Use of neutron radiography and tomography to ... - Springer Link

Materials and Structures (2013) 46:105–121 DOI 10.1617/s11527-012-9887-1

ORIGINAL ARTICLE

Use of neutron radiography and tomography to visualize the autonomous crack sealing efficiency in cementitious materials Kim Van Tittelboom • Didier Snoeck • Peter Vontobel • Folker H. Wittmann • Nele De Belie

Received: 26 September 2011 / Accepted: 11 June 2012 / Published online: 3 July 2012 Ó RILEM 2012

Abstract Penetration of moisture into building materials is at the origin of several damage mechanisms. In the case of cement-based materials crack formation is a common problem and highly accelerates the ingress of water and aggressive substances. Crack repair may be needed, however, repair works are expensive and in some cases cracks are even not accessible. Therefore, in this research we aim at autonomous crack sealing. Upon crack appearance, damage is sealed autonomously by the release of encapsulated agents. Visualization of the water uptake by means of neutron radiography for samples with manually and autonomously sealed cracks showed that in both cases ingress of water into the crack can be prevented depending on the type of agent. The K. Van Tittelboom D. Snoeck N. De Belie (&) Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Technologiepark Zwijnaarde 904, 9052 Ghent, Belgium e-mail: [email protected] P. Vontobel ASQ Division, Paul Scherrer Institut (PSI), WBBA/107, 5232 Villigen, Switzerland F. H. Wittmann Aedificat Institute Freiburg, Schlierbergstrasse 80, 79100 Freiburg, Germany F. H. Wittmann Centre for Durability and Sustainability Studies, Qingdao Technological University, Fushun Road 11, Qingdao 266033, China

efficiency of three different agents was examined and it was shown that the use of polyurethane or a water repellent agent were most promising. Neutron tomography scans demonstrated that poor results were obtained when encapsulated methyl methacrylate was used, since one component of the agent hardened inside the capsules before crack appearance. From the results we can conclude that autonomous sealing of cracks is feasible and that neutron radiography and tomography are suitable non-destructive test techniques to visualize the autonomous crack sealing efficiency. Keywords Mortar Glass tubes Polymers Bending cracks Capillary water uptake Neutron beam attenuation

1 Introduction Water penetration into porous building materials is the cause for many damage mechanisms. The humid environment promotes the growth of micro-organisms leading to biodeterioration. Salts dissolved in the penetrating water cause efflorescence during dry periods. Due to freezing, the penetrated water expands and causes material failure. Especially in the case of concrete, water penetration causes leaching of lime, reducing the alkalinity and the strength of the matrix. The humid environment inside cementitious materials can also induce alkali-silica reaction. Moreover, when

106

the intruding water contains dissolved chemicals such as sulfate, chloride or carbon dioxide, this leads to sulfate attack, chloride attack and carbonation. At their turn chloride attack and carbonation might cause reinforcement steel corrosion. At that instant not only the durability but also the structural integrity is impaired. In the case of concrete, cracking is inevitable and these cracks highly accelerate the ingress of water into the matrix. Even though the allowable crack width is limited, it is shown by Zhang et al. [1] that the water penetration is highly accelerated even in the presence of fine cracks due to higher capillary forces. Therefore, cracks need to be repaired if water tightness of the structure is an issue. Sometimes, the cracked surface is treated with a water repellent agent, in other cases cracks are injected with polyurethane, epoxy resins, silicones or methyl methacrylate or the deteriorated zone is chopped off and a repair mortar is applied. However, crack repair becomes difficult when cracks are not visible or accessible, for example for underground structures. Moreover, costs related to repair works amount to half of the annual construction budget [2]. In addition to the direct costs due to maintenance and repair, also the indirect costs due to loss in productivity and occurrence of traffic jams carry a severe economic penalty. Accordingly, autonomous sealing of cracked concrete would be highly beneficial. Autonomous crack repair is actually an old and well known phenomenon for concrete as it possesses some natural autogenous healing properties. Due to ongoing hydration of clinker minerals or carbonation of calcium hydroxide cracks may heal after some time. However, autogenous healing is limited to small cracks, is only effective when water is available and is difficult to control. Therefore, concrete is modified in order to build in autonomous crack repair. In 1969, self-healing properties were for the first time built-in inside polymeric materials. In 1979 and 1981 publications about self-healing in thermoplastic and cross-linked systems appeared. In the 90s, Dry started to work on self-healing concrete [3] and polymers [4]. Although, it was only in 2001 when White et al. [5] published their paper in Nature about self-healing in polymer-based materials that the research on self-healing materials started to attract a lot of attention and became a hot topic. In this research, a mechanism is embedded inside the cementitious material allowing immediate crack

Materials and Structures (2013) 46:105–121

sealing after formation through release of an agent from capsules that break upon crack formation. In this way, cracks should become watertight. To evaluate the sealing efficiency the attenuation of electromagnetic radiation such as gamma-rays [6–8] or X-rays [8–11] or the attenuation of neutrons [1, 12– 19] can be utilized. As neutrons offer the highest hydrogen detection sensitivity, neutron radiography was used in this study to visualize the water uptake in cement-based samples. The application of neutron radiography to study cementitious materials dates back to 1972. At that time Reijonen and Pihlajavaara [20] made use of this technique to monitor carbonation in concrete. Neutron radiography has also been employed to visualize cracks in concrete. In order to enhance the contrast samples were impregnated with a high neutron absorbing substance [21]. De Beer et al. [22, 23] used this technique to study the drying process of concrete and to determine the porosity of concrete samples with different water-cement ratio and different curing time. Other researchers [24, 25] made use of neutron radiography to visualize the impregnation depth of polymerized consolidants and water repellent agents into building materials. When the sample is rotated during the measurements, three-dimensional neutron tomographs are obtained [12, 26, 27]. Trtik et al. [28, 29] used neutron tomography to visualize the water release from super absorbent polymers to the surrounding cement matrix in order to obtain internal curing. In the current study, the self-sealing efficiency of three types of agents was studied. The influence of the released agent on water penetration was investigated by means of neutron radiography and a comparison was made between manual and autonomous crack treatment. Moreover, for some of the agents under investigation, the amount of crack filling with the agent was visualized by means of neutron tomography.

