Study of ’real’ shrinkage by ESEM observations and digital image
analysis
D. Jankovic
Faculty of Civil Engineering, Delft University of Technology, The Netherlands
Keywords: Portland cement paste, ’real’ shrinkage, internal restraint, microcracking, ESEM, Digital Image Analysis
ABSTRACT: Defining the ’real’ shrinkage values of concrete is still a subject of much debate. In shrinkage experiments size effects are inherently present. Through an attempt to determine the real shrinkage of cement-based materials, these size effects have to be eliminated or at least reduced as much a possible. In this contribution drying shrinkage experiments on thin cement paste samples are discussed. Thin-cast Portland cement paste specimens (2 mm) are used, polished to 1 mm with specially designed tools. Samples with and without aggregate inclusions (glass pearls) are considered. The specimens are dried in Environmental Scanning Electron Microscope (ESEM) from 100% to 20% relative humidity (RH). ESEM images are acquired during drying and analyzed by means of Digital Image Analysis. Both drying shrinkage and microcracking are determined. The analyses show a non-linear relation between RH and shrinkage deformations in the whole drying range (from 100% to 20%).
1 INTRODUCTION
Initial, early-age drying microcracking, induced by differential volume changes and subsequent shrinkage of the cement paste, may jeopardize the durability of concrete structures. In order to study the early-age behaviour of cement-based materials, various research techniques have been applied in the past, mostly on larger, prismatic samples. It was concluded that microcracking was likely to occur around inclusions, i.e. sand or coarse aggregate particles, affecting the strength and stiffness of the concrete (Hsu 1963, Slate & Olsefski 1963). The formation of cracks in larger samples emerges due to moisture gradient (’apparent’ shrinkage) and complicates the interpretation of shrinkage tests. Therefore, cracking should be avoided as much as possible by testing of thin specimens and determining unrestrained ’real’ shrinkage (Wittmann 1982, Young et al. 1988).
The real shrinkage is considered a material property and should not dependent on the specimen’s size. Sabri & Illston (1982) and Ferraris & Wittmann (1987) investigated real shrinkage on 1-2 mm thick cement paste cylinders (up to 28 days of age), while investigating shrinkage mechanisms of C-S-H xerogel. Hansen (1987) tested slabs of a few mm thickness (age 165 days). One of the conclusions was that avoiding the moisture gradients in thin samples, true
shrinkage could be obtained. By applying modern techniques such as ESEM, real drying shrinkage deformations on a thin specimen can be observed ’in situ’ (Jennings & Xi 1993, Neubauer 1997), thanks to the high magnification (2 µm and higher) and change of environmental conditions in ESEM.
Even though the specimen size for drying shrinkage examinations is still not standardized, the size effect in drying shrinkage experiments remains an issue for different reasons. Could shrinkage of a small, thin specimen be an accurate reference to a shrinkage of larger samples? Or, how to treat various phenomena that appear at micro-scale but which are neglected at macro-scale?
For determination of the drying shrinkage strains the ESEM and Digital Image Analysis were used. Due attention is given to the description of the sample preparation. The objective was the coupling of the numerical modelling of moisture flow and ESEM tests to gain better understanding of drying shrinkage of plain cement paste and samples with rigid cast-in inclusions.
