Boone, Matthieu

De Belie, Nele

Goethals, Wannes

Snoeck, Didier

English

Ghent University Academic Bibliography

Analysing dynamic neutron tomography of self-healing concreteusing piecewise constant functionsWannes Goethals∗1, Didier Snoeck†2, Nele De Belie‡2, and Matthieu Boone§11Department of Physics and Astronomy - UGCT, Ghent University, Ghent, Belgium2Department of Structural Engineering - Magnel Laboratory for Concrete Research, Ghent University, Ghent,BelgiumKeywords: dynamic neutron tomography; self-healing concrete; super-absorbent polymers; fluid flowSummary: This work presents the analysis of a dynamic neutron tomography experiment. The goal is tovisualize the internal water flow in a variety of cement-based materials that contain superabsorbent polymers.During hardening of the cement paste, the polymers seem to release their entrained water for internal curingpurposes. Per-pixel analysis of the time attenuation curve using piecewise-constant functions clearly visualizesthis feature and may lead to interesting results and optimization of the material.1. INTRODUCTIONConcrete is a very popular material in construction due to its high compressive strength. But due to structuraltension or autogenous shrinkage during the drying phase, cracks may form. This will make the material moresusceptible to gas or water ingress, weakening the structure by deterioration of the cementitious material orcorrosion of the reinforcement. The use of superabsorbent polymers (SAPs) is widely investigated to improveself-healing behaviour in cement [1].A previous study has been conducted using neutron radiography on a small thin mold [2]. It was found thatthe imaging technique can be used to study the influences of SAPs in cement given sufficient resolution and theuse of tomography. In earlier research [3], the smaller polymers are found to be more ideal in terms of mitigatingself-dessication and autogenous shrinkage. Now 3D tomography is performed to give more information on thewater kinetics from the SAPs towards the cementitious matrix. Furthermore, nuclear magnetic resonance testingclearly showed a difference when differently sized SAPs are added [4]. Their kinetics differ and the mechanismshould be visualized.A local change in attenuation coefficient is related to the transport of water inside the volume element orvoxel. A reduction from the time attenuation curve to piecewise constant functions in each single voxel givesmore accessible information to derive the local kinematics.2. EXPERIMENTAL METHODMaterials In the neutron imaging set-up, five cups with different mixtures were positioned in a vertical stage.The compositions of the different mixtures are equal to those stated in a previous experiment [4]. The cupshave a height of 10mm and an internal diameter of 5.5mm. Two cement pastes are used as reference (R), whiletwo other preparatives contain different types of SAPs (Ae and Be). One final cup is filled with solely water.The mixtures are labelled from top to bottom H2O, R0.30, Ae, Be and R0.354.Neutron tomography A neutron tomography experiment has been performed in the course of 57 hours toimage the structural behaviour of water in drying cement paste. The sample has made 57 rotations of one houreach, and 47 projections were taken each 360◦ rotation from regular angle intervals. The quasi-parallel beamprojection of each cylindrical cup is about 300 pixels wide on the detector plate with pixel size of 21µm.∗Wannes.Goethals@UGent.be†Didier.Snoeck@UGent.be‡Nele.DeBelie@UGent.be§Matthieu.Boone@UGent.beReduction to piecewise constant functions A method developed for reconstruction improvement of dy-namic CT [5] was implemented in GPU code as a stand-alone tool, used to analyse the outcome of a dynamicexperiment. The simplicity of this technique makes it a great tool for a fast check of internal processes. Eachspatial element independently converts its time attenuation curve (TAC) to a piecewise constant (PWC) func-tion of four parameters and a significance measure D. The jumps from low (gl) to high (gh) attenuation phaseoccur at time instances ta, or vice versa at tb.3. RESULTSIn figure 1 one can see a small selection of the PWC analysis in symbolic renders for the two SAP filled samplesAe and Be. Disappearing water (dropping attenuation coefficient) is indicated in blue, adding water in red afterthe first transition. Only the regions with most significant transitions are shown, such that no apparent changewas left in the water reference. These correspond well to the SAP regions, which illustrates the convenience ofthe PWC technique to detect interesting processes. Surprisingly, in Ae the surroundings seem to collect waterbefore the SAPs are in the low phase (blue). This happens when the water outflow occurs at a slow pace: thePWC algorithm indicates approximately the middle time tb of the phase transition, not the start. In Be thisflow is more synchronized, which can hint to faster dynamics. Starting at tb = 13 hours after the scan beginning,the SAPs start to shrink and send their water to the surrounding matrix over the course of the next 12 hours.Figure 1: Result of the data processing on the two SAP filled cementitious materials for five time steps.4. Conclusion- The PWC analysis is a valuable and fast tool for processing of dynamic 3D processes.- In slow processes the results should be interpreted carefully, further analysis is needed in Ae.- The behaviour of SAPs in cementitious materials has been visualised with neutron tomography.- SAP sizes change the internal dynamics, as seen in figure 1.References[1] A. Mignon, D. Snoeck, P. Dubruel, S. Van Vlierberghe, N. De Belie, Crack mitigation in concrete: superab-sorbent polymers as key to success?, Materials 10 (3) (2017) 237.[2] D. Snoeck, N. Alderete, B. Van Belleghem, P. Van Den Heede, K. Van Tittelboom, N. De Belie, Internalcuring of cement pastes by SAPs studied by means of neutron radiography, no. 107, 2017.[3] D. Snoeck, O. M. Jensen, N. De Belie, The influence of SAPs on the autogenous shrinkage properties ofcement pastes with supplementary cementitious materials, Cement and Concrete Research 74 (2015) 59–67.[4] D. Snoeck, L. Pel, N. De Belie, The water kinetics of superabsorbent polymers during cement hydration andinternal curing visualized and studied by NMR, Scientific reports 7 (1) (2017) 9514.[5] G. V. Eyndhoven, K. J. Batenburg, D. Kazantsev, V. V. Nieuwenhove, P. D. Lee, K. J. Dobson, J. Sijbers,An iterative CT reconstruction algorithm for fast fluid flow imaging, IEEE Transactions on Image Processing24 (11) (2015) 4446–4458.

Analysing dynamic neutron tomography of self-healing concrete using piecewise constant functions

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Analysing dynamic neutron tomography of self-healing concrete using piecewise constant functions

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