2 Materials 2.1 Agents used for crack repair Three different types of agents were investigated on their appropriateness to be used in self-sealing cementitious materials. The first is a commercially available polyurethanebased agent (MEYCO MP 355 1K, BASF The


Chemical Company) which is used in practice to make cracks watertight and to cut off running water. This agent consists of two components, one being a prepolymer of polyurethane and the other being an accelerator which shortens the reaction time. When both components come into contact with water, an expanding reaction starts after several minutes and a hard foam-like material is formed. The advantages of the expanding foaming reaction were already demonstrated in previous research [30]. Due to the expansion, a bigger crack space can be filled while only a small volume is occupied when being encapsulated. Moreover, the expansion acts as a driving force for the agent to come out of the capsules. The second agent is a non-commercial, methyl methacrylate based, agent which was synthesized in cooperation with the Polymer Chemistry and Biomaterials Group (PBM) of Ghent University. This is also a two-component agent from which the first component consists of a mixture of methyl methacrylate, an initiator (benzoyl peroxide) and poly (methyl methacrylate) in order to increase the viscosity. The second component consists of methyl methacrylate, an activator (dimethyl-p-toluidine) and poly (methyl methacrylate) to increase the viscosity. When both components come into contact, a polymerization reaction starts and a hard Plexiglas-like, transparent polymer forms within a few hours. The advantage of this agent is that the tensile strength of pure poly (methyl methacrylate) is higher than the tensile strength of cementitious materials. So by using this agent it should be feasible to create a crack repair which is stronger than the initial material. In that case, further cracking will occur at another location within the material where there is still encapsulated agent available and thus several sealing actions may occur. Moreover it has been shown [31] that the polymerization reaction of this agent is relatively insensitive to the mix ratio of both components which makes it suitable for autonomous crack sealing. The third agent is a commercial water repellent agent (Rubson Invisible Terasses, Henkel) which is used as an invisible treatment for porous materials such as concrete, bricks, natural stones, etc. to make them dirt- and water-repellent. Due to the nonpolar character of this agent, the surface tension of the material is lowered and liquid water cannot enter the pores while vapour permeability of the material is not diminished [25]. This agent consists of one-

107

component which is fully cured 24 h after application. The efficiency of this agent to restore the liquid tightness of concrete cracks in an autonomous way was not yet evaluated before but the advantage of this type of agent is that cracks can become watertight while no leaking agent is visible from the outside of the structure, which may be an advantage in case visual appearance of the structure is important. While polyurethane and methyl methacrylate seal the crack by filling it, the water repellent agent prevents the ingress of water into the crack by making the crack faces water repellent. 2.2 Encapsulation of the agents Borosilicate capillary glass tubes, with an inner diameter of 3.00 mm, an outer diameter of 3.35 mm and a length of 75 mm, were used to carry the agents (Hilgenberg, Germany). For the two-component agents, being polyurethane and methyl methacrylate, half the tubes were filled with the first component and the other half were filled with the second component. For the water repellent agent, consisting of only one component, the same total amount of tubes was filled with this agent. First, the tubes were sealed at one end by means of a fast curing adhesive (X60, HBM, Germany) in case of the polyurethane or water repellent agent, or by means of plasticine and a droplet of cyanoacrylate glue in case of the methyl methacrylate agent. Then, the tubes were filled with (the components of) the agent, which was/were injected by means of a syringe with a needle. When all tubes were filled, the other ends were sealed with plasticine and/or adhesive. Finally, two tubes, filled with each of both components of the agent or two times the same component in case of the water repellent agent, were fixed to each other. 2.3 Mortar samples with and without autonomous crack sealing properties All specimens were prepared using the mortar composition given in Table 1. Ordinary Portland cement (CEM I 52.5 N) and DIN standard sand with a maximum grain size of 2 mm were mixed with tap water according to the standard NBN EN 196-1 [32]. Moulds with dimensions of 100 mm 9 100 mm 9 300 mm were used for preparation of eight series of mortar beams, as shown in Table 2. All mortar beams

108


were reinforced with six smooth steel bars having a diameter of 2 mm. Three steel bars were positioned at a height of 25 mm, the remaining three bars were positioned at 75 mm height (Fig. 1a). For beams of

the test series ‘AUT’, tubular capsules were first positioned into the moulds. Therefore, four steel wires were attached to the sides of the moulds. Two at a height of 10 mm and two at a height of 40 mm. Six couples of tubes were glued onto the lower and the higher steel wires (Fig. 1a). This was done in order to be sure that the position of the tubes inside the mortar specimens would not change during casting. Each time a tube filled with one component of the agent was positioned next to a tube filled with the other component. For the water repellent agent two tubes filled with the agent were placed next to each other. Moulds were filled with mortar in several layers and each layer was compacted by means of vibration. Besides the three ‘AUT’ series, five more test series were prepared in the same way as described above, however, samples belonging to these series contained only reinforcement bars. After preparation, all beams were placed in an air conditioned room with a temperature of 20 °C and a relative humidity of more than 95 %. Specimens were demoulded 24 h later and then stored again under the same conditions until the time of testing. Before crack creation, the steel reinforced prisms were cut with a diamond saw into three slices along the long axis of the prisms (Fig. 1a) so specimens with dimensions of 30 mm 9 100 mm 9 300 mm were obtained (Fig. 1b).

Fig. 1 Test specimens used during the experiments. Mortar beams (100 mm 9 100 mm 9 300 mm) with(out) encapsulated agent were sawn into three prisms (a). The obtained prisms (30 mm 9 100 mm 9 300 mm) were subjected to a crack-width controlled bending test. During the test an LVDT was attached at the bottom of the prism. After crack creation the

middle part of the prism was sawn out for further testing (b). The water absorbed by (cracked) specimens (30 mm 9 100 mm 9 100 mm) was visualized by means of neutron radiography (c). The crack filling efficiency of prisms (30 mm 9 20 mm 9 100 mm) was studied by means of neutron tomography (d)

Table 1 Composition of the mortar mix 3

Material

Amount/m (kg)

Sand 0/2

1,530

CEM I 52.5 N

510

Water

255

Table 2 Different test series under investigation Code

Description

UNCR

Uncracked beams

REF

Reference beams (no crack sealing)

MAN_PUR MAN_MMA

Manual crack sealing with polyurethane Manual crack sealing with methyl methacrylate

MAN_WRA

Manual crack sealing with water repellent agent

AUT_PUR

Autonomous sealing with polyurethane

AUT_MMA

Autonomous sealing with methyl methacrylate

AUT_WRA

Autonomous sealing with water repellent agent


3 Experimental methods 3.1 Crack creation At the age of 14 days, in samples from each test series except the series ‘UNCR’, cracks were created by means of a crack width controlled three-point-bending test. The crack width was measured by means of a linear variable differential transformer (LVDT, Solartron ATR/5/S) with a measurement range of ±5 mm and an accuracy of 5 lm. This LVDT was attached at the bottom of the sample and measured the displacement over a distance of 8 cm. During the bending test, mortar samples were placed onto two steel bars creating a span of 280 mm. The force was applied, by means of a third steel bar, positioned in the middle of the specimen (Fig. 1b). The crack width was increased with a velocity of 0.5 lm/s until a crack of 400 lm was reached. At that point, the specimen was unloaded giving cause to a decrease in crack width. The resulting crack width (w) was measured at the bottom of each sample at three different locations by means of a stereo microscope (Leica S8 APO) with a camera (DFC 295) and amounted to approximately 280 lm.

