Joseph J. Assaad · Kamal H. Khayat. Received: ... The design of formwork systems for wall and column .... bridge pier abutment wall sections, the casting rate of.
Materials and Structures (2006) 39:333–341 DOI 10.1617/s11527-005-9042-3
Effect of casting rate and concrete temperature on formwork pressure of self-consolidating concrete Joseph J. Assaad · Kamal H. Khayat
Received: 09 December 2004 / Accepted: 31 January 2005 C RILEM 2006
Abstract An experimental program was undertaken to determine the effect of casting rate and concrete temperature on formwork lateral pressure that can be developed by self-consolidating concrete (SCC). Concrete mixtures prepared with initial temperatures varying from 10 to 30◦ C or with high early-strength cement and set-accelerating admixture were evaluated. The concrete was placed either continuously at casting rates varying between 5 and 25 m/h or by stopping the placement for some predetermined periods of time. Test results show that the increase in casting rate from 5 to 25 m/h can lead to 15% increase in initial formwork pressure; however, no significant effect on the rate of pressure drop with time is observed. The variations in fresh concrete temperature have limited effect on the initial lateral pressure; however, the rate of pressure drop was significantly increased with the increase in temperature. The time for the cancellation of pressure is directly affected by the concrete temperature and materials in use, and occurs shortly after the end of the dormant period of cement hydration.
temp´erature du b´eton sur le d´eveloppement des pressions lat´erales des b´etons autoplac¸ants (BAP). Cinq m´elanges ayant diff´erentes temp´eratures variant entre 10 et 30◦ C ou incorporant un ciment a` prise rapide et agent acc´el´erateur de prise ont e´ t´e e´ valu´es. Le b´eton a e´ t´e plac´e soit continuellement a` diff´erentes vitesses de coulage variant entre 5 et 25 m/h ou en arrˆetant le coulage pour des temps de repos pr´ed´etermin´es. Les r´esultats obtenus ont montr´e que l’augmentation de la vitesse de coulage de 5 a` 25 m/h induit une augmentation de pression de 15% de l’hydrostatique sans, toutefois, un effet significatif sur la chute des pressions avec le temps. Par ailleurs, la variation de la temp´erature initiale du b´eton a un effet mineur sur les pressions lat´erales d´evelopp´ees par le BAP juste apr`es le coulage. En revanche, la chute des pressions avec le temps augmente consid´erablement avec l’augmentation de la temp´erature initiale du b´eton. L’annulation des pressions est due a` un effet chimique peu apr`es la fin de la p´eriode dormante du ciment, et est affect´ee par la temp´erature initiale du b´eton ainsi que les mat´eriaux utilis´es.
R´esum´e Une e´ tude exp´erimentale a e´ t´e r´ealis´ee pour d´eterminer l’effet de la vitesse de coulage et la 1. Introduction Joseph J. Assaad Holderchem Building Chemicals S.A.L, P.O. Box 40206, Baabda, Lebanon Kamal H. Khayat Universit´e de Sherbrooke, Faculty of Engineering, J1K 2R1, Sherbrooke (Qc), Canada
The design of formwork systems for wall and column elements is governed by the level of lateral pressure exerted by the plastic concrete. The American Concrete Institute (ACI) Committee 622 and 347 analyzed wide range of data and field measurements and concluded
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that the magnitude and shape of lateral pressure envelope is dependent on at least 13 factors, including the casting rate and concrete temperature [1, 2]. In order to determine the influence of the casting rate on the development of lateral pressure, Ritchie [3] conducted a series of experiments on concrete made with cement-tocoarse aggregate ratios of 1:3 and 1:6. Lateral pressure was determined using an experimental column measuring 2400 mm in height and 150 × 150 mm2 cross section. The slump consistency of the concrete ranged between low and high levels, and the casting rate was varied from 1 to 20 m/h. Irrespective of the composition and workability of the concrete, lateral pressure was found to increase with the casting rate. For example, for the 1:6 concrete mixture of high consistency, the maximum lateral pressure exerted on the formwork was shown to decrease from 38 to 24 and 10 kPa when the casting rate was reduced from 20 to 3.5 and 1 m/h, respectively. The reduction in casting rates can increase the time available for the concrete to develop shear strength and wall friction, thus limiting the development of lateral formwork pressure [4]. Maxton (from Rodin [5]) studied the coupled effect of the casting rate and concrete temperature on the lateral pressure envelope for conventional concrete. Different series of low-slump concrete mixtures placed at casting rates varying between 0.6 and 2 m/h were investigated. The concrete temperature varied from 4.5 to 27◦ C. Maximum lateral pressure was found to increase with the increase in the casting rate and/or decrease in concrete temperature. Irrespective of the tested parameters, the pressure envelope was reported to be hydrostatic from the free surface to a certain maximum value, and then remained constant until the bottom of the formwork. For formwork design purposes, ACI Committee 622 [1] proposed the following design equations for column and wall elements, both of which take into account the rate of casting and concrete temperature: For columns, Pmax = 7.19 +
