International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 3, March - 2013
Experimental Study Of Core Diameter Varing H/D Ratio On Concrete
Core Strength
Pawan Kumar Aggarwal1, Sanjay Sharma2,Sanjeev Naval3,
1
(Post Graduate Student DAVIET Jalandhar, India)
2
( Professor and Head ,Nat io na l I n st it u te o f T ech n ic a l T rai ni n g &
Re se ar c h, Chandigarh)
3
(Associate Professor and Head ,Department of Civil Engineering,DAVIET Jalandhar, India)
100mm. However is not always possible to obtain
ABSTRACT cores of this diameter, with required minimum H/D
In this paper an effort has been made to find the ratio of 1.0.This may be due to limitation of
effect of H/D ratio on the strength characteristics of dimensions of the member or critical reinforcement
the core. Cubes of 150mm x 150mm x 150mm were location. To overcome this problem CSTR 11,1976[7]
casted and cured for 28 days, desired core samples has allowed the use of 50mm and 75mm diameter
having diameter 50mm and 75 mm have been cores.BS:1881, part (120) 1983[3], and ASTMC 42-
prepared from these cubes having different H/D 90,1994[2], allows a minimum core diameter of
ratios of 1, 1.25, 1.5, 1.75 and 2 respectively. The 102mm, provided that diameter of core is at least
core samples were tested on compression testing 3times greater than maximum aggregate size of
machine. It was found that with the increase in the concrete mix. As per Bartlett F.M. et.al 1994[4], the
H/D ratio and decrease in the diameter of the core, diameter of core affects the compressive strength of
compressive strength of the core increase. It has also cores. Arioz , O. et.al, 2006[1], concluded through his
been observed that the strength of core samples was research that though , smaller diameter cores should
less than those of the standard cubes. give higher average strength than larger diameter
cores for the same H/D ratio, due to size effect, but is
1. INTRODUCTION not always true , since the concrete is susceptible to
micro cracking during drilling operation.
The compressive strength of these specimens is
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From the above discussion it is evident that core
generally tested at the age of 7 days and 28 days. strength is dependent of H/D ratio and diameter of
However 28 days strength is generally taken as the core specimen.IS:1199-1959[15] recommends the
compressive strength of concrete. H/D ratio of core as 2, and IS: 516-1959[14], gives the
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But this 28 days strength of standard specimen is not correction factor to be applied , if the H/D ratio is
always the actual strength of concrete because: less than 2.0.But this relation does not give particular
values of different core diameter. In this paper the
i) It depends upon curing and compaction effect of age and H/D ratio ranging between 1.0 and
of concrete 2.0 on cores of diameter of 50mm and 75mm has
ii) It is not possible to prepare and test been analyzed. To find the in situ strength of
standard specimens at a later stage, if concrete, generally core strength test is used. In this
the current strength of structure is to be method cores are drilled from the structure, and are
determined to check whether or not tested for compressive strength. Compressive
strength and durability of concrete is strength of the core is then converted into equivalent
adequate. cube strength. Though it seems to be a simple
procedure, it is not easy to interpret the results. The
To overcome this problem one of the method is core
following are some of the factors which affect the
testing of suspected structural elements. In this
accuracy of results:
method cores are drilled from the structure as per IS:
[15]
1199-1959 and tested for compressive strength. As 1. Height to Diameter Ratio (H/D) ratio.
per BS: 1881: part 4, 1970(2)[5], the need for 2. Moisture condition at the time of testing.
compressive strength of cores to yield an estimate of 3. Age of concrete.
strength of suspected concrete is well established. 4. Strength level of concrete.
This code also contains recommendation for core 5. Disturbance during drilling operation of
testing of concrete. Also more detailed and cores.
comprehensive recommendations for core testing is
provided by Concrete society technical report Also the condition of placing, compacting and curing
no.11 of 1976[7].This report gives detailed procedure may not be the same at site which were used for
for testing and interpretation of results of core testing. preparing standard specimens. J.H. Nevile
Both BS:1881 and the report of concrete society et.al.(1956) [17], suggested that measured core
recommends the diameter of cores as 150mm and strength increases with the decrease of core size. The
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 3, March - 2013
smaller size cores give more variable results. shown that concrete strength is a further factor which
Mcintyre.et. al(1990) [20]through their studies may influence the behavior of a core, and it is
concluded that with the decrease in diameter, the possible that this also may affect the relative behavior
homogeneity of the material in test specimen of small and large cores. The complexity of these
diminishes, thereby affecting the internal failure problems contributed to the decision to confine the
mechanisms of the specimen. investigation to a single small core size
IndelicatoF.et.al(1993) [9], showed that with the .M.Yaqub.et.al (2006) [19] in their research
decrease in diameter, the assessment of correct established the relation between core compressive
results become hard. The core strength obtained by strength and cube compressive strength of hardened
converting these to standard cube or cylinder may not concrete in existing structures, and concluded that
give true value. Neville.et.al (1995) [21] , showed that core strength of 75mm cores is 69% of compressive
as the core diameter decreases, the volume the strength of 150mmX150mmX150mm cubes. I. M.
specimen also decreases significantly for a given H/D Nikbin et.al (2009) [16] revealed that the age of the
ratio. Hence the strength of specimen decreases with concrete was found to be an effective factor in the
the increase in its size. BungeyJ.H. Et.al (1996) [6] , interpretation of the core strength results. Test results
showed that the properties of insitu concrete will vary also revealed that the H/D ratio of the specimen is
with in a member , due to difference in compaction more effective for small diameter cores. The
and curing as well as non uniformity of material. The coefficient of variation of strength values was noticed
effects of drilling are more pronounced for small to be somewhat higher for 50 mm diameter cores.
