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NEW LIMIT STATES IN CONSTRUCTION

3.1 Construction Optimisation

Although the structural cost forms a relatively small portion of the overall development cost,

the structural component is a critical path activity for multi-storey building construction.

Therefore it is imperative that the structural design focuses on a solution that considers

construction time as a leading parameter.

The optimisation process of the construction process can be broken down into three broad

areas:

Optimisation of labour and time to complete the building.

Optimisation of construction time. Design for construction time.

Optimisation of labour and time

Studies have revealed that labour can account for as much as 50% of the total cost of

construction. For concrete structures the labour can be minimised in two ways. The first is by

extensive use of factory made prefabricated materials which typically use lower man-hours.The second is to minimise the labour associated with formwork, by using systems which can

be reused many times.

Optimisation of Construction time

There are tremendous financial benefits of early completion. Early completion not only lowers

the interest bill, but will also attract an early income stream in the form of rental income frompotential building occupants.

Recent trends in the construction procedures to increase speed of construction include:

Pre-Conference Workshop on High Performance Concrete Design and Applications January 2012

2nd International Conference on Sustainable Built Environment (ICSBE) 201214th, 15th, 16th December 2012Kandy, Sri Lanka


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Carrying out demolition of existing structures, bulk excavations, piling and excavation

protection structures over the site, concurrently with the design and documentation of

the redevelopment.

Use of sophisticated formwork system to construct the lift cores as quick as possible,

well advanced from the remainder of the construction, providing more time for the

critical activities of lift installation and vertical service distribution. Structurally the core

typically forms part of the necessary vertical and lateral support system during the

construction phase.

Early completion of finishes generally governs building completion. There is a growing

trend to complete floor levels, clear of all formwork, as quick as possible providing the

various trades with a clean working platform to proceed with the labour intensive

service installation.

Construction systems incorporating High-performance concrete provide a highly-effective

solution to address the above objectives.

With reference to Fig 3.1, it can be seen that for nearly 80% of the construction period the

construction of structure is the critical activity.

Fig 3.1 Simplified chart of High Rise construction

From these facts it may be concluded that structural designers need to focus their efforts in

developing structural systems and details to suit faster construction. The traditional least cost

structures rarely optimise construction speed and more importantly rarely minimises total cost.

A cost premium on the structural components may be involved, and therefore a cost benefit

study should be carried out to ensure that there is a cost benefit on the total development

cost.




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Construction systems incorporating High-performance concrete (HPC) provide a highly-

effective solution to address the above objectives.

Construction of some key components is discussed below.

Core Construction

This section will discuss aspects of core construction

For tall buildings the core forms the principal structural element for both the gravity

load-resisting system and the lateral load-resisting system, with the latter becoming

increasing important with increasing height of the building. The size and location of

the core is predominantly governed by architectural considerations.

The cost of the core for a typical tall building is estimated to be 38% of the total

structural cost or 4 to 5% of the total development cost. The core is typically the critical construction activity for high-rise development. Any

delay in the core construction will not only delay construction of the other structural

components but will also delay all other activities associated with architectural and

mechanical-electrical services.

Important factors that govern core construction time are:

- To employ an efficient formwork which is readily available and requires minimum

set-up time

- To minimise transitions in the shape and size, up the height of building

- To minimise the number of transitions in wall thickness. Transitions take up

valuable construction time. This can be achieved by greater use of varying

concrete strength and reinforcement content to increase wall strength.

In Australia, and many other countries, introduction of high strength concrete with 28

day strengths of up to 120MPa has made reinforced concrete a feasible option for tall

buildings which demand large force-resistant capacities in the core walls.

- Compared to Steel, Materials for reinforced concrete are readily available with

minimum lead times. Also strengths can be modified at short notice without

changing wall thickness, which may have architectural implications, by changingthe reinforcement content or concrete strength.

- With the core construction being on the critical path, design changes are

inevitable. Reinforced concrete can cater for modifications with relative ease.

The importance of speed of core construction is reflected by the fact that advance

formwork systems have been developed specifically for core construction. Although




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the initial capital cost of such formwork system is high, up to $1M, the savings

associated with reduction in construction time will more than offset this penalty.

The two most common advance formwork systems employed for core construction

are:

- Slip-form - where the concrete is poured continuously whilst the formwork is

continuously slipped up, by use of hydraulic jacks anchoring off the set concrete

below. This is a 24 hour per day operation requiring skilled labour force. All

stages of operation such as placing of reinforcement, concrete supply etc., must

be reliable and efficient. Unplanned stops in the operation can be costly (Fig 3.1).

HPC with appropriate setting times for concrete is essential for slip-form

construction.

- Jump-form- a more traditional formwork system, where a storey height of core is

poured at a time. The formwork is supported off the poured core below. On

completion of the pour the formwork is jumped by another storey height, for the

next cycle.

These advanced formwork systems do not rely on the status of the construction of

other structural elements, as it is a self-supporting system. Therefore it is prudent to

complete the core as quick as possible, so on completion the costly system can be

removed from site and utilised on another project. HSC is essential for these systems.

Now innovative jump form systems have been developed (e.g. Eureka Tower

Melbourne) to jump 2 storeys at a time (Fig. 3.2).

Fig 3.2 Typical example of an advanced formwork system (Eureka Tower Melbourne)




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When combined with use of high strength concrete it becomes a very efficient column

system in terms of strength and buildability. Fig 3.3 illustrates how this form of

construction can be effectively utilised, with discrete activities taking place at several

levels simultaneously, in an orderly and uncluttered environment.

Fig 3.3 High-strength Concrete filled Steel tube column in High Rise applications




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3.2 Early-Age Concrete Including Maturity Method to Predict the

Strengths

With the need of speed in construction, an accurate prediction of early age properties of high

strength concrete is important. The following section discusses the available methods in

predicting compressive strength of concrete at early age using a case study.

