Evaluation and Analysis of LTPP Pavement Layer Thickness Data

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The main purpose of this chapter is to characterize the extent of differences in the layer thickness data between as-designed and as-constructed (measured) thicknesses for the newly constructed SPS layers. Only these new SPS layers have design thicknesses accurately documented.

Data sources for the analysis are discussed first, followed by an overview of as-designed thicknesses for the newly constructed SPS layers. After that, typical thickness deviations from the target thicknesses are summarized, as well as their distribution types. Finally, the results of the statistical analysis are presented.

Data Sources

Two thickness data sources with multiple measurements on a given layer exist in the LTPP database:

• Elevation measurements in SPS*_LAYER_THICKNESS tables for experiments SPS-1, SPS-2, SPS-5, SPS6, SPS-7, and SPS-8
• Pavement core measurements in testing tables TST_AC01_LAYER and TST_PC06.

According to the SPS construction guidelines [35-40], rod and level survey measurements are to be taken at a minimum of five offset locations (edge, outer wheel path, midlane, inner wheel path, and inside edge of lane) at longitudinal intervals no greater than 15 m (50 ft). Typically, 55 elevation measurements are available for each regular SPS test section.

The number of cores taken at each section depends on experiment and layer type and is defined in the corresponding Sampling and Testing Guide [6-11]. The number of cores per section ranges between 1 and 9.

All sections with available thickness data in either one of these tables are studied to quantify as-designed versus as-constructed variations in layer thickness.

For the section/layer combination, an analysis cell is defined to represent a specific layer in a test section, for which the target thickness was documented. The following fields from TST_L05B or EXPERIMENT_SECTION table in LTPP database along with the design target layer thickness define a unique analysis cell:

• EXPERIMENT_NO (Experiment number).
• LAYER_TYPE (Layer type).
• MATL_CODE (Material type description)
• Target layer thickness.

Design Thicknesses

For newly constructed SPS layers, the design thicknesses are defined in the corresponding SPS Experimental Designs [12-17]. The design thicknesses are available for the following layer types:

• SB - AC surface and binder thickness (SPS-1, SPS-5, SPS-6, SPS-8).
• DGATB - Dense-graded asphalt-treated base (SPS-1).
• PATB - Permeable asphalt-treated base (SPS1, SPS-2).
• PCC - Portland cement concrete (SPS-2, SPS-7, SPS-8).
• LC - Lean concrete (SPS-2).
• DGAB - Dense-graded aggregate base (SPS-1, SPS-2, SPS-8).

The design thicknesses for all these SPS experiments and layer types are presented in tables 38 through 43.

Table 38. Design layer thicknesses for the SPS-1 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	DGAB	PATB	DGATB	SB
0101	203 (8)			178 (7)
0102	305 (12)			102 (4)
0103			203 (8)	102 (4)
0104			305 (12)	178 (7)
0105	102 (4)		102 (4)	102 (4)
0106	102 (4)		203 (8)	178 (7)
0107	102 (4)	102 (4)		102 (4)
0108	203 (8)	102 (4)		178 (7)
0109	305 (12)	102 (4)		178 (7)
0110		102 (4)	102 (4)	178 (7)
0111		102 (4)	203 (8)	102 (4)
0112		102 (4)	305 (12)	102 (4)
0113	203 (8)			102 (4)
0114	305 (12)			178 (7)
0115			203 (8)	178 (7)
0116			305 (12)	102 (4)
0117	102 (4)		102 (4)	178 (7)
0118	102 (4)		203 (8)	102 (4)
0119	102 (4)	102 (4)		178 (7)
0120	203 (8)	102 (4)		102 (4)
0121	305 (12)	102 (4)		102 (4)
0122		102 (4)	102 (4)	102 (4)
0123		102 (4)	203 (8)	178 (7)
0124		102 (4)	305 (12)	178 (7)

Table 39. Design layer thicknesses for the SPS-2 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	DGAB	PATB	LC	PCC
0201	152 (6)			203 (8)
0202	152 (6)			203 (8)
0203	152 (6)			279 (11)
0204	152 (6)			279 (11)
0205			152 (6)	203 (8)
0206			152 (6)	203 (8)
0207			152 (6)	279 (11)
0208			152 (6)	279 (11)
0209	102 (4)	102 (4)		203 (8)
0210	102 (4)	102 (4)		203 (8)
0211	102 (4)	102 (4)		279 (11)
0212	102 (4)	102 (4)		279 (11)
0213	152 (6)			203 (8)
0214	152 (6)			203 (8)
0215	152 (6)			279 (11)
0216	152 (6)			279 (11)
0217			152 (6)	203 (8)
0218			152 (6)	203 (8)
0219			152 (6)	279 (11)
0220			152 (6)	279 (11)
0221	102 (4)	102 (4)		203 (8)
0222	102 (4)	102 (4)		203 (8)
0223	102 (4)	102 (4)		279 (11)
0224	102 (4)	102 (4)		279 (11)

Table 40. Design layer thicknesses for the SPS-5 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	SB
0501	0
0502	51 (2)
0503	127 (5)
0504	127 (5)
0505	51 (2)
0506	51 (2)
0507	127 (5)
0508	127 (5)
0509	51 (2)

Table 41. Design layer thicknesses for the SPS-6 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	SB
0601	0
0602	0
0603	102 (4)
0604	102 (4)
0605	0
0606	102 (4)
0607	102 (4)
0608	203 (8)

Table 42. Design layer thicknesses for the SPS-7 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	PCC
0701	0
0702	76 (3)
0703	76 (3)
0704	76 (3)
0705	76 (3)
0706	127 (5)
0707	127 (5)
0708	127 (5)
0709	127 (5)

Table 43. Design layer thicknesses for the SPS-8 experiment.

SHRP_ID	Design Layer Thickness, mm (in)
	DGAB	PCC	SB
0801	203 (8)		102 (4)
0802	305 (12)		178 (7)
0803	203 (8)		102 (4)
0804	305 (12)		178 (7)
0805	203 (8)		102 (4)
0806	305 (12)		179 (7)
0807	152 (6)	203 (8)
0808	152 (6)	279 (11)
0809	152 (6)	203 (8)
0810	152 (6)	279 (11)
0811	152 (6)	203 (8)
0812	152 (6)	279 (11)

Study Methodology

For both the elevation and core as-constructed thickness measurements, typical mean layer thickness deviations are established by the following:

• Descriptive summary statistics of the average thicknesses deviations between as-designed and as-constructed values for the layers with the same layer material type and same design thickness.
• Kurtosis and skewness tests of the distribution of the mean thicknesses for the layers with the same layer material type and the same design thickness.
• Two types of comparisons are made in relation to their as-designed thicknesses or target values:
• Evaluation of the percent of the individual measurements that are either within or outside specific values from the target thickness.
• Statistical analysis of the measured mean thickness values versus the designed values.

