MEASURING THE FORMWORK PRESSURE OF SELF-COMPACTING CONCRETE

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1 th International RILEM Symposium on Self-Compacting Concrete MEASURING THE FORMWORK PRESSURE OF SELF-COMPACTING CONCRETE Niki Cauberg, Jan Desmyter BBRI, Belgian Building Research Institute, Belgium Abstract Estimating the exact formwork pressure for self-compacting concrete (SCC) remains an unsolved issue for building contractor and formwork supplier. In practice, it is often advised to use the hydrostatic model for formwork calculations. This sometimes leads to an overestimation of the formwork configuration and extra costs, especially in the case of high construction elements. On the other hand, some research projects describe the successful attempts to use special types of admixtures for obtaining a decrease of this formwork pressure. In those cases, the thixotropic properties in the concrete may eliminate part of the pressure. In this research project, the BBRI evaluated the practical application of the following questions: the influence of some relevant mixture properties on the pressure, the potential thixotropic behaviour decreasing the formwork pressure and the feasibility of measuring this pressure during the casting process. Several measuring devices were used and evaluated, in order to propose an efficient monitoring tool for the building site. Monitoring the pressure could be an economical and efficient tool to use the full potential of the formwork configuration, without having the risk of serious deformations or even failure. The developed devices were used for monitoring the casting of real-scale walls, in order to evaluate the influence of parameters such as casting rate, reinforcement ratio and type of SCC. INTRODUCTION The market share of Self-compacting concrete (SCC) for use on site remains rather limited compared to the situation in the precast industry. The higher cost of the mix is usually named as the main reason, besides the lack of experience with this concrete []. On site, this brings along quite some questions and uncertainties for the contractor. One of the technical questions concerns the formwork stability due to the strongly increased lateral pressure. The formwork pressure of fresh SCC, and its evolution in time is inadequately known and sometimes difficult to check. A safe approach is the simple application of hydrostatic formwork pressure, but this may result in overestimation and extra costs (which make the whole even more expensive). On the other hand, some sources state that formwork pressure may decrease considerably when using admixtures that promote the thixotropic behaviour of the concrete. The BBRI carried out research in this regard and focussed first of all on the possibilities of measuring this formwork pressure in a simple way on site. A suitable system would after-all not only facilitate (non-destructive) measurements, but also could allow for a monitoring during the casting process. Furthermore, the main parameters influencing formwork pressure were 03

2 th International RILEM Symposium on Self-Compacting Concrete evaluated in large-scale tests. These parameters were: rising rate of the concrete in the formwork, concrete type and viscosity, reinforcement ratio. In practice, the formwork pressure is generally calculated by the formwork suppliers. This is realised on the basis of existing models that take into account different parameters. These (usually empirical) models were developed for traditional types of concrete and are distinguished according to the number of parameters used and the way in which they quantify them. The models from the CIRIAreport ([] and Figure ) and the German standard [3] are primarily applied to normal concrete (in Europe). It is clear that at least some additional recommendations should be formulated for SCC. 2 MEASURING EQUIPMENT Figure : Formwork pressure according to [], and indication of the changes due to SCC Several types of pressure and load cells were used for the measurements, taking accuracy, impact on the formwork and specific boundary conditions in the concrete (alkalinity, hydration heat) into consideration. Adapted fittings allowed for efficient placement and recovery: flush with the formwork panel for the pressure sensors, on the tie bars for the load cells. For the latter, it is not always obvious to recalculate pressure values starting from these measured forces on the tie bars: it is clear that the interpretation of the measurements is strongly related to the configuration of the formwork. Calibration using water pressure and a reference sensor allowed correct interpretation of the test results and monitoring of possible damage of the sensors after each test (Figure 2). The measurement performance of the sensors was furthermore tested using a small formwork filled with water or concrete (Figure 3). These tests showed a relatively good correlation between the different types of sensor. Figure 2: Calibration of the sensors Figure 3: Small-scale testing 0

3 th International RILEM Symposium on Self-Compacting Concrete Figure : Load cell on a tie bar Figure : Installed pressure sensor on the formwork 3 CONCRETE COMPOSITIONS AND TEST SETUP As mentioned before, different types of concrete were used, subdivided into four classes, with different rheological characteristics (Table ). The various compositions were refined in advance in the laboratory. Special attention was paid here to the choice of superplasticizer, since a long pumping time was necessary (at least minutes), which is not obvious for most superplasticizers. Furthermore, the superplasticizer was added in situ. Full characterisation of the fresh concrete was performed before casting (see Table and Table 2). Two casting rates ( m/h and m/h), two reinforcement percentages (% and %) and four concrete mixes (three SCC types and one normal concrete) were considered. Table : Target values for concrete for the large-scale tests Mix Slump flow [mm] Funnel Time [mm] Yield Value* [Pa] Viscosity* [Pa.s] C reference mix 70 2 C2 powder content C3 incorporation VMA N normal concrete S3 * Measurements were done with a coaxial viscometer. Table 2: Setup parameters for the large-scale tests, slump flow results and summarized results for the formwork pressure. The dimensions of the wall: m height,.2 m wide, 0.3 m thick. Placement parameters Concrete composition Vertical reinforcement percentage (%) Casting rate (m/h) Slump flow [mm] Slump flow loss at end of casting [mm] Formwork pressure, immediately after casting [kn/m²] at 20 cm wall height in [%], compared to hydrostatic pressure Formwork pressure, 200' after casting [kn/m²] at 20 cm wall height in [%], compared to hydrostatic pressure M C M2 C M3 C M C M C M6 C M7 N 70 M8 C M9 C

