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Advances in Building Technology, Volume 1 M. Anson, J.M. Ko and
E.S.S. Lam (Eds.) 2002 Elsevier Science Ltd. All rights
reserved
17
ADVANCES IN CONCRETE TECHNOLOGY
M.F. Cyr and S.P. Shah
Center for Advanced Cement Based Materials, Department of Civil
Engineering, Northwestern University, Evanston, IL
60208 USA
ABSTRACT
A survey of recent advances in concrete technology, with a focus
on research performed at the Center for Advanced Cement Based
Materials (ACBM Center) at Northwestern University, is presented.
Ultra-high-strength concrete (UHSC), with compressive strength of
200MPa, has been developed. The properties and applications of
reactive powder concrete, one type of UHSC, are discussed. Fiber
reinforcement is used to overcome the inherent brittleness and
increase the tensile strength of concrete, especially high- and
ultra-high-strength concrete. Fiber-reinforced cementitious
composites can be designed for specific applications with the use
of special processing techniques, such as extrusion, and hybrid
fiber reinforcement. Significant reductions in drying shrinkage are
achieved with a newly developed shrinkage reducing admixture.
Construction costs can be reduced with the use of self-compacting
concrete (SCC), which does not require vibration at placement. The
design of SCC is facilitated with a newly developed rheological
model. A nondestructive evaluation technique has been developed to
monitor the hardening process of fresh concrete.
KEYWORDS
High-performance concrete, ultra-high-strength concrete, fibers,
extrusion, shrinkage, self-compacting concrete, nondestructive
evaluation
INTRODUCTION
As concrete technology developed, an initial goal was to
increase its strength. High-strength concrete (HSC) columns were
first used in the construction of high-rise buildings in the 1970s.
The same changes that increased the strength also improved the
durability and other aspects of concrete performance. The term
high-performance concrete (HPC) began to be used. Today, HPC refers
to concrete with many different attributes. It is produced with
specifically designed matrices, often containing special chemical
and mineral admixtures and fiber reinforcement. HPC performance
criteria include high strength and elastic modulus, improved
toughness and impact resistance, high
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early-age strength, high durabilityincluding low permeability,
resistance to chemical attack and free-thaw damageand ease of
placement and compaction without segregation.
A selection of current research in HPC, with an emphasis on work
performed at the Center for Advanced Cement Based Materials (ACBM)
at Northwestern University, is presented. Ultra-high-strength
concrete, with a compressive strength of 200 MPa, has been produced
for specialized applications. The ductility of concrete, especially
high- and ultra-high-strength concrete, has been enhanced with
fiber reinforcement. Hybrid fiber reinforcement and special
processing techniques, such as extrusion, have enabled the
optimization of composite performance for specific applications. A
shrinkage reducing admixture has been developed to improve
shrinkage cracking resistance. A model to facilitate the design of
self-compacting concrete has been developed. In addition to
improving concrete performance, new nondestructive techniques have
been employed to monitor the setting of fresh concrete and to
assess early deterioration in hardened concrete.
ULTRA-HIGH-STRENGTH CONCRETE
The strength of brittle materials, such as concrete, is related
to the porosity of the material. As the porosity decreases, the
strength exponentially increases (Mindess and Young 1981). Powers
and Brownyard (1948) showed that decreasing the water-to-cement
(w/c) ratio reduced the porosity of the concrete, increasing the
strength. This reduction in porosity also makes the concrete more
durable. In addition to having a lower water-to-binder (w/b) ratio,
HSC usually contains superplasticizer, and mineral admixtures, such
as silica fume or fly ash. Its compressive strength is around 100
MPa compared to compressive strengths of 20-40 MPa for
normal-strength concrete (Kosmatka et al. 2001).
Recently, special processing techniques have been used to
produce concrete with even higher compressive strengths.
Ultra-high-strength concrete (UHSC) can reach compressive strengths
of 200 MPa. Two commonly produced UHSCs are macro-defect-free (MDF)
cement and reactive powder concrete (RPC). Macro-defect free cement
is a mixture of cement and a water-soluble polymer. High shear
mixing causes a mechano-chemical reaction between the cement and
the polymer resulting in tensile strengths of up to 200 MPa (Shah
and Weiss 1998a).
Reactive powder concrete typically has a compressive strength of
200 MPa, although strengths as high as 810 MPa have been recorded
(Semioli 2001). Its high strength and low porosity are obtained by
optimizing particle packing and reducing water content. The mixture
contains no coarse aggregates. Instead fine powders, such as sand,
crushed quartz, and silica fume, with particle sizes ranging from
0.02 to 300 |nm are used. The grain size distribution is optimized
to increase the matrix density. Superplasticizer is used to reduce
the w/b to 0.2 as compared with w/b of 0.4-0.5 for typical
normal-strength concrete. Steel and synthetic fibers are typically
added to improve the ductility, and a post-set heat treatment is
applied to improve the microstructure.