2 ESEM DRYING SHRINKAGE TEST 2.1 Specimen preparation method
Portland cement (type I, 32.5 R) is mixed with demi-water in the weight ratio w/c 0.5. Mixing is done by hand for 1.5 min using steel ’spoon’ (spatula) in the porcelain cup at a room temperature (about 22°C). Sawing of a larger sample as well as handling of very thin samples is often difficult, especially at young age. Instead of casting prismatic/slab samples and cutting it into slices, a new sample preparation method is developed. The ’homogeneous’ (plain cement paste) and ’heterogeneous’ samples (with embeddment of one or two glass pearls, φ4, as aggregates) are cast (Fig. 1). All samples are initially cast small-sized (10x10x2 mm) in specially designed steel mould (Fig. 2a). During casting of ’heterogeneous’ samples, the glass pearl(s) are placed in the middle of the sample (Fig. 1a) or diagonally (Fig. 1b). To avoid segregation, the mould is vibrated for about 20 s. After 24 h, the samples were demoulded. They are cured in desiccators with lime-saturated water for about 28 days. The desiccators are placed in the climate chamber at environmental conditions 20°C and 95% RH. Prior to the experiments, the samples are manually polished in wet environment to a thickness of 1 mm (Fig. 2b). A specially designed cylindrical tool was used for polishing. Detailed sample-preparation procedure, including casting and polishing of the cement paste samples was described earlier (Jankovic & Van Mier 2002).
2.2 Drying procedure in ESEM
The drying shrinkage is induced only by variations in relative humidity in the ESEM chamber - and thus in the sample itself - from a fully saturated condition (100% RH) to a relatively dry (20% RH). No mechanical load is applied. The relative humidity in the ESEM chamber has been decreased step-wise from 100% to 80, 60 etc. with step of 20%, such that temperature in ESEM was kept constant, while pressure decreased. After each step-wise decrease, the equilibrium was established in the chamber and in the sample.
Even though drying took place only from the top surface of the sample (Fig. 2c), uniform drying was assumed due to the 1 mm thick specimen (Fig. 2b). A thin layer of a conductive paste was placed locally at the bottom of cement paste sample in order to slightly
fix the specimen to the surface of a newly created cooling stage (Fig. 2c). In this way, sliding of the sample during the measurements was avoided and the temperature (9-10°C) at the bottom of the sample, was successfully controlled.
2.3 ESEM test observations in plain cement paste and samples with inclusion
ESEM images are acquired using Gaseous Secondary Electron Detector (GSED), which is a special detector that can function in a non-vacuum environment. It was observed that drop of RH to 40%, and further on to 20%, in all images induced darkening. In some cases, retrieving initial brightness and especially contrast at low chamber pressure at 10°C, became an impossible task. The image darkening can be explained by the presence of less water vapour at low pressure, but probably also by some other processes that take place at the atomic structure of cement paste. A higher magnification of cement paste microstructure (to 10
µm or 2 µm) revealed layered microstructure of a paste
(C-S-H), with a porous but firm upper layer and plenty of unhydrated moist cement structure underneath.
In plain cement paste samples, rare microcracks (width 0.1 µm) could be found but mostly after drying to 20%. In the samples with an embedded obstacle (Figs. 3, 4) no radial microcracks were found. Mostly observed are initial separations between cement paste and aggregate (in the ITZ) at 100% RH, as random gaps. In Fig. 3b, a gap approximately 10 µm long and 1
µm wide, is visible at some spots along the embedded
aggregate. During drying, the gap did not intensely grow (Fig. 3c) or caused complete sample fracture. The reason could be the fact that the sample was not exposed to the extreme environmental conditions, such as drying to a very low RH (5 or 10%) or subsequent re-wetting cycles to 100% RH, which would cause total cracking of the sample. In the sample with two-embedded obstacles (Fig. 4), which are at the shortest distance of 500 µm (Fig. 4a), similar gaps are visible in ITZ also at 100% RH. The gaps, slighly increase during drying to 40% (Fig. 4b). This is probably due to the shrinkage in ITZ between two closely spaced aggregates. A microstructure between two aggregates (Fig. 4a) is enlarged in Fig. 4c. Hydration products are visible, i.e. C-S-H and CH hexagonal plates. No microcracks are found.
(a) (b)
Figure 1. Schematic presentation of cement paste sample (10x10x1 mm) with inclusion of a glass pearl (a) and two glass pearls (b).
