Construction QC/QA Control Charts 1

Quality control/quality assurance

Control Charts

Senior/GraduateHMA Course

Construction QC/QA Control Charts 2

Control

Charts

Construction QC/QA Control Charts 3

Variation

- Chance Causes- Assignable Causes

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Chance Causes

1. Everything varies2. Individuals are unpredictable3. Groups from a constant system tend to be predictable

8-31

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Example: Chance Causes

1. People live to different ages

2. No one knows how long he or she will live

3. Insurance companies can predict the percentage of people who will live to certain ages

Construction QC/QA Control Charts 6

Assignable Causes

If identified -

It can be eliminated

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Benefits of Control Charts

- Early Detection of Trouble- Decrease Variability- Establish Process Capability- Reduce Price Adjustment Costs- Decrease Inspection Frequency

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Benefits of Control Charts

• Basis for Altering Specification Limits

• Permanent Record of Quality

• Provide a Basis for Acceptance

• Instill Quality Awareness

Construction QC/QA Control Charts 9

Control Chart Analysis

AASHTO

“To Develop a Quality Control/Quality

Assurance Plan for Hot Mix Asphalt”

Construction QC/QA Control Charts 10

1 2 3 4 5 6 7Lot Number

Upper control limit

Lower control limit

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Data Points

Statistical Control Chart

Upper Control Limit

Lower Control Limit

Target

Value

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Five lines on a statistical process control chart

• Target value• Warning upper control limit• Warning lower control limit• Action upper control limit• Action lower control limit

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Upper Control Limit

Lower Control Limit

Mean

1 2 3 4 5 6 7Lot Number

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Statistical Control Charts

Upper and Lower Warning Control Limits

UWCL = X + { 2 (s) / (n)1/2 }

LWCL = X + { 2 (s) / (n)1/2 }

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Statistical Control Charts

Upper and Lower Action Control Limits

UWCL = X + { 3 (s) / (n)1/2 }

LWCL = X + { 3 (s) / (n)1/2 }

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Example Problem

• Given:– Target asphalt binder content: 5.7 %– Standard deviation – 0.25 %– The rolling average is based on five data

points

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Example Problemwarning limits

UWCL = X + { 2 (s) / (n)1/2 }

UWCL = 5.7 + { 2 (.25) / (5)1/2 }

UWCL = 5.9

Construction QC/QA Control Charts 18

Example Problemwarning limits

LWCL = X - { 2 (s) / (n)1/2 }

LWCL = 5.7 - { 2 (.25) / (5)1/2 }

LWCL = 5.5

Construction QC/QA Control Charts 19

Example Problemwarning limits

UACL = X + { 3(s) / (n)1/2 }

UACL = 5.7 + { 3 (.25) / (5)1/2 }

UACL = 6.0

Construction QC/QA Control Charts 20

Example Problemwarning limits

LACL = X - { 3 (s) / (n)1/2 }

LACL = 5.7 - { 3 (.25) / (5)1/2 }

UWAL = 5.4

Construction QC/QA Control Charts 21

Look for Assignable Cause If:

• One point is outside of control limits

• Eight consecutive points are on one side of the target value

Interpreting

X & R

Charts

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Action Limit (UCL)

Warning Limit

Action Limit (LCL)

Chart

0

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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

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s

(UCL)

(LCL)

Range (R) Chart

Control ChartsRules of Thumb

Lack of Control Occurs When:

Change in , R Constant

Change in R, Constant

Change in Both & R

X

X

X

Sustained Shift in

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Action Limit

Warning Limit

Mean Increases

Trend in

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3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Production "Upward Drift"

Action Limit

Warning Limit

Irregular Shift in

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Action Limit (UCL)

Warning Limit

Action Limit (LCL)

Sudden Change in R

0

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8

9

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

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s

(UCL)

(LCL)

Gradual Change in R

0

1

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5

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7

8

9

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

(UCL)

(LCL)

Irregular Shift in & R

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Action Limit (UCL)

Warning Limit

Action Limit (LCL)

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

Sample Number

% A

ir V

oid

s

Eight Points Below Target

Single Point AboveAction LimitAction Limit

Warning Limit

Lack of Control?

Construction QC/QA Control Charts 33

Questions?