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In-situ Concrete Strength Assessment based on Ultrasonic (UPV), Rebound, Cores and the SONREB Method

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The strength of concrete in a new structure is sometimes called into question. This may be due to cylinders not being taken, poor cylinder production, transport or testing, the actual concrete strength being low or the suspicion that in-situ cylinders are not representative of in-situ concrete. Whatever the reason testing of the in-situ concrete is generally called for. This paper gives a brief outline of the key aspects of strength assessment including a review of concrete supply and testing records, extent of testing required, assessment by cores to AS 1012.14 (1) and AS 3600 (2), ultrasonic (direct and indirect methods) and rebound hammer testing and analysis of in-situ strength using cores and NDT results (including published methods such as EN 12504 (3), EN 13791 (4), BS 6089 (5) and those under consideration by RILEM). If the concrete strength is found to be low, a structural analysis has to be undertaken. The paper includes information of six projects assessed. In two, strengths were low across the whole mine site but in each case only one structure required strengthening. In another, very low strength was identified at an early stage and a risk assessment identified the structure should be replaced. In the others, strengths were found to be adequate.
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In-situ Concrete Strength Assessment based on Ultrasonic (UPV),
Rebound, Cores and the SONREB Method
Frank Papworth1, David Corbett2, Reuben Barnes3, Joseph Wyche4 and Jonathon Dyson5
1 BCRC (WA); 2 Proceq; 3 PCTE; 4 Wyche Consulting; 5 BCRC (NSW).
Synopsis:
The strength of concrete in a new structure is sometimes called into question. This may be due to
cylinders not being taken, poor cylinder production, transport or testing, the actual concrete strength being
low or the suspicion that in-situ cylinders are not representative of in-situ concrete. Whatever the reason
testing of the in-situ concrete is generally called for. This paper gives a brief outline of the key aspects of
strength assessment including a review of concrete supply and testing records, extent of testing required,
assessment by cores to AS 1012.14 (1) and AS 3600 (2), ultrasonic (direct and indirect methods) and
rebound hammer testing and analysis of in-situ strength using cores and NDT results (including published
methods such as EN 12504 (3), EN 13791 (4), BS 6089 (5) and those under consideration by RILEM). If
the concrete strength is found to be low, a structural analysis has to be undertaken. The paper includes
information of six projects assessed. In two, strengths were low across the whole mine site but in each
case only one structure required strengthening. In another, very low strength was identified at an early
stage and a risk assessment identified the structure should be replaced. In the others, strengths were
found to be adequate.
Keywords: Concrete, compressive strength, ultrasonics, UPV, rebound, cores, structural assessment,
strengthening.
1. Introduction
For many years CSTR 11 (7) was the revered document on assessment of concrete compressive strength
from cores. This was updated in 1987 (8). In 2002 CIA Z11 (9) was published and became the Australian
reference for assessment of core strengths as a supporting document for AS 1012.14 (1). AS1012.14
provides performance requirements on sampling, conditioning, and reporting on cores and calls on AS
1012.9 (6) as the method of testing concrete samples for compressive strength. However, in 2004 a
project report on core strengths by the Concrete Society (10) provided updated information on tests from a
range of samples and findings were subsequently incorporated in an updated BS 6089 (5) which is
complimentary to BS EN 13791 (4), the European standard on testing concrete strength in structures.
Consequently CSTR 11 (7,8), and AS1012.14 (1) and CIA Z11 (9) which are at least partly based on them
should be used with caution as some of the analysis methods have been superseded. The Concrete
Society issued Advice Note 43 (11) in 2013 which summarises some of the approaches in the new
European standards. In this paper some of the differences between European Standards and the methods
used in Australia based on CSTR 11 are given.
BS EN 13791 now provides clear guidance on the use of ultrasonic pulse velocity and rebound hammer
testing. These methods enable rapid scanning of the concrete to detect variations in strength and BS EN
13791 provides guidance on how their use can be incorporated into reducing the number of cores
required. In this paper the methods are outlined with specific reference to direct and indirect UPV
measurements and combined use of rebound, UPV and core strengths using the ‘SonReb’ method.
