Concrete International, 28 (10), 39-45, 2006
Water Movement during Internal Curing: Direct Observation
Using
X-ray Microtomography
Dale P. Bentz
Phillip M. Halleck
Abraham S. Grader
John W. Roberts
The goal of
internal curing of concrete, via the utilization of saturated lightweight fine
aggregates (LWA) for example, is to provide internal reservoirs of water that
are readily available to supply the hydrating cement paste with the water
needed to maintain saturation within its capillary porosity throughout the
first days or weeks of hydration. It has
been suggested that this water availability is especially critical during the
first day of hydration in high-performance concretes, as without it,
significant autogenous shrinkage and possibly cracking can occur.
In this paper, water movement during the
internal curing of a high-performance mortar containing saturated LWA is directly
observed using three-dimensional x-ray microtomography with a voxel dimension
of about 20 μm (8x10-4 in).
With this technique, the emptying of individual "pores" within the LWA
can be readily observed. For the mixture
proportions employed in this study, much of the LWA reservoir water was removed
within the first 24 h of hydration at 30 oC (86 oF).
The observations of water movement are
supported by more conventional measures of performance including degree of
hydration based on loss-on-ignition measurements and compressive strength of
mortar cubes.
Introduction
In recent years,
quantitative three-dimensional characterization of the microstructure of
cement-based materials using x-ray computed tomography (CT) has become a
reality.1-3 X-ray absorption
techniques have also been successfully applied to examining water movement in
cement-based materials during early-age curing.4,5
One application where such water movement is
critical is for the internal curing of concrete.6-9
For successful internal curing, the water
contained in the internal reservoirs (saturated lightweight fine aggregates (LWA)
or superabsorbent polymer particles, for example) must readily evacuate the
reservoirs and participate in the ongoing cement hydration reactions.
Cusson et al. have indicated that the supply
of this water during the first day of curing is especially critical for
high-performance concretes, if substantial autogenous shrinkage and possible
cracking are to be avoided.8,9
In this paper, x-ray CT is applied to directly observing the migration
of water from initially saturated LWA particles to the surrounding hydrating
cement paste in a high-performance mortar, throughout the first two days of
isothermal 30 oC (86 oF) hydration, illustrating the
power and potential of this experimental technique for examining dynamic
processes occurring within the concrete microstructure.
To supplement the x-ray CT observations, the
performance of the mortar with internal curing was contrasted to that of a
control with respect to achieved degree of hydration and compressive strength
development at ages up to 8 d.
Mixture
Proportions
The mortars were
prepared using a Type I/II portland cement obtained from a local manufacturer,
with a measured specific gravity of 3.22 +/- 0.01 and a potential Bogue
composition of 66 % tricalcium silicate, 12 % dicalcium silicate, 6.5 %
tricalcium aluminate, and 11 % tetracalcium aluminoferrite by mass fraction.
The particle size distribution of the cement,
as measured by laser diffraction, is provided in Fig. 1.
For
both the control and the internally-cured (IC) mortar, a blend of four (normal
weight) sands (specific gravity = 2.61) that has been shown to provide improved
particle packing for high-performance mixtures was employed.
The complete mixture proportions for the
control and IC w/c=0.35 mortars are
provided in Table 1. Because of the
intended microtomography study, the expanded shale LWA had been sieved to remove any particles
retained on a No. 8 sieve or passing a No. 30 sieve, the former to allow mixing
in a conventional laboratory mixer and the latter to avoid the presence of very
fine LWA particles in the microtomography images, which could complicate their
interpretation. The other four sands had
also been sieved (as necessary) to remove any particles retained on a No. 8
sieve or passing a No. 100 sieve. For
the IC mortar, portions of the normal weight 20-30 and S16 sands were selected
to be replaced by equivalent volumes of the LWA, consistent with the selected
particle size distribution of the (sieved) LWA as provided in Table 2.
