Effect of structural buildup at rest of self-consolidating concrete on mechanical and transport properties of multilayer casting

Construction and Building Materials 196 (2019) 626–636

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Effect of structural buildup at rest of self-consolidating concrete on mechanical and transport properties of multilayer casting Wael A. Megid a,b, Kamal H. Khayat c,⇑ a

Université de Sherbrooke, 2500 Boul. de l’Université, Sherbrooke, QC J1K 2R1, Canada Menoufia University, Shebeenelkom, MNF, Egypt c Missouri S&T, 500 W. 16th St., Rolla, MO 65409, United States b

h i g h l i g h t s
Delays in multilayer casting of thixotropic SCC can decrease the flexural strength.
Delays in placing successive lifts of thixotropic SCC can increase the permeability.
SCC with low level of thixotropy can improve the performance of multilayer casting.
The increase in static yield stress can decrease the roughness of bonded interphase.

a r t i c l e

i n f o

Article history: Received 2 August 2018 Received in revised form 9 November 2018 Accepted 14 November 2018

Keywords: Flexural strength Lift lines Multilayer casting Permeability Rheology Structural buildup at rest Thixotropy

a b s t r a c t Multilayer casting of self-consolidating concrete (SCC) can be critical in situations involving delays between the placements of successive lifts. In the absence of mechanical consolidation, the increase in structural buildup at rest (SBR) of existing SCC lift prior to the placement of a successive lift can lead to lift lines. Furthermore, the bond strength deteriorates across the bonded interphases. In this study, eight SCC mixtures with different levels of SBR were considered. The thixotropy of concrete was determined using standard workability test methods and inclined plane test. Flexural strength and water permeability were determined using composite specimens cast in two SCC lifts. The second lift was placed after given periods of rest up to 60 min. Composite specimens developed residual flexural strengths and water permeability resistance with minimum values of 56% and 2%, respectively, compared to reference samples. To secure 90% residual flexural strength and impermeability, delays up to 25 and 10 min can be tolerated, respectively, depending on SBR level of the existing SCC layer. Statistical models were established to predict the residual flexural strength and water permeability resistance between successive lifts. SCC with low level of SBR, having static yield stress up to 250 Pa after 15 min of rest and rate of SBR up to 2.5 Pa/min evaluated using the inclined plane test, can secure relatively high level of bond strength and impermeability providing that the delay time before casting successive lifts is limited to 20 and 5 min, respectively. The interphases bond of successive SCC lifts was found to be affected by intermixing level of the layers. Both can be increased by the reduction in SBR of existing lift when concrete is cast with a given free fall height. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Casting of multiple concrete lifts is required for the construction of large concrete elements, such as vast foundation and long and/or deep wall elements. As mechanical consolidation is not needed for casting SCC, distinctly weak surfaces such as lift/fold lines can be generated between the successive lifts that are still in the plastic ⇑ Corresponding author. E-mail address: [email protected] (K.H. Khayat). https://doi.org/10.1016/j.conbuildmat.2018.11.112 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

state. This can lower the mechanical properties of the final composite product compared to elements cast in a single lift or where proper mechanical consolidation is applied to the existing lift prior to casting of a successive lift. Deterioration in performance of SCC subjected to multilayer casting can be a result when the structural buildup at rest (SBR) of the SCC is high or when the concrete is subjected to some delays before casting a successive lift, especially in hot environment that can accelerate cement hydration. Many studies [1–5] have recommended reducing the casting rate of SCC in deep elements or enhancing concrete thixotropy to