According to code procedures

The Australian Standard on Specification and supply of concrete AS1379 (2007) requires

concrete samples to be tested in accordance with AS1012 Parts 1, 8 and 9. According to

AS1012 (1999), the concrete samples are cured under standard-moist conditions where the

samples are subjected to constant temperatures of 23 2C and 27 2C for a standard

temperate zone and standard tropical zone respectively. In ASTM C192/C192M (ASTM

2007), the curing temperature is set at 23 2C. On the other hand, the Australian Concrete

Structures Standard, AS3600 (2009) and ACI 318 (2011) state that accompanying cylinder

specimens of the same concrete batch should be stored and cured under conditions similar to

those of the in-situ concrete. What constitute conditions similar to the in-situ concrete are

open to interpretation.

Temperature Matched Curing

The standard method of testing according to AS1012 (1999) prescribes a curing regime and

boundary conditions that are essentially different from those under which the in-situ concrete

is cured. Slabs have a high surface to volume ratio and while the top surface is exposed to

variable temperature profiles, the other surfaces are sealed by formwork. It is well known that

the curing temperature of concrete is an important factor governing early-age strength

development.

Temperature Matched Curing (TMC) is well established for assessing strength in-situ and is

widely used in the industry. TMC is a process by which test specimens of concrete, from a

representative sample of the concrete that is placed in an in-situ or precast element, are

cured by keeping them in a chamber at the same temperature as a pre-selected point in the

element. The TMC system can function under isothermal, near adiabatic, pre-recorded, and

even real-time profile conditions. The system is capable of accurately simulating the in-situ




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Postcastinginput

temperatureRealtime

tem erature

OR

Water

bath

Heater/

cooler

Process

ControlOn/off

Temperature

Concretesamples

curing temperature variation by controlling the temperature of a water bath for estimating the

real strength development in concrete. The main components of a TMC system are depicted

in a schematic drawing (Fig 3.4).

The system has an embedded process controller to precisely regulate the curing temperatureprofile. Accurate temperature matching is enabled using proportional, integral and derivative

control of feedback temperature data obtained from sensors embedded into one or two

concrete specimens within the conditioning chamber. The sensors are part of the eight

thermocouple inputs located at the temperature controller. A thermocouple connected to one

of the eight thermocouple inputs is also permanently positioned in the chamber to provide

water temperature measurements. More details about the particular system can be found in

reference (Mak and Torii, 1995).

Fig 3.4 Components of TMC systems

Extensive investigation has shown that TMC is able to predict the in-situ strength. However,

the technology remains costly and inaccessible to small to medium manufacturers.

Maturity method

The maturity method is a common method used in predicting the early-age of in-situ concrete.

The maturity method is used to account for the combined effect of temperature and time on

the development of the hydration reaction and the mechanical properties of concrete. The

nonlinear maturity function based on the Arrhenius law, which is also known as equivalent

age function is defined by Eq. (3.1):




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1 1

0

a

r

Et

R T T

et e t

(3.1)

Where:

te= the equivalent age at the reference temperature, Days

Tr= reference temperature, K

Ea= apparent activation energy, J mol-1

,

R= universal gas constant, 8.314 J mol-1

K-1

T= temperature of the concrete during interval ( t ), K

The reference temperature (Tr) is the standard cured temperature at which test specimens are

cured. In many parts of the world it is 20oC (293K). In Australia it is 23C in temperate zones

and 27C in tropical zones. Once the equivalent age is found, strength maturity functions

(presented in Table 3.1) can be used to find the strength at a particular age.

The procedure to use the maturity method in predicting the compressive strength of concretes

at early age is outlined below (Carino and Lew, 2001):

1. Cast cylinder specimens in accordance with AS1012 (1999). The specimens should

be cured under isothermal conditions (15, 23, 35 and 50C). Conduct compressive

tests at regular age intervals (1, 2, 3, 4, 7 and 28 days).

2. Plot the compressive strength of concrete from the tests (step 1) against the concrete

age for each curing temperature (15, 23, 35 and 50C).3. Conduct linear regression analyses to curve fit the strength maturity function to the

experimental data (step 2). From the curve-fitting process, the values of the

parameters kT, t0 and S are obtained for each curing temperature (15, 23, 35 and

50C).

4. Construct strength maturity curve for the reference temperature of 23C. This curve

provides prediction in concrete strength at different age when the concrete is cured at

a constant temperature of 23C. In order to use this curve to predict the compressive

strength of concrete that is subjected to various temperature conditions, the age at

which the compressive strength is predicted needs to be converted into an equivalent

age with a reference temperature of 23C.

5. To obtain the equivalent age (Eq.3.1), it is necessary to establish the activation

energy Ea. The activation energy can be obtained by plotting the natural logarithms of

kT versus the inverse of the curing temperature (15, 23, 35 and 50C). The negative

of the slope of the straight line equals the activation energy (Ea) divided by the gas




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constant (R). In the absence of experimental data, Ea/R of 4200 as suggested by Day

(2006) can be used.

6. Using Eq. (3.1), Ea/R from step 5 and the in-situ temperature profile, determine the

equivalent age.

7. The compressive strength of concrete can be predicted using the equivalent age

calculated in step 6 and the strength maturity curve constructed in Step 4.

Table 3.1 Strength-maturity Functions and Parameters

Function Name Strength-maturity function Parameters

Linear

Hyperbolic

S=)

0(1

)0

(

ttT

k

ttT

k

S

(Tank and Carino, 1991)

S = strength at age t,

S = limiting strength,

kT = rate constant, 1/day,

t0 = the age at the start of

strength development

Parabolic

HyperbolicS=

)0

(1

)0

(

ttT

k

ttT

k

S

Knudsen (1982)

As listed above

Exponential S= S

te

Freiesleben and Pedersen

(1985)

t = age,

= a time constant (value 1 is

the rate constant kTfor this

function),

= shape parameter

PlowmanS= )log(Mba

(Plowman, 1956)

a= strength for maturity index

M= 1

b= slope of the line

M= maturity index




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A case study

In this section, a case study of a post-tensioned slab of an apartment building in Melbourne is

presented to illustrate the maturity method. The prediction of concrete compressive strength

using the method will be compared with the in-situ compressive strength.

