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Title:Effect of Early Curing Conditions on Permeability of Concrete
Author(s):Ludirdja, Darmawan
Doctoral Committee Chair(s):Young, J.F.
Department / Program:Civil Engineering
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Civil
Abstract:The potential effects of damage during early curing of Portland cement concrete have been investigated by measuring the coefficient of permeability. Damage can occur due to temperature gradients caused by heat of hydration or drying shrinkage due to premature loss of water.
A simple low pressure apparatus to measure permeability on a routine basis has been developed successfully. The system uses a 1 foot head of water, and does not require complicated or expensive tools to build and operate. The permeability data is evaluated using Darcy's Law for falling head permeability.
The flow of water through hardened cement pastes was investigated. The lower the water to cement ratio, the lower is the permeability coefficient, as expected. The permeability coefficient continues to decrease during the test. This behavior is attributed to continued hydration which changes the pore structure of the cement paste. Early drying of the cement paste caused an increase in the permeability coefficient. This is attributed to cracking and changes in pore structure. As water reintroduced during the test, the pore structure is recovered, and microcracks are healed as the result of continuation of hydration. After 14 days paste showed no effects of initial drying, even in the extreme case of oven drying.
In mortars and concretes the flow through interface region has been predicted using simple composite theory (the parallel model). The interfacial contribution is about 50-70% of the permeability coefficient but depends on the total aggregate content. Contributions from both fine and coarse aggregate can be determined. This quantification is important for prediction of the permeability coefficient from the mix design.
Model concrete was used to determine the effects of thermal mismatch of aggregate and matrix, caused by different linear thermal coefficients of expansion. An elastic analysis which accounts for the changes in the modulus of elasticity of cement paste at early stages was used. In this way the width of interfacial cracks could be predicted and the flow of water through these cracks could be calculated by the Cubic Law. It was found later that spacing between aggregates has an important role in determining whether cracking occurs also in the matrix. Thermal mismatch with plexiglass is so great that cracking causes extensive damage.
Thermal gradients in simulated mass concrete using cubes and beams have no effect on the flow of water. This may be explained by the localized or discontinuous microcracks which are healed during the permeability test.
It is concluded that damage to concrete at early ages either due to temperature rise or premature drying is not permanent. Subsequently hydration can provide sufficient healing to prevent the flow of water through a continuous system of cracks.
Issue Date:1993
Description:142 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.
Other Identifier(s):(UMI)AAI9411698
Date Available in IDEALS:2014-12-17
Date Deposited:1993

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