and aggregate surface roughness that hinder concrete deformability [7,9,12]. For example, the L-box ratio decreased from 0.92 for the control 0.5-w/b mix to 0.88 and 0.75 with 50% and 100% RCA additions, respectively. From the other hand, mixtures containing increased RCA replacement levels

Table 2 Mixture composition of SCC containing different RCA replacement levels. Mixture codification 3

Binder, kg/m w/b Sand, kg/m3 NCA, kg/m3 RCA, kg/m3 VMA, % of binder HRWR, % of binder Qw, % Unit weight, kg/m3 Slump flow, mm L-box Segregation resistance, % Bleeding, % τ0 at t = 0 min, Pa η at t = 0 min, Pa s AThix, Pa/s

0.5-NCA

0.5-50%RCA

0.5-100%RCA

0.44-NCA

0.44-75%RCA

0.38-NCA

0.38-75%RCA

375 0.5 795 970 0 0.4 1.6 0.761 2315 705 0.92 17.6 7.2 28.8 10.13 0.055

375 0.5 795 460 460 0.4 1.65 2.654 2295 680 0.88 17 6.3 31 13.44 0.096

375 0.5 795 0 870 0.4 1.95 4.339 2270 700 0.75 12.2 3.9 33.5 17.4 0.126

410 0.44 790 960 0 0.2 2.2 0.761 2395 710 0.91 13.5 6 34.6 12.1 0.084

410 0.44 790 0 860 0.2 2.45 3.509 2345 700 0.79 10.3 4.5 41 19.06 0.127

450 0.38 780 955 0 0.1 2.35 0.761 2410 705 0.84 9.3 2.6 45.2 18.8 0.169

450 0.38 780 0 670 0.1 2.65 3.54 2380 685 0.74 7.5 1.9 48 27.57 0.215

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formwork were trimmed to reduce their diameter to around 17 mm, and using highly elastic silicon, re-mounted in their position flush with the inner/outer sides of the tube. This set-up allowed the slots to move freely due to pressure after material filling. It is to be noted that the idea of measuring strains to determine concrete lateral pressure is not new. For instance, Shunk (from Gardner [31]) inserted 230-mm diameter cylinder fitted with freely moving piston into wood formwork to monitor concrete pressure. By means of weighted lever arm, the load required to prevent the piston from moving under the applied pressure could be determined. Rodin and Macklin (from Gardner [31]) deducted concrete pressure by deflection of steel plate or wood sheathing by means of dial-type micrometer mounted on bridge arrangement through the formwork width. Khayat and Assaad [32] determined SCC pressure using strain gages welded onto 8-mm diameter steel rods that were used to support a slotted area created in the formwork. The tested SCC mixtures were cast continuously without vibration at a rate hovering 10 m/h. The strains at each slot location were picked-up using linear variable differential transformers (LVDTs) and high-precision digital micrometer strain gages. It is to be mentioned that the micrometer gages are found pretty practical, especially for field-testing, as this allows real-time readings while reducing the hassle of wire connections and data acquisition. The LVDTs and gages were mounted on rigid stand frames, separate from the test tube, to avoid interference of strain measurements due to concrete filling. The conversion of strains into stresses was realized by calibrating the tube with water.

(i.e., higher Qw) led to improved stability, which is reflected by reduced bleeding and aggregate segregation. For example, the segregation index decreased from 17.6% to 12.2% when Qw increased from 0.761% to 4.339%, respectively (i.e., RCA increased from 0% to 100%), for mixtures made with 0.5-w/b. The corresponding bleeding decreased from 7.2% to 3.9%, respectively. This can be attributed to the concurrent effect of different phenomena including higher percentage of fine particles (i.e., material finer than 75-μm is 0.88 vs. 0.42), increased possibility of hydraulic activity due to such fines, as well as greater capacity of aggregates to absorb free water (i.e., absorption rate is 7.2% vs. 0.6%) and reduce moisture exchange in the plastic concrete [7,9,12,14]. 2.3.2. Rheological properties The static yield stress (τ0) was determined at various elapsed resting times (trest) after mixing (i.e., 0, 15, 30, 45, and 60 min) using fourbladed slotted vane connected to rotational rheometer [18]. The testing protocol consisted on subjecting the material to very low rotational speed of 0.3 rpm and recording the changes in torque as a function of time. All profiles showed linear elastic region followed by yielding moment where the torque exerted on the vane shaft reaches a maximum value, indicating that the majority of bonds and inter-particle links are broken. Also, as Assaad detailed [12], the plastic viscosity (η) of freshly mixed SCC was determined using the modified-Bingham model by considering the vane rotating in infinite medium. Considering stresses uniformly distributed along the inner and outer cylinders circumscribed by the slotted vane, the transformation of torque measurement to τ0 was made in accordance to Ref. [30]. Table 2 summarizes τ0 and η registered right after mixing, along with AThix determined by considering the slope of regression lines for τ0 at various trest. As can be seen, AThix increased for SCC made with combinations of increased binder content and reduced w/b; this varied from 0.055 to 0.084 and 0.169 Pa/s for control mixtures made with 0.5, 0.44, and 0.38 w/b, respectively. From the other hand, the substitution of NCA by higher RCA replacement levels led to increased τ0, η, and AThix measurements, which can be mostly attributed to greater internal friction that renders the plastic concrete stiffer requiring increased shear stresses to initiate flow [11,12,18]. Hence, for example, AThix reached 0.126 Pa/s in 0.5-w/b mixture containing 100% RCA, and 0.215 Pa/s for the 0.38-w/b mix prepared with 75% RCA.

