CHAPTER 3
RESEARCH METHODOLOGY
3.1
Introduction
In this chapter the methodology of the experimental works engaged are
discussed. The laboratory programme and framework are illustrated in Figure 3.1. The
tests performed in the laboratory were the creep test, shrinkage test, compressive
strength test and modulus of elasticity test. The materials used, specimens preparations,
curing of specimens and testing procedures are described in the following sections.
The creep and shrinkage tests were performed under the temperature of 27 C
and 80 % of relative humidity to represent the Malaysian environmental conditions.
Two different grades of normal strength concrete that widely used in current Malaysian
construction field were chosen in these experimental works; they were grade 25 and 30.
Since creep and shrinkage changes are more rapid during early stage, therefore the
creep and shrinkage were measured at an age of initial loading for 7 days up to 39 days.
ASTM C512 (1987) was mentioned that creep is proportional to stress from 0 to 40 %
of concrete compressive strength but Neville et al. (1983) suggested that in terms of
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stress-strength ratio an upper limit between about 0.30 and 0.75 has been observed.
Therefore, in this study 30 % stress-strength ratio was used as applied load for creep
test.
3.2
Test Parameters
The two testing parameters involved in this study are grades of concrete and
maximum aggregate sizes. A grade of concrete was chosen in this study for determining
the various data for normal strength concrete. The maximum aggregate sizes was also
chosen as the test parameter here for the reason as mentioned in sections 2.4.5 and
2.5.4.2, i.e. aggregate is one of the factors that influencing creep and shrinkage. The
summary of the testing parameters is presented in Table 3.1.
MAIN TEST
(i)
(ii)
Creep test
Shrinkage test
PARAMETERS
(i)
(ii)
Grades of concrete
Maximum
aggregate sizes
ENGINEERING
PROPERTIES TEST
(i)
(ii)
Compressive
strength test
Modulus of
elasticity test
Figure 3.1: Framework of laboratory programme
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Table 3.1: Summary of testing parameters for the experimental work
Maximum
Grade of Concrete
aggregate sizes
Types of Experimental Work
(mm)
10
25
20
10
30
20
3.3
Materials
3.3.1
Cement
(i)
Creep
(ii)
Shrinkage
(iii)
Engineering properties
(i)
Creep
(ii)
Shrinkage
(iii)
Engineering properties
(i)
Creep
(ii)
Shrinkage
(iii)
Engineering properties
(i)
Creep
(ii)
Shrinkage
(iii)
Engineering properties
Ordinary Portland cement (OPC) (ASTM Type 1) supplied by Tenggara Cement
Manufacturing Sdn. Bhd., Johor, Malaysia that complies with MS 522: Part 1 (2003)
was used throughout the experimental work in this study. The chemical compositions
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and physical characteristics for the cement are presented in Tables 3.2 and 3.3
respectively.
Table 3.2: Typical chemical compositions of Portland cement
Chemical compositions
Percentage
Silica, SiO2
20.0 – 22.5
Alumina, Al2O3
4.8 – 6.0
Ferum Oxide, Fe2O3
2.4 – 2.5
Calcium Oxide, CaO
Min 62.0
Magnesium Oxide, MgO
Max 3.5
Sulphuric Anhydrite, SO3
2.1 – 2.4
Insoluble Residue, IR
Max 2.5
Loss of Ignition, LOI
Max 2.0
Density
Max 0.4
Lime Saturated Factor, LSF
Min 0.85
* Source: Tenggara Cement Manufacturing Sdn. Bhd.
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Table 3.3: Typical physical characteristics of Portland cement
Fineness:
Surface area (m2’kg)
290 – 325
90 micron (%)
1.5 – 2.5
45 micron (%)
15.0 – 2.0
Setting time (minute):
Initial set
90 -0180
Final set
180 - 270
Compressive strength (N/mm2):
1 day
20
3 days
30
7 days
40
28 days
50
Soundness (mm)
Max 10
* Source: Tenggara Cement Manufacturing Sdn. Bhd.
3.3.2
Aggregate
3.3.2.1 Fine Aggregate
Uncrushed fine aggregate which comply with the MS 30: Part 2 (1995) was used
in this study. The fine aggregate used in this study is sand, which was produced by
Gunung Raya quary, Johor, Malaysia. Sieve analysis was performed in accordance to
ASTM C136, prior for casting to determine the percentage of fine aggregate passing
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the 600 m sieve needed for the mix design calculations. The fine aggregate was
maintained in a saturated surface dry condition for 24 hours prior to use.
3.3.2.2 Coarse Aggregate
Coarse aggregate used in this study is a crushed type aggregate with 10mm and
20 mm maximum size. Coarse aggregate used in this study was produced by Gunung
Raya quary, Johor, Malaysia and it complies to MS 29 (1995).
