H. Nassif
N. Suksawang
A. Mohamed
Weryfikacja modeli pełzania i kurczenia się
betonu wysokowartościowego
VERIFICATION OF CREEP AND SHRINKAGE MODELS OF HIGH
PERFORMANCE /HIGH STRENGTH CONCRETE
Streszczenie
Niniejszy referat przedstawia wyniki testów na pełzanie i kurczenie wykonane na
różnych mieszankach betonu wysokowartościowego (HSC) (o wytrzymałości na ści-
skanie do 90 MPa). Wyniki porównano z danymi z różnych modeli do przewidywania
zachowań. Zbadano też wpływ pucolany na pełzanie i kurczenie. Wyniki wskazują, że
podczas gdy popiół lotny zwiększa pełzanie betonu, opary krzemionki je zmniejszają.
Ponadto, obecne modele pełzania i kurczenia się należy zweryfikować dla mieszanki
betonu BBW (HSC).
Abstract
This paper presents results from creep and shrinkage tests performed on different High
Strength Concrete (HSC) mixes (with compressive strengths up to 90 MPa). Results were
compared with those from various Code prediction models. The effects of pozzolanic
materials on the creep and shrinkage were also investigated. Results show that while
fly ash increases the compressive creep of concrete, silica fume decreases it. Moreover,
current creep and shrinkage prediction models need to be revised for the HSC mixture.
H. Nassif  Department of Civil an Environmental Engineering, Rutgers-The State University of New
Jersey, USA
N. Suksawang  Department of Civil an Environmental Engineering, Rutgers-The State University of New
Jersey, USA
A. Mohamed  Department of Civil an Environmental Engineering, Rutgers-The State University of New
Jersey, USA
Verification of Creep and Shrinkage Models of High Performance ...
1. Introduction
Creep and shrinkage are highly complex phenomena that are very sensitive to the sur-
rounding environment and gel content. Because of these complexities, the calculation
for creep and shrinkage cannot be simply derived from available models that are based
on data from conventional concrete. High strength concrete (HSC) properties are very
different than those of conventional concrete. As a result, the creep and shrinkage of
concrete changes, and therefore, new experimental data are needed to verify current
models. This study investigates the creep and shrinkage behavior of HSC containing
pozzolanic materials. Available creep and shrinkage models used I two design building
codes are also evaluated and compared with the experimental data to determine their
accuracy and validity.
Pozzolanic materials such as silica fume, fly ash, and slag have been used in the
United States to improve the quality of concrete (Goodspeed et al. 1996). These materials
make HSC denser and impermeable to chemical attacks and, at the same time, alter its
mechanical properties. Despite advancements in concrete material, the design of HSC
structures, or more specifically creep and shrinkage predictions, are still based on equations
that were founded on properties of conventional concrete. Moreover, most creep and
shrinkage prediction models have limitations when dealing with high strength concrete
(HSC) or more specifically concrete with a compressive strength in excess of 80 Mpa. This
is partly a result of the limitation of data available from the RILEM data bank, which is
used by most models for their calibration and validation. Thus, more experimental data
are needed for HSC.
Many previous studies have dealt with creep and shrinkage of high performance
concrete (HPC) as summarized by Brooks (2000). Brooks (2000) concluded that more
experimental data obtained from HPC were still needed to fully understand the effect of
pozzolanic materials on creep and shrinkage properties. Moreover, there were no data on
mixes with ternary blended cement, (e.g., combination of cement, silica fume, and fly ash),
which are becoming more common in North America. In addition, with the exception of
mixes that contain silica fume, most of the data summarized by Brooks (2000) were from
normal strength (30-60 MPa) concrete.
