|M.Sc Student||Mazuz Ofer|
|Subject||Measurement and analysis of lateral pressure of concrete on|
vertical formwork using geotechnical tools
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Mark Lloyd Talesnick|
|Professor Emeritus Aviad Shapira|
|Full Thesis text - in Hebrew|
The objective of this research is to measure and analyze the distribution of lateral pressures that develop between concrete and vertical forms, as a function of form height and time, using novel geotechnical instrumentation.
The technical literature and relevant standards offer a wide variety of formulae and empirical relationships that predict lateral pressures exerted by concrete on vertical forms based on various measurement methods and tools and involving multiple parameters that affect those lateral pressures. The ways in which those parameters exert their effect are at times unclear, and in such cases they are estimated on an empirical rather than analytical basis.
Methods used to measure lateral pressures exerted by concrete on forms can be classified into two groups: direct and indirect. In both methods, pressures are calculated based on the stresses and distortions created in the form. Examining the concrete hardening process, however, reveals that the concrete "carries" itself from a certain point onward, and then the form in fact "leans" on it; hence, the stresses that are measured at this stage represent the equilibrium between stiffness of the form and stiffness of the concrete.
During its early stages of life, concrete can be considered as a collection of water-saturated grains, similar to soil. This observation led us to the idea that its behavior may be examined from a geotechnical perspective.
In the present study, a unique gauge was used that can measure the change in the pressure exerted by the concrete on the form during both casting and concrete hardening. The gauge is based on the null pressure principle. Another type of gauges used in the present research was tension gauges. These gauges were used to compare and verify results obtained by the null gauge.
The research method applied to measure the lateral pressures between concrete and its form in the current study was a field experiment, conducted on an active construction site that included casting a long wall in several sections. The wall was 4.8 m high and 35 cm wide. A total of four experiments were executed on four casting sections, each 15-25 m long.
The findings of the study demonstrate the advantage of the null pressure method over the tension gauge method when the material exerting the measured pressures is a granular and thixotropic material. The effect of form stiffness on the tension gauge measurements as opposed to the null pressure gauge measurements was evident throughout the entire experiment. An additional significant difference was seen between the two gauge types during the concrete hardening stage, when decreases in pressure are observed; only the null pressure gauges measured a decrease to zero pressure.
Two additional phenomena were identified during the experiments: the first was the sedimentation of aggregates in the concrete that leads to friction between the concrete in the form. This phenomenon was found to affect both lateral pressure values and their change over time. The second phenomenon is a process that resembles consolidation and is accompanied by what is called “water bleeding”.