3.3.3
Water
Water is a key ingredient in the concrete production. The properties of water that
was used in the concrete work is be potable, free from oil and other organic impurities
accordance to MS 28 (1985). Therefore, ordinary tap water was used as mixing water
throughout the study.
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3.4
Mix Design
The mix proportion in this study was designed for characteristic strength of 25
N/mm2 and 30 N/mm2 based on ‘Design of normal concrete mixes, Department of the
Environment (1986) (DOE)’. In this study, constant slump of 30 – 60 mm was designed
for all mixes. The calculation for the concrete mix design is presented in Appendix A.
3.5
Concrete Mixing
Before mixing the concrete, cement was kept dry and placed in a moisture-proof
container to prevent the initiation of hydration and difficulties in handling. Fine and
coarse aggregate was maintained in a saturated surface-dry condition 24 hours prior to
use. All the concrete materials were stored at room temperature in the range of 20 to 30
C in accordance with ASTM C 192-90a (1990) ‘Making and Curing Concrete Test
Specimens in the Laboratory’.
It is important to have proper mixing to ensure all surfaces of the aggregate
particles were coated with cement paste and the ingredients were blended into a uniform
mass. In this study, the drum type mixer was used. Figure 3.2 shows the mixing process
of concrete specimens in the laboratory.
The workability tests adopted in this investigation was slump test for the
concrete. The slump test was carried out in accordance to ASTM C143-90a (1990) ‘Test
Method for Slump of Hydraulic Cement Concrete’.
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3.6
Curing of Specimens
Concrete must be properly cured to develop its optimum properties. To prevent
evaporation of water from the unhydrated concrete, the specimens were immediately
covered with wet gunny sack after molded. The specimens were removed from the
molds after 24  8 hours (ASTM C192, 1990), moist cured at 23  1.7C until the age
of 7 days in accordance with ASTM C 512-87.
After the completion of moist curing, the specimens were loaded for creep test
and stored at the control room until completion of the test. The control room used in this
study was set at a temperature of 27 C and at a relative humidity of 80%.
DRY MIXING
MIXING
Coarse aggregate
+
Fine aggregate
+
Cement
Coarse aggregate
+
Fine aggregate
+
Cement
+
Water
WORKABILITY
TEST
Figure 3.2: Mixing process of concrete specimens
Slump test
CASTING
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3.7
Engineering Properties
3.7.1
Compressive Strength Test
Cylinder specimens with 100 mm in diameter and 200 mm in height were
prepared for compressive strength test. The specimens prepared were the same as
specimens prepared for creep test and was tested at the age of 7 days.
The compressive test was performed in accordance with ASTM C 39 (1993).
The specimens were tested using a ‘Tonipac 3000’ compression machine and with the
rate of loading of 3.0 kN/sec as shown in Figure 3.3. The load applied continuously and
without shock until the specimen fails and the maximum load carrying by the specimen
during the test was recorded.
3.7.2
Modulus of Elasticity Test
Cylinder specimens with 100 mm in diameter and 200 mm in height were
prepared for modulus of elasticity test. The specimens was prepared the same manner as
specimens prepared for the creep test.
The specimen was placed in the testing machine with the wire strain gauges
attached on the four opposite sides of the concrete specimen prior testing to obtain the
strain of the specimen when loading as shown in Figure 3.4. The specimen was load at
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least twice during the first time loading without any data taken. Then the load was
applied continuously up to 30% of the ultimate load determined from the compressive
strength test. Three readings were recorded for each specimen and the strain value at
30% of the ultimate load was determined from the average of two specimens. The
detailed testing procedure for modulus of elasticity test is accordance with Standard
Test Method ASTM C 469 (1992).
Figure 3.3: Compressive strength test
41
Figure 3.4: Modulus of elasticity test
3.7.2.1 Calculation
The required values for calculation of modulus of elasticity were obtained from
the stress-strain diagram. The secant modulus of elasticity was calculated using the
following equation:-
30%
E=
Where:
E
=
30% =
30%
[3.1]
modulus of elasticity (GPa)
compressive strength corresponding to
approximately 30% of ultimate load (MPa)
30% =
longitudinal strain corresponding to 30%
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3.8
Creep and Shrinkage Tests
3.8.1
Specimen Preparation
For creep and shrinkage tests, vertically cast cylindrical specimens were
prepared in accordance to the provisions of ASTM C 512 (1987). The size of specimens
prepared is 100 mm in diameter and 200 mm in height. The numbers of two specimens
were prepared for total creep (loaded specimens) and two specimens for shrinkage test
(unloaded) and used as a control.
The concrete were placed in two approximately equal layers and each layer was
compacted 25 times uniformly over the cross section of the mold. After consolidation,
the top surfaces were finished by fitting the ends with Perspex plates normal to the axis
of the cylinder to get the flat surfaces. Then the specimens were covered with wet
gunnysack to protect water from evaporation and then the curing process was performed
as mentioned in Section 3.6.