The slab has the surface area of approximately 570 m2. The thickness of the slab is 230 mm.

Thin copper-constantan type double T reusable thermocouple wires with cold junction

compensation and linearisation were used to measure the temperature variation at various

locations. Fig 3.5 shows the arrangement of the thermocouples in the slab cross section (100,

150, 220 mm, measured from the surface of the slab), one thermocouple measuring the

ambient temperature variation next to the slab and another one embedded in the ambient

cured cylinders cast beside the slab to obtain compressive strength predictions in accordance

with AS3600 and ASTM C31/C31 M. The measurement was obtained during the winter time

as the range of average daily temperature could be lower than that of standard 23 C. In this

case, the standard testing procedure following the Australian Standards AS1012 may not

accurately represent the in-situ concrete strength. In this standard procedure, the concrete

samples are cured under standard-moist conditions at constant temperatures of 23 2C for a

standard temperate zone.

Fig 3.5 Arrangement of thermocouple wires in the slab

Atypical

temperature

profile measured from the site is shown in Figure (see Fig 3.6). Some

small differences in temperature recorded across the slab depth can clearly be seen. The

thermocouple deeper inside the slab (Channel 3) records higher temperatures. The ambient

cured cylinder temperatures were generally lower than the temperatures recorded inthe in

situconcreteslab.

Timber

formwork

Datalogger

Thermocouple

measuringambient

conditions

Thermocouple

positions(mm)

Moulded

Cylinder

Channel1 10Concreteslab

Channel2Channel3

15

22




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Fig 3.6 Typical temperature profiles obtained for the thermocouple arrangements

To best represent the temperature across the slab section, the in-situ concrete temperature

profiles (Channels 1, 2 and 3) were averaged to represent a single profile in each case. The

average profile for the third day is duplicated to extend the profiles to a seven day period.

These averaged profile of is presented in Fig 3.7.

Fig 3.7 Average concrete slab temperature profiles

Cylinder specimens were tested at 1 to 28-day age. The concrete mix used is similar to the

concrete mix used on-site. The concrete specimens cured in the laboratory were batched at

the plant and truck-mixed in order to obtain a mix that is similar to the one that was

dispatched to the site. All concrete specimens were ground and conditioned before being

tested at 1 to 28-day age. The specimens were cured under standard and ambient conditions,

isothermal conditions and temperature matched curing (TMC) methods.

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80

Tempera

ture(oC)

Time (hrs)

Channel 1 Channel 2

Channel 3 Ambient

Cylinder

0

5

10

1520

25

30

35

0 40 80 120 160

Temperature(C)

Time (hrs)




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The specimens were cured in accordance with AS1012 (1999) and AS3600 (2009) for

standard and ambient conditions respectively. The average compressive strengths of the

cylinder specimens are presented in Table 3.2. To evaluate the in-situ strength development

of concrete, some specimens were subjected to representative temperature profiles recorded

from the two building projects using the TMC method. The specimens were cured under

average temperature profiles shown in Figure 3.7. The average compressive strengths are

presented in Table 3.1.

Table 3.2 Standard and Ambient Cured Cylinder Compressive Strengths (fc)

Age

(Days)

C1b, fc(MPa)

Standard cured

23oC

C2b, fc(MPa)

Ambient

cured

TMC C2c,

fc(MPa)

in-situ

1

23

4

7

28

11.2

-24.2

26.4

31.7

42.5

13.4

20.421.9

23.9

28.8

40.3

14.0

21.025.4

26.7

32.3

41.8

The equivalent age can be calculated using Eq. (3.1) based on the temperature profile shown

in Fig 3.7. The equivalent age profile is presented in Fig 3.8. The ratio of the activation energy

against the gas constant, /E R at 4200 with a Tr value of 23C was assumed (as suggested

by Day (2006)). The temperature profiles result in a slightly higher equivalent age for the

channels positioned deeper in the slab section. .

Fig 3.8 Equivalent age profiles for slab at different depths

Fordetermining the strength-maturity function parameters, a linear regression analysis was

conducted to curve fit the strength-maturity functions in Table 3.1 to the compressive strength

010

20

30

40

50

60

70

80

0 20 40 60 80

Equivalentage(hrs)

Elapsed time (hrs)

Channel 1

Channel 2

Channel 3




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measured for cylinder specimens cured under a reference temperature of 23C. For brevity,

only the result for the Linear Hyperbolic function (Eq. 3.2) is shown. The result of the

regression analysis for the reference temperature of 23C is presented in Fig 3.9. The values

of the parameters kT, t0 and S are 0.43, 0.18 and 43 respectively.

Fig 3.9 Strength predictions from regression analyses

To obtain the compressive strength of concrete subjected to various temperature conditions,

the values of equivalent age should be used. For example, a 3-day age concrete (72 hrs)

subjected to variation of temperature shown in Fig 3.7 has an equivalent age of about 2.7 day

(65 hrs). Using the strength-maturity curve shown in Fig 3.10, the compressive strength of the

concrete is about 22MPa.

Fig 3.10 shows the comparison between the compressive strength prediction using the

maturity method (linear hyperbolic function shown by the thick line in Fig 3.10), the standard

cured approach and the in-situ (denoted TMC C2c) compressive strength. It is shown that the

strength function can reasonably predict the in-situ strength of concrete. The difference

between the strength predictions using the maturity functions and the in-situ strengths were

found to be less than 10% with the exception of the 1 day age.

4

8

12

16

20

24

28

0 1 2 3 4

strength(MPa)

te(days)

Linear Hyperbolic

Standard cured




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Fig 3.10 Comparison of strength functions with specimens cured under different temperature profiles

1. Development of other mechanical properties over time

Maturity method can equally be used to predict other concrete properties such as tensile

strength and modulus of elasticity over time. However, similar to that reported for

compressive strength prediction, that there exists no single value for the activation

energy (Ea) for all the properties of concrete or different concrete formulations (Zhang et

al., 2008).

