Short-term length-change isotherms and desorption isotherms of matured hardened cement paste were measured at different temperature conditions, namely, 20, 30, 40, 50, and 60℃. For the short-term length-change isotherms, higher temperature conditions resulted in a smaller drying shrinkage. This trend was largely controlled by the microstructural reorganization of the calcium-silicate-hydrates in the cement paste at elevated temperatures. The experimental results suggest that a portion of the evaporable water did not contribute to the shrinkage. In addition, based on the relation-ships between the incremental strain and the incremental evaporable water content, with a starting relative humidity of 5%, consistent trends were observed for the incremental strain and the incremental evaporable water when the incremental evaporable water content was less than 0.08 g/g-dried hcp. The microstructural reorganization pathways for elevated temperature and/or for drying were confirmed to be identical.
A variation of material properties with depth is observed in a massive, seismic-resistant internal wall of Unit 1 of the Hamaoka Nuclear Power Plant (NPP) 45 years after its construction. The maximum values are attained in the center of the structural element and gradually decrease towards the surface, while still well exceeding the required performance. It is found that this trend can be mostly attributed to the reaction between cement hydrates and rock-forming minerals of a feldspar group, which releases silica and alumina oxides into the pore solution, where they react with portlandite to form new hydrates. No evidence of expansive distress similar to the alkali-silica reaction (ASR) is found in the material, owing to this reaction. Possible implications for the aging management of existing concrete structures are discussed.
Tensile fracture of fiber reinforced cement-based composites (FRCC) with rebar was investigated via a mesoscale analysis using discretized short fibers. Herein, the effects of fiber volume fraction, steel reinforcement ratio, FRCC–rebar bond characteristics, and fiber distribution on tensile fracture behavior were investigated. In some cases, localized crack was observed in the post-yield range of rebar. The localization mechanism was numerically explained and then inhibited by focusing on the bridging forces of the fibers and rebar. The effectiveness of steel reinforcement in enhancing the strain capacity of strain-hardening cement-based composites was confirmed. This paper is based on an original paper (Ogura et al. 2016) written in Japanese.
Cement mortars are extensively used in tunnels and underground facilities, where the risk of high temperatures due to fires and explosions triggered by traffic accidents, earthquakes or electrical malfunctions cannot be ruled out. It is necessary, therefore, to know the mechanical properties of cement mortars at high temperature in both static and dynamic conditions. The dynamic mechanical behavior of pre-stressed cement mortars at high temperature is investigated in this research project, from 25 to 300°C, by means of the Hopkinson bar. The microstructure of cement mortars is also studied via Scanning Electron Microscopy-SEM. The properties under investigation are: density, coefficient of thermal expansion, dynamic compressive strength, peak strain, dynamic elastic modulus and damage variables, as well as the longitudinal wave velocity. The tests indicate that the physical and dynamic mechanical properties of cement mortars are significantly affected by high temperature. In the range 25 - 300°C, because of the increasing number and size of the microdefects (microcracks), the density and the longitudinal wave velocity exhibit a gradual less-than-linear decline, while the decline of the compressive strength and elastic modulus is more marked (roughly linear), the peak strain and damage almost increase linearly.
The primary goal of the present paper is to investigate the influence of cracking on water transport by capillary suction of UHPFRC. Prismatic specimens were firstly loaded under four-point bending up to specific crack open displacement (COD). Target COD, under loading, was varied between 200 and 400 μm, in steps of 50 μm. After unloading, a COD recovery was observed with residual COD ranging between 116-334 µm and 75-248 μm for UHPFRC-1.5% and UHP-FRC-3.0% specimens, respectively. The crack pattern created was characterised (number of cracks and crack width) before capillarity testing. Sorptivity results of cracked UHPFRC-1.5% and UHPFRC-3% specimens remained in the range of 0.024 to 0.044 mg/(mm2.min0.5), which are about 2 to 4 times higher than the sorptivity results of non-cracked UHPFRC specimens. However, the maximum sorptivity observed on cracked UHPFRC is relatively low as compared to typical sorptivity results found in good quality conventional concrete or engineered cementitious composites (ECC).
Delayed ettringite formation (DEF) is known as the deterioration phenomenon that occurs in mortar or concrete cured at high temperature. It has been proposed that DEF expansion is affected by calcium-silicate-hydrates (C-S-H), although the mechanism has not yet been clarified. The present study experimentally examined the relationship between the expansion characteristics of heat-cured mortar specimens used cement, anhydrite and supplementary cementitious materials (SCMs) during water curing and the origins of ettringite formation. The proportional change in length and the phase compositions of the heat-cured specimens were acquired. The respective amounts of ettringite generated from cement and SCMs were determined. The expansion of the mortar specimens using cement, anhydrite, silica fume and fly ash occurred continuously during water curing, while the continuous expansion did not occur in the mortar specimens using cement, anhydrite and blast furnace slag. These expansion characteristics could not be explained solely by the change in the amounts of ettringite. There was a certain correlation between the amount of ettringite generated from cement and the expansion characteristics of the specimens. It is one of the evident that origins of ettringite formation or coexisting materials such as C-S-H phase can affect expansion characteristics due to DEF.