Effects of concrete cracking and measures to prevent it from happening by curing it internally using light weight aggregate.

1. Internal Curing using
Light Weight Aggregate
Mohammad Alidousti
Quality Manager
Leca
www.leca.ae

2. Modern concretes;
generally possess some highly advantageous
properties compared to traditional concrete, for
example good workability in the fresh state, high
strength, and low permeability. However, these
types of concretes have also shown to be more
sensitive to early‐age cracking than traditional
concrete
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For many years, we have
been curing concrete from
the outside in, internal
curing is for curing from the
inside out. Internal water is
generally supplied via
internal reservoirs, such as
saturated lightweight fine
aggregates, superabsorbent
polymers, or saturated
wood fibers

4. Internal curing
Conventional curing procedures of water ponding, as used for drying shrinkage, are not effective in the case of autogenous shrinkage. They
may eliminate the autogenous shrinkage in small cross‐sections only, because the penetration of water from the external surface is limited.
Moreover, external curing might be difficult to apply to some surfaces. In view of this limitation, different strategies have been developed in
recent years, based on the use of internal water reservoirs [RILEM TC‐ICC 2002]. One strategy, which has been investigated more extensively,
is based on the use of lightweight aggregates (LWA) [Hammer 1992], while the other is based on the use of water absorbing polymers [Jensen
& Hansen 2001a]. Both water‐saturated porous aggregates and water‐saturated polymers are able to act as internal reservoirs, providing a
source of curing water to the paste volume in their vicinity. This process of internal curing should offset self‐desiccation and help avoiding
self‐desiccation shrinkage.
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With the primary benefit of IC being improved hydration and improved SCM reaction many other concrete
properties are also improved. For example the addition of a relatively small proportion of fine lightweight
aggregate (FLWA) to a regular mixture in a bridge deck reduces dramatically the amount of plastic
shrinkage cracking. The relatively small proportion of FLWA also causes small but collectively significant
changes to several key mixture properties, which have been documented in both laboratory and field
studies and include the following:
• Reduction in unit weight.
• Reduction in elastic modulus.
• Increase in strength.
• Reduction in coefficient of expansion.
• Reduction in shrinkage.
• Reduction in permeability.
POTENTIAL BENEFITS

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Particularly in HPC, it is not easily possible to provide curing water
from the top surface (for example) at the rate that is required to satisfy
the ongoing chemical shrinkage, due to the extremely low
permeabilities that are often achieved in the concrete as the capillary
pores depercolate.

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Early-Age Cracking Contributors
Thermal Effects
Hydration heat can raise concrete temperature
significantly (causing expansion), subsequent
thermal contraction during cooling can lead to early-
age (global or local) cracking if restrained (globally or
locally)
Autogenous Shrinkage
In lower w/cm concretes, if sufficient curing water
can not be supplied externally, the chemical
shrinkage that accompanies the hydration reactions
will lead to self-desiccation and significant
autogenous shrinkage (and possibly cracking)

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IC distributes the extra curing water
throughout the entire 3-D concrete
microstructure so that it is more readily
available to maintain saturation of the
cement paste during hydration, avoiding
self-desiccation (in the paste) and
reducing autogenous shrinkage.
Because the autogenous stresses are
inversely proportional to the diameter of the
pores being emptied, for IC to do its job, the
individual pores in the internal reservoirs
should be much larger than the typical sizes
of the capillary pores (micrometers) in
hydrating cement paste and should also be
well connected (percolated).
How does IC work?

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• Light Weight Aggregate:
Stephen Hayde discovered that LWA could be
manufactured from shale, clay and slate
– Observed that some bricks bloated during
burning
– Began developing rotary kiln process in
1908
– Patent for rotary kiln process was granted
in 1918
• First use of LWA was for LWC to build ships in
World War I
– Launching of the
USS Selma in June 1919

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LWA production process in a rotary kiln

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Quantify the effectiveness of IC
By direct and indirect experimental measurements including
internal relative humidity (RH)
autogenous deformation
compressive strength development
degree of hydration
restrained shrinkage or ring tests
3-D X-ray microtomography

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Internal RH Results
Mortars with Internal Curing
LWA = saturated lightweight aggregates
SAP = superabsorbent polymer
FSF = control with fine silica fume
85
90
95
100
0 5 10 15 20
Time (days)
Relativehumidity(%)
SAP
FSF
LWA20
LWA08

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Autogenous Deformation Results
IC added via fine LWA to increase total “w/c”
from 0.30 to 0.38 or 0.40
Note – chemical shrinkage of pozzolanic reaction
of silica fume with CH is ~0.22 g water/g silica fume or about 3.2 times that of cement

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Degree of Hydration and Strength
0
14
28
42
56
70
0 1 2 3 4 5 6 7 8
Age (d)
CompressiveStrength
(MPa)
0.00
0.16
0.32
0.48
0.64
0.80
Degreeofhydration
IC strength
Control strength
IC hydration
Control hydration
0
25
50
75
100
125
0 10 20 30 40 50 60
Time (d)
Strength(MPa)
Control
IC
30
40
50
60
70
80
0 10 20 30
Time (days)
Compressivestrength(MPa)
FSF
LWA20
LWA08
SAP
w/cm = 0.35 mortars, sealed curing
w/cm = 0.3
HPM with
silica fume
blended
cement

16. THANK YOU
For more question:
alidousti@leca.ae
www.leca.ae