Search results

Rebar corrosion in reinforced concrete is the major reason for the durability degradation, especially under harsh environment. This paper presents an experiment conducted to investigate the influence of freeze-thaw cycles on the rebar corrosion in reinforced concrete. The purpose of this paper is to provide fundamental information about rebar corrosion under frost environment and improvement measures.

The related elastic modulus and compressive strength of different concrete specimens were measured after different freeze-thaw cycles. The accelerated rebar corrosion test was carried out after different freeze-thaw cycles; additionally, the value of calomel half-cell potential was determined. The actual rebar corrosion appearance was checked to prove the accuracy of the results of calomel half-cell potential.

The results show that frost damage aggravates the rebar corrosion rate and degree under freeze-thaw environment; furthermore, the results become more obvious with the freeze-thaw cycles increasing. Mixing the air-entrained agent into fresh concrete to prepare air-entrained concrete, increasing the cover thickness and processing the surface of concrete with a waterproofing agent can significantly improve the resistance to rebar corrosion. From the actual appearance of rebar corrosion, the results of calomel half-cell potential can well reflect the actual rebar corrosion in reinforced concrete.

The durability of reinforced concrete is mainly determined on chloride penetration that brings about rebar corrosion in chloride environments. Furthermore, the degradation of concrete durability becomes more serious in the harsh environment. As the concrete exposure to the freeze-thaw cycles environment, the freeze-thaw cycles accelerate the concrete damage, and the penetration of chloride into the concrete becomes easier because of the growing pore and crack sizes. In addition, rebar corrosion caused by chloride is one of the major forms of environmental attack on reinforced concrete. The tests conducted in this paper will describe the rebar corrosion in reinforced concrete under freeze-thaw environment.

This study purposes to study the influence of artificial freezing on the liquefaction characteristics of Nanjing sand, as well as its mechanism.

was studied through dynamic triaxial tests by means of the GDS dynamic triaxial system on Nanjing sand extensively discovered in the middle and lower reaches of the Yangtze River under seismic load and metro train vibration load, respectively, and potential hazards of the two loads to the freezing construction of Nanjing sand were also identified in the tests.

The results show that under both seismic load and metro train vibration load, freeze-thaw cycles will significantly reduce the stiffness and liquefaction resistance of Nanjing sand, especially in the first freeze-thaw cycle; the more freeze-thaw cycles, the worse structural behaviors of silty-fine sand, and the easier to liquefy; freeze-thaw cycles will increase the sensitivity of Nanjing sand's dynamic pore pressure to dynamic load response; the lower the freezing temperature and the effective confining pressure, the worse the liquefaction resistance of Nanjing sand after freeze-thaw cycles; compared to the metro train vibration load, the seismic load in Nanjing is potentially less dangerous to freezing construction of Nanjing sand.

The research results are helpful to the construction of the artificial ground freezing of the subway crossing passage in the lower reaches of the Yangtze River and to ensure the construction safety of the subway tunnel and its crossing passage.

Polyurethane concrete has a high strength-to-weight ratio in the short term, and the strength-to-weight ratio stage during the maintenance period is critical. Freeze-thaw cycles have a noticeable damaging effect on the durability of polyurethane concrete. The engineering specification of polyurethane concrete with incomplete hydration reaction must be studied, as well as the development of internal structure during curing. In this paper, the polyurethane concrete tests were set up under eight distinct maintenance settings based on the climate features of the northern area and the service environment. The test results were evaluated to determine the effect of the number of early freeze-thaw cycles and the time node of early freeze-thaw cycles on the mechanical characteristics of polyurethane concrete, which revealed that the time node of freeze-thaw damage impacted the freeze-thaw resistance of polyurethane concrete susceptible to early freeze-thaw damage.

The early-age freeze-thaw damage polyurethane concrete was experimentally studied by controlling the time node of the freeze-thaw cycle and the curing environment. The test considered the time node, frequency of freeze-thaw damage of polyurethane concrete and the influence of subsequent curing environment and observed the mass change, relative dynamic elastic modulus, relative durability index, compressive strength and apparent damage of polyurethane concrete. The early mechanical properties of polyurethane concrete were studied by analyzing the change of numerical value. The microscopic mechanism of strength formation of polyurethane concrete was analyzed by XRD, FTIR and SEM image.

The closer the time of freeze-thaw damage was to the specimen hardening, the worse the mechanical properties and structure were, according to SEM photographs. For specimens with serial number of 12-groups, its compressive strength is only 82.39% of that of the standard group, even if the curing process continues after 20 times thawing, which increased early environment exacerbate strength loss in polyurethane concrete and also reduced freeze-thaw resistance. The findings of the tests reveal that curing can restore the freeze-thaw resistance of damaged polyurethane concrete. Curing in water has a better recovery impact than curing in air; the mechanical properties can be restored by sufficient re-curing time and good re-curing conditions.