(a) (b) (c)
Figure 2. (a) Designed mould for casting of cement paste samples (10x10x2 mm), (b) polished cement paste sample (10x10x1 mm), (c) improved cooling ’house’ of XL30 ESEM chamber (1), with cooling stage and inbuilt thermocouple sensors and conductive paste (2), for temperature measurements on the surface of a sample (3) and integrated circuits inside the polyvinyl box (4), (5) pipes for liquid circulation, (6) metal clamps, and (7) plastic frame around sample.
(a) (b) (c)
Figure 3. Digital image (1424x968 pixels) of 32 days old cement paste sample (CEM I 32.5 R, w/c 0.5), with an embedded obstacle (glass pearl) at initial stage (100% RH), resolution of 20 µm (a). Shrinkage deformations of individual cement hydration productions in the ITZ are visible in the enlarged part of (a) to a resolution of 5 µm at (b) 100% and (c) 40% RH.
(a) (b) (c)
(a) (b)
Figure 5. Digital image (1424x968 pixels) of 32-days old wet-cured cement paste sample (CEM I 32.5 R, w/c 0.5) at 50 µm with marked Area-of-Interest (AOI), used for the analysis of strains in the Vic-2D program. Strain distribution (εxx) in AOI at: (a) 80% RH
varies min (-0.0015) to max 0.0004) and (b) from min (-0.0047) to max 0.0001 at 20% RH.
(a) (b)
Figure 6. Digital image (1424x968 pixels) of 28-days old wet-cured cement paste (CEM I 32.5 R, w/c 0.5) at 50 µm, with marked Area-of-Interest (AOI) for the analysis of strains using the Vic-2D program. Strain distribution in AOI at 40% RH: (a) εxxvaries from
min (-0.012) to max 8.34x10−5and (b) εyyvaries from min (-0.082) to max (-0.0042).
3 DIGITAL IMAGE ANALYSIS
The response of cement paste on drying is ex-tremely complex. In order to determine shrinkage deformations between 100% and 20% RH, a digital image analysis (mapping technique) is performed. As an example, the samples from Figs. 3 and 4 are analyzed in Figs. 5 and 6. According to Chu et al. (1985) the application of various mapping techniques and digital image correlation techniques can lead to good results. For displacement measure-ments in cement-based materials Xi et al. (1994) and Neubauer et al. (2000) developed mapping tech-niques (Image-Intensity-Matching-Technique, IIMT and Deformation-Mapping-Technique, DMT) respec-tively. The mapping technique employs a digital imag-ing system where images are constructed from discrete components (pixels).
The principle of the mapping technique is searching for a similar pixel-based intensity pattern in the reference image (initial ESEM image at 100% RH) and in the ’deformed’ image, i.e. an acquired ESEM image at a particular drying rate (80, 60, 40 or 20%). The intensity pattern is used as the basis for the calculation of displacements and strains as a 2-D projection of
the object. The resulting displacements are obtained by correlating intensity pattern and deformations. The analyzed Area-Of-Interest (AOI) in the applied Vic-2D code is the arbitrarily chosen image area. AOI may cover small or large part of the sample. In principle, analyses of AOIs in different sizes (small or large) may result in different strain values.
In the analyses of the samples with embedded aggregates, the focus is on the behaviour in ITZ. That is because ITZ is extremely porous at an early age, which means intensive moisture transport and higher shrinkage than in the bulk paste. In Figs. 5 and 6, AOI covers the area of cement paste (on the left-hand side), including the ITZ. The AOI is marked sufficiently close to the aggregate, avoiding the zone of gaps or cracks. The 2-D ’in-situ’ displacements and subsequently developed strains during drying (εxx, εyy
and εxy) are calculated for the chosen AOI, based
Figure 7. Drying shrinkage strains (εxx, εyyand εxy) in 29 days old cement paste sample (CEM I 32.5 R, w/c 0.5), calculated at image
resolution of 50 µm.
(a) (b)
Figure 8. Drying shrinkage strains (εxx, εyyand εxy) in cement paste samples (CEM I 32.5 R, w/c 0.5) at 50 µm: (a) 32 days old, with
inclusion (glass pearl) and (b) 28 days old, with two inclusions (glass pearls).
