Measurement of variation
• Mean strength
• Standard deviation
• Coefficient of variation
cv =
σ
x
4
Producer’s Risk and Consumer’s Risk
Producer’s Risk
• Rejection of concrete that is upto-standard
• Higher cost price
Consumer’s Risk
• Acceptance of concrete that is
sub-standard
• Stringent acceptance criteria
5
Review on Standards for Acceptance
Testing and Criteria of Concrete
Foreign codes
• American Standard ACI 214R-11
• European Standard BS EN 206: 2013
Local codes
• Code of Practice for Structural Use of Concrete: 2013
• General Specification for Civil Engineering Works: 2006
• Construction Standard CS1: 2010
6
ACI 214R-11
• Specimens for each test result:
• 2 concrete cylinders of 150 × 300 mm or;
• 3 concrete cylinders of 100 × 200 mm
• Evaluation of standard of quality control through:
• Standard deviation
• Coefficient of variation
(see Table 4.3 and Table 4.4)
7
Table 4.3
f’cube= 43.7 MPa
8
Table 4.4
f’cube= 43.7 MPa
9
Estimation of standard deviation or
coefficient of variation
• A minimum of 30 test results
• Conservative approach required when
test results < 30
• ACI 318 allows a minimum of 15 test results
• the sample standard deviation should be increased by up to 16% to account
for greater uncertainty in the estimated population standard deviation
10
Acceptance criteria
• Acceptance criteria
• Average of required
strength not exceeding the
specified strength by a
certain multiple of the
standard deviation, as
given in table
11
BS EN 206: 2013
• Acceptance testing and criteria are give in Section 8.2
• Clause 8.2.1.2
• Test result = individual specimen OR the average of test values from
two or more specimens from one sample tested at same age
• Disregard the test result when
range of test values > 15% of the mean
• Calculation of standard deviation
• Most recent 35 consecutive test results
• No maximum limit
12
Acceptance criteria
• Given in Clause 8.2.1.3
1. Individual test result > (Specified grade strength – 4) MPa
2a. In initial production (until 35 test results are obtained)
Mean strength of 3 consecutive results
≥ (Specified grade strength + 4) MPa
2b. In continuous production (when 35 test results are available)
Mean strength of the consecutive test results
≥ (Specified grade strength + 1.48 × standard deviation MPa
13
Code of Practice for Structural Use of
Concrete: 2013
• Acceptance testing and criteria given in Clause 10.3.4.2
• Clause 10.3.4.2
• 2 cubes shall be made in accordance to CS1
• Test result = average compressive strength of the pair of cubes
• Disregard test result when difference > 20% of test result
• Calculation of standard deviation
• 40 consecutive test results
15
Acceptance criteria
16
Non-compliance circumstances
• Suspension of concrete production and concreting
• For concrete grade < C60,
Cubes of 150 mm
• standard deviation > 8.0 MPa
Cubes of 100 mm
• standard deviation > 8.5 Mpa
• For concrete grade > C60,
coefficient of variation > 14%
17
General Specification for Civil
Engineering Works: 2006
• Acceptance testing and criteria given in Clauses 16.58 to 16.62
• Clause 16.59
• 2 cubes shall be made in accordance to CS1
• Test result = average compressive strength of each pair of cubes made from
the sample
• Disregard test result when difference > 15% of test result
• Concrete cores might be necessary if the acceptance requirements are not
satisfied
• Calculation of standard deviation
• 40 consecutive test results
18
Acceptance criteria
19
Non-compliance circumstances
• Suspension of concrete production and concreting
• Cubes of 150 mm
• standard deviation > 8.0 MPa
Cubes of 100 mm
• standard deviation > 8.5 Mpa
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Comparison
Different requirements
Standards
ACI 214R-11
BS EN 206: 2013
Code of Practice
for Structural Use
of Concrete 2013
General Specs. For
Civil Engineering
Works: 2006
Specimens
Cylinder
Cube / Cylinder
Cube
Cube
Maximum
allowable range of
difference
Nil
15%
20%
15%
Calculation of
standard deviation
OR coefficient of
variation
30 test results
required
35 test results
required
40 test results
required
40 test results
required
Limit imposed on
standard deviation
OR coefficient of
variation
No
No
Yes (Table 10.2)
Yes (Table 16.10)
21