An outline of the strength assessment on seven projects is given in the paper to show the variety of
approaches that may be appropriate. For each project a durability assessment was also undertaken for
each structure but this is not included here as the paper’s focus is strength and structural assessment.
The methods employed for strength assessment range from reliance on 28 day strengths to the use of
cores, UPV and rebound testing. The subsequent analysis of the structures included a full structural
analysis to determine structural reliability and risk assessment if there is a reduced reliability.
2. Methods of Testing
2.1 Cores
To maximise the number of cores meeting AS 1012.14 requirements and to minimise damage to the core
and structure, the engineer should ensure that the coring contractor has adequate experience in using his
equipment to take cores for the specified purpose. On one project in remote Botswana the contractor
arrived with a brand new coring machine, no hold down anchors and 5 people. His first core failed the
requirements of AS 1012.14 because the machine was not properly secured and because of stop start
coring. Being a remote location it took two days to get the team working.
Before coring the reinforcement locations are carefully mapped and marked with due allowance for bar
diameter. The upper reinforcement layer can generally be identified using a covermeter but this may miss
saddles which may be identified using ground penetrating radar.
Assessment of core strength can be based on BS EN 13791 and BS 6089. A comparison of these with
current Australian standards is given in Table 1.
Table 1 : Australian and European Standard Requirements for Assessing In-situ Concrete Strength
Not Yet Harmonised with European
Practice (based on CSTR 11)
State of the art documents on in-situ
strength assessment
AS 1012.14 CIA Z11 BS EN 13791/BS6089
Core diameter 75mm 75min or 2.5 x
agg size
100mm (increase number of cores if
using diameters down to 50mm)
Core length:diameter ratio Close to 2:1 1 to 2 (ideally 1.9
to 2)
1 to 2
Actual Strength Corrections
Correction l:d k1 = 1 for l/d =2 reducing in steps to 0.87
for l/d=1
Kcyl=2.0/(1.5+1/(l/d))
Correction rebar present - k2 by formula Avoid rebar
Correction core axis - k3= 1 perp. to
casting direction
and 1.08 if parallel
Influence noted but UK Annex NA says
no allowance to be made
Core locations limits
Vertical pours - Not from top 20%
up to 300mm
Exclude top 300mm. Core top third
Face of concrete - Not within 50mm
of face
Not within 50mm of face
Number of cores where
suspect batch
- 3 from each
suspect batch
15 with no NDT, 9 with NDT. Move to
reduce the number with NDT
Conditioning Wet 3d or air 7d Dry (7d) as AS
3600
Air 3d but allowance can be made for wet
cure
End preparation AS 1012.9 Sulphur cap or
ground ends
Ground ends recommended
Provision for outliers - No Yes
NDT assessment - - UPV, pullout and rebound accepted as
means of reducing core numbers
2.2 Rebound
The original rebound hammer measured the hardness of a material by the degree of rebound. The
instrument worked by firing a known mass using a standard spring loading to impact on a rod held in
contact with the surface being tested. When the mass hits the rod it rebounds to an extent that depends
on the hardness of the surface in contact with the rod. The original rebound hammer measured the
rebound value R mechanically as a distance that the mass rebounded from the concrete surface.
Extensive testing carried out in the late 1950’s gave correlation curves between rebound value and
concrete compressive strength.
In 2007 an electronic version of the original rebound hammer was introduced. This instrument measures
rebound value (now Q) as the quotient between the velocity of the hammer mass just before and after
impacting the rod. The validity of the new measurement principle has been recognized by the major
standards bodies.
It should be noted that Q-values and R-values are not interchangeable. Correlation curves developed for
the original rebound hammers cannot be used with Q-value and correlation curves developed for the new
hammer cannot be used with the classical hammer. The manufacturer provide a strength relationship
between Q values and compressive strength based on a lower 10th percentile curve. They do not provide
direct relationships between Q values and R values, as testing on various types of concrete have shown
that the relationship is not constant.