4 DISCUSSION
A linear relationship between strain and RH has been found in the RH-range from 100% to 40% (L’Hermite 1960) for shrinkage measured on large-size samples. In this study, such a linear correlation was not found for thin cement paste samples (see Fig. 7). Shrinkage gradually increases with decreasing RH, while the significant change occurs at 40% RH. This change was observed during drying as a change of brightness and contrast in ESEM images. Drying to 40% RH emerges, in principle, as the threshold after which shrinkage sharply increases with a further drop of the RH from 40% to 20%. The possible reason for that lays in the change of dominating shrinkage mechanism. Namely, mechanisms of capillary tension in combination with disjoining pressure are active in a wet condition, between 100% and 40-50% RH. Below 40% other mechanisms (Gibbs-Bangham shrinkage) i.e. change in surface energy, prevail (Wittmann 1985). Another reason for non-linearity of the strain-RH relationship, is the inhomogeneity of the microstructures of the
cement paste. It can happen that in two AOIs, which may cover larger or smaller portions of the microstructure, extreme shrinkage or even swelling occurs.
This emphasizes the strong localization of the shrinkage deformations at various relative humidity, which at micro-scale appear heterogeneous, while varying qualitatively and quantitatively from point to point. This is probably due to existence of two types of hydration products (Jennings et al. 2000): low-density (LD) and high-density (HD) S-H. Low-density S-H shrinks while being restrained by high-density C-S-H. It is probably not only the HD C-S-H areas that act as restraint but also other hydration products such as CH hexagonal plates, which are observed in the ITZ microstructure (Fig. 4c). We can additionally assume that the conductive paste (at the bottom of the tested sample), may contribute to local restraining although the layer of the paste was extremely thin.
cement paste at 50 µm resolution. Other analyses are performed on ’smaller’ AOI on image resolutions of 5, 10 or 20 µm. Since shrinkage deformations varied in smaller AOI compared to large AOI, it can be concluded that size of AOI as well as image resolution, influence the resulting strains of cement matrix. The shrinkage sometimes appear asymmetric i.e. strains (εxx, εyy and εxy) have different values,
both in cement paste samples and in the samples with aggregate inclusions. This could be due to the presence of microcracks (in cement paste) and initial random gaps i.e. separation of cement paste from the aggregate surface and early paste shrinkage at 100% (in samples with inclusion).
In the case of a sample with two aggregates (28 days old), Fig. 8b, as well as in plain cement paste sample (29 days old), carbonation shrinkage develops. The relation of shrinkage vs. RH has a typical curved shape with a characteristic reversible part at 40% RH, due to a carbonation shrinkage (Verbeck 1958). If we exclude possible technical errors during tests, then the influence of carbonation or autogeneous shrinkage to drying shrinkage cannot be excluded.
Regarding the behaviour of ITZ microstructure during drying, various influences should be considered such as cement type and w/c ratio, age of a sample, and microstructure (in particular C-S-H) changes. Thus, the presented results can not be fully related to cement paste with other properties (different cement type, w/c ratio, dry-cured samples, etc).
It can be concluded that Digital Image Analysis of ESEM images is succesfully applied on cement-based samples during drying. The accuracy of shrinkage can be approved by averiging of strain, determined at several spots in the sample. In these tests strains at a spot in a middle of sample were observed. Still, an absolute determination of the ’real’ (unrestrained) shrinkage of thin cement paste specimens is a demanding task, since always some, even small internal restrains exist, which cause microcracks or gaps to emerge.
ACKNOWLEDGEMENTS
I gratefully acknowledge suggestions of Prof. Dr. Van Breugel from TU Delft in reviewing the article. I also express my gratitude to STW and TU Delft for the funding of my Ph.D. research, and to Microlab for usage of the ESEM and the laboratory.
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