109

due to capillary forces. Leakage of the polyurethane is shown in Fig. 2a. Upon contact of both components, a foam was formed, resulting in crack sealing. When encapsulated methyl methacrylate was embedded inside the mortar beams, no leakage of the agent was noticed. However, during the bending test the sound of breaking tubes was noted together with the smell of methyl methacrylate. When water repellent agent was encapsulated, sometimes leakage of the agent was noticed at the crack faces (Fig. 2b). Cracks of the samples belonging to the test series ‘MAN’ were manually treated after crack formation. For the cracks which were manually sealed with polyurethane, the prepolymer was mixed first with water and accelerator in the same proportions as encapsulated in the tubes. Next, the mixture was injected into the crack by means of a syringe with a needle. Injection was stopped when the crack was completely filled with sealing agent. For the other agents under investigation, (a mixture of both components of) the agent was also injected into the crack by means of a syringe. 3.3 Visualization of the crack sealing efficiency 3.3.1 Neutron radiographic facility

3.2 Crack treatment Cracks of the specimens containing encapsulated agent were autonomously treated. The embedded tubes broke during crack formation and (both components of) the agent was/were released into the crack

Experiments were performed at the thermal neutron radiographic facility, called NEUTRA, which is part of the Swiss spallation source SINQ of the Paul Scherrer Institute (PSI) in Switzerland [33]. It is a steady state neutron source, driven by a proton beam from a ring

Fig. 2 Leakage of polyurethane (a) and water repellent agent (b) at the crack faces

110


cyclotron. The neutron beam then passes by a collimator which is a beam forming assembly, defining the geometric properties of the beam and containing filters to influence the energy spectrum of the beam and to reduce beam pollution by gamma-rays [16]. The collimation ratio (L/D, where L is the length of the collimator and D is the diameter of the entrance aperture) amounted to 550. The diameter of the beam at the position of the object was 400 mm and the average proton beam current measured 1.5 mA so neutron flux was 3.96 9 106 cm-2 s-1 mA-1. The mean energy of the neutrons measured 25 meV. When the neutrons passed through the object, they were recorded by a 100 lm thick neutron sensitive 6LiF/ZnS scintillation screen, which converted the neutrons into light. The detectors field of view measured 310 mm 9 310 mm. The light emitted from the scintillation screen was then deflected by a mirror and recorded by a cooled slowscan CCD camera with a 50 mm AF-S NIKKOR lens and a size of 2,048 pixels 9 2,048 pixels resulting in a nominal pixel size of 0.1515 mm at full resolution. The test setup is shown in Fig. 3. 3.3.2 Visualization of water absorption by neutron radiography The principle of radiography consists of recording the radiation passing through an object by a position sensitive detector. The detector records an image which contains spatial information on the intensity of radiation. Because the attenuation of the radiation in the object depends on the isotopic number density and the geometry of the sample, the image contains qualitative and quantitative information on the structure and composition of the object. Contrary to X-rays, neutrons are attenuated by some light materials, but

penetrate many heavy materials. Pleinert et al. [16] showed that the attenuation of neutrons by water is about 10 times higher than that of frequently used building materials such as concrete and brick. Therefore, neutron radiography perfectly serves the purpose to visualize water inside cementitious materials. After complete polymerization of the agent, the center part of all (cracked) specimens, was sawn out so samples with dimensions of 30 mm 9 100 mm 9 100 mm were obtained (Fig. 1c). After the specimens were sawn out, they were dried in an oven at 50 °C. Brew et al. [12] reported that drying in an oven at 50 °C minimizes microstructural damage and yet allows significant water transport in the capillary pore structure. After 1 week, specimens were removed from the oven and sample dimensions were determined. Before contact of one small surface (30 mm 9 100 mm) with water, the square surfaces (100 mm 9 100 mm) and two opposite small surfaces (30 mm 9 100 mm) were covered with self-adhesive aluminum foil in order to impose unidirectional moisture movement during the test. Before starting the test, specimens were weighed using a Sartorius BP 3100 S balance. During each neutron radiography measurement four samples were scanned simultaneously. Therefore they were placed in a test frame as shown in Figs. 3 and 4a. Two specimens, with their cracked surface downwards, were placed onto two line supports in an aluminum container which was positioned at the highest level of the frame while two more specimens were placed onto line supports at the lower level of the frame in an aluminum container. Before the containers were filled with water, reference images of the samples were taken in the dry state. Three reference images were taken at short exposure time (3 s) resulting in a pixel

Mirror Samples Darkroom

CCD camera

Fig. 3 Test setup used during the neutron radiography experiments. The neutron beam leaving the collimator was attenuated by the samples and detected by the scintillator. When passing

Scintillator screen

Collimator

the scintillator screen the beam was turned in the direction of the CCD camera by means of a mirror


111

Fig. 4 Test setup used for performance of neutron radiography (a) and neutron tomography (b) scans

size of 0.302 mm and three images were taken at higher exposure time (30 s) resulting in a smaller pixel size of 0.1515 mm. Subsequently, water was automatically supplied into the containers by means of a water valve which was controlled from the outside of the shielded bunker. When the water level raised ±4 mm above the bottom of the samples, water supply was turned off. First, 200 radiographs with an exposure time of 3 s were taken in time steps of approximately 5 s (3 s exposure time ? process delay). After this, 40 images were taken consecutively with an exposure time of 30 s. Then, images with an exposure time of 30 s and a waiting time of 270 s were taken till the samples were in contact with water for 4 h. At that time, samples were removed from the containers and weighed. After this action, one sample of each test series was immersed again and when immersed for 24 h one more radiographic scan was taken with an exposure time 30 s. In total, three samples were examined for each test series. 3.3.3 Quantification of water content in cracked samples If neutrons pass through the sample, most of them are scattered by the atomic nuclei and only a few neutrons are absorbed. Scattered neutrons are not necessarily removed but can reach the scintillator and appear in the radiographs. Hence the transmission of the neutron beam through the sample is overestimated. Thus the

attenuation and the sample density are underestimated, unless the scattering contribution is taken into account. Hassanein et al. [34] stated that without a scatter correction the calculated water content would be 50 % lower than the real content. For quantitative evaluation of the radiographs, a scattering correction based on point scattered functions (PScF) was applied [34–36]. Besides the scatter correction, a CCD dark current, background scattering, spectral effect, intensity and flat field correction was taken into account by using the image correction tool ‘quantitative neutron imaging’ (QNI) [37]. Based on the corrected images, quantification of the water content in the samples can be performed according to the method described below. In general, the relationship between the incident and attenuated neutron beam is expressed by Beer’s Law as given in Eq. (1). I ¼ I0 expðR dÞ