diameter cores, as suggested by Barlet F.M. et
al.(1997) [4]They showed that is because damaged 2. Materials and Methods
parts of the core have a constant thickness In this work, Experimental study of core diameter
independent of core diameter. They used 102mm varying H/D ratio on concrete core strength is
diameter and 204mm height of the cores as standard investigated by:
specimen and concluded that a factor of 0.98 and
1.06 has to be applied for cores of 152mm and 51mm Using a same type of aggregate and a same
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respectively. IndelicatoF.et.al(1997) [10]through their type of cement for standard and core
tests on cores , showed that choice of smaller specimens.
diameter of cores is motivated by the need to reduce Curing the standard cubes specimens
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costs and minimize the damage to the structure, and and the testing
by possibility of drilling out samples more easily by core specimens in the same conditions.
means of smaller tools for these reasons concrete Taking cores from the cubes.
society has recommended the ise of 50mm and 75 Using regression analysis to define the
mm diameter cores.Indelicato F.et.al (1998 and correction factor curves.
1999) [11,12], concluded that potential strength of
concrete is related to quality of material used and is Testing equipment :The equipment used are those
an estimate of standard test specimen. However in available in the structural laboratory of NITTR,
situ strength of the concrete as it exists in CHANDIGARH.
construction is the end result of quality of concrete Core drilling machine: The Cores of 75mm and
used in the construction. Erdogan T.Y. et. al(2003) 50mm were drilled out this machine.
[8]
, indicate that variability of strength of small Measurement of compressive strength :For cubes
diameter cores is greater than that of large diameter and core drilled are tested at various rates of
cores , even when strength are very close to each loading with a compression machine with a
other . JeeNamyong .et.al(2004) [18] in their study maximum capacity of 3000 KN.
presented a regression equation for predicting Materials: Locally available Ordinary Portland
compressive strength of in-situ concrete based upon Cement (OPC) and sand was used in this study
mix proportions and concluded that , water cement and locally available crushed coarse aggregate
ratio, cement contents, cement aggregate ratio are the with a nominal maximum aggregate size of 20.0
main influential factors for reliability prediction of mm was used.
compressive strength. Tuncanet.al(2006) [22] has
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 3, March - 2013
diamond- tipped core-cutter and trimmed to give
over- all H/D ratios between 1.0 and 2.0. The H/D
ratios of capped core specimens were 2.0, 1.5, 1.25
and 1.0. Fig.1 shows core drilling operation
The compressive strength of the standard specimens
and the cores was determined by a Compression
testing machine. The compressive strength test results
were taken as the average of four specimens. A total
of 4 standard and 40 core specimens were tested in
this investigation.
.
. RESULTS AND DISCUSSION
In this study for each mix the average values of
measured core strength, were compared for each
different value of H/D and expressed in terms of a
core with H/D = 2.0. Based on the results obtained
for H/D ratios an over-at least-squares regression was
performed.
Table 1: Correction factor for 75mm diameter
Figure 1. Core drilling machine core:
Strength(Mpa)
Compressive
Correction
H/D Ratio
Diameter
Factor
(mm)
Core
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No.
1. 50 1 24.16
0.85
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2. 50 1.25 25.93
0.91
3. 50 1.5 26.66
0.93
4. 50 1.75 27.6
0.97
5. 50 2 28.54
1.00
Table 2: Correction factor for 50mm diameter
core:
Strength(Mpa)
Compressive
igure 2. Compression Testing Machine
Correction
H/D Ratio
Diameter
Factor
(mm)
Core
No.
Concrete Mix Proportions
Nominal mix concrete of M25 grade was used.
Specimens 1. 75 1 24.08
0.86
Testing procedureThe compressive strength of the 2. 75 1.25 24.94
standard specimens and the cores was determined by 0.89
Compression testing machine. The compressive 3. 75 1.5 25.82
0.92
strength test results were taken as the average of four
4. 75 1.75 27.13
specimens. A total of 4 standard and 40 core 0.96
specimens were tested in this investigation. 5. 75 2 28.15
1.00
In all 44 numbers of 150X150X150 mm (were casted The following graphs are obtained between the
and cured in the laboratory condition.50 and 75 mm Correction factors and different Height/Diameter ratio
diameter cores were cut from the cubes by using a , for 50mm and 75mm diameter cores
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 3, March - 2013
The following graphs are obtained between the
Correction factors and different Height/Diameter k = 0.2127ln (λ) + 0.8512 (1)
Ratio, for 50mm and 75mm Diameter cores. k= 0.2094ln (λ) + 0.8457 (2)
Where k is the correction factor of core strength for a
core with H/D = λ
1.05
50mm Diameter 4..SUMMARY AND CONCLUSION
1.00 For small cores of 50 and 75 mm diameters, the
following conclusion may be drawn:
k = 0.2127ln(λ) + 0.8512 1 The compressive strength of cores increased with
Correction Factor
0.95 R² = 0.9896
50mm the decrease in H/D ratio of the core.
Diameter 2 The effect of H/D ratio was more pronounced for
0.90 50 mm diameter cores.
3 The strength of cubes is generally higher than that
Log. of cores drilled of the same concrete mix.
0.85 (50mm
Diameter) 5. REFERENCES
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Fig. 5.1: Correction factor for H/D ratio of core No. 2, March 2006, pp. 117-122.
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testing drilled cores and sawed beams of concrete.
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[3]BS 1881, Part 120 (1983).Method for
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1.00 [4] Bartlett, F. M., And Macgregor, J. G., “Effect of
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 2 Issue 3, March - 2013
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