When assessing in-situ compressive strength using cores, EN 13791 requires at least 15 cores to be
taken to establish the insitu concrete strength. The number of cores to be taken may be reduced to 9
when used in combination with NDT tests such as rebound hammer or ultrasonic pulse velocity.
In the German national annex to EN 13791 there is also the possibility to assign a compressive strength
class based on rebound hammer testing alone, as in many cases, it is not allowed to take cores. EN
13791 is currently under review and there are proposals for reducing the number of cores further when
used in combination with NDT testing and also to generally accept the method described in the German
national annex.
Testing should be undertaken in accordance with EN 12504-2. A test location should be a minimum 100m
thick, 300x300mm test area, minimum of nine readings, impact points >25mm apart, surface clean and
smooth. Nine such test locations are required for a test region as described in EN 13791. A core should
also be taken at each test location to establish the correlation. The method then uses the core correlation
to shift a base correlation curve upwards. The same method may be used with ultrasonic pulse velocity.
2.3 Ultrasonic Pulse Velocity
UPV can be determined in number of ways direct, indirect and semi-direct. Only the first two are
discussed here. Direct UPV is the most reliable as the velocity is measured across the entire element and
gives an average velocity for a large thickness of concrete. Direct UPV can also be measured on cores to
give a direct correlation between UPV and core strength.
y = 0.2126x - 2.59
R² = 0.9995
y = 0.2308x - 7.67
R² = 0.9967
30
35
40
45
50
55
60
65
70
75
80
150 200 250 300 350 400
Pulse Time (S)
Head Specing (mm))
B2 B3
Figure 1 : Indirect Pulse Velocity Using Yaman's Five Point Method
However in cases where access to only one face is possible a direct velocity measurement is not
possible. Yaman (16) developed a method of measuring the indirect velocity over 4 head spacings (200,
250, 300 and 350mm). This is very similar to the surface velocity method described in Annex A of EN
12504-4. The slope of a straight line plot of time vs head spacing (Figure 1) gives an indirect velocity that
is very close to the direct velocity for homogeneous specimens. The principle difference between direct
and indirect UPV’s is that direct measurements are largely through the bulk concrete and indirect
measurements are largely through the near surface but Yaman’s five point method avoids direct surface
effects such as carbonation and finishing. This is particularly useful for assessing slabs. It should be noted
however, that typically concrete is an inhomogeneous material and the difference between indirect and
direct pulse velocities can vary significantly if surface effects are deep.
In the indirect mode a particular issue is the low signal level compared with direct measurements. The first
pulse may not trigger the timer unless the gain is suitably increased. For such measurements, it is
advisable to use a waveform display to be certain of correct triggering. This may or may not be apparent
from the best fit line through the four data points. Testing should otherwise be undertaken in accordance
with EN 12504-4.
2.4 SonReb
One issue with combined use of NDT and cores for strength assessment is the number of core
correlations required according to EN BS 13791. By reducing uncertainty by combining two NDT
measurements the number of correlation points decreases. Breysse (15) describes the SonReb method
of combined UPV and Rebound measurement as discussed by RILEM Technical Committee TC 207-INR
as follows: “This combination has received the name of SonReb, for Sonic and Rebound. Rebound and
ultrasonic pulse velocity measurements can be carried out quickly and easily. The underlying concept is
that if the two methods are influenced in different ways by the same factor, their combined use can cancel
the effect of this factor and improve the accuracy of the estimated strength.” Breysse describes two
approaches to the combined assessment based on best fit of data but the multivariate approach A is
preferred, i.e.: Approach A : fc=aVbRc where a,b and c are constants (1.15x10-10; 2.6 and 1.3 respectively);
V is UPV; R rebound number.
Although standard values for a, b and c are given project specific values are determined.
Generally, the SonReb method provides an increase in correlation accuracy when compared with using
either the rebound method or UPV method in isolation.
The data, presented in Figure 2 was collected by the rebound hammer manufacturer to establish a
SonReb curve using the Q-value. It illustrates the benefit of using the combined method. The SonReb
method has been established as a national standard in several countries including Italy and China in
particular and is currently being considered by the RILEM TC-249-ISC committee dealing with in-situ
compressive strength estimation.
