Creep Of Concrete at High Age of Loading

Abstract

There are numerous publications about the creep behavior of young age loaded concrete. However, knowledge about the creep behavior of older concrete is rare. But this knowledge is important for existing bridge structures that are subsequently strengthened by external prestressing and thus subject to an additional creep load at high concrete age. If the deformation behavior of the old concrete is not adequately represented, it may lead to an underestimation of the loss of the prestressing force resulting in a considerable damage of the structure. At the Institute of Concrete Construction (IfMa) at Leibniz University Hannover, the creep behavior of old concrete was systematically investigated on the basis of drill cores from a deconstructed precast concrete bridge. At the time of the experimental investigations, the concrete was already more than 40 years old. The drill cores were tested under a constant continuous load for 6 months. The applied load, the specimen deformation and in addition the temperature and humidity were continuously measured. The research focused on the creep behavior of concrete at high load age and the influence of a preload. The old concrete still showed notable creep deformations. However, a significant influence of the preload on the creep behavior could not be detected. The test results were then evaluated and compared with results from literature and common creep prediction approaches of EN 1992-1-1 and Model Code 2010. Based on the test results, it cannot be assumed that the prediction approaches are capable of realistically representing the creep behavior of old concretes. The creep deformations are clearly underestimated by both approaches.

Full text

CREEP OF CONCRETE AT HIGH AGE OF LOADING
Boso Schmidt1), Max Herbers2), Gregor Schacht3), Steffen Marx2) and Vincent Oettel1)
1) Leibniz University Hannover, Institute of Concrete Construction (IfMa)
Appelstraße 9A, 30167 Hannover, Germany
2) Technical University Dresden, Institute of Concrete Structures (IMB)
August-Bebel-Straße 30/30A, 01219 Dresden, Germany
3) MKP GmbH
Werftstraße 17, 30163 Hannover, Germany
ABSTRACT
There are numerous publications about the creep behavior of young age loaded concrete. However,
knowledge about the creep behavior of older concrete is rare. But this knowledge is important for
existing bridge structures that are subsequently strengthened by external prestressing and thus subject
to an additional creep load at high concrete age. If the deformation behavior of the old concrete is
not adequately represented, it may lead to an underestimation of the loss of the prestressing force
resulting in a considerable damage of the structure.
At the Institute of Concrete Construction (IfMa) at Leibniz University Hannover, the creep behavior
of old concrete was systematically investigated on the basis of drill cores from a deconstructed
precast concrete bridge. At the time of the experimental investigations, the concrete was already
more than 40 years old. The drill cores were tested under a constant continuous load for 6 months.
The applied load, the specimen deformation and in addition the temperature and humidity were
continuously measured. The research focused on the creep behavior of concrete at high load age and
the influence of a preload.
The old concrete still showed notable creep deformations. However, a significant influence of the
preload on the creep behavior could not be detected. The test results were then evaluated and
compared with results from literature and common creep prediction approaches of EN 1992-1-1 and
Model Code 2010. Based on the test results, it cannot be assumed that the prediction approaches are
capable of realistically representing the creep behavior of old concretes. The creep deformations are
clearly underestimated by both approaches.
Keywords: Creep, old concrete, creep tests, material laws, creep prediction, deformation of concrete
1 INTRODUCTION
Numerous bridges will have to be replaced or strengthened in the next few years due to their age
and/or due to increased traffic volumes. An approved and widely used subsequent strengthening
method on existing prestressed concrete bridges is the application of an additional external