W.A. Megid, K.H. Khayat / Construction and Building Materials 196 (2019) 626–636

reduce formwork pressure. Thixotropy can be defined as the increase of viscosity in a state of rest and decrease of viscosity when the material is submitted to a constant shearing stress [6]. Thixotropy is reversible and involves structural breakdown phase when subjected to a given shear rate and the SBR when the shear rate is removed [7–12]. However, low casting rates and use of viscosity- or thixotropy-enhancing admixtures can increase the SBR (or static yield stress) of the existing concrete, thus leading to multilayer casting of different lifts during SCC placement. Lift/fold line was observed between the castings of successive lifts of SCC in a large structural wall [13]. The wall was cast in two lifts using a highly thixotropic SCC to reduce lateral pressure. The casting of upper lift was delayed by 25 min. The upper concrete layer spread over the existing one with limited degree of intermixing. Fresh SCC layer exhibits structural buildup after a period of rest that leads to an increase in the static yield stress. SBR can significantly affect the contact characteristics between successive SCC layers. SBR can be evaluated using rheometric or empirical test methods, including the portable vane and inclined plane test methods as well as standard workability tests [13–19]. During SCC casting, a concrete layer has a short time to rest and flocculate before the second layer is cast on top of it [20]. If the fine particles can flocculate, the SBR increases beyond a critical value, where the two layers cannot intermix at all. This leads to the formation of lift/fold line which can be observed on the surface of the final product. In addition, loss of bond strength greater than 40% has been reported [21]. Similar findings were reported when the delay time between the casting of two successive layers of highly thixotropic SCC exceeds 60 min [14,15]. Megid and Khayat [13] pointed out that depending on the degree of SBR of an existing lift of SCC, bond strength can range from 80% to 95% when determined under slant shear stress and from 55% to 90% for direct shear stress when the delay time between the castings of two layers is 30 min. Moreover, employing SCC with relatively low level of SBR can secure high residual interlayer bond. Such concrete can have an initial slump flow greater or equal to 630 mm, a rate of slump flow drop with time at rest limited to 1.20 mm/min, an initial T50 less or equal to 1.6 s, and a maximum rate of increase in T50 with time at rest of 0.045 s/min. It was reported that an increase in the dosage of retarder decreased the growth of static yield stress with rest time and therefore, improved the mechanical strength of distinct-layer casting SCC [22]. Bond between two successive layers depends on the interphase adhesion, friction, aggregate interlock, and time-dependent factors [23]. It was reported that bond strength is sensitive to interlocking. A 12.7-mm interlocking resulted in increasing bond strength by 17% and 26% for uniaxial compression and splitting tensile strength, respectively [24]. Friction and aggregate interlocking are often associated with aggregate size, shape, and texture [25]. The contribution of aggregate interlock to shear capacity of SCC was found to be affected by the volume of coarse aggregate [26]. Moreover, bond strength in multilayer casting determined under direct shear stress can increase due to the increase in aggregate interlock across a bonded interphase [13]. In addition to these factors, the bond strength results are highly dependent on the specimen size and geometry and the state of stress applied at bonded interphases that are quite dependent on the selected test method. Mechanical properties between two successive concrete layers can be investigated using a variety of test methods that can apply direct or indirect shear, tension, or flexural stress at bonded interphases [13,22,24,25,27–34]. These tests can also be used to evaluate bond strength across lift/fold lines in multilayer casting. The authors [13] concluded that bond strength determined using the direct shear stress is more adversely affected by multilayer casting than that evaluated using the slant shear stress.

627

Improving the service life and durability of concrete structures is essential due to the increased emphasis on life-cycle cost analysis for building projects. Durability is most influenced by the transport properties of concrete, such as diffusivity, permeability, and sorptivity. Relevant transport properties include ionic and gas diffusion, gas and liquid permeability, and liquid sorption. It is necessary to consider the liquid barrier properties of multilayer concrete because the true single layer case is, in fact, relatively unusual [35]. In practice, casting concrete against formwork results in the formation of surface layer of fine, cement-rich material, which has different properties compared to that of the bulk concrete. In such case, the concrete should be considered to consist of two distinct layers of materials. A more complex case arises when a concrete is cast in several different layers. The drop in quality of the interphases between successive lifts can also lead to a sharp increase in local permeability of the concrete. There is a critical need to evaluate the mechanical and transport properties developed across lift/fold lines formed in multilayer casting which could result in evident defects on the concrete surface and deterioration in the structure behavior. In order to eliminate such deficiencies, designing SCC mixtures with adapted rheological characteristics including thixotropy and minimizing the delay between concrete lifts are essential. This investigation offers guidelines and recommendations for multilayer casting of SCC to reduce the impact of thixotropy and delays between successive layers on the performance and durability of the structure. These guidelines should be helpful for quality control and construction engineers.

2. Experimental program 2.1. Materials Eight SCC mixtures were designed using various materials to secure different levels of SBR. The mixture design and fresh properties of the investigated mixtures are reported in Table 1. The initial slump flow values extended from 630 to 700 mm. Five mixtures were prepared with a ternary cement (CSA Type GUbS/ SF) containing 22% granulated blast-furnace slag, 6% silica fume, and 72% general use cement (Type GU), by mass. SCC2 mixture was prepared using a quaternary binder system where a manufactured calcium carbonate with a specific gravity of 2.7 was also employed. Three mixtures were prepared with Type GU cement that met the chemical and physical requirements of CSA A3000 and ASTM C 150. The coarse aggregates were continuously graded crushed limestone with nominal maximum size of 10 and 14 mm. The fine aggregate was a river-bed siliceous sand. The particle-size distributions of the coarse aggregates and sand met the CSA A23.1 recommendations. The fineness modulus of combined coarse aggregate and sand was 6.4 and 2.5, respectively. They had bulk specific gravities of 2.71 and 2.67, respectively, and water absorption rates of 0.38% and 0.6%, respectively. Three types of high-range water reducing admixtures HRWRAs (PCP1, PCP2, and PCP3), having specific gravity of 1.05, were used. Polycarboxylate-based HRWRAs complying with CSA3-A266.6M85 specifications were used. A set retarder (SR), formed by lignosulfonic acids and having a specific gravity of 1.22, was employed in five mixtures. Two types of polysaccharide-based viscosity modifying admixtures VMAs (VMA1 and VMA2) in liquid form were used. They had specific gravities of 1.0 and 1.2, respectively. An air-entraining agent (AEA) was added in SCC3 mixture. Three batches were prepared for each of the investigated mixtures for the evaluation of SBR and the fabrication of test samples

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Table 1 Mixture proportioning and fresh properties of investigated concretes. Mixture

SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

w/cm w/p Ternary cement, kg/m3 GU cement, kg/m3 Limestone filler, kg/m3 S/A, by volume Sand (0–5 mm), kg/m3 Coarse aggregate, kg/m3

