Spalling, which is a phenomenon encountered when high-performance concrete is exposed to high temperature, can lead to large economical damage and can be a major safety hazard. Moisture distribution ...in concrete during exposure to fire is of paramount importance for understanding the complex mechanism of this phenomenon. To capture in its fullness this mechanism, it is crucial to account for the heterogeneous nature of concrete.
In this paper, the first 3D analysis of moisture distribution in concrete at high temperature through in-situ neutron tomography is presented. The world-leading flux at the Institute Laue Langevin in Grenoble, France allowed capturing one 3D scan per minute, which is sufficient to follow the fast dehydration process. The paper describes the experimental setup with the heating system and discusses in detail the framework of the neutron tomography test. Quantitative analysis showing the effect of the aggregate size on the moisture distribution is presented.
This communication explores the influence of boundary effects, embedded sensors and crack opening on high-temperature experiments of concrete as revealed by in situ neutron tomography. The hypotheses ...routinely taken about these experimental aspects in common practice are hereby reassessed in light of the insight given by noninvasive full-field measurements. Notably, we directly assess the heat and moisture insulation techniques and reveal the influence of temperature and gas pressure monitoring on the testing conditions, opening new perspectives toward their improvement.
This article presents the results of an inter-laboratory study performed by six international research groups in the framework of RILEM Technical Committee 260 RSC “Recommendations for use of ...superabsorbent polymers in concrete construction”. Two commercially available superabsorbent polymer (SAP) samples with different chemical compositions were tested in terms of their ability to mitigate plastic shrinkage cracking of concrete. The SAP mixtures showed a clear reduction of plastic shrinkage cracking in conventional concrete. On the contrary, if only additional water is added and no SAP, the area of plastic shrinkage cracks increases. This suggest the ability of SAP to mitigate plastic shrinkage cracking. Upon addition of the predetermined amount of SAP and additional water, the compressive strength decreased on average by 3% for the mixtures with 0.15% SAP (by mass of cement) and by 10% for the mixtures with 0.30% SAP.
New methods are proposed for the verification of the presence of superabsorbent polymers (SAP) in freshly mixed concrete and estimation of SAP quantity. The methods are in general based on flushing ...concrete with excess water. They allow separating the light, water-sorbed hydrogel particles from the mineral components in the fresh concrete and making these particles available for further tests. Two types of tests are proposed: Test 1 serves for a visual verification of the presence of SAP (qualitative test), while Test 2 enables quantifying the mass of the collected SAP as a proxy of their concentration in concrete (quantitative test). Different procedures are proposed for these two test methods and their performance is evaluated. The testing procedures were scrutinized in an interlaboratory study carried out by 14 participants from 12 countries. All participating groups detected the presence of SAP in the mix using the qualitative procedures (Test 1). Based on this outcome, we suggest that this method should be applied in the field. In contrast, while most participants obtained reasonably reliable results with the quantification procedure of Test 2, some participants reported large errors. Therefore, the quantification method needs to be further refined, starting from the experience gained in this interlaboratory study.
This study investigates the absorption behavior of superabsorbent polymer (SAP) with different chemical structures and their effect on cement hydration, early-age autogenous shrinkage and mechanical ...properties of cement paste. SAP with high density of anionic functional groups absorbed the cement pore solution quickly, and then released it because the anionic groups on the network of SAP complexed with multivalent cations in the pore solution (e.g., Ca2+). Much less release was measured for SAP with low density of anionic groups. Furthermore, SAP with either both anionic and cationic groups or with only non-ionic groups did not release the liquid. Despite their different behavior in solutions, all SAP were able to counteract autogenous shrinkage. SAP with either both ionic groups or high density of anionic groups showed excellent internal curing effect. The internal curing had no negative effect on the compressive strength of the paste when the total cement-to-water ratio was considered.
An emerging strategy to remove CO2 from the atmosphere and compensate for the greenhouse-gas emissions of cement and concrete is based on incorporating biochar into concrete. With this approach, ...concrete can be turned into a functional carbon sink (C-sink). Until now, biochar has been mainly used without modification to replace part of the cement or of the aggregates in concrete. However, this technology comes with a number of practical problems, which include the high water absorption of the biochar (due to its high specific surface) and hazards (dust, risk of combustion).
