CENTER FOR TRANSPORTATION RESEARCH THE UNIVERSITY OF TEXAS AT AUSTIN Center for Transportation Research The University of Texas at Austin Project Summary Report 0-4382-S Project 0-4382: Establish an Acceptable Pavement Thickness Tolerance to Allow for Non-Destructive Continuous Concrete Pavement Thickness Measurements Authors: Seong-Min Kim and B. Frank McCullough October 2002 ACCEPTABLE CONCRETE PAVEMENT THICKNESS TOLERANCE

This research project was conducted to investigate if the current thickness tolerance for concrete pavements can be loosened and to provide TxDOT with the acceptable thickness tolerance so that non-destructive testing (NDT) methods can be used with con? dence for thickness measurements. TxDOT’s current tolerance limit for concrete slab thickness was developed in the 1950s based on engineering judgment and experience; no additional study on tolerance limit has been conducted since. The tolerance limit is currently 5 mm (0. 2 in. ) for full payment.

This tolerance limit is too tight to allow use of existing NDT methods for slab thickness determination because these methods are not as accurate as direct measurement from coring. If the current tolerance can be shown Number of load application (ESALs) SUMMARY to have minimal impact on the pavement performance, then the tolerance limit can be loosened so that NDT methods can be used for thickness determination. NDT methods are less time-consuming and more cost-effective than coring. Moreover, the slab thickness measured continuously by NDT methods will represent the pavement more adequately than spotchecking by coring.

In this research, the sensitivity of pavement performance to slab thickness has been investigated based on various models including the AASHTO model, a mechanistic distress prediction model, and fatigue failure models. The controlling performance indicator from the sensitivity study has been compared with the measured variability of pavement thickness in the ? eld, and to the accuracy of the NDT devices. From these comparisons, a reasonable tolerance limit of the slab thickness has ? nally been obtained. REPORT What We Did …

This research was conducted with four different phases as follows: • Review of current thickness tolerance limits for concrete pavements in Texas and other states. • Sensitivity analysis of concrete pavement thickness based on various models such as the AASHTO model, mechanistic distress prediction model, and fatigue failure models. • Investigation of ? eld variability of concrete pavement thickness and accuracy of NDT devices. • Determination of acceptable thickness tolerance. The thickness tolerance Pavement thickness Design life Thickness tolerance PROJECT

AASHTO equation Design life Allowable loss of life Design thickness Allowable loss of life Allowable design life AASHTO equation Pavement thickness Allowable thickness Figure 1: Thickness Tolerance Determination with AASHTO Equation Thickness tolerance Project Summary Report 0-4382-S –1– mechanistic models. The concept of determining pavement thickness sensitivity to pavement Pavement stress life by using the AASHTO equation is illustrated in Figure 1. The pavement design life can be obtained from the Stress level pavement design thickness by using the Fatigue failure equation AASHTO equation.

Then, an allowable Design life loss of the pavement life is selected, and the allowable design life is obtained by Allowable loss of life subtracting the allowable loss of life Allowable design life from the design life. The correspondFatigue failure equation ing allowable pavement thickness can then be obtained by using the AASHTO Allowable stress level equation inversely, and ? nally the thickness tolerance for the allowable loss of Allowable stress life can be obtained by subtracting the allowable thickness from the design Westergaard equation thickness.

Allowable thickness The concept of determining pavement thickness sensitivity to pavement life by using the fatigue failure equations Thickness tolerance is similar, as shown in Figure 2. Another Figure 2: Thickness Tolerance Determination concept for ? nding pavement thickness with Fatigue Failure Equations sensitivity to pavement life is based on limits and corresponding penalties distresses such as cracks and punchouts. should be related to the loss of pavement If a pavement with a thickness de? ciency life caused by the thickness de? ciency. oes not induce more cracks as compared Three different approaches have been with the plan pavement, the thickness used to ? nd the relationship between de? ciency can be acceptable with this the pavement thickness de? ciency and concept. To predict crack and punchout the loss of pavement design life. The formations, a mechanistic model, CRCP? rst is based on the change in present 10, has been used. serviceability index (PSI) to predict the pavement life, which includes the What We Found … AASHTO pavement life prediction The ? ndings of this research are as equation.

The second is based on the follows: fatigue failure, which includes a number • The current thickness tolerance limits of fatigue failure equations. The third is are independent of the design pavebased on distresses such as cracks and ment thickness. If the same thickness punchouts, which can be predicted by Pavement thickness Westergaard equation de? ciency is established for different design thicknesses, the contractor for the pavement with a thicker thickness should pay higher a penalty because the penalty is determined as a percentage of the contract price corresponding to the de? iency and the contract price for the thicker pavement is generally more expensive than that for the thinner pavement. • The sensitivity analysis of the pavement thickness based on the AASHTO design equation to predict the pavement life shows that the tolerance increases as the thickness increases for a given percent allowable loss of design life. The relative (percent) tolerance remains almost constant when the pavement thickness is greater than about 10 inches. The tolerance increases as the elastic modulus of concrete decreases or the modulus of subgrade reaction increases.