Despite the need for more experimental data, few papers had experimental results on
creep and shrinkage of HSC. Moreover, only two papers presented the results of studies
addressing material resources available in North America (Huo et al. 2001, Mokhtarzadeh &
French 2000). Huo et al. (2001) presented results of three different HPC mixes, out of which
two mixes possess HSC characteristics. However, because the mixture varied greatly, the
effect of pozzolanic materials on creep and shrinkage could not be concluded. Also, in
their study, Mokhtarzadeh & French (2000) considered the effect of the type of aggregates
instead of the pozzolanic materials on the time-dependent properties of HSC.
The main focus of this paper is to investigate the effect of pozzolanic material on
the creep and shrinkage behavior of HSC using readily available resources in the State
of New Jersey. A total of eight mixes were cast and tested for creep and shrinkage; these
mixes consisted of three mixes with varying percentages of silica fume, three mixes with
varying percentages of fly ash (Class F), and two mixes with different combinations of
silica fume and fly ash.
3
H. Nassif, N. Suksawang, A. Mohamed
2. Code prediction models
Several creep and shrinkage prediction models exist, but only two main code models
are most commonly used in the United States and Europe, ACI 209 and CEB 90, respec-
tively.
2.1. ACI 209
The ACI 209 model is based on the creep and shrinkage models proposed by Branson
& Christiason (1982). This model has been incorporated in most building codes in the
United States, as well as other countries. It is a general-purpose model and does not set
any limitation on the strength of the concrete. The model takes the following parameters
into account: 1) the relative humidity, 2) the specimen size, 3) the type of curing method
used, and 4) the age at the end of curing duration.
The model takes into account relative humidity, specimen size, curing type and age at
the end of curing. Only steam curing and moist curing are considered as type of curing
methods. The ACI 209 equation for predicting unrestrained shrinkage strain at any time
under standard conditions:
Shrinkage after age 7 days for moist cured concrete:
(1)
where = ultimate shrinkage strain in in/in or mm/mm = ; t = time in
days; and = product of correction factors.
Correction factors are applied for conditions other than the standard conditions like
length of curing, relative humidity conditions, specimen size and type of curing.
ACI 209 gives the creep coefficient, vt as follow:
(2)
where ; t = time in days after loading; and = correction factors associated
with loading age, relative humidity, specimens size, and concrete compositions.
2.2. CEB 90
The CEB 90 model is adopted by the CEB-FIP Model Code 1990 (Euro-International Con-
crete Committee and International Federation for Prestressing), and it is based on the work
by Muller and Hillsdorf (1990). The input parameters for this model differ from those of
the ACI 209 model in compressive strength and type of curing method. The strain due to
shrinkage at normal temperature may be calculated as:
(3)
where = notional shrinkage coefficient; = coefficient to describe development of
shrinkage with time; t = age of concrete (days); and t = age of concrete (days) at the be-
s
4
Verification of Creep and Shrinkage Models of High Performance ...
ginning of shrinkage. Correction factors are applied for relative humidity, specimen size
and 28-day compressive strength.
CEB 90 gives the creep coefficient, as follow:
(4)
where = relative of humidity; = compressive strength correction factors;
= loading age correction factors; and = creep coefficient time functions
3. Experimental invesitagtion
3.1. Materials and Mix Proportioning
The materials used in this project were readily available resources in the State of New
Jersey. The binding materials consisted of ordinary Portland cement (OPC) Type I, silica
fume (SF), and Class F fly ash (F). All mixes contained superplasticizer and air-entraining
agent to ensure good workability and freeze-thaw resistance, respectively. River sand
and crushed granite were used as the fine and coarse aggregate, respectively. The fine
aggregate (FA) had a unit weight, fineness modulus, and water absorption of 1621 kg/m3,
2.56, and 0.36%, respectively. The coarse aggregate (CA) had a maximum size aggregate
of 10 mm. Its unit weight, specific gravity, and water absorption were 1572 kg/m3, 2.81,
and 1%, respectively.