Descriptive Summary Statistics of the Thickness Deviations

The mean thickness difference between as-designed and as-constructed thicknesses was computed for each layer using both core and elevation thickness measurements.

The following statistical indicators were computed:

• Total number of sections or layers.
• Mean thickness deviation.
• Minimum thickness deviation.
• Maximum thickness deviation.
• Standard deviation of thickness deviation.
• COV of thickness deviation.

The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements.

Layer Thickness Deviation Distribution Type

Mean thickness deviations from layers or sections were analyzed to determine whether they follow typical statistical distributions. Skewness and kurtosis analyses were conducted for this purpose, using the methodology outlined in chapter 5.

Percentage Distribution of the Individual Measurements

To evaluate the variation between as-designed and as-constructed thicknesses, deviations of the individual measurements in relation to the target values are computed for each analysis cell. These deviations are then summarized into three deviation levels: 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in), for different material types and target thickness values.

This evaluation provides information regarding variations between as-constructed and as-designed thicknesses at individual measurement level.

Statistical Analysis of Sample Measurement Means

Statistical analysis is performed to evaluate variations for each analysis cell. The goal of statistical analysis is to assess deviation of the measurement population means from the target thicknesses. Two types of the thickness comparison are performed for both data sources:

Two-sided t-tests with 95 percent confidence level for each section and layer, to determine whether the differences between as-designed and as-constructed thicknesses are significant.

The null hypothesis for this test is that average of core or elevation thickness data is equal to the target thickness, i.e.: If the null hypothesis is rejected (i.e., the result of the two-sided t-test is significant), then the measured mean thickness is different from the design thickness at the 95 percent confidence level. On the other hand, if the null hypothesis is not rejected or the test result is not significant, then there is no evidence that the measured mean thickness is different from the design value.

One-sided t-tests with 95 percent confidence level for the difference between as-designed thickness and the mean as-constructed thickness and for tolerance level of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in). The null hypothesis is that the absolute value of the difference between the mean and target thickness is less than or equal to the tolerance level with the alternative hypothesis being that the absolute value of the difference is greater than the tolerance level. For example, for elevation data, for allowance of 6.35 mm (0.25 in), the null and alternative hypotheses are:

If the null hypothesis is rejected (i.e., the result the one-sided t-test is significant), then the measured mean thickness deviates from the design thickness by more than the specified allowance (in this example 6.35 mm) at a 95 percent confidence level. On the other hand, if the null hypothesis is not rejected or the test result is not significant, then there is no evidence that the measured mean thickness deviates from the designed value by more than the specified allowance value, in other words, that the mean thickness is within the allowance value (in this case 6.35 mm) from the designed thickness.

Typical Deviations between Mean Measured and the Design Thicknesses

Descriptive Summary Statistics

Mean layer thickness data for SPS experimental sections with newly constructed layers were obtained from the TST_AC01_LAYER and TST_PC06 tables (core thickness), and from the SPS*_LAYER_THICKNESS tables (elevation thickness), to compute measured thickness deviation from the design value. The analysis was done for the sets of data grouped by target design thickness, material, and layer type. The following statistical indicators were computed:

• Total number of sections or layers
• Mean thickness deviation
• Minimum thickness deviation
• Maximum thickness deviation
• Standard deviation of thickness deviation
• COV of thickness deviation

The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements. Table 44 summarizes layer thickness deviations by different layer and material types based on analysis of elevation measurements. Table 45 summarizes mean core examination layer thickness deviations from their designed values by different layer and material types.

Figures 47 through 61 present the frequency distributions of the thickness deviations for different layer types and target thicknesses for both core and elevation thickness measurements.

The following observations are made based on these summary statistics:

• The computed description statistics using elevation measurement data are different from those using core examination data. However, based on statistical analyses, the differences in the mean layer thicknesses and standard deviations at the section or layer level are not significant for a majority of the layers.
• The mean constructed layer thicknesses for PCC layers and lean concrete base layers are generally above the designed values.
• For the same layer and material type, the mean constructed layer thicknesses tend to be above the designed value for the thinner layers, and below the design value for the thicker layers.

Table 44. Summary of differences between mean elevation thickness measurements and target thicknesses.

Mat. Type	Target Thickness		Total Number of Sections	Mean Difference		Standard Deviation		Min. Difference		Max. Difference
	mm	in		mm	in	mm	in	mm	in	mm	in
DGAB	102	4	84	0.4	0.01	10.3	0.40	-28.6	-1.13	33.4	1.32
	152	6	55	-1.2	-0.05	14.4	0.57	-51.5	-2.03	38.2	1.51
	203	8	40	0.9	0.04	12.7	0.50	-26.8	-1.05	45.2	1.78
	305	12	40	-6.0	-0.24	30.0	1.18	-173.3	-6.82	34.9	1.37
DGATB	102	4	27	1.8	0.07	8.0	0.31	-12.0	-0.47	21.1	0.83
	203	8	42	0.5	0.02	16.3	0.64	-62.5	-2.46	28.9	1.14
	305	12	28	-2.1	-0.08	15.9	0.63	-35.1	-1.38	38.1	1.50
LC	152	6	48	5.5	0.22	10.6	0.42	-25.8	-1.02	36.9	1.45
PATB	102	4	129	1.2	0.05	10.5	0.41	-17.1	-0.67	41.9	1.65
PCC	76	3	12	18.2	0.72	11.5	0.45	3.4	0.13	42.6	1.68
	127	5	12	16.5	0.65	11.6	0.46	5.1	0.20	39.0	1.53
	203	8	76	5.4	0.21	12.2	0.48	-32.6	-1.28	53.3	2.10
	279	11	77	4.7	0.18	11.0	0.43	-24.8	-0.98	39.0	1.54
SB	51	2	46	4.8	0.19	19.9	0.78	-27.8	-1.10	67.9	2.67
	102	4	125	-2.2	-0.09	18.5	0.73	-58.9	-2.32	31.7	1.25
	127	5	46	-4.4	-0.17	20.1	0.79	-70.6	-2.78	38.3	1.51
	178	7	95	-8.2	-0.32	23.9	0.94	-73.3	-2.89	59.4	2.34
	203	8	7	-2.7	-0.11	22.9	0.90	-36.9	-1.45	36.3	1.43

Table 45. Summary of differences between mean core thickness measurements and target thicknesses.

