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Effect of Age on Concrete Core Strength Results

Authors:
  • University of Technology, Iraq, Baghdad
  • University of Technology - Iraq

Abstract and Figures

Assessment of in-situ concrete strength by means of cores cut from hardened concrete is accepted as the most common in-situ nondestructive method, however the assessing of the concrete in the existing buildings, particularly in the troubleshooting of problems with new construction, If the strength of standard compression test specimens found to be below the specified 28 days value, frequently, cores tests are undertaken at later ages exceeding the 28 days. This study includes an attempt to find the influence of the long-term concrete age and strength level on the compressive strength development for the standard concrete core. This study involves laboratory investigation were number of specimens including concrete panels and cubes with specified compressive strength ranging from 25-55 MPa were prepared and tested at concrete age of 28, 60, 90, 120, 180,and 270 days by in-situ nondestructive tests (cores) and destructive tests (cubes). The test results obtained from core specimen were compared with those of standard specimens. The test results showed that the core compressive strength increases as the age of concrete increase, but the core strength is somewhat higher than 28-day cube compressive strength even up to the age 270 days in moderate concrete, while the core compressive strength remains lower than 28-day cube compressive strength in the higher strength level even up to the age 270 days.
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The 2nd International Conference of Buildings, Construction and Environmental Engineering (BCEE2-2015)
13
AbstractAssessment of in-situ concrete strength by means of
cores cut from hardened concrete is accepted as the most common
in-situ nondestructive method, however the assessing of the
concrete in the existing buildings, particularly in the
troubleshooting of problems with new construction, If the
strength of standard compression test specimens found to be
below the specified 28 days value, frequently, cores tests are
undertaken at later ages exceeding the 28 days. This study
includes an attempt to find the influence of the long-term concrete
age and strength level on the compressive strength development
for the standard concrete core.
This study involves laboratory investigation were number of
specimens including concrete panels and cubes with specified
compressive strength ranging from 25-55 MPa were prepared
and tested at concrete age of 28, 60, 90, 120, 180,and 270 days by
in-situ nondestructive tests (cores) and destructive tests (cubes).
The test results obtained from core specimen were compared with
those of standard specimens.
The test results showed that the core compressive strength
increases as the age of concrete increase, but the core strength is
somewhat higher than 28-day cube compressive strength even up
to the age 270 days in moderate concrete, while the core
compressive strength remains lower than 28-day cube
compressive strength in the higher strength level even up to the
age 270 days.
Index TermsCompressive strength, Concrete cores,
Concrete age, Strength level.
I. INTRODUCTION
he strength of concrete is traditionally characterized by the
28 days value. However, strength of concrete is expected to
increase with time at continuously diminishing rate.
Knowledge of the strength-time relationship is of importance
when a structure is subjected to certain type of loading at a
later age. Many factors can significantly influence the
compressive strength of the concrete. These include cement
type, water-cement ratio, aggregate content, water curing
period, and exposure conditions [1].
Asst. Lec. Mushtaq Sadiq Radhi is with College of Engineering,
Civil Engineering Department, University of Kerbala, Kerbala, Iraq
(phone: 009647801136645; e-mail: mushtaq79811@ yahoo.com).
Prof. Dr. Shakir Ahmed Al-Mishhadani is with the Building and
Construction Engineering Department, University of Technology,
Baghdad, Iraq .
Asst. Prof. Dr. Hasan Hamodi Joni is with the Building and
Construction Engineering Department, University of Technology,
Baghdad, Iraq .
Locally, the in-situ nondestructive tests like core
compressive strength, ultrasonic pulse velocity, and rebound
hardness hammer are widespread in the assessing the concrete
in the existing buildings, particularly in the troubleshooting of
problems with new construction, if the strength of standard
compression test specimens found to be below the specified 28
days value, frequently, these nondestructive tests are
undertaken at later ages exceeding the 28-days.
One of the well-known in situ nondestructive tests to
estimate compressive strength of the concrete is core test. Core
test is done by extracting cylindrical usually from existing
concrete structure using a rotary cutting tool with diamond
bits. Core testing is the most direct method to determine the
compressive strength of concrete in a structure. Generally,
cores are obtained either to assess whether suspect concrete in
a new structure complies with strength-based acceptance
criteria or to evaluate the structural capacity of an existing
structure based on the actual in-place concrete strength. In
either case, the process of obtaining core specimens and
interpreting the strength test results is often confounded by
various factors that affect either the in-place strength of the
concrete or the measured strength of the test specimen [2]. The
measured core compressive strength is mainly affected by the
following factors [2-5] Moisture condition, Length-diameter
ratio of core, Diameter of core, Direction of drilling, Effect of
Age, Microcracking, and Reinforcement.