160
140
120
100
80
60
40
20
3000 4000 5000
Compressive Strength (MPa)
20 30 40 50 60 70 80
Q ValueUPV
(
m/sec
)
Compressive Strength
EN13791 Base
Expon. (Compressive Strength)
Compressive Strength
Expon. (Compressive Strength)
y = 4.1974e
0.0458x
R
2
= 0.9065
y = 0.1129e
0.0013x
R
2
= 0.5022
SonReb Com
p
ressive Stren
g
th
20 40 60 80 100
y = 1.001x + 0.4476
R
2
= 0.9308
ISI13311 Concrete Classification
<3000 m/s doubtfu l
3000-4500 m/s medium
3500-4500 m/s good
>4500 m/s excell ent
a) UPV correlation to compressive strength b) Q-value correlation to
compressive strength
c) SonReb correlation to
compressive strength
Figure 2 : Data from 240 Cubes to Establish SonRep Calibration Curves for Q Value
3. Structural Assessment of New Concrete Structures in a Ghana Mine
Shortly after construction deterioration of the primary crusher approach slab led to a preliminary
investigation of the strength of concrete. This indicated that there was cause to suspect that the strength
of concrete could be lower than design. Subsequently a detailed investigation of the concrete to identify if
there are any significant deficiencies in construction was undertaken. For cores testing SANS standards
were followed but as there were no SANS standards for non-destructive tests international standards were
followed. The specified strengths varied and are shown together with core strength results in Table 2.
Table 2 : Core Test Results From Ghana Mine
Element Orientation Vertical Horizontal
Specified Strength 40MPa 25MPa 40MPa 25MPa
No of Cores 3 4 - 19
No of Compressive Strength Tests 3 7 - 28
Average Strength (MPa) 34.0 16.1 - 17.8
Standard Deviation (MPa) - - 6.3
% Failing Individual Result 0% 57% - 54%
% Failing Average Result 0% 100% - 71%
The large fraction of cores that failed the individual and average strength requirements, and the margin by
which some cores failed, was a serious concern.
Rebound hammer and core testing results from the same location are shown in Figure 3. The best fit
relationship is achieved at compressive strength = 2.161e0.043x where x is the Q value. The rebound results
are from the surface of slabs and the finishing may account for why a high Q value is achieved for a given
compressive strength but more likely was that carbonation had occurred hardening the surface strength.
This effect means rebound results on the top of the slab are not an effective method of assessing bulk
strength. Consequently the assessment was based on core strengths.
Figure 3 : Core Strength Vs Rebound Hammer
Based on the core strength tests an equivalent characteristic cylinder strength of 15MPa was determined
for the 25MPa concrete and 25MPa for 40MPa concrete. The structural assessment was undertaken in
conjunction with the original designers with free and unfettered access to the design calculations. Jointly
with the designer critical elements were identified and checked based on the strength of 15MPa and 10
year design life. The structural review identified all structures behaved acceptably at the actual in-situ
strengths except for one tall conveyor support. Additional cores from this structure indicated there were
still some concerns and hence strengthening was instigated.
4. Structural Assessment of New Concrete Structures in a Botswana Mine Plant
25MPa and 32MPa concrete was specified for footings/slabs and walls/columns respectively but review of
the cube test results showed a much lower strength was achieved in practice. Cores were taken from
various locations and results were consistent with the cube results. Multiple cube results from the same
batch tested at the same age were quite consistent and 7 day results were reasonably consistent at 67%
of the 28 day results. This indicates that, failing an unexpected consistent error in testing, the results are a
realistic and able to be used as a true indication of the strength of concrete supplied. The results are
summarised in Table 3.
Table 3 : Summary of 28day Compressive Strength Test Results
Period 4th-9th May Remainder
f’c cube (MPa) 11 19
f’c cylinder (MPa) 9 15
Compressive strengths from 2 cores taken from each of three different bases gave a characteristic in-situ
cylinder strength of 17.5MPa. This was marginally higher than the 28day cube results and may have an
improvement due to the aging.