2.5. Vertical steel configurations Fig. 2 summarizes the five different vertical steel configurations tested in this study (with illustrations given for three of these configurations). The vertical steel consisted of deformed Ø20 bars having 20mm diameter and theoretical weight of 2.47 kg/m; these were arranged symmetrically along the two opposite 400-mm sides of the form, while maintaining constant concrete cover of 25 mm. The center-to-center spacing (Sv) for 4, 6, 8, 10, and 12 vertical steel bars is 350, 175, 116.6, 87.5, and 70 mm, respectively (i.e., clear spacing between bars was at least 3-times greater than the nominal maximum aggregate size of 12.5 mm). The resulting percentage of vertical steel (ρsv) calculated as the ratio between total cross-sectional area of vertical bars to gross column area (i.e., 80,000 mm2) was 1.57%, 2.36%, 3.14%, 3.93%, and 4.71%, respectively. It is to be noted that SCC pressure was also tested in case the formwork contained no vertical reinforcement, which indicates that ρsv varied from 0% to relatively high reinforcement level of 4.71% (note that, for example, ACI 318 [33] recommends maximum 8% vertical steel in columns). The Eff(ρsv) and Δ(ρsv) shown in Fig. 2 are defined later in text. As earlier mentioned, this paper evaluates the effect of vertical steel reinforcement on RCA-modified SCC pressure, while the combined effect of vertical and transverse bars is treated in follow-up publication. Hence, to minimize the effect of transverse bars on pressure readings within the context of this study, the ties were placed at maximum distance of 450 mm accepted by ACI 318 for column and wall elements [33]. Deformed Ø10 bars having 10-mm diameter and theoretical weight of 0.62 kg/m were used to provide adequate positioning of vertical bars without any movement due to concrete filling. Hence, the first tie was placed at 50 mm from the base (i.e., 50 mm below the first row of pressure sensors), while the other 3 ties distributed at 500, 950, and 1400 mm from the base.

2.4. Description of set-up used for formwork pressure testing A plexiglass acrylic rectangular form measuring 1600-mm height, 400-mm length, and 200-mm width was developed for measuring the SCC lateral pressure (Fig. 1). The 15-mm thick walls are transparent, which allowed visual monitoring of filling process and eventual anomalies such as arching effect or blocking due to reinforcing bars in the vicinity of strain sensors. The tube faces were fabricated using groove-tongue system to prevent leakage and facilitate assembling as well as dismantling for subsequent cleaning. Five steel straps are wrapped at different elevations along the height, to provide rigid form capable of withstanding the concrete pressure. The 40-mm thick double-layered base was grooved along the wall perimeter, and extended 120-mm beyond the rectangular tube surface to ensure stable formwork during and after concrete filling, without the need of external supports. On each of the 400-mm side faces of formwork, six slots of 20-mm diameter each were laser-cut along two horizontal rows (i.e., 3 slots per row distributed at equal distances of 100 mm); the rows are located at 100 and 300 mm from the base (Fig. 1). The idea behind such disposition of slots per row was to capture whether the SCC lateral pressure is affected by the positioning of vertical steel bars. The six other slots perforated on the opposite-side of formwork were used to confirm the pressure readings at given location, when needed. For measuring SCC pressure, the same slots created along the

3. Test results and discussion Table 3 summarizes the lateral pressure characteristics determined using the middle strain gage located at 100-mm from the base for SCC mixtures cast in formwork containing ρsv ranging from 0% to 4.71%. This includes the maximum initial pressure (kPa), σmax/hyd (%), and rate of pressure drop over time (%/min). Note that the nomenclature 162

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Fig. 1. Photo of the column set-up used for SCC pressure measurements.