RPC is produced commercially by two French construction
companies (Semioli 2001). Beton Special Industriel (BSI) is
produced by the Eiffage Group (EGI) in conjunction with Sika Corp.,
and Ductal is made by Bouygues Construction in partnership with
Lafarge Corp. Ductal is reinforced with high-strength steel
microfibers to improve ductility. It has a compressive strength of
200 MPa, a tensile strength of 8 MPa and a flexural strength of
40-50 MPa. Ductal is 100 times more resistant to water diffusion
than normal-strength concrete and 10 times more resistant than
typical French HPC. It has zero shrinkage after setting and 85-95%
less creep than conventional concrete. The relatively low tensile
strength requires the use of prestressing in more severe
applications. With Ductal, very strong, lightweight and durable
thin sections can be produced. Currently, there are plans to use
Ductal in a 120-m long, slender arch pedestrian bridge near Seoul,
Korea. The bridge will consist of six post-tensioned segments with
a 30-mm thick walkway. In the United States, the Federal Highway
Administration (FHWA) is currently testing Ductal in prestressed
concrete girders (FHWA 2002).
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BSI has been used in French nuclear power plants. The producers
of RPC also see potential for its use in pipes, tunnel and canal
linings, paving, floors, liquid storage structures, nuclear waste
containment, and long-spanning, slender, self-supporting structures
such as stadium domes.
The high strength of UHSC is due to the homogeneity and low
porosity of the matrix. These same characteristics also cause the
material to be extremely brittle and increase the likelihood of
shrinkage cracking. Microfiber reinforcement can increase the
ductility of the concrete and improve shrinkage cracking
resistance. The use of shrinkage reducing admixture also improves
shrinkage performance.
FIBER-REINFORCED CONCRETE
Microfiber reinforcement
Microfiber reinforcement reduces the inherent brittleness of
concrete, especially UHSC. Fibers spaced at the micron scale can
interact with microcracks, delaying localization and increasing the
tensile strength of the matrix (Shah 1991), in ways that steel
reinforcing bars with spacing at the millimeter scale cannot. In
addition, microfiber reinforcement delays the age of the first
visible crack and reduces the crack width in restrained shrinkage
tests (Grysbowski and Shah 1990).
The performance of fiber-reinforced concrete (FRC) is governed
by the ratio of the elastic modulus of the fiber to the elastic
modulus of the matrix, the strength of the fiber-matrix bond, the
aspect ratio (fiber length/fiber diameter) of the fiber, and the
material properties of the fiber (Bentur and Mindess 1991).
Different fibers types and geometries yield different composite
performance. Only relatively small amounts, usually less than 1% by
volume, of fiber reinforcement can be added to conventional
concrete because the fibers significantly reduce the workability of
the fresh concrete. At this dosage, fibers can improve shrinkage
cracking resistance and slightly enhance ductility. To maximize the
benefits of fibers, including increasing the tensile strength and
ductility of the composite and producing a strain-hardening
response, larger doses must be added to the matrix. In cement-based
materials, this requires special processing techniques. These
composites are referred to as high-performance, fiber-reinforced
cementitious composites (HPFRCC). The relative performance of
plain, normal-strength concrete, conventional FRC, and HPFRCC is
shown in Figure 1.
Tensile Stress
Strain
Figure 1: Tensile response of concrete, FRC, and HPFRCC.
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Extruded fiber-reinforced cementitious composites
Extrusion, a common processing technique for polymers and
ceramics, was recently adapted for the production of HPFRCC at ACBM
(Shao and Shah 1997 and Shao et al. 1995). A highly viscous,
dough-like mixture of cement paste and fibers (2%-10% by volume) is
forced through a die to produce an element of desired
cross-section. Production is continuous, and a variety of shapes,
such as thin sheets for siding and roofing tiles, pipe, and
cellular sections can be extruded, as shown in Figure 2. The high
compressive and shear forces required to extrude the composite
result in a dense matrix, a strong fiber-matrix bond and alignment
of fibers in the direction of extrusion. The extruded composites
are stronger and tougher than a cast composite of the same
material, as shown in Figure 3 (Shah et al. 1998b).
Figure 3: Flexural response of extruded and cast composites
(Shah et al. 1998b).