785R < 23.5H 17.78 + T
or
143.7
For walls, 785R 17.78 + T < 23.5 H or 95.8
R < 2.14 m/h Pmax = 7.19 +
1155 17.78 + T 244R + 17.78 + T < 23.5 H or 95.8
2.14 < R < 3 m/h : Pmax = 7.19 +
R > 3 m/h : Pmax = 23.5 H < 95.8
where Pmax : maximum lateral pressure, kPa R: rate of casting, m/h T: concrete temperature, ◦ C H: head of concrete, m Self-consolidating concrete (SCC) is a new type of high-performance concrete that does not require any mechanical vibration to achieve proper consolidation. To date, limited information exists regarding the effect of the casting rate of SCC on the development of lateral pressure. Casting rates of such concrete can vary with the size of the cast element and placement method. For example, GTM Construction Company [6] evaluated the effect of casting rates varying from 10 to 150 m/h on the development of lateral pressure on formwork of different dimensions with length varying from 1.25 to 2.5 m, height from 2.8 to 5.6 m, and width from 0.25 to 0.40 m. Two types of SCC mixtures made with or without viscosity-enhancing admixture (VEA) were investigated. The sand-to-total coarse aggregate ratio was fixed at 0.46, and the water content was adjusted to secure slump flow values varying between 700 and 880 mm. For most of the tested mixtures, the measured pressure was found to be close to the hydrostatic pressure [6]. The CEBTP [7] carried out a large-scale field experiment on diaphragm wall elements measuring 12 m in height to evaluate the formwork pressure envelope. The concrete was cast either by pumping from the bottom of the formwork on average casting rate of 25 m/h or by bucket from the top at 10 m/h. The concrete had slump flow consistency of 700 mm and water-to-cementitious materials ratio (w/cm) of 0.46. The maximum lateral pressure at the base of the experimental walls was found to correspond to 65% of hydrostatic pressure in the case of concrete cast with bucket from the top and 70% for that cast by pumping from the bottom [7]. It is important to note that these casting rates are quite high and can be as low as 2 to 4 m/h, depending on the length of the cast section [8]. This is the case when SCC is employed for casting
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3.8 3.8 3.9 3.7 3.3 – – – 1000 – ∗∗
TER-10 TER-20∗ TER-30 TER-20-ACC T30-20 10 20 30 20 20
mixture tested at five different casting rates. mixture code refers to: Type of cement – Initial concrete temperature – Presence of set-accelerator.
260 260 260 260 260 740 740 740 740 770 180 180 180 180 180 – – – – 450
Mixture codification∗∗ Concrete temperature, ◦ C
450 450 450 450 –
Water, kg/m3 (w/cm = 0.40) Type 30 cement, kg/m3 Ternary cement, kg/m3
Table 1 Mixture proportions of evaluated SCC
Commercially available ternary cement and high earlystrength cement (CSA Type 30 cement) were used. The ternary cement contained 6% silica fume, 22% fly ash, and 72% CSA Type 10 cement. The Type 30 cement, Type 10 cement, and fly ash had Blaine specific surface values of 600, 325, and 410 m2 /kg, respectively. The silica fume had a B.E.T specific surface of 20,250 m2 /kg. Continuously graded crushed limestone aggregate with nominal size of 10 mm and well-graded siliceous sand were employed. The coarse aggregate and sand had fineness moduli of 6.4 and 2.5, bulk specific gravities of 2.71 and 2.69, and absorption values of 0.4% and 1.2%, respectively. Polycarboxylate-based high-range water-reducing admixture (HRWRA) of 1.1 specific gravity and 27% solid content was used. A high molecularweight cellulosic-based material was employed for the VEA to enhance stability of mixtures proportioned with 0.40 w/cm. The liquid-based VEA had a specific gravity of 1.12 and solid content of 39%. A nonchloride-based set-accelerating admixture and a synthetic detergent-based air-entraining agent (AEA) were used.
Sand (0-5 mm), kg/m3
2.1. Materials
∗
2. Experimental program
870 870 870 870 900
Coarse aggregate, (5-10 mm), kg/m3
VEA, mL/ 100 kg of cement
Set-accelerator, mL/100 kg of cement
HRWRA, L/m3
AEA, mL/ 100 kg of cement
wall elements for residential or commercial type of construction. In some repair operations of columns or bridge pier abutment wall sections, the casting rate of SCC can approach 10 m/h given the limited width of the repair sections [8]. For the evaluation of the effect of casting rate and concrete temperature on the development and variations of lateral pressure exerted by SCC, five mixtures with 650 ± 15 mm slump flow consistency were evaluated. The concrete was cast at three different temperatures, and was proportioned with high early-strength cement and set-accelerating admixture to investigate the influence of the kinetics of cement hydration on the pressure envelope. The SCC was placed at casting rates ranging between 5 and 25 m/h. The variations in lateral pressure during the plastic stage were determined using an experimental column of 2800 mm in height. Another column measuring 1100-mm in height was employed to monitor pressure drop until cancellation which occurs during the setting stage of the concrete.