By studying the freeze-thaw cycle test and test results of polyurethane concrete in different curing time nodes, the relationship between the mechanical properties of polyurethane concrete and the time node, number of freeze-thaw cycles, and subsequent maintenance environment was explored. Considering the special mechanism of strength formation of polyurethane concrete, the polyurethane concrete damaged by freeze-thaw has the ability to continue to form strength under subsequent maintenance. This experimental study can provide an analytical basis for the strength formation and reconditioning of polyurethane concrete structures subjected to freeze-thaw environments during the curing time under extreme natural conditions in fall and winter in actual projects.

In the freeze-thaw zone, the pre-stressed concrete of bridge structure will be damaged by freezing-thawing, the bearing capacity of structure will decrease and the safety will be affected. The purpose of this paper is to establish the time-dependent resistance degradation model of structure in the freeze-thaw zone, and analysis the structural reliability and remaining service life in different freeze-thaw zones.

First, according to the theory of structural design, a calculation model of the resistance of pre-stressed concrete structures in f freeze-thaw zone is established. Second, the time-dependent resistance model was verified by the test beam bending failure test results done by the research group, which has been in service for 20 years in freeze-thaw zone. Third, using JC algorithm in MATLAB to calculate the index on the reliability of pre-stressed concrete structure in frozen thawed zones, forecasting the s remaining service life of structure.

First, the calculation model of the resistance of pre-stressed concrete structures in freeze-thaw zone is accurate and it has excellent applicability. Second, the structural resistance deterioration time in Wet-Warm-Frozen Zone is the earliest. Third, once the pre-stressed reinforcement rusts, the structural reliability index will reach limit value quickly. Finally, the remaining service life of structure meets the designed expectation value only in a few of freeze-thaw zones in China.

The research will provide a reference for the design on the durability of a pre-stressed concrete structure in the freeze-thaw zone. In order to verify the security of pre-stressed concrete structures in the freeze-thaw zone, engineers can use the model presented in this paper for durability checking, it has an important significance.

Epoxy resin emulsions are used in water‐based coatings for surface protection of concrete and metal. An unfortunate drawback for most emulsions is their poor freeze‐thaw stability. Epoxy emulsions are indeed unstable below 0°C, ‐5°C or ‐10°C, depending on the type of resin. In this study, other factors capable of influencing the freeze‐thaw behaviour were investigated: e.g. solids content, amount and type of emulsifiers, solvents, protective colloids. Discusses methods for testing the quality of thawed emulsions as well as the physics involved in their destabilization. Freeze‐thaw performance can be improved by different means but mostly not without sacrificing resistance properties of the coating. Therefore, avoiding temperatures below 0°C is still the best advice.

The purpose of this paper is to develop a method for predicting the chloride ingress into concrete structures, with an emphasis on the low temperature range where freeze-thaw cycles may cause damage.

The different phenomena that contribute to the rate and amount of transported chlorides into concrete, i.e., heat transfer, moisture transport and chloride diffusion are modeled using a two-dimensional nonlinear time dependent finite element method. In modeling the chloride transport, a modified version of Fick’s second law is used, in which processes of diffusion and convection due to water movement are taken into account. Besides, the effect of freeze-thaw cycles is directly incorporated in the governing equation and linked to temperature variation using a coupling term that is determined in this study. The proposed finite element model and its associated program are capable of handling pertinent material nonlinearities and variable boundary conditions that simulate real exposure situations.

The numerical performance of the model was examined through few examples to investigate its ability to simulate chloride penetration under freeze-thaw cycles and its sensitivity to factors controlling freeze-thaw damage. It was also proved that yearly temperature variation models to be used in service life assessment should take into account its cyclic nature to obtain realistic predictions.

The model proved promising and suitable for chloride penetration in cold climates.

From recent laboratory research monofilament and fibrillated polypropylene fibres were used in structural concrete and have been tested against 150 freeze/thaw cycles. The findings show monofilament fibres to play a significant role in protecting the concrete matrix against the forces encountered. External cube integrity was shown to be a poor indicator of structural condition. A significant aspect of the work is the range of tests applied to the freeze/thaw concrete cubes against the control sample. Strong evidence of condition was obtained from ultrasonic, compressive strength and weight loss. Surface scaling was not a satisfactory indication of the structural condition of the concrete.

In order to unravel the evolution of microstructure characteristics and the change of mechanical properties of bituminous mixture in the freezing and thawing environment in cold region, this study starts from macroscopic experiments and analyzes the changes in mechanical properties of asphalt mixtures before and after freezing and thawing in detail. On this basis, the displacement of key particles in the structure of asphalt mixture under the action of external forces (before and after freezing and thawing) is simulated through the combination macroscopic and microscopic methods.

