- Problem Definition
The common method today for pavement thickness assessment after execution [both asphalt and concrete] compare to contract specification is using core drill method. Consultant Supervision and Client determine method assessment in the bid for payment work item with tolerance according to standard. Field core drill test can be used also to assess pavement quality since we have samples from test points. Both parameters are the guideline for declaring work progress for Contractor.
Figure 1 Field Core Drill Test
- Identify the Feasible Alternative
There are three possible progress payment conditions adapted from core drill test result: 1. 100% applied contract unit rate, 2. Reduction contract unit rate and 3. Reject working progress.
- Development of the Outcome for Alternative
Core drill points taken along the length of asphalt road pavement with mean value and standard deviation calculation as present below:
Figure 2 Core Drill Data, Mean and Standard Deviation
Next thing to do is establishing control limits X-chart,  &  as present below:
Figure 3 Core Drill Test-X Chart
- Selection Criteria
Contract specification requirement for surface thickness is 36 mm. If there is thickness reduction then applied 80% reduction unit price until max 5 mm reduction. If more than 5 mm reduction then Client has the right not to pay or to reject Contractor work progress based on overall quality assessment. Client will not pay any excess of surface thickness [if any] to Contractor.
- Analysis and Comparison of the Alternative
From data table present in Figure 2, present in Figure 4 unit price payment application as follow:
Figure 4 Unit Price Payment Application
As shown in Figure 4 there are seven excess thicknesses that become Contractor risk. Also there are four points of 80% payment reduction and one point less than requirement that required overall quality assessment.
For overall quality three steps produce for assessment as follows:
- Refer to mean data Contractor provides good quality pavement surface thickness [36.71 mm > 36 mm]
- Checking for any internal cause for out of control situation refer to some criteria:
- One or more points fall outside the control limits [outside +/- three sigma]
- Two points, out of three consecutive points, are on the same side of the average between +/- two and three sigma or beyond
- Four points, out of five consecutive points, are on the same side of the average between +/- one and two sigma or beyond
- Nine consecutive points are on the same side of average
- There are six consecutive points, increasing or decreasing
- There are 14 consecutive points that alternate up and down
- There are 15 consecutive points between mean and +/- one sigma and mean
The result on Figure 3 shows no internal process causes for out of control situation that proves the deliverable compliance according to specification.
Figure 5 Cp and Cpk Factors for Process Variability Assessment
Both Cp and Cpk factors indicate that process variability from actual executions meets specification [Cpk factor use to check if there is difference between specification with mean or actual data spread instead only using Cp factor].
- Selection of the Preferred Alternative
Based on assessment as described previously Client can accept 100% Contractor working progress but with 80% points unit price reduction for five points locations.
- Performance Monitoring and the Post Evaluation of Result
Core drill test can be used as well for checking asphalt content using testing Marshall from the sample of asphalt pavement, but for concrete pavement usually Client perform testing Compressive for assess concrete strength. The best common practice for quality assessment of pavement structure is combining this test with thickness test for progress payment approval.
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