The measured specific gravity of the
saturated surface dried (SSD) sieved LWA was 1.7.
Fig. 1: Measured
particle size distribution for the Type I/II portland cement used in the
experiments.
The IC
mortar was proportioned according to a previously outlined procedure that
balances the water demand of the hydrating cement paste with the water supply
from the saturated lightweight fine aggregates.7
Thus, the necessary LWA mass was calculated
as7:
(1)
where: LWA =
mass of (dry) LWA needed in mortar mixture (g or lb),
Cf =
cement content for mortar mixture (g or lb),
CS =
chemical shrinkage of cement (grams of water per gram of cement or lb/lb),
αmax
= maximum expected degree of hydration of cement,
S = degree
of saturation of aggregate (0-1), and
LWA
= absorption of LWA (kilograms of water per kilogram of dry LWA or lb/lb).
Table
1. Mixture proportions for the control and IC high-performance mortars.
|
Material |
Control Mortar Mass |
IC Mortar Mass |
|
Cement |
984.6 g (2.17 lb) |
953.3 g (2.10 lb) |
|
Water |
344.6 g (0.759 lb) |
333.7 g (0.735 lb) |
|
Sand (total) |
1870.8 g (4.12 lb) |
1529.4 g (3.37 lb) |
|
F95 fine sand |
467.7 g (1.03 lb) |
452.8 g (0.997 lb) |
|
Graded sand (ASTM
C778) |
355.4 g (0.783 lb) |
344.1 g (0.758 lb) |
|
20-30 sand (ASTM C778) |
355.4 g (0.783 lb) |
287.8 g (0.634 lb) |
|
S16 coarse sand |
692.2 g (1.52 lb) |
444.7 g (0.980 lb) |
|
LWA (SSD) |
--- |
183.6 g (0.404 lb) |
|
Water in LWA |
--- |
35.2 g (0.0775 lb) |
Table
2. Selected size distribution of LWA utilized in the experimental program.
|
Sieve (Opening in mm/in) |
Fraction Retained |
|
8 (2.36/0.093) |
0.000 |
|
10 (2.0/0.079) |
0.216 |
|
12 (1.7/0.067) |
0.166 |
|
16 (1.18/0.046) |
0.270 |
|
20 (0.85/0.033) |
0.201 |
|
30 (0.6/0.024) |
0.146 |
|
40 (0.425/0.017) |
0.000 |
The
chemical shrinkage of the cement paste, w/c
= 0.35 cured at 30 oC (86 oF), was directly measured
using the ASTM C1608 test method,10 and a value of 0.06 grams of
water per gram of cement (0.06 lb of water per lb of cement) was obtained when
extrapolating to complete hydration.
Throughout this study, curing at 30 oC (86 oF) was
employed to accelerate the cement hydration reactions, so that the
microtomography experiment could be completed in a reasonable 2 d to 3 d time
period. The absorption/desorption of the
LWA was measured by saturating representative masses (2 g (0.07 oz.) to 3 g
(0.106 oz.)) of the sieved LWA to an SSD condition, and then measuring their
desorptions when exposed to salt solutions of potassium sulfate (97 % RH) or
potassium nitrate (93 % RH) or when dried in a dessicator (0 % RH), similar to
the procedure described in the ASTM C1498 test method.11
A total SSD absorption of 25 % by mass fraction
was measured for the sieved LWA and 95 % of this total absorbed water was
removed when exposed to the potassium nitrate salt solution.
Finally, the decision was made to only supply
enough saturated LWA to maintain the saturation of the capillary pores in the
hydrating cement paste during the first three days of hydration with a 1.1
safety factor, as it was envisioned to freeze the microtomography specimen
following the first few days of hydration to try to monitor internal water
movement during freezing (with the purpose of examining whether the pores in the
now empty LWA might function in a manner similar to air voids).
The measured degree of hydration, based on
loss-on-ignition measurements,12 of a w/c = 0.35 cement paste cured under either saturated or sealed
conditions at 30 oC (86 oF) is provided in Fig. 2.
