evaluation of probing vs. coring for …docs.trb.org/prp/10-1630.pdfallison, whited, hanna, nasief 1...

Allison, Whited, Hanna, Nasief 1

EVALUATION OF PROBING VS. CORING FOR DETERMINATION OF PCC 1 PAVEMENT THICKNESS 2

3 Submission Date: July 31, 2009 4 Word Count (Text): 4,921 5 Number of Tables: 4 6 Number of Figures: 5 7 8 9 Gyude W. Allison 10 Former Graduate student 11 University of Wisconsin-Madison 12 Dept. of Civil & Environmental Engineering 13 1 Basil Ct., Madison, WI 53704 14 Phone: 608-445-0112 15 Email: [email protected] 16 17 Gary C. Whited, P.E. 18 Program Manager 19 Construction & Materials Support Center 20 University of Wisconsin-Madison 21 Dept. of Civil & Environmental Engineering 22 Madison, WI 53706 23 Phone: 608-262-7243 24 Email: [email protected] 25 26 Awad S. Hanna, Ph.D., P.E. 27 Director of Construction & Materials Support Center 28 Professor and Chair of Construction Engineering & Management 29 University of Wisconsin-Madison 30 Dept. of Civil & Environmental Engineering 31 Madison, WI 53706 32 Phone: 608-263-8903 33 Email: [email protected] 34 35 Haidy Gerges Nasief (Corresponding Author) 36 Ph.D. Candidate, Construction Engineering and Management, 37 University of Wisconsin-Madison 38 Dept. of Civil & Environmental Engineering 39 Madison, WI 53706 40 Phone: 304-282-3707 41 Email: [email protected] 42

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TRB 2010 Annual Meeting CD-ROM Paper revised from original submittal.


ABSTRACT 1 2 The Wisconsin Department of Transportation (WisDOT) has used contractor probing 3 measurement of fresh Portland Cement Concrete Pavement (PCCP) to determine pavement 4 thickness since 1998. Prior to that, WisDOT used coring to measure thickness and determine 5 payment for pavement. The probing method is non-destructive and has financial advantages for 6 WisDOT, as it eliminates the costs of coring and reduces the expenses of contract 7 administration. Probing also provides the contractor with immediate feedback on the depth of 8 the pavement being constructed. A study was conducted to verify the reliability of probing 9 measurement for determining pavement thickness, as well as verifying contractors’ compliance 10 with design requirements. To determine whether probing was still a viable method for use by 11 WisDOT in construction contract administration, core depths were compared to field measured 12 probe depths for twelve projects constructed between 2006 and 2008 across the state of 13 Wisconsin. Sample means were compared, statistically calculated estimates of differences were 14 examined, and a statistical analysis at the 95% confidence interval was carried out. Based upon 15 these analyses, probing does provide an acceptable measure of pavement thickness. All 16 contractor probe measurements and over 80% of the WisDOT core samples showed that 17 constructed pavement thickness exceeded the design thickness. 18

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TABLE OF CONTENT 1 2 ABSTRACT……………………………………………………………………………… 2 3 TABLE OF CONTENT…………………………………………………………………………...3 4 LIST OF FIGURES ………………………………………………………………………………4 5 LIST OF TABLES………………………………………………………………………………...4 6 INTRODUCTION ………………………………………………………………………..5 7 BACKGROUND ………………………………………………………………………... 5 8 PROBLEM STATEMENT ……………………………………………………………8 9 SCOPE…………………………………………………………………………………… 9 10 PREVIOUS WISDOT STUDY…………………………………………………………..9 11 DATA DISCRIPTION…………………………………………………………………....9 12 METHODOLOGY………………………………………………………………………..10 13

Test Statistics Used………………………………………………………………..10 14 F-test Statistics……………………………………………………………..10 15 T-test Statistics……………………………………………………………..11 16 Skewness test……………………………………………………………....11 17 Kurtosis test……………………………………….……………………….11 18

RESULTS………………………………………………………………………………....12 19 STATISTICAL QUALITY CONTROL ………………………………………………....15 20 CONCLUSIONS………………………………………………………………………….16 21 LIST OF REFERENCES………………………………………………………………….17 22