The relationship between the tolerance (both absolute and relative) and the percent allowable loss of life is almost linearly proportional. • The thickness sensitivity analysis based on various power fatigue failure equations shows that the absolute tolerance increases and the relative tolerance remains almost constant as the pavement thickness increases for a given percent allowable loss of design life. The tolerance is affected little by the concrete elastic modulus and the modulus of subgrade reaction. Both the absolute and relative tolerances increase as the percent allowable loss of life increases, and the relationship is Thickness = 6 in. Tolerance (in. ) Tolerance (in. ) 2 AASHTO 1. 5 1 0. 5 0 Thickness = 12 in. Thickness = 18 in. 1. 5 1 0. 5 0 Power formula Linear formula 0 10 20 30 Loss of life (%) 40 50 0 10 (a) 20 30 Loss of life (%) 40 50 (b) Figure 3: Relationship between Thickness Tolerance and Loss of Pavement Life Project Summary Report 0-4382-S –2– 10 Mean crack spacing (ft) 8 6 4 2 0 radar (GPR) system, a non-destructive testing device, generally slightly overestimates the pavement thickness. The relative average error of the measurement is about 2% of the pavement thickness.

The Researchers Recommend … The ? ndings from this research clearly indicate that the current thickness tolerance limits can be loosened for thicker pavements. It is recommended that the thickness tolerance limits be dependent on the design thickness and the linearly proportional relationship between the tolerance and the design thickness be used. The payment adjustments should be dependent on the thickness de? ciency and the linearly proportional relationship between the payment adjustment and the thickness de? ciency may be used beyond the no-penalty tolerance limit.

Several approaches would be acceptable for determining the payment adjustments to the thickness de? ciency. One would be to use the current thickness de? ciency adjustment table for a 10-inch pavement, with the relative tolerance limits used for other pavement thicknesses, because the current thickness tolerance was developed when the pavement thickness was mostly less than 10 inches. Then, a proposed thickness de? ciency adjustment table can be obtained as shown in Table 1. 3 6 9 12 15 18 21 Pavement thickness (in. )

Figure 4: Sensitivity of Crack Spacing to Thickness De? ciency almost linear. • The thickness sensitivity study based on various linear fatigue failure equations shows that both the absolute and relative thickness tolerances increase with increasing the pavement thickness for a given percent allowable loss of life. The tolerance increases as the elastic modulus of concrete or the modulus of subgrade reaction increases. As the percent allowable loss of life increases, both the absolute and relative tolerances become larger, and the relationship is almost linear. The results from the sensitivity analysis based on the AASHTO equation and various fatigue failure equations show that the thickness tolerance increases as the pavement thickness increases and as the allowable loss of pavement design life increases, as shown in Figure 3. • The sensitivity analysis of the pavement thickness based on distresses shows that as the pavement thickness increases, the thickness de? ciency that induces more cracks becomes larger. The thickness de? ciency of 0. 2 in. does not affect the CRC pavement performance when the pavement thickness is greater than about 10 inches.

The thickness de? ciencies that do not affect the CRC pavement performance are 0. 6, 0. 8, and 1. 2 in. (5, 5. 3, and 6. 7%) for 12-, 15-, and 18inch-thick pavements, respectively, as shown in Figure 4. • The ? eld variability of the thickness shows that the average thickness is generally 3 to 7% larger than the design thickness and the average thickness difference from the average thickness is about 3%. • The current ground-penetrating Table 1: Thickness De? ciency Adjustment Table De? ciency in thickness (%) 0–2 2–3 3–4 4–5 5–8 Project Summary Report 0-4382-S

Percent of contract unit price allowed 100 80 72 68 57 –3– Research Supervisor: For More Details… B. Frank McCullough, Ph. D. , P. E. , (512) 232-3141 email: [email protected] utexas. edu TxDOT Project Director: Moon C. Won, Ph. D. , P. E. , (512) 506-5863 email: [email protected] state. tx. us The research is documented in the following reports: 4382-1 Reconsideration of Thickness Tolerance for Concrete Pavements October 2002 To obtain copies of a report: CTR Library, Center for Transportation Research, (512) 232-3138, email: [email protected] cc. utexas. edu

TxDOT Implementation Status January 2004 The Concrete Branch of the Construction Division of TxDOT is evaluating the recommendations of this project for implementation in TxDOT speci? cations. The recommendations of this project will be evaluated again in conjunction with the capabilities of a new generation Ground Penetrating Radar (GPR) that has greater accuracy in thickness measurements. Project 5-4414-01 is implementing the new GPR equipment. The Concrete Branch of the Materials and Pavements Section will be responsible for the future implementation.

For more information, contact: German Claros, P. E. , Research and Technology Implementation Of? ce, (512) 465-7403 or e-mail [email protected] state. tx. us. Your Involvement Is Welcome! This research was performed in cooperation with the Texas Department of Transportation and the U. S. Department of Transportation, Federal Highway Administration. The contents of this report re? ect the views of the authors, who are responsible for the facts and accuracy of the data presented herein.

The contents do not necessarily re? ect the of? cial view or policies of the FHWA or TxDOT. This report does not constitute a standard, speci? cation, or regulation, nor is it intended for construction, bidding, or permit purposes. Trade names were used solely for information and not for product endorsement. The engineer in charge was Dr. B. Frank McCullough, P. E. (Texas No. 19914). The University of Texas at Austin Center for Transportation Research 3208 Red River, Suite #200 Austin, TX 78705-2650 Disclaimer