The pore structure is one of the major factors affecting the mechanical properties of waste fiber recycled concrete. In this article, the pore structure and strength performance of waste fiber recycled concrete are experimentally studied. The design variables are water–cement ratio, recycled aggregate replacement rate, waste fiber length, and volume fraction of waste fibers. The pore structure characteristic parameters of waste fiber recycled concrete are investigated using mercury intrusion porosimetry test and fractal theory. The complex distribution of pore structure in space is quantitatively described by fractal dimension, and the pore structure is comprehensively evaluated. The results show that the water–cement ratio has the largest influence on the pore structure, and the fiber length has the least influence. The optimum volume fraction of waste fibers is 0.12%. There is an obvious linear relationship between the pore volume fractal dimension and strength. With the increase in the fractal dimensions, the compressive and splitting tensile strengths increase. Macroscopic mechanical properties of waste fiber recycled concrete can be predicted by the pore structure.

This paper aims to study the effectiveness of adding waste polypropylene fibers into recycled aggregate concrete (RAC) on shrinkage cracking. The influences of fiber properties (length and content) on the shrinkage performance of RAC are investigated. Firstly, through the plat-ring-type shrinkage test and free shrinkage test, both of the early age and long-term shrinkage performance of waste fiber recycled concrete (WFRC) were measured. Then, X-ray industrial computed tomography (ICT) was carried out to reflect the internal porosity changes of RAC with different lengths and contents of fibers. Furthermore, the compressive strength and flexural strength tests are conducted to evaluate the mechanical performance. The test results indicated that the addition of waste fibers played an important role in improving the crack resistance performance of the investigated RAC specimens as well as controlling their shrinkage behaviour. The initial cracking time, amount and width of cracks and shrinkage rate of fiber-reinforced specimens were better than those of the non-fiber-reinforced specimen. The addition of waste fibers at a small volume fraction in recycled concrete had not obviously changed the porosity, but it changed the law of pore size distribution. Meanwhile, the addition of waste fibers had no significant effect on the compressive strength of RAC, but it enhanced the flexural strength by 43%. The specimens reinforced by 19-mm and 0.12% (volume fraction) waste fibers had the optimal performance of cracking resistance.

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This study focuses on the relationship between the complexity of pore structure and capillary water absorption of concrete, as well as the connection behavior of concrete in specific directions. In this paper, the water absorption of concrete with different binders was tested during the curing process, and the pore structure of concrete was investigated by mercury intrusion porosimetry (MIP). The results show that the water absorption of concrete with mineral admixtures is lower, mainly due to the existence of reasonable pore structure. The effect of slag on concrete modification is more remarkable comparing with fly ash. In addition, the analysis shows that the pore with different diameters has different fractal characteristics. The connectivity probability and water absorption of unidirectional chaotic pore are linearly correlated with the pore diameter of 50–550 nm, and the correlation coefficient reaches a very significant level, and detailed analysis was undertaken to interpret these results based on fractal theory.

Vegetation growing recycled concrete (VGRC) is a relatively new building material that has both biocompatibility and engineering function. The basic performance of VGRC was investigated by experimental analysis, and the hydration products and pore structure of different VGRC mix proportions were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), and industrial computed tomography (CT). The results show that ultrafine slag can reduce Ca(OH)2 content in cementing material and has a filling effect on micropores. VGRC has the best performance; the internal pore distribution is uniform when porosity is 20–25%, and the ultrafine slag content is 40%. The compressive strength of VGRC is greatly damaged by the quick-freezing method, while the degree of damage from natural freeze–thaw cycles is relatively small. Soaking in acid solution can effectively reduce the internal pore alkalinity of VGRC. Most plants can grow normally in vegetation concrete, and plant roots can penetrate 6-cm thick concrete blocks after being planted for 60 days. The compressive strength of VGRC decreased after turf planting of 30 days and then increased slowly. The permeability coefficient of VGRC increases with the increase in porosity and aggregate size and decreases after planting and covering. The frost resistance of VGRC is enhanced, and the influence of aggregate size and porosity is small after turf planting.

Freeze-thaw (F-T) damage is the major factor destroying the bond behavior of reinforced concrete in the cold areas of China. The bond behavior between recycled fiber recycled concrete (RFRC) and reinforcement after F-T cycles was investigated in this paper. The pull-out tests were undertaken with the replacement rate (0, 50%, and 100%) of recycled aggregates (RA) and volume content (0, 0.12%, and 0.24%) of recycled fibers (RFs) as test variables. The results demonstrate that the F-T cycles will reduce the bond strength between RFRC and reinforcement. Bond strength decreases by 69.41% after 150 cycles. Moreover, RF can improve the bond strength between RFRC and reinforcing steel. Bonding strength increases by 11.35% with the addition of 0.12% RF. A simplified two-phase bond-slip model between RFRC and reinforced steel after F-T cycles was eventually established, and it correlated well with the experimental results. This research presents a theoretical basis for the application of RFRC in building structures in cold areas.

Permeability is one of the major performances for recycled aggregate concrete, which affects the durability and service life of concrete structures. In most cases, the main factor affecting the permeability of recycled aggregate concrete is the pore structure. Considering water-cement ratio, replacement rate of recycled aggregates, waste fiber length, and volume fraction of waste fibers as the design variables, pore structure and gas permeability were studied experimentally. In addition, fractal theory was here used to assess the pore structure of waste fiber recycled concrete and study the effects of pore structure on permeability. The results showed that the pore size distribution had a small impact on the permeability with the water-cement ratio and replacement rate of recycled aggregates increasing. The fractal dimension can be used to describe the complexity of the pore structure quantitatively. There is an obvious linear relationship between fractal dimension and gas permeability. The larger the pore volume fractal dimension, the better the impermeability of waste fiber recycled concrete.

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