28 rebound hammer results provided had an average cube strength of 28MPa and a standard deviation of
5.6MPa to give a characteristic cube strength of 18.8MPa.
A review of the mix indicated low coarse aggregate volume and a high proportion of crushed sand. The
mix would have had a high water demand and it was concluded that to achieve a workable concrete extra
water would have likely been added.
Hence for the project it was determined that:
a) No further NDT testing was generally required as adequate data was available to give a
reasonable estimate of the concrete strength. Originally the client had asked for more detailed
NDT testing to show which elements had low strength concrete.
b) A 28 day characteristic compressive cylinder strength of 15MPa was to be used for structural
assessment of all bases except those produced between 4-9th May.
c) For the concrete poured 4-9th May the location of the concrete placed was to be identified and
design undertaken for strengthening to take the full design loads.
The structural assessment was undertaken of all structures by review of the original designs. This showed
that in general the 15MPa cylinder strength was adequate. The exceptions were:
a) Primary Crusher Base Slab – Development lengths up to 400mm to short due to low strength and
low cover. Strengthen to reduce stresses or effectively increase development lengths
b) Primary Crusher Rear Wall – Will crack as applied moments are 3 times the moment capacity
5. Structural Assessment of New Concrete in an Australian Mine Plant
On this project the concrete strength was called into question when the supervisor recorded that after
taking cylinders for strength assessment the concrete water was added to the mix for placement. Cores
and NDT were used to establish the actual characteristic strength of 18MPa for most elements, well below
the minimum specified strength of 32MPa.
Structural analysis was undertaken to show if these low strengths might be acceptable. A foundation slab
was considered as 1m wide strips with actual load points imposed and resisted by soil spring nodes.
Although the concrete strength was only 18MPa the authors believe that it is reasonable to extrapolate
outside the range of characteristic compressive strengths of 20 MPa to 100 MPa, specified in Clause 1.1.2
of AS3600, provided that a reliable coherent set of core results can be obtained. Of note is the anchorage
bond length where requirements will increase beyond those in AS3600 in highly stressed elements. The
analysis undertaken indicated that although capacities were significantly reduced most parameters were
predicted to be acceptable except for shear and hence a risk assessment was undertaken.
Risk is conventionally assessed by ISO 31000 (12) based on a combination of consequence and
likelihood of failure. For some failure modes full probabilistic modelling can be undertaken to assess
failure likelihood quite accurately. Where these methods are not possible a qualitative approach can be
followed. The consequence of structural failure can vary significantly depending.
Discussions with the owner identified that the increased likelihood of failure was unacceptable because
when combined with the consequence of failure the risks during operations became unacceptable.
Consequently the concrete was replaced.
6. Strength Assessment of an Old Australian Shopping Centre Floor Slab
The strength of a concrete slabs on grade had been called into question. The strength of the surface layer
is highly affected by finishing and curing and hence assessment using rebound hammer testing is not
recommend as the results are very dependent on near surface properties. It was agreed to undertake
widespread testing using indirect ultrasonic pulse velocity to show potential variations in strength. Cores
were undertake for calibration of the UPV. Rebound hammer results were also taken over a wide area to
determine if it gave any further insight into concrete performance.
UPV results were taken using the five point indirect method. Results were recorded on a spreadsheet
which automated a linear regression analysis to check the four results gave adequate correlation (Table
4). The velocities, and the rebound results, were plotted in a spreadsheet using conditional formatting to
highlight variations (Table 5).
Table 4 : Indirect UPV Data for Shopping Centre Floor Slab. D is the Transmitter/Receiver Separation and T the
Corresponding Wave Transit Time
Table 5 : Colour Coded Plot of Rebound and UPV Test Results
In the case of the rebound results there appears to be two distinct areas where the 10 percentile strengths
are 24.9 and 38.2MPa. These variations are not seen in the UPV results. The Rebound variations may be
due to variations in finishing and curing of the two pours and be limited to surface effects.