0.42 – 475 – – 0.5 783 810 – 3.25 – – 420 – 190 – 700 2300 3.9

– 0.37 415 – 183 0.5 766 157 628 – 5.0 – – – – – 640 2350 1.6

0.42 – – 425 – 0.5 816 – 835 – – 5.82 – 220 – 140 630 2330 4.5

0.39 – – 475 – 0.5 803 830 – 3.35 – – – – 190 – 650 2360 3.0

0.37 – – 475 – 0.5 844 173 692 8.0 – – 750 – – – 700 2400 1.4

0.39 – 475 – – 0.5 803 830 – 4.0 – – – – 190 – 670 2350 1.9

0.37 – 475 – – 0.5 803 830 – 4.96 – – – – 190

0.34 – 475 – – 0.5 803 830 – 6.7 – – 580 – 100 – 630 2340 4.1

HRWRA, l/m

3

VMA, ml/100 kg of cement

5–10 mm 5–14 mm PCP1 PCP2 PCP3 VMA1 VMA2

SR, ml/100 kg of cement AEA, ml/m3 Slump flow, mm Unit weight, kg/m3 Air content, %

required to determine the residual flexural strength and water permeability. In total, 24 batches were prepared. 2.2. Evaluation of structural buildup at rest 2.2.1. Standard workability tests The initial slump flow (ASTM C 1611) and J-ring flow (ASTM C 1621) values were determined 8 min after the initial contact of cement and water. After the initial sampling, the concrete was kept inside the mixer without any agitating, and the mixer was covered to minimize any water evaporation. After 17 min of rest (25 min of age), the concrete was sampled to determine the second set of slump flow and J-ring flow values. The testing molds were filled in a single layer without any consolidation. This procedure was repeated after 34 and 52 min of rest to determine the third and fourth sets of workability values, respectively. 2.2.2. Inclined plane test Four inclined plane (IP) test apparatuses were used as shown in Fig. 1. The IP test apparatuses consist of two PVC surfaces, where the upper surface is slowly lifted allowing the surface to pivot,

670 2330 3.8

resulting in flow of a concrete sample that was placed onto the upper surface [19]. The concrete flow is initiated when the static yield stress of the material is exceeded [18,36]. A sheet of fine sand paper having a grit number of 600 is glued on the top pivoting surfaces of the IP test apparatuses. Four transparent polymethyl methacrylate cylinders measuring 60 mm in diameter and 120 mm in height with opened ends were placed onto the upper plates and filled with concrete. The open-ended cylinders were gently lifted, allowing the concrete to spread. The height of the concrete sample was then measured, and the sample was covered and maintained undisturbed. After 15 min of rest, the upper surface of the first IP test apparatus was gently lifted until the concrete started to shear and slide; the inclination angle corresponding to the initiation of the sliding was recorded. This protocol was repeated for the other three IP test apparatuses after 30, 45, and 60 min of rest. The static yield stress at rest evaluated using the IP test (ss-IP, in Pa) was determined using Eq. (1) [18,19].

ssIP ¼ qg h sinh

ð1Þ 3

where q is the density of the mixture (g/cm ), g is the gravitational acceleration (9.81 m/s2), h is the height of the tested sample at the conclusion of lifting the open-ended cylinder (mm), and h is the inclination angle (degree) corresponding to sliding initiation of the concrete sample. 2.3. Assessment of flexural strength in multilayer casting

Fig. 1. Inclined plane test [19].

The flexural strength test (ASTM C78/C78M) was employed to investigate the effect of SBR and the delay time (DT) between the castings of two successive layers of SCC on mechanical properties of multilayer casting. The test involved the casting of composite beam prisms having two identical halves bonded along a vertical phase located at the midpoint of the prism length, as shown in Fig. 2a and b. PVC material was used to build up all the mold specimens. In total, 15 prisms measuring 100 mm in width and height and 400 mm in length were prepared for each mixture without applying a release agent. A small notch at the midpoint of the prism length was created during casting to prompt the failure to take place at the interphase between the two sample halves. The molds were vertically arranged, and the beam prisms were horizontally tested, as shown in Fig. 2c. The DT between the castings of the first and second lifts was set to 15, 30, 45, and 60 min to evaluate the loss of bond strength for each of the eight SCC mixtures. The second (top) layer of SCC consisted of concrete that

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629

Fig. 2. Flexural strength: (a) casting, (b) dimensions, (c) beam prism under testing, and (d) failure pattern.

was cast without any rest. The SCC in the second lift was allowed to drop a height of 200 mm above the upper surface of the existing concrete. Three samples per testing condition were cast to determine the average flexural strength value. Another three samples were cast in a single lift to secure monolithic casting conditions and were considered as control samples. The flexural test consisted of applying four loading points to the beam prism after seven days of moist curing. The load was continuously applied without shock until failure using hydraulically operated machine with a loading rate within the range of 0.15– 0.35 MPa/s. The designated loading rate was maintained during the testing cycle. The dimensions of the beam prism and failure load were recorded. The type of failure and the appearance of the concrete were noted, as shown in Fig. 2d. The flexural strength of the specimen (d) was determined using Eq. (2). 2