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LIST OF FIGURES 1 2 FIGURE 1 Drilled Core Sample - http://www.tfhrc.gov/pubrds/02jul/04.htm (April 2008) ....... 6 3 FIGURE 2 WisDOT thickness measurement device……………………………………………6 4 FIGURE 3 Illustrated Probe Measurement Method (Allison, 2008) ….…………………………7 5 FIGURE 4 Probing as Done in the Field (Courtesy of WisDOT, 2008) ………………………...8 6 FIGURE 5 Quality Control X-bar Chart for USH 151, Columbia Co. ………………………....16 7

8 9

LIST OF TABLES 10 11 TABLE 1 List of Study Projects ……………………………………………………………… 10 12 TABLE 2 Mean, Standard Deviation, Variance and Sample Size …..……….……………….. 12 13 TABLE 3 F-test results ………………………………………………………………………… 13 14 TABLE 4 Hypothesis Test - All Projects (Unequal Variance) ………………………………....14 15

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INTRODUCTION 1 The current trend in State Highway Agencies (SHA) is to move toward utilizing quality control 2 data for quality assurance purposes as a way of coping with staff reductions and budget cutbacks 3 (Allison, 2008). Portland Cement Concrete (PCC) pavement thickness has traditionally been 4 measured by coring for quality assurance and payment determination. Several states have moved 5 to other methods for measuring pavement thickness including probing and Impact- Echo 6 methods. Contractors often use probing as a method for thickness quality control during 7 construction. If contractors’ quality control data is used by SHAs for quality assurance purposes, 8 cost and resources would have been saved. However, verification studies must be done to assure 9 that probing produces adequate results when compared to coring for measuring the thickness of 10 the pavement. 11

12 BACKGROUND 13 Coring is a destructive measurement method that requires cutting out a segment of the pavement 14 and refilling that section. Figure 1 shows a sample of a drilled core. According to ASTM C174, 15 ASTM C42 and AASHTO T-24 the cores extracted from the constructed pavement for 16 determining the pavement thickness must have a diameter of 4 inches. The Wisconsin 17 Department of Transportation (WisDOT) does coring according to AASHTO T-24 standards and 18 evaluates the results according to AASHTO T-148. The results obtained from the core 19 measurements are used for determining payment to the contractor. Coring is the accepted 20 standard and represents the most accurate means for determining pavement thickness at a 21 specific point. Additionally, randomly located cores are taken to ensure that the contractor 22 constructs the required concrete thickness. For this reason, coring is used for calibrating non 23 destructive measuring techniques [3&8]. WisDOT classifies the results obtained from coring 24 into: conforming - if the thickness is greater than or equal the plan thickness minus 3/8 inch; non 25 conforming - if the thickness is greater than or equal the plan thickness minus 1 inch but less 26 than 3/8 inch; unacceptable - if the thickness is less than the plan thickness minus 1 inch. 27 WisDOT uses a measuring device to measure the thickness, as shown in Figure 2 below. Once 28 the core is removed WisDOT uses a chisel and lightly chips away any base course material that 29 is not firmly attached, thus removing the loosely attached material. Nine measurements are taken 30 for each core, one in the center, and eight additional measurements around the circumference of 31 the 4” diameter core. These measurements are recorded to the nearest 0.05 inch; averaged and 32 rounded to the nearest tenth inch. If the average falls in the middle, WisDOT rounds the odds up 33 and the evens down. The device WisDOT uses is calibrated by the University of Wisconsin-34 Madison annually. 35



1 2

Figure 1 Drilled Core Sample - http://www.tfhrc.gov/pubrds/02jul/04.htm (April 2008) 3