For completeness, some of the results reproduced here for discussion. The reader

should refer to the original source for in-depth discussion of the results and analysis.

The detailed mixture proportions of the high performance concrete are provided in Table

3.3, which also indicates the concrete slump, air content and density measured after

casting, and the compressive strength measured at 7 days (50MPa). Over 105 concrete

cylinders (100200 mm) were made from one single batch of concrete to minimize the

variability in the test results.

0

5

10

15

20

25

30

0 1 2 3 4

streng

th(MPa)

te(days)

Linear Hyperbolic

Standard cured

TMC C2c




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Table 3.3 The mix composition details

Proportions (kg) Properties

Max water 213 Slump 185mm

Cement

(ASTM Type I)

62.5 Air Content 4%

Fine aggregate

(5mm max,), dry

125 Density 2452

kg/m3

Coarse aggregate

(20mm max,), dry

125 fc@ 7 Days 50MPa

Superplasticiser, dry 2.1

Approximately 3 h after setting, when the concrete gains enough strength, they are

subject to different curing environments. For this, three equal groups of concrete

samples (with their moulds and covers) are stored in three environmental chambers with

temperatures previously set to 10 C, 25 C and 40 C and maintained as such until the

time of testing. This procedure ensured that the concrete samples had the same

properties at the onset of strength development for a better comparison.

Fig 3.11 Temperature in concrete samples and environmental chambers

Fig 3.11 presents the concrete temperatures monitored for the 3 groups of cylinders and

their respective curing temperatures during the first day (with curing temperatures

maintained constant until testing). Test results were obtained for the compressive

strength, splitting tensile strength, and modulus of elasticity measured over time at the 3

curing temperatures, respectively.




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Regression analyses were then conducted to model the development of these properties

as functions of time. Since the early-age development of HPC properties are of prime

interest in this study, the analyses presented in this paper focus mainly on the results

measured during the first 7 days. This ensured more accurate analyses and predictions

for the period of interest, during which the properties of HPC developed relatively fast.

The 28-day test results will be presented later in the paper as additional information. The

following empirical exponential equation, adapted from the well-known relationship

suggested by the (CEB-FIP, 1990)[20], was used to model the development of each

selected property of concrete:

1 (3.2)

where P(t) represents a given property as a function of time, P7 is the property value at 7

days, tis the time elapsed after the setting time, and A and Bare constants obtained by

regression analysis (Figs 3.12 and 3.13). The obtained regression curves for the

development of these properties were then used to determine the activation energy

factors and develop the property-maturity relations. Note that by using the time elapsed

after setting as the time zero in the above equation ensures the validity of the model for

concretes that may have different setting times.

Fig 3.12 Splitting tensile strength measured for 3 curing temperatures




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Fig 3.13 Modulus of elasticity measured for 3 curing temperatures

To estimate the equivalent age, the range of values presented for Ea presented may be

assumed.

To determine the activation energy for the properties such as tensile strength and

modulus of elasticity, the followings steps are recommended:

1. The determination ofK(T) from the initial slopes of the relative property P/Pu curves

for different temperatures (at t=to), where the Pu is the ultimate value of the property;

2. The activation energy can be obtained by plotting the natural logarithms of kT versus

the inverse of the curing temperature (15, 23, 35 and 50C). The negative of the slope

of the straight line equals the activation energy (Ea) divided by the gas constant (R).

3. The equivalent age can be determined using

1 1

0

a

r

Et

R T T

et e t

(3.3)

And the properties of concrete for different temperature profiles can be predicted using

the equivalent age and the curve for the reference temperature.

Zhang et al., 2008 report experimental results conveying important information on the

activation energy: (i) different properties may have different Ea for a given concrete; (ii)

different concretes may have different Ea for a given property; and (iii) different

development stage of a given property of a given concrete may have different Ea. The

experimental investigation demonstrates that the properties of Ea/R vary as the




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properties develop over time. The range of values listed is varying from 3763 K to 7781 K

for tensile strength and from 7145 K to 8435 K for modulus of elasticity.

2. Effect of Cementitious Materials on Concrete Strength Development

A recent investigation of the accuracy of the maturity method to estimate the strength of

concrete with different cementitious materials such as fly ash and slag is reported in

(Brooks et al, 2007). The reference discusses the strength development of mortar cubes,

made from Type I cement and various replacement levels of Class C fly ash, Class F fly

ash, and ground-granulated blast-furnace slag which are cured at isothermal curing

temperatures of 8, 23, and 40C. The temperature and activation energy values were

determined for these cementitious systems. Table 3.4 lists a summary of mixtures tested:

Table 3.4 Summary of Mixtures Tested during This Study (Courtesy: Brooks et al, 2007)

Data from the reference (Brooks et al, 2007) is used to make up the Excel spreadsheet

(strength prediction of concrete with varying fly ash content.xls). The excel file makes

use of the data provided in the reference to estimate the equivalent age for each of the

mixes listed in Table 3.4. For the calculations, the average concrete slab temperature for

Fig 3.7 is assumed. The reference temperature is set to 23C. Analysis results

determined with exponential strength-age linear hyperbolic function according to (Tank

and Carino, 1991, see Table 3.1) is used for the strength calculations. The model

parameters which are based on experimental results are listed in Table 3.5. These are

obtained following the steps listed in Section 3 (Maturity Method).




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Table 3.5: Analysis Results Determined with Strength-Age Function (Courtesy: Brooks et al, 2007)

The results of the experimental study show that composition of the cementitious

materials in the system significantly influences the maturity method to accurately

estimate the strength development. The effect of addition of fly ash of different types on

concrete strength development is shown in Fig 3.14. As can be noted, the Class F fly ash

results in significantly lower strength development of concrete when compared to that of

Class C fly ash.

Fig 3.14 Effect of addition of fly ash of different types on concrete strength development

The amount of longterm strength reduction, due to curing at high temperatures

compared with curing at room temperature, is equally found to be influenced by the type

of cementitious system used in the concrete mixture.