d ¼ P L=ðb d Þ

ð2Þ

where d is the flexural strength (MPa), P is the failure load (N), L is the loading span (m), and finally b and d are the width (m) and height (m) of the beam prism, respectively. 2.4. Assessment of water permeability in multilayer casting The CRD-C48-92 [37] water permeability test was employed to investigate the effect of SBR and the DT between the castings of two successive layers of SCC on transport properties of a composite sample. The test involved the casting of composite cylinders having two layers bonded along a horizontal plane at mid-height, as

shown in Fig. 3. In total, eight cylinders measuring 150 mm in diameter and 300 mm in height were prepared for each mixture. The sample had a central channel measuring 25 mm in diameter and 300 mm in height to collect the convergent flow of water during the testing. The channel was created using a tube. A central hole was machined in the base and the cover of the mold to keep the tube vertical during concrete casting. The tube was gently pulled out from the specimen shortly after setting. The DT between the castings the first and the second layers was set to 20, 40, and 60 min. The second layer of SCC consisted of concrete that was properly mixed without any rest. The SCC in the second lift was allowed to drop from a height of 150 mm above the upper surface of the existing concrete. Two specimens per testing condition were prepared to determine the average water permeability coefficient. Two other specimens were cast in a single lift and were considered as control samples. All samples were subjected to 28 days of moist curing before water permeability testing. The water permeability test consisted of applying water pressure to the outer surface of the cylindrical specimens forcing a convergent flow migration through the concrete mass towards the inner central channel. As illustrated in Fig. 3, the water permeability test apparatus was composed of a steel cylindrical vessel with a retainer ring at the bottom and a flange at the top. The apparatus had a water reservoir made with a glass pipe connected with suitable fittings, valves, and regulators to permit the admission of water for the filling of the pressure vessel and the subsequent application of air pressure. The top and bottom of the specimen were sandblasted to remove the cement paste at the extremities of the test samples. The specimen was lowered into the pressure

Flange

Steel cylindrical vessel

Retainer ring Fig. 3. Water permeability: casting, dimensions, and testing apparatus.

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vessel and firmly seated using metal rods serving as guides and a lever between the specimen and container to assist in centering the specimen, as shown in Fig. 3. The testing pressure values were set to 2.1, 3.4, and 5.5 MPa, and the variations in water reservoir level with the time were recorded. The concrete permeability coefficient (K) was determined using Darcy’s law for fluid in a permeable medium, as expressed in Eq. (3), below:

K ¼ M L=ðA hÞ

ð3Þ 3

where K (cm/h) is the permeability coefficient, M (m /h) is the flow rate determined by dividing the water volume that migrated through the concrete by the elapsed time, L (cm) is the length of the flow path and was determined using Eq. (4), Dcy is the diameter of the cylindrical specimen, Dch is the diameter of the central channel, A (cm2) is the area of the permeable medium perpendicular to the flow that was determined using Eq. (5), and h (m) is the hydraulic head and was determined by dividing the water pressure by the water density.

L ¼ ðDcy  Dch Þ=2

ð4Þ

A ¼ 1:57 HðDcy þ Dch Þ

ð5Þ

3. Experimental results and discussion 3.1. Structural buildup at rest The values of the slump flow and J-ring flow determined after rest periods of 0, 17, 34, and 52 min are summarized in Table 2. The reported properties are mean values of three measurements carried out on three separate concrete batches required for the casting of the flexural strength and water permeability test specimens. The variations between the slump flow and J-ring flow values were limited to 50 mm, which correspond to the tolerance value stipulated by EFNARC [38]. The J-ring flow values of the SCC1, SCC4, and SCC6 mixtures were higher than the corresponding slump flow values. This observation may due to the J-ring flow test was conducted after the slump flow test. Sampling the concrete from the mixer to conduct the slump flow test may result

in generating some disturbing energy, which in turn, can increase the J-ring flow values. As expected, the values of slump flow and J-ring flow decreased with the increase in rest time referring to the loss of fluidity and self-consolidation ability at rest resulted from SBR. The results of slump flow and J-ring flow tests were used to determine two indices to evaluate the degree of SBR of the concrete. The filling ability index (FAI) was calculated by multiplying the initial slump flow value (S.flow(0)) by the average rate of loss in slump flow at rest time (RS.flow) between 0 and 52 min [19]. The normalized FAI values reported in Table 2 varied from 341 mmmm/min for the SCC1 mixture that has the lowest thixotropy level to 2111 mmmm/min for the SCC8 mixture with highest thixotropy level. The passing ability index (PAI) was calculated by multiplying the initial J-ring flow (J-ring(0)) by the average rate of drop in J-ring flow with rest time (RJ-ring) [19]. The PAI values, reported in Table 2, varied from 415 mmmm/min for the SCC1 mixture to 1908 mmmm/min for the SCC8 mixture. The static yield stress (ss) values determined from 15 to 60 min of rest using the IP test method are summarized in Table 2. The reported properties are mean values of three measurements carried out on the three separate batches required for casting the flexural strength and water permeability samples. The values of ss increased with the increase in rest time. The values of ss were used to evaluate the degree of SBR that correspond to the thixotropy level of the concrete. The index of SBR (Athix) was calculated as the static yield stress value after 15 min of rest (ss(15)) multiplied by the rate of SBR (RSBR). The RSBR is the rate of the increase in ss over 60 min of rest, expressed in Pa/ min. The values of Athix of the investigated mixtures determined using the IP test method are presented in Table 2.