3.1. Effect of RCA on SCC pressure in non-reinforced formwork

describing tested mixtures refers to w/b-Aggregate-Sv; for example the 0.38-75%RCA-175Sv refers to SCC prepared with 0.38 w/b, 75% RCA replacement rate, and cast in formwork containing 6 bars Ø20 (i.e., Sv of 175 mm).

The lateral pressure variations determined using the middle sensor located at 100-mm from the base for SCC mixtures made with or Fig. 2. Various vertical steel configurations used for testing.

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Table 3 Measured formwork pressure of tested mixtures (using the middle sensor located at 100mm from the base). ρsv, %

0.5-NCA 0.5-50%RCA 0.5-100%RCA 0.44-NCA 0.44-75%RCA 0.38-NCA 0.38-75%RCA 0.5-NCA-175Sv 0.5-NCA-87Sv 0.5-NCA-70Sv 0.5-50%RCA87Sv 0.5-100%RCA116Sv 0.5-100%RCA70Sv 0.44-NCA350Sv 0.44-75%RCA350Sv 0.44-75%RCA87Sv 0.38-NCA116Sv 0.38-NCA-70Sv 0.38-75%RCA350Sv 0.38-75%RCA87Sv 0.38-75%RCA70Sv

Sv, mm

Initial pressure, kPa

hyd,

σmax/ %

Pressure drop, %/min

0 0 0 0 0 0 0 2.36 3.93 4.71 3.93

0 0 0 0 0 0 0 175 87.5 70 87.5

33.45 31.91 30.43 34.82 30.95 34.47 30.43 33.52 32.7 30.42 29.75

98.2 94.5 91.1 98.8 89.7 97.2 86.9 98.4 96 89.3 88.1

– – – – – – – – – – –

3.14

116.6

30.9

92.5

– 0.19

4.71

70

28.19

84.4

– 0.188

1.57

350

34.19

97

– 0.089

1.57

350

31.13

90.2

– 0.238

3.93

87.5

28.16

81.6

– 0.262

3.14

116.6

33.87

95.5

– 0.183

4.71 1.57

70 350

31.92 30.54

90 87.2

– 0.157 – 0.278

3.93

87.5

27.63

78.9

– 0.256

4.71

70

26.62

76

− 0.204

0.062 0.104 0.166 0.103 0.23 0.162 0.271 0.07 0.064 0.082 0.117

Fig. 4. Typical pressure variations for SCC cast in formwork containing different steel configurations (data registered from the middle sensor located at 100-mm from the bottom).

percentage of fine particles and water absorption that improve build-up of SCC skeleton at rest [7,10,18], with reduced transformation of vertical stresses into lateral ones. It should also be noted that the concrete unit weight decreased with RCA additions (for example, from 2395 to 2345 kg/m3 for 0.44-w/b mix), which reduces the absolute magnitude of pressure developed on the form.

3.2. Effect of vertical steel on formwork pressure Fig. 4 plots the lateral pressure variations for 0.5-NCA and 0.38-75% RCA mixtures cast in formwork containing different vertical steel configurations (the middle sensor located at 100-mm from the base was used for the plots). Regardless of RCA additions, the increase in vertical steel density (i.e., reducing Sv) led to reduced initial maximum pressure; for example, this decreased from 33.52 to 32.7 and 30.42 kPa when the 0.5-NCA mixture was cast in formwork containing vertical steel positioned at Sv of 175, 87.5, and 70 mm, respectively (this corresponds to σmax/hyd of 98.4%, 96%, and 89.3%, respectively, for ρsv of 2.36%, 3.93%, and 4.71%, respectively). This can be attributed to the reinforcement cage that confines the inner concrete volume and carries part of its load, thus reducing the net lateral pressure exerted on the formwork. The lowest initial pressure registered in this testing program was 26.62 kPa for the 0.38–75%RCA mix cast in the most congested formwork containing 12 bars Ø20 (i.e., ρsv and Sv of 4.71% and 70 mm, respectively); the resulting σmax/hyd was 76% (Table 3). The relationship between ρsv and normalized initial pressure is illustrated in Fig. 5; the normalized pressure is the ratio between σmax/hyd obtained from mixtures cast in reinforced column with respect to those determined from similar concrete cast in non-reinforced column. As can be seen, the normalized decreased gradually for mixtures cast in columns containing ρsv less than about 3%, but then dropped significantly at higher ρsv with acceptable correlation coefficient (R2) of 0.72.