prestressing. Thereby, the existing (old) concrete is exposed to an additional creep-generating
permanent load.
There is a large number of studies on the creep behavior of concrete with an initial load age of a few
days to several months, which is particularly relevant for new constructions. Whereby studies on the
creep of old concrete are rare. Whether existing creep prediction approaches reliably describe the
long-term deformation behavior due to an additional creep-generating load at high concrete age is
largely unknown.
2 STATE OF RESEARCH
2.1 General
An overview of the current state of knowledge on concrete creep can be found for example in Bazant and
Jirasek (2018) [1] and Müller et al. (2021a) [2]. Numerous publications consider the creep of young
concrete over a short period of time. Creep under long-term loading is investigated in Troxell et al. (1958)
[3], Probst and Stöckl (1978) [4], Russel and Larson (1989) [5] and Brooks (2005) [6]. Important
investigations on creep of old concrete with a high load age are presented below.
2.2 Investigations according to Nasser et al. (1967)
Nasser and Neville [7] carried out creep tests on 50-year-old concrete cylinders
(h/d = 23.50/7.62 cm) at temperatures ranging from 21.1°C to 96.1°C. The test specimens were
obtained by core drilling at an angle of 60° to 80° to the horizontal from the pier of a railroad bridge
in the province of Saskatchewan in Canada. It should be noted that in the conducted creep tests
existed an unfavourable ratio between the maximum aggregate diameter (about 5 cm) and the drill
core diameter (about 7.5 cm). For comparison, 1-year-old concrete specimens of nearly the same
concrete, the same shape and size and with similar behavior to the old concrete were tested. The
mean compressive strength of concrete was determined on three 14-days-old cylindrical specimens
to 39 N/mm². Until creep load application, the specimens were placed in a water basin for 14 days at
a temperature corresponding to the subsequent test temperature. All specimens remained submerged
during the test procedure so the measured deformations can be attributed to basic creep. The tests
were conducted at a temperature of 21.1°C (70 °F), 46.1°C (115 °F), 71.1°C (160 °F), and 96.1°C
(205 °F). The applied load corresponded to a load level of about 45% of the mean compressive
strength of the old concrete. Figure 1 shows the creep deformations for the 1-year-old concrete (left)
and for the 50-year-old concrete (right) over the duration of loading in linear axis scaling.
Figure 1 – Creep deformations of young (left) and old (right) concrete as a function of the duration of loading
and temperature (indicated in Fahrenheit) from [7]
The qualitative creep behavior of older, submerged concrete specimens is similar to that determined
on younger submerged concrete specimens. This finding leads to the assumption that the mechanisms
of basic creep are the same in old and young concrete. After a loading duration of 225 days, the basic
creep of the old concrete of 210 μm/m (70 °F) is significantly lower than the basic creep of the
comparative specimens. The creep deformations of the young concrete were about 70% higher than

those of the old concrete. It is generally assumed that the creep deformations (with lower creep rate)
increase with the load duration, thus there tends to be no final creep rate. Nasser and Neville have
found that an increased temperature tends to increase the creep deformations, which will not be
considered further below. When interpreting the results, the unfavourable ratio between the
maximum aggregate diameter and the drill core diameter must be taken into account.
2.3 Investigations according to Wesche et al. (1978)
Wesche, vom Berg and Schrage [8] carried out creep tests on cylindrical concrete specimens
(h/d = 80/20 cm) stored under nearly constant climatic conditions that are first loaded at concrete
ages ranging from 3 days to 9 years. The concrete composition and the mean value between
fc,cube,min = 37 N/mm² and fc,cube,max = 56 N/mm² of cube compressive strength can be taken from [8].
The time-dependent development of the stress-related deformation of the concrete specimens are
shown for different loading ages in Figure 2. The values at the right edge of the figures represent the
final stress-related deformations calculated by extrapolation.
Figure 2 – Development of the stress-dependent deformation over the duration of loading (left) and over the
time after unloading (right) with varying load age from [8]
The test results of Wesche et al. show a more pronounced creep behavior of young concretes
compared to older concretes. Moreover, it can be observed that the age difference of the specimens
with a higher concrete age of approx. 6.8 years (= Belastungsalter 6/280) and 8.4 years
(= Belastungsalter 8/135) has not an insignificant effect on the creep affinity. Wesche et al. explain
this with the progressive hardening process of the concrete.
They assume that it exists a finite point in time for a concrete age greater than 9 years at which the
additional deformations are negligible. The authors justify this by the fact that for a concrete age of
8.4 years, when first loaded, the expected final value of the creep deformation (using the
extrapolation method according to Ross (1937) [9]) is only about 14% above the 28-day creep
deformation.
2.4 Investigations according to Trost et al. (1978)
Trost, Cordes and Abele [10] carried out creep and relaxation tests on drill cores (h/d = 30/15 cm).
A total of 10 drill cores were obtained from two different non loaded and approximately 40-year-old
concrete elements, which show a dense (concrete no. 1: 6 specimens) and a porous (concrete no. 2:
4 specimens) microstructure. Consequently, the specimens of concrete no. 1 have a higher mean
compressive strength (fcm = 50.5 N/mm²) than the specimens of concrete no. 2 (fcm = 37.6 N/mm²).
The creep tests were conducted in a climatic room under a load level of 30% to 45% of the mean
compressive strength of the concrete for 350 days. Creep recovery was measured over 300 days.
Trost et al. evaluated the creep coefficient φt and φv (creep recovery) of concrete no. 1 and no. 2 over

