International Concrete Abstracts Portal

To make full use of recycled aggregate concrete (RAC), carbon fiber (CF) and nano metakaolin (NMK) were mixed into RAC to improve their mechanical properties and microstructure. The effects of NMK content, CF content, recycled aggregate (RA) replacement rate, and CF length on the compressive strength, split tensile strength, and tension-compression ratio of RAC were studied by the orthogonal test method, then the test results were analyzed. The results show that the NMK content and RA replacement rate have significant effects on the compressive strength of RAC, while the CF content has significant effects on the split tensile strength and the tension-compression ratio. Through the synergistic effect of NMK and CF, the pore structure characteristics of RAC are improved, and the bonding strength of the interfacial transition zone (ITZ) of CF-mortar is increased, which further enhances the strengthening effect of CF, thus the mechanical properties of RAC are continuously enhanced.

Compared to external curing, internal curing enables the judicious use of available water to provide additional moisture in concrete for more effective hydration, and improvement in the performance of concrete structures. However, certain challenges with the incorporation of internal curing materials (ICMs) still need to be addressed as its effectiveness depends on several factors. Furthermore, sustainable construction demands the use of recycled materials, and this paper discusses the comparative evaluation of recycled aggregate (RA) as an ICM with two other types of ICMs on various properties of high-performance concrete in the hardened state under two curing conditions. Concrete mixes were prepared with pre-wetted recycled aggregates (RA), superabsorbent polymers (SAPs), and pre-wetted lightweight volcanic aggregates (LWVA) as ICMs. Concrete performance was compared through the investigation on the strength development, shrinkage, mass loss, and volumetric water absorption. In addition, the change in internal humidity of concrete with time at different stages of hardening was determined. The compressive strength results showed that RA and LWVA are more efficient in the early days, and the performance of SAP is better in the later age due to its slow water-releasing capabilities. Compared to the control mixture, the least reduction in strength of 4% and 8% at 28 days and 90 days, respectively could be observed for the mixes containing RA under both air and water curing.

The attached cement paste in recycled concrete (RC) aggregateleads to its potential reactivity against sulfate ions. Several testmethods were evaluated to find a suitable, reliable, and accuratemethod to evaluate the potential reactivity of aggregates. Differentquality RC aggregates were used to apply those methods. Thestudies included evaluations of concrete cores drilled from sourceconcrete (SC), RC aggregates, recycled mortar bars under differentexposures, and RC prisms exposed to external sulfate attack (ESA).The concrete core test allowed qualifying SC as potentially reactiveagainst sulfate in a short time. Tests on recycled aggregates andrecycled mortar bars showed variable sensitivity levels. Resultsfrom concrete prisms showed an effective reactivity of recycledaggregates when the replacement is higher.

A high cement content is often found in concrete mixture designs to achieve the unique fresh-state behavior requirements of three dimensional (3-D) printable concrete (3DPC) to ensure rapid stiffening of an extruded layer without collapsing under the stress applied by the following layers. Some materials with high water absorption, such as recycled concrete aggregates, have been incorporated in concrete mixture designs to minimize environmental impact; nevertheless, the fine powder fraction that remains from the recycled aggregate processing still poses a challenge. In the case of 3DCP, few studies are available regarding mixture designs using recycled concrete powder (RCP) for 3-D printing. In this context, this study presents the use of RCP as a filler to produce a printable mixture with low cement content. An RCP with 50 μm average particle size was obtained as a by-product from recycled concrete aggregate production. Portland cement pastes were produced with 0, 10, 20, 30, 40, and 50% of cement mass replacement by RCP to evaluate its effects on the hydration reaction, rheology, and compressive strength. It was found that the studied RCP replacement was not detrimental for the hydration reaction of portland cement during the initial hours, and at the same time, it was capable of modifying the rheological parameters of the paste proportionally to the packing density of its solid fraction. The obtained results indicated the viability of 3DCP with up to 50% cement replacement by RCP. It was concluded that RCP presents good potential for decreasing the cement consumption of 3DPC, which in turn could decrease its associated environmental impact while providing a destination for a by-product from recycled concrete aggregate production.

The use of construction waste to prepare recycled micro powder can improve the use of construction waste resources and effectively reduce carbon emissions. In this paper, researchers used waste concrete processing micro powder to prepare foam concrete (FC) and quantitatively characterized the performance and pore structure of FC by scanning electron microscopy (SEM), pore and fissure image recognition and analysis system (PCAS), and mechanical property testing methods with different mixing ratios of micro powder. The results showed that the effect of single mixing of micro powder or fly ash is better than the composite mixing test, and the optimal proportion of compressive strength of single mixing of micro powder is higher than that of single mixing of fly ash. The optimum mixing ratio is 6:4 between cement and micro powder, and the best effect is achieved when the micro powder mixing amount is 40%. Single or double mixing can fill the pores between the foam and strengthen the performance of the substrate. The tests of single-mixed or compound-mixed micro powder showed that the fractal dimension decreased with the increase of porosity; when the fractal dimension of the specimen increased, the average shape factor became smaller, the compressive strength decreased, and the water absorption rate increased.