Fig. 3. Typical pressure variations for SCC cast in non-reinforced formwork (data registered from the middle sensor located at 100-mm from the bottom).

without RCA additions are plotted in Fig. 3 (the formwork contained no vertical reinforcement). The slump flow values determined after 60 min from mixing are shown. It is important to note that the pressure plots could be replicated whether the middle or edge sensors, located on either side of the 400-mm length, are used for measurements. This practically implies that the lateral pressure is identical, at given concrete head, along the length of non-reinforced formworks. In concordance with previous data obtained using 150-mm diameter column [18], the effect of incorporating RCA led to reduced initial maximum pressure as well as accelerated rate of pressure drop over time. For example, the initial pressure decreased from 34.82 to 30.95 kPa when 75% RCA were used in 0.44-w/b mix (the resulting σmax/hyd decreased from 98.8% to 89.7%); the rate of pressure drop increased from – 0.103 to – 0.23%/min. This can be directly related to RCA intrinsic properties including higher surface roughness that increases internal friction and plastic viscosity as well as higher

Fig. 5. Relationship between ρsv and normalized initial pressure.

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Fig. 6. Effect of gage location on initial pressure measurement (formwork containing 4 bars Ø20).

The rates of pressure drop over time were not altered due to increased vertical steel density (Fig. 4); this varied within – 0.07%/ min ± 0.01%/min for the 0.5-NCA mixture cast in formwork containing different vertical steel configurations. Such results are in complete agreement with other researchers [21,22,32] who concluded that pressure drop is mostly governed by the concrete intrinsic properties (such as AThix, Qw, and cement hydration), rather than the presence of reinforcing bars.

Fig. 7. Determination of Eff(ρsv) to improve conservativeness of predictive models for SCC pressure.

3.3. Suitability of models to predict initial maximum pressure

the closer Ψ-factor to 1.0 reflects accurate prediction of σmax/hyd. Also, the standard deviation (i.e., St.D) between measured-to-predicted values is given in Table 4, which reflects the spread of values about the mean.

3.3.1. Development of indices that reflect density of steel bars Contrarily to the conclusion drawn earlier in non-reinforced formwork, the maximum SCC pressure recorded at end of casting in reinforced formworks was affected by the location of strain gages (i.e., middle vs. edge). This was particularly noticed in the lightly reinforced formwork containing 4 bars Ø20, where Sv and ρsv are equal to 350 mm and 1.57%, respectively. As can be seen in Fig. 6, the σmax/hyd values picked-up using the middle gage were consistently higher than those determined using the right- or left-edge gages, albeit in some cases such variations remained within the repeatability of testing estimated to ± 2% of hydrostatic value. The relative increase in SCC pressure recorded from the middle gage can logically be attributed to reduced density of steel bars in the vicinity of that gage (although ρsv is constant in instrumented column), leading to increased σmax/hyd. In other words, the use of ρsv in given predictive model would under-estimate the developed lateral pressure in locations where the vertical steel is spaced farther apart, which ultimately may lead to collapse of formwork. To improve appropriateness as well as conservativeness of vertical steel index when predicting formwork pressure exerted by SCC, an effective ρsv index (refereed to as Eff(ρsv)) is proposed as follows:

Eff(ρsv ) =

2A v St 2A v = eS v St eS v

3.3.2. Correlations with Ovarlez and Roussel [20] model Weak R2 of 0.41 exists between measured σmax/hyd to predicted values using the original Ovarlez and Roussel model given in Eq. (1) (Fig. 8); the H, e, and R are taken 1.5 m, 0.2 m, and 10 m/h, respectively. As earlier explained, this is related to the fact that such model under-estimates the increase in internal friction due to recycled aggregates as well as presence of vertical steel that helps carrying part of the vertical load. The R2 of measured-to-predicted σmax/hyd relationships gradually improved to 0.53 when Qw was incorporated in the model (as given in Eq. (3)), and then to 0.75 when both Qw and Eff(ρsv) are included (as given in Eq. (8) below). The 0.027-coefficient was determined by iterative regression analysis, in a way to minimize the standard deviation of ratios between measured-to-predicted values while maximizing the accuracy of correlation. As can be seen in Table 4, the Ψ-factor steadily increased from 0.919 to 0.946 and 1.001 when Eqs. (1), (3), or (8) are used, respectively; the corresponding St.D decreased from 0.063 to 0.048 and 0.038, respectively.