time and compared the creep curves with the expected values of the current guideline of prestressed
concrete from that time, the DIN 4227:1960-05 (1960) [11] (Figure 3). The creep coefficients φt and
φv describe the ratio of the creep deformations to the elastic deformations. According to the
extrapolation method of Ross [9], a final creep coefficient of φt = 0.77 (Figure 3, left) for concrete
no. 1 and φt = 0.51 for concrete no. 2 (Figure 3, right) can be concluded. These were significantly
higher than the value of the prestressed concrete guideline at that time (φ∞ = 0.40).
Figure 3 – Course of the determined creep coefficients φt and φv for concrete no. 1 (left) and concrete no. 2
(right) and the predicted values of the prestressed concrete guideline DIN 4227:1960-05[11] from [10]
Concrete no. 1, which has a denser concrete microstructure and a lower load level, shows larger creep
coefficients φt. This contradicts the idea that the creep tendency increases with increasing porosity
and the test results of Troxell et al. [3] on young concrete, according to which the specific creep
deformations also increase with the height of the load levels. With their investigations, Trost et al.
show that significant creep deformations occur even in 40-year-old concrete.
2.5 Investigations according to Bazant et al. (2011)
Bazant, Hubler and Yu (2011) [12] collected the deflection histories of 56 prestressed box-girder
bridges built using the balanced cantilever method. They plotted the deflection as a function of the
time since span closing. After an initial period of about 1,000 days since span closing, the deflections
increase systematically as a straight line for the logarithmic time scale. Bazant et al. found no
indication of a convergence to a limit value in their investigations. The maximum acceptable
deflection is assumed to 1/800 of the respective spans. According to the measured deflections and
their straight-line extrapolations, the deflections of 43 of the 56 evaluated bridges exceed the
maximum acceptable deflection in less than 100 years, which is the normally required minimum
lifetime. The deflections of 33 of the 56 bridge spans exceed in less than 40 years and the deflections
of 20 of the 56 bridge spans exceed in only 25 years the acceptable deflection. Bazant et al. conclude
from the results that the creep calculation methods are not suitable for segmentally erected large span
box girders. These structures have a high creep sensitivity and the use of a realistic creep approach
is particularly important. Bazant et al. does not address possible further or other causes for the
disproportionate deformations, cf. Müller et al. (2021b) [13].
3 OWN CREEP INVESTIGATIONS ON DRILL CORES OF OLD CONCRETE
3.1 Drill core taking and storing
For own creep investigations on old concrete drill cores (h/d = 30/9.4 cm) were taken from a 43
years old precast concrete bridge girder of the type BT 50 N-10B built in 1978 in Bütow (Germany).
The experimental investigations were carried out on a total of 12 drill cores. 8 of the 12 drill cores
were taken orthogonal to the tendon direction and 4 of the 12 drill cores were taken parallel to the