ð1Þ

where I is the flux transmitting through sample and reaching detector (cm-2 s-1 mA-1), I0 the flux leaving collimator and penetrating into sample (cm-2 s-1 mA-1), R the attenuation coefficient (cm-1), and d is the thickness of sample (cm). The transmitted neutron beam through the sample in dry and wet state can be expressed as shown in Eqs. (2) and (3). Idry ¼ I0 exp Rdry d ð2Þ ð3Þ Iwet ¼ I0 exp Rdry d Rw dw

112


where Idry is the flux transmitting through dry material (cm-2 s-1 mA-1), Iwet the flux transmitting through wet material (cm-2 s-1 mA-1), Rdry the attenuation coefficient of the dry material (cm-1), Rw the attenuation coefficient of liquid water (cm-1), and dw is the thickness of fictitious water layer in agreement with water content in sample (cm) (Fig. 5). The water content W (kg/m3) is defined as shown in Eq. (4). W¼

qw Vw qw dw ¼ V d

ð4Þ

where qw is the density of water (kg/m3), Vw the volume of water (m3), and V is the volume of sample (m3). Using the latter equation, Eq. (3) can be rewritten. Rw W d Iwet ¼ Idry exp ð5Þ qw This results in expression (6), which can be used to calculate the water content. q Iwet W ¼ w ln ð6Þ Rw d Idry From this it can be concluded that the water content can be calculated by logarithmically dividing an image obtained in the wet state by an image in the dry state and multiplying this value by the density of water and dividing it by the attenuation coefficient of water (Rw = 3.64 cm-1) and the thickness of the sample. To verify the calculation method, the water content in the water basin, which should be equal to 1 g/cm3, was determined. Using the above mentioned calculation method, the water content in the basin amounted to Fig. 5 The water content in the samples is obtained by logarithmically dividing an image of the sample in the dry state (Idry) by an image of the sample in the wet state (Iwet). The attenuation of the water inside the sample can be seen as the attenuation of a fictitious layer of water with thickness dw which is placed behind the sample

0.1 g/cm3. As this value was 10 orders of magnitude smaller than what would be expected, a calibration factor of 10 was included [38].

3.3.4 Visualization of crack filling efficiency by neutron tomography Neutron tomography complements other computed tomography techniques like X-ray computed tomography due to the specific attenuation characteristics of neutrons by light elements like hydrogen or polymeric materials [27]. This makes the technique suitable to detect the polymerized polyurethane and methyl methacrylate inside the cracks of mortar specimens. Previous attempts to visualize the agent inside the crack by means of X-ray computed tomography [30] were successful although quite some effort had to be undertaken to distinguish between air and polymeric materials inside the crack. Specimens used for neutron tomography were obtained by sawing a mortar beam of the test series MAN_PUR, MAN_MMA, AUT_PUR and AUT_MMA into prisms with dimensions of 30 mm 9 20 mm 9 100 mm as shown in Fig. 1d. These prisms were positioned vertically onto a rotation stage which was placed in between the collimator and the scintillation screen (Fig. 4b). A series of neutron radiographs with an exposure time of 30 s were taken while the sample was rotated over an angle of 180° with rotation steps of 0.5°. Reconstruction of the data was performed using the LabView based package ‘‘Octopus’’ developed at the Centre for X-ray Tomography of Ghent University (UGCT) [39].

detector

detector

I dry

Iwet

detector Iwet dw Idry d

I0

I0 I0

source

source

source


4 Results 4.1 Visualization of water absorption by neutron radiography In Fig. 6 (middle), for one representative sample of each test series, a differential radiographic image, obtained after 30 s (pixel size 0.302 mm) and 24 h (pixel size 0.1515 mm) of contact with water, is shown. Dark grey zones correspond to places where water is available. On the left a picture of the samples themselves is shown on which the crack location, crack width and the zone used in the quantitative analysis are indicated. In the graphs on the right, the moisture profile is shown when the sample was in contact with water for 30 s, 1 min, 5 min, 15 min, 30 min, 1 h, 2 h, 4 h and 24 h. The moisture profile was determined as the average water content over a band width of 160 pixels (pixel size 0.302 mm) symmetric around the crack and perpendicular to the moisture front (see indicated rectangles in Fig. 6, left). In the radiographic images with a pixel size of 0.1515 mm of samples from the test series ‘AUT_PUR’, ‘AUT_MMA’ and ‘AUT_WRA’ the positions of the capsules can clearly be noticed. Also in the graphs the capsule locations are noticed as positions with reduced water content. Negative water content values in ‘‘dry’’ zones of the sample are artifacts due to in-sufficient scatter correction. However, in the wet regions, which are most important in this study, more correct values of the water content are obtained after use of the scattering correction. It is noticed from the radiographic images obtained after the samples were for 30 s in contact with water that the untreated crack immediately fills with water. When the crack is manually treated with methyl methacrylate, the crack is filled with water till a limited height but when the crack is autonomously sealed with methyl methacrylate, water enters into the crack along the whole height. However, when the crack is manually or autonomously treated by means of polyurethane or by making use of the water repellent agent, after 30 s no water enters into the crack. Only at the crack mouth of the autonomously treated samples little water entry is seen. After 24 h of contact, all samples contain a certain amount of water due to capillary water absorption along the matrix pores of the cementitious material. However, the amount of water taken up is dependent