The UPV results were correlated with strength and gave the following relationship:
Strength (MPa) = 8.5344e0.3173v where v is ultrasonic pulse velocity in m/s.
Converting the UPV to strength results gave a plot of strength as shown in Table 6. The colour coding is
based on highlighting areas where the strength is less than the required 32 MPa.
Table 6 : Plot of Compressive Strength estimated using UPV measurements
7. Strength Assessment of New Columns and Walls of an Australian Office Tower
S65 concrete was poured in columns and stair well walls. Cylinder results were marginal and hence
strength assessment was requested. An assessment of in-situ strength was undertaken using the SonReb
method with UPV and rebound measurements calibrated against core strength tests. The process used
for the SonReb method was:
1) Mark out reinforcement grid on the concrete surface using GPR so that UPV test results avoided the
influence of reinforcement as far as possible.
2) Mark out sixteen measurement points for each element. These comprised eight pairs of measurement
points precisely on opposite faces of the element. Typically four measurement points were at 0.5m, 1m,
1.5m and 2.0m above base level so that strength with height could be assessed. No significant variation
was found (Figure 4)
Figure 4 : Plot of UPV and Rebound With Height for a Shaft Wall
3) Assess the path length for the measurement points.
4) Take direct UPV measurements using digital UPV equipment with the transmitter on one face and
receiver on the other for each of the eight pairs of measurement points. The transmitter and receiver
were then swapped at each pair of points to give sixteen direct measurements. The UPV’ equipment’s
built in measuring process takes multiple measurements to give an average of several reading for each
result. Results were also verified by the strength of the received signal and only highly reliable results
were recorded.
5) Prepare the concrete surface using a grinding stone at each of the sixteen points and take rebound
measurements using the digital rebound hammer. Ten rebounds were used to give one Q value.
6) Enter the data in the results spreadsheet prepared for the project to verify results were sensible and
consistent. The formula for strength generally used is fck =a.Vb.Qc where a, b and c are constants V is
the ultrasonic pulse velocity in m/s. Q is the rebound value as given in RILEM (14)
8. Strength Assessment of 50 Year Old Basement Columns in an Australian Building
On this city centre project the building height was to be increased adding several new floors of office
space. This translated to additional load in the basement columns and diaphragm wall. The existing
structure had reached its 50 year design life and hence a durability and strength assessment was
undertaken to confirm that the load capacity was adequate and that the structure would provide an
additional 50 year life commensurate with the requirements for the new structure.
The strength assessment was in two parts. Cores were taken from walls, columns and slabs. For the more
critical columns a wider assessment of strength was required. This was achieved by correlating direct
UPV results with core strengths and then using UPV results to give an indication of in-situ strengths. The
correlation was based on compressive strength =1.0828e0.8201v where v is UPV.
Table 7 : UPV Correlation with Core Strengths
Col Pulse Velocity (m/s) Core Strength (MPa) UPV Strength (MPa)
Bottom Middle Top Bottom Middle Top
B10 4.781 4.817 4.849 48.51 63.52 56 43.5 45.1 46.7
G8 4.919 4.953 4.944 58.53 Broke 58.5 50.1 51.9 51.4
Piles were tested using a force vibration test as developed by Davis and Dunn (17). Testing was
undertaken by measuring the response from a series of hammer blows on the concrete surface using a
geophone held on the concrete surface adjacent to the hammer. Where there was no pile response the
test location was moved along until a response was found. Having located the pile the test results were
recorded. The received signal was put through a fast Fourier transform to give Mechanical Admittance
and Frequency. Mechanical Admittance gives the load deflection curve based on wave theory. Frequency
gives parameters of the pile model. Information obtained includes pile length, minimum pile diameter,
presence of an end bulb, concrete modulus and safe pile load.
9. Strength Assessment of Old Shopping Centre Columns in Australia
Testing was undertaken on ground floor columns of a shopping centre in Queensland testing to assess
the capacity for planned extensions. No coring was allowed in the structure and so the completely non-
destructive NDT method was proposed. NDT results depend on the materials used and so cylinders
made using local materials were used to create a calibration between NDT (UPV and rebound) and
cylinder strengths.