tendon direction, with the tendons arranged longitudinally to the component axis. The extraction of
the specimens was carried out in accordance with EN 12504-1 (2021) [14] by using the wet drilling
method. The specimens taken parallel to the tendon direction are assumed to be preloaded in the
direction of the creep load due to the prestressing and are marked with “H” (preloaded). The
specimens taken orthogonal to the tendon direction were assumed to be largely non-preloaded in
the direction of the creep load and are marked with “V” (non-preloaded), see Table 1. Afterwards
the drill cores were cut flat, grinded at the end faces and measured according to EN 12390-1 [15].
The specimens were stored and tested in an acclimated room at a largely constant temperature of
22°C and a constant relative humidity of 60%.
3.2 Test program and experimental procedure
The test programme of the series 0 to 3 can be taken from Table 1. Before starting the creep tests,
the compressive strength (series 0, V_7-1, V_7-10, H_12-1) and the modulus of elasticity (series 0,
V_7-10, H_12-1) of the concrete were determined on 3 and 2 drill cores, respectively. The mean
compressive strength of the concrete of series 0 was used to determine the creep loads. The creep
tests are carried out in 3 hydraulic test rigs for creep investigations (series 1, 2 and 3). In series 2 and
3, the duration of loading  − 0 was 182.5 days. The specimens of series 1 are still being tested (at
the time of submission). The duration of loading of series 1 is planned for 365 days. In each test rig,
three specimens were arranged one above the other and aligned in such a way that uniform load
application could be ensured (Figure 4, left). The stress-strength ratio is calculated from the constant
creep load at each creep test rig and the compressive strength of each specimen after the creep tests.
In addition, deformations due to temperature and shrinkage were measured on an unloaded
compensation specimen, see Figure 4 (right).
Table 1 – Program and results of the accompanying tests (series 0) and the creep tests (series 1, 2, 3)
Series Specimen
Date / start
of test t - t0
Stress-strength
ratio  
[d]
[%]
[N/mm²]
[N/mm²]
0
V_7-1
10.03.2020 - -
89.55 -
V_7-10 76.54 41,034.54
H_12-1 79.97 37,371.51
Mean value:
82.02
39,203.25
1
V_7-7
17.05.2021 365 1)
35.1 2)
- 1) - 1) V_7-8 34.9 2)
V_7-9 35.1 2)
2
V_7-2
17.05.2021 182.5
34.6
78.52 3)
38,636.90 3)
V_7-4
42.5
64.00
3)
37,128.88
3)
V_7-6
34.4
80.08 3)
40,141.16 3)
3
H_12-2
17.05.2021 182.5
34.1 75.28 3) 36,895.59 3)
H_12-3 36.3 78.65 3) 38,613.59 3)
H_12-4 34.9 82.55 3) 38,407.94 3)
1) Still being tested (at the time of submission).
2) Stress-strength ratio based on the mean value of the compressive strength test results of serie 0, 2 and 3, because serie 1
is still being tested (at the time of submission).
3) After the creep tests.