113

on the crack treatment. When the crack has been left untreated, the water front reached the upper side of the sample. There, the water starts evaporating and an equilibrium arises as the same amount of water evaporates at the top of the sample as is absorbed at the bottom. Moreover, it can be seen that the upper part of the sample is darker than the rest. This means that more water is available at that position. This is due to the fact that the matrix is more porous at that position due to micro cracks which formed at the upper part of the beam where the force was located during the bending test. This was also noticed by Zhang et al. [40]. When cracks are manually treated with polyurethane, after 24 h of contact with water, an evenly distributed amount of water is absorbed by the matrix. The amount of absorbed water is comparable to the amount absorbed by the uncracked sample. When the crack is manually sealed with methyl methacrylate, after 24 h, an extra amount of water is absorbed by the crack and the zone around it. The water rapidly entered the crack and from that time, it penetrated horizontally into the cementitious matrix. When cracks are treated manually by making use of the water repellent agent, the crack and the zone around the crack seem to be completely free of water. The water repellent agent has a very low viscosity and is therefore able to penetrate deeply into the matrix of the material. Due to this, the efficiency of this product is not only noted in the crack itself but also in the zone around it. When encapsulated polyurethane is embedded inside the matrix, water will not enter the crack even after 24 h of contact with water. However, in comparison with the uncracked sample, a higher amount of water penetrated in the matrix just beside the crack. This is possibly because the mortar matrix was less dense in the zone under the capsules as the capsules prevented the air bubbles, arising during vibration, to escape. When cracks are autonomously sealed with methyl methacrylate, the crack and the zone around are completely filled with water after 24 h. Constrictions in the water front appear at the capsule locations. It seems that the crack is only filled at the location of the capsules and that the methyl methacrylate agent was not able to fill the crack completely upon rupture of the capsules. Nevertheless, for one sample of this test series complete crack sealing was observed. From this it follows that it is possible to seal the crack completely with encapsulated methyl methacrylate, however, release of this type of agent and curing was

114

UNCR

24h

Sample height [cm]

10

30s

30s 1min 5min 15min

30 min 24h 1h 2h 4h

30s

24h

30s

24h

30s

24h

30s

24h

30s

24h

30s

24h

Sample height [cm]

24h

0 10

Sample height [cm]

30s

0 10

Sample height [cm]

MAN_MMA w = 360 µm

MAN_PUR w = 197 µm

REF w = 163 µm

0 10

Sample height [cm] Sample height [cm]

0 10

Sample height [cm]

0 10

0 10

Sample height [cm]

AUT_MMA w = 161 µm

AUT_PUR w = 278 µm

MAN_WRA w = 223 µm

0 10

AUT_WRA w = 258 µm

Fig. 6 Neutron radiographs obtained after 30 s and 24 h of contact with water for one representative sample of each test series (field of view amounts 100 mm 9 100 mm). The moisture profiles determined at different time intervals are shown on the right. Closed dots represent the position of the reinforcement, open dots render the position of the capsules. On the left a picture of the sample under investigation is shown together with the location of the crack, the crack width and the zone in which the average water content was calculated


0 -0.1 -0,01

0.0 0

0.1 0,01

0.2 0,02

Water content [g/cm³]

0.3 0,03


not noted for all samples. Therefore, it was thought that for some samples premature hardening of the agent inside the capsules occurred and that this caused the poor behaviour of these specific samples. When water repellent agent is encapsulated, only a little water enters the crack mouth and the amount of water absorbed in the matrix around it is comparable to the amount absorbed by the uncracked sample. In order to prevent reinforcement corrosion, it is important that the water front does not reach the reinforcement location. For an uncracked sample the water does not reach the reinforcement even after 24 h of continuous contact with water. For a cracked sample which is left untreated the water front reached the reinforcement already after a few seconds of contact with water and thus at that time corrosion may initiate, especially when the water contains dissolved chlorides. When cracks are treated manually or autonomously with polyurethane or water repellent agent, the reinforcement remains in the dry zone even after 24 h of contact with water. When methyl methacrylate is used, the water front will reach the reinforcement location. The poor behaviour of the methyl methacrylate agent is ascribed to micro crack formation inside the polymerized agent due to shrinkage and possibly early curing of the agent inside the capsules. Further modification of the agent can make it more suitable to seal concrete cracks. Note that in these tests, water ingress was accelerated as samples were dried before exposure to the water bath. Therefore, the drawn conclusions can not immediately be extrapolated to practice. As already mentioned, the efficiency of a water repellent agent for autonomous restoration of the liquid tightness of cracked cementitious materials was not yet evaluated before. From the first results it can already be concluded that the water repellent agent seems to behave as well as the polyurethane when encapsulated and embedded in mortar samples. Moreover, when enough agent is provided (manual crack treatment with water repellent agent), it is able to penetrate into the porous matrix and to make also the zone around the crack watertight. However, the efficiency of manual water repellent treatments was already shown before [41]. Although this agent was selected based on its potential to make cracks watertight without being visible at the crack faces, the zone where the agent leaked out of the crack became visible when the specimens were wet (Fig. 7). Completely

115

Fig. 7 Visibility of the water repellent agent when the mortar samples are wet

invisible crack treatment seems not to be possible with this type of agent but its efficiency was shown as it is able to penetrate into the matrix and to make not only the crack but also the zone around it watertight. Moreover, as long as the construction remains dry, leakage of the agent would not be visible. 4.2 Crack sealing by the release of encapsulated agents The autonomous crack sealing efficiency was evaluated for all samples containing encapsulated agent, after 4 h of contact with water (Fig. 8). The water profile was determined in a rectangular zone located below the lower capsules (rectangle in white dashed line, white curve) and another zone located above the upper capsules (rectangle in black dashed line, black curve). For the samples containing encapsulated polyurethane, a drop in water content is noticed at the crack location (indicated by an arrow) in the zone below the capsules. This shows that the crack is already filled with polyurethane and thus sealed at the crack mouth. However, the drop in water content is very local and water is still absorbed by the matrix around the crack, even in the near vicinity of the crack. For one of the samples containing encapsulated water repellent agent (second sample of test series ‘AUT_WRA’ shown in Fig. 8), the water content is also reduced in the zone