In this paper we present an alternative approach, in which biochar is first processed into lightweight aggregates in a cold-bonding process. To this end, biochar is pelletized together with water and a small amount of hydraulic binder forming round pellets that further harden with hydration. In this way, carbon-rich lightweight aggregates (C-LWA) are obtained that are easier to handle than the original biochar. The C-LWA pellets have similar porosity and strength as conventional LWA and can be used for similar applications. Yet, the CO2 emissions from sintering traditional LWA are avoided and the C-LWA are instead an effective C-sink. We demonstrate that it is possible to incorporate in the pellets and eventually in the concrete a sufficient amount of carbon to compensate for the original emissions of concrete. The net-zero emissions concrete obtained with this approach possesses mechanical performance sufficient for typical structural applications in buildings.
•Biochar is processed into lightweight aggregates in cold-bonding process.•The biochar-rich lightweight aggregates added to concrete act as a safe carbon sink.•Structural concrete with net-zero greenhouse gas emissions becomes possible.
An interface sample between Portland concrete and Opalinus Clay with a contact time of 10 years recovered from a field experiment was investigated by SEM-EDX and X-ray CT. The concrete side showed a ...large chloride ingress from the claystone alongside a decalcification and an opening of the porosity. Additional XRD, TGA and leaching experiments of the concrete at few centimetres (∼5 cm) away from the interface confirmed the chloride ingress. The interface was then subjected to a long-term percolation experiment accompanied with repeated X-ray CT-scans. Injection of synthetic claystone pore water proceeded into the claystone-part of the sample, and through the concrete part, whereby the outflow was continuously sampled. The bedding joints that were partially desaturated rapidly saturated, while hydraulic conductivity steadily decreased to values similar to unaltered claystone. The analysis of the exfiltrating aliquots shed light on the advective/diffusive properties of water transport and multi-component solute transport.
In high-performance concrete (HPC) subjected to high temperature during tunnel fires, the build-up of vapor pressure due to dehydration of the cement hydration products cannot be relieved due to the ...very low porosity and permeability of this type of concrete, often resulting in explosive spalling. Explosive spalling may cause devastating damage of the tunnel structure, which threatens both, civilians and emergency response units.
This work suggests a potential application of silica aerogel in the protection of concrete linings, which consists in decorating the surface of HPC structures with a highly-insulating aerogel-cement mortar layer, with the aim of delaying the heating of the HPC and extending the performance of the main concrete structure of the tunnel under fire. The main aim of this study is investigating the impact of the microstructure, with special focus on the pore structure, on the thermal conductivity and the mechanical properties of the aerogel mortars. In particular, the integrity of the aerogels in the mortars, both in the mixing process and the possible long-term chemical degradation, was a main concern. Finally, a preliminary test of the performance against fire spalling was performed. While aerogel-containing mortars were able to protect HPC cubes from fire spalling under a specific thermal loading protocol, the thermal conductivity and the mechanical properties obtained were similar to those of cellular concrete in the same range of total porosity. A possible explanation of the lower-than-expected insulation performance is the partial degradation of the aerogel filler by reaction with the alkaline pore solution of the mortars. Implications of these new findings on aerogel-cement mixtures are also discussed.
The drying process has a prominent impact on the volume changes, crack propagation and durability of concrete structures. This study is to quantify the moisture distribution in real-time drying ...cement mortars. Mortar prisms with different water-to-cement ratios (w/c) and superabsorbent polymers (SAP) amounts were cut into slices and prepared with distinct lengths of cut notches. They were subjected to accelerated drying during neutron radiography measurements. In mortars with higher w/c, the coarser pores accelerate water transport and lead to more rapid drying. The large voids and the SAP in the bulk of the specimens are emptied well before the exposed surface starts to dry out. The presence of notches enhances drying. However, the moisture loss through the notches is less efficient than through the external surfaces. The competition between evaporation from the surfaces and moisture transport within the material, in both liquid and gas forms, governs the moisture distribution evolution.
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