The direct utilization of recycled components derived from meticulously demolished old concrete members in new structural elements holds significant potential for saving materials, reducing energy consumption, and lowering carbon emissions. However, the performance of such new members requires systematic investigation for quantitative understanding. As a preliminary exploration, this paper investigated the flexural performance of precast concrete floor slabs containing recycled components. Flexural experiments were conducted under both sagging and hogging moments. The influences of factors such as the surface treatment methods of recycled components and the strength difference between new and old concrete on the sagging moment flexural performance was examined.Furthermore, the effect of the configuration of locally added connecting reinforcement on the hogging moment flexural performance was studied. A calculation methods for the flexural capacity of these slabs was proposed. The results show that: (1) the sagging moment flexural capacity of the recycled-component slabs is significantly higher than that of monolithic cast reference slabs; (2) Different surface treatment methods for the recycled components generally had a limited impact on the sagging moment capacity; (3) the addition of local connecting reinforcement at the ends of the recycled components notably enhanced the hogging moment flexural capacity; for practical engineering, the configuration using directly laid lapped reinforcement is recommended. The results of the proposed methods for calculating flexural bearing capacities are generally conservative.

Large amount of hole slag will be produced in the construction of punched cast-in-place piles for highway bridges. It is an effective countermeasure to apply the hole slag to concrete components of highway bridges nearby. Based on the recycling use of recycled coarse aggregates (RCAs) and considering that concrete structures in coastal areas are greatly affected by chloride corrosion, this paper prepared the recycled aggregate concrete mixed with the hole slag by replacing natural sand and natural coarse aggregates with the hole slag and RCAs, respectively. To investigate the effects of the hole slag replacement ratio (0, 50%, 70%, 100%) and RCAs replacement ratio (0, 50%, 70%, 100%) on the compressive and chloride resistance properties of such concrete, this paper carried out compression and chloride resistance tests of 96 specimens (48 cylinders with Φ 150×300 mm for compression test, and 48 cylinders with Φ 100×50 mm for chloride resistance test) made of such concrete, and mercury intrusion tests of the hole slag, river sand, and corresponding mortar. The test results show that: the compressive strength of such concrete decreases gradually with the increase of the replacement ratios of the hole slag and RCAs, and the reductions caused by the two are roughly the same; compared with RCAs, the influence of the hole slag on the elastic modulus and peak strain of such concrete is relatively limited; the chloride ion migration coefficient of such concrete increases gradually with the increase of the replacement ratios of the hole slag and RCAs on the whole, but the influence of the hole slag is obviously lower than that of RCAs; with the increase of the replacement ratio of the hole slag, the porosity of such concrete generally shows a trend of increasing gradually from fast to slow. When the hole slag and RCAs are used simultaneously in practical projects, it is suggested that the maximum replacement ratios of both should be limited to 50%.

Compressive creep tests on thirty-seven recycled lump/aggregate concrete (RLAC) specimens were carried out to reveal the creep behavior of RLAC by taking the replacement ratio of demolished concrete lumps (DCLs), replacement ratio of recycled coarse aggregates (RCAs), replacement ratio of recycled sand rooted from alluvialproluvial (A-P) soil, stress level and reinforcement ratio as parameters. The results show that the specific creep of reinforced/unreinforced RLAC is greater than that of reinforced/unreinforced recycled aggregate concrete (RAC). The increase rate of RLAC is 10.1% when unreinforced and the increase rate is 13.4% and 11.5% when the reinforcement ratio is 1.16% and 1.57%, respectively. As the replacement ratio of RCAs in new concrete increases from30% to 50%, the specific creep of reinforced/unreinforced RLAC increases by 7.4% and 11.4% respectively; when the fine aggregate (i. e., river sand) of new concrete is completely replaced by the recycled sand from A-P soil, the creep behavior of reinforced/unreinforced RLAC shows almost no change, but both of the shrinkage deformation decrease. As the reinforcement ratio increases from 1.16% to 1.57%, the specific creep of RLAC and RAC decrease by 5.0% and 6.6%, respectively. Reinforcement reduces the shrinkage deformation of RAC and RLAC, and the reduction grows with the increase of reinforcement ratio. As the replacement ratio of RCAs in new concrete increases from 30% to 50%, the elastic modulus of RLAC have little change and it is also almost free from the impact of replacing the fine aggregate (i. e., river sand) of new concrete with the recycled sand from A-P soil; and when the stress level is less than 0.4, the specific creep of RLAC is nearly deemed as irrelevant to the stress level.