4 5

Figure 2 WisDOT’s thickness measurement device 6 7

Probes taken in fresh PCC pavement directly measure the thickness and are an alternative 8 procedure to coring for the determination of pavement thickness. WisDOT’s probing method is 9 detailed in the WisDOT Construction and Materials Manual (CMM 4-25-70). Probing involves 10 placing a base plate in a selected location and securing it with an anchoring spike. A probing rod 11 with a top plate assembly attached is then inserted into the concrete, perpendicular to the 12 pavement surface, until the rod strikes the base plate. The top plate is then slid down the rod 13 until it makes contact with the pavement surface and locked in place. The probing device is then 14 retracted and the distance between the underside of the top plate and the end of the probing rod is 15 measured to the nearest 1/8 inch. 16

The probing rod is a non-flexing rod with a minimum diameter of 3/8 inch and sufficient 17 length to completely penetrate the pavement. The top plate can be circular or square with a 18 minimum area of 16 square inches. The top plate must be at least 1/16 inch thick and 19 sufficiently ridged to maintain a surface planeness of at least 1/8 inch across the widest 20 dimension intended to be in contact with the concrete pavement surface. The base plate can be 21



circular or square with a minimum area of 80 square inches and must be of such rigidity, when in 1 place, to allow for the probing rod to be pushed against it without flexing [2]. 2

Probing is also used by Texas DOT (TXDOT), under specification Tex-423-A., where 3 they used a rigid straight steel rod of about 5/8 inch diameter and at least 6 inches longer than the 4 pavement thickness to take the measurements. The rod is inserted into the concrete, removed, 5 and the full depth of the pavement is measured using a tape measure readable to 1/16th of an 6 inch. TXDOT takes the average of three readings obtained from points located at one, half and 7 three quarters across the width of the pavement. Texas procedures require measuring to the 8 nearest 1/16th instead of the 1/8th inch used by WisDOT. [3&7] 9

WisDOT utilizes contractors’ QC probing measurements of the freshly placed concrete as 10 the primary method for determining thickness. Two probes are required for each paving unit 11 which is defined as being one lane wide and 250 feet long. Probes are taken at randomly 12 selected longitudinal points within the section and at pre-selected transverse locations as agreed 13 on by the engineer and contractor. For each ½ day of paving at least one verification test is done 14 by the engineer. This involves observing the contractor’s probing operation and verifying the 15 measurement taken by the contractor. 16

The switch to the use of probes rather than cores was largely based upon the results of a 17 1998 study conducted by WisDOT. In this study they compared the results of both methods on 18 eight construction projects. WisDOT concluded that probing could be considered an acceptable 19 method of measuring PCC pavement thickness and recommended that a standard methodology 20 for conducting probing be developed. Figure 2 shows the probe measurement method and 21 Figure 3 shows the probe measurement as it is done in field by WisDOT. 22

23

24 25

26 Figure 3 Illustrated Probe Measurement Method (Allison, 2008) 27

28



1 2

Figure 4 Probing as Done in the Field (Courtesy of WisDOT, 2008) 3

PROBLEM STATEMENT 4 Coring is a destructive technique that can only be performed after the concrete has hardened. 5 Therefore, the time lag between pavement constructions and getting the test results is 6 problematic. It also requires expensive coring equipment and must be operated by trained 7 personnel. Additional costs are incurred as refilling the holes requires extra labor hours. 8 Furthermore, due to the cost and destructive nature of coring, only a limited number of cores can 9 be done. Thus conventional coring represents a very limited sample size upon which major 10 decisions are made regarding acceptability and payment for the as-constructed pavement. 11 Improvements to these methods of assessing in-place PCC pavement properties would benefit 12 both contractors and SHA [3]. 13

Probing is a non-destructive method that requires no extractions from the pavement. The 14 probe measurements taken during construction provide immediate feedback to field engineers 15 and contractors so they can make real time adjustments of the paving operation to control 16 thickness. Probing saves cost by eliminating cores and the refilling of core holes; at the same 17 time it requires less contract administration resources for the SHA. WisDOT requested that the 18 Construction and Materials Support Center (CMSC) at University of Wisconsin-Madison 19 conduct a follow up study to WisDOT’s original 1998 study to determine if probing still 20 provided an acceptable method for measuring PCC pavement thickness. 21