Mat. Type	Target Thickness		Total Number of Sections	Mean Difference		Standard Deviation		Min. Difference		Max. Difference
	mm	in		mm	in	mm	in	mm	in	mm	in
DGATB	102	4	22	-0.9	-0.04	10.9	0.43	-22.9	-0.90	20.3	0.80
	203	8	34	1.1	0.04	21.5	0.85	-64.3	-2.53	38.1	1.50
	305	12	22	-5.4	-0.21	25.1	0.99	-88.9	-3.50	21.0	0.83
LC	152	6	36	8.2	0.32	12.6	0.50	-19.1	-0.75	38.9	1.53
PATB	102	4	32	-19.7	-0.78	39.4	1.55	-87.2	-3.43	113.5	4.47
PCC	76	3	10	20.3	0.80	10.7	0.42	5.9	0.23	35.9	1.41
	127	5	12	13.4	0.53	13.5	0.53	-9.9	-0.39	37.1	1.46
	203	8	71	9.8	0.39	14.0	0.55	-22.5	-0.89	52.3	2.06
	279	11	71	-0.7	-0.03	28.3	1.12	-94.7	-3.73	31.8	1.25
SB	51	2	45	16.2	0.64	21.4	0.84	-17.1	-0.68	59.7	2.35
	102	4	114	5.2	0.20	17.0	0.67	-63.5	-2.50	47.0	1.85
	127	5	47	9.1	0.36	23.6	0.93	-39.4	-1.55	73.2	2.88
	178	7	94	-4.3	-0.17	21.8	0.86	-96.5	-3.80	65.4	2.58
	203	8	6	-18.4	-0.73	51.6	2.03	-118.1	-4.65	16.5	0.65

These summary statistics for the differences between as-designed and mean as-constructed layer thicknesses can be used as benchmarks for use in pavement design reliability and other research studies.

Layer Thickness Deviation Distribution Type

Mean thickness deviations from layers or sections were analyzed to determine whether they follow typical statistical distributions. Skewness and kurtosis analyses were conducted for this purpose. The statistical test results are presented in table 46 for both the elevation and core mean layer thicknesses. Examples of the thickness deviation distributions are shown in figures 62 and 63.

Table 46. Distribution of the mean thickness deviations from the design thickness based on kurtosis and skewness tests.

Mat. Type	Target Thickness		Elevation Measurement Data		Core Examination Data
	mm	in	No. Layers	Distribution Type	No. Layers	Distribution Type
DGAB	102	4	84	Normal	No Data
	152	6	55	Wide spread and skewed left
	203	8	40	Wide spread and skewed right
	305	12	40	Wide spread and skewed left
DGATB	102	4	27	Normal	22	Normal
	203	8	42	Wide spread and skewed left	34	Normal
	305	12	28	Normal	22	Wide spread and skewed left
LC	152	6	48	Normal	36	Normal
PATB	102	4	129	Skewed right	32	Normal
PCC	76	3	12	Normal	10	Normal
	127	5	12	Normal	12	Normal
	203	8	76	Wide Spread	71	Normal
	279	11	77	Normal	71	Wide spread and skewed left
SB	51	2	46	Skewed right	45	Normal
	102	4	125	Skewed left	114	Wide spread and skewed left
	127	5	46	Normal	47	Normal
	178	7	95	Skewed left	94	Wide spread and skewed left
	203	8	7	Not enough data	6	Not enough data

As shown in table 46, there are some discrepancies between the distribution types drawn from elevation data and core data. For the layers with both elevation and core data, the distribution of the thickness deviation derived from the core data is normal for more layer type and design thicknesses than from the elevation data.

The conclusions drawn from both the descriptive statistics and the kurtosis and skewness tests of their distribution types will be useful for pavement designers and researchers. They will be especially useful in reliability based mechanistic-empirical pavement performance analysis and design.

Figure 62: Chart. Example of normally distributed thickness deviations (elevation data, LC, target thickness 152 mm [6 in]).

Figure 63: Chart. Example of a skewed distribution for layer thickness deviation (core data, PCC, target thickness 279 mm [11 in]).

Statistical Analysis of Elevation Measurements

Analysis of the Percentage Distribution

The overall percentage distribution of elevation measurements as a function of the three tolerance levels is presented in table 47.

Table 47. Percentage distribution summary of the elevation thickness measurements.

Measured Layer Thickness, t	Difference Between As-Constructed and As-Designed Thickness
	Diff = 6.35 mm (0.25 in)		Diff = 12.7 mm (0.5 in)		Diff = 25.4 mm (1.0 in)
	Number of Measurements	Percent of Measurements	Number of Measurements	Percent of Measurements	Number of Measurements	Percent of Measurements
t < TV1 - Diff	15557	30.30	8481	16.52	3656	7.12
t within TV ± Diff	17788	34.65	32542	63.38	44324	86.33
t > TV + Diff	17996	35.05	10318	20.10	3361	6.55
Total	51341	100	51341	100	51341	100
Notes: 1Target value

The distribution of measurements by layer type for tolerance levels of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in) are presented in tables 48, 49, and 50, respectively.

Table 48. Percentage distribution of individual elevation measurements by layer type and design thickness for a tolerance level of 6.35 mm (0.25 in).

Layer Type	Target Thickness		Thickness <TV-6.35 mm (0.25 in)		Thickness Within TV ± 6.35 mm (0.25 in)		Thickness >TV+6.35 mm (0.25 in)		Total Number of Measurem.
	mm	in	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.
DGAB	102	4	1376	31.9	1686	39.0	1256	29.1	4318
	152	6	820	31.1	1046	39.7	772	29.3	2638
	203	8	675	32.0	679	32.2	756	35.8	2110
	305	12	809	37.8	722	33.7	609	28.5	2140
DGATB	102	4	370	25.9	597	41.8	461	32.3	1428
	203	8	700	30.3	700	30.3	907	39.3	2307
	305	12	570	37.6	499	32.9	446	29.4	1515
LC	152	6	342	13.9	1034	42.1	1082	44.0	2458
PATB	102	4	2059	30.6	2554	37.9	2124	31.5	6737
PCC	76	3	7	1.5	96	21.0	355	77.5	458
	127	5	10	2.2	85	18.5	365	79.3	460
	203	8	706	18.5	1296	33.9	1821	47.6	3823
	279	11	713	18.3	1460	37.5	1721	44.2	3894
SB	51	2	655	27.3	810	33.8	932	38.9	2397
	102	4	2286	33.9	2203	32.6	2259	33.5	6748
	127	5	1107	46.2	617	25.8	671	28.0	2395
	178	7	2201	42.9	1589	30.9	1345	26.2	5135
	203	8	151	39.7	115	30.3	114	30.0	380
Total			15557	30.3	17788	34.6	17996	35.1	51341

Table 49. Percentage distribution of individual elevation measurements by layer type and design thickness for a tolerance level of 12.7 mm (0.5 in).