Seven mixes out of a total of eight mixes were made with a constant water-to binder
ratio (w/b) of 0.27. The mix proportions are presented in Table 1. All mixes had the
same amount of water, FA, and CA. Three mixes had varying SF contents of 5%, 10%,
and 15%. The other three mixes had varying F contents of 10%, 20%, and 30%. The last
mix was ternary blended concrete that contained 5% SF and 20% F. The concrete was
made in accordance to ASTM C192. Slump and air-content tests were also performed
on the fresh concrete in accordance to ASTM C143 and ASTM C173, respectively. After
the concrete specimens were cast, they were sealed with plastic wrap to prevent loss of
moisture. For each mix, specimens were made for compression, drying shrinkage, and
creep tests.
Table 1. Mix Proportion
5
H. Nassif, N. Suksawang, A. Mohamed
3.2. Compression Test
The compression test was performed in accordance to ASTM C39. For each mix, 21 plain
concrete cylinders, each with a diameter of 102 mm and a length of 203 mm, were made
and cured in a water tank. Three specimens were tested at the age of 1, 3, 7, 14, 28, 56,
and 90 days.
3.3. Drying Shrinkage Test
The drying shrinkage test was performed in accordance to ASTM C157. For each mix,
three plain concrete prisms, each with a dimension of 76 mm × 76 mm × 279 mm, were
made for testing the drying shrinkage. Immediately after the specimens were demolded,
the specimens were submerged in water at room temperature before they were tested.
The change in length was measured using a length comparator at the age of 1, 3, 7, 14,
28, 56, and 90 days. The specimens were dry cured in a 5 m × 9 m environmental cham-
ber with a constant ambient temperature and relative humidity of 24 Ä… 1°C and 50 Ä… 5%,
respectively.
3.4. Compressive Creep Test
The compressive creep of concrete was performed in accordance to ASTM C512 using a
custom-design creep rig (Figure 1). The creep rig was designed to accommodate three 152
mm × 305 mm cylinders. The sustained creep load was applied using five double-coiled
springs. The rig was designed to apply a maximum stress of 48.3 MPa on to the concrete
specimens. Assuming that the applied load is 30% of the ultimate strength of concrete,
the rig is capable of testing concrete with a compressive strength of 158.6 MPa.
For each mix, eight 152 mm × 305 mm cylinders were made to perform the compressive
creep test. Two of the eight cylinders were used to determine the compressive strength
of concrete on the testing day. Five cylinders had embedded bolts, which were used for
attaching vibrating wire strain gages (VWSGs). Out of the five cylinders, three cylinders
were loaded and the other two cylinders were kept unloaded and used as control spe-
cimens. The creep and shrinkage strains of the loaded and controlled specimens were
measured by the VWSGs. Three external VWSGs were customized by Rutgers University
so that they could be externally bolted onto the specimens, as well as to accommodate high
compressive strain (as high as 4000 µµ); they were installed around the cylinder at 120°
to account for eccentric loading effects. The average strain of the three external VWSGs
was used to calculate the total strain. The load was measured using an 890 kN load cell
located between the specimens and the steel loading plate. The load was carefully moni-
tored and adjusted (if necessary) on a regular basis (hourly for the first 8 hours, daily for
the first week, and weekly thereafter) to ensure that the load did not deviated more than
2%. The VWSGs, as well as the load cell, were attached to the data loggers that automa-
tically collected strain readings every 10 minutes. However, data collection is controlled
from a computer terminal and can be adjusted depending on the test duration. The last
cylinder was cut in half and placed above and below the three loaded cylinders. The two
half cylinders were used for eliminating stress concentrations on the loaded specimens
that can lead to localized failure of the cylinder. The loaded specimens and the two half
cylinders were covered with a capping compound before loading to ensure surface flat-
ness. The compressive creep test was performed on moist-cured specimens at 28 days,
with the applied load ranging from 30% to 35% of the ultimate load.
6
Verification of Creep and Shrinkage Models of High Performance ...