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The Eff(ρsv) is inspired from ACI 318 [33] building code when accounting the boundary conditions in structural walls subjected to flexure and axial loads. It reflects the influential steel area in given concrete volume (as schematically presented in Fig. 7), whereby Av, e, Sv, and St refer to cross-sectional area of vertical steel, wall thickness, center-to-center spacing of vertical steel, and center-to-center spacing of transverse steel, respectively. As summarized in Fig. 2, the Eff(ρsv) is lower than ρsv; however, by different rates depending on formwork configuration. For example, the ρsv of 1.57% dropped significantly to Eff(ρsv) of 0.9% (i.e., by 74%) in case of formwork containing 4 bars Ø20, while in contrast, such decrease was limited to only 5% in case of congested formwork containing 12 bars Ø20. Table 4 summarizes the Eff(ρsv) and various predicted σmax/hyd using different proposed models. The accuracy of measured-to-predicted values was determined using Ψ-factor calculated as the sum of measuredto-predicted values divided by the number of measurements (i.e., 21);

1 HAThix ⎞ σmax/hyd,% = ⎜⎛ 0.05 ⎟⎞ ⎜⎛1− ⎟ (1 − 0.027Eff(ρ )) sv ⎝ Qw ⎠ ⎝ ρg e R ⎠

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3.3.3. Correlations with Omran and Khayat [22] model The same previous analysis was performed using Omran and Khayat model (Dmin is taken 0.2 m). As can be seen in Fig. 9, the original model given in Eq. (4) over-estimates the formwork pressure, with weak R2 of 0.39 between measured-to-predicted σmax/hyd values. Yet, such correlation improved to 0.55 when Qw is incorporated (as given in Eq. (9)), and then to 0.81 when both Qw and Eff(ρsv) are included (as given in Eq. (10)). As earlier, the 0.017-coefficient assigned to Eff(ρsv) was determined by iterative regression analysis. The Ψ-factor increased from 0.945 to 0.973 and 1.007 when Eqs. (4), (9), or (10) are used, respectively. 165

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Table 4 Predicted σmax/hyd (%) of tested mixtures. Eff (ρsv), %

0.5-NCA 0 0.5-50%RCA 0 0.5-100%RCA 0 0.44-NCA 0 0.44-75%RCA 0 0.38-NCA 0 0.38-75%RCA 0 0.5-NCA-175Sv 1.79 0.5-NCA-87Sv 3.59 0.5-NCA-70Sv 4.49 0.5-50%RCA-87Sv 3.59 0.5-100%RCA-116Sv 2.69 0.5-100%RCA-70Sv 4.49 0.44-NCA-350Sv 0.9 0.44-75%RCA-350Sv 0.9 0.44-75%RCA-87Sv 3.59 0.38-NCA-116Sv 2.69 0.38-NCA-70Sv 4.49 0.38-75%RCA-350Sv 0.9 0.38-75%RCA-87Sv 3.59 0.38-75%RCA-70Sv 4.49 Ψ-factor St.D of measured-to-predicted

Following Ovarlez and Roussel model [20]

Following Omran and Khayat model [22]

Eq. (1)

Eq. (3)

Eq. (8)

Eq. (4)

Eq. (5)

Eq. (9)

Eq. (10)

Eq. (11)

99.4 98.9 98.5 99 98.5 98.1 97.5 99.4 99.4 99.4 98.9 98.5 98.5 99 98.5 98.5 98.1 98.1 97.5 97.5 97.5 0.919 0.063

100.7 94.2 91.5 100.4 92.5 99.4 91.5 100.7 100.7 100.7 94.2 91.5 91.5 100.4 92.5 92.5 99.4 99.4 91.5 91.5 91.5 0.946 0.048

100.7 94.1 91.5 100.4 92.5 99.4 91.5 95.8 91 88.5 85 84.9 80.4 98 90.3 83.5 92.2 87.4 89.3 82.7 80.5 1.001 0.038

