In this contribution, the microstructure features of cement mortar exposed to various temperatures (105 °C, 200 °C, 400 °C, 600 °C, 800 °C) was investigated by combining Mercury Intrusion Porosimetry ...(MIP) and multi-scale X-ray computed tomography. The influence of exposure temperature and resolution of X-ray CT on the determination of microstructure parameters of heat-treated mortar was focused. Based on results of MIP test, it was found the porosity and pore size increased slightly when the exposure temperature varied from 105 °C to 200 °C and significant pore coarsening and micro-damage occurred once the temperature exceeded 400 °C. Bimodal pore size distribution (PSD) of the heat-treated mortar specimens was observed when the temperature reached 400 °C. To interpret the results of MIP test, the microstructure of heat-damaged mortar specimens was imaged using X-ray CT with a reconstructed voxel size of ~4.0 μm3 and then local volume inside the specimen was focused and scanned with a reconstructed voxel size of 1.5 μm3. A method was proposed to select proper threshold based on the MIP results for segmenting the void space from the X-ray CT images. The fracture aperture, 2D/3D fractal dimension, connectivity and tortuosity of the heat-damaged mortar specimens were further determined at different scale. By analyzing the fracture aperture determined from X-ray CT images, it was found the bimodal PSD revealed by MIP test can be associated with the creation of thermal micro-fractures. The fractal dimension increased remarkably when exposure temperature was raised from 400 °C to 600 °C while it varied slightly from 600 °C to 800 °C. Linear dependences between the fractal dimension and the volume fraction/tortuosity of micro-scale pores and fractures were found. The scale-dependent fractal properties of the heat-treated mortar were revealed with the capillary pressure data measured by MIP. The fractal dimension of micro-scale pores and fractures measured by MIP exhibited good consistency with that determined based on by X-ray CT images with a reconstructed voxel size of 1.5 μm3.
•3D fracture aperture of heat-damaged mortar was presented at multi-scale.•Microstructure parameters measured by MIP and X-ray CT images with different voxel sizes were compared.•The connectivity, tortuosity and fractal features of micro-fractures were analyzed.
Display omitted
•Electrical capacitance volume tomography (ECVT) with the assembled square sensor can monitor the 3D moisture flow inside concrete and cracked mortar.•The response of ECVT to moisture ...is not affected by the coarse aggregates inside the concrete.•ECVT can locate crack and water sources in singled cracked mortar.•The ECVT reconstructions agreed well with the unsaturated water flow simulations.
Moisture and other dissolved ions can easily enter cracked cement-based materials, leading to a series of degenerating processes. Electrical capacitance volume tomography (ECVT) is a powerful approach for detecting and visualizing the three-dimensional (3D) water distribution inside cement-based materials. In this paper, the water ingress in mortar and concrete was monitored and visualized in 3D by ECVT. The feasibility of ECVT was first verified by conducting a moisture transfer experiment inside uncracked mortars. Then, the water ingress process into cracked mortar and concrete was monitored and visualized using this technique, which further confirmed the ability to image 3D volumetric water content in materials with highly heterogeneous permittivity. In addition, the water distribution inside the cracked and uncracked mortars was simulated utilizing two finite element models. The simulated results agree well with the ECVT reconstructed results, supporting the applicability of ECVT to image 3D water transfer in uncracked and cracked cement-based materials.
Service life and durability of concrete structures have become one of the most concerning issues in civil engineering because of their economic and ecological implications. Most durability problems ...for concrete structures can be associated with the presence and migration of water. Capacitance measurements have been proven to be a powerful non-destructive testing technique for investigating water distribution in cement-based materials. Capacitance measurements are very sensitive to moisture because of the high contrast in relative permittivity between water and dried concrete. This review focuses on specific applications of capacitive techniques in studying the durability and degradation of cement-based materials. Examples of results obtained by qualitative and quantitative investigations of moisture content and water migration in such materials are given. Studies on using capacitive techniques for detecting chloride content, rebar corrosion, crack development and carbonation in concrete are discussed. Capacitance measurements are a feasible method for real-time and non-destructive testing of the durability of building materials. Current challenges and potential for further research in capacitive techniques are outlined and discussed.