4 th International RILEM Symposium on Self-Compacting Concrete The formwork setup can be found in Figure 6, together with the test results for wall M. Heigth [cm] Tie bars with load cells Concrete heigth [cm] Measuring height for the pressure sensors [cm] cm 0 cm 20 cm 30 cm 00 cm Increasing concrete height 6 0 Tie bars with strain gauges Formwork Tie bars panels hinge Pressure sensors Pressure [kn/m²] Figure 6: Illustration of test setup and result for wall M. Left side: Complete test configuration with an indication of the placing of pressure sensor and load cells. Frontal view with dimensions above, cross section below. Right side: test results for wall M, with the evolution at different time steps. The legend indicates the formworkpressure at an indicated concrete height. At a concrete height of 00 cm (complete filling), a nearly hydrostatic pressure is measured. INTERPRETATION OF THE RESULTS Figure 6 illustrated already the results for the measurements of one single wall, and the nearly hydrostatic build-up is clear. Starting from those diagrams for all the walls, Figure 7 summarises the results for the sensors at two different wall heights. From this, the impact of the varying parameters in the casting (casting rate, reinforcement percentage, type of mixture) can be assessed. 2 3 Formwork pressure [kn/m²] Pressure at wall height 20 cm Hydrostatic pressure at wall height 20 cm Pressure at wall height 20 cm Wall 6 Hydrostatic pressure at wall height 20 cm Figure 7: Summary of the large-scale tests: for each wall, the results at 2 different wall heights are given in the diagram, and this for the completely filled wall. At a wall height of 06

5 th International RILEM Symposium on Self-Compacting Concrete 20 cm, all walls show a nearly hydrostatic pressure, as expected. At a wall height of 20 cm, the specific configuration (type of mixture, concrete rising rate,...) influences the results. The most remarkable points are presented hereafter : The formwork pressure for normal concrete was high (M7, 79% of the hydrostatic pressure). There was no significant influence of reinforcement ratio or concrete rising rate. Especially for the latter, this does not mean concrete rising rate is not important. At very low casting rates (for instance < m/hour), the influence could be important, as mentioned sometimes in literature. However, at higher rising rates ( or m/hour as used during this project), the influence could be less important. The negative effect of a superplasticizer overdose (and consequently a too liquid SCC) is illustrated by the results for wall M8: 97% of the hydrostatic pressure at the end of casting, and 7% after 200 minutes. Not only maximum formwork pressure is important, the decrease in function of the time is interesting as well. Figure 8 and Figure 9 present the huge difference between wall M6 and M8: for the wall M8, an overdose of superplasticizer maintained the high formwork pressures for several hours. The pressure regression for wall M6 however occurs so fast that no real accumulation of pressure would occur, in case higher walls would be casted (with a moderate concrete rising rate). In general, all walls show rather high maximum formwork pressure, all between 80% and 0% of the hydrostatic pressure. The influence of mixture type may not be overestimated: when comparing the maximum formwork pressure for the wall series, the maximum difference does not exceed % (wall M9 compared to wall M7). Concrete (Formwork) height [cm] Concrete (Formwork) height [cm] Pressure [kn/m] Figure 8: Decrease of formwork pressure at several time steps after casting for wall M6, until 00 minutes after casting. Pressure [kn/m] Figure 9: Decrease of formwork pressure at several time steps after casting for wall M8, until 00 minutes after casting. 07

6 th International RILEM Symposium on Self-Compacting Concrete CONCLUSIONS AND FURTHER STUDY The application of self compacting concrete (SCC) at the building site still raises a number of technical questions for the contractor. These concern both the general characteristics of the material and specific aspects such as carrying out inspection tests on the fresh concrete, placing methods and formwork pressures associated with SCC []. Based on an extensive study of the literature and on the results of a testing campaign, the BBRI addressed the issue of the formwork pressure caused by SCC. Main question is the need to take hydrostatic pressure into account or not and the general differences with normal concrete. During the test campaign an attempt was made to measure the formwork pressure using different sensors and to assess the impact of placement parameters such as casting rate, reinforcement ratio and the type of mixture. Two casting rates ( m/h and m/h), two reinforcement percentages (% and %) and four concrete mixes (three SCC types and one normal concrete) were considered. An easy to use measuring system was developed as well. It has been showed that the influence of lower casting rates ( m/h instead of m/h) or a higher reinforcement ratio (% instead of %) on the formwork pressure is limited. From the point of view of the mix composition, the results have highlighted that this aspect is the most important one regarding to the influence on the formwork pressure. Still, for all real-scale experiments, rather high pressures were measured, ranging from 78% to 97% of the hydrostatic pressure. Finally, on the basis of the results presented in this paper and within the range of the tested parameters, it is therefore still recommended to calculate formworks based on the hydrostatic pressure load of the concrete, unless extremely low casting rates are being used. If the use of this hydrostatic pressure requires an unacceptable modification to the formwork configuration, a monitoring of the placement with sensors could be used to check real pressure in the structure during casting. ACKNOWLEDGEMENTS The authors would like to express their appreciation to the IWT Flanders for the financial support of this research project. REFERENCES. Desmyter J., Barriers to the application of cast-in-place self-compacting concrete, Proceedings of the th International RILEM Symposium on Self-Compacting Concrete, Belgium, 2007 (to be published). 2. CIRIA, Report 8: Concrete pressure on formwork, DIN 8 28 Frischbetondruck auf lotrechte Schalungen, September Cauberg N., Desmyter J., Pierard J., Formwork pressure for Self-Compacting Concrete, BBRI Contact, BBRI-files, nr. 3/2006, issue 7, p. 08