For creep and shrinkage tests, DEMEC gauges were used to measure the
deformation of the concrete as shown in Figure 3.5. Before testing, four DEMEC
measurements point (studs) were glued at 150 mm distant apart on four opposite sides
of the concrete specimens.
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Figure 3.5: DEMEC gauge measurement apparatus for strain
3.8.2
Apparatus
According to ASTM C 512-87 (1987), the loading frame used for creep test
must be capable of applying and maintaining the required load on the specimens. The
loading frame is consists of header plates bearing on the ends of the loaded specimens, a
load-maintaining element either spring or a hydraulic capsule or ram, threaded rod to
take the reaction of the loaded system.
In this study, the creep test was carried out using the creep test rig as shown in
Figure 3.6. Coil spring loading system was selected as the loading frame. The coil
spring was installed between lower base plate and upper base plate. The sustained stress
was applied by tightening the four tie rods and a load cell was permanently installed in
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the frame when the load was applied. The creep test rig was able to hold two concrete
specimens in series. Figure 3.7 shows a schematic representation of a loading frame for
the creep test.
3.8.3
Testing Procedure for Creep Test
For this study, creep tests were carried out at age of loading of 7 days. Before
loading the creep specimens, the compressive strength of the specimen was determined
in accordance with Standard Test Method ASTM C 39 (1993). The average ultimate
compressive strength of two specimens was used to determine a stress which was being
applied to specimens for creep test. The manual loading system for creep test was
subjected to a stress of 30% of average ultimate compressive strength as stress-strength
ratio.
The specimens were placed in the loading frame as shows in Figure 3.6. The
centre point of each plate was determined and the specimens were placed with caution
to avoid eccentricity. The actual load applied was monitored using a load cell, which
was connected to the data logger as shown in Figure 3.8. The load was measured every
time before each strain reading was taken to ensure the correct value of loading was
applied. The strain reading was taken immediately before and after loading, two to six
hours later, and then daily for 1 week, weekly until the end of one month and monthly
until the end of the testing. Direct reading of the strain was obtained by multiplying the
reading shown on the DEMEC gauge by a calibration of 1.09 x 10-5.
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3.8.4
Testing Procedure for Shrinkage Test
At the same time of creep test, measurement of strain for shrinkage specimens
(unloaded specimens) was also made to obtain the shrinkage strain at the same as the
creep test as shown in Figure 3.9. The initial strain reading was taken at a same time as
creep specimens reading was taken. Then the reading were recorded at the following
ages; two to six hours later, and then daily for 1 week, weekly until the end of one
month and monthly until the end of the testing. Direct reading of the strain was obtained
by multiplying the reading shown on the DEMEC gauge by a calibration of 1.09 x 10-5.
In this study, the measurement was made for 39 days.
Figure 3.6: The creep test rig
46
Nut
Upper load plate
Load cell
Data
logger
Lower load plate
Temperature and
Humidity
controller
Upper base plate
100 mm X 200 mm
specimens for total
deformation
Lower base plate
Spring
Stand box
Figure 3.7: Schematic representation of a loading frame
47
Figure 3.8: Data logger connected to the load cell
Figure 3.9: Specimens for shrinkage test
48
3.8.5
Calculations
3.8.5.1 Shrinkage
Shrinkage strain was determined from an average of eight measurements points.
The shrinkage strain was calculated by the Equation [3.2]:-
( t1, t0) = [ (t) - (tsh,0)] x M
Where:
[3.2]
(t1, t0) =
shrinkage at time, t measured from the start of t0
(t)
shrinkage at time t
=
(tsh,0) =
shrinkage at time t0
M
coefficient of DEMEC gauge
=
3.8.5.2 Creep
The total creep strain was determined from an average of six measurements
points. The value of total creep was obtained from the calculation of following formula
at a given time and days.
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(i)
Total creep
The total creep strain was obtained by subtracting the instantaneous elastic strain
and the strain for the shrinkage specimen from the total deformation strain as given in
Equation [3.3].
c(t1, t0) = [ t(t1) - ie(t0) - su(t1)] x M
Where:
(ii)
[3.3]
c(t1, t0) =
total creep at time t1 due to a stress applied at t0
t(t1)
total deformation at t1
=
ie(t0) =
instantaneous elastic strain at time t0
su(t1) =
strain for shrinkage specimen at t1
M
coefficient of DEMEC gauge
=
Creep coefficient
After the creep value obtained from Equation [3.3] the creep coefficient was
obtained as a ratio of creep to the instantaneous elastic strain at any age.
(t1,t0) =
Where:
c(t1, t0)
ie(t0)
[3.4]
(t1,t0) =
creep coefficient at t1 due to a stress applied at t0
c(t1, t0) =
creep at time t1 due to a stress applied at t0
ie(t0) =
instantaneous elastic strain at time t0