97.1 96.4 95.9 96.6 95.8 95.1 94.3 97.1 97.1 97.1 96.4 95.9 95.9 96.6 95.8 95.8 95.1 95.1 94.3 94.3 94.3 0.945 0.063

97.1 96.4 95.9 96.6 95.8 95.1 94.3 81.6 66.2 58.4 65.7 73 57.7 88.9 88.2 65.3 72.4 57.2 86.8 64.3 56.7 1.187 0.209

98.4 91.8 89.1 97.9 90 96.4 88.5 98.4 98.4 98.4 91.8 89.1 89.1 97.9 90 90 96.4 96.4 88.5 88.5 88.5 0.973 0.048

98.4 91.8 89.1 97.9 90 96.4 88.5 95.4 92.4 90.9 86.2 85 82.3 96.4 88.6 84.5 92 89.1 87.2 83.1 81.8 1.007 0.034

98.4 91.8 89.1 97.9 90 96.4 88.5 82.8 67.1 59.2 62.5 67.8 53.6 90.1 82.8 61.3 73.4 58 81.5 60.3 53.3 1.223 0.209

Ψ-factor = Σ (measured-to-predicted values)/nb. of measurements = Σ (measured-to-predicted values)/21.

1 σmax/hyd,% = ⎜⎛ 0.05 ⎟⎞ (95.9 − 3.84H + 0.71R + 4.1Dmin ⎝ Qw ⎠ − 0.29PVτ0rest (t))(1 − 0.017Eff(ρsv ))

(10)

It is to be noted that the similarity of plots shown in Figs. 8 and 9 reflects the analogy between Ovarlez and Roussel (Eq. (1)) and Omran and Khayat (Eq. (4)) models, especially knowing that both account for similar parameters including head of concrete, rate of cast, width of form, and thixotropy. 3.4. Suitability of Eq. (5) – ongoing research Eq. (5) proposed by Omran and Khayat was validated using 1400mm high plywood formwork containing vertical steel (i.e., ρsv ranging from 0% to 4%) attached together with rectangular transverse ties 5 Ø8 mm/m (i.e., 1 tie per 200 mm) [22]. Weak correlation with R2 of 0.32 exists between measured σmax/hyd values and those predicted using this equation (Fig. 10). On average, the predicted values are 18.7% lower (i.e., Ψ-factor) than actual readings, which practically implies that Eq. (5) is not conservative and under-estimates the SCC pressure. The R2 slightly improved to 0.47 when Qw resulting from the presence of recycled aggregates is incorporated (as given in Eq. (11) below); yet,

Fig. 8. Relationships between measured-to-predicted initial pressure (following Ovarlez and Roussel model).

Fig. 9. Relationships between measured-to-predicted initial pressure (following Omran and Khayat model).

1 σmax/hyd,% = ⎜⎛ 0.05 ⎟⎞ (95.9 − 3.84H + 0.71R + 4.1Dmin − 0.29PVτ0rest (t)) Q ⎝ w ⎠ (9) Fig. 10. Relationships between measured-to-predicted initial pressure (following Omran and Khayat model accounting the effect of steel bars).

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formworks registered relatively higher pressure than the edge sensors, which was attributed to reduced density of steel bars in the vicinity of the middle sensor. This makes the use of ρsv index in given predictive model non-conservative as it may under-estimate the developed SCC pressure, particularly in locations where vertical steel bars are spaced farther apart from each other. Hence, the Eff(ρsv) was proposed in Eq. (7) to improve conservativeness of vertical steel index. The original Ovarlez and Roussel [20] and Omran and Khayat [22] models over-estimate the lateral pressure developed by RCA-modified SCC, especially when casting takes place in formworks containing vertical steel. The inclusion of suitable factors accounting for RCA materials (Qw) and presence of vertical steel (Eff(ρsv)) proved efficient to match the measured-to-predicted σmax/hyd values. The resulting Eqs. (8) and (10) are valid for minimum transverse steel, i.e. when placed at maximum distance of 450 mm acceptable by ACI 318 building code.

Fig. 11. Effect of transverse steel bars on initial pressure for selected SCC mixtures.

the predicted values decreased by 22.3% (i.e., Ψ-factor) than actual σmax/hyd.

