Embed Size (px)
Citation preview
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 1 of 26
What is Quality Control ?
Jason Weiss, [email protected], Purdue UniversityJack and Kay Hockema Professor & Director of the Pankow Materials Laboratory
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 2 of 26
Henry Ford with his Model T car in 1912
Ford Changes How We View Quality
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 3 of 26
Fords Approach to Quality
Advent of mass production has shifted the emphasis to the reproducibility of the size, shape, strength, etc of a product to become a quality issue.
Consistency
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 4 of 26
Quality Control/Quality Assurance (Ensure adequate confidence)
• Owner establishes requirements (mean and range typically)
• Contractor develops a quality control plan• Contractor takes samples and compares
them to targets using this as feedback to control the process by making adjustments
• Owner performs random tests (random in time or location) to ‘assure’ the contractor is in compliance with the established minimum requirement. No feedback –materials are either accepted or rejected
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 5 of 26
Constituent Materials (ASTM 169C Survey)
Cement most variable
Reasonably variablefiner more so
Fly ash moderately variable
Slag, SF low variability
low variability
Constituent Materials
• Cement• Aggregates• Water• Min. Admix. • Chem. Admix.
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 6 of 26
Steps in Processing
Processing• Batching• Mixing• Transporting• Placing• Finishing • Curing
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 7 of 26
Measuring Concrete Properties
Concrete ‘Properties’• Rheological• Mechanical• Chemical• FT-Resistance• Electrical
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 8 of 26
Variability Comes From Each of These Factors
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 9 of 26
Every Job May Not Be Glamorous But Quality Control is Important
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 10 of 26
Establishing Criteria
• Establish a level of quality in your procedures and set up methods to DEMAND that your suppliers do the same
• Establish material specifications and quality expectations
• Identify procedures for unsatisfactory materials
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 11 of 26
Double Checking Never Hurts
• Inspect materials when materials arrive to make sure they meet your specifications
• Marking and recording process to clearly identify lots and supplies
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 12 of 26
Keeping Records
• Constituent material records– Cement mill certificates– Aggregate reports (gradation/ moisture)– Chemical & mineral admixture certificates– Reinforcement supplier reports – Accessory supplier documents– Batch records or RMC delivery tickets
• Work orders, product drawings, operation report
• Equipment calibration records– Cement and aggregate scales (water meter), – Cylinder compression tester– Test equipment (air meter, therm., clocks)
• Test results (slump, temp., unit w, air, strength)• Personnel training and certifications
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 13 of 26
How Do We Measure Quality ?
• Obviously the first thing that we all want to do is build a cylinder
• 4795 psi• Lets say we do it again• 4925 psi• The average strength• 4860 psi
• Then we want to break it and determine the strength
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 14 of 26
How Do We Assess Variability
• Averages do not tell us everything
The Mississippi River is “on average”about 11 inches deep
Frequency distributions are more common
600 650 700 750 800 850Flexural Strength (psi)
0
4
8
12
16
Num
ber o
f Occ
uren
ces
Frequency distributions are more common
600 650 700 750 800 850Flexural Strength (psi)
0
4
8
12
16
Num
ber o
f Occ
uren
ces
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 15 of 26
Defining Variability More Precisely
• Distribution of concrete properties is typically assumed to follow a normal (Gaussian) distribution
nx
1
12
2 2
nx
s
nx
s
i
nx
ii
1s = 68.2%2s = 95.5%3s = 99.7%12 16 20 24 28 32 36
Compressive Strength (MPa)
0
5
10
15
20
25
Prec
ent o
f Tes
ts (%
) f’cr = 25.7 MPaStd dev. =3.1 MPa
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 16 of 26
Quality in the Ideal World
• Monitored by someone not involved in the production or marketing
• Comprehensive system (quality manual) for all processes involved
• Detailed record of inspections• Effective training manual or
procedures• Formal procedure for rectifying
substandard goods or operations
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 17 of 26
Statistical Process Control
• A diagnostic tool that allows you to determine “special” versus “common” causes of variation.
• Common variation – Inherent in every process describing why nothing is ever exactly the same.
• Special variation - something happens that is not usually part of the process.
• SPC allows identification of when special variations occur allowing them to be controlled without overreacting to common variability.
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 18 of 26
Simple Ways to Describe Variability
• The product variability refers to the spread in some product quality parameter.
• Common measure of product variability is 2 sigma (Std. Dev.)
• In a normally distributed data set, 95% of all data points will fall within ± 2 standard deviations of the mean.
1s
2s
3s
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 19 of 26
Quality Control Charts
• Economical method based on statistics and probability
• Provide a method to assess on a continuous basis
• Graphical Representation
Required Average Strength f’cr= f’c +ts
Required Strength f’r
0 15 30 45 60Number of Samples (n)
600
700
800
900
Flex
ural
Str
engt
h (p
si)
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 20 of 26
Proper QC Helps to Avoid Big Surprises in QA
• Here we see samples tested by the contractor in “their schedule”
600 700 800 900Flexural Strength (psi)
0
10
20
30
40
Num
ber o
f Occ
uren
ces
• When the agency tested
• mean - 752 psi• std dev – 45 psi
• mean - 762 psi• std dev – 49 psi
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 21 of 26
Quality Control is an Active Process
• “We don’t take QC tests for practice !!!” Dick Newell
• Tests provide information – to better
understand material
– to verify production as compared to target and set points
– to make adjustments
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 22 of 26
Testing for Compliance Versus Implementing Quality Control
• Option #1 – Truly Compliance Testing to Insure that You are Within a Band
Required Average Strength f’cr= f’c +ts
0 15 30 45 60Number of Samples (n)
600
700
800
900
Flex
ural
Stre
ngth
(psi
)
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 23 of 26
Testing for Compliance Versus Implementing Quality Control
• Option #2 – Better • Method for Quality
Control is to Checking the test as compared to ‘Design’
• As Tests Pass the Red Line Some Action is Needed
• Adjust As Needed • Caution do not
adjust on every test 0 15 30 45 60
Number of Samples (n)
600
700
800
900
Flex
ural
Stre
ngth
(psi
)
Individual tests are less sensitive to changes in quality but fluctuate
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 24 of 26
Testing for Compliance Versus Implementing Quality Control
• Option #2 – Better • Compare each test
to the design• On low tests begin
checking more closely, take additional samples
• Here a running average is used (5 previous samples)
• As the red line is passed some action is needed
Use running average to change processand return the measure to design
0 15 30 45 60Number of Samples (n)
600
700
800
900
Flex
ural
Str
engt
h (p
si)
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 25 of 26
500 600 700 800 900 1000Flexural Strength (psi)
Num
ber o
f Occ
uren
ces
Spec
ified
Stre
ngth
, f’ r
What Are the Practical Implications of Having Tighter Quality Control
• We can not design structures based on the mean measured strength but we will use it for designing the mixture
• Assumption: 1/1000 tests are below the design strength (f’r=570 psi)
• Consider two contractors with measured variation
X = 680psi = 45 psi
X = 810psi = 100 psi
February 13th, 2014 IN-ACPA Quality Control: Section 2 – What is Quality Control Slide 26 of 26
Summary
• Quality Control – Really All About Consistency – Same Material Day/ Pavement/Structure Day After Day
• Comparing Sample Data to Targets to Make Adjustments
• Statistical Process Control Allows to Separate Common and Special Variations
• Enables Cost Savings in Design if Control is Improved
Consistency
Main Factors
Not Glamorous
Items to Consider
Measuring Quality
Statistical Control
Active Process
Summary
