Prediction of SCC Formwork Pressure1

Kamal Khayat, Missouri University of Science and Technology, Ahmed Omran, Université de Sherbrooke, Matthew D’Ambrosia, CTLGroup

The use of SCC enables rapid placement and labor savings; however, lack of information regarding formwork pressure variations during casting and pressure decay after placement has prompted contractors and engineers, as recommended by ACI 347 (Guide to Formwork for Concrete), to design SCC formwork for full hydrostatic pressure leading to cost increase. This article summarizes an extensive project aimed at developing formulation expertise and guidelines to better understand lateral pressure of SCC. This included the development of test methods to predict lateral pressure exerted by SCC and field tests to assess rheological properties of SCC. The study investigated the role of material constituents, casting characteristics, and formwork geometry on SCC form pressure.

Pressure device to simulate SCC formwork pressure²

The device is equipped with two flush-mounted pressure sensors. It is filled with 0.5 m of concrete at given rate, and then air pressure is introduced, at the same rate, to simulate pressure increase up to 13 m. The device is used to monitor lateral pressure during casting and early pressure decay during plastic stage (2 to 3 h). The device reflects the influence of mix design and enables prediction of lateral pressure on elements cast at rates of 2 to 22 m/h.

Field-tests to evaluate structural build-up at rest of SCC³

Six field-oriented tests were developed to evaluate structural build-up at rest (thixotropy) of concrete, which significantly affects lateral pressure characteristics. Among these tests, the portable vane (PV) and inclined plane (IP) tests showed good repeatability, low relative error, and comparable results to measurements obtained from concrete rheometer.

The PV test consists of four sets: each has cross-shaped vane that is centered vertically in square container before filling the container with concrete up to the total vane’s height. The concrete in the four containers is maintained undisturbed for 15, 30, 45, and 60 min, respectively, before measuring the maximum torque (T in N.m) to shear the material. T values are converted to static shear stress (T_0rest) using vane’s geometry G [τ_0rest ]. The T_0restat15min [PVT_0rest@15min] or change in shear with time [PVτ_0rest (t)] or PVτ_0rest@15minxPVτ_0rest(t) is used to quantify structural build-up at rest. The IP method involves casting concrete in a cylindrical mold onto a horizontal plate of a given roughness, then lifting the plate to initiate flow of the material. The angle (α) necessary to initiate flow is used to determine τ_0rest [IPτ_0rest = ρ.g.h.sinα, where ρ: material density, g: gravitation constant, h: characteristic height of spread sample]. Four tests are performed after different rest periods to evaluate shear growth at rest .

Experimental investigation for parameters affecting SCC formwork pressure^{4, 5}

A comprehensive testing program was undertaken to evaluate key parameters affecting formwork pressure exerted by SCC. The investigated factors included mix design (slump flow, dosages of HRWRA and VMA, volume of coarse aggregate, sand-to-total aggregate ratio, paste volume, nominal maximum size of aggregate MSA); casting characteristics (depth, placement rate, concrete temperature, and waiting period between consecutive lifts WP); and minimum formwork dimension d. The pressure device and the PV and IP test methods were employed in the testing program.

Prediction models for SCC formwork pressure⁶

The shear growth at rest of SCC is used to estimate the maximum lateral pressure (P_max), as shown below.

where;
P_max : maximum lateral pressure, KPa (0 – 350 kPa)
γ_c : concrete unit weight (18 – 26 kN/m³)
H : height of placement, m (1 – 13 m)
R : rate of placement, oC (2 – 30 m/h)
D_min : Equivalent parameter to d, m
• d < 0.2 m, D_min = 0.2 m
• 0.2 < d < 0.5 m, D_min = d
• 0.5 < d < 1.0 m, D_min = 0.5 m
f_MSA : factor depending on MSA
• SCC with MSA of 10 mm and PVτ_{0rest@15min@22ᴼC} ≤ 700 Pa; 1 ≤ f_MSA ≤ 1.10 for 4 ≤ H ≤ 13 m
• SCC with MSA of 14 and 20 mm; f_MSA = 1
f_wp : factor accounting for delay between successive lifts:
• f_wp = 1 for SCC with any thixotropic level cast continuously.
• f_wp = 1 – 0.85 for SCC with PVτ_{0rest@15min@22ᴼC} = 50 – 1000 Pa, respectively, when placement interrupted with 30-min waiting period in the middle of casting period
PVτ_{0rest@15min@Ti} : at a given concrete temperature (Ti) (0 – 2000 Pa)
The predicted-to-measured P_max results of using this above equation lie within 90% confidence interval .

Validation of model

Measured P_max values monitored using the pressure sensors mounted at different casting depths of six wall panels with heights of 3.7 and 4.4 m, widths of 0.2 m, and lengths of 5.5 m cast at the Université de Sherbrooke, QC, Canada as well as eight column elements with diameters of 0.61 m, heights of 3.66 m cast at CTLGroup, Skokie, Il, USA are compared to predicted values. The comparison yielded excellent correlation

Conclusions

An approach based on the measurement of the structural build-up at rest of SCC measured using field-oriented test methods can be used to estimate form pressure exerted by SCC. The study shows good correlation with field results and contributes to growing need to update form pressure prediction equations for flowable concrete that assume full hydrostatic pressure.

¹The project was sponsored by RMC Research & Education Foundation and the Strategic Development Council of the American Concrete Institute.

²Omran AF, Khayat KH (2012) Portable Pressure Device to Evaluate Formwork Pressure Exerted by Flowable Concrete. J. of Mat. in Civil Eng. (ASCE): 37 (accepted and pending publication in June 2013 issue, available online since Aug. 27th 2012).

³Khayat KH, Omran AF, Naji S, Billberg P, Yahia A (2012) Field-Oriented Tests to Evaluate Structural Build-up at Rest of Mortar and Flowable Concrete. Journal of Materials and Structures (RILEM), 45(10):1547-1564.

⁴Omran AF, Khayat KH, Elaguab YM (2012) Effect of SCC Mixture Composition on Thixotropy and Formwork Pressure. Journal of Materials in Civil Engineering (ASCE), 24(7):876-888.

⁵Khayat KH, Omran AF (Dec. 2009) State-of-the-Art Review of Form Pressure Exerted by Self-Consolidating Concrete. Final Report, Ready-Mix Concrete (RMC) Research and Education Foundation, American Concrete Institute (ACI), and Strategic Development Council (SDC):549. (http://www.concretesdc.org/projects/SCCreport.htm).

⁶Khayat KH, Omran AF (June 2011) Field Validation of SCC Formwork Pressure Prediction Models. Journal of Concrete International. 33(Issue 6):33-39.

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