Sixteen cylinders were tested. Direct UPV measurements were taken on the cylinders and then once the
cylinder has been compressed to 3.5MPa the rebound values were taken using an original rebound
hammer. The cylinders were then crushed. The calibration co-efficient were assed using the equation
proposed by Samirin (18), i.e. f’c = aR +bV4. Samin’s approach was an early form of combined ultrasonic
pulse velocity and rebound for strength assessment and the SonReb equation would be recommended
today.Excel’s ‘Solver’ function was used to give the values for a and b of 1.0695 and 4.96x10-15
respectively. The correlation achieved was 0.973.
Ten columns were tested. Two rebound hammer tests were carried out in accordance with BS 1881:202
on each face. Two UPV measurements (BS1881:203) were taken between each of the two sets of
opposing faces. Care was taken to avoid any reinforcing steel by locating this first using a covermeter.
Column 1 2 3 4 5 6 7 8 9 10
Rebound Number 47.8 54.4 52.1 51.4 53.3 49.8 50 48.5 53.6 54.8
UPV (m/se) 4457.5 4300 4298.3 4173.3 4155 4295 3776.7 4175 4207.5 4100
Calculated Strength (MPa) 53.0 59.8 57.4 56.4 58.5 54.9 54.5 53.3 58.8 60.0
All of the columns were found to have strengths in excess of the 40MPa required for the extension.
10. Conclusions & Recommendations
The paper identifies recent changes in the state of the art for assessing the in-situ strength of structures
and outlines some of the principles used on various structures. The principles include core testing,
rebound hammer and ultrasonic pulse velocity use to support in-situ strength assessment, methods of
structural assessment and use of risk assessment to determine acceptability of reduced reliability due to
low strength concrete. AS 3600, AS 1012.14 and CIA Z11 represent the current recommendations for
structural assessment in Australia. These are now inconsistent with overseas documents on which they
were based due to recent developments with the overseas documents. They do not incorporate the use of
NDT and do not give structural assessment guidelines. It is recommended that AS 1012.14 and CIA Z11
are updated to reflect the current state of the art and be expanded to give guidance from testing to
structural assessment.
Acknowledgements
The authors wish to thank their clients for involvement in their projects.
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... Also for multivariable power regression model, the experimental stability and flow values are used. Initially, the natural logarithms for the data have been analyzed, after which 'LINEST' function in Microsoft Excel is applied to estimate the best fits straight lines for the data using the method of least squares [22]. ...
Article
This paper examines the potential use of small-diameter cores (D < 100 mm), for estimating the in situ cube strength of concrete. The assessment is based on systematic analysis and evaluation of data sourced from published work since 1961, by 243 researchers working at 128 institutions in 32 countries, that yielded a matrix of 32 953 data points. The effects of core diameter (D), length to diameter ratio (L/D) and core diameter to maximum aggregate size (A) ratio (D/A) formed the main basis of the analysis. It is shown that there appears to be no technical justification for having requirements that limit D to 100 mm and/or D/A ≥ 3. Conversion factors are proposed for estimating, from the test results of small cores, the strength of cores having D = 100 mm and L/D between 1·0 and 1·2, which is currently the required size of cores to estimate the in situ compressive cube strength of concrete in the UK. It is shown that, in complying with the test data variability requirement, damage to structural elements through the coring can be reduced by adopting the use of small-diameter cores, down to 25 mm, where the damage is 10% that of 100 mm dia. cores.