4. Results and discussion
4.1. Compressive Strength
The compressive strength of concrete is shown in Table 2; the table illustrates that an
increase in SF increases the compressive strength of concrete whereas an increase in F
content produces lower compressive strength. However, the ternary blended mix atta-
ined the highest compressive strength compared with any of the mixes with an equal
amount of individual pozzolanic material (i.e., the mix containing 5% SF and 20% F had
a higher compressive strength than the mix that only contained 5% SF and the mix that
only contained 20% F.
4.2. Drying Shrinkage
Figure 2 shows the effect of pozzolanic materials on drying shrinkage. It is observed that
both SF and F concretes had similar shrinkage strains after 90 days. However, at an early
age (between 1 to 7 days), F concrete shrinks less than SF concrete because SF is more
reactive and has a higher water demand than F. For the ternary blended concrete mix, the
F content slows the reaction at an early age, resulting in a lower shrinkage strain compared
with the concrete containing SF only. It should be noted that the results presented in this
paper are based on drying shrinkage and not the total shrinkage. The total shrinkage for
HP/HSC would have been higher and even more for mixes containing SF because these
mixes have high autogenous shrinkage. Nassif et al (2003) showed that the autogenous
shrinkage of SF concrete is as high as drying shrinkage.
Table 2. Compressive Strength
7
H. Nassif, N. Suksawang, A. Mohamed
Figure 1. Compressive Creep Test Setup
Figure 2. Drying Shrinkage of HSC
Figures 3 and 4 illustrate the comparison of the ACI 209 and CEB 90 shrinkage pre-
diction models, specifically comparing measured and calculated shrinkage strains. Both
models under-predict the drying shrinkage of HSC. However, the shrinkage strain given
by the ACI 209 model is better than the CEB 90.
8
Verification of Creep and Shrinkage Models of High Performance ...
Figure 3. Comparison of Measured Drying Shrinkage to ACI 209 Prediction Models
Figure 4. Comparison of Measured Drying Shrinkage to CEB 90 Prediction Models
9
H. Nassif, N. Suksawang, A. Mohamed
4.3. Creep
Figure 5 illustrates the effect of pozzolanic materials on creep. For comparison of the
creep behavior of various mixes, the specific creep per unit stress is plotted, rather than
the creep strain. The specific creep per unit stress is used because all mixes have different
compressive strength; thus different mixes have different applied load. It is observed that
an increase in the pozzolanic material also increases the specific creep. In addition, the
mixes with SF have lower specific creep than the mixes with F only. Furthermore, the
addition of SF to F concrete (e.g., Mix 5SF-20F) decreases the specific creep of concrete.
Figure 5. Specific Creep of HSC
As mentioned earlier, creep of concrete is one of the most complex phenomena to
analyze because many parameters affect the results. Furthermore, there are several the-
ories and hypotheses that lead to no definite conclusion (Neville 1970). However, one
of the most common theories and hypotheses that contributed to creep is the presence
of evaporable water. The presence of evaporable water is related to the seepage of the
absorbed water to the outside of the concrete (only for drying creep), or the evaporable
water could also undergo an internal seepage (in the direction of the load) to an absor-
bed layer, such as capillary voids. Therefore, seepage of water causes creep, and the
presence of evaporable water can be used to explain the increase in creep with a higher
content of pozzolanic materials. For SF concrete, as the amount of SF content increases,
the water demand of the SF becomes higher; this causes internal tensile forces that pull
the water from the load-bearing water (Powers hypothesis), which results in higher creep.
However, this creep is still relatively smaller than in concrete containing F because the
capillary voids in SF concrete are smaller compared with F concrete; hence, the amount
of evaporable water presence in SF concrete is lower. For the F concrete, the mix with
the highest F content has the highest creep because the mix has a lower water demand.
Nevertheless, because the water content in all mixes remains constant, the mixes contain
more evaporable water, leading to higher creep.
10
Verification of Creep and Shrinkage Models of High Performance ...