SCOPE 1 The intent of this study was to determine if the contractor’s quality control data (probe 2 measurements) provided comparable results to cores obtained by WisDOT. The project scope 3 was limited to comparing PCC pavement thickness measured in the field by the paving 4 contractor at the time of construction to the core depths taken and measured by WisDOT. 5 Coring and probing data from twelve projects were included in the study. The projects studied 6 included urban and rural paving projects constructed in 2006 and 2008, while the coring data was 7 obtained in 2007 and 2008. All the study projects were constructed with the PCC pavement 8 placed directly on a compacted granular base course. 9 10 11 PREVIOUS WISDOT STUDY 12 In January 1998, WisDOT prepared an unpublished internal report titled “Alternative Methods 13 for Determining PCCP Thickness.” The study statistically compared contractor’s probe 14 measurements to WisDOT core measurements for eight projects. WisDOT concluded that 15 “probing can give an acceptable estimate of pavement thickness; and based on the analysis of 16 variance; WisDOT coring cannot be directly replaced with probing in the current specifications” 17 [6]. The findings also recommended that probing be considered an acceptable method for 18 measuring PCC pavement thickness and that a standard methodology for conducting probing be 19 developed and incorporated in a new acceptance specification [6]. In summary, the probe data 20 according to this research is statistically similar to the core data at a confidence level of 95%, 21 except for variances which could be due to the differences in number of samples or human error 22 in the measuring process. 23

DATA DESCRIPTION 24 The goal of this research was to correlate the probe measurements taken by the contractor and 25 core measurements taken by WisDOT. The probe data was obtained from WisDOT project 26 construction records or from the contractors’ files when the data had not been retained by 27 WisDOT. The core data was provided by WisDOT from coring they did with their equipment 28 and staff. Core locations were randomly determined for each of the projects. The twelve 29 projects selected by WisDOT for the study are shown in Table 1. The size of projects ranged 30 from 25,350 to 327,277 square yards of pavement and plan thicknesses varied from 8 to 10 ½ 31 inches. The projects were constructed by five different paving contractors, with one contractor 32 building six of the projects, another contractor constructing three projects, and three contractors 33 building one project each. 34

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TABLE 1 List of Study Projects 1 WisDOT Contract ID

Project Description

Year of Construct-ion

Plan Thickness (Inches)

Pavement Area (SY)

Contractor

20070213045 STH 95, Jackson Co. 2007 8.00 54,748 A

20060613038 S. Cray Sr. BLVD, Chippewa Co.

2006 9.00 44,653 D

20060411001 USH 151, Columbia Co.

2006 10.50 68,083 B

20060711005 USH 14, Lacrosse-Vernon Co.

2006 8.50 155,500 E

20060214019 STH 31, Racine Co. 2006 8.50 55,600 A 20060314045 N 91st St., Milwaukee

Co. 2006 8.50 25,350 A

20060314016 STH 100, Milwaukee Co.

2006 9.00 125,640 B

20060411013 S 11th St., Milwaukee Co.

2006 8.00 32,500 C

20070213022 STH 60, Washington Co.

2007 10.50 28,330 A

20060214015 STH 190 , Milwaukee Co.

2006 8.50 65,400 A

20071211021 USH 57 NB, Door Co. 2008 9.00 327,277 A

20070814008 USH 45, Winnebago

Co.

2008 9.00 77,700 B

2 3 METHODOLOGY 4 A statistical analysis was done on each of the projects to determine if the core and probe 5 measurements yielded the same results for each project. The mean and variance for each project 6 (core and probe) were determined and compared. In all cases there were more probe 7 measurements (np) taken than cores (nc). This is understandable given the time and expense of 8 coring done after construction, as compared to probing which is done during construction. 9

Test Statistics Used 10 The F-test, T-test, Skewness test, and Kurtosis tests were all run on both the probe and core data 11 sets for each project. The confidence level chosen for this study was 95%, since it is an 12 assumption often used in statistical analysis. A brief description of each statistical test is 13 provided. 14 15 F-test Statistics 16 This test is used to determine whether the two population variances are equal. This is done by 17 comparing the ratio of two variances. The null hypothesis test is that the two independent 18



samples come from normal distributions with the same variance, while the alternative hypothesis 1 is that they come from normal distributions with different variances. Thus the null hypothesis is 2 true if the ratio of the variances is 1(the variances are equal). 3