Successful extrusion requires the mixture to be soft enough to
flow through the extruder but stiff enough to maintain its shape
after exiting the die. The highly specialized cementitious matrix
has a low w/b (w/b ~ 0.25) with admixtures, and mineral additives.
The rheology of the extrudate has been improved with the
replacement of a portion of the cement with Class F fly ash (Peled
et al. 2000a). The spherical fly ash particles make the paste
easier to extrude. Fly ash is also less expensive and more
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environmentally friendly than cement. It improves the flexural
performance of fiber-reinforced extruded composites by increasing
the likelihood of fiber pullout instead of fiber fracture at
failure. Fly ash also improves the durability of
glass-fiber-reinforced extruded composites by reducing the
alkalinity of the cement matrix (Cyr et al 2001)
Hybrid fiber reinforcement
Material performance can be optimized for given applications by
combining different types of fiber reinforcement in hybrid fiber
composites. For example, glass fibers tend to be strong but
relatively brittle and form a strong fiber-matrix bond. Polyvinyl
alcohol (PVA) fibers are weaker, but more ductile than glass
fibers. These fibers were combined in extruded composites to
produce composites that were both strong and tough with strain
hardening behavior (Peled et al. 2000b). In addition, it was shown
that a portion of the PVA fibers can be replaced with less
expensive polypropylene (PP) fibers without any significant
reduction in performance. The performance of extruded single-fiber
composites and extruded hybrid-fiber composites is shown in Figure
4.
0 1 2 3 4 0 1 2 3 4 Deflection (mm) Deflection (mm)
Figure 4: Flexural response of single-fiber and hybrid-fiber
reinforced extruded composites. Total Vf = 5% for hybrids (Peled et
al. 2000b).
Different size fibers can also be combined to enhance
performance. Lawler (2001) found that combining 0.5% steel
macrofibers (500 urn diameter, 30 mm length) with 0.5% PVA
microfibers (14 l^ m diameter, 12 mm length) significantly improved
both the pre- and post-peak performance of mortar. The microfibers
bridge microcracks as they form, preventing them from coalescing
and increasing the tensile strength of the composites. As the
cracks grow, the steel macrofibers bridge the larger cracks,
increasing the ductility of the composite. This hybrid combination
also significantly reduces the permeability of cracked
concrete.
SHRINKAGE REDUCING ADMIXTURES
It is well known that concrete shrinks as it dries. If the
concrete is restrained, tensile stresses will develop and cracking
can occur. This is of particular concern in pavements, bridge
decks, and industrial floors where the volume-to-surface ratio is
low. The likelihood of shrinkage cracking depends on the free
shrinkage, the creep relaxation, age-dependent material properties,
such as tensile strength, and the degree of restraint of the
concrete. Shrinkage cracking is of greater concern in HSC where
increased early-age free shrinkage, reduced creep and increased
brittleness result in earlier cracking (Wiegrink et al 1996).
Attempts to reduce shrinkage cracking have included the use of
secondary reinforcement to keep cracks from widening, the use of
fiber reinforcement to prevent microcracks from coalescing, the use
of expansive cementa cement that expands during hydration creating
a compressive prestress that counteracts the tensile stresses that
develop under restrained shrinkageand reducing the w/c. Research at
the ACBM Center has included the development of
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experimental techniques and theoretical models to assess the
cracking potential due to shrinkage (Grysbowski and Shah 1990,
Weiss et al. 1998, Shah et al. 1998c). In addition, a shrinkage
reducing admixture (SRA) has been developed to improve the
shrinkage cracking resistance of concrete (Shah etal 1992).
The SRA is a propylene glycol derivative sold by Grace
Construction Products as Eclipse that reduces free shrinkage by
reducing the surface tension of water. One cause of drying
shrinkage in concrete is the surface tension that develops in small
pores as water evaporates (Balogh 1996). As cement reacts with
water, calcium silicate hydrate (CSH) forms in water-filled spaces.
These spaces are not completely filled by the CSH so a capillary
pore network develops. As the water evaporates, a meniscus forms in
the pores. The surface tension of the water pulls the pore walls
inward causing the concrete to shrink. This phenomenon occurs pores
with radii from 2.5 nm to 50 nm. The reduction in the surface
tension of the water by the SRA reduces the capillary pore forces
that cause shrinkage, thus reducing the drying shrinkage of the
concrete.
