Moisture curling in concrete with fine lightweight aggregates

concrete pavements;moisture gradient;concrete curling

Amirkhanian, Armen N.

关键词: concrete pavements;    moisture gradient;    concrete curling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/90858/AMIRKHANIAN-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Moisture gradient development of concrete that incorporates saturated fine lightweight aggregates (FLWA) is not well understood. When a concrete beam or slab is exposed to external drying, water is transmitted through the surface pores and an internal drying front forms. If additional internal water can be provided, the development of the drying front can be minimized and delayed especially during early-age drying periods. With a delay, the concrete material can gain sufficient strength to resist cracking and hypothetically reduce the rate and magnitude of moisture curling.In order to measure the impact of external drying on concrete moisture curling, a comprehensive Box-Wilson design of experiments setup was constructed to examine three critical factors: water to cementitious ratio, FLWA content, and moist curing duration. The concrete mixtures were characterize by utilizing a high-aspect ratio beam geometry. The beams were placed on a near frictionless foundation perpendicular to their direction of curling in order to eliminate the effects of creep and self-weight. Unrestrained curling deflections were measured for the experimental combinations with the results indicating that moist curing duration and FLWA content have the highest impact on the concrete curling magnitude. In fact, greater curling occurred for mixtures that were moist cured longer. Concrete mixtures with 27% FLWA by volume of fine aggregates had a 50% reduction in unrestrained curling deflection relative to concrete mixture without FLWA.In order to observe the effects of creep and self-weight, several of the concrete mixtures from the unrestrained beam testing were tested on an elastic foundation. An analytical solution was developed to calculate the deflections and moments in the beams given a certain curling moment, which also allowed for beam-foundation separation. Experimental results indicate that very early-age drying of restrained beams, i.e. 24 hours after hydration begins, leads to lower curling magnitudes than longer moist curing e.g., 6.5 days. Both the larger surface porosity and the high creep values for early age concrete result in this curling behavior. After a 6.5 day moist curing duration, the concrete mixtures curled more and had significantly less creep with the concrete containing FLWA reducing the beam curling magnitude.In order to directly calculate the strain profile through the concrete beam specimen and the impact of partial replacement of fine aggregates with FLWA, an existing analytical solution was combined with measured relative humidity data. An existing cement hydration model was modified to incorporate the inclusion of FLWA and the effects of the additional water contributed to the microstructure during external drying. The modifications allowed for the pore-free bulk modulus of each tested concrete mixture to be calculated. With an accurate pore-free bulk modulus, the moisture strain gradients in the tested specimens could be calculated over a 28 day drying period. The analysis revealed that concrete without moist curing benefits greatly from the inclusion of FLWA. When moist curing is applied, the benefits of FLWA decrease with respect to the moisture strain gradients.In order to assess the influence FLWA has on the near surface cracking potential, an air-coupled acoustic emission technique was developed to quantify cracking events occurring during early age hydration couple with external drying. The new experimental method utilizes microelectromechanical sensor (MEMS) technology to passively listen for cracking events. This type of air-coupled acoustic emission test has never been successfully implemented. The MEMS-based AE system was able to record cracking events over an 8 hour period at frequencies greater than 15kHz and sound intensities greater than -53dB for mortar specimens with and without FLWA. The experimental results indicated that the FLWA-modified mixtures had more cracking events than a natural sand or blended mortar. This suggested that the surface strains of a FLWA modified mixture are higher than an unmodified mixture, which was confirmed by the theoretical strain profile calculations. However, these high strain levels for FLWA mixture are only at the near surface and do not continue further into the bulk cross section like the virgin concrete mixtures. Thus, it is hypothesized that more cracking events occur for FLWA mixtures, which are shallower depth, where 100% natural sand mixtures had less cracks which propagated deeper into the specimen.