Sirivivatnanon et al. (2009) presented results from research of two concrete mixes

supplied to two building projects over a 16-month period covering two winter seasons in

Sydney. In this investigation, real-time in-situ concrete temperatures in post-tensioned

slab were simulated in a temperature matched curing tank in a laboratory. Concrete

0.0

10.0

20.0

30.0

40.0

0.0 50.0 100.0 150.0

f'c(Mpa)

te (hr)

ClassCflyashC15

C25

C35

IA

0.0

10.0

20.0

30.0

40.0

0.0 50.0 100.0 150.0

f'c(Mpa)

te (hr)

ClassFflyashIA

F15

F25

F35




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cylinders sampled from building sites were subjected to curing conditions identical to

those experienced in the slabs in order to determine as accurate as possible the in-situ

concrete strength development. Additional concrete cylinders cast from the same

concrete were cured using four different curing methods: air curing, insulated air-curing,

semi-adiabatic curing and standard AS 1012 water curing. Comparisons were made

between the cylinder strengths cured with the four methods and the estimated in-situ

concrete strengths for twenty concrete slabs cast in ambient temperatures ranging from

10C to 25C. The anchorage performance of post-tensioning strands at initial and final

stressing was also monitored. It was found that the 4-day air-cured strengths consistently

underestimated in-situ concrete strength by about 10% and had the poorest correlation

to in-situ strength of any of the curing regimes investigated. This finding suggests that

strengths derived from air cured cylinders cannot necessarily be considered to provide

curing conditions similar to those of concrete in the work as noted in AS3600 (Section 17

in 2009 Edition). The 1-day and 4-day in-situ strength of 9 MPa and 24.5 MPa was found

to be satisfactory for the initial and final stressing of 12.7mm diameter prestressing

strands for both types of concrete without the need for prescriptive binder specifications.

Summary

Benefits of Maturity

The benefits of using maturity as contrasted with traditional quality control procedures

are:

It provides a real-time, in-place indication of the strength of the concrete.

It is a non-destructive testing method as contrasted to breaking cylinders in the

laboratory.

Itprovidesearlyqualityverificationoftheinplaceconcrete,oftenwithinhoursof

itsplacement.

Itacceleratestheconstructionprocessbyallowingthepavementtobeopenedto

traffic

or

formwork

stripped

from

structures

(or

for

posttensioned

construction

helpsearlystressingsequence).

Itreducesthequantityandcostofsamplingandtestingbyreducingthenumberof

cylindersthatneedtobecastandbrokentodeterminestrength.

Thematuritymethod isreadilyassessabletomostmaterials laboratoriesbecause

itisbasedontraditionalcylindercompressivestrengthtestsforitsdevelopment.




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Weaknesses of Maturity

The maturity method also has its weaknesses. Changes in the brand of cement, the

source and type of pozzolonic admixtures, the source of the aggregate or the water to

cement ratio can result in a change in the strength-maturity relationship and require anew calibration curve. The method also cannot account for humidity conditions during

curing, that is, if there is not enough moisture present for hydration to occur the strength

gain will not be realized as predicted by the maturity curve. It is not accurate when there

are large temperature swings during the curing process. The method also cannot

account for concreting practices that result in inadequate consolidation, poor placement

techniques, inadequate curing, lack of protection during early ages, or fluctuations in air

content.

Volume Changes at Early Age

If the heat produced during the hydration reaction is not transmitted to the environment at the

same rate as it has been produced, the temperature rise in the concrete member might

become significant. Like any other material, concrete is susceptible to volume changes when

subjected to varying temperatures. The thermal expansion coefficient of concrete is about 10-

1510-6

per C (Neville, 1996). Temperature is one of the main causes of volume changes in

the early age concrete, besides autogenous deformation.

Temperature raises as a consequence of exothermic hydration reaction and the results

volume changes. If the deformation due to the volume change is restrained, stresses develop

in the member, often referred to as thermal stresses. In practice, restraints can be provided by

other adjacent structural members such as (the building core, columns or beams).

Reinforcements can also provide restraining effects internally. If concrete member was set

without any restraints, the volume changes would occur freely and no stresses would develop

in the member. In case of a restrained member, compressive and tensile stresses could

potentially develop due to heat development and cooling phases, respectively.




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Input parameter for thermal analysis

Parameter Value

Thermal conductivity 2.6 W/mK

Volumetric specific heat 2400 J/m3K

Convection coefficient in boundary 10 W/m2K

Arrhenius Constant 6000

Initial temperature 20 C

Coefficient thermal expansion () 15 10-6 per C

Properties of the concrete mix such as conductivity and capacitance are often assumed

constant. In reality these properties vary over time and are dependent on concrete maturity.

The thermal conductivity of concrete is reported to decrease over time as the concrete

matures (Gibbon and Ballim, 1998). The value depends on the moisture content and

temperature evolution, as well as the type of cement and aggregate of the concrete mix. Theparameters commonly adopted for concrete are summarized in Table given here.

Early Age Creep

Important factors that may contribute to cracking and failure of the early-age concrete are the

ambient conditions (e.g. temperature, humidity) and properties such as creep and shrinkage.

For a restrained concrete element, creep can occur due to thermal expansion and shrinkage.

Creep is considered as a time dependent deformation of concrete due to the imposed

external mechanical load or as a result of thermal expansion and shrinkage associated with

the hydration reaction. Immediate deformations due to applied load are referred to as the

nominal elastic strains (Neville 1996). This is not completely correct since there will always be

some early creep, but it is in practice good enough for most purposes. Creep can then be

taken as the increase in strains as a function of time, after this point as shown in Fig 3.15.

Additionally, there will also be time-dependent shrinkage, unaffected of whether the concrete

is loaded or not. Shrinkage is a result of chemical and physical changes in the concrete

volume during the hydration process. It can be divided into plastic shrinkage and drying

shrinkage. Plastic shrinkage occurs while the concrete is in the plastic phase. Drying

shrinkage is mostly affected by environ-mental conditions such as wind speed, temperature

and relative humidity. As shown in the figure, there are two types of creep which can make up

the total creep, depending on the moisture content of the environment. If there is no moisture

movement between the concrete and the environment, it can roughly be assumed that all

creep is basic creep, even though this is also a simplification.