0.7

0.6

0.6

0.6

0.5 0.4 0.3 0.2 0.1

0.5 0.4 0.3 0.2 0.1


0.7


0.7

0.5 0.4 0.3 0.2 0.1 0.0

-0.1

-0.1

-0.1

0.7

0.7

0.7

0.6

0.6

0.6

0.5 0.4 0.3 0.2 0.1

0.5 0.4 0.3 0.2 0.1


0.0


0.0

0.5 0.4 0.3 0.2 0.1

0.0

0.0

-0.1

-0.1

-0.1

0.7

0.7

0.7

0.6

0.6

0.6

0.5 0.4 0.3 0.2 0.1

0.5 0.4 0.3 0.2 0.1


0.0






AUT_WRA

AUT_MMA

AUT _PUR

116

0.5 0.4 0.3 0.2 0.1

0.0

0.0

0.0

-0.1

-0.1

-0.1

Fig. 8 Water profile after 4 h of contact with water for samples containing encapsulated sealing agent in a zone located below the lower capsules (rectangle in white dashed line, white curve)

and a zone located above the upper capsules (rectangle in black dashed line, black curve)

around the crack, proving that the agent is able to penetrate into the matrix and has a wider efficiency. Moreover, for this specific sample, the water content inside the crack amounts 0 g/cm3, proving that the crack and the zone around are completely free of water. However, for the two other samples of this series this positive effect was not noted. When the samples contained encapsulated methyl methacrylate, for one sample a pronounced drop in water content is seen (second sample of test series ‘AUT_MMA’ shown in Fig. 8), while for the two other samples of the same test series the water content contains a peak

at the crack location. For these samples, the crack colours black in the neutron radiographs indicating that they were almost completely filled with water. These findings correspond with our thought that for some of the samples premature hardening of the methyl methacrylate in the capsules occurred. The water content in the zone above the upper capsules is equal to 0 g/cm3 when polyurethane or water repellent agent is encapsulated. For the test series with methyl methacrylate as agent, this is only the case for one of the three samples. For the two other samples of that test series, the crack was filled with


4.3 Differences in the amount of capillary absorbed water The weight of each sample was determined before starting the water absorption measurement and after the samples had been in contact with water for 4 h. The difference in weight between the wet and dry samples results in the amount of water absorbed by the sample in a time frame of 4 h. The amount of absorbed water can also be calculated from the neutron radiographic measurements. When the mean thickness of the water layer in the wet region over the whole width of the sample (not only in the zone around the crack) is determined, the total amount of water present in the sample at a given contact time with water can be calculated. This calculation was made for the radiographic scans taken after 4 h and the results were compared with the gravimetrical measurements. No significant difference was noticed between the values obtained when using both techniques (Fig. 9). This served as a proof that the water content can be calculated accurately from the neutron radiographs. This was also noted by several other researchers [12, 22, 23]. De Beer et al. [23] stated that a big advantage of neutron radiography, compared to gravimetric measurements, is that quantitative distribution information can be obtained non-destructively instead of a bulk measurement. From the results shown in Fig. 9, a comparison can be made between the amount of water absorbed by each of the different test series. A significant difference is seen between the efficiency of untreated cracks or cracks autonomously treated with methyl methacrylate and the other test series under investigation. While the mean amount of water absorbed by untreated cracks or autonomously sealed cracks with methyl methacrylate amounted to 7.6 g (gravimetric measurements) or 7.3 g (neutron radiography) and 7.9 g (gravimetric measurements) or 7.8 g (neutron radiography), respectively, the remaining test series absorbed much less water. The standard deviation obtained for samples containing encapsulated methyl methacrylate is higher than that obtained when

20

Amount of water absorbed [g]

water till the crack tip and the water horizontally migrated into the matrix around the crack. A water content of almost 0.2 g/cm3 is noticed in a zone of 50 mm around the crack. This means that for the latter samples the reinforcement is located in the wet zone and thus the risk of reinforcement corrosion exists.

117

Balance Neutron radiography

18 16 14 12 10 8 6 4 2 0 0 5 UNCR REF

10

MAN15

20

25 AUT

30

PUR MMA WRA PUR MMA WRA

Fig. 9 Amount of water absorbed by the samples after 4 h contact with water, determined by means of gravimetrical measurements and by analyzing corrected neutron radiographs. Dots represent the mean value (n = 3), standard bars represent the standard error

weighing the other test series. This conclusion could also be drawn when analyzing the neutron radiographic images. While some samples of that test series seemed to be watertight, others absorbed a high amount of water. While self-sealing with methyl methacrylate does not result in a better protection against water ingress compared to untreated cracks, the water tightness of the other techniques under investigation does not differ significantly compared to uncracked samples. As no significant difference was obtained when comparing the gravimetrical measurements and the calculated amount of absorbed water based on the radiographs taken after 4 h of contact with water, the water content was also calculated for other contact times. In Fig. 10, the amount of water absorbed is shown in function of the square root of time. An almost linear relationship is noticed. While the water content of untreated cracks and cracks autonomously sealed with methyl methacrylate increases rapidly, the other test series show the same progress as uncracked samples. This finding is in agreement with previous conclusions. 4.4 Visualization of crack filling efficiency by neutron tomography In the rendered 3D image (Fig. 11a) of the sample manually sealed with polyurethane, it is seen that for this specific sample the crack runs over the complete height. This is most clear when the rendered image is

118


cut (Fig. 11b). Moreover it is noticed that the complete crack is coloured light grey. As polyurethane contains lots of hydrogen bonds, it is highly attenuated by neutrons and will therefore colour lighter. Because the complete crack colours light grey it is proven that the crack is completely filled with polyurethane. From the image shown in Fig. 11c it is noted that the polyurethane does not form a continuous layer. However one must be careful when drawing this conclusion as the pixel size of the scans is limited and thus it is possible that a biased view was obtained. For the sample which was autonomously sealed with polyurethane and scanned by means of neutron tomography the crack runs till half height of the sample (Fig. 12a, b). Over the whole length of the crack it is coloured light grey which shows that also in the case of autonomous crack sealing, the crack is completely filled with agent. In Fig. 12a it is even shown that some agent

leaked out of the crack and flew over the surface of the sample. Another interesting notice is that the capsules seem to be completely empty. Due to breakage of the capsules during crack formation both components of the agent came out of the tubes and filled the crack where they started to polymerize. Capsules filled with methyl methacrylate, were not completely empty after crack formation (Fig. 13). Each time one of the two tubes is still completely filled with agent as it colours light grey on the rendered images (Fig. 13b). When preparing the mortar samples, each time the same component was positioned at the right and the other at the left of the sample. As it is each time the tube at the right which is still filled, the authors suppose that the poor behaviour of the methyl methacrylate agent is due to the fact that one of both components polymerized inside the tubes before crack formation and therefore only one component could flow into the crack and cause the smell of methyl methacrylate during the bending test. As only one component was released, polymerization of the agent and crack sealing were impossible. In this sample, the crack coloured black, like the pores in the sample. This shows that the crack was filled with air instead of agent. The authors believe that the component which was released during crack formation evaporated after some time as no polymerization reaction could take place. In Fig. 14 a 3D rendered image of a sample manually sealed with methyl methacrylate is shown. Polymerized methyl methacrylate can be clearly seen inside the crack. This shows that it is possible to visualize poly(methyl methacrylate) inside a cementitious matrix and that no polymer filled the crack when encapsulated methyl methacrylate was provided.