•Capacitance measurements are very sensitive to moisture in cement-based materials.•Capacitance measurements can detect moisture and chloride content, rebar corrosion, crack development and carbonation.•Calibration procedures are essential to achieve quantitative measurements.•Simultaneous detection of multiple media changes is a worthwhile research direction.
Water repellent treatment is one of the effective means to improve the durability of concrete. This paper aims to investigate the water repellency of cement-based materials treated with silane-based ...water repellent agent under different exposure environments. Five different exposure conditions were applied in this experiment, namely, standard moist curing room, in-door laboratory atmosphere, pre-oven-dried condition, and out-door natural environment with or without shelter. Three different types of mortars with water-to-cement ratios of 0.4, 0.5 and 0.6 were prepared. And three different dosages (200, 400, 600 g/m2) of silane gel were utilized on the surface of specimens. The impregnation depth and water absorption after hydrophobic treatment have been measured. Results indicate that the initial moisture condition of mortar has a significant influence on the efficiency of surface silane impregnation. The depth of silane impregnation gradually decreases with the increase of initial moisture condition. The water repellency of surface impregnation of cement-based materials was well exhibited under the condition of about 50% relative humidity. It illustrated that the moisture within the cement-based materials affects the hydrolysis reaction process and the formation of hydrophobic membrane. Thus, the water repellency of cement-based materials greatly depends on the initial moisture content and the thickness of hydrophobic layer.
The ingress of mass in a harsh chloride environment leads to the corrosion of reinforcing steel bars and deterioration of the structural performance. This paper deals with a two-dimensional (2D) ...numerical model for water and chlorides transport in the cracked-unsaturated concrete on the mesoscale level, in which concrete is modeled as a composite material with impermeable coarse aggregates embedded in the porous matrix which is separated by vulnerable interfacial transition zone (ITZ). Coupled with the unsaturated flow theory and the cubic law of water through a single crack, the transport coefficients for water and chloride ions in a single crack are developed by treating the crack as a parallel-plate configuration. The lattice network developed on the basis of Voronoi tessellation is presented to investigate mass transport process in the cracked-unsaturated concrete subjected to drying-wetting (D/W) cycles. The above proposed transport model coupled with the mesoscale lattice approach is validated by comparison with the available experimental findings from the literature. The numerical results indicated that the computational models are able to well represent the mass (water and chloride) movement within the cracked-unsaturated concrete. Furthermore, the cyclic D/W action and crack width/length within concrete are crucial for the transport properties of unsaturated concrete.
•Mass transport process in cracked-unsaturated concrete was numerically studied.•Mesoscale lattice network model of cracked concrete was developed.•Diffusivities of water and chloride through a single crack were proposed.•Mass distribution depends on the drying-wetting cycles and cyclic mechanism.•Effect of main crack parameters and saturation on mass transport was discussed.
Solid waste building material treatment is an essential means of disposal and an active research area. Despite this, the multisource characteristics, large volumes and complex compositions of solid ...waste necessitate new requirements for its treatment. In this paper, a typical resource-based coastal city is considered the research object. Fly ash (FA), furnace slag (FS) and mineral powder (MP) are taken as samples to investigate the influence of admixtures on the properties, pore structure and durability of foam concrete by three-factor and three-level orthogonal tests. Results indicate that foam concrete with different admixtures can have various mechanical and thermal insulation properties. The compressive strength of foam concrete can increase to 3.90 MPa, and its thermal conductivity can reach 0.1347 W/M·k. MP has the largest apparent effect on the compressive strength, while FS has the most obvious impact on the thermal conductivity. The durability analysis shows that the number of freeze-thaw cycles foam concrete can withstand is 25–100 cycles. The collaborative disposal of multisource inorganic solid waste has high potential for being applied as inner partition walls and thermal insulating components in buildings. This paper provides valuable guidance for the sustainable and clean utilization of multisource inorganic solid waste.
