Moisture curling in concrete with fine lightweight aggregates

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

Design Methodology for Partial Volumes of Internal Curing Water Based on the Reduction of Autogenous Shrinkage

AbstractHistorically, the use of internal curing began with a low water-to-cement ratio (w/c) and high-strength concrete. More recently, the benefits of reduced autogenous shrinkage and improved hydration have been recognized over a wider range of mixtures. In North America, the internally cured mixtures are typically made using prewetted fine lightweight aggregates (FLWA). The volume of FLWA used in an internally cured concrete is usually determined on the principle that the FLWA provides a volume of internal curing water that is equivalent to the volume of chemical shrinkage. This study examines whether it is possible to reduce the volume of the FLWA while still achieving the benefits of internal curing with respect to the reduced autogenous shrinkage and increased relative humidity. An alternative mixture methodology is presented that utilizes the pore-size distribution of the cementitious paste to calculate the amount of internal curing water needed to maintain a specific relative humidity (that is, to provide water sufficient to keep pores of a certain size filled) in the mixture. The proposed approach can be used to calculate the volume of water required to fill the pores of a specific size that empty, so that the effects of self-desiccation are minimized to an acceptable level. This approach maintains a high relative humidity to reduce the autogenous shrinkage and increase the early-age hydration. Although many applications such as bridge decks are still designed with the more conventional design approach, it is hypothesized that this assumption generally overestimates the amount of water needed to effectively reduce the autogenous shrinkage of a concrete mixture. Other applications where larger volumes of materials are involved, such as pavements, may benefit from a design approach that optimizes the volumes of FLWA, due to the reduction in the raw materials required, which has benefits from the perspectives of both the cost and the staging

Flexural Capacity of Rigid Pavement Concrete Slabs with Recycled Aggregates

Few studies have focused on the effect of recycled materials on the concrete slab load capacity. This study used virgin and recycled aggregates—fractionated reclaimed asphalt pavement (FRAP) and recycled concrete aggregate (RCA)—and by-product cementitious materials—ground granulated blast furnace slag and fly ash—to cast and test the load capacity of single- and two-lift concrete slabs. Five concrete mixtures were examined, which were virgin aggregate (the control) and four different replacements of coarse aggregate: 45% FRAP, 45% FRAP with macrofibers, 100% RCA, and a blend of 45% FRAP and 55% RCA. For all laboratory specimens tested, the virgin aggregate concrete had the highest strength (compression, split tension, and flexural) and modulus of elasticity, and the mix with 45% FRAP and fibers resulted in the lowest properties, which was attributed to the relatively high air content of the fresh concrete. With the exception of the mix with 45% FRAP and fibers, the critical stress intensity factor and initial and total fracture energies of the recycled aggregate concretes were not statistically different than the virgin aggregate concrete using the single edge notched beam specimen. For the 16 large-scale concrete slabs (6 ft x 6 ft x 6 in. thick) cast using both single and two-lift designs, the results indicated that the concrete slab load-carrying capacity is significantly underpredicted by the beam flexural strength measurements, and that concrete with recycled aggregates had similar flexural load capacity to the virgin concrete slabs. Using 2D finite element analysis, the ratio between slab flexural capacity and beam strength was found to be significantly higher for the recycled aggregate concrete relative to the virgin aggregate concrete. The concrete slab flexural load capacity was governed more by the concrete fracture properties and slab geometry then by conventional strength criteria. Two additional topics presented in the appendices are a literature review and evaluation of recycled washout water (grey water) from concrete batch plants as mixing water in fresh concrete. Based on the literature, grey water should be suitable for use in fresh concrete, provided that the solids content is not excessive and ASTM C1602 guidelines are followed. A laboratory study evaluating the use of recycled tire cord steel as fiber reinforcement in concrete revealed that the fibers could be beneficial for improving the concrete toughness, but a cost-benefit analysis is required.Illinois State Toll Highway Authoritypublished or submitted for publicationnot peer reviewe

Activation Energy of Conduction for Use in Temperature Corrections on Electrical Measurements of Concrete

The formation factor obtained through electrical resistivity measurements is becoming a popular method to determine transport properties of concrete. Resistivity measurements are dependent on multiple factors, including degree of saturation, pore solution conductivity, and temperature. The Arrhenius equation is used to correct electrical resistivity for temperature effects using an activation energy of conduction (Ea-cond). This parameter has been measured on a wide variety of materials, including pore solutions, pastes, mortars, and concretes (with a variety of saturation states). The reported values of Ea-cond typically range from 9 to 39 kJ/mol. This article examines the factors affecting Ea-cond in order to select an appropriate temperature correction. In this study, Ea-cond was determined from data measured on various concrete mixtures used in transportation infrastructure applications as well as extracted and simulated pore solutions. The Ea-cond of pore solutions remains relatively constant (an average value of 13.9 ± 1.5 kJ/mol) for typical pore solutions and was slightly lower than the Ea-cond of saturated specimens (an average value of 15.8 kJ/mol). It was found that Ea-cond increases as the degree of saturation of the specimen is reduced. Drying increases the ionic concentration of the fluid in the pores; however, this does not explain the changes in Ea-cond. The effects of drying were determined to be primarily due to a change in the volume of the conductive fluid film in the concrete and in the connectivity of the fluid-filled pores. While it is better to directly measure the Ea-cond of a concrete mixture, this is not always feasible or practical. In such cases, for pore solutions, a value of 13.9 kJ/mol can be used, and for saturated concretes, a value of 15.8 kJ/mol can be used. For concretes with a varying degree of saturation, the Ea-cond can be estimated using the developed equation