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Interpretation of in-situ concrete strength test results
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NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
Interpretation of in-situ concrete strength test results
Marios SOUTSOS1, John BUNGEY1, Adrian LONG2 and Geoff TICKELL1
1
Department of Engineering, The University of Liverpool, UK
School of Planning, Architecture and Civil Engineering, Queen’s University Belfast,
UK
2
Abstract
A full scale insitu reinforced concrete building frame built in a laboratory provided an
opportunity to assess the use of the in-situ pull-out test on a systematic basis during
construction. Interpretation of strength correlations should take into account the uncertainties
that exist and therefore should be made by the use of standard statistical procedures. Two
procedures recently adopted by BS EN 13791:2007 “Assessment of in-situ compressive
strength in structures and precast concrete components” for estimating the in-situ
characteristic strength of a test region from in-situ test results have been examined. Both
procedures account for the uncertainty of the strength correlation by requiring either the lower
95% confidence limit curve or a curve shifted downwards by a margin to be used. Overly
conservative estimates of the in-situ characteristic strength are found to result.
Résumé
Le test d’arrachement fut utilisé systématiquement pendant la construction d’un bâtiment
poteaux/poutres en béton armé in-situ de taille réelle. L’interprétation des corrélations de
force doit prendre en compte les incertitudes qui existent et, de ce fait, doit être faite au
travers de l’usage de procédures statistiques standardisées. Deux procédures récemment
adoptées par le BS EN 13791 :2007, pour estimer la force caractéristique in-situ, ont été
examinées mais ont donné des estimations sécuritaires.
Keywords
In-situ non-destructive tests, pullout (Lok) tests, strength correlation, statistical analysis.
1
Introduction
An experimental full scale seven-storey in-situ advanced reinforced concrete building
frame was constructed in a large airship hangar at Cardington in the UK, operated as a
laboratory by Building Research Establishment Ltd (BRE). This encompassed a range of
different mixes, including high strength concretes, and advanced construction techniques. It
provided a focus for a number of construction-phase research investigations involving several
Universities. These included the project reported in this paper, which was concerned with the
capabilities and practicality of use on site of the Lok test, the Danish version of the pull-out
test which conforms to ASTM C900 [1]. BS EN 13791:2007 [2] has been written mainly for
assessment of in-situ compressive strength of existing structures. Nonetheless, the
methodology described was adapted as much as possible so that it could be used for
estimating the in-situ characteristic strength during construction. The requirement of at least
fifteen in-situ tests at a particular location is unrealistic to expect if this is to be carried out
during construction. The analysis of only a limited number of in-situ tests is presented herein
but this represents the situation commonly met during construction.
NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
2
Experimental Programme
It was important to balance the ‘research’ requirements with the practical and commercial
requirements of the contractor to complete the work with a minimum of delay and disruption.
One consequence was that limitations were placed upon the number of tests performed.
Pull-out testing, involving preplanned inserts cast in the concrete, is particularly suitable
for direct in-situ measurements of early age strength utilising cut-out panels in shutters where
appropriate. One key feature is the good sensitivity of correlations to compressive strength
and the relative insensitivity to mix variables such as aggregate type.
Six different mixes were covered in this study which could be grouped into two strength
classes with minimum 28-day characteristic cube strengths of 37 and 85 MPa. The principal
variables for the concretes covered by this study were: concrete grade (C37N-10 and C37N11 with different cement contents and C85), chemical admixtures (plasticiser and
superplasticiser) and aggregate type (gravel and limestone).
Tests were performed at different levels of the structure, on columns at different heights
(top, middle and bottom), and on slabs, both adjacent to columns and in mid bay (top and
bottom). In-situ tests using the Lok-test, were undertaken at 1 day (or as soon as practicable),
3 days, 7 days and 28 days. Results for corresponding air cured 150mm cubes were also made
available which permitted the development of Lok-test strength correlations. The detailed
results of this comprehensive study can be found elsewhere [3].
The establishment of good correlations between a measured property and the strength is a
key to successful in-situ testing, and for normal practice these should ideally be prepared in
advance of the construction starting date. In this case, however, bearing in mind the number
of mixes involved, the short time-scale for preparatory work, the BRE intention to set up a
laboratory on site, and the need to avoid duplication of effort, correlation specimens were cast
on-site during concreting of the structure. In each case these comprised four 200mm timber
moulded cubes with four Lok-inserts attached to the vertical sides together with cubes cast
using standard 150mm steel moulds which were subsequently tested by a commercial test
house for compressive strength. All the specimens were air-cured adjacent to the building.
The strength correlations were therefore determined using 150mm cubes rather than cores as
is the requirement of BS EN 13791:2007 [2].