3.2. Mechanical and transport properties across boundary of successive layers 3.2.1. Residual bond strength The flexural strength results are reported in Table 3. The relative flexural strength values were calculated by dividing the individual results obtained after various periods of rest by the corresponding

Table 2 Variations of slump flow, J-ring flow, and static yield stress with rest time. Mixture S.flow(t), mm

SCC1 t* = 0 min t = 17 min t = 34 min t = 52 min

RS.flow, mm/min FAI = S.flow(0)  RS.flow, mmmm/ min J-ring(t), mm

t = 0 min t = 17 min t = 34 min t = 52 min

RJ-ring, mm/min PAI = J-ring(0)  RJ-ring, mmmm/ min

ss-IP(t), Pa

t = 15 min t = 30 min t = 45 min t = 60 min

RSBR, Pa/min Athix-IP = ss-IP(t)  RSBR, PaPa/min *

t is rest time

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

Slump flow values 695 640 690 630 680 615 670 605 0.49 0.69 341 442

SCC2

630 600 570 540 1.73 1090

650 620 590 560 1.73 1125

705 675 645 610 1.82 1283

665 625 590 550 2.2 1463

665 625 585 545 2.31 1536

630 570 515 455 3.35 2111

J-ring flow values 715 630 705 610 695 590 685 570 0.58 1.16 415 731

590 565 545 520 1.33 785

640 620 595 570 1.36 870

685 660 635 610 1.45 993

660 635 605 580 1.56 1030

645 610 575 540 2.02 1303

580 525 465 410 3.29 1908

IP test values 159 181 204 226 1.49 237

239 277 314 351 2.49 595

250 327 405 482 5.16 1290

287 408 530 652 8.11 2328

325 445 565 685 8.00 2600

519 621 723 825 6.80 3529

577 698 819 940 8.06 4656

211 236 262 288 1.71 361

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W.A. Megid, K.H. Khayat / Construction and Building Materials 196 (2019) 626–636 Table 3 Results of flexural strength and water permeability coefficient. Mixture **

SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

Flexural strength, d, MPa

t = 0 min t = 15 min t = 30 min t = 45 min t = 60 min

5.10 4.85 4.62 4.30 4.10

4.50 4.30 4.05 3.71 3.44

4.00 3.80 3.56 3.29 3.00

4.80 4.54 4.18 3.90 3.55

5.10 4.70 4.49 3.97 3.70

4.60 4.30 3.82 3.60 3.14

6.00 5.28 4.70 4.20 3.65

6.60 5.74 5.02 4.38 3.70

Water permeability coefficient, K  108, cm/h

t = 0 min** t = 20 min t = 40 min t = 60 min

0.60 0.70 0.84 1.00

0.93 1.24 1.50 1.86

0.50 0.70 1.00 1.67

1.10 1.92 2.80 6.20

0.40 0.75 1.50 3.33

0.60 1.36 3.33 8.57

0.40 1.05 2.85 8.90

0.50 1.72 6.10 21.90

t is rest time. Control sample

values obtained for the control samples cast monolithically in a single layer. These values were used to evaluate the residual bond strength, which can be developed between successive layers under the impact of flexural stress. The variations of residual bond strength for the flexure test (RB) of the investigated SCC mixtures cast with a DT between the casting of successive lifts are plotted in Fig. 4. The results show that the values of RB decrease with the increase in DT. The SCC1 mixture designed to develop the lowest level of SBR exhibited a RB value of 95% after 15 min of rest. In the case of SCC8 mixture that had the highest level of SBR, the RB value dropped drastically to 56% after 60 min of rest. The effect of increasing the degree of SBR of the lower SCC lift prior to the casting of the upper lift on the RB values is illustrated in Fig. 5. The degree of SBR, corresponding to the thixotropy level of the concrete, is expressed using the index of SBR (Athix). For a relatively short DT of 15 min between the castings of the two successive lifts, the RB values for the SCC1 and SCC8 mixtures were 95% and 87%, respectively. These two mixtures exhibited the lowest and highest Athix values, respectively. For a DT of 60 min, the values of RB for the SCC1 and SCC8 mixtures were 80% and 56%, respectively. 3.2.2. Residual water permeability resistance The results of water permeability coefficient are given in Table 3. The relative values of water permeability coefficient were determined by dividing the values obtained for the control samples cast monolithically in a single layer by the corresponding individual results obtained after various periods of rest. These relative values were used to evaluate the residual water permeability resistance (RP) that can be developed between successive layers. The water permeability resistance of concrete is defined as its ability to withstand the water ingress under a pressure gradient. The high value

DT = 0 min R² = 0.89

90

80

y = -0.002x + 96.4

R² = 0.91

15

R² = 0.94

30

y = -0.003x + 91.6

45

y = -0.004x + 85.4

60

y = -0.005x + 80.1

R² = 0.93 70

60

1

2

50 0

1000

2000

3000

4000

5000

Structural buildup at rest, Pa·Pa/min Fig. 5. Variations in residual bond strength with delay time and structural buildup at rest determined using IP test.