In creep tests, the test load should be applied quickly to minimize the amount of creep deformation
during load application. In the tests carried out, the test load was applied via an electric hydraulic
pump within 15 to 65 seconds, which in some cases is above the required 30 seconds according to
EN 12390-17 (2019) [16]. During the tests, the oil pressure decreases due to the compression of the
concrete cylinders, especially in the first few days. To prevent a drop in force, a bladder pressure
accumulator was installed at each hydraulic jack, containing an oil-nitrogen mixture. The
compressibility of the nitrogen bubble allows energy to be stored in the system. If the oil pressure in
the hydraulic jack drops after installation, this can then be compensated by the expansion of the
nitrogen bubble. In this way, the load level could be maintained with a deviation of ± 3% for the
duration of the tests.
Figure 4 – Creep test rig (left) and compensation specimen (right)
The time-dependent concrete strain is measured with inductive displacement transducers over a
measuring length of 20 cm at half of the specimen height, see Figure 4. In the creep tests, the total
deformations were measured, which consist of initial elastic deformations and time-dependent creep,
shrinkage and temperature strains. Shrinkage and temperature deformations were (also) determined
on the compensation specimen and were taken into account in the total deformations of the creep
specimens. The long-term measurements are related to a zero-point. The deformations were recorded
with a measurement interval of one second for the entire test duration. To determine the applied creep
load, oil pressure sensors were used. In addition to the test loads and strains, the temperature and
relative humidity were measured for the entire test duration. The carried out tests are so-called single-
stage tests with constant load.
3.3 Evaluation of the test results
The stress-strain curves for the load application in the creep test rigs are shown in Figure 5. For
comparison, the linear stress-strain relationship based on Hooke's material law is also shown (black
line). This is the mean value of the modulus of elasticity m = 39,203 N/mm² determined for the test
specimens V_7-10 and H_12-1 (cf. Table 1) before starting the creep tests.
The experimentally determined deformations of the tests approximately correspond to the values
calculated on the basis of Hooke's material law. This result confirms observations by Müller et al.
(2021c) [17]. For increasing concrete strengths they found, the initial plastic deformation decreases
in relation to the elastic strain. In high-strength concretes of concrete grade C80/95 or higher, initial
plastic deformation almost no longer occur. The initial plastic deformation are predominantly caused
by increasing micro-cracking after the start of loading. Since the inductive displacement transducers
were fixed to the specimens at a distance from the bearing plates, deformations due to settlements
between the specimens and the bearing plates can be neglected.

Figure 5 – Stress-strain curve of the creep test specimens during application of the test load as well as elastic
modulus from the preliminary investigations
Based on the extraction direction, the drill cores can be divided into preloaded and largely non-
preloaded specimens, see chapter 3.1. Below, the influence of a preload on the creep behaviour is
investigated. The specific creep for preloaded (“H”), non-preloaded (“V”) and all specimens is shown
in Figure 6 in box plots for 28 days (left) and 182.5 days (right). The median (red line), upper and
lower quartiles and extreme values outside the interquartile range (whisker) for the specific creep are
shown separately. The length of the whisker is limited to 1.5 times the interquartile range.
It can be seen from Figure 6 that the tested preloaded specimens have a slightly higher specific creep
after 28 days as well as after 182 days. It is also shown that the scatter of the specific creep increases
with continuous loading. Taking into account the small number of specimens tested and the scattering
of the specific creep, no clear influence of the preload (“H”) or the extraction direction can be
detected on the basis of the presented investigations.
Figure 6 – Box plots of the specific creep after 28 days (left) and 182.5 days (right)
Since nearly no influence of preloading on the creep tendency can be identified and only a small
number of test specimens were available, the further evaluations were carried out with all (preloaded
and non-preloaded) test specimens.
In a next step, the results of the own creep investigations will be compared on the basis of the specific
creep  according to Eq. (1) with results from the literature as well as with creep approaches from
common guidelines.
(,0)=cc(,0)
c(0) (1)
Where
(,0) is the creep coefficient;
(0) is the creep-generating constant stress applied at time 0.

The codes EN 1992-1-1 (2011) [18] and Model Code 2010 (2013) [19] contain creep prediction
approaches that are particularly valid for young concretes. These approaches considere the mean
concrete compressive strengths and modulus of elasticity at the age 28 days. Within the range of
service stresses ||≤0.4 ∙ 
(0), creep is assumed to be linearly related to stress. For a constant
stress c(0) applied at time 0 this leads to the creep strain:
(,0)=c(0)