The climate in China exhibits high complexity and diversity, divided into five zones based on the temperature difference from south to north. Considering that the significant effect of geography and natural climate on the design, construction and maintenance of asphalt pavement, the criterion for the road construction at different areas should be highly different.

The results show that the mechanical properties of asphalt mixture greatly decrease due to the influence of freeze-thaw, and the displacement of key particles in the structure of asphalt mixture (several representative particle sizes were selected through experiments) is obviously observed because of the action of external force. By analyzing the variation of several key particle sizes after freezing-thawing cycle, the gradation standard of asphalt mixture aggregate suitable for cold area was obtained. The research results have certain theoretical and practical value for the design and application of asphalt mixture in cold area.

The results show that the mechanical properties of asphalt mixture greatly decrease due to the influence of freeze-thaw, and the displacement of key particles in the structure of asphalt mixture (several representative particle sizes were selected through experiments) is obviously observed because of the action of external force. By analyzing the variation of several key particle sizes after freezing-thawing cycle, the gradation standard of asphalt mixture aggregate suitable for cold area was obtained. The research results have certain theoretical and practical value for the design and application of asphalt mixture in cold area.

The purpose of this paper is to study the effects of water immersion‐freeze‐thaw treatment on the physical properties, flexural strength (FS) and morphology of wood‐polypropylene composites containing pigments.

Wood‐polypropylene composites containing brown, green and grey pigments were compounded in a conical twin‐screw extruder. A composite manufactured without any pigment addition was used as a reference. The amount of pelletized wood, polypropylene and coupling agent (MAPP) was kept constant. The moisture content, thickness swelling (TS), FS and surface colour of the composites were measured before and after water immersion‐freeze‐thaw cycling. Scanning electron microscopy (SEM) was used to study the morphology of the composites.

FS and dimensional stability were reduced after exposure to water immersion‐freeze‐thaw cycling for all composites. The surface properties (colour and roughness) of the composites also changed after exposure to water immersion‐freeze‐thaw cycling. The degree of change depended on the presence of pigment and the type of polypropylene (neat or recycled), however.

This study is a part of an ongoing study on weathering of wood‐polymer composites (WPC) containing different additives. The results of this study were obtained from accelerated laboratory experiments.

Inorganic pigments are widely used as additives in plastics, because they have an excellent UV absorption, good IR‐reflective properties and heat stability. The research revealed that metal‐containing pigments had an effect on degradation in quality of wood‐polypropylene composites exposed to water immersion‐freeze‐thaw cyclic treatment. The addition of metal‐containing pigments to composite formulation resulted in a higher susceptibility of wood‐polypropylene composites to water absorption, and as a consequence to a higher drop of FS compared to composites made without pigment. The polymer matrix plays an important role in the protection of WPC against weathering.

This paper will help in understanding possible problems in the durability of wood‐polypropylene composites compounded with metal‐based pigments when they are exposed to water immersion‐freeze‐thaw cyclic treatment.

To investigate the effects of moisture and freeze‐thaw cycling on the absorption and flexural properties of rice‐hull‐polyethylene (PE) composite.

Various rice‐hull‐PE composite specimens were submerged in water at various temperatures and subjected to various freeze‐thaw cycles. Various characterisations including water absorption, bending strength and stiffness, Fourier transform infrared spectroscopy and scanning electron microscope imaging were performed.

High temperatures accelerated the water sorption of the rice‐hull‐PE composite and increased the equilibrium moisture content. The uncoated surface was not significantly more easily permeated than the coated surface, contrary to expectations. However, more water was absorbed from the cut surface than from the original extruded surface. This was attributed to the tiny checks left on the surface by the sawing action, which indicated the importance of protecting the original surface layer from scraping or other damage. Bending strength and stiffness of the rice‐hull‐PE composite decreased significantly after the freeze‐thaw cycling treatment. The modulus of elasticity decreased more than the modulus of rupture. Compared to the effect of water immersion alone, freeze‐thaw cycling treatment slightly accelerated this decrease.

The results of this study were obtained from accelerated laboratory experiments. Further research could be carried out to evaluate the properties of this rice‐hull‐PE composite in practical application.

The research revealed a possible degradation in quality when the rice‐hull‐PE composite is used in moist or freezing conditions. The resin layer on the extruded surface provides an important protection.

In China, rice‐hull powder is widely used as a reinforcing component in plastic composite. However, the durability of rice‐hull/PE composites has rarely been investigated. Results from this study will help users apply rice‐hull‐PE composites correctly and encourage the development of other agro‐fibre/polymer materials.