Layer Type	Target Thickness		Thickness <TV-12.7 mm (0.5 in)		Thickness Within TV ± 12.7 mm (0.5 in)		Thickness >TV+12.7 mm (0.5 in)		Total Number of Measurem.
	mm	in	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.
DGAB	102	4	589	13.6	2990	69.2	739	17.1	4318
	152	6	447	16.9	1796	68.1	395	15.0	2638
	203	8	425	20.1	1284	60.9	401	19.0	2110
	305	12	560	26.2	1168	54.6	412	19.3	2140
DGATB	102	4	104	7.3	1087	76.1	237	16.6	1428
	203	8	384	16.6	1419	61.5	504	21.8	2307
	305	12	370	24.4	851	56.2	294	19.4	1515
LC	152	6	168	6.8	1661	67.6	629	25.6	2458
PATB	102	4	790	11.7	4774	70.9	1173	17.4	6737
PCC	76	3	2	0.4	159	34.7	297	64.8	458
	127	5	2	0.4	214	46.5	244	53.0	460
	203	8	323	8.4	2549	66.7	951	24.9	3823
	279	11	338	8.7	2745	70.5	811	20.8	3894
SB	51	2	374	15.6	1420	59.2	603	25.2	2397
	102	4	1360	20.2	4031	59.7	1357	20.1	6748
	127	5	747	31.2	1241	51.8	407	17.0	2395
	178	7	1380	26.9	2971	57.9	784	15.3	5135
	203	8	118	31.1	182	47.9	80	21.1	380
Total			8481	16.5	32542	63.4	10318	20.1	51341

Table 50. Percentage distribution of individual elevation measurements by layer type and design thickness for a tolerance level of 25.4 mm (1 in).

Layer Type	Target Thickness		Mean Thickness < TV-25.4 mm (1in)		Mean Thickness Within TV ± 25.4 mm (1in)		Mean Thickness >TV+25.4 mm (1in)		Total Number of Measurem.
	mm	in	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.	Number of Measurem.	Percent of Measurem.
DGAB	102	4	181	4.2	3910	90.6	227	5.3	4318
	152	6	187	7.1	2310	87.6	141	5.3	2638
	203	8	124	5.9	1807	85.6	179	8.5	2110
	305	12	260	12.1	1688	78.9	192	9.0	2140
DGATB	102	4	9	0.6	1403	98.2	16	1.1	1428
	203	8	134	5.8	2038	88.3	135	5.9	2307
	305	12	170	11.2	1249	82.4	96	6.3	1515
LC	152	6	33	1.3	2228	90.6	197	8.0	2458
PATB	102	4	108	1.6	6378	94.7	251	3.7	6737
PCC	76	3	0	0.0	308	67.2	150	32.8	458
	127	5	0	0.0	336	73.0	124	27.0	460
	203	8	64	1.7	3474	90.9	285	7.5	3823
	279	11	45	1.2	3593	92.3	256	6.6	3894
SB	51	2	72	3.0	1970	82.2	355	14.8	2397
	102	4	925	13.7	5512	81.7	311	4.6	6748
	127	5	298	12.4	1866	77.9	231	9.6	2395
	178	7	983	19.1	3987	77.6	165	3.2	5135
	203	8	63	16.6	267	70.3	50	13.2	380
Total			3656	7.1	44324	86.3	3361	6.5	51341

The graphical presentations of percentage distributions of elevation measurements are shown in figures 64, 65, and 66 for different tolerance levels.

The following conclusions may be drawn based on the percentage distributions of the elevation measurements:

• Overall, about 35 percent of the measurements are within ± 6.35 mm (0.25 in) of the target value, with about 30 percent lower than the target value and about 35 percent higher than the target value by more than 6.35 mm (0.25 in).
• Thickness measurements for asphalt concrete surface and binder layers and thin bonded PCC layers consistently show the highest deviations from the target values.
• The percentage of thickness measurements that is greater than the target value for jointed PCC and lean concrete base layers is significantly higher than the percentage of the measurements that are lower than the target value. Only 2 percent of thickness measurements are lower and almost 80 percent are higher than the target value by more than 6.35 mm (0.25 in) for thin PCC bonded layers (76-mm- [3-in-] and 127-mm- [5-in-] thick).
• Thickness measurements for PATB are more evenly distributed around the target value than the thickness measurements for other layer types.

Figure 64: Chart. Percentage distribution of the elevation measurements for a tolerance level of 6.35 mm (0.25 in) for different material types and design thicknesses.

Figure 65: Chart. Percentage distribution of the elevation measurements for a tolerance level of 12.7 mm (0.5 in) for different material types and design thicknesses.

Figure 66: Chart. Percentage distribution of the elevation measurements for a tolerance level of 25.4 mm (1 in) for different material types and design thicknesses.

Statistical Analysis of the Elevation Measurements

Two-sided t-test

After removing the outlying data points (as discussed in chapter 4), t-tests are performed to evaluate whether the mean constructed thicknesses are close to the designed thicknesses. Many of these tests are highly significant, meaning that the mean constructed thickness is significantly different from the designed thickness.

The following notes apply to tables 51 to 56 and tables 61 to 66:

• "Number of layers" is used to summarize number of layers (which can be different layer types and belong to the same or different sections) falling into certain tolerance range. This is normally an overall summary.
• "Number of sections" is used to summarize number of sections with the specified layer type and design thickness falling into certain tolerance range. This is used for summarizing results by layer type and design thickness.

Results of two-sided t-test with 95 percent confidence are presented in table 51. The results of the two-sided t-tests by layer material type and target thickness are given in table 52.

Table 51. Summary of the results of the two-sided t-tests (95 percent confidence level) using elevation measurements.

Mean Thickness	Number of Layers	Percentage of Layers
Significantly lower than the target value	357	36.10
No significant difference from the target value	196	19.82
Significantly higher than the target value	436	44.08
Total	989	100

The following observations are based on the results of the two-sided t-test for the elevation measurements:

• Overall, only about 20 percent of the layers had mean constructed thicknesses not significantly different from their target thicknesses.
• All 24 sections with 76-mm (3-in) or 123-mm (5-in) target thicknesses for bonded PCC overlays are constructed significantly thicker.
• For only 4 to 15 percent of the sections with SB layers and target thicknesses between 51 mm (2 in) and 178 mm (7 in), the as-constructed mean thickness is not significantly different from the as-designed thickness.
• The lowest deviations from as-designed thickness are observed for DGAB layers, for which more than 30 percent of sections have as-constructed mean thickness not significantly different from the target value.

Table 52. Results of the two-sided t-test for different material types (95 percent confidence level) by layer type and design thickness using elevation measurements.

