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The use of pore water pressure to follow the evolution from fresh to hardened concrete



Title: The use of pore water pressure to follow the evolution from fresh to hardened concrete
Author(s): T.A. Hammer
Paper category : conference
Book title: 2nd International RILEM Symposium on Advances in Concrete through Science and Engineering
Editor(s): J. Marchand, B. Bissonnette, R. Gagné, M. Jolin and F. Paradis
Print-ISBN: 2-35158-003-6
e-ISBN: 2351580028
Publisher: RILEM Publications SARL
Publication year: 2006
Nb references: 10
Language: English


Abstract: Measurement of the pore water pressure (PWP) is a good tool in order to follow the evolution of some important properties of paste, mortar or concrete in the fresh state and through time of setting. There are two main periods; before and beyond “point of self-support” shortened to “PSS”.
In the time before PSS PWP corresponds to the hydrostatic pressure of the liquid paste-mortar-concrete given by the height over the point of measuring and the density, as long as there is no moisture exchange with the surroundings (sealed condition). Then, the pressure decreases with time as result of the formation of a "self-supporting" body. At the time when the paste-mortar-concrete is able to support its own weight, PWP corresponds to the water pressure at the depth of measuring. Thus, PWP in this initial period may be used to assess workability retention, period of consolidation and period of formwork pressure.
At external drying, e.g. of horizontal surfaces, PWP reflects capillary tension which is the origin of plastic shrinkage. It has been confirmed by tests that there is a good correlation between PWP and plastic shrinkage: Plastic shrinkage starts when PWP turns into tension and the two coincide well in the period shortly after. Also, it has been shown a good correlation between PWP and tensile strain capacity in this period. By measuring at different depths it is possible to follow the “thickening” of the surface skin when exposed to external drying.
In the period beyond PSS, PWP reflects quite good the stiffness evolution of the paste-mortar-concrete (at sealed condition). This is seen as increasing pore water tension, at a low rate until the period when setting starts and then at progressively increasing rate in the setting period. It has been shown that the point in time when PWP has reached this high rate coincides fairly well with the time of setting.
Tests have shown that the friction in slipforming is suction mainly, caused by pore water tension in the time until setting, and thus, that PWP is a good measure for friction in slipforming operations.


Online publication: 2006-08-02
Classification: 3.2 Theme 2: From Fresh to Hardened Concrete
Publication type : full_text
Public price (Euros): 0.00
doi: 10.1617/2351580028.061