of water permeability coefficient indicates the concrete has low resistance to water permeability. The variations of RP, expressed in percent, of the investigated SCC mixtures cast with different DT between the castings of successive lifts are plotted in Fig. 6. The results show that the values of RP decreased with the increase in DT. The SCC1 mixture, with the lowest level of SBR, exhibited a RP value of 86% after 20 min of rest. In the case of SCC8 mixture that had the highest level of SBR, the RP value dropped drastically to 2% after 60 min of rest. The effect of increasing the thixotropy level of SCC, that causes an increase in the degree of SBR of the lower concrete lift at the time of casting the upper lift, on the RP values is illustrated in Fig. 7. The thixotropy level is expressed using the index of SBR (Athix). The loss of RP was determined for each of the eight SCC mixtures cast after three DT values. Samples cast monolithically were

100

Residual permeability resistance, %

100

Residual bomd strength, %

100

Residual bond strength, %

* **

*

90

80

70 1

2

60 SCC1

SCC2

SCC3

SCC4

SCC5

SCC6

SCC7

SCC8

SCC1 SCC5

90

SCC2 SCC6

SCC3 SCC7

SCC4 SCC8

80 70 60 50 40 30 2 1

20 10 0

50 0

10

20

30

40

50

60

Delay time, min Fig. 4. Variations in residual bond strength with delay time of various SCC mixtures.

0

10

20

30

40

50

60

Delay time, min Fig. 6. Variations in residual water permeability resistance with delay time of various SCC mixtures.

Residual permeability resistance, %

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DT = 0 min 2

80

1

60 y = -17.3ln(x) + 180.5 R² = 0.97 40

20 y = -20.9ln(x) + 185.8 R² = 0.99

20

40

y = -19.3ln(x) + 160.5 R² = 0.96

0 0

1000

2000

60 3000

4000

5000

Structural buildup at rest, Pa·Pa/min Fig. 7. Variations in residual water permeability resistance with delay time and structural buildup at rest determined using IP test.

considered as the control samples. For a relatively short DT of 20 min between the castings of the two successive lifts, the RP values for the SCC1 and SCC8 were 86% and 29%, respectively. On the other hand, for a DT of 40 and 60 min, the RP values for the SCC1 mixture were 71% and 60% and were 8% and 2%, respectively, for the SCC8 mixture. 3.3. Effect of structural buildup at rest on performance of multilayer SCC Given the performance of multilayer casting of SCC in terms of mechanical and transport properties at the interphase between adjacent lifts, the degree of SBR of the previously placed lift, and DT between successive lifts, SCC mixtures can be classified into three categories. As indicate in Table 4, SCC mixtures belonging to Category I are expected to yield relatively low RB and RP in multicasting situation, such as the case of SCC8 mixture. The Athix, FAI, and PAI values of SCC mixtures belonging to Category I can be equal or greater to 4000 PaPa/min, 1800 mmmm/ min, and 1600 mmmm/min, respectively. SCC belonging to Category II, such as the SCC4, SCC5, SCC6 and SCC7 mixtures, have Athix values ranging between 1000 and 4000 PaPa/min, FAI values ranging between 800 and 1800 mmmm/min, and PAI values between 600 and 1600 mmmm/min. These mixtures can lead to relatively moderate RB and RP in multilayer casting. On the other hand, SCC mixtures belonging to Category III, such as the SCC1, SCC2, and SCC3 mixtures, are expected to secure relatively high RB and RP with Athix, FAI, and PAI values less or equal to 1000 PaPa/min, 800 mmmm/min, and 600 mmmm/min, respectively. Category III (low thixotropy) concrete is therefore recommended for casting elements of large dimensions or those where a certain delay prior to casting successive layers is expected. Such SCC can have a ss up to 250 Pa determined using the IP test after 15 min of rest and a maximum rate of SBR of 2.5 Pa/min. In addition, the concrete belonging to Category III can exhibit relatively

high filling ability with an initial slump flow of at least 630 mm and a maximum rate of loss in slump flow at rest of 1.2 mm/min, as well as a relatively high passing ability with a minimum initial J-ring flow of 630 mm and a rate of drop in J-ring flow values at rest of 0.8 mm/min or less. When the DT between casting successive layers of SCC belonging to Category III is 15 min, the RB determined under flexural stress can be expected to be greater than 95%, as indicated in Table 4. This value can decrease to 88% when the DT increases from 15 to 30 min. Therefore, the use of concrete with workability characteristics belonging to Category III can develop a RB greater than 90% when the DT between successive layers does not exceed 25 min. In addition, when the DT between casting successive layers of SCC belonging to Category III is 20 min, the minimum RP can be 64%, as indicated in Table 4. This value decreases to 45% when the DT increases from 20 to 40 min. Therefore, a RP greater than 50% can be developed when a concrete with thixotropy characteristics belonging to Category III is used and the DT between successive layers does not exceed 30 min. In other words, to achieve a RP greater than 90%, a DT between successive layers should not exceed 5 min when the Category III SCC is in use. Category I concrete, which exhibits high rate of SBR, can have a ss of at least 550 Pa determined using the IP test after 15 min of rest and a minimum rate of SBR of 7.5 Pa/min. Such concrete can exhibit relatively low filling ability with an initial slump flow up to 650 mm and a minimum rate of loss in slump flow at rest of 2.8 mm/min as well as relatively low passing ability with a maximum initial J-ring flow of 620 mm and a rate of drop in J-ring flow values at rest of 2.6 mm/min or more. The use of SCC with such rheology characteristics can develop a maximum RB and RP in multilayer casting of approximately 88% and 34% if the DT between successive layers does not exceed 15 and 20 min, respectively. The RB and RP can be improved by applying adequate external mechanical consolidation to the concrete layer that is previously placed immediately prior to casting a new layer. Care should be taken to avoid extensive mechanical consolidation that can lead to segregation, which can adversely affect the mechanical and transport properties across the boundary of successive layers. Therefore, the external mechanical consolidation, if necessary, should be adapted according to the rheological properties of the concrete, its segregation resistance, and the expected DT between the castings of successive layers. The water permeability resistance was found to be much more sensitive to multilayer casting of SCC compared to mechanical bond strength. Using Category II concrete in multilayer casting can lead to RB ranging between 60% and 74% when a DT between casting successive layers is 60 min. On the other hand, the same casting characteristics can lead to RP ranging between 4% and 23%. 4. Models for SCC performance in multilayer casting Eqs. (6) and (7) can be used to estimate the RB determined under the impact of flexural stress and the RP between two succes-