Layer Type	Target Thickness		Significantly Lower than the Target Value		No Significant Difference		Significantly Higher than the Target Value		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGAB	102	4	28	33.3	27	32.1	29	34.5	84
	152	6	20	36.4	14	25.5	21	38.2	55
	203	8	12	30.0	15	37.5	13	32.5	40
	305	12	16	40.0	13	32.5	11	27.5	40
DGATB	102	4	10	37.0	5	18.5	12	44.4	27
	203	8	15	35.7	12	28.6	15	35.7	42
	305	12	14	50.0	3	10.7	11	39.3	28
LC	152	6	9	18.8	11	22.9	28	58.3	48
PATB	102	4	48	37.2	26	20.2	55	42.6	129
PCC	76	3	0	0.0	0	0.0	12	100.0	12
	127	5	0	0.0	0	0.0	12	100.0	12
	203	8	20	26.3	11	14.5	45	59.2	76
	279	11	16	20.8	21	27.3	40	51.9	77
SB	51	2	19	41.3	4	8.7	23	50.0	46
	102	4	50	40.0	16	12.8	59	47.2	125
	127	5	29	63.0	2	4.3	15	32.6	46
	178	7	48	50.5	14	14.7	33	34.7	95
	203	8	3	42.9	2	28.6	2	28.6	7
Total			357	36.1	196	19.8	436	44.1	989

One-sided t-test

Three one-sided t-tests with a confidence level of 95 percent were performed to evaluate whether the absolute differences between as-constructed and as-designed thicknesses are greater than 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in), respectively. The results of the overall analysis of all data points for all layers are presented in table 53.

The results of the analysis by layer material type for different tolerance levels are presented in tables 54 to 56.

Table 53. Summary of the results of one-sided t-tests using elevation measurements.

Level of Significance (TV - Target Value)	Difference Between the Mean As-Constructed and As-Designed Thickness
	6.35 mm (0.25 in)		12.7 mm (0.5 in)		25.4 mm (1.0 in)
	Number of Layers	Percent of Layers	Number of Layers	Percent of Layers	Number of Layers	Percent of Layers
Significantly lower than TV	181	18.3	102	10.3	50	5.1
No significant difference from the TV	562	56.8	760	76.8	908	91.8
Significantly higher than TV	246	26.9	127	12.8	31	3.1
Total	989	100	989	100	989	100

Table 54. Results of one-sided t-test for tolerance level of 6.35 mm (0.25 in) by layer type and design thickness using elevation measurements.

Layer Type	Target Thickness		Mean Thickness <TV-6.35 mm (0.25 in)		Mean Thickness Within TV ± 6.35 mm (0.25 in)		Mean Thickness >TV+6.35 mm (0.25 in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGAB	102	4	12	14.3	59	70.2	13	15.5	84
	152	6	9	16.4	40	72.7	6	10.9	55
	203	8	5	12.5	28	70.0	7	17.5	40
	305	12	9	22.5	25	62.5	6	15.0	40
DGATB	102	4	3	11.1	20	74.1	4	14.8	27
	203	8	8	19.0	22	52.4	12	28.6	42
	305	12	8	28.6	12	42.9	8	28.6	28
LC	152	6	3	6.3	28	58.3	17	35.4	48
PATB	102	4	21	16.3	81	62.8	27	20.9	129
PCC	76	3	0	0.0	2	16.7	10	83.3	12
	127	5	0	0.0	3	25.0	9	75.0	12
	203	8	5	6.6	41	53.9	30	39.5	76
	279	11	8	10.4	44	57.1	25	32.5	77
SB	51	2	9	19.6	24	52.2	13	28.3	46
	102	4	31	24.8	65	52.0	29	23.2	125
	127	5	19	41.3	17	37.0	10	21.7	46
	178	7	29	30.5	47	49.5	19	20.0	95
	203	8	2	28.6	4	57.1	1	14.3	7
Total			181	18.3	562	56.8	246	24.9	989

Table 55. Results of one-sided t-tests for tolerance level of 12.7 mm (0.5 in) by layer type and design thickness using elevation measurements.

Layer Type	Target Thickness		Mean Thickness <TV-12.7 mm (0.5 in)		Mean Thickness Within TV ± 12.7 mm (0.5 in)		Mean Thickness >TV+12.7 mm (0.5 in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGAB	102	4	5	6.0	73	86.9	6	7.1	84
	152	6	6	10.9	47	85.5	2	3.6	55
	203	8	3	7.5	33	82.5	4	10.0	40
	305	12	6	15.0	32	80.0	2	5.0	40
DGATB	102	4	0	0.0	25	92.6	2	7.4	27
	203	8	5	11.9	29	69.0	8	19.0	42
	305	12	6	21.4	19	67.9	3	10.7	28
LC	152	6	1	2.1	40	83.3	7	14.6	48
PATB	102	4	5	3.9	111	86.0	13	10.1	129
PCC	76	3	0	0.0	7	58.3	5	41.7	12
	127	5	0	0.0	6	50.0	6	50.0	12
	203	8	4	5.3	60	78.9	12	15.8	76
	279	11	2	2.6	62	80.5	13	16.9	77
SB	51	2	6	13.0	29	63.0	11	23.9	46
	102	4	20	16.0	88	70.4	17	13.6	125
	127	5	10	21.7	29	63.0	7	15.2	46
	178	7	21	22.1	66	69.5	8	8.4	95
	203	8	2	28.6	4	57.1	1	14.3	7
Total			102	10.3	760	76.8	127	12.8	989

Table 56. Results of one-sided t-test for tolerance level of 25.4 mm (1 in) by layer type and design thickness using elevation measurements.

Layer Type	Target Thickness		Mean Thickness <TV-25.4 mm (1in)		Mean Thickness Within TV ± 25.4 mm (1 in)		Mean Thickness >TV+25.4 mm (1in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGAB	102	4	0	0.0	83	98.8	1	1.2	84
	152	6	4	7.3	50	90.9	1	1.8	55
	203	8	0	0.0	39	97.5	1	2.5	40
	305	12	2	5.0	38	95.0	0	0.0	40
DGATB	102	4	0	0.0	27	100.0	0	0.0	27
	203	8	2	4.8	39	92.9	1	2.4	42
	305	12	1	3.6	26	92.9	1	3.6	28
LC	152	6	0	0.0	46	95.8	2	4.2	48
PATB	102	4	0	0.0	127	98.4	2	1.6	129
PCC	76	3	0	0.0	10	83.3	2	16.7	12
	127	5	0	0.0	10	83.3	2	16.7	12
	203	8	1	1.3	73	96.1	2	2.6	76
	279	11	0	0.0	75	97.4	2	2.6	77
SB	51	2	0	0.0	41	89.1	5	10.9	46
	102	4	18	14.4	104	83.2	3	2.4	125
	127	5	4	8.7	39	84.8	3	6.5	46
	178	7	17	17.9	76	80.0	2	2.1	95
	203	8	1	14.3	5	71.4	1	14.3	7
Total			50	5.1	908	91.8	31	3.1	989

The results of the one-sided t-tests for the elevation measurements are shown in figures 67, 68, and 69 for the three different tolerance levels.

