An investigation has been performed on the drying shrinkage and tensile drying creep characteristics of a non-proprietary ultra-high performance concrete (UHPC) mixture. The mixture was formulated using metakaolin as supplementary cementitious material (SCM) and limestone powder as mineral filler. Cylindrical specimens with dimensions of 52 x 400 mm (2.05 x 16 in.) were fabricated and loaded at 7 and 11 days from casting to various stress levels for 90 days. Additional specimens were fabricated from a proprietary mixture with a silica fume-ground quartz formulation to study the effects of mixture composition. Simultaneous free-drying shrinkage measurements were recorded in accompanying specimens placed in the same room environment. Attention was given to the effect of the casting orientation, age at loading, and mixture composition on the drying shrinkage and drying creep behavior of the samples. These tests show that the metakaolin-limestone powder mixture has significantly lower drying shrinkage and specific drying creep than the silica fume-ground quartz mixture. Additionally, the age at loading influences primary creep behavior while not affecting secondary creep at the same stress level. It seems that fiber orientation plays a significant role in the drying creep behavior of UHPC and that cracked UHPC under constant tensile stress undergoes a significant amount of fiber slip.

Post-fire rehydration is an interesting method to recover the structural performance of fire-damaged concrete. This paper evaluated the viability of using cementitious materials containing carbon nanotubes (CNT) or carbon-black nanoparticles (CBN) for damage recovery detection and self-monitoring of strain and stress of fire-damaged structures subjected to post-fire curing. Nanomodified mortars were subjected to high temperatures, rehydration, and measurements of capacitive behavior, electrical resistivity, and self-sensing properties. After 600ºC and rehydration, mortars with 9.00% CBN presented the ability of self-detection of damage recovery, as also verified in mortars with 0.4-1.20% CNT and 6.00% CBN after 400ºC and rehydration. The post-fire curing method filled the pores and microcracks of the cementitious matrix with non-conductive rehydration products, increasing their electrical resistivity. Mortars with 0.80% and 1.20% CNT presented self-monitoring of strain and stress after 400ºC and rehydration, as also observed in mortars with 9.00% CBN after 600ºC and rehydration. The post-fire curing process also increased the self-sensing properties because non-conductive rehydration products obstructed conductive stretches, improving tunneling conduction mechanisms rather than contacting conduction. These self-sensing materials are promising alternatives to evaluate post-fire curing processes and self-monitor the strain and stresses of next-generation smart structures.

This paper presents a set of procedures and a recently developed direct tension test for determining the uniaxial tensile strength and the full stress-strain behavior of ultra-high performance concrete (UHPC). The proposed set of procedures is aimed to establish an upper and lower bound for the tensile strength based on preferential casting orientation. Results from this research show that an upper and lower bound of strength could be established when properly executed casting procedures are in place. On the other hand, the proposed direct tension test is able to capture the full stress-strain behavior of the material at pre- and post-cracking stages, for both, strain-hardening and strain-softening samples. Results from the direct tension tests performed during this research, favor the use of contactless extensometers to avoid stress concentrations that induce early localization at the regions close to the attachment points when using traditional measuring methods.

This research focuses on the evaluation of the sustainability of recycled ultra-high-performance concrete (R-UHPC), from a life cycle analysis perspective and with reference to a case study example dealing with structures exposed to extremely aggressive environments. This involves the assessment of the self-healing capacity of R-UHPC, as guaranteed by the recycled UHPC aggregates themselves. Recycled aggregates (RA) were created by crushing four-month-old UHPC specimens with an average compressive strength of 150 MPa. Different fractions of recycled aggregates (0 to 2 mm) and two different percentages (50 and 100%) were used as a substitute for natural aggregates in the production of R-UHPC. Notched beam specimens were pre-cracked to 150 µm using a three-point flexural test. The autogenous self-healing potential of R-UHPC, stimulated also by the addition of a crystalline admixture, was explored using water absorption tests and microscopic crack healing at a pre-determined time (0 days, 1 month, 3 months, and 6 months) following pre-cracking. Continuous wet/dry healing conditions were maintained throughout the experimental campaign. The specimens using recycled UHPC aggregates demonstrated improved self-healing properties to those containing natural aggregates, especially from the 2nd month to the 6th month. To address the potential environmental benefits of this novel material in comparison to the conventional ones, a Life Cycle Assessment (LCA) analysis was conducted adopting the 10 CML-IA baseline impact categories, together with a Life Cycle Cost (LCC) analysis to determine the related economic viability. Both LCA and LCC methodologies are here integrated into a holistic design approach to address not only the sustainability concerns but also to promote the spread of innovative solutions for the concrete construction industry. As a case study unit, a basin for the collection and cooling of geothermal waters has been selected. This is meant as representative of both the possibility offered, in terms of structural design optimization and reduction of resource consumption, and of reduced maintenance guaranteed by the retained mechanical performance and durability realized by the self-healing capacity of the R-UHPC.

An experimental campaign, performed on different types of UHP-FRCC, made with four replacement rates (0%, 20%, 50%, and 70%) of cement with fly ash and cured for 1, 4, and 13 weeks, is described in this paper. Specifically, 72 cylinders were tested for measuring compressive strength and Young’s modulus of elasticity; stress-strain relationships were obtained from 72 dumbbell-type specimens subjected to uniaxial tension; and 12 beams, tested in four-point bending, provided the moment-curvature diagrams. The best UHP-FRCC was selected through an eco-mechanical analysis, capable of combining the mechanical performance with the environmental impact of concrete. When the ultimate bending moment of a beam is the functional unit of this analysis, the higher the replacement rate of cement the better the beam performance, although material properties and structural ductility show opposite trends.