International Concrete Abstracts Portal

This research examines the hole-drilling method per ASTM E837, known for its minimal invasiveness, for measuring in situ stress in reinforcing bars embedded within concrete structures. The primary objective is to ascertain the applicability of this method in estimating non-apparent stresses, such as those resulting from the external loads, creep, shrinkage, or alkali-silica reaction, that are needed for structural assessment. Systematic experiments on reinforced concrete beams are conducted to validate the method’s viability in identifying these critical in situ stresses. The findings highlight the potential of the hole-drilling method to enhance structural health monitoring practices, offering an accurate tool for assessing stress states crucial for the maintenance and safety of concrete structures. The results demonstrated that while the hole-drilling method is robust for moderate-stress evaluations (up to about 70% of the nominal yield stress), it overestimates the stress in the reinforcing bar under high-stress conditions near the 100% nominal yield stress. This study contributes to the field by confirming the limits and applicability of the ASTM E837 standard for estimating the existing stress in the embedded reinforcing bars.

Ultra-high-performance concrete (UHPC) features tensile strain-hardening behavior and a high compressive strength. Existing studies on the shear behavior of UHPC structural members have been focused on prestressed UHPC girders. More experimental data of the shear behavior of non-prestressed UHPC beams are necessary to quantify the safety margin of shear designs for structures. Moreover, while the UHPC members in most studies did not contain coarse aggregate to strengthen their microstructure, the inclusion of coarse aggregate has been shown to substantially reduce the autogenous shrinkage and enhance the elastic modulus for UHPC materials, which is beneficial for structural applications of UHPC. This study experimentally investigated the shear failure behavior of eighteen non-prestressed rectangular UHPC beams. The experimental variables included the volume fraction of fibers, shear span-to-depth ratio of the beams, and coarse aggregate. The detailed shear failure responses of the UHPC beams were discussed in terms of the damage pattern, shear modulus, shear strength, shear strain, and strain energy. The test results showed that the inclusion of coarse aggregate increased the beam shear strength, and its enhancement became more significant with a higher volume fraction of fibers and a lower shear span-to-depth ratio of the beam. In addition to the experimental investigation, a shear strength model for non-prestressed rectangular UHPC beams that accounts for the interactive effect of the key design parameters was developed. An experimental database of the shear strength of the UHPC beams in existing studies was established to assess the performance of the proposed model. It was shown that the proposed model reasonably predicted the shear strength of the UHPC beams in the database with a higher accuracy and lower scatter compared to the results of existing models.

The management of municipal solid waste incineration fly ash (MSWI FA) has become a critical issue as its generation increases rapidly along with the global population growth. In this study, MSWI FA was treated via water-washing, and then the untreated and water-treated MSWI FAs (RFA and WFA) were blended with mainstream supplementary cementitious materials (SCMs), including ground granulated blast-furnace slag (GS), silica fume (SF), and limestone powder (LS). The MSWI FASCMblends were used as a cement replacement in a mortar. The content of MSWI FAs was set at 10% (by weight of binder) for all mortar mixtures. The content of GS and LS was also set at 10%, while the SF content was 2.5%. Flowability, setting time, isothermal calorimetry, compressive strength, and free-drying shrinkage tests were performed. The results showed that mortars containing raw (untreated) fly ash (RFA) had reduced strength, whereas mortars containing water-treated fly ash (WFA) displayed comparable or even higher strength than the control mortar (made with 100% cement) after 28 days. While mortars containing RFA showed increased drying shrinkage, mortars containing WFA exhibited diminutive or no increase in drying shrinkage when compared to the control mortar. Based on the test results, the mixture with a cement:WFA:GS ratio of 80:10:10 was the optimal binder for concrete applications.

This paper examines the influence of harvested fly ash on the properties of mortar and concrete. Class F and harvested fly ash were used at the substitution rate of 20% by weight of Portland cement. The investigated properties included heat release, consistency, setting time, compressive strength at different testing ages, absorption, the volume of permeable voids, surface resistivity, and drying shrinkage. The results revealed that the harvested fly ash produced the lowest released heat of hydration and longest setting times. Mixtures containing harvested fly ash displayed lower strength at all curing ages. Compared to traditional fly ash, harvested fly ash showed inferior transport properties for both absorption rate, permeable voids, and surface resistivity. Mixtures containing harvested fly ash showed comparable 120-day drying shrinkage when compared with the companion mortars made with traditional fly ash.

Geopolymers are amorphous mineral materials manufactured from aluminosilicates and a strongly basic alkaline solution. One of their advantages is a lower carbon footprint than conventional cementitious binders. However, they are subject to significant drying shrinkage. In this study, geopolymer samples are produced from metakaolin, silica fume, and a potash solution. The mixture proportions are selected to reach the molar ration Si/Al=1.8, K/Al=1.15, and H₂O/K=5.3 leading to satisfactory rheology while mixing. Cylindrical samples (70 mm diameter, 40 mm height) are exposed to various curing conditions (temperature and relative humidity). A protocol including 3D scanning and instrumented macroindentation is used to monitor the drying shrinkage and hardening kinetics. Sample volume change and hardness are measured periodically until sample mass stabilization. It appears that the hardest samples are also the most cracked. Covering the sample for 5 days at 23°C or 24 hours at 40°C limits the shrinkage to ~1% but leads to a large decrease of the hardness compared to the hardest samples. An optimal geopolymerization requires a minimal amount of water which decreases with the progress of the reaction. Optimal curing conditions are identified. Thus, covering the sample for 3 days at 23°C allows to limit the shrinkage to 3% without cracking while reaching satisfactory mechanical properties.