The following observations are drawn based on the results of the one-sided t-test for the elevation measurements:

• The AC surface and binder layers have the greatest number of sections with the mean constructed thickness tested to deviate more than their target values plus or minus all three tolerance levels (6.35 mm [0.25 in], 12.7 mm [0.5 in], and 25.4 mm [1 in]).
• For most sections (about 70 percent), the mean constructed thicknesses for the dense-graded aggregate base layers are within ±6.35 mm (0.25 in) of their target thickness values.
• For portland cement concrete slabs and lean concrete bases, a much higher percent of sections had mean thicknesses greater than the target values plus tolerance levels than the ones below the target values. For thin bonded PCC overlays (76-mm- [3-in-] and 127- mm- [5-in-] thick) there are no sections with an as-constructed thickness significantly lower than the target value for all three tolerance levels.
• For all layer material types, except AC surface and binder layers and thin bonded PCC slabs, more than 90 percent of sections have mean layer thicknesses tested within ±25.4 mm (1 in) from their target values.

Figure 67: Chart. Results of one-sided t-tests for the differences between mean elevation and design thicknesses for a tolerance level of 6.35 mm (0.25 in).

Figure 68: Chart. Results of one-sided t-tests for the differences between mean elevation and design thicknesses for a tolerance level of 12.7 mm (0.5 in).

Figure 69: Chart. Results of one-sided t-tests for the differences between mean elevation and design thicknesses for a tolerance level of 25.4 mm (1 in).

Statistical Analysis of the Core Thickness Data

Analysis of the Percentage Distribution

The percentage distribution of core data as a function of different tolerance levels is presented in table 57.

Table 57. Summary of the percentage distribution of the individual core thickness measurements versus the design thickness.

Measured Layer Thickness t	Difference Between As-Constructed and As-Designed
	Diff = 6.35 mm (0.25 in)		Diff = 12.7 mm (0.5 in)		Diff = 25.4 mm (1.0 in)
	Number of Measurements	Percent of Measurements	Number of Measurements	Percent of Measurements	Number of Measurements	Percent of Measurements
t < TV1 - Diff	617	19.04	368	11.35	179	5.52
t within TV ± Diff	1117	34.46	2026	62.51	2720	83.92
t > TV + Diff	1507	46.50	847	26.13	342	10.55
Total	3241	100	3241	100	3241	100
Notes: 1Target value

The distributions of measurements by layer type for tolerance levels of 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in) are presented in tables 58, 59, and 60 for different layer types and target thickness values.

Table 58. Percentage distribution of core thickness measurements by layer type and design thickness for a tolerance level of 6.35 mm (0.25 in).

Layer Type	Target Thickness		Thickness <TV-6.35 mm (0.25 in)		Thickness Within TV ± 6.35 mm (0.25 in)		Thickness >TV+6.35 mm (0.25 in)		Total Number of Cores
	mm	in	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores
DGATB	102	4	23	25.0	46	50.0	23	25.0	92
	203	8	42	29.6	46	32.4	54	38.0	142
	305	12	36	40.4	20	22.5	33	37.1	89
LC	152	6	24	13.2	50	27.5	108	59.3	182
PATB	102	4	86	60.6	39	27.5	17	12.0	142
PCC	76	3			5	6.8	68	93.2	73
	127	5	11	6.8	26	16.1	124	77.0	161
	203	8	48	10.2	159	33.8	263	56.0	470
	279	11	67	15.2	182	41.4	191	43.4	440
SB	51	2	10	6.0	63	38.0	93	56.0	166
	102	4	63	11.8	213	39.9	258	48.3	534
	127	5	64	22.1	85	29.4	140	48.4	289
	178	7	134	30.5	180	41.0	125	28.5	439
	203	8	9	40.9	3	13.6	10	45.5	22
Total			617	19.0	1117	34.5	1507	46.5	3241

Table 59. Percentage distribution of core thickness measurements by layer type and design thickness for a tolerance level of 12.7 mm (0.5 in).

Layer Type	Target Thickness		Thickness <TV-12.7 mm (0.5 in)		Thickness Within TV ± 12.7 mm (0.5 in)		Thickness >TV+12.7 mm (0.5 in)		Total Number of Cores
	mm	in	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores
DGATB	102	4	8	8.7	76	82.6	8	8.7	92
	203	8	22	15.5	90	63.4	30	21.1	142
	305	12	28	31.5	40	44.9	21	23.6	89
LC	152	6	15	8.2	105	57.7	62	34.1	182
PATB	102	4	69	48.6	63	44.4	10	7.0	142
PCC	76	3	0	0.0	25	34.2	48	65.8	73
	127	5	8	5.0	66	41.0	87	54.0	161
	203	8	24	5.1	300	63.8	146	31.1	470
	279	11	38	8.6	315	71.6	87	19.8	440
SB	51	2	4	2.4	96	57.8	66	39.8	166
	102	4	29	5.4	387	72.5	118	22.1	534
	127	5	22	7.6	166	57.4	101	34.9	289
	178	7	93	21.2	290	66.1	56	12.8	439
	203	8	8	36.4	7	31.8	7	31.8	22
Total			368	11.4	2026	62.5	847	26.1	3241

Table 60. Percentage distribution of core thickness measurements by layer type and design thickness for a tolerance level of 25.4 mm (1 in).

Layer Type	Target Thickness		Thickness TV-25.4 mm (1 in)		Thickness Within TV ± 25.4 mm (1 in)		Thickness >TV+25.4 mm (1 in)		Total Number of Cores
	mm	in	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores	Number of Cores	Percent of Cores
DGATB	102	4	2	2.2	90	97.8	0	0.0	92
	203	8	10	7.0	123	86.6	9	6.3	142
	305	12	15	16.9	70	78.7	4	4.5	89
LC	152	6	2	1.1	171	94.0	9	4.9	182
PATB	102	4	45	31.7	90	63.4	7	4.9	142
PCC	76	3	0	0.0	51	69.9	22	30.1	73
	127	5	0	0.0	123	76.4	38	23.6	161
	203	8	10	2.1	419	89.1	41	8.7	470
	279	11	32	7.3	387	88.0	21	4.8	440
SB	51	2	0	0.0	123	74.1	43	25.9	166
	102	4	13	2.4	476	89.1	45	8.4	534
	127	5	7	2.4	205	70.9	77	26.6	289
	178	7	37	8.4	379	86.3	23	5.2	439
	203	8	6	27.3	13	59.1	3	13.6	22
Total			179	5.5	2720	83.9	342	10.6	3241

The graphical presentation of the percentage distributions of core thickness measurements is shown in figures 70, 71, and 72 for the three different tolerance levels.