The shrinkage reducing admixture was tested in both normal- and
high-strength concrete at 1% and 2% by weight of cement (Weiss et
al. 1998). In general, 2% SRA showed much greater improvements over
mixtures without SRA than 1% SRA did. With 2% SRA, the free
shrinkage at 49 days was reduced by 42% in both normal- and
high-strength concrete. Because the SRA greatly reduced the free
shrinkage, the age of cracking of restrained ring specimens
containing SRA was increased. Rings of normal-strength concrete
cracked 10 days after casting, on average. With 2% SRA, only one of
the rings cracked before the end of the tests, 50 days after
casting. For high-strength concrete, the mixture without SRA
cracked at 3.2 days, while 2% SRA delayed cracking until 11.6 days.
The rings that cracked at later ages also had much smaller
cracks.
SELF-COMPACTING CONCRETE
Self-compacting concrete (SCC) is concrete that is designed to
flow under its own weight. This eliminates the need for vibration,
making it easy to place in dense reinforcement and complicated
formwork and reducing construction time and costs. SCC must be
fluid enough to fill a mold without vibration but not segregate.
Viscosity agents, such as superplasticizer, and fine mineral
admixtures are commonly used.
SCC is characterized by deformability, segregation resistance,
and passing ability. Deformability, represented by the flow or
fluidity of the SCC, is a measure of yield stress (Ozawa et al.
1992). It is quantified as the slump flow diameter, which is
obtained from a modified slump test (Takada 2000). The segregation
resistance is sufficient if the aggregates are uniformly
distributed throughout the cement paste. It is evaluated using a
penetration apparatus (Bui 2000). The segregation of aggregates has
been modeled using Stoke's Law at the ACBM Center (Saak et al
2000). The passing ability indicates how well the fresh SCC can
flow through the spaces between rebar. It is measured using an
L-box apparatus (Tangtermsirikul and Khayat 2000). These properties
of SCC are affected by the rheology of the cement paste and the
average diameter and spacing (Dss) of aggregates.
To facilitate the design of SCC, a paste rheology model was
developed at the ACBM Center (Bui et al 2001). The goal was to
determine the rheology of cement paste required to obtain SCC with
sufficient deformability and segregation resistance for given
aggregate properties. The paste rheology is characterized by yield
stress, measured as the paste flow diameter, and the viscosity,
measured with a standard rheometer. The model establishes, for
given aggregate properties and doses, the minimum apparent paste
viscosity, the minimum paste flow and the optimum flow-viscosity
ratio required to achieve SCC with acceptable segregation
resistance and deformability.
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In a recent study at the ACBM Center, the parameters for the
paste rheology were examined by varying the total aggregate ratio,
the paste volume, the w/b, and the cement, fly ash and
superplasticizer contents (Shah et al. 2002). These mixes had
different degrees of deformability and segregation resistance.
Plots of Dss vs. flow diameter and Dss vs. apparent viscosity were
obtained for a constant average aggregate diameter (Shah et al.
2002). For a constant aggregate diameter and a given Dss, there
exists a minimum paste flow diameter below which the SCC exhibits
poor deformability and a minimum viscosity below which the SCC
segregates. Fresh SCC with a larger Dss requires a smaller paste
flow diameter and higher viscosity to achieve acceptable
performance. To increase the aggregate spacing (Dss), the paste
volume of the mixture is increased, or the aggregate volume is
decreased. This results in reduced friction between aggregates (Bui
and Montgomery 1999). A higher viscosity is required to hold the
aggregates together, and a smaller paste flow diameter is necessary
to achieve good deformability.
NONDESTRUCTIVE EVALUATION
Nondestructive evaluation (NDE) uses stress waves to determine
mechanical properties, the presence, location, and extent of
damage, or the degree of hydration of concrete structures. Stress
pulses are applied to the structure, and the transmission or
reflection of the resulting waves or the vibration response of the
structure is measured. Early NDE techniques relied on the
transmission of a wave through a structure, which required access
to both sides of the structure, making them inappropriate for
concrete pavements or slabs. They were also unable to detect early
stages of deterioration, i.e., the presence of microcracks,
resulting from damage due to freeze-thaw cycling, sulfate attack,
or rebar corrosion.
Recently, several new NDE techniques have been developed at the
ACBM Center (Shah et al. 2000). These techniques are sensitive to
the early stages of damage. A self-calibrating, one-sided
technique, suitable for use on concrete pavements, was developed
and shown to be sensitive to the presence of cracks (Popovics et
al. 1998). The structural vibration frequency response can be
tracked during test loading (Subramaniam et al. 1998) and was used
to predict the remaining life of a specimen subjected to fatigue
loading. Another newly developed technique, discussed in detail
here, uses the reflection of wave energy at a steel-concrete
interface to monitor the setting of fresh concrete.
A one-sided, ultrasonic technique has been developed to monitor
the hardening and setting of fresh concrete (Ozttirk et al. 1999,
Rapoport et al. 2000, and Akkaya et al. 2001). The change in
ultrasonic shear wave reflections over time between a steel plate
and hardening concrete are measured to monitor the setting process.