Fig. 11 3D rendered images of the sample from which the crack was manually sealed with polyurethane. Render of the complete sample (a). Render where part of the sample was cut

off. The white grey line corresponds with the polyurethane inside the crack (b). Visualization of the polyurethane inside the part of the sample which was cut off in the image (c)

Amount of water absorbed [g/cm³]

9

UNCR

8

REF

7

MAN_PUR

6

MAN_MMA MAN_WRA

5

AUT SHC_PUR

4

AUT SHC_MMA

3

AUT SHC_WRA

2 1 0 0.0

0.5

1.0

1.5

2.0

Time [ h]

Fig. 10 Increase in the amount of water absorbed for each of the different test series. Symbols represent the mean value (n = 3)


119

Fig. 12 3D rendered images of the sample from which the crack was autonomously sealed with polyurethane. Render of the complete sample where leakage of the polyurethane out of the crack can be seen. The position of the tubes is also visible

(indicated with arrow) (a). Render where part of the sample was cut off. The white grey line corresponds with the polyurethane inside the crack (b)

Fig. 13 3D rendered images of the sample from which the crack was autonomously sealed with methyl methacrylate. Render of the complete sample where it becomes visible that the

crack is not filled in this case (a). Render where the upper part of the sample was cut off. It is clear that each time one of both tubes is still filled with sealing agent (indicated with arrows) (b)

Fig. 14 3D rendered image of sample from which the crack was manually sealed with methyl methacrylate

5 Conclusions It can be concluded that neutron radiography is a very efficient non-destructive test method to evaluate the

crack sealing efficiency by visualizing the capillary water uptake. After correction of the neutron images, it is possible to calculate the water content very accurately as no significant difference was noted between the gravimetric measurements and the water content calculated from the neutron radiographs. From the neutron radiographs, it was shown that the polyurethane agent and the water repellent agent completely resist water ingress when applied manually or when encapsulated in the matrix. The polyurethane agent seals the crack by filling it with a foamlike material and thus prevents ingress of liquids and gasses which may contain harmful substances. The water repellent agent makes the crack fasces water repellent and prevents the ingress of liquids. One should notice that when using the latter type of agent

120

gasses such as CO2 can still penetrate and possibly cause carbonation. It was the first time that the efficiency of a water repellent agent to make concrete cracks watertight in an autonomous way was investigated. Results seem very promising as no significant difference was noticed between the efficiency of cracks manually or autonomously treated with water repellent agent and uncracked samples. As the leaked water repellent agent became visible when specimens were wet, the aim of this agent, to provide an invisible watertight crack protection, is not fulfilled. Nevertheless, it will be used in further experiments, as it is able to penetrate into the matrix and protect also the zone around the crack. Moreover, as long as the specimen remains dry, leakage of this agent is not visible. When methyl methacrylate was applied manually, some water ingress into the crack was noticed, possibly because shrinkage cracks formed inside the polymerized matrix. When methyl methacrylate was encapsulated, for two out of three specimens no improvement was noticed compared to untreated cracks. On neutron tomographic scans, it was seen that when methyl methacrylate was encapsulated, one component cured in the capsules before crack formation. This explains the poor behaviour of this agent. Besides visualization of the remaining agent inside the capsules, also the polymerized agent inside the crack could clearly be visualized by means of neutron tomography and complete crack filling was noticed for samples manually and autonomously sealed with polyurethane. As a tiny polymeric layer can be visualized inside a cementbased material, neutron tomography is highly appropriate to evaluate the crack filling efficiency in case of self-sealing of cementitious materials by means of polymers. Acknowledgments The authors would like to thank the Neutron Imaging and Activation Group of Paul-ScherrerInstitute (PSI), Switzerland, for experimental support during operation of neutron radiography and tomography. They also want to thank Manuel Dierick from the Centre for X-ray Tomography of Ghent University (UGCT) for reconstruction of the tomography scans. This research project has been supported by the European commission under the 7th Framework Programme through the ‘Research Infrastructures’ action of the ‘Capacities’ Programme, contract No: CP-CSA_INFRA2008-1.1.1 and the Research Foundation Flanders (FWOVlaanderen) (Project No. G.0157.08).


References 1. Zhang P, Wittmann FH, Zhao TJ, Lehmann EH, Tian L, Vontobel P (2010) Observation and quantification of water penetration into Strain Hardening Cement-based Composites (SHCC) with multiple cracks by means of neutron radiography. Nucl Instrum Methods Phys Res Sect A 620(2–3):414–420 2. Cailleux E, Pollet V (2009) Investigations on the development of self-healing properties in protective coatings for concrete and repair mortars. In: Paper presented at the 2nd international conference on self healing materials, Chicago, USA, 29 June–1 July 2009, p 120 3. Dry C (1994) Matrix cracking repair and filling using active and passive modes for smart timed release of chemicals from fibers into cement matrices. Smart Mater Struct 3(2):118–123 4. Dry CM, Sottos NR (1993) Passive smart self-repair in polymer matrix composite materials. In: Paper presented at the symposium on smart structures and materials 5. White SR, Sottos NR, Moore J, Geubelle P, Kessler M, Brown E, Suresh S, Viswanathan S (2001) Autonomic healing of polymer composites. Nature 409:794–797 6. Nielsen AF (1972) Gamma-ray-attenuation used for measuring the moisture content and homogeneity of porous concrete. Build Sci 7(4):257–263 7. Nizovtsev MI, Stankus SV, Sterlyagov AN, Terekhov VI, Khairulin RA (2008) Determination of moisture diffusivity in porous materials using gamma-method. Int J Heat Mass Transf 51(17–18):4161–4167 8. Roels S, Carmeliet J, Hens H, Adan O, Brocken H, Cerny R, Pavlik Z, Ellis AT, Hall C, Kumaran K, Pel L, Plagge R (2004) A comparison of different techniques to quantify moisture content profiles in porous building materials. J Therm Envelope Build Sci 27(4):261–276 9. Baker PH, Bailly D, Campbell M, Galbraith GH, McLean RC, Poffa N, Sanders CH (2007) The application of X-ray absorption to building moisture transport studies. Measurement 40(9–10):951–959 10. Roels S, Carmeliet J (2006) Analysis of moisture flow in porous materials using microfocus X-ray radiography. Int J Heat Mass Transf 49(25–26):4762–4772 11. Roels S, Vandersteen K, Carmeliet J (2003) Measuring and simulating moisture uptake in a fractured porous medium. Adv Water Resour 26(3):237–246 12. Brew DRM, de Beer FC, Radebe MJ, Nshimirimana R, McGlinn PJ, Aldridge LP, Payne TE (2009) Water transport through cement-based barriers—a preliminary study using neutron radiography and tomography. Nucl Instrum Methods Phys Res Sect A 605(1–2):163–166 13. Cnudde V, Dierick M, Vlassenbroeck J, Masschaele B, Lehmann E, Jacobs P, Van Hoorebeke L (2008) High-speed neutron radiography for monitoring the water absorption by capillarity in porous materials. Nucl Instrum Methods Phys Res Sect B 266(1):155–163 14. El Abd A, Czachor A, Milczarek J (2009) Neutron radiography determination of water diffusivity in fired clay brick. Appl Radiat Isot 67(4):556–559 15. Kanematsu M, Maruyama I, Noguchi T, Iikura H, Tsuchiya N (2009) Quantification of water penetration into concrete