4 T-test Statistics 5 This test provides a statistical approach for indicating the confidence that can be placed in 6 conclusions drawn from relatively small numbers of samples of the population. For this study, 7 the sample sizes were quite large, meaning the t-test is a very rigorous standard for evaluating 8 the confidence levels used to compare the two population means. It also provides useful 9 information in comparing and predicting the difference between two population means, and it 10 was done for all the projects. Such hypothesis testing is an essential part of statistical inference 11 and was utilized to evaluate whether the probe and core data gave comparable results. The null 12 hypothesis used in testing the means was “the mean of the contractor’s probe measurements is 13 equal to the mean of the WisDOT core measurements” and the alternative hypothesis was then 14 “the mean of the contractor’s probe is not equal to the mean of the WisDOT core.” In other 15 words, the null hypothesis was µp - µc = 0 and the alternative hypothesis was µp - µc ≠ 0. 16

One significant aspect for understanding this data is the frequency distribution, a method 17 for condensing and summarizing data [4]. Of paramount interest is the position of the mean and 18 the spread or dispersion of the observations about the mean. 19

20 Skewness test 21 The goal of the skewness test is to determine the normality or lopsidedness of the sample 22 distribution [5]. The skewness is a measure of symmetry, or the extent to which the 23 observations group themselves more on one side of the central value (i.e. mean) than the other. 24 The coefficient of skewness is used to measure the lopsidedness of a sample frequency 25 distribution. According to statistical principles, skewness is a pure number and may be either 26 positive or negative [5]. For symmetrical distributions, the coefficient of skewness is zero. For 27 non-symmetrical distributions, the coefficient of skewness is negative if the long tail of the 28 distribution extends to the left (toward the smaller values on the scale of measurement) and 29 positive in the opposite case. 30 31 Kurtosis test 32 The measure of kurtosis relates to the tendency of a distribution to have a sharp peak in the 33 middle and excessive frequencies on the tails, as compared with a normal distribution, which is 34 relatively flat in the middle with little or no tails [5]. The peakedness and tail excess of a sample 35 frequency distribution is generally measured by the coefficient of kurtosis. 36

37



RESULTS 1 The initial evaluation consisted of looking at the means (µ) of both the core and probe data sets. 2 The result of that analysis is shown in Table 2. 3

In all cases the mean of the probe data was greater than the plan thickness while the mean 4 of the cores exceeded plan thickness on ten of the twelve projects. A comparison of the means 5 of the core and mean of the probes show close agreement for most of projects, the largest 6 differences coming from two projects where the probe depths exceeded the plan thickness and 7 the cores were less than the plan thickness. Of the twelve projects, eleven had a difference 8 between means of less than 1/4 inch and eight of the projects had differences less than 1/8 inch. 9 While a simple comparison of the means shows little noticeable difference, this approach can be 10 misleading because the data is based upon two different measuring techniques with differing 11 numbers of measurements taken at different locations. Statistical hypothesis testing allows us to 12 judge if the means of two sample populations are equal. 13

14 TABLE 2 Mean, Standard Deviation, Variance and Sample Size 15

SUMMARY OF WISDOT PROJECTS

Probe Core

Project

Plan Depth (Inches)

Mean (µp)

Std. Dev. (σp)

Var. ( )

Sample Size (np)

Mean (µc)

Std. Dev. (σc)

Var. ( )

Sample Size (nc)

µp - µc (Inches)

STH 95 , Jackson Co.

8.00 8.07 0.18 0.03 1174 8.12 0.22 0.05 102 -0.05

S. Cray Sr. BLVD, Chippewa Co.

9.00 9.38 0.35 0.12 176 9.31 0.40 0.16 27 0.07

USH 151 , Columbia Co.

