Vacuum environment engineering has attracted significant attention due to its revolutionary applications in ultra-high-speed transportation, space technology, and frontier physics experiments. Concrete, as the most widely used construction material globally, remains an ideal choice for constructing ultra-high-speed vacuum pipelines and space bases. However, under harsh vacuum conditions, concrete faces severe challenges such as rapid dehydration, increased brittleness, drying shrinkage cracking, and reduced durability. To effectively enhance the vacuum service performance of concrete, this study designs a type of polymer-fiber modified concrete that leverages the film-forming toughening effects of polymers and the high strength and high elastic modulus characteristics of fibers to effectively mitigate the brittle behavior of concrete when exposed to vacuum. Utilizing acoustic emission techniques, the study examines how vacuum conditions, polymers, and fibers affect the mechanical properties of the modified concrete, further elucidating the underlying mechanisms with scanning electron microscopy (SEM) imaging.

Based on the principles of dense particle packing and gradational hydration, a mix design was developed for the binder system (including cement, fly ash, slag, and silica fume). Redispersible latex powder (RDP) and styrene-butadiene latex-modified emulsified asphalt (MEA) were selected as the polymer modification materials, while ultra-high molecular weight polyethylene fibers (PEF) and carbon fibers (CF) were chosen as the fiber reinforcement materials. To balance the strength and toughness of the concrete, the polymer dosage was set at 3.5% by mass of the binder, and the fiber volume fraction at 0.8%. A polycarboxylate-based superplasticizer was used at 1.2% by mass of the binder to ensure the workability of the fresh paste. The concrete was mixed using a dual-axis horizontal mixer. Initially, the coarse and fine aggregates and binder were dry-mixed for 1 min, then water and superplasticizer were added and mixed for 1 min. Finally, the polymer and fibers were incorporated and mixed for an additional 2-3 min to produce a fresh paste. The specimens were cured under standard conditions for 28 days before being exposed to either atmospheric exposure (AE) or vacuum exposure (VE) conditions.

AE was conducted in an ambient pressure room maintained at a temperature of (20±2) ℃ and a relative humidity of 55%-65%. VE was simulated using a homemade vacuum chamber with a vacuum pressure of approximately 100 Pa, a relative humidity of <10%, and a temperature essentially identical to that of the AE environment. In accordance with GB/T 50081-2019 "Standard Test Methods for the Physical and Mechanical Properties of Concrete," uniaxial compression tests and four-point bending tests were performed on specimens before and after exposure. Extensometers were used to record the axial and mid-span displacements of the specimens, and the corresponding stress-strain curves and load-displacement curves were plotted. During the mechanical performance tests, acoustic emission monitoring was conducted simultaneously. The mechanical damage characteristics of the concrete specimens were analyzed based on the acoustic emission signal intensity, b-value, and RA-AF plots. Finally, SEM was employed to observe the microstructure of the samples, revealing the degradation mechanism of concrete under vacuum exposure as well as the modification mechanism of the polymer-fiber combination.

VE is beneficial to the compressive strength of the concrete but detrimental to its bending strength. Compared with SC, the concrete in the control group under VE shows a 3.3% increase in compressive strength and a 22.8% decrease in flexural strength. For the polymer-fiber modified concrete, the compressive strength increased by 6%-31%, whereas the flexural strength decreased by 9%-16%. The modified concrete also exhibited significantly improved compression and flexural toughness under VE conditions. Notably, the combination of RDP and PEF yielded the greatest enhancement in mechanical properties, increasing compressive energy absorption capacity over two-fold and flexural energy absorption capacity more than ten-fold compared to the control group. Acoustic emission test results indicate that VE can reduce the damage-warning time by approximately 40%. The RA-AF correlation features show that the proportion of shear cracks increases significantly during the failure stage, which is attributed to intensified fiber-matrix interfacial debonding and the propagation of microcracks in the interfacial transition zone (ITZ). SEM images revealed that vacuum exposure promotes RDP/MEA film formation, creating an interpenetrating network of rigid cement paste and flexible film. This film bonds strongly with hydration products, reducing internal stresses and preventing microcracks. Additionally, PEF enhances post-peak toughness through its three-dimensional network and sliding energy dissipation, while the low fracture elongation and strong matrix adhesion of CF lead to brittle pull-out, thereby reducing concrete toughness.

The main conclusions of this paper are summarized as follows. The compressive and flexural toughness of the control group concrete under VE is significantly reduced, while modification with polymers and fibers can effectively avert this issue. Vacuum exposure enhances the film-forming effect of the polymer, effectively bridging hydration products and forming a high-toughness three-dimensional network structure together with the fibers, thereby inhibiting microcrack propagation. In particular, the combination of RDP and PEF exerts the optimal synergistic toughening effect, allowing the specimens to better retain their post-peak toughness. The synergistic mechanism of "film toughening-sliding energy dissipation" effectively reduces the brittleness of the concrete and improves its performance under vacuum conditions. These findings provide practical references for the optimized design of the new ultra-high-speed vacuum pipeline maglev transportation system as well as materials for deep space engineering.