| Introduction |  |
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Variation exists in all material- and construction-related acceptance quality characteristics (AQC’s). The implementation of performance-related specifications (PRS) requires considerable knowledge about the variability of these AQC’s. The goal of this specific investigation was to establish the "typical" variability associated with the AQC’s included in the PRS for concrete pavements. The information obtained from this study will provide guidance to agencies when selecting PRS target as-designed variabilities.
A comprehensive literature search was conducted (described in appendix C) to identify previous studies where variations of concrete pavement AQC’s were reported and documented in an unbiased manner. The information was evaluated so that those AQC’s for which variability was not well-documented could be targeted for field testing and evaluation. In order to establish additional information on typical levels of variation, a field testing program was developed for the identified AQC’s. Using the work plan developed for this investigation, data were collected from actual field construction projects. The specific AQC variabilities investigated during this research project include:
- Concrete strength—28-day flexural strength (predicted using maturity and measured directly).
- Slab thickness—measured from cores and ground-penetrating radar (GPR).
- Entrained air content—measured in the plastic concrete using an air pressure meter and in the hardened concrete slab using linear traverse tests of cores.
- Initial smoothness—measured as the initial profile index using a California profilograph.
- Transverse and longitudinal sawcut depth—measured with a ruler prior to any joint sealing.
- Consolidation level at doweled joints—measured using relative density tests of cores.
- Longitudinal tie bar depth—measured using GPR.
All testing was performed in accordance with standards developed by the American Society for Testing and Materials (ASTM). The information collected and analyzed during this research will help identify allowable tolerances for each of the investigated AQC’s. These tolerances are based on the observed variability limits contractors are currently able to achieve. The identified AQC tolerances may then be used by agencies to select appropriate AQC target standard deviations.
A detailed discussion of the field/laboratory investigations (including projects studied, sampling and testing procedures, data analysis procedures, and a summary of results for each of the targeted AQC’s) is provided in the following sections.
| Discussion of Variance | |
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Because the PRS approach determines the performance of a pavement on a lot basis, the most important variability of interest for the procedure is the overall within-lot variability. The procedure assumes that the pavement lot quality is represented by AQC's that are normally distributed. Each AQC is measured by testing randomly selected samples located within the pavement lot. Each sample is the mean of all replicate test values obtained at the randomly selected sample location. All samples representing a given lot are assumed to estimate a distribution of the AQC. For each AQC, the value targeted during construction of the lot (specified by the governing agency) is the mean of the distribution. The tolerance value for each AQC (minimum acceptable value without penalty) allowed by the governing agency was determined during this investigation as the currently achievable within-lot variability. For each AQC studied in this investigation, the total within-lot variability was calculated as the standard deviation of all samples collected within the lot. If replicate values were used to represent the samples, the total within-lot variability was calculated using equation 10.
sTOTAL WITHIN-LOT = sMEASURED WITHIN-LOT * n0.5 (10)
where
sTOTAL WITHIN-LOT = The total within-lot standard deviation for the AQC (materials/process and testing).
sMEASURED WITHIN-LOT = The standard deviation between all representative samples in the lot. Each sample is the mean of all replicate test values obtained at the sample location.
n = Number of replicate test values obtained at each AQC sample location.
For each AQC required by the governing agency, the within-lot variability is an estimate of the total variability of the AQC over the entire lot. This total within-lot variability is comprised of testing variation and materials/process variation. The testing variation, or within-test variability, is a measure of the repeatability of a particular testing method. This variability is calculated as the standard deviation of the test values representing a sample for a random sample location. As long as the standard deviation is independent of the mean, it can be used to represent the variability of the AQC being evaluated. The materials/process variability represents the variation due to actual AQC point-to-point material differences and process control. It is the difference between the total within-lot variability and the within-test variability.
The relationship of the components of variance is shown in equation 11.
s2TOTAL WITHIN-LOT = s2WITHIN-TEST + s2MATERIALS/PROCESS (11)
where
s2TOTAL WITHIN-LOT = The total within-lot variance for the AQC.
s2WITHIN-TEST = The estimated variance of the testing method (testing repeatability).
s2MATERIALS/PROCESS = The estimated variance of the materials and process (material variance).
Calculating Equivalent Variability
When implementing a PRS, it is important that the selected AQC target variabilities correspond with the sampling and testing methods being used during construction. Each of the AQC variabilities recommended in this report represents the variability associated with evaluating individual samples (comprised of one actual test value) within a given lot. If an agency elects to collect replicate test values to represent a sample (for example, three cast cylinders from the same sample location tested for 3-day compressive strength), the variability must be adjusted using equation 12.
sTARGET = sRECOMMENDED / n0.5 (12)
where
sTARGET = The targeted within-lot standard deviation for the AQC.
sRECOMMENDED = The recommended within-lot standard deviation for the AQC (materials/process and testing).
n = Number of replicate test values obtained at each AQC sample location.
| Identification of Field Projects | |
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To evaluate the variability of construction parameters achievable by the contractor, testing was performed on field construction projects during the 1995 and 1996 construction seasons. The project team established criteria for the field sites as follows:
- Jointed plain air-entrained concrete pavement.
- Located on a highway in a rural or suburban area.
- Construction duration of at least 3 days.
- Located within a 1-day drive of Skokie, IL (because test samples and materials had to be transported to the laboratory facilities).
- Willingness of the State highway agency (SHA) and contractor to allow the project to be included in the study.
The established criteria for the field studies limited the search to northern midwestern States. SHA’s were contacted and a testing schedule was planned. As projects were let, close contact with field engineers was required to schedule beneficial field visits to the projects. When possible, the field evaluations were conducted once the contractor had established a smooth, trouble-free paving operation. It was desirable to work with several SHA's to collect data for this investigation. The construction projects selected for evaluation in this study were performed in Illinois, Iowa, Michigan, Minnesota, Nebraska, Ohio, and Wisconsin. A list of the specific projects, along with the construction parameters evaluated, is shown in table 45.