International Concrete Abstracts Portal

This study evaluates the potential to use shrinkage-reducing admixture (SRA) and pre-saturated lightweight sand (LWS) to shorten the external moist-curing requirement of ultra-high-performance concrete (UHPC), which is critical in some applications where continuous moist-curing is challenging. Key characteristics of UHPC prepared with and without SRA and LWS and under 3 days, 7 days, and continuous moist curing were investigated. Results indicate that the combined incorporation of 1% SRA and 17% LWS can shorten the required moist-curing duration because such a mixture under 3 days of moist curing exhibited low total shrinkage of 360 με and compressive strength of 135 MPa (19,580 psi) at 56 days, and flexural strength of 18 MPa (2610 psi) at 28 days. This mixture subjected to 3 days of moist curing had a similar hydration degree and 25% lower capillary porosity in paste compared to the Reference UHPC prepared without any SRA and LWS and under continuous moist curing. The incorporation of 17% LWS promoted cement hydration and silica fume pozzolanic reaction to a degree similar to extending the moist-curing duration from 3 to 28 days and offsetting the impact of SRA on reducing cement hydration. The lower capillary porosity in the paste compensated for the porosity induced by porous LWS to secure an acceptable level of total porosity of UHPC.

This paper describes an approach to predict the mechanical and fracture behavior of cement-based systems by combining thermodynamic and finite element analysis models. First, the reaction products in a hydrated cementitious paste are predicted using a thermodynamic model. Second, a pore partitioning model is used to segment the total porosity into porosity associated with gel pores and capillary pores. A property-porosity relationship is used to predict the elastic modulus, tensile strength, and fracture energy of the hardened cement paste. The paste’s modulus, fracture energy, and tensile strength, along with information on the aggregate properties and interfacial transition zone properties, are used as inputs to a finite element analysis model to predict the flexural strength and fracture response of mortars.

The apparent density of lightweight aggregate (LWA)-modified ultra-high-performance concrete composite is 2080 kg/m3, and the compressive strength is not less than 110 MPa at 28 days. Lightweight ultra-high-performance concrete (LUHPC) not only has light weight and high strength, but also reduces the consumption of raw materials and the section size of the structure, thus reducing the cost. The macroscopic properties are closely related to the pore structure characteristics, but the structural nature of LUHPC under different curing regimes and the LWA on their pore structure remain unclear. To comprehensively understand the pore structure of LUHPC and then control its properties, capillary absorption method, low-field nuclear magnetic resonance (LF-NMR), computed tomography (CT), and nitrogen adsorption (BET) technologies were used to characterize the pore structure characteristics of LUHPC. The experimental results show that there are many nanoscale pores (mainly harmful and more-harmful pores) in LUHPC. With the increase of water absorption of the added LWA, the porosity of LUHPC and the proportion of less-harmful pores increase, thus changing the pore structure of LUHPC. With the increase of temperature and pressure, the internal curing effect of LWA is accelerated. Heat treatment promotes the formation of dense additional hydrates such as tobermorite and xonotlite, and the average chain length of the hydrates and the pozzolanic reaction between supplementary cementitious material and Ca(OH)2. Steam curing increases the total porosity and coarsens the pore size while accelerating the hydration of cementitious paste. Autoclaved curing can stimulate the pozzolanic activity of inert SiO2, promote the formation of secondary hydration products, and fill the pores in the matrix. The evolution of the pore structure of LUHPC plays a key role in improving its performance due to the curing regimes and presence of LWA.

This study evaluates the acid resistance of cement mortar using granulated ferronickel slag (FNS) as fine aggregate and fly ash or ground FNS (GFNS) as a supplementary cementitious material (SCM). The deterioration was evaluated by visual inspection, and changes of mass and strength after immersion in 1% sulfuric acid solution for up to 180 days. Acid resistance was marginally reduced when 50% volume of sand was replaced by FNS. While the control specimens suffered significant spalling and strength loss, the use of fly ash or GFNS considerably reduced the deterioration. This is attributed to the formation of a protective zone and densification of microstructure by the pozzolanic reaction, as confirmed by strength activity index, permeable voids, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Overall, the specimens with FNS aggregate and GFNS or fly ash showed less deterioration than the control specimens after prolonged acid exposure.

Internal curing can reduce alkali-silica reaction (ASR) damage in concrete. Alkali reactions with chert in natural sand caused damaging ASR in plain concrete. However, ASR damage was minimal in companion internally cured (IC) concrete in which a portion of the sand was replaced with pre-wetted lightweight aggregate (LWA). IC improved paste quality through a quantitative reduction in paste porosity and unhydrated cement. This was assessed using quantitative paste characterization including image analysis of backscattered electron (BSE) images, quantitative fluorescent intensity assessment, resistivity measurements, and qualitative analyses using SEM-EDS and polarized light microscopy. In concretes with the same water-cement ratio (w/c), IC concrete has denser paste microstructure from increased hydration due to additional water from pre-wetted LWA. Less permeable paste reduced fluid ingress, ASR reaction, and crack propagation. This demonstrates the potential of internal curing as a mitigation tool in reducing damage from ASR when high cement content and potentially reactive aggregates are used.