120 120 120 135 170
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Table 2 Properties of evaluated SCC mixtures Slump flow, mm Air content, % Initial concrete temperature, ◦ C Unit weight, kg/m3 h2 /h1 of L-box test Surface settlement, % Initial set time, min Final set time, min
2.2. Mixture proportions As summarized in Table 1, the investigated mixtures were prepared with 450 kg/m3 of binder content and w/cm of 0.40. The effect of concrete temperature on lateral pressure variations was evaluated by testing mixtures prepared at 10, 20, and 30 ± 2◦ C for the TER-10, TER-20, and TER-30 mixtures, respectively. Ambient temperatures during the sampling and testing were 14, 20, and 27◦ C, respectively, to minimize heat loss of the tested concrete. The effect of using Type 30 cement and setaccelerating admixture on the variations in lateral pressure was investigated, as they have marked effect on the rate of cement hydration. The dosage of the setaccelerator was set at 1000 mL/100 kg of binder. The T30-20 and TER-20-ACC mixtures prepared with Type 30 cement and set accelerating admixture, respectively, were proportioned at 20 ± 2◦ C and tested at 20◦ C ambient temperature. The VEA dosage was fixed at 260 mL/100 kg of binder, and the sand-to-total aggregate ratio remained constant at 0.46 for all tested mixtures. The HRWRA and AEA concentrations were adjusted to secure initial slump flow of 650 ± 15 mm and air content of 6 ± 2%. 2.3. Instrumented column systems Two experimental columns were used to determine the lateral pressure exerted by plastic concrete. The first column measures 2800 mm in height and 200 mm in diameter, and was used to evaluate pressure variations of the plastic concrete. The lateral pressure was determined using five pressure sensors mounted at 50, 250, 450, 850, and 1550 mm from the base. Given that this column had to be emptied prior to stiffening, the monitoring of lateral pressure distributions did not extend
TER-10
TER-20
TER-30
TER-20-ACC
T30-20
655 6.5 9.6 2230 0.84 0.48 690 780
665 4.3 21.7 2265 0.81 0.34 610 705
645 5.9 30.1 2190 0.85 0.32 585 660
645 4.5 20.8 2315 0.82 0.29 440 480
640 6.2 21.7 2335 0.85 0.15 425 470
until the hardened state. Therefore, the monitoring of pressure tests were stopped once the concrete had a slump consistency of approximately 150 mm. In order to enable the evaluation of pressure variation up to the hardening of the concrete, a shorter column measuring 1100 mm in height and 200 mm in diameter was used. Three pressure sensors similar to those employed in the former column were mounted at 50, 250, and 450 mm from the base. Both experimental columns were made of PVC with a smooth inner face to minimize friction with the concrete. The pressure sensors of 100-kPa capacity were placed flush with the inside of the formwork, and were calibrated using a free head of water prior to use. 2.4. Fabrication and testing program Two batches of 100-L each were prepared using an open-pan mixer for the cast of each set of experimental columns. The mixing sequence consisted of homogenizing the coarse aggregate and sand for one minute before introducing one third of the mixing water along with the AEA. The cement was then added and followed by the HRWRA diluted in one third of the water. After three minutes of mixing, the VEA and remaining water were introduced, and the concrete was mixed for two additional minutes. The slump flow, concrete temperature, unit weight, air volume, L-box flow characteristics, surface settlement, and setting time were determined, and the results are summarized in Table 2. The L-box test is used to determine the passing ability of the concrete [9]. In this test, the SCC is cast in a vertical compartment of an L-shaped apparatus and left at rest for one minute before removing a gate separating the vertical and horizontal compartents. This enables then the concrete to flow out through closely spaced reinforcing bars of 12-
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mm diameter located at the bottom of the section. The ratio of the heights of concrete remaining in the leading edge (h2 ) and vertical section (h1 ) is determined to evaluate the self-levelling characteristic of the concrete. The surface settlement was assessed by casting the concrete in a PVC column measuring 200 mm in diameter and 800 mm in height [10]. The surface settlement was monitored using a dial gauge fixed at the top of a thin plate positioned and anchored at the concrete surface. The initial and final setting times were determined in compliance with ASTM C403 using mortar samples extracted from the fresh concrete. Concrete temperature rise was monitored using two thermocouples placed at the center of the 1100-mm high experimental column used for lateral stress measurements. In order to evaluate the effect of casting rate on pressure variations, the TER-20 mixture was cast using five different casting procedures. Three of the procedures consisted of discharging the concrete continuously from the top at a rate of rise of 5, 10, and 25 m/h. In the other two casting procedures, the 2800-mm column was cast at 10 m/h in three lifts separated by two resting periods of either 10 or 20 minutes each. Therefore, the total durations for the filling of the 2.8 m high columns were 17, 37, and 57 minutes when the concrete was cast continuously or when cast in three lifts with 10- or 20-minute waiting periods, respectively.
3. Test results and discussion 3.1. Fresh concrete properties All SCC mixtures had L-box blocking ratios (h2 /h1 ) greater than 0.80 indicating adequate passing ability, and relatively low surface settlement (