The following are observed based on the percentage distributions of the individual core thickness measurements:

• Overall, less than 35 percent of core measurements are within ± 6.35 mm of the design thickness value. For some material types and target thickness values, such as thin PCC layers (76 mm [3 in] or 123 mm [5 in] thick) and 203-mm- (8-in-) thick SB layers, this percentage is below 20.
• For LC and PCC layers, a much larger percentage of cores have thicknesses higher than designed. For PATB, the situation is just the opposite.
• For DGATB, SB, and PCC layers, the percentage of sections with as-constructed thicknesses below the target value increases with target thickness. For PCC layers, the percentage of sections with as-constructed thickness above the target value decreases with increasing target thickness.

Figure 70: Chart. Percentage distribution of core measurements by layer type and design thickness for a tolerance level of 6.35 mm (0.25 in).

Figure 71: Chart. Percentage distribution of core measurements by layer type and design thickness for a tolerance level of 12.7 mm (0.5 in).

Figure 72: Chart. Percentage distribution of core measurements by layer type and design thickness for a tolerance level of 25.4 mm (1 in).

Statistical Analysis of the Core Data

Two-sided t-test

The results of the two-sided t-tests with 95 percent confidence are presented in table 61. The distribution of differences by different surface type and target thickness is presented in table 62.

Table 61. Summary of the results of the two-sided t-test (95 percent confidence level) using core thickness data.

Difference	Number of Layers	Percentage of Layers
Significantly lower than the target value	90	15.38
No significant difference from the target value	268	45.81
Significantly higher than the target value	227	38.80
Total	585	100

Table 62. Distribution of differences by layer type and design thickness (two-sided t-test, 95 percent confidence level) using core thickness data.

Layer Type	Target Thickness		Significantly Lower than the Target Value		No Significant Difference		Significantly Higher than the Target Value		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGATB	102	4	3	15.8	15	78.9	1	5.3	19
	203	8	5	16.1	16	51.6	10	32.3	31
	305	12	3	15.8	11	57.9	5	26.3	19
LC	152	6	2	5.7	13	37.1	20	57.1	35
PATB	102	4	13	41.9	15	48.4	3	9.7	31
PCC	76	3	0	0.0	2	20.0	8	80.0	10
	127	5	1	8.3	2	16.7	9	75.0	12
	203	8	6	8.5	21	29.6	44	62.0	71
	279	11	12	17.1	32	45.7	26	37.1	70
SB	51	2	3	7.7	18	46.2	18	46.2	39
	102	4	13	11.8	49	44.5	48	43.6	110
	127	5	11	23.9	15	32.6	20	43.5	46
	178	7	17	19.8	54	62.8	15	17.4	86
	203	8	1	16.7	5	83.3	0	0.0	6
Total			90	15.4	268	45.8	227	38.8	585

The following are observed based on the results of the two-sided t-test for the core thickness measurements:

• Overall, the mean constructed thickness for more than 45 percent of layers is not significantly different from the target thickness. The percentage is highest for DGATB and lowest for PCC and LC.
• DGATB has the highest number of sections (61 percent) with mean constructed thicknesses not different from the target values. For almost 80 percent of the sections with DGATB and 102-mm (4-in) target thickness, the constructed thickness is not significantly different from the designed thickness.
• PCC and LC layers have the fewest number of layers (between 34 and 37 percent) with mean constructed thicknesses not significantly different from the target values. For thin PCC slabs, this percentage is 20 or below.

One-sided t-test

Three one-sided t-tests (95 percent confidence level) were performed to check whether the difference between as-constructed and as-designed thickness is lower than 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in), respectively. The results of the overall analysis of all data points for all layers are summarized in table 63.

The results of the analysis by layer type for different tolerance levels are presented in tables 64 through 66.

Table 63. Summary of the results of the one-sided t-tests using core thickness data.

Level of Significance	Difference Between As-Constructed and As-Designed Thickness
	6.35 mm (0.25 in)		12.7 mm (0.5 in)		25.4 mm (1.0 in)
	Number of Layers	Percent of Layers	Number of Layers	Percent of Layers	Number of Layers	Percent of Layers
Significantly lower than the target value	58	9.91	34	5.81	22	3.76
No significant difference from the target value	378	64.62	473	80.85	533	91.11
Significantly higher than the target value	149	25.47	78	13.33	30	5.13
Total	585	100	585	100	585	100

Table 64. Results of the one-sided t-test (95 percent confidence level) by layer type and design thickness for tolerance level of 6.35 mm (0.25 in) using core thickness data.

Layer Type	Target Thickness		Mean Thickness <TV-6.35 mm (0.25 in)		Mean Thickness Within TV ± 6.35 mm (0.25 in)		Mean Thickness >TV+6.35 mm (0.25 in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGATB	102	4	1	5.3	17	89.5	1	5.3	19
	203	8	4	12.9	20	64.5	7	22.6	31
	305	12	3	15.8	12	63.2	4	21.1	19
LC	152	6	2	5.7	22	62.9	11	31.4	35
PATB	102	4	13	41.9	16	51.6	2	6.5	31
PCC	76	3	0	0.0	2	20.0	8	80.0	10
	127	5	0	0.0	5	41.7	7	58.3	12
	203	8	2	2.8	40	56.3	29	40.8	71
	279	11	6	8.6	50	71.4	14	20.0	70
SB	51	2	1	2.6	20	51.3	18	46.2	39
	102	4	7	6.4	75	68.2	28	25.5	110
	127	5	5	10.9	27	58.7	14	30.4	46
	178	7	12	14.0	68	79.1	6	7.0	86
	203	8	2	33.3	4	66.7	0	0.0	6
Total			58	9.9	378	64.6	149	25.5	585

Table 65. Results of the one-sided t-test (95 percent confidence level) by layer type and design thickness for tolerance level of 12.7 mm (0.5 in) using core examination data.