10.50 10.54 0.42 0.18 68 10.32 0.41 0.17 48 0.22

USH 14, Lacrosse- Vernon Co.

8.50 8.81 0.33 0.11 674 8.68 0.35 0.12 71 0.13

STH 31, Racine Co.

8.50 8.60 0.27 0.07 264 8.31 0.40 0.16 12 0.29

N 91st St. , Milwaukee Co.

8.50 8.63 0.28 0.08 80 8.66 0.61 0.37 12 -0.03

STH 100 , Milwaukee Co.

9.00 9.13 0.31 0.09 309 9.18 0.19 0.04 24 -0.05

S 11th St , Milwaukee Co.

8.00 8.41 0.22 0.05 164 8.43 0.52 0.27 14 -0.02

STH 60, Washington

10.50 10.55 0.20 0.04 60 10.61 0.23 0.05 20 -0.06



Co. STH 190 , Milwaukee Co.

8.50 8.66 0.63 0.40 233 8.84 0.48 0.23 32 -0.18

USH 57 NB,

Door Co. 9.00 9.17 0.22 0.05 225 9.28 0.38 0.15 109 -0.11

USH 45,

Winnebago

Co.

9.00 9.12 0.21 0.04 72 9.05 0.44 0.19 28 0.07

1 The simple measurement of variance shows that the results obtained from cores and 2

probes are very close. By applying the f-test we can determine whether the t-test should be 3 performed assuming equal or unequal variances. The results of the f test are shown in Table 3. 4 5 TABLE 3 F-test results 6

Project

Variance Probe ( )

Variance Core ( )

F-test

Agree with null

hypothesis “equal variances”

STH 95 , Jackson Co. 0.03 0.05 0.0093 NO Seymour Cray Sr. BLVD, Chippewa Co. 0.12 0.16 0.2998 YES USH 151 , Columbia Co. 0.18 0.17 0.8834 YES USH 14, Lacrosse-Vernon Co. 0.11 0.12 0.5959 YES STH 31, Racine Co. 0.07 0.16 0.0277 NO N 91st St. , Milwaukee Co. 0.08 0.37 2.86e-5 NO STH 100 , Milwaukee Co. 0.09 0.04 0.0112 NO S 11th St , Milwaukee Co. 0.05 0.27 1.01e-7 NO STH 60, Washington Co. 0.04 0.05 0.3297 YES STH 190 , Milwaukee Co. 0.12 0.23 0.0567 YES USH 57 NB, Door Co. 0.05 0.15 0.2783 YES USH 45, Winnebago Co. 0.04 0.19 8.55e-12 NO

7 Six of the projects reject the hypothesis of equal variances in favor of the hypothesis of 8

unequal variances. Based upon those results, the research team decided to perform the t test 9 assuming unequal variances for the two sample populations. The two sided t-test is used to 10 compare cores and probes based upon a chosen confidence interval. The confidence intervals 11 calculated for a 95% interval level are shown in Table 4. If the p-value falls below 0.05, then the 12 test rejects the null hypothesis at a default significance level of 0.05. 13

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TABLE 4 Hypothesis Test - All Projects (Unequal Variance) 1 TWO SAMPLE T-TEST

(Assuming Unequal Variance) 95% CI

Project

Lower

Upper

p-value

Estimated Difference (inches)

(µp - µc)/ µc Percent

Agree with null hypothesis ("means are equal")

STH 95 , Jackson Co. -0.082 -0.007 0.019 -0.0558 -0.59% NO S. Cray Sr. BLVD, Chippewa Co. -0.079 0.254 0.295 0.0728 0.78% YES USH 151 , Columbia Co. 0.069 0.382 0.005 0.2091 1.99 % NO USH 14, Lacrosse-Vernon Co.

0.043 0.225 0.004 0.1342 1.54% NO STH 31, Racine Co. 0.044 0.553 0.026 0.2984 3.59% NO N 91st St. , Milwaukee Co. -0.419 0.360 0.87 -0.0296 -0.34% YES STH 100 , Milwaukee Co. -0.134 0.042 0.295 -0.0458 -0.50 % YES S 11th St , Milwaukee Co.