As would be expected, at later ages (7 d),
the hydration of the saturated specimen exceeds that of the sealed one, due to
the chemical shrinkage and self-desiccation accompanying the ongoing hydration
in the latter case. The degree of
hydration obtained after three days is observed to be 0.56.
Substituting the numeric values from Table 1
and above into Eq. (1), one obtains MLWA = 953.3 * 0.06 *
(0.56*1.1)/(0.25*0.95) = 148.4 g (0.327 lb) dry LWA (or 183.6 g (0.404 lb) SSD
LWA as in Table 1). For the IC mortar,
the necessary mass of LWA was pre-saturated overnight in a sealed plastic jar with
the exact amount of water needed for its 25 % SSD absorption capacity.
The jar was shaken and rotated vigorously by
hand on several separate occasions prior to the final preparation of the mortar
mixture.
Prior to the
microtomography experiments, control and IC high-performance mortars were
prepared in the NIST laboratory and cured under sealed conditions at 30 oC
(86 oF). Their air contents
were measured (3 % for the control mortar and 2 % for the IC mortar), and 50 mm
(2 in) cubes were prepared for the measurement of compressive strength after 1
d, 3 d, and 8 d of sealed curing. After
breaking each set of cubes, the broken fragments of one cube from each mixture
were retained for the determination of the degree of hydration of the cement
based on loss-on-ignition analysis, with an estimated expanded uncertainty of
0.01 for mortars.13
Fig. 2: Degree of
hydration for w/c = 0.35 cement paste
hydrated under saturated or sealed curing conditions at 30 oC (86 oF).
The expanded uncertainty in the computed
degrees of hydration for cement pastes, 0.004, is well within the size of the
symbols employed in the graph.
Degree of Hydration and
Compressive Strength of High-Performance Mortars
The
measured results for cube compressive strengths and degree of hydration are
provided in Fig. 3. Two points are worth
comment. First, it can be noticed that
the measured 1 d compressive strengths are already quite high, being over 40
MPa (5800 psi). This is likely due to a
combination of the optimized sand particle packing, the curing temperature of
30 oC (86 oF) that was employed, and the somewhat unique
particle size distribution of the cement used (Fig. 1), containing basically no
particles larger than 30 μm (0.0012 in) in diameter.
Second, IC is seen to clearly enhance
hydration (3 d and beyond) and compressive strength (8 d).
The maintenance of saturated conditions
within the hydrating cement paste 3-D microstructure via the water supplied by
the IC reservoirs (LWA) leads to both a more complete hydration and a greater
strength development, as has been observed in previous studies.6,8
Fig. 3: Measured
compressive strengths and degrees of hydration after 1 d, 3 d, and 8 d of
sealed curing for the control and IC high-performance mortars.
Error bars for compressive strength
correspond to measured standard deviation in testing 3 cubes of each mortar
mixture at each age.
X-ray Microtomography
The IC mortar for the
microtomography experiment was prepared in the microtomography laboratory at
the
Volumetric
x-ray CT data were collected using the facility's microfocus x-ray source with a
voltage setting of 120 kV and a tube current of 200 μA, to minimize the focal
spot size (to about 10 μm (4x10-4 in)) and thus optimize the spatial
resolution. For this study, all of the
microtomography data sets were acquired with voxel dimensions of dx = dy = 18 μm
(7.1x10-4 in) and dz = 19 μm (7.5x10-4 in), each of
which is larger than the focal spot size quoted above.
Each data set consisted of a 1024 x 1024 x 246
array of 16-bit x-ray absorption values on an arbitrary scale.
Each of the 246 individual two-dimensional
slices was available as a 16-bit tiff-format image for further processing as
described below.
Representative
two-dimensional and three-dimensional views of the IC mortar microstructures as
observed using x-ray microtomography are provided in Figs. 4 and 5.