In-situ strength is the strength of the concrete as it exists
in the element at the time of sampling and is the end result of
the quality materials characteristics, construction techniques,
workmanship and exposure. Tapkin et al. [6] were made a
study to examine a method to evaluate concrete core strengths
by using artificial neural networks. Eight different concrete
mixtures were prepared by using two different aggregates of
four different maximum sizes. Beam specimens were cast by
prepared mixtures. Cores with different diameters and length-
to-diameter ratios were drilled from beam specimens.
Compressive strength tests were carried out on core specimens
at different ages. The parameters influencing the strength of
cores were used as input for neural network architecture and
the core strengths were evaluated. The core strength test results
were analyzed by means of multi-layer feed forward back
propagation neural network model. The authors concluded that
the average compressive strengths of concrete cores
determined by the artificial neural networks and by destructive
tests during the investigation were very similar to each other. It
Effect of Age on Concrete Core Strength
Results
Asst. Lec. Mushtaq Sadiq Radhi, Prof. Dr. Shakir Ahmed Al-Mishhadani , and Asst. Prof. Dr. Hasan
Hamodi Joni.
T
Effect of Age on Concrete Core Strength Results
14
indicates that the estimations were representative of the real
results.
ACI 318 [7] and ACI 301[8] stated that concrete shall
be considered adequate (as specified) when the average of
three cores is equal to at least 85% of the specified strength of
concrete and no single core is less than 75% of the specified
strength of concrete .No allowance for age or curing history is
mentioned. The value of 85% in ACI 318 is not very different
from the value used in BS 6089[9], which require the
“estimated in-situ cube strength” to be 83% of the specified
characteristic strength of concrete, to which the partial safety
factor for design strength is applied.
Bartlett and MacGregor [10] submitted a relationship between
the average in situ strength and the specified strength is
determined from the analysis of result test data. The average in
situ strength at 28 days, as corrected for the effect of the core
moisture condition and damage sustained during drilling,
approximately equals the average cylinder strength. The
average cylinder strength is usually greater than the specified
strength, because the concrete producer desires assurance that
the product will meet specifications. The relationship obtained
by regression analysis between the average in situ strength at
28 days and the specified strength is:
where , is an indicator variable equal to 0 for elements less
than 450 mm high or 1 otherwise
This study includes an attempt to find the influence of
the long-term concrete age on concrete properties by using the
well known locally in-situ nondestructive test method core
compressive strength. Many researches have been conducted
to assess or expect the mechanical properties of normal
concrete using nondestructive tests. Though, locally little work
has been published about the influence of the long-term
concrete age and strength level on the compressive strength
development for the standard concrete core.
II. EXPERIMENTAL STUDY
In order to achieve the aim of this study, three concrete
mixes with 28 days specified strength ranging from 25 to 55
MPa were produced by using ordinary Portland cement
manufactured by united cement company commercially known
(Tasluja-Bazian) , AL-Ekhaider natural sand of 4.75mm
maximum size with grading limited zone 2, Crushed gravel of
20 mm maximum size from Al-Nebai quarry and Tap water
and superplasticizer admixture. Three mixes are prepared
according to Building Research Establishment method, the
mixes were designed to have a 28 days potential compressive
strength of 25, 40, and 55 MPa. According to the design and
trial mixes the cement content was 350, 400, and 450 kg / m3,
while the water-cement ratio 0.5, 0.45, and 0.35 respectively
.The details of the mixes used throughout the laboratory work
are given in Table 1. By using these mixtures, 100 mm cube
specimens were cast and water cured until being tested. An
average of three specimens was considered for each age. Also
1000×1000×250 mm unreinforced concrete slabs were cast
and moist cured for 7 days. 100 mm diameter cylindrical core
specimens were extracted from the slabs at different ages,
prepared, and tested at the same age of the cubes (The general
procedures for core cutting and testing are demonstrated on the
following headlines, Drilling, Trimming, Core Capping, and
Compression testing..
TABLE (1) DETAILS OF THE MIXES USED THROUGH
THIS INVESTIGATION.
III. RESULTS AND DISCUSSION
The core compressive strengths developments for all mixes
moist cured for 7 days schemed in Figure (1). Depending on
the test results, it can be seen that the core compressive
strength of all mixes increases gradually with the progress of
concrete age due to the continuity of hydration process in
concrete in the outdoor circumstances. The relative humidity
in the climate, the rain falls, and the lubricating water during
drilling cores supplied the concrete core panels with additional
water for curing. The form of the core compressive strength
increasing is comparable to the compressive strength growing.
Figure (2) shows the normalized values of core compressive
strength, all the values were divided by the corresponding 28
days values to make the 28 days values equal to unity. The
percent of increase at age of 90 days are 37%, 22%, and 14%
for C25, C40, and C55 respectively. The percent of increase at
age of 180 days are 46%, 31%, and 18% for C25, C40, and
C55 respectively. It is clear that mixes with higher water-
cement ratio and lower cement content higher in strength
development above the 28 days core compressive strength.
