Electrical and relative humidity measurements of concrete and their relation to transport properties

Philip Kompare, Purdue University

Abstract

Many of the issues that affect the service life of concrete are related to intrusion of water into the system. Freeze-thaw cycle deterioration, corrosion of steel reinforcement, alkali-silica reaction, and sulfate attack are all detrimental to concrete by reducing its service life, and all can only occur if water is present. In order to design more durable concrete mixtures, it is important to know how a fluid, such as water, moves through concrete. The elements intrinsic to how a fluid moves through a concrete system are known as its transport properties. In this research, the transport properties are studied and measured using two separate methods. The first method uses Electrical Impedance Spectroscopy (EIS) which can measure how easily electrons flow through a medium. As concrete is a mixture of aggregate, cement, and water, the liquid phase of concrete, known as pore fluid, is the only electrically conductive component in the system. The change of electrical conductivity over time can be correlated to how easily fluid can pass through the system using an analytical model of concrete called the parallel model. This research measures the conductivity of different concrete mixtures to determine how changes in water to cement ratio (w/c), aggregate volume fraction, and common admixtures affect the results of the EIS experiment. The research also measures the impact of temperature on the EIS measurements, specifically the temperature impact on the conductivity of the pore solution. Pore solution was extracted from samples to determine the conductivity at different ages and to derive the activation energy of conduction. From this method it was determined, for standard concrete ranges, a low w/c is important for keeping fluid transport low in concretes whereas the volume fraction of aggregate had little impact on fluid transport after only 1 weeks time. The second method used to study fluid transport properties of concrete uses the measured value of relative humidity (RH) within concrete itself. RH is a measure of the moisture in vapor form within the atmosphere. The internal RH measurements were achieved by creating tubular cavities in concrete slabs and placing sensors capable of measuring RH within them. The slabs were placed outdoors to experience real atmospheric conditions. The cavities were placed at different depths within the slab for the purpose of measuring the RH at specific distances away from the top of the slab which received exposure to the atmospheric conditions. By separating the cavities by known distances away from the exposure side, a profile of the moisture content along the depth of the slab could be determined. Multiple conditions of exposure were examined; a covered slab, a vertical slab, a slab on a drainable base, a slab on a non-draining base, and a submerged slab. Shortcomings of the experimental setup were realized, but conclusions were made; first, moisture from precipitation does not penetrate any deeper than 1 inch from the surface of a well drained concrete, and secondly, the RH of a concrete 2 inches and deeper from an exposed surface that is subjected to ambient environmental conditions found in the Midwest does not drop below 80%. The findings of this research will hopefully aid in creating more durable concrete mixture designs. At a minimum, the research should help better understand how transport properties change for concrete at early ages and how moisture infiltrates concrete when exposed to real atmospheric conditions.

Degree

M.S.C.E.

Advisors

Weiss, Purdue University.

Subject Area

Civil engineering

COinS