Investigation of Air-void Structure Characteristics and Bubble Evolution Mechanisms in Concrete under Low Atmospheric Pressure

低氣壓下混凝土氣孔結構特徵及氣泡演變機制研究

Student thesis: Doctoral Thesis

View graph of relations

Author(s)

Related Research Unit(s)

Detail(s)

Awarding Institution
Supervisors/Advisors
  • Ruiqin ZHANG (Supervisor)
  • Xiaohui ZENG (External person) (External Supervisor)
Award date27 Dec 2023

Abstract

With the promotion of national strategies such as "Belt and Road" and "Transportation Power", the construction of infrastructures such as railways and airports have been on the rise in high-altitude regions in western China in recent years. However, in the low atmospheric pressure environment at high-altitude regions, the freeze-thaw damage of concrete has intensified, severely reducing the service life of concrete. Air-entraining agents can improve the freeze-thaw resistance of concrete by introducing many tiny and evenly distributed bubbles. However, in low atmospheric pressure, abnormal air-void structures such as decreased air content and larger bubble sizes occur in concrete. Clarifying the air-void structure characteristics of concrete and the stability mechanism of bubbles in low atmospheric pressure is crucial for improving the freeze-thaw resistance of concrete in high-altitude regions. Currently, the research on the influence of low atmospheric pressure on the stability of bubbles in concrete is still in its early stages, and there is a large difference in the results of existing literature, with no unified conclusion.

To clarify the characteristics of the air-void structure of concrete and the mechanism of bubble evolution at low atmospheric pressure, this thesis firstly conducted on-site experiments on concrete in different altitudes and studied the influence of low atmospheric pressure on the air-void structure characteristics and the microscopic morphology of concrete; secondly, used the high-precision automatic interfacial tension meter to study the impact of low atmospheric pressure on the surface tension of pore solutions of cement pastes; thirdly, established the bubble dynamic evolution equation of cement paste, and analyzed the influence of low atmospheric pressure on bubble parameters, bubble deformation, and bubble instability; then, revealed the instability mechanism of bubbles at low atmospheric pressure; finally, propose a technology to improve bubble stability of concrete in low atmospheric pressure based on the nano-bubble water and nano-silica. The main innovative achievements obtained from this thesis are as follows:

(1) It has confirmed that in high-altitude regions, the air content of concrete decreases, the diameter of bubbles increases, and the bubble shell becomes thinner under low atmospheric pressure. Compared with standard atmospheric pressure (0.1 MPa), the air content of fresh concrete and hardened concrete under low atmospheric pressure decreased by 13.8%~41.3% and 20%~53%, respectively; the spacing coefficient of bubbles and the average air-void diameter increased by 11.6%~37.4% and 14.6%~30.3%, respectively; the proportion of air voids in the range of 400~2500μm increased by 7%~21%; and the bubble shell became thinner and more fragile.

(2) It has clarified that low atmospheric pressure (0~0.1 MPa) only has a minimal effect on the surface tension of cement paste pore solutions. Surface tension is crucial for bubble stability, but there is still controversy over whether low atmospheric pressure affects the surface tension. This thesis proves that low atmospheric pressure has a minimal effect on the surface tension of cement paste pore solutions, with an impact of less than 2%. Therefore, surface tension is not a significant factor that causes a decrease in bubble stability at low atmospheric pressure in high-altitude regions.

(3) It has revealed the mechanism of bubble instability under low atmospheric pressure (0~0.1 MPa). This thesis has established the bubble dynamic evolution equation in cement paste and identified the key factors that affect bubble deformation. The impact of low atmospheric pressure and initial radius on bubble parameters, deformation, and instability are analyzed. When standard atmospheric pressure P is reduced to low atmospheric pressure 0.2P, the "restoring force" that hinders bubble expansion decreases by 50.0%~57.7%, the time required to prevent bubble expansion increases by 94%~137%, the increasement of bubble radius increases by 122%~140%, and the bubble is more prone to instability and rupture. Finally, the key to improving bubble stability in low atmospheric pressure is put forward: reducing the initial radius of bubbles and increasing the strength of the bubble film.

(4) It has revealed the mechanism of improving bubble stability in cement-based materials based on nanobubble water (NBW). This thesis clarified the evolution of bubbles in air-entraining agent solutions at low atmospheric pressure, the average size of bubbles in the solution is larger than that at standard atmospheric pressure, and that the bubbles evolve faster over time at low atmospheric pressure. NBW reduces the average size of bubbles by 24.5% and 48.8% at standard atmospheric pressure and low atmospheric pressure, respectively, and significantly slows down the evolution of bubbles over time. This thesis has revealed the mechanism of NBW in reducing the initial radius of bubbles and promoting the nucleation of C-S-H gel.

(5) It has proposed a technology to improve the bubble stability of concrete at low atmospheric pressure based on nanobubble water (NBW) and nano-silica (NS). The air-void parameters of hardened cement paste samples at low atmospheric pressure are abnormal, and there is a significant mass loss after freeze-thaw cycles. By using NBW or adding 0.5wt.% NS to reduce the initial radius of the bubbles and enhance the strength of the bubble film, the stability of the bubbles can be improved, the air-void distribution of the hardened paste can be optimized, and the freeze-thaw resistance of the samples can be improved.

In summary, this thesis has clarified the air-void structure characteristics of concrete at low atmospheric pressure environments in high-altitude regions; established the bubble dynamic evolution equation of cement paste; revealed the mechanism of bubble instability at low atmospheric pressure; and proposed a technology to improve the bubble stability of concrete in low at low atmospheric pressure environments based on NBW and NS. This thesis provides a theoretical basis and technical support for the freeze-thaw resistance design of concrete infrastructure in high-altitude regions.

    Research areas

  • Low atmospheric pressure, Concrete, Air-void structure, Bubble stability, Nano-bubble water, Nano-silica