Figure (3) shows the proportion of core compressive
strength at different ages to the corresponding 28 days cube
compressive strength. The proportions for C25 are 71.38 %,
88.41 %, 98.54 %, 101.15%, 103.46 %, and 107.78% at core
ages of 28, 60, 90, 120, 180 and 270 days respectively.
While the proportions for C40 are 68.64 %, 79.69 %, 83.8 %,
87.92%, 89.72%, and 92.54% at core ages of 28, 60, 90,
120, 180 and 270 days respectively . Also the proportions for
C55 are 58.76 %, 64.07 %, 66.73 %, 67.61%, 69.38 %, and
73.81% at core ages of 28, 60, 90, 120, 180 and 270 days
Mix
Cement
Content
(kg / m3)
Fine
Aggregate
(kg / m3)
Coarse
Aggregate
(kg / m3)
Water
Content
(kg / m3)
Water-
Cement
Ratio
C25
350
800
1040
192.5
0.55
C40
400
665
1135
180
0.45
C55
450
610
1140
157.5
0.35
The 2nd International Conference of Buildings, Construction and Environmental Engineering (BCEE2-2015)
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respectively . Although the strength of core increases with age,
but the rate of increase is very low when compared with 28
days compressive strength. This can be explained by the
combined effect of various factors on core strength in
unpredictable degrees, they are: -
(a) The difference in compressive strength between the cube
and the cylinder, according to BS 1881 part 120 [11] the
compressive strength of cylinder (standard core) is about 0.8
of the compressive strength of a cube. However there is no
simple relation between the strengths of the specimens of the
two shapes. Shetty[12] has confirmed that the ratio of
compressive strength cylinder to cube specimens increases
significantly with the increase of their strengths and become
nearly 1 in high strength concrete .
(b) Destruction of drilling operation, where the drilling
operation weakens the bonds between the aggregate and the
surrounding hardened cement paste. Also in high strength
concrete, the bonds between cement paste and aggregate are
higher and more cohesive. This results more resistance during
the coring operation and causes greater shearing between the
coring bit and the concrete surface, which would cause greater
damage to higher strength concrete as compared to low
strength concrete.
(c) The difference in exposure conditions and curing
between the core and the standard specimens. Panels of core
were moist cured for 7 days, while standard specimens were
moist cured up to the age of test. Petersons, cited by Neville
[1], stated that the ratio of core strength to standard cylinder
strength is always less than 1, and decreases with an increase
in the concrete strength level. Approximate values of this ratio
are: just under 1 when the cylinder compressive strength is 20
MPa and 0.7 when it is 60 MPa. Neville [1] identified a
difficulty in separating out the effect of drilling operation and
curing history effect. The difficulty is exacerbated by the fact
that the exact curing history of a structure is, usually difficult
to determine so that the effect of curing on the strength of
cores is uncertain, and he suggested a reduction of drilling
between 5-7%.
(d) The consequence of soaking the core in water causes
the reduction in compressive strength. Bartlett and MacGregor
[13] found that, on average, the strength of cores dried in the
air for 7 days is 14% higher than the strength of cores soaked
in water for, at least, 40 hours. Another research carried out by
Bartlett and MacGregor [14]. They observed a more severe
strength loss in 50 mm diameter cores compared with 100 mm
diameter cores from the same element. In addition, they found
that extending the soaking period beyond 40 hours duration
could cause further reduction of the core strength.
It is agreed that as the concrete compressive strength
increases, the core compressive strength is also increased but
at a decreasing rate. A number of factors affect the relation
between the two strengths such as diameter, length/diameter
ratio and moisture condition of the core specimen, the
direction of drilling, presence of reinforcement steel bars in the
specimen and even the strength level of the concrete.
FIGURE (1) THE DEVELOPMENT OF THE CONCRETE CORE
STRENGTH WITH AGE.
FIGURE (2) VARIATION OF NORMALIZED CONCRETE
CORE STRENGTH WITH AGE FOR ALL MIXES.
Effect of Age on Concrete Core Strength Results
16
FIGURE(3) CORE STRENGTH TO 28 DAYS COMPRESSIVE
STRENGTH RATIO FOR ALL MIXES AT DIFFERENT AGES.
IV. CONCLUSION
Based on the results of this investigation, the conclusions that
can be drawn are given below:
1. The strength of all concrete mixes, tested by destructive or
nondestructive methods, increases with age in diminishing
rate.
2. The water-cement ratio of concrete mix is a significant
factor that affects on the rate of development of strength with
age. Concrete mixes with low water-cement ratio achieved the
long-term values more rapidly than concrete mixes with high
water-cement ratio. The higher water-cement ratio and the
higher in the development above the normalized 28-day
values. The development of compressive strength for mix with
w/c 0.35 (C55), and w/c 0.55(C25) at age 120 days is 95 %,
and 90 % respectively, expressed as percentage of the 270
days corresponding values. The development of compressive
strength for the same above mixes at age 120 days is 17 %,
and 87 % respectively, when expressed as percentage of the 28
days corresponding values.