Four separate microstructural components are
clearly observed. From darkest to
brightest, they are the air voids, the LWA particles, the normal weight sand
grains, and the hydrating cement paste.
For some of the LWA particles, individual (dark) empty pores can be
observed within the particle structure. Looking
carefully at the top two images in Fig. 4, one can observe that the circled LWA
particles have become darker between the original (after mixing) and the 1 d
images. Also, in a few cases, the
emergence of new empty pores within some of the circled LWA particles in the 1
d image can be detected. In contrast,
little further change is observed between the 1 d and 2 d images in Fig. 4,
suggesting that most of the empty porosity within the (originally) “saturated”
LWAs was created during the first day of hydration at 30 oC (86 oF)
(consistent with when most of the measured hydration occurred as indicated in
Fig. 3). In addition, it is worth noting
that some empty pores are observed within the LWA particles even in the data
set obtained just after mixing and casting of the microtomography
specimen. Two possibilities exist
regarding the observation of these empty pores.
The first is that the pores were empty within the SSD LWA particles that
were added to the mortar mixture, either because they are inaccessible to an
exterior source of water (isolated pores) or because the employed
(pre)saturation method was only partially effective at filling all of the
interconnected pores with water. The
second possibility is that these empty pores were initially filled with water
by the
employed saturation method, but
emptied during the initial mixing and casting of the specimen.
In the future, a more quantitative analysis
of the moisture contents of the individual LWA particles during the course of
the hydration will be conducted to compare against the bulk measured nominal 25
% water absorption at SSD saturation.
Fig. 4: Two-dimensional
slices (13 mm x 13 mm or 0.51 in x 0.51 in) from 3-D microtomographic data sets
corresponding to: upper left- mortar immediately after mixing, upper right-
mortar after about 1 d of hydration, lower left- mortar after about 2 d of
hydration, and lower right- subtracted color-coded image of 1 d - original
slices. In the color-coded image, aqua
indicates regions of drying and red indicates regions of wetting.
Fig. 5:
Three-dimensional image (4.6 mm x 4.6 mm x 4.7 mm or 0.181 in x 0.181 in x
0.185 in) of a portion of the microtomography data set for the IC mortar
obtained immediately after mixing and casting.
To quantitatively determine the
locations where water is being drawn out of the LWA particles during the first
day of the cement hydration, a 3-D subtraction of the 1 d and original data
sets was performed, after first removing random noise from both data sets using
a 5 x 5 x 5 median filter (each considered voxel within the specimen volume is
replaced by the median value for all voxels within a 5 x 5 x 5 cube centered at
the voxel being considered). The results
are illustrated in Figs. 6 and 7. In
Fig. 6, the aqua regions correspond to those voxels where the largest negative
difference was obtained when subtracting the original microstructure data set
from that obtained after 1 d of hydration.
The differences are negative because an empty pore has an x-ray
absorption signal that is significantly less than that of a water-filled
one. In Fig. 7, these same aqua regions
are superimposed on a magnified portion of one 2-D slice from the original
microtomography data set, allowing a direct verification that the aqua regions
are generally contained within the boundaries of the LWA particles.
Also, in Figs. 6 and 7, the shapes of some of
the individual (newly emptied) pores (highlighted in aqua) within several of
the LWA particles can be readily observed.
Fig. 6:
Three-dimensional color-coded image of original data set subtracted from that
obtained after 1 d of hydration. Aqua
volumes indicate regions where the LWA particles have lost water (to the
surrounding hydrating cement paste). 3-D
volume is 4.6 mm x 4.6 mm x 4.7 mm (0.181 in x 0.181 in x 0.185 in).
Fig. 7: Two-dimensional
image (4.6 mm x 4.6 mm or 0.181 in x 0.181 in) of a portion of the original
mortar microstructure with the locations of the evacuated water (in aqua)
superimposed.