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Fig 3.15 Time-dependent strains in concrete subjected to external loading over a time, Courtesy: Neville

(1996)

Estimation of the early age creep effects is essential in accurately estimating the development

of stresses that are induced in the concrete member. The stresses can be caused by thermal

effects, autogenous deformations or by application of external actions such as post-tensioning

loads. It is reported that creep deformation of concrete at early ages may lead to stress

fluctuations that reach magnitudes of 50% or higher (see for eg., Faria et al., 2006). At the

same time, it is important to note that with the evolution of the hydration reaction makes the

material properties of concrete such as the elastic modulus, compressive and tensile

strengths to be dynamic.

Elastic materials have the ability to fully regain the initial shape after being deformed.

Concrete at early ages cannot be assumed to be elastic. A visco-elastic material exhibits both

viscous and elastic characteristics under deformation. While elastic deformations are always

recoverable upon unloading, viscous deformations are never recoverable (Neville 1996).

Considering Fig 3.16, the instant that the stress is being applied to the concrete member, the

strain is elastic and thus recoverable if the stress is removed immediately. Obviously, if the

magnitude of the applied load is close to the concrete strength, this would not be the case due

to cracking and plastic deformations.




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ElasticStrain

t

Fig 3.16 Elastic and time dependent strains of concrete due an imposed load [After stergaard et al.

(2001)]

A strong aging tendency of concrete material in the first two days after casting has been

observed at-several occasions (Altoubat and Lange 2001; Bissonnette and Pigeon 1995).

Aging can be explained as the concrete hardening and gaining strength. The concrete is very

susceptible to creep at early ages, but quickly ages and therefore becomes more resistant as

it hardens.

According to the linear theory, the strain is proportional to the applied stress. Hence, the strain

at time tcaused by a constant uniaxial stress applied at time tis given by:

)',(.)( ttJt

The compliance function J(t, t) for a specimen subject to constant stress is given by the

following expression:

J(t, t) = q1+C0 (t, t)+Cd (t, t,t0)

Where, q1 is the instantaneous strain due to unit load, C0(t, t) is the basic creep compliance

function, and Cd (t, t, t0) is the drying creep compliance function, t is the age of concrete at

time of loading in days and tis the current age of concrete in days.

For a variable stress history, the principle of superposition is assumed to be valid. There is a

great many number of creep models proposed in the literature. For consideration of early age

effects, however, Bazants B3 model and the Double Power Law (DPL) is commonly used.

The creep response to a load applied at slightly different ages changes rapidly at this stage of

the hydration process due to the strong aging. Higher creep compliance is reported for

concrete at earlier ages. Prior to one day, even a light load was observed to produce twice the




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amount of creep of that only a few hours later, using identical concrete mixes (Atrushi, 2003,

Ostergaard, 2003).

Construction Issues and the role of Early Age Concrete

Although high early strength concrete (HESC) concrete is an obvious choice for construction,

particularly for multi-storey buildings, many instances of localised failures have been reported

during construction. These are partly due to wrong estimates of the strength prediction of the

concrete strengths. A wide range of other defects such as excessive floor sagging, cracking,

spalling, honeycombs, increased long term deflections, bulges and misalignments are also

considered as failures that result in serious serviceability problems (Feld, 1964; Aalami and

Kelly, 2001). In the following section, typical examples of failures related to both precast and

post-tensioned industries are presented.

Post-tensioned slabs Anchorage failures

In post-tensioned systems, the prestressing force is transmitted to the concrete mainly by the

direct bearing of a steel anchorage plate or casting on the concrete. An external anchorage is

used for stressing the strand (live end) and an internal anchorage is used to resist the loads

by stressing of the strand (dead-end). A typical slab edge anchorage assembly is presented in

Fig 3.17. At the dead-end, the strands coming out of the duct tube are spread out and are

bent with their ends to form the shape of an onion. The anchorage zone of a post-tensioned

concrete member is a critical portion of the member where the concentrated PT forces spread

to a uniform stress distribution in the concrete.




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Fig 3.17 Anchorage in slab edge (dimensions in mm)

The common criteria for acceptance of concrete used in post-tensioned applications having

12.7 mm diameter strands in Australia are;

Minimum 28 day characteristic cylinder strength (fc) of 32 MPa

Compressive strength at final stress transfer of 22 MPa typically at 3 to 5 days after

concrete

placement

Compressive strength of 7 MPa at 24 hours after concrete placement for initial

stressing at

25% of the ultimate stress.

At these times, i.e., 1-7 days after the concrete pour when the post-tensioning procedure is to

be carried out, while the concrete is curing and the properties of concrete evolve over time

(Choong et al., 2005). One major issue with the post-tensioning of slabs is the failure of the

concrete material at the anchorage zone locations. Failures happen due to stress

concentrations in spite of factors of safety and heavy reinforcement, usually in the form of

loops or spirals, as shown in Fig 3.17. A typical anchorage zone failure in a post-tensioned

suspended slab is shown in Fig 3.18. The failures, which mostly happen during the post-

tensioning process, can be serious and explosive in nature. They happen at the second stage

of post-tensioning at either of live or dead-end anchorages. As can be seen (Fig 3.18), they

require resources for replacement and repair which can substantially increase the

construction cost. Reinforcement congestion in the local zone, poor concrete quality and

inappropriate workmanship has been blamed for the failures. Inadequate strength prediction




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of in-situ concrete at the time of application of the post-tensioning has been another prevalent

hypothesis [2] (CPN29, 1996). Anchorage failures have been occurring at the dead and live

ends, resulting in loss of load in the strand, delamination and spalling of concrete around the

anchorage. In many instances, where the concrete cover is broken off, grooves can be

observed demonstrating slippage of wires and strand assembly with respect to surrounding

concrete.