AbstractThe synergetic effects of the environmental and loading actions on mass transport process in concrete play an important role for the deterioration of mechanical properties and durability ...problems of reinforced concrete (RC) structures. Water is either the main agent responsible for the deterioration of concrete or the principal transport medium carrying aggressive species to ingress into concrete. This paper presents the results of an experimental investigation on the effect of short-term sustained uniaxial compressive or tensile loadings on water absorption by hollow cylindrical concrete specimens. An improved test apparatus aimed for the coupled effect of sustained loading on water absorption was designed to measure in real time the amount of water absorbed by the concrete specimen. The cumulative water content and axial/lateral strain of specimens under various stress levels (approximately 0–50%) were recorded. It was noted that under the short-term sustained tensile loading, the cumulative water content at a given exposed time generally increases with an increase of the stress level, while for the compressive case it initially decreases and then increases. The initial and secondary sorptivities, which are defined for characterizing the tendency of concrete to absorb water by the capillarity versus the compressive stress level, conformed to the polynomial function, except for the tensile loading case. which exhibited a linear relation. The ratio between initial and secondary sorptivity, as well as the quantitative correlations between the two sorptivities and stress level were proposed to account for the sustained loading effect on water absorption of concrete. Based on the unsaturated flow theory (UFT) model and experimental data, two forms of diffusivity, i.e., the exponential law and power functions, were both adopted to predict water distribution within a concrete sample under compressive/tensile stress states after 32-h absorption. The results indicated that the predicted water penetration depths by means of both exponential law and power functions in the UFT model increases with the increase of sustained compressive/tensile stress levels. The validation of the proposed model shows good correlation between the predicted water zpenetration depth and the test data.
This paper presents a comprehensive investigation of the bond characteristics of steel bar reinforced geopolymer concrete (GPC). The ASTM A944 beam end tests were conducted on GPC beams reinforced ...with plain or ribbed bars. The bond-slip curves and the bond strength of GPC beams were obtained. The relationship between the bond stress and relative slip in plain and ribbed bar reinforced GPC has been represented by empirical formulae. The bond testing results were compared with those of corresponding ordinary Portland cement concrete (OPC) using statistical hypothesis tests. The results of hypothesis testing showed that GPC was significantly superior to OPC in terms of bond capability with plain bars and bond stiffness with ribbed bars. The statistical analysis indicated that the bond-slip relations derived for OPC are inapplicable to GPC; thus, new bond-slip relations are suggested to estimate the development of bond stress and relative slip between GPC and steel bars.
The purpose of this study is to explore the possibility of gold tailings as an alternative to natural river sand for preparing concrete, to solve the problems of rapid increase in gold tailings and ...insufficient natural river sand stock. Additionally, gold tailings upgraded by centrifugal separation are compared with untreated gold tailings. The performances and modification mechanisms of gold tailings on the mechanical strength and durability performance of concrete were investigated. The results demonstrate that concrete composed of gold tailings exhibits commendable mechanical and durability properties, with the optimal replacement level of natural river sand being 30%. Replacing river sand with gold tailings enhances the compressive and split tensile strengths of concrete, reduces water absorption, and improves its resistance to freeze-thaw cycles and chloride ion permeability, primarily owing to the pozzolanic activity and filling effect of the gold tailings. The upgrading of gold tailings not only changed altered the particle size but also changed the active SiO2 and Al2O3 content, thus, further enhancing the mechanical and durability of concrete. The study establishes that utilizing gold tailings in construction can serve as a viable substitute for river sand, thereby mitigating potential environmental repercussions associated with gold tailings. The findings hold promise for diminishing the reliance on river sand, thereby markedly enhancing resource sustainability.
Alkali-activated slag concrete (ASC) is regarded as one of the most promising sustainable construction materials for replacing ordinary Portland cement concrete (OPC) due to its comparable strength ...and outstanding durability in challenging environments. In this study, the corrosion of steel bars embedded in ASC and OPC was studied by means of an electrically accelerated corrosion test of steel bars in concrete. Meanwhile, the bond performance of the corroded steel bars embedded in ASC was tested and compared with corresponding OPC groups. The results showed that ASC and OPC behaved differently in terms of bond deterioration. The high chemical resistance of ASC decreased the corrosion of steel bars and, thus, increased the residue bond strength and the bond stiffness.
PDF