Table 4 Effect of structural buildup at rest and delay time on SCC performance in multilayer casting. Performance Key

Bond strength under flexural stress

SCC category Level of SBR Athix, PaPa/min FAI, mmmm/min PAI, mmmm/min Performance, %

I High 4000 1800 1600 88 – 77 – 68 59

DT = 15 min DT = 20 min DT = 30 min DT = 40 min DT = 45 min DT = 60 min

II Moderate 1000–4000 800–1800 600–1600 89–94 – 78–87 – 69–81 60–74

Water permeability resistance III Low 1000 800 600 95 – 88 – 82 75

I High 4000 1800 1600 – 34 – 11 – 3

II Moderate 1000–4000 800–1800 600–1600 – 35–63 – 12–44 – 4–23

III Low 1000 800 600 – 64 – 45 – 24

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sive layers. These two statistical models were developed using multi-regression analysis performed on the experimental data where idealized forms for the models were considerably presented. The estimated RB and RP are represented in percent and take into consideration the DT between successive layers and the SBR of the existing concrete evaluated by the FAI and PAI indices. Contour diagrams representing the models expressed in Eqs. (6) and (7) are plotted in Figs. 8 and 9, respectively. When placing a SCC mixture having PAI and FAI of 1200 mmmm/min in multilayer casting conditions, the RB and RP of the final product can be estimated to be 84% and 38%, respectively, if a 30 min delay between successive lifts is expected.

RBð%Þ ¼ 100  0:1745 DT  0:0003 DT  PAI

ð6Þ

RPð%Þ ¼ ð0:4 DT þ 100Þ=e0:00003 DTFAI

ð7Þ

where RB (%) is the residual bond strength between successive layers determined under the impact of flexural stress; RP (%) is the residual permeability resistance between successive layers; DT (min) is the delay time between successive layers; PAI (mmmm/ min) is the index of SBR evaluated by J-ring flow test; and FAI (mmmm/min) is the index of SBR evaluated by slump flow test. In order to secure specific levels of RB and RP, the critical delay time (DTC) between casting successive lifts of SCC can be estimated using Eqs. (8) and (9), respectively. When SCC mixtures with Athix of 1000 PaPa/min are employed in multilayer casting, the DTC between successive lifts should not exceed 22 and 5 min to secure at least 90% RB and 90% RP of the final product, respectively.

DTC ¼ 280:91  2:8091 RB% þ ð0:0003 RB%  0:0329Þ Athix

ð8Þ

DTC ¼ ð17875  3881 Ln RP%Þ=A0:65 thix

ð9Þ

where DTC is the critical delay time (min) to secure a RB% and RP%, and Athix (PaPa/min) is the SBR evaluated using IP test. The variations of DTC corresponding to a RB and RP of 90% with the Athix are plotted in Fig. 10. The DTC between successive SCC layers is shown to decrease with the increase in SBR of the freshly cast concrete. The DTC determined under the impact of flexural stress

2 1

Fig. 9. Contour diagrams of variations in residual water permeability resistance with delay time and structural buildup at rest determined using slump flow test.

was shown to be much higher than that corresponding to water permeability for a given SCC thixotropy level. The difference between DTC values determined using the flexural stress and water permeability test methods is found to be approximately 15 min when the SCC mixture has a low level of SBR, Category III SCC. However, when the SCC has high thixotropy level, Category I, the spread in DTC should decrease considerably to secure adequate mechanical and transport properties in multilayer casting. 5. Surface roughness across boundary of successive layers In multilayer casting, the newly cast concrete layer can penetrate the existing concrete lift if a sufficient free-fall height (FFH) is employed during the casting of the top layer [13]. Moreover, the penetration of new layer into the existing layer is affected by thixotropy level of concrete and DT between placing two successive layers. The surface roughness between concrete layers is influenced by how much the new layer can intermix with the existing layer. The roughness profile of the surface can be obtained with digital image processing [13,39,40] or using optical measuring device [41–43]. In order to investigate the effect of SBR and time on the roughness of bonded interphases between successive con-

1

2

Critical delay time, min

30 Residual bond strength

25 20 15

Residual water permeability resistance

10 5 0 0

1000

2000

3000

4000

5000

Structural buildup at rest, Pa·Pa/min

Fig. 8. Contour diagrams of variations in residual bond strength with delay time and structural buildup at rest determined using J-ring flow test.