While it
has been observed to be relatively straightforward to detect the water leaving
the SSD LWA during hydration using the x-ray microtomography technique, during
the first 2 d of hydration, it is not possible to isolate where the water goes.
At early ages, due to the high permeability
of the hydrating cement paste, water is able to move rapidly over a distance of
at least several millimeters.4,14
Thus, it would be expected that the hydrating cement paste would remain
saturated within a distance of at least 2 mm (0.079 in) from an internal
reservoir. Simulations of this water
availability distribution were conducted using programs15 similar to
those available for free access at http://ciks.cbt.nist.gov/lwagg.html
and the resulting simulated spatial distribution of water availability is shown
in Fig. 8. From the 3-D simulation that
incorporated the IC mortar mixture proportions and the LWA particle size
distribution provided in Tables 1 and 2, 97 % of the total cement paste volume
was found to be within a 2 mm (0.079 in) distance of the surface of an LWA
particle. This would support the
observation that the water coming from the LWA internal reservoirs is basically
uniformly redistributed throughout the entire 3-D mortar microstructure, so
that no regions of increased brightness (density) can be readily isolated in
the microtomography data sets. For
example, in the lower right image of Fig. 4, the voxels with the largest positive
difference between the 1 d hydration and the original microstructure data sets
(indicating those regions that have undergone the greatest increase in density
by imbibing the most water) are highlighted in red.
These quite small red regions are observed to
be homogeneously distributed throughout the entire specimen area (volume).
At later ages (28 d and beyond), as the
permeability of the cement paste decreases by several orders of magnitude,
water movement may become limited to distances of 100 μm (0.004 in) to 200
μm (0.008 in),16 and one might expect to observe brighter
(hydration) rims around the LWA particle reservoirs in IC mortars with
sufficient volumes of internal curing water.
Fig. 8: Two-dimensional
slice from three-dimensional simulated water availability distribution for IC mortar.
Image size is 10 mm by 10 mm (0.39 in by 0.39
in); red indicates normal weight sand, yellow indicates LWA, and concentric
rings of various shades of blue indicate hydrating cement paste within 0.1 mm
(0.004 in), 0.2 mm (0.008 in), 0.3 mm (0.012 in), 0.5 mm (0.02 in), 1.0 mm
(0.04 in), and 2.0 mm (0.08 in) of an LWA surface, respectively.
Prospectus
The power
and potential of applying x-ray microtomography to quantitatively visualize
dynamic processes within cement-based materials has been demonstrated for the
case of water movement during the internal curing of a high-performance
mortar. Clear visual evidence that many
of the initially water-filled pores within the SSD LWA particles empty during
the first day of hydration (during the critical time window for avoiding early
age cracking)8,9 was observed.
These observations were supported by conventional measures of the degree
of hydration and compressive strength development of companion mortar
specimens. The x-ray microtomography technique
should be equally applicable to other internal curing materials, such as
superabsorbent polymers or water-absorptive fibers.
It had been envisioned to extend the current
experiment by freezing the specimen to observe whether water returns from the
hydrating cement paste into the pores within the LWA (and the air voids) during
the expansive ice formation process.
Unfortunately, it was not possible to decrease the temperature of the
specimen lower than about -8 oC (17.6 oF), which is in
the range where freezing may not have occurred within the mortar microstructure,
due to supercooling effects. In the
future, it will be of interest to repeat the experiment with equipment that can
achieve a lower temperature on the order of -20 oC (-4 oF),
at which point the water within the percolated capillary pores of the hydrating
cement paste should have definitely frozen.
Acknowledgements
The authors
would like to thank Mr. Max Peltz and Mr. John Winpigler of the Building and
Fire Research Laboratory at the National Institute of Standards and Technology for
their assistance with materials characterization and specimen preparation.
They would also like to thank Mr. Jeffrey
Hook of the Lehigh Portland Cement Company for supplying materials for this
study and Prof. Maria Juenger of the
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