Scientifically grasping the durability performance of the recycled lump/aggregate concrete (RLAC) is of great significance for promoting the engineering application of this type of concrete. In this paper, to reveal the carbonation properties of the stone-mortar interface and the new mortar-old mortar interface in RLAC, rapid carbonation tests of these two kinds of interfaces and the mortar matrix were carried out, and the porosity of the new mortar-old mortar interfacial transition zone was investigated through backscatter electron images. The results show that the carbonization depth of the interfacial transition zone is greater than that of the mortar matrix, and the closer to the interface, the greater the degree of carbonization, showing an obvious two-dimension carbonization superposition phenomenon. The interface carbonization depths of the stone-mortar specimens vary from 26 to 46 mm, while those of the mortar-mortar specimens are only 7～15 mm. As compared with the new mortar-old mortar interface, the carbonization depth of the stone-mortar interface is much greater, indicating the latter interface having a weaker carbonization resistance. The water-to-cement ratio of the new mortar has a great influence on the carbonation performance of the new mortar-old mortar interface, and the related reduction in carbonization depth is between 15% and 52%, while the water-to-cement ratio of the old mortar has no obvious effect on the carbonation performance of such interface. When the water-to-cement ratio of the new mortar is fixed, the porosity of the new mortar-old mortar interface is almost unchanged with the water-to-cement ratio of the old mortar. However, when the water-to-cement ratio of the old mortar is fixed, the porosity of such interface decreases greatly with the reducing of the water-to-cement ratio of the new mortar. There thus comes to the conclusion that, in practical engineering, the carbonation performance of the new mortar-old mortar interface can be effectively improved by controlling the water-to-cement ratio of the new mortar.

In order to reduce the amount of natural aggregates and cementitious material in 3D printed concrete, this study used recycled fine aggregate (RFA) to partially replace natural fine aggregate and clay brick powder (CBP) to replace cement. The fluidity and compressive strength test of cast-in-situ concretes with RFA only (0, 25%, 50%, 75% and 100% replacement ratio), with CBP only (0, 5%, 10%, 15%, 20% and 30% replacement ratio) and with both of them were firstly carried out to obtain the optimal replacement ratio of RFA and CBP. Then, it investigated the effect of the addition of 50%RFA and 10%CBP and the corresponding mix proportion adjustment methods (adding extra water and increasing water reducer dosage) on the fresh and harden properties of 3D printed concrete. The test results indicate that the decreasing amplitude of the compressive strength of cast-in-situ concrete is within 10% when the replacement ratio of RFA is less than 50%. With the increase of CBP replacement ratio from 0 to 30%, the compressive strength of cast-in-situ concrete increases first, then decreases and then slightly increases. The specimen with CBP content of 10% exhibits the highest compressive strength. Compared to the concrete with 50%RFA only, the concrete with 50%RFA and 10%CBP has higher strength, while the fluidity is almost kept unchanged. For 3D printed concrete, the addition of 50% recycled fine aggregate and 10% brick powder and additional water can keep the initial expansion of 3D printed concrete unchanged and improve the buildability of concrete, but it can decrease the slump, opening time, compressive and split tensile strength and increase intensity anisotropy. However, increasing water reducer dosage in the mixture can not only significantly improve the fluidity, opening time and compressive strength of 3D printed concrete, but also reduce the intensity anisotropy.

Textile reinforced mortar (TRM) strengthening is a method of using mortar as an inorganic adhesive to stick textile onto the surface of components to form a strengthening layer. It has advantages such as light weight, high strength, minimal change in cross-sectional dimensions, good high-temperature resistance, and excellent durability, and thus has gained widespread attention in recent years. TRM typically uses polymer-modified cement mortar as the adhesive, but the production of cement has high energy consumption and carbon emissions. To achieve the “dual carbon” goals, this paper proposed to replace cement with geopolymer, which has much lower production energy consumption and carbon emissions while offering mechanical properties similar to cement, thus forming a textile reinforced mesh-enhanced geopolymer mortar (TRGM) strengthening method. This paper employed carbon textile reinforced geopolymer mortar to strengthen two-way reinforced concrete slabs. The flexural performance tests and finite element analysis were conducted on the unstrengthened and strengthened slabs with different aspect ratios and different numbers of TRGM layers. The strengthening effect of TRGM, the contribution of bidirectional fibers to bearing capacity, and the force transmission mechanism of the strengthened slabs were investigated. The results show that TRGM strengthening can effectively improve the post-cracking stiffness and flexural carrying capacity of two-way slabs and inhibit crack propagation, especially the widthwise cracks. The strengthening effect of TRGM increases with the increase in the aspect ratio of the two-way slabs. The bearing capacity of the strengthened slab with one layer of TRGM was greatly influenced by the overlap of the textile and strengthening construction quality, which made the strengthening efficiency of one layer of TRGM lower than that of two layers. The overlap of the fiber mesh may affect the strength of the fibers, and the design should ensure that the fibers have sufficient overlap length. After the widthwise reinforcement yielding, the ratio of bending moment borne in the widthwise direction to the lengthwise direction gradually decreased, since the contribution of the longitudinal reinforcement and fibers to the bearing capacity gradually increased. As the mid-span deflection increases, the proportion of tensile force borne by the fibers shows a wave-like trend, first decreasing, then increasing, and subsequently decreasing again.