Conference Paper
In 2016, the CIA withdrew its 2002 publication Z11 on 'Evaluation of Concrete Strength by Testing Cores' from use. This was primarily because CSTR 11, on which it is substantially based, has been withdrawn. With improvements in global understanding of how best to predict in-situ concrete strength over time and the publication of more modern methods such as those described in EN 12504, EN 13791 and BS 6089, the methods described in Z11 are now known to be inaccurate and therefore potentially unsafe to use. AS 1012.14 for testing of cores from hardened concrete is currently being updated, there is currently no comprehensive Australian guideline on how to assess in-situ concrete strength of existing structures using cores and NDT. This has meant Australian engineers conducting in-situ strength assessments currently have to rely on international codes, or in some cases are still using out of date methods such as Z11. In this paper, an Australian guide to standardise the industry approach to strength assessment of existing concrete structures and eradicate the use of out of date methods by way of an update to CIA Recommended Practice Z11 is proposed and discussed. The update would be based around state of the art practices in use in current international codes and incorporate learnings from the recent international research and ongoing updates to AS 1012.14, EN 13791 and the RILEM 'in-situ strength assessment of concrete' guidelines.
Article
Full-text available
The relationship between velocities of ultrasonic stress waves transmitted along direct and indirect paths was investigated. Tests were conducted on plain concrete slabs of dimensions 1000 x 1500 mm, with a thickness of 250 mm. Direct ultrasonic wave transmission tests were conducted between top and bottom surfaces of the slabs and indirect tests were conducted along the slab surface. A test procedure, described in BS 1881 to determine indirect wave velocities, was refined by defining the number and spacing of transducers. Comparisons were made between direct and indirect wave velocity measurements using statistical analysis. The statistical analysis revealed that direct and indirect wave velocities could be used interchangeably in evaluating the properties of the concrete. The minimum number of tests points required for a reliable estimate of indirect wave velocity was studied and recommendations are provided
Article
An empirical equation derived from nonlinear, multi-regression analysis, relating compressive strength, ultrasonic pulse velocity and Schmidt rebound number of concrete made with natural dense weight aggregate is presented.
Article
The paper gives an outline of the principle of a non-destructive pile testing method by vibration and presents a summary of the parameters which may be obtained from velocity response graphs for cylindrical concrete piles excited by a vibrator. Consideration of the behavior of ideal piles provides a means of interpreting these parameters. The effect of important defects in piles on the shape of their response graphs is considered by computer analysis of an electrical analogue of their mechanical behavior. Examples of vibration test results on piles at five sites are given (two in France and three in Great Britain).
AS 1012.14 Method of testing concrete. Methods for securing and testing cores from hardened concrete for compressive strength
  • Standards Australia
Standards Australia "AS 1012.14 Method of testing concrete. Methods for securing and testing cores from hardened concrete for compressive strength" Standards Association of Australia, 1991, Homebush, Australia.
AS3600-2009 Concrete Structures Standards Association of Australia
  • Standards Australia
Standards Australia " AS3600-2009 Concrete Structures. " Standards Association of Australia, 2009 Homebush, Australia.
Testing concrete in structures. , Part 1 Cored specimens -taking, examining and testing in compression Part 2 Non-destructive testing. Determination of rebound number (2012) Part 3 Determination of pull out force
  • Bs En
BS EN 12504 " Testing concrete in structures., Part 1 Cored specimens -taking, examining and testing in compression, (2009) Part 2 Non-destructive testing. Determination of rebound number (2012). Part 3 Determination of pull out force (2005). Part 4 Determination of ultrasonic pulse velocity (2004). BSI, London, UK.
Assessment of compressive strength in structures and precast concrete component
  • Bs En
BS EN 13791 " Assessment of compressive strength in structures and precast concrete component " British Standards Institute, 2007, London, UK.
BS6089 Assessment of in-situ compressive strength in structures and precast concrete components -complementary guidance to BS EN 13791
  • British Standards
British Standards "BS6089 Assessment of in-situ compressive strength in structures and precast concrete components -complementary guidance to BS EN 13791" British Standards Institute, 2010, London, UK.
AS1012.9 -Method of testing concrete. Methods for the determination of the compressive strength of concrete specimens
  • Standards Australia
Standards Australia "AS1012.9 -Method of testing concrete. Methods for the determination of the compressive strength of concrete specimens" Standards Association of Australia, 1999, Homebush, Australia.
Concrete Society Concrete Testing for Strength
Concrete Society " Concrete Testing for Strength " Technical Report 11, Concrete Society, 1976, Camberly, UK.