-0.321 0.283 0.895 -0.0188 -0.22% YES STH 60, Washington Co.

-0.179 0.059 0.311 -0.0642 -0.61% YES STH 190, Milwaukee Co.

-0.366 0.011 0.065 -0.1524 -1.75% YES USH 57 NB, Door Co. -0.036 -0.191 0.005 -0.1134 -2.4 % NO USH 45, Winnebago Co. -0.105 0.246 0.420 0.0705 0.77 % YES

2 Table 4 shows that with the assumption of unequal variances, 7 of the 12 projects met the 3

test for equivalent measures. In the 1998 WisDOT study which compared probe data to core 4 data for 8 paving projects, WisDOT assumed a 95% confidence level with equal variances and 5 only 2 of the 8 projects met the test for equivalency. 6

The most important outcome from using the hypothesis test in this study was the 7 consideration of the predicted differences in the means based upon the statistical modeling, and 8 shown in the fifth column of Table 4. The predicted difference was significant in this study 9 because of the large number of measurements taken for the probes and the cores. The difference 10 between the probe and the core was predicted to be less than 1/4 inch for 11 of the 12 projects 11 and the highest percentage difference was 3.59%. These results show that the two data sets, 12 probes and cores, compare very closely to each other. 13

The skewness test measures symmetry, while the kurtosis test measures peakedness and 14 tail excess of a sample frequency distribution. According to statistical principles, skewness is a 15 pure number and may be either positive or negative. Skewness value is zero for symmetrical 16 distribution, and for non-symmetrical distribution, it is either negative if the long tail of the 17 distribution extends to the (toward the smaller values on the scale of measurement) or positive if 18



otherwise. Kurtosis is a dimensionless number and may be either positive or negative [6]. A 1 positive value of kurtosis is said to be leptokurtic, a negative value is said to be platykurtic, and a 2 value of zero is said to be mesokurtic. The contractor’s probe values of skewness ranged from 3 0.37 to 2.71 with an average of 1.20 while WisDOT’s core values on the other hand ranged from 4 -0.41 to 2.48 with an average of 0.35. the contractor’s values are more lopsided than the 5 WisDOT values in that, even though the long tails of both distributions extend to the right, the 6 WisDOT distribution is more normal (closer to the value of zero-symmetry) than the contractor’s 7 values. The contractor’s probe values exhibited kurtosis values ranging from 0.69 to 14.36 with 8 an average of 3.77 while WisDOT’s core values exhibited kurtosis values ranging from -1.46 to 9 6.85 with an average of 1.02. Neither data set exhibited a mesokurtic tendency overall. However, 10 the WisDOT values show a tendency to be slightly more normal than the contractor’s values. 11 90% of the normality test results fail to reject the hypothesis that they follow a normal 12 distribution, whereas 80% of the normality of the probe results were rejected in favor of the 13 hypothesis that they did not follow a normal distribution. The kurtosis and skewness test results 14 validate the assumption to use the t-test and the unusually high values observed for some of the 15 projects need to be studied further to determine why they occur and if they offer any insights 16 regarding construction control processes. 17

STATISTICAL QUALITY CONTROL 18 In October 1979, the U.S. Department of Transportation Federal Highway Administration 19 (FHA), in conjunction with The Sigma Partnership, published a course notebook called Practical 20 Applications of Statistical Quality Control in Highway Construction, in order to assist engineers 21 and technicians in highway construction [4]. The FHA notebook discusses the control chart 22 technique, which is of primary interest to the contractor and should be of interest to the agency 23 inspection team as well. It identifies two causes of variability: the “chance cause” inherent in any 24 particular method of production and inspection, and the “system of assignable causes” which can 25 be controlled or removed. In order to remove an assignable cause, one must first establish that it 26 is working on the system, using the Statistical Control Chart to determine whether there is a lack 27 of control in the process. 28