1 σmax/hyd,% = ⎜⎛ 0.05 ⎟⎞ (95.9 − 3.84H + 0.71R + 4.1Dmin ⎝ Qw ⎠ 106.3 ⎞ ⎞ − 0.29PVτ0rest (t)) ⎛1−Eff(ρsv ) ⎛4.63+ cover ⎠ ⎠ ⎝ ⎝

Acknowledgments This research project is funded by the Lebanese University, Lebanon, within the Research Support Program. References

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The discrepancy between measured-to-predicted values in Fig. 10 could be attributed to different formwork surfaces (plywood sheathing vs. plexiglass) that affect interfacial friction with placed concrete, and mostly importantly, different transverse steel percentages used in experimental set-ups (1 tie per 200 vs. 450 mm). To quantify the effect of transverse steel, three selected SCC mixtures were cast in the instrumented reinforced column, but the spacing between rectangular ties reduced from 450 to 150 mm. As can be seen in Fig. 11, the initial σmax/ hyd dropped significantly with increased density of transverse steel, regardless of mixture composition. For example, the pressure of 0.3875%RCA-70Sv mixture decreased from 76% to 65.6% when the distance between ties reduced from 450 to 150 mm, respectively, thus making the measured σmax/hyd values quite close to those predicted using Eq. (11). In other words, as suggested by Perrot et al. [19], this implies that concrete pressure is affected by both vertical and transverse reinforcing bars. The combined effect of both steel directions on SCC pressure will be presented in follow-up paper.

[1] J. de Brito, J. Ferreira, J. Pacheco, D. Soares, M. Guerreiro, Structural, material, mechanical and durability properties and behavior of recycled aggregates concrete, J. Build. Eng. 6 (2016) 1–16. [2] G.B. Fonteboa, M.F. Abella, Shear strength of recycled concrete beams, Constr. Build. Mater. 21 (4) (2007) 887–893. [3] M. Etxeberria, A.R. Mari, A.E. Vazquez, Recycled aggregate concrete as structural material, Mater. Struct. 40 (2007) 529–541. [4] A.B. Ajdukiewicz, A.T. Kliszczewicz, Comparative tests of beams and columns made of recycled aggregate concrete and natural aggregate concrete, J. Adv. Concr. Technol. 5 (2007) 259–273. [5] L.A. Pereira-de-Oliveira, M.C.S. Nepomuceno, J.P. Castro-Gomes, M.F.C. Vila, Permeability properties of self-compacting concrete with coarse recycled aggregates, Constr. Build. Mater. 51 (2014) 113–120. [6] J.J. Assaad, Y. Daou, Behavior of structural polymer-modified concrete containing recycled aggregates, J. Adhes. Sci. Technol. 31 (8) (2017) 874–896, http://dx.doi. org/10.1080/01694243.2016.1235750. [7] E. Güneyisi, M. Gesŏglu, Z. Algin, H. Yazici, Rheological and fresh properties of selfcompacting concretes containing coarse and fine recycled concrete aggregates, Constr. Build. Mater. 113 (2016) 622–630. [8] I. González-Taboada, B. González-Fonteboa, J. Eiras-López, G. Rojo-López, Tools for the study of self-compacting recycled concrete fresh behaviour: workability and rheology, J. Clean. Prod. 156 (2017) 1–18. [9] K.H. Yang, H.S. Chung, A.F. Ashour, Influence of type and replacement level of recycled aggregates on concrete properties, Acids Mater. J. 105 (2008) 289–296. [10] Z.J. Grdic, G.A. Curcic, I.M. Despotovic, N.S. Ristic, Properties of self-compacting concrete prepared with coarse recycled concrete aggregate, Constr. Build. Mater. 24 (2010) 1129–1133. [11] F. Faleschini, C. Jiménez, M. Barra, D. Aponte, E. Vazquez, C. Pellegrino, Rheology of fresh concretes with recycled aggregates, Constr. Build. Mater. 73 (2014) 407–416. [12] J.J. Assaad, Influence of recycled aggregates on dynamic/static stability of selfconsolidating concrete, J. Sustain. Cem.-Based Mater. (2017), http://dx.doi.org/10. 1080/21650373.2017.1280427. [13] C.S. Poon, C.S. Kou, L. Lam, Influence of recycled aggregate on slump and bleeding of fresh concrete, Mater. Struct. 40 (2007) 981–988. [14] O. Kebaïli, M. Mouret, N. Arabi, F. Cassagnabere, Adverse effect of the mass substitution of natural aggregates by air-dried recycled concrete aggregates on the selfcompacting ability of concrete: evidence and analysis through an example, J. Clean. Prod. 87 (2015) 752–761. [15] J.J. Assaad, K.H. Khayat, Effect of coarse aggregate characteristics on lateral pressure exerted by self-consolidating concrete, Acids Mater. J. 102 (2005) 145–153. [16] J.J. Assaad, K.H. Khayat, Kinetics of formwork pressure drop of self-consolidating concrete containing various types and contents of binder, Cem. Concr. Res. 35 (2005) 1522–1530. [17] S. Teixeira, A. Santilli, I. Puente, Analysis of casting rate for the validation of models developed to predict the maximum lateral pressure exerted by self-compacting concrete on vertical formwork, J. Build. Eng. 6 (2016) 215–224. [18] J.J. Assaad, J. Harb, Formwork pressure of self-consolidating concrete containing recycled coarse aggregates, Acids Mater. J. 114 (3) (2017) 491–500. [19] A. Perrot, A. Amziane, G. Ovarlez, N. Roussel, SCC formwork pressure: influence of steel rebars, Cem. Concr. Res. 39 (2009) 524–528. [20] G. Ovarlez, N. Roussel, A physical model for the prediction of lateral stress exerted by self-compacting concrete on formwork, Mater. Struct. 39 (2) (2006) 269–279. [21] A. Santilli, S. Teixeira, I. Puente, Influence of temperature and concrete reinforcement on vertical formwork design, Constr. Build. Mater. 88 (2015) 188–195.