Layer Type	Target Thickness		Mean Thickness <TV-12.7 mm (0.5 in)		Mean Thickness Within TV ± 12.7 mm (0.5 in)		Mean Thickness >TV+12.7 mm (0.5 in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGATB	102	4	0	0.0	18	94.7	1	5.3	19
	203	8	0	0.0	27	87.1	4	12.9	31
	305	12	2	10.5	14	73.7	3	15.8	19
LC	152	6	0	0.0	27	77.1	8	22.9	35
PATB	102	4	10	32.3	20	64.5	1	3.2	31
PCC	76	3	0	0.0	4	40.0	6	60.0	10
	127	5	0	0.0	6	50.0	6	50.0	12
	203	8	1	1.4	58	81.7	12	16.9	71
	279	11	6	8.6	59	84.3	5	7.1	70
SB	51	2	0	0.0	29	74.4	10	25.6	39
	102	4	5	4.5	97	88.2	8	7.3	110
	127	5	1	2.2	33	71.7	12	26.1	46
	178	7	7	8.1	77	89.5	2	2.3	86
	203	8	2	33.3	4	66.7	0	0.0	6
Total			34	5.8	473	80.9	78	13.3	585

Table 66. Results of the one-sided t-test (95 percent confidence level) by layer type and design thickness for tolerance level of 25.4 mm (1 in) using core examination data.

Layer Type	Target Thickness		Mean Thickness <TV-25.4 mm (1 in)		Mean Thickness Within TV ± 25.4 mm (1 in)		Mean Thickness >TV+25.4 mm (1 in)		Total Number of Sections
	mm	in	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections	Number of Sections	Percent of Sections
DGATB	102	4	0	0.0	19	100.0	0	0.0	19
	203	8	0	0.0	31	100.0	0	0.0	31
	305	12	1	5.3	18	94.7	0	0.0	19
LC	152	6	0	0.0	34	97.1	1	2.9	35
PATB	102	4	7	22.6	23	74.2	1	3.2	31
PCC	76	3	0	0.0	8	80.0	2	20.0	10
	127	5	0	0.0	10	83.3	2	16.7	12
	203	8	0	0.0	68	95.8	3	4.2	71
	279	11	6	8.6	64	91.4	0	0.0	70
SB	51	2	0	0.0	32	82.1	7	17.9	39
	102	4	3	2.7	103	93.6	4	3.6	110
	127	5	0	0.0	37	80.4	9	19.6	46
	178	7	4	4.7	81	94.2	1	1.2	86
	203	8	1	16.7	5	83.3	0	0.0	6
Total			22	3.8	533	91.1	30	5.1	585

The graphical presentations of one sided t-test results of core thickness measurements are shown in figures 73, 74, and 75 for the three different tolerance levels.

The following conclusions may be drawn based on results of the t-test for the core thickness measurements:

• The PCC layers have the highest percentage of sections with mean measured thicknesses above their target thicknesses for all three tolerance levels. This percentage decreases with the increased PCC target thickness. For thin bonded PCC layers (76-mm- [3-in-] or 123-mm- [5-in-] thick), there are no sections with layer thicknesses significantly lower than the target value. For very thin bonded PCC overlays (76-mm- [3-in-] thick), 80 percent of the sections have mean thicknesses significantly higher than the target value for more than 6.35 mm (0.25 in). This percentage decreases with increasing target thickness.
• For all material types except PATB and 178-mm- (7-in-) and 203-mm- (8-in-) thick SB layers, a much larger percentage of layers have a mean thickness significantly higher than designed. For PATB, the situation is just the opposite, with more than 40 percent of layers having values that are significantly lower than the target value for more than 6.35 mm (0.25 in). For 203-mm- (8-in-) thick SB layers, there are no sections with a mean measured thicknesses significantly higher than designed.
• For DGATB and SB layers, the number of sections with mean thicknesses below target thickness increases with the design thickness.
• All sections with DGATB and LC layers, except one, have thicknesses within ± 25.4 mm (1 in) of the target thickness.

Figure 73: Chart. Results of one-sided t-tests for the differences between core measurements and design thicknesses for tolerance level of 6.35 mm (0.25 in).

Figure 74: Chart. Results of one-sided t-tests for the differences between mean core and design thicknesses by layer type and design thickness for tolerance level of 12.7 mm (0.5 in).

Figure 75: Chart. Results of one-sided t-tests for the differences between mean core and design thickness by layer type and design thickness for tolerance level of 25.4 mm (1 in).

Summary

In this chapter, the as-constructed core and elevation grid layer thickness measurements were compared to the design thicknesses for newly constructed SPS layers.

The mean thickness difference between as-designed and as-constructed thicknesses was computed for each layer using both core and elevation thickness measurements and typical thickness deviations from the target thicknesses are summarized, as well as their distribution types.

For both data sources, two types of comparisons are made in relation to their as-designed thicknesses or target values. First, both data sources were evaluated for the percentage of individual measurements either within or outside specific values from the target thickness. Second, a statistical analysis was performed to compare the measured mean thickness values with the designed values. Two types of the thickness comparisons are performed for both data sources. The two-sided t-test with 95 percent confidence level was used for each section and layer to determine whether differences between as-designed and as-constructed thicknesses are significant. One-sided t-tests with 95 percent confidence level were used for each layer to determine if the difference between as-designed thickness and the mean as-constructed thickness had significant allowances of more than 6.35 mm (0.25 in), 12.7 mm (0.5 in), and 25.4 mm (1 in), respectively.

Based on the analysis of both data sources, the following conclusions can be made:

• The computed description statistics using elevation measurement data are different from those using core examination data. However, based on statistical analyses, the differences in the mean layer thicknesses and standard deviations at the section or layer level are not significant for a majority of the layers.
• For the same layer and material type, the mean constructed layer thicknesses tend to be above the designed value for the thinner layers and below the design value for the thicker layers.
• The majority of the LC and PCC layers have constructed or measured thicknesses greater than the design values. This is particularly true for thin (76-mm- [3-in-] and 127-mm- [5- in-] thick) PCC slabs.
• Thin PCC and AC surface and binder layers have the highest number of sections with a mean as-constructed thickness that significantly deviates from the design thicknesses.
• Elevation thickness measurements for PATB are more evenly distributed around the target value. However, the core measurements for PATB show that a significant number of sections have thicknesses lower than the target thickness. It appears that for some cores the entire thickness of PATB layer was not obtained. The analysis shows the values currently stored in the database. A feedback report was submitted regarding these questionable data. In some cases, core thicknesses were less than 25.4 mm (1 in), even though the target thickness is 102 mm (4 in).
• About 60 percent of all section/layers have mean thickness within ±6.35 mm (0.25 in) from the target thickness. For a tolerance level of 25.4 mm (1 in) this percentage is above 90 for most layer types and target thickness values.

A comparison between analysis results from the elevation and core thickness measurements shows that the percentage of measurements within the selected limits is approximately the same for all three tolerance levels. However, the percentage of measurements lower than the target value is consistently higher for core measurements than for elevation grid measurements.

Based on elevation measurements, it is observed that more than 70 percent of sections with DGAB have as-constructed thickness within ±6.35 mm (0.25 in) from the design value.

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