16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

through cracks by neutron radiography. Nucl Instrum Methods Phys Res Sect A 605(1–2):154–158 Pleinert H, Sadouki H, Wittmann FH (1998) Determination of moisture distributions in porous building materials by neutron transmission analysis. Mater Struct 31(4):218–224 Zhang P, Wittmann FH, Zhao T-J, Lehmann EH, Vontobel P (2011) Neutron radiography, a powerful method to determine time-dependent moisture distributions in concrete. Nucl Eng Des 241(12):4758–4766 Zhang P, Wittmann FH, Zhao T, Lehmann EH (2010) Neutron imaging of water penetration into cracked steel reinforced concrete. Physica B 405(7):1866–1871 Zhang P, Wittmann FH, Zhao TJ, Lehmann E, Vontobel P, Hartmann S (2009) Observation of water penetration into water repellent and cracked cement-based materials by means of neutron radiography. Int J Restor Build Monum 15(2):91–100 Reijonen H, Pihlajavaara SE (1972) On the determination by neutron radiography of the thickness of the carbonated layer of concrete based upon changes in water content. Cem Concr Res 2:607–615 Pugliesi R, Andrade MLG (1996) Study of cracking in concrete by neutron radiography. Appl Radiat Isot 48(3):339–344 de Beer FC, le Roux JJ, Kearsley EP (2005) Testing the durability of concrete with neutron radiography. Nucl Instrum Methods Phys Res Sect A 542(1–3):226–231 de Beer FC, Strydom WJ, Griesel EJ (2004) The drying process of concrete: a neutron radiography study. Appl Radiat Isot 61(4):617–623 Cnudde V, Dierick M, Vlassenbroeck J, Masschaele B, Lehmann E, Jacobs P, Van Hoorebeke L (2007) Determination of the impregnation depth of siloxanes and ethylsilicates in porous material by neutron radiography. J Cult Heritage 8(4):331–338 Nemec T, Rant J, Apih V, Glumac B (1999) Study of building materials impregnation processes by quasi-realtime neutron radiography. Nucl Instrum Methods Phys Res Sect A 424(1):242–247 Masschaele B, Dierick M, Cnudde V, Van Hoorebeke L, Delputte S, Gildemeister A, Gaehler R, Hillenbach A (2004) High-speed thermal neutron tomography for the visualization of water repellents, consolidants and water uptake in sand and lime stones. Radiat Phys Chem 71(3–4):807–808 Vontobel P, Lehmann EH, Hassanein R, Frei G (2006) Neutron tomography: method and applications. Physica B 385–386(1):475–480 Trtik P, Muench B, Weiss WJ, Herth G, Kaestner A, Lehmann E, Lura P (2010) Neutron tomography measurements of water release from superabsorbent polymers in cement paste. In: Paper presented at the international RILEM

121

29.

30.

31.

32. 33.

34.

35.

36.

37. 38.

39.

40.

41.

conference on material science, Aachen, Germany, 6–10 Sept 2010, pp 175–185 Trtik P, Mu¨nch B, Weiss WJ, Kaestner A, Jerjen I, Josic L, Lehmann E, Lura P (2011) Release of internal curing water from lightweight aggregates in cement paste investigated by neutron and X-ray tomography. Nucl Instrum Methods Phys Res Sect A 651(1):244–249 Van Tittelboom K, De Belie N, Van Loo D, Jacobs P (2011) Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent. Cem Concr Compos 33(4):497–505 Van Tittelboom K, Adesanya K, Dubruel P, Van Puyvelde P, De Belie N (2011) Methyl methacrylate as a healing agent for self-healing cementitious materials. Smart Mater Struct 20(12):125016 NBN EN 196-1 (2005) Methods of testing cement—part 1: determination of strength Lehmann EH, Vontobel P, Wiezel L (2001) Properties of the radiography facility NEUTRA at SINQ and its potential for use as European reference facility. Nondestruct Test Eval 16(2–6):191–202 Hassanein R, Meyer HO, Carminati A, Estermann M, lehmann E, Vontobel P (2006) Investigation of water imbibition in porous stone by thermal neutron radiography. J Phys D 39(19):4284–4291 Hassanein R, Lehmann E, Vontobel P (2005) Methods of scattering corrections for quantitative neutron radiography. Nucl Instrum Methods Phys Res Sect A 542(1–3):353–360 Kardjilov N, de Beer F, Hassanein R, Lehmann E, Vontobel P (2005) Scattering corrections in neutron radiography using point scattered functions. Nucl Instrum Methods Phys Res Sect A 542(1–3):336–341 Hassanein R (2006) Correction methods for the quantitative evaluation of thermal neutron tomography. PhD Joos A, Schmitz G, Mu¨hlbauer MJ, Schillinger B (2010) Investigation of moisture phase change in porous media using neutron radiography and gravimetric analysis. Int J Heat Mass Transf 53(23–24):5283–5288 Dierick M, Masschaele B, Van Hoorebeke L (2004) Octopus, a fast and user-friendly tomographic reconstruction package developed in LabView (R). Meas Sci Technol 15(7):1366–1370 Zhang P, Wittmann FH, Lehmann E, Zhao TJ (2011) Water absorption of cracks in neat and in water repellent concrete. In: Paper presented at the ASMES international workshop, Lausanne, Switzerland, 28–29 July 2011, pp 291–302 Wittmann FH, Zhao T-J, Guo P-G, Ren Z-J (2008) Penetration of chloride into cracked concrete. In: Paper presented at the international conference on durability of concrete structures, Hangzhou, China