WisDOT utilizes quality control charts to insure construction processes are controlled 29 and the materials being supplied by contractors meet specifications for many of the products 30 used in construction. This technique, however, is not used in monitoring the thickness of PCC 31 pavement being constructed. There are many benefits in using a statistical control chart, 32 including: 33 early detection of process trouble before rejections occur 34 decrease in product variability 35 establishment of process capabilities 36 provision of a rational basis for altering specification requirements 37 sense of “quality awareness” in the construction team 38

As an adjunct to the data analysis, statistical quality control charts were created for the 39 probe data to determine if the construction processes on the study projects were under control. 40 For this analysis, the Average, or X-bar Chart, was used. Since WisDOT has no established 41 criteria for thickness greater than plan depth, the upper control limits (UCL) and lower control 42 limits (LCL) were calculated using statistical methods. The target value was the plan thickness. 43 The UCL and LCL levels were calculated using statistical methods. An example of the resulting 44 statistical control chart for the USH 151, Columbia Co. project is shown in Figure 5. 45



To judge whether the paving process was under control, the “theory of runs” was utilized. 1 This theory states that if there are seven consecutive points on the same side of the center (target 2 value) line, there is a lack of control. All twelve projects passed this criterion and were judged 3 to have been in control. This may explain the close agreement between the probe and core 4 measurements. 5 6 7

1715131197531

11.4

11.2

11.0

10.8

10.6

10.4

10.2

10.0

Sample

Sam

ple

Mea

n

__X=10.526

UCL=11.070

LCL=9.981

1

Xbar Chart of Probe

8 9

FIGURE 5 Quality Control X-bar Chart for USH 151, Columbia Co. 10 11 CONCLUSIONS 12 A total of twelve projects were analyzed to determine if probing fresh concrete was a valid 13 quality assurance method for determining pavement thickness as compared to coring hardened 14 concrete. Sample means were compared, statistically calculated estimates of differences were 15 examined, and a statistical analysis at the 95% confidence interval was carried out. Based upon 16 these analyses, probing provides an acceptable measure of pavement thickness. 17

Use of statistical quality control charts has many benefits, and SHAs should consider 18 adopting them for use on future PCC paving projects. It would give both contractors and owner 19 agency personnel a tool for determining if the paving process is under control and would provide 20 further assurances that probe measurements are providing adequate results. The upper control 21 limits (UPC) and lower control limits (LCL) should be established based upon contract 22 requirements. 23

24



LIST OF REFERENCES 1 1. Allison, G.W. Evaluations of Probing Measurement for Determination of Portland 2

Cement Concrete Pavement Thickness. Master of Science Thesis, University of 3 Wisconsin-Madison, Madison, Wisconsin, May 2008. 4

2. State of Wisconsin Department of Transportation Standard Specifications for 5 Highway and Structure Construction, 2008. 6

3. Nazarian, S., et al., Acceptance Criteria of Airfield Concrete Pavement Using Seismic 7 and Maturity Concepts, Programs Management Office, Skokie, IL, USA, May 2006, 8 IPRF REPORT 01-G-002-02-2. 9

4. The Sigma Partnership, Practical Applications of Statistical Quality Control in 10 Highway Construction, Course Notebook, FHA, US Department Of Transportation 11 1979. 12

5. American Society for Testing and Materials (ASTM), Manual on Presentation of 13 Control Chart Analysis, 6th Edition, 1916 Race Street, Philadelphia, PA 19103, 1992 14

6. Wisconsin Department of Transportation, Methods for Determining PCCP Thickness, 15 Report # WI/SPR-11-97 Alternative, 3502 Kinsman Blvd., Madison, WI 53704, 16 January 8, 1998 17

7. Texas Department of Transportation, Determining Pavement Thickness by Direct 18 Measurements, TXDOT DESIGNATION: TEX-423-A, 2008 19

8. American Association of State Highway and Transportation Officials (AASHTO), 20 Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, T 24M/T 24, 21 2007 22

9. Crawford, Gary L., et al., On the Road Testing Roads, Vol. 66 · No. 1, FHA, United 23 States Department of Transportation, http://www.tfhrc.gov/pubrds/02jul/04.htm, 24 Accessed April 2008. 25


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