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The electrochemical tests commonly used for evaluating reinforcement corrosion in Portland cement-based concrete may not be appropriate for testing alkali-activated slag (AAS) concrete due to its different pore structure and pore solution composition. In this article, corrosion behaviour of the steel bars in 12 AAS concrete mixes with different alkali concentration and modulus of sodium silicate solution was monitored by using gravimetric mass loss measurement. The results obtained from other electrochemical tests were compared to the gravimetric mass loss to determine their suitability for assessing the corrosion of steel in AAS concrete. It was found that only 7.8–28% of the mass loss was accounted for by the macrocell current for AAS concretes, indicating that this type of test underestimates the corrosion of the steel in AAS concrete. The steel bars in the AAS concretes gave a much higher negative half-cell potential value, presumably due to the influence of sulfides. In summary, it has been established that the criteria for assessing the onset and progress of corrosion of steel in Portland cement concrete are not suitable for the same purpose in the range of AAS concretes studied. [ABSTRACT FROM AUTHOR]

Abstract: The technology development of concrete and demand for high strength construction materials give momentum to the development of Ultra-High Performance Concrete (UHPC). Current UHPC preparation methods require costly materials and relatively sophisticated technology. To overcome these weaknesses, this paper focused on the preparation of UHPC with common technology and ordinary raw materials. Influence of binder content, water/binder ratio, ground granulated blastfurnace slag (GGBS) content, and limestone powder (LP) replacement on fluidity and compressive strength of concrete were researched, respectively. The test results show that the addition of superplasticizer and fine mineral additives enabled the UHPC to be produced at an extremely low water/binder ratio of 0.14–0.18, achieving excellent workability with a maximum slump of 268mm and compressive strengths of 175.8MPa at 90d and 182.9MPa at 365d. [Copyright &y& Elsevier]

The addition of coarse aggregate in ultra-high performance concrete offers several benefits, including the reduction of self-shrinkage, cost reduction, and increased modulus of elasticity. However, it also affects the dispersion space of steel fibers, resulting in a combined effect that both enhances and limits their respective advantages. An image method was used to describe the dispersion of steel fibers and coarse aggregate in the specimens along their length, width and height directions, respectively. The impact of dispersion of steel fibers and coarse aggregate on bending performance was investigated by four-point bending tests using various surfaces as the bottom tensile zone. Furthermore, the damage occurrence mode and crack development process are discussed using micro-morphology and cross-sectional graphs. The results show that the combined effect of steel fibers and coarse aggregate is not conducive to dispersion in the length direction but prevents settling due to gravity during vibration. Differences in bending properties among various surfaces in the bottom tension zone was significant, but the synergy between steel fibers and coarse aggregate reduced these differences by 1.5–2.4 times. Steel fibers in ultra-high performance concrete containing coarse aggregate act as crack arrestors, resulting in ductile damage. Cracks develop partly through the coarse aggregate and partly along the interface transition zone, indicating that the addition of coarse aggregate does not compromise flexural strength. • Dispersion variability was observed in the length, width, and height directions, influenced by the combined effect. • The combined effect mitigated the settling of steel fibers in ultra-high performance concrete. • The coarse aggregate has a limited effect on the flexural strength but diminishes post-peak performance. • The dispersion of steel fibers is the primary factor influencing post-peak performance. • The combined effect reduced the variation in bending parameters for different surfaces acting as the bottom tension zone. [ABSTRACT FROM AUTHOR]

Highlights • Design a new field test method to determine the permeability of structural concrete. • Establish the steady state flow net theory for air permeability test. • Assess the effect of specimens on flow features to calculate the calibration factor. • Justify the reliability and repeatability of the new air permeability test. Abstract An in situ air permeability test method that does not require assumptions for the often-difficult unidirectional flow has been developed to calculate the air permeability coefficient of the near surface concrete. The proposed method involves applying a constant pressure head to a surface mounted ring and measuring the steady state air flow rates. The analysis is based on modification of the flownet theory, which needs a calibration factor accounting for the influence of specimen and ring geometries. Effects of test area, width of seal, depth and width of test specimen were investigated using numerical simulation of the air flow. The repeatability of the proposed test method was assessed by the signal noise ratio (SNR) and discrimination ratio (DR). A new formula is offered and it only requires the steady state air flow rate to calculate the coefficient of air permeability, thus assisting engineers and researchers to quickly determine this property of structural concretes. [ABSTRACT FROM AUTHOR]

Drying shrinkage of alkali-activated slag concrete (AASC) is significantly greater than that of concrete made with ordinary Portland cement (OPC). It limits the large-scale application of AASC in field engineering. This study investigates the effect of early age-curing methods, including water curing, curing in elevated-temperature water, and CO2 curing, on drying shrinkage of AASC. Scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric (TG-DTG), and mercury intrusion porosimetry (MIP) were carried out to analyze the composition and microstructure of hydration products, to provide deeper understanding of drying shrinkage of AASC. The results show that water curing decreased drying shrinkage of both C30 and C50 AASC moderately compared to air curing, while it was more effective for C30 AASC. Curing in water of elevated temperature and CO2 curing were very beneficial to mitigate drying shrinkage of AASC. Heat curing decreased drying shrinkage of AASC up to 80%. SEM and TG-DTG results show that denser microstructure formed because of the accelerated hydration, resulting in lower porosity and lower proportion of pores smaller than 25 nm that contributed to the reduction of drying shrinkage. In addition, under high-temperature curing, most autogenous shrinkage of AASC occurred in the first few days because hydration was accelerated. After measurement of drying shrinkage was started, recorded autogenous shrinkage of AASC cured in elevated-temperature water should be much less than that of AASC cured at normal temperature. It is another important reason for the reduction of drying shrinkage. Carbonation occurring in the CO2 curing period led to the decalcification of C-(A)-S-H gel; it coarsened the pore-size distribution and decreased the total porosity. Therefore, drying shrinkage of C30 and C50 AASC was declined by 49% and 53% respectively. [ABSTRACT FROM AUTHOR]