∙ (,0) (2)
Where
(,0) is the creep coefficient;
 is the tangent modulus of elasticity at the age 28 days.
The creep coefficient can be calculated according to EN 1992-1-1 (Annex B) from Eq. (3).
(,0)=0∙ (,0) (3)
Where
0 is the creep number;
(,0) is the development of creep with time.
The Model Code 2010 takes into account the physical behaviour and separates the total creep into
the components of basic creep and drying creep. The creep coefficient can be calculated according
to Model Code 2010 from Eq. (4).
(,0)=(,0)+ (,0) (4)
Where
(,0) is the basic creep coefficient;
(,0) is the drying creep coefficient.
For further evaluation, it should be taken into account that the compressive strengths of the drill cores
(cf. Table 1) are significantly higher than the expected strengths of the original structural calculation
of the Bütow precast concrete bridge girder. The concrete grade in the original structural analysis
corresponds to today’s concrete grade C30/37 (according to BMVi (2011) [20]). This phenomenon
is common for older existing bridges, see Gebauer et al. (2021) [21]. Reasons therefore are the
progressive hydration and the partly use of concretes with a higher strength than required. Such
reserves in the concrete compressive strength allow additional subsequent prestressing while
observing the stress limits in the concrete.
Figure 7 (left) shows the specific creep  of the conducted creep tests over the duration of loading
 − 0 in comparison to the results of Trost el al. and Nasser and Neville. It should be noted that the
comparability of the test results shown in Figure 7 (left) is limited, because the concrete mixtures,
concrete strengths, load levels and environmental conditions of the experimental tests differ from
each other (see section 2).
Figure 7 (right) shows the specific creep  of the conducted creep tests in comparison to the creep
prediction approaches according to EN 1992-1-1 and Model Code 2010. The mean compressive
strength of the concrete were taken to fcm = 82.02 N/mm² (see Table 1) and the concrete age was set
to t0 = 43 a. The approach of a concrete compressive strength at the age of 28 days (back-calculated
on the basis of the experimentally determined strength) provides almost identical results for the
specific creep according to EN 1992-1-1 and Model Code 2010 after 182.5 days.

Figure 7 – Comparison of the test results with results from the literature on old concretes (left) and the creep
prediction approach according to EN 1992-1-1 and Model Code 2010 (right)
The own results differ from the results of Nasser and Neville because their specimens were stored and
tested under water. Therefore, the creep deformations observed by Nasser and Neville include only
basic creep. The water temperature during their experiments was 21.1 °C. Despite the limited
comparability, still tendencies can be identified. In particular, the development of the specific creep
according to Trost et al. and the own investigations agrees very well despite the different concrete
compressive strength. It can be seen that the 40 to 50 year old concretes show in Figure 7 (left) still
exhibit significant creep deformations and do not indicate a final creep rate. However, the creep rates
only reach marginal values at high concrete ages. The qualitative course of the prediction approaches
according to EN 1992-1-1 and Model Code 2010 also agree very well with the experimental results.
However, the creep deformations of the investigated old concrete tend to be underestimated by these
prediction approaches for young concretes. The approach according to EN 1992-1-1 seems to be more
suitable to represent the specific creep of the investigated old concrete, although even this approach
still shows deviations of almost 45% at a loading duration of 182.5 days.
4 CONCLUSION AND OUTLOOK
In order to investigate the creep of concrete at high loading age, creep tests were carried out on
preloaded and non-preloaded 43-year-old drill cores. The results were compared with results from
the literature as well as current prediction approaches of EN 1992-1-1 and Model Code 2010. Based
on these investigations, the following conclusions can be drawn:
− An influence on the creep behavior due to a preload in the direction of the creep load
(specimens “H” vs. “V”) cannot be clearly determined.
− The test results, like the results of Trost et al., do not indicate a final creep rate. However,
the creep rates only reach marginal values at high concrete ages.
− Existing prediction approaches according to EN 1992-1-1 and Model Code 2010 are only
limited suitable for describing the creep behavior of old concrete and in comparison with the
presented test results tend to underestimate the creep deformations.
− Compared to the Model Code 2010, the prediction approach according to EN 1992-1-1
seems to be better suited to describe the creep of old concrete, although even this approach
still shows deviations of almost 45% at a loading duration of 182.5 days.
Since numerous prestressed concrete bridges need to be strengthened in the next years, reliable
findings on the creep of old concrete are required and further creep tests should be carried out. This
should include long loading duration and a measurement of the creep behavior after unloading. The
creep recovery of the specimens (series 1) is still being tested at the Institute of Concrete Construction
(IfMa) at Leibniz University Hannover (see Table 1). Further investigations on sealed test specimens
(or water stored and loaded specimens) will allow further findings on the basic creep of old concrete.

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