Fig. 10. Variations in critical delay time needed to achieve 90% residual bond strength and water permeability resistance with structural buildup at rest determined using IP test.

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crete layers, a green pigment was used during the fabrication of composite prisms used for flexural strength testing using the SCC8 mixture. The green pigment was added and mixed well with the concrete used for casting the upper layer in order to identify the interphase between lower and upper layers of the composite prism. The DT between casting the two layers was set to 30 and 60 min. The FFH of the upper layer was set to 200 mm. The increase in the ss values of the existing layer of SCC8 mixture from 698 to 940 Pa resulted in a reduction in the RB from 76% to 56% when the top layer was placed after a DT of 30 and 60 min, respectively. The decay in RB with the increase in ss can be related to a reduction in surface roughness of bonded interphase due to relatively low intermixing of the newly cast SCC into the existing material. Fig. 11a and b show the interphase between the two concrete lifts corresponding to the two composite prisms captured after the failure at the conclusion of the flexural test. The close-shot pho-

tograph was treated using the Computer Aided Design (CAD) software to remove the background and identify the boundary of the bonded interphase and the green spots. The intermixing level of the new layer into the existing layer is represented by the surface area occupied by the green color in Fig. 11a-right and 11bright, which correspond to the images shown in Fig. 11a-left and 11b-left, respectively. The number and surface area of green spots are shown to decrease with the increase in ss at rest and DT. An intermixing level index was determined using CAD and is expressed as a percentage of the total area occupied by the green color to the area of bonded interphase. The intermixing level index was adjusted according to the scale of each image. This scale was the ratio between the actual dimensions of the concrete element and the corresponding dimensions in the close-shot photograph. The results indicated that the intermixing level index of bonded interphases shown in Fig. 11a and b were 18% and 7%, respectively. The increase in the intermixing level index, resulting from the

(a) Delay time = 30 min, Static yield stress = 698 Pa, Intermixing level index = 18%, and Residual bond strength = 76%

(b) Delay time = 60 min, Static yield stress = 940 Pa, Intermixing level index = 7%, and Residual bond strength = 56% Fig. 11. Variant intermixing levels at multilayer bonded interphases with static yield stress and delay time.

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decrease in the ss at rest and the reduction in the DT, improved the RB in multilayer SCC casting. This observation demonstrates that the mixtures belonging to Category III can increase the intermixing level of the newly cast SCC material into the existing lift, which in turn enhances the RB in multilayer casting. Therefore, Category III SCC is recommended to be used in multilayer casting when a relatively long DT is expected to occur before casting of the next concrete lift.

635

Acknowledgments The authors wish to thank the financial support of the National Science and Engineering Research Council of Canada (NSERC) and the 17 industrial partners participating in the NSERC Chair on High Performance Flowable Concrete with Adapted Rheology at the Université de Sherebrooke (2008–2013). References

6. Conclusions This paper investigated the thixotropy at rest of eight SCC mixtures that were designed with different levels of SBR. The thixotropy index was determined using standard workability tests and the IP test. Flexural strength and water permeability characteristics were determined using composite specimens cast with two lifts of SCC where the second lift was placed after a given rest periods varying between 15 and 60 min. Based on the results presented here, the following conclusions can be drawn: 1. The flow of SCC over freshly cast lift of concrete without any mechanical consolidation applied to the existing material can lead to the formation of distinctive layers with reduction in the bond strength and increase in the permeability across the fold/lift line. 2. Water permeability is shown to be significantly affected by multilayer casting than flexural bond strength. Depending on the degree of SBR of SCC, RB between the successive lifts can range from 88% to 96% for mixtures cast with a DT of 15 min, compared to strength of monolithic specimens. In the case of RP, the corresponding values were 41% to 91%. 3. The RB and RP of multilayer SCC decreases with the increase in SBR of the existing concrete lift. The deviation between the values of DTC needed to secure 90% RB and RP is limited to 15 min and it decreases with the increase in SBR. 4. Relationships are proposed to estimate the RB and RP as functions of DT between successive layers and SBR evaluated by standard workability test methods and IP test. Similar correlations were proposed to estimate the DTC required to secure a specific performance level in terms of mechanical and transport properties. 5. Three categories for SCC are identified based on the level of SBR before casting a successive lift. Category III mixtures with relatively low level of SBR can secure relatively high residual interlayer bond and permeability resistance values. Such SCC can be defined with a maximum ss of 250 Pa determined using IP test after 15 min of rest and a maximum SBR rate of 2.5 Pa/min. Such low thixotropy level can be secured when the initial slump flow and J-ring flow are greater or equal to 630 mm and the maximum decay rate in filling ability and passing ability at rest are 1.2 and 0.8 mm/min, respectively. 6. At a given free fall drop of the concrete during multilayer casting, SCC having a relatively low ss can improve the intermixing level of a newly layer into an existing material, hence resulting in an increase in the roughness of bonded interphase. An increase of 36% in the RB between successive layers can be obtained by improving the interphase roughness corresponding to an increase in intermixing level index resulted from a reduction of 26% in ss of the previously placed concrete layer.

Conflict of interest None.

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