High-frequency, ultrasonic shear waves are transmitted through the
steel plate into the fresh concrete. As the wave reaches the
steel-concrete interface, a portion of it is transmitted through
the concrete and the rest is reflected back to the transducer. The
wave energy reflected at the steel-concrete interface is called the
wave reflection factor (WRF). It depends on the differences in the
acoustic impedance, the product of material density and wave
velocity, of the steel and the hardening concrete. Initially, the
value of the WRF is unity because the concrete is in a liquid
state, which cannot transmit shear waves, so all of the wave energy
is reflected at the interface. As the concrete stiffens, more of
the wave energy is transmitted into the concrete and the WRF
decreases. A typical WRF vs. time plot is shown in Figure 5. In the
initial stage, the WRF equals one. At the end of the induction
period, there is a sharp drop in the WRF as the concrete begins to
harden. The WRF eventually approaches a final asymptote. The
significance of this asymptote is not yet known. The WRF test setup
is shown in Figure 6.
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1.00
0.90 u. | 0.80 CO
0.70
0.60 0 4 8 12 16 20 24 28 32 36
time (hours)
Figure 5: A typical WRF curve (Rapoport et al. 2000).
^ H j ^ ^ ^ ^ l l i l ^ ^ e r s ':' computer
Figure 6: The WRF test apparatus.
Initial tests demonstrated the sensitivity of the WRF method to
the presence of different admixtures, including accelerators,
retarders, superplasticizer, and silica fume (Rapoport et al.
2000). WRF, pin-penetration measurements and dynamic modulus tests
were performed simultaneously. The results were correlated and
critical points on the WRF curve were shown to correspond to
critical points in set time, temperature and dynamic modulus curves
(Rapoport et al. 2000).
Additional work has further demonstrated the sensitivity of the
technique to changes in mixture design and curing conditions and
correlated the wave energy attenuation, or the inverse of the WRF,
with early-age strength gain. Akkaya et al. (2001) measured wave
energy attenuation for two different mixtures at three different
curing temperatures. It is well understood that temperature affects
hydration (Mindess and Young 1981). This trend is evidenced in WRF
attenuation measurements. Akkaya et al. (2001) also compared the
wave energy attenuation in mortar and concrete. Three batches of
each mixture were tested. The mortar specimens showed good
repeatability, while the concrete did not. The attenuation curves
for mortar had the same shape and approached the same final
asymptote. The shape of the attenuation curves for concrete was
consistentdifferent stages of hydration, as indicated by distinct
points on the curve, were reproducedbut each sample approached a
different asymptote. This might be due to local differences in the
concrete, and suggests that the homogeneity of the mixture and the
final attenuation value are related.
Compressive strength tests and ultrasonic measurements were
performed simultaneously to correlate wave energy attenuation with
early-age strength gain (Akkaya et al. 2001). The results were
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25
correlated to predict strength evolution from the change in wave
energy attenuation. The relationship between strength and
attenuation is linear up to three days. The linearity is not
affected by changes in curing temperature or mixture design.
Several tests performed outdoors demonstrated that fluctuating
ambient temperatures also did not influence the linearity of the
relationship between strength evolution and change in attenuation.
This procedure requires the determination of one or two compressive
strength values at the beginning of strength evolutionwithin the
first dayto calibrate the strength-change-in-attenuation
relationship.
CONCLUSIONS
Some of the most recent developments in concrete technology were
discussed. Reactive powder concrete, one form of UHSC, has a
compressive strength of 200 MPa. It is currently used in long,
slender pedestrian bridges and nuclear power plants and has
potential for use in pipes, tunnel linings, and nuclear waste
containment. Micro fiber reinforcement is used to overcome the
inherent brittleness of concrete. Fiber-reinforced cementitious
composites for specialized applications are produced with special
processing techniques, such as extrusion, and hybrid-fiber
reinforcement. Significant reductions in drying shrinkage, and thus
the potential for shrinkage cracking, are achieved with the use of
a newly designed shrinkage reducing admixture. The design of SCC is
facilitated with a recently developed rheological model. Finally, a
new nondestructive technique is used to monitor the setting and
predict the strength gain of fresh concrete.
ACKNOWLEDGEMENTS
Much of the work presented here was funded by the Center for
Advanced Cement Based Materials at Northwestern University. In
addition, the research on extrusion is currently being funded by a
grant from the National Science Foundation in support of the
Partnership for Advancing Technology in Housing. The assistance of
these organizations is gratefully acknowledged.
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