Fig 3.18 Failed anchorage in slab edge photographed on construction site

For the live end, the duct covers the strands entirely and the PT load is transmitted directly to

the anchorage plate. As regards to dead-end anchors, which constitute a more critical case,

the PT load is resisted initially by bonding effect of the straight part of the strand. This straight

part of the strand, about 800 ahead of the anchor plate is bonded to the concrete (see, Fig

3.17). When the bond mechanism of the strands is exhausted, the load is directed to the

anchorage assembly and transmitted to the surrounding concrete. Clearly, there are two

contributing factors in resisting the PT load: 1) the bond strength of the straight part of the

strand; and, 2) the anchorage action due to the anchorage plate. In details, the onions which

are themselves made up of bent wires, are equally bonded to the surrounding concrete.

A comprehensive research project is carried out at the University of Melbourne sponsored by

Australian Research Council and Holcim to investigate this problem. In view of the anchorage

description, it is noted that extensive number of variables are involved. The preferred option

was to investigate the problem within the framework of finite elements. Sofi et al. (2007) report

the results of the experimental work and the modelling approach adopted to investigate the

behaviour of anchorage zone concrete. The experiments were carried out in an attempt to

obtain the input parameters necessary for the finite elements and to validate the models. In

this paper, a short literature review of bond mechanism is presented first followed by a

description of the experimental work. This is followed by a discussion of the models and the

results (see Sofi et al., 2011).




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In recent times, questions have been raised as to the possible negative impacts of fly ash

inclusions into concrete and the potential for increased risk of anchor failure as a result of

impact on early age concrete properties relevant to when final stressing is applied. Key

concerns appear to include potential loss of bond between strands and concrete, lower early

age tensile properties and compressive strength. Some specifications limit the percentage of

fly ash allowed in a post tensioned concrete mix to be no greater than 10% by weight of

Portland cement for these reasons.

Sirivivatnanon et al. (2009) conducted an experimental study on implications of using fly ash

in post-tensioned concrete.

They concluded that;

Fly ash concretes can be easily designed to meet early age requirements specified for

post tensioning applications.

Concrete strength gain characteristics for control (non fly ash) and fly ash concrete

mixes are well understood and are often optimised for particular design and

constructional applications.

The research indicates that the bond strength (as determined by pullout load) between

tendons and concrete is a function of the compressive strength of the concrete and not

the composition of the cementitious component. In particular, fly ash inclusions of up to

30% appear to have no adverse effect on bond at the same compressive strength.

There is no evidence in the technical literature to support the hypothesis that increasing

fly ash contents in concrete (certainly to levels up to 30%) will reduce bond strengths

between tendons and concrete.

Precast Walls Support and Anchorage Systems during Construction

The use of precast allows not only the speedy erection of the structure, but also flexibility and

overall program shortening. This is achieved by allowing the production of components at the

same time the footing system is being prepared1.

A minimum amount of propping allows the following trades to commence work on the

structure earlier than conventional construction methods. During construction the lateral load

1http://www.npcaa.com.au/images/file/Skeletal%20frame%20struct.pdf




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path for a building under construction is incomplete. Therefore, external supports in the form

of vertical struts are provided to resist the lateral wind loads and to provide stability for the

newly installed precast elements.

For large panels, the joints between panels are generally located at rafter centres so that theroof framing (or portal frames when panels are used as cladding only) laterally supports both

panels. For smaller panels, an intermediate strut can be provided to laterally support the

panels at the intermediate joint location. Early age concrete strength is very important and

some failures are reported due to problems in reaching specified strength.

Fig 3.19 Rafter to panel connection at joint

Cast-in headed anchors provide a means of making bolted or welded connections to other

concrete members. According to the National Precast Concrete Association of Australia

(NPCCA), the usual forms of these anchors are illustrated in Fig 3.20.

Fig 3.20 Types of anchors, (Courtesy, NPCAA Manual)

Ferrules, which are internally-threaded, steel sleeves to take a bolt. They are anchored by

transverse reinforcement attached to the base by welding, or, through a transverse hole in the

base of the sleeve, or, with a J-bolt screwed or butt-welded to the base. A common




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application for a ferrule is the restraint fixing for a wall panel. Ferrules for structural

connections are typically 75100 mm long and the usual bolt diameters are 20 and 24 mm.

Plates with stud anchors provide a means to weld a fixing bracket in its correct position after

erection of the member. Such brackets should reach their yield strength prior to the load

reaching the calculated capacity of the cast-in stud assembly.

Fig 3.21 Failure modes of anchors (Courtesy, NPCAA handbook)

Figure 3.21 depicts failure modes of anchors asreported by the National Precast Concrete Associationof Australia. The following failure modes of an anchorare possible (NPCAA):

Steel failure of the anchor shaft or bolt in tension

Steel failure of the anchor shaft or bolt in shear

Breakout of a prism of concrete surrounding the

anchor in tension

Breakout of a prism of concrete towards an edge in

shear

Crushing of the concrete over the bearing area at

the base of the anchor (pullout)

Rotational pryout of an anchor body subjected to

shear

Side-face blowout when the anchor is located close

to an edge in conjunction with deep embedment

(greater than 200 mm)

Splitting of the concrete in the vicinity of the anchor.




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The type of failure cones in high strength concrete are generally deeper than those in normal

strength concrete. This is an indication that higher compressive strength of concrete reduces

the possibility of local crushing and thus enhances the interlock effects. Some useful results

are also available in Joshi (2001) and Yin (2002).

REFERENCES

1. Aalami, BO and Kelley, GS 2001, Structural design of post-tensioned floors, Concrete

International, v.23(1), Pp.31-36.

2. ACI318-11, Building code requirements for structural concrete (ACI 318-11) and Commentary

(ACI 318R-11)..

3. Altoubat, SA and Lange, DA 2001, 'Tensile Basic Creep: Mea-surements and Behaviour at

Early Age', ACI Materials Journal, vol. 98, no. 5, pp. 386-93.

4. AS 1012.8.1, Method of testing concrete: Method of making and curing concrete -

compression and indirect tensile test specimens. Committee BD-042, Australian Standards

1999.