Figure 5 shows the comparison of results between the creep prediction models and
experimental data. Both the ACI 209 and CEB 90 models provided accurate creep pre-
diction but the ACI 209 slightly outperforms the CEB 90 model. Overall, both models
over-predicted the creep of HSC.
5. Conclusions
The mechanism of creep and shrinkage (especially creep) is highly complex because of the
nature of concrete. Concrete is a composite material that comprises different ingredients
that affect creep. In addition, other parameters, such as the surrounding environment,
loading age, and applied stress, all contribute to creep. Because of this complex nature,
its mechanism is difficult to understand; therefore, it is challenging to conclude the actual
behavior of creep using available theories. The best way to understand creep is through
measured data.
Figure 6. Comparison of Measured Specific Creep to ACI 209 Prediction Model
11
H. Nassif, N. Suksawang, A. Mohamed
Figure 7. Comparison of Measured Specific Creep to CEB 90 Prediction Model
An experimental program was conducted to determine the effect of pozzolanic material
on the creep and shrinkage of HP/HSC. From the experimental results, the following
conclusions could be made:
1. The drying shrinkage of concrete is influenced by the loss of water. The water loss
is a result of drying, as well as the hydration process. Because SF has a high water
demand, the drying shrinkage increases as the SF content increases. For F concrete,
the F content has a lower water demand; therefore, the drying shrinkage is reduced
as the F content increases.
2. The creep of concrete is also highly dependent on the loss of water, or more specifically
the presence of evaporable water. Because SF concrete has a lower capillary void than
F concrete, the former has lower creep than the latter. Hence the creep in F concrete
could also be reduced by adding SF.
3. The Code models for creep and shrinkage do not include parameters that take the pro-
perties of HSC into account. A correction factor is needed to account for the additional
pozzolanic constituents of HSC.
References
[1] ACI Committee. 209 1992. Prediction of creep, shrinkage and temperature effects in concrete structures..
Report No. ACI 209 R-92, ACI Publication: 47.
[2] Branson, D.E., & Christianson, M.L. 1982. Time-dependent concrete properties related to design-
strength and elastic properties, creep, and shrinkage , Designing for Creep and Shrinkage in Concrete
Structures : A Tribute to Adrian Pauw, SP-76, ACI Publication: 257-277.
[3] Brooks, J.J., 2000. Elasticity, creep, and shrinkage of concretes containing admixtures. The Adam Neville
Symposium: Creep and Shrinkage  Structural Design Effects, SP-194, ACI Publications: 283-360.
[4] Comité Euro-International du Béton (CEB), CEB-FIP Model Code 1990: Design Code. London, Thomas
Telford, 1993, p. 480.
12
Verification of Creep and Shrinkage Models of High Performance ...
[5] Goodspeed, C.H., Vanikar, S., & Cook, R.A., 1996. High-performance concrete defined for highway
structures. Concrete International, 18(2): 62-67.
[6] Huo, X.S., Al-Omaishi, N., & Tadros, M.K. 2001. Creep, shrinkage, and modulus of elasticity of high
performance concrete. ACI Materials Journal, Vol. 98, No. 6,: 440-449
[7] Mokhtarzadeh, A., a& French, C.,  Time-dependent properties of high-strength concrete with consi-
deration for precast applications , ACI Materials Journal, Vol. 97, No. 3, 2000, pp. 263-271
[8] Muller, H.S. & Hillsdorf, H.K. 1990. CEB bulletin d information, No. 199, evaluation of the time
dependent behavior of concrete, summary report on the work of general task group 9
[9] Nassif, H.H., Suksawang, N., and Mohammed, M. 2003.  Effect of curing methods on the early-age
and drying shrinkage of high-performance concrete , Transportation Research Record, No. 1804, TRB,
National Research Council, Washington, D.C.:48-58.
[10] Neville, A.M. 1970 Creep of Concrete: Plain, Reinforced, and Prestressed. North-Holland Publishing
Company, Amsterdam.
13