3
Results and Discussion
To make an in place strength estimation it is necessary to have a known relationship
between the result of the in-place test and the strength of the concrete for the particular mix
concerned. The usual practice is to treat the average values of the replicate air-cured cube
compressive strength and in-place test results at each strength level as one data pair. These are
plotted using the in-place test value as the independent value (or X variable) and the
compressive strength as the dependent value (or Y variable). Regression analysis is performed
to obtain the best-fit estimate of the strength relationship, see Figure 1. The envelope of the
best fit lines, allowing for possible errors in intercept and slope, determined using the least
squares method, indicated that the general correlation for all mixes would be encompassed by
these [4]. The possible errors in intercept and slope values is affected by the number of sets of
data which will also affect the confidence limits of any strength predictions made with these
correlations. In order to get a wide range of values, all the data points from the different mixes
were grouped together. The equation of the combined strength correlation, based on air-cured
cube compressive strengths, was found to be:
Measured Cube Compressive Strength (MPa)
NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
110
Correlation for
ALL mixes
(Linear Regression)
C85MS
C85MK
C37N-10
C37N-11
C37F
C37P
100
90
80
Manufacturer's
Recommended
Correlation
70
60
C37N-10
C85MS
50
40
C37F
30
C37N-11
C85MK
20
C37P
10
MS is for microsilica
MK is for metakaolin
F is for superplasiticizer
P is for plasticizer
0
0
10
20
30
40
50
60
70
Measured Lok Force (kN)
Figure 1: Lok test strength correlations [3]
f c = −4.71 + 1.59 Fu
(1)
where fc is the mean cube compressive strength (N/mm2) and Fu is the pull-out force (kN).
This equation was used for all in-situ strength predictions and this was justified since BS EN
13791:2007 [2] allows “well-established relationships” despite recommending the linear
relationship:
f F = 1.33(F − 10)
10 ≤ F ≤ 60
(2)
where fF is the in-situ core (N/mm2) and F is the pull-out force (kN).
This is very close to the Manufacturer’s Correlation [5] which has been derived from a
wide range of concrete mixes made with gravel, limestone and granite aggregates. The
wisdom of assuming that all the data obtained from the different mixes belong to one
population seems to be justified by the observed similarity to the Manufacturer’s
recommended correlation.
BS EN 13791:2007 [2] requires “the standard error of estimate” to be computed and “the
confidence limits for the best fit line” to be determined. The 95% confidence limit, since none
other was specified, has been used and this is shown in Figure 2. Grouping all the mixes
together resulted in twenty-two data sets and this undoubtedly had the effect of narrowing the
95% confidence limit interval. The mean in-situ compressive strength of n test results, fm(n),is,
estimated using the lower 95% confidence limit curve was then used to estimate the in-situ
characteristic strength of the test region, fck,is, from the following equation:
(3)
f ck ,is = f m (n ),is - k 1s
where s is the standard deviation of the test results or 3.0 N/mm2, whichever is the highest
value, and k1 is a coefficient dependent on the number of in-situ tests. The value of k1 is given
as 1.48 in BS EN 13791:2007 [2] since it assumes/requires fifteen test locations. Values of k1
given for less than fifteen tests only go down to nine tests and these are plotted in Figure 3.
These appear to be slightly different from values published for one-sided tolerance factors for
ten percentile strengths at 75% confidence. Nonetheless, since the number of in-situ tests was
either 3 or 4, values of k1 had to be obtained from tables given in Natrella [6]. These k1 values
are also plotted in Figure 3 and they were 2.501 and 2.134 for 3 and 4 test locations
NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
110
Cube Compressive Strength (N/mm2)
100
Correlation for
ALL mixes
(Linear Regression)
90
80
95%
Confidence
Limit
70
60
50
85% of
characteristic
strength of
standard specimens
40
30
20
Estimated In-situ
Characteristic Strength
10
0
0
10
20
30
40
50
60
70
Measured LOK Force (kN)
Figure 2: Characteristic strength estimates of in-situ concrete.
respectively.
The values used for Figure 1 and 2 to obtain the strength correlation, are the mean
compressive strengths of cubes taken from the same batch of concrete. A margin to allow for
production variability was taken as: M = 1.64 x8 = 13.12 N/mm 2 based on 5% defectives and
an estimated standard deviation of 8 N/mm2 [7]. To obtain the characteristic compressive
strength from the target or in-situ mean strength, the correlation in Figure 1 needs to be
shifted downwards by: f c = f m − M = 13.12 N/mm 2 . BS EN 13791:2007 [2] recognizes that
the in-situ characteristic strength may be less than that measured on standard test specimens
taken from the same batch of concrete, and allows for a ratio of 85% which is part of the
safety factor, γc, in EN 1992-1-1:2004 [8]. This is shown in Figure 2 as a long dashed line
3.5
95% Confidence Level
3.0
k value
75% Confidence Level
2.5
2.0
1.5
BS EN 13791:2007
9 to 15
4 to 9
1.0
0
5
10
15
20
25
30
Number of tests
Figure 3: One-sided tolerance factor for 10% defective level.
NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
Cube Compressive Strength (N/mm2)
110
100
Correlation for
ALL mixes
(Linear Regression)
90
80
95%
Confidence
Limit
70
60
Correlation
Shifted
by -k1.s
50
85% of
characteristic
strength of
standard specimens
40
30
20
Estimated In-situ
Characteristic Strength
10
0
0
10
20
30
40
50
60
70
Measured LOK Force (kN)
Figure 4: Characteristic strength estimates of in-situ concrete based on a shift of the
strength correlation of –k1s.
after having allowed for the margin, M, of 13.12 N/mm2. The placing of the concrete, despite
being in a real structure, employed strict quality control procedures and strength correlations
were obtained from the same batches that were placed in the structure. The quality of the
concrete was therefore known and the strength correlation for the pull-out tests represents the
average in-situ concrete strength. Converting this into characteristic strength should therefore
have indicated that all the concrete placed met the acceptance criteria. The in-situ
characteristic strength of the test region, fck,is, was estimated from Equation 3 with the use of
the mean in-situ compressive strength of n test results, fm(n),is. The dashed line showing the
85% characteristic strength of standard specimens appears as if it is a regression line through
the estimated in-situ characteristic strengths, but it is worrying that almost half the estimates
do not meet the acceptance criteria. It is not that the in-situ concrete did not have the required
strength. The characteristic strength estimates are significantly lower than the in-situ average
strength estimates and this is a consequence of using only 3 or 4 test locations for the Lok
tests. The use of the corresponding k1 values for such small number of test locations requires
k1 values to be relatively high. A larger number of test locations would reduce the k1 values
and assuming that the standard deviation, s, value did not increase then the predicted in-situ
characteristic strengths would not only have less scatter but they would all shift closer to the
strength correlation. BS EN 13791:2007 [2] accounts for the uncertainty of the strength
correlation by requiring the lower 95% confidence limit curve to be used to estimate the insitu characteristic strength of the test region, fck,is. Another way of allowing for this
uncertainty is by shifting downwards the strength correlation by k1s and this is shown in
Figure 4. The k1s in this case refers to the correlation specimens and therefore k1 can be taken
as 1.48 since twenty two data sets were used. The standard deviation, s, was calculated to be
3.42 N/mm2. The similarity of this line to the lower 95% confidence limit is evident. The
subsequent analysis of converting in-situ mean strengths to in-situ characteristics strengths
again requires use of Equation 3 but in this case the k1 and s refer to the tests carried out insitu. The values of in-situ characteristic strength predicted with this alternative procedure are
not dissimilar from those estimated from the lower 95% confidence limit. In estimating the insitu characteristic strength, both procedures assume that the variability of the in-place
compressive strength is the same as the variability of the in-place test results. ACI Committee
NDTCE’09, Non-Destructive Testing in Civil Engineering
Nantes, France, June 30th – July 3rd, 2009
228 [9] acknowledges that this assumption will make the estimates of the in-situ
characteristic/tenth-percentile strength overly conservative. It has been acknowledged that the
within-test variability of in-place test results is generally greater than compressive test results.
4
Conclusions
Interpretation of in-situ non-destructive test results is not simply their averaging and then
computing the equivalent compressive strength by means of an established strength
relationship. It is necessary to design a concrete mix to have a mean strength greater than the
specified characteristic strength, by an amount termed the margin, in order to account for the
variability of concrete in production. The in-situ characteristic strength of the test region can
be estimated from in-situ test results by the use of standard statistical procedures that take into
consideration the quality control, number of tests and the required confidence level in the
predictions. The use of as few as three or four test results, as is the norm during construction,
results in overly conservative estimates of the in-situ characteristic/tenth percentile strength.
This is exacerbated by the fact that the procedure assumes that the variability of the in-place
compressive strength is the same as the variability of the in-place test results when it has been
acknowledged that the latter has a within-test variability generally greater than the former.
Acknowledgements
The authors gratefully acknowledge the helpful collaboration of the Building Research
Establishment, Byrne Bros., and Ready Mixed Concrete Ltd. The successful onsite testing for
this project was carried out by Dr. G. D. Henderson and this is gratefully acknowledged.
References
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