5. AS 3600, Concrete structures. Committee BD-002, Australian Standards 2001.

6. ASTM C192/C192 M-10, Making and Curing Concrete Test Specimens in the Field, American

society for testing and materials (ASTM). West Conshohocken, Pennsylvania; 2007.

7. Atrushi, DS 2003, 'Tensile and Compressive Creep of Early Age Concrete: Testing and

Modelling', Doctoral thesis, The Norwegian University of Science and Technology.

8. Baant, ZP and Panula, L 1978, 'Practical prediction of time de-pendent deformations of

concrete', Materials and Structures, vol. 11, no. 65, pp. 307-28.

9. Bissonnette, B and Pigeon, M 1995, 'Tensile creep at early ages of ordinary, silica fume and

fiber reinforced concretes', Ce-ment and Concrete Research, vol. 25, no. 5, pp. 1075-85.

10. Brooks, AG1; Schindler, AK and Barnes, RW., 2007 Maturity method evaluated for various

cementitious materials, Journal of materials in civil engineering, v. 19(12), pp. 1017-1025.

11. Brundtland Report: 1987, The Report of the Brundtland Commission, Our Common Future,

published by Oxford University Press..

12. Carino, N. J. and Lew, H. S., The Maturity Method: From Theory to Application. In: Chang, P.

C., editors. Structures Congress and Exposition. Washington, D.C.: American Society of Civil

Engineers; 2001. p. 19.13. CED-FIB, CEB-FIP Model Code 1990, Information Bulletin No. 213/214, Euro-International

Concrete Committee, Lausanne, 1993, 437 pp.

14. Choong, J., Mendis, P.A., Mak, S.L., and Baweja, D., More Accurate Concrete Strength

Predictions for Post-Tensioned Slab Construction, Proceedings, Concrete Institute of

Australia, 22nd Biennial Conference, Concrete 2005, Melbourne, Australia, October, 2005.

15. CPN29, Prestressed concrete anchorage zones. Concrete Institute of Australia 1996; 1-13.




33/34

134

16. Day, K. W., Concrete mix design, quality control and specification. London: E. and F. N.

Spon; 2006.

17. Faria, R, Azenha, M and Figueiras, (2006), 'Modelling of con-crete at early ages: Application

to an externally restrained slab', Cement and Concrete Composites, vol. 28, pp. 572-85.

18. Feld, J., 1964, Lessons from failures of concrete structures, American Concrete Institute

monograph, v.1(1), Pp. 179.

19. Flower, J., Sanjayan, G., and Baweja,D, Environmental Impacts of Concrete Production and

Construction, Proceedings, Concrete Institute of Australia Biennial Conference, Concrete

2005, Melbourne, Australia, October, 2005

20. Freiesleben Hansen, P. and Pedersen, E. J., Curing of concrete structures, Comit euro-

international du bton (CEB) Information Bulletin, (1985)166.

21. Gibbon, GJ and Ballim, Y 1998, 'Determination of the thermal conductivity of concrete during

the early stages of hydration', Magazine of Concrete Research, vol. 50, no. 3, pp. 229-35.

22. Jenkins, D, Baweja, D, Portella, J., Optimising Building Design for Sustainability Using High

Performance Concrete , 2011.

23. Joshi, T., Behaviour Experimental Investigation of PostInstalled Anchors in Highstrength

Concrete,MastersThesis(supervisedbyP.Mendis),TheUniversityofMelbourne,2001

24. Knudsen, T., Modelling hydration of Portland cement: Effect of particle size distribution,

Engineering Foundation Conference on Characterization and Performance Prediction of

Cement and Concrete, Henniker, N. H. (Ed.), (1982)125.

25. Lin,Y.,BehaviourofScrewAnchorsinHighstrengthConcrete,MastersThesis(supervisedby

P.Mendis),TheUniversityofMelbourne,2002

26. Mak, S. L. and Torii, K., Strength development of high strength concretes with and without

silica fume under the influence of high hydration temperatures. Cement and ConcreteResearch 1995; 25(8); 1791-1802.

27. Neville, AM 1996, Properties of Concrete, 4th and final edn, Wiley, Harlow, Essex.

28. Ngo, T., Mendis, P., Aye, L., Gammampila, R., Crawford, R., Life cycle assessment of steel

and concrete framed high rise commercial buildings, International Conference on

Sustainable Built Environment, ICSBE 2010, Sri Lanka

29. NPCAA *(2009), Precast Concrete Handbook, 2nd edition, National Precast Concrete

Institute of Australia.

30. Plowman, J. M., Maturity and the strength of concrete, Magazine of Concrete Research,

8(22), (1956)13.

31. Sirivivatnanon, V, Baweja, D. and Khatri, R.P., Evaluation of In-situ Concrete Strengths for

Post-Tensioning of Concrete Slabs, 2009

32. Sofi, M., Mendis, P.A., Baweja, D. and Mak, S.L., Behaviour of Post-Tensioning Anchors in

Early Age Concrete Slabs, Proceedings, Concrete Institute of Australia 23rd Biennial

Conference, Concrete 2007, Adelaide, Australia, October, 2007.




34/34

33. Sofi, M., Mendis, P.A., Baweja, D. and Elvira, E., Bond Performance of Strand and Wire in

Early Age Concrete, 9th International Symposium on High Performance Concrete:Design,

Verification and Utilisation, 9-11 August 2011, New Zealand.

34. Stergaard, L, Lange, DA, Altoubat, SA and Stang, H 2001, 'Tensile basic creep of early-age

concrete under constant load', Cement and Concrete Research, vol. 31, no. 12, pp. 1895-9.

35. Tank, R. C. and Carino, N. J., Rate constant functions for strength development of concrete',

ACI Materials Journal, 88(1), (1991) 74-83.

36. Zhang, J., Cusson, D., Monteiro, P. and Harvey, J. (2008), New perspectives on maturity

method and approach for high performance concrete applications, Cement and Concrete

Research, 3814381446.


limit states in construction_mendis

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