4. Summary and conclusions The main objective of this paper is to assess the effect of RCA additions and presence of vertical steel bars on formwork pressure developed by freshly placed SCC; the transverse steel was kept to the minimum level accepted by ACI 318 building code (i.e., center-tocenter spacing of 450 mm). Based on foregoing, test results have shown that mixtures incorporating RCA exhibited reduced initial pressure and accelerated rates of pressure drop over time. This was attributed to greater RCA surface roughness that increases internal friction, associated with higher water absorption that improves build-up of SCC skeleton at rest, thus leading to reduced transformation of vertical stresses into lateral ones. The increase in vertical steel density led to remarkably reduced initial pressure, given that the reinforcement cage confines the inner concrete volume and carries part of its load. The initial pressure dropped to 76% of hydrostatic value in case of 0.38-75%RCA mixture cast in the most congested formwork containing 12 Ø20 bars (ρsv of 4.71%). The rates of pressure drop over time were not altered because of vertical steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. In non-reinforced formwork, the magnitude of lateral pressure at given concrete head is not dependent by the location of strain sensor (i.e., middle vs. edge). In contrast, the middle sensor in reinforced 167

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[28] EPG, The European Guidelines for Self-Compacting Concrete Specification Production and Use, May 2005. 〈http://www.efnarc.org/pdf/SCCGuidelinesMay 2005.pdf〉. [29] ASTM C232-14, Standard Test Method for Bleeding of Concrete, West Conshohocken, PA. [30] J.J. Assaad, J. Harb, Y. Maalouf, Effect of vane configuration on yield stress measurements of cement pastes, J. Non-Newton. Fluid Mech. 230 (2016) 31–42. [31] N.J. Gardner, Pressure of concrete on formwork—a review, J. Proc. Civ. Eng. 82 (1) (1985) 744–753. [32] K.H. Khayat, J.J. Assaad, Measurement systems for determining formwork pressure of highly flowable concrete, Mater. Struct. 41 (2008) 37–46. [33] ACI 318, Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, USA, 2016.

[22] A.F. Omran, K.H. Khayat, Progress to understand influence of reinforcement density on SCC lateral pressure, Mater. Struct. 50 (2017) 1–15. [23] A. Abbas, G. Fathifazl, O.B. Isgor, A. Razaqpur, B. Fournier, S. Foo, Proposed method for determining the residual mortar content of recycled concrete aggregates, J. ASTM Int. 5 (2008) 1–10. [24] BS 812–110, Testing Aggregates, British Standards Institute, London, 1990, p. 18. [25] J.J. Assaad, K.H. Khayat, Effect of viscosity-enhancing admixtures on formwork pressure and thixotropy of self-consolidating concrete, Acids Mater. J. 103 (2006) 280–287. [26] J.J. Assaad, Y. Daou, Cementitious grouts with adapted rheological properties for injection by vacuum techniques, Cem. Concr. Res. 59 (2014) 43–54. [27] K.H. Khayat, J.J. Assaad, J. Daczko, Comparison of field-oriented test methods to assess dynamic stability of self-consolidating concrete, Acids Mater. J. 101 (2004) 168–176.

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