3. The core compressive strength increases as the age of
concrete increase, but the core strength is somewhat higher
than 28-day cube compressive strength even up to the age 270
days in moderate concrete (C25), while the core compressive
strength remains lower than 28-day cube compressive strength
in the higher strength level (C40 and C55) even up to the age
270 days. This means that the core strength is affected by the
other factors (strength level, curing conditions, and moisture
testing conditions) more than age of core. So that no age
adjustment should be used in the assessment of the strength of
core especially in the lack of definite moist curing.
REFERENCES
[1] Neville, A. M., “Properties of Concrete,” Fourth and Final
Edition Standards updated to 2002, Pearson, 2005,844 p, printed
in Malaysia.
[2] Neville, A. M., “In My Judgment, Core Tests: Easy to Perform,
Not Easy to Interpret,” Concrete International, Vol.23, No.11,
November 2001, pp.59-68.
[3] Bungey, J. H., Millard, S. G., Grantham, M. G., “Testing of
Concrete in Structures,” Taylor and Francis, 4th edition, 2006,
pp.35-139.
[4] ACI 214.4R-03, “Guide for Obtaining Cores and Interpreting
Compressive Strength Results,” Reported by ACI Committee 214,
ACI Manual of Concrete Practice, American Concrete Institute,
2004,16 p.
[5] ASTM C42-03, “Standard Test Method for Obtaining and Testing
Drilled Cores and Sawed Beams of Concrete,”Annual Book of
ASTM Standards American Society for Testing and Materials,
Vol.4.2, 2006, 5 p.
[6] Tapkin, S., Ariöz,O., Tuncan,M., Tuncan,A. and Ramyar,K.,
“Use of Neural Networks for The Evaluation of Concrete Core
Strengths,” 4th Faculty of Architecture and Engineering
International Symposium, European University of Lefke, Turkey,
2006 , pp.195-202.
[7] ACI 318M-08, “Building Code Requirements for Structural
Concrete,” Reported by ACI Committee 318, ACI Manual of
Concrete Practice, American Concrete Institute, 2008.
[8] ACI 301M-99, “Specifications for Structural Concrete,”
Reported by ACI Committee 301, ACI Manual of Concrete
Practice, American Concrete Institute, 2004, pp.9.
[9] BS1881: part 6089:1981, “Guide to Assessment of Concrete
Strength in Existing Structures,” British Standards Institution,
1999, 15 p.
[10] Bartlett, F. M., and MacGregor, J. G., “Equivalent Specified
Concrete Strength from Core Test Data,” Concrete International,
Vol. 17, No. 3, Mar.1995, pp. 52-58.
[11] BS1881:part120:1983, “Method for Determination of The
Compressive Strength of Concrete Cores,” British Standards
Institution, 1983, 5p.
[12] Shetty, M.S., “Concrete Technology, Theory and Practice,” 6th
multicolour edition, 2009, pp.428.
[13] Bartlett, F. M., and MacGregor, J. G., “Effect of Moisture
Condition on Concrete Core Strengths,” ACI Materials Journal,
Vol. 91, No. 3, May-June 1994, pp. 227-236.
[14] Bartlett, F. M., and MacGregor, J. G., “Cores from High
Performance Concrete Beams,” ACI Materials Journal, Vol. 91,
No. 6, Nov.-Dec.1994, pp. 567-576.
... As for the mix prepared with 100% NFA, the amount of strength gain during the same period was about 16%. The results are consistent with the guidelines of Portland Cement Association [65] and findings of Radhi et al. [72] that concrete mixes prepared with Portland cement Type I and a w/ c ratio close to the value considered in this study, can exhibit about 15-20% increment in compressive strength at 90 days in comparison to the corresponding value at 28 days. Furthermore, the enhancement in compressive strength was more obvious in fiber-reinforced mixes where the 90-day compressive strength was up to 25% higher than the 28-day compressive strength. ...
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Core Tests: Easy to Perform, Not Easy to Interpret
  • A M Neville
Neville, A. M., " In My Judgment, Core Tests: Easy to Perform, Not Easy to Interpret, " Concrete International, Vol.23, No.11, November 2001, pp.59-68.
Concrete Technology, Theory and Practice
  • M S Shetty
Shetty, M.S., "Concrete Technology, Theory and Practice," 6 th multicolour edition, 2009, pp.428.
Properties of Concrete
  • A M Neville
Neville, A. M., "Properties of Concrete," Fourth and Final Edition Standards updated to 2002, Pearson, 2005,844 p, printed in Malaysia.