Quality Control (QC) is the implementation, measurement, and enforcement of sound construction practices and jobsite inspections to ensure construction quality. Quality Assurance (QA) is the inspection and testing of the completed product, in accordance with specifications intended to verify the quality of the completed structure.
QC programs intended to address construction quality of decks routinely include (a) visually inspecting the forms and deck reinforcement at regular intervals during construction (and carefully repairing nicks and scratches found in epoxy-coated bars within the reinforcing cage with fresh epoxy); (b) careful field and laboratory testing of the quality of materials used in its construction; and (c) testing and inspection activities, both in the field and laboratory, associated with placing and curing the concrete. Results are carefully measured and archived as a permanent part of the job record and/or used to modify field (construction and inspection) practices and take corrective action while construction is still underway.
Quality control of construction practices and materials selection contributes to a deck that can last at least as long as its design life, from meticulous construction and inspection of the reinforcing cage to care in designing, mixing, placing, finishing and curing the concrete, along with proper monitoring during each of these processes. This quality-control series of activities is consistently practiced, in general, throughout most of the industry.
However, the quality assurance component, particularly as it relates to inspection of the internal condition of the deck after construction, generally receives much less attention and care, because it seems to be physically more difficult to do. In addition, when accuracy or reliability of QA inspections come into doubt, QA can often be viewed as an added construction cost with low, or indeterminate, perceived value. Well-written QA verification specifications in terms of the desired inspection outcomes and consequences related to noncompliance often become seemingly meaningless when inspection capabilities appear to fall short of expected results. This statement is particularly true when the inspection methods cannot be effectively critiqued or corrected, resulting in an inherent ineffectiveness in their ability to be used for enforcement or improvement of QA policies.
Ultimately, it is not how well a deck appears to have adhered to standard practice prior to completion that matters in the end. More importantly, the design elements intended to be built into the structure must actually exist internally once it is finished. In other words, excellent QC is not much good without adequate QA. The only way to ensure compliance to construction requirements is to construct and adopt a meaningful, high-quality QA program that undeniably verifies the deck has been properly built. This includes development and implementation of good specifications, backed by selection of appropriate inspection methods that are accurate and repeatable for verifying compliance, to inspire confidence in both the construction community and the owners that the QA program is sound.
Geophysical methods, such as ground penetrating radar and other electromagnetic or magnetic devices, are often employed to verify that the amount, size, layout, spacing, and depth of reinforcement meet design specifications. Often, the QA specifications or verification standards for the inspection process are more stringent than the capability (accuracy and/or repeatability of the selected method) to measure these properties.
An example of this statement is the preponderance of QA specifications related to verification of reinforcement layout and depth (concrete cover), specifications requiring accurate measurement and reporting of these dimensions to within 0.25 to 0.635 cm of actual position in the deck. Yet, most of these specifications require that the measurements be made using a Pachometer or similar device. There is nothing wrong with the specifications requiring that a structure be properly built and verified, and the tolerances listed above are not unrealistic to achieve. Nor is there anything wrong (technically) in specifying a Pachometer for measurement of rebar position or depth. However, a Pachometer does not accurately or reliably measure these parameters, nor are the measurements repeatable, particularly within the stated measurement tolerances for accuracy and/or repeatability.
This problem has caused several DOT's to stop enforcing their QA verification inspections for cover or discontinued any attempt to measure it after the structure is built, relying solely on field inspections of the reinforcing cage, formwork, and placement operation-the QC component of their QC/QA program. Other states have dropped the cover verification specification altogether, or avoided adopting one because confidence in the reliability and accuracy of this instrument is not high.
Ground penetrating radar (GPR), only when used with a recently built, digital data acquisition system (post-1996) and a shielded 1.5 GHz ground-coupled antenna, has been proven by New Hampshire Department of Transportation (NHDOT) to accurately and routinely measure cover on a new bridge deck to within 0.25 cm. NHDOT's (Pachometer-based) specification was revised to allow only the Geophysical Survey Systems Model 5100, 1.5 GHz sensor (or a newer data acquisition unit that can provide as good, or better results) to be used for this purpose. NHDOT specifications, originally designed to reward contractors based on performance (mean cover depth and variability within specified tolerances to determine degree of compliance or non-compliance), are implemented in that fashion, and contractors can receive a pay factor on the portion of the contract covering deck construction (typically 40% of the bridge construction cost) as high as 1.05 (5% bonus). Overall contractor performance on the QC end in New Hampshire has also improved as a result. During the "non-enforcement" period, all contractors received a pay factor of 1.0, simply because no one could confidently determine the level of performance.
Problems exist in quality assurance (QA) verification of deck construction, even when stringent quality control (QC) initiatives are implemented and improved as part of a well-planned QC/QA strategy. Mostly, the inability to accurately and nondestructively verify reinforcement position (horizontally and vertically), layout, and quantity within a concrete deck, as well as to measure deck thickness to the degree specified or desired by most structure designers and/or owners as they develop and improve their QA methods, is systematically below acceptable standards in most areas of the country. QC/QA on new deck structures, by critically impacting how well they are constructed, are implemented so that future deck deterioration problems can be delayed and/or reduced, minimizing the quantity and cost of future maintenance on these decks during their service lives. The use of GPR as a primary evaluation tool for QA regarding placement (including cover), pattern and density of rebar is recommended, and should be seriously considered for verification purposes. Also, seismic methods should be used for concrete quality verification. Guidance for comparison of test methods, specification development, and implementation can be obtained by following New Hampshire DOT's lead in the area of GPR.
Ground Penetrating Radar (GPR)
For detailed description of GPR in this application, please refer to Ground Penetrating Radar - Pavements.
Data Acquisition: Vehicle mounted GPR systems, discussed in detail in Vehicle Mounted Ground Penetrating Radar Systems, can be used on bridge decks for overall assessment spotting areas of problems. However, vehicle mounted GPR systems are not generally the preferred method for accurate location of rebar. One of the portable systems with a 1.5 GHz antenna is more appropriate for that application.
Figure 65. Ground Penetrating Radar image of rebar.
Interpretation: As indicated in figure 65, locating rebar is done by noting hyperbolic shapes in the GPR image. The apex of each hyperbola locates a rebar. Lateral separation can be determined to sub-centimeter accuracy as long as care is taken in the lateral location control of the instrument. Likewise, the rebar depths can be measured to a similar depth as long as local calibration of the EM wave velocity is carried out.
Advantages: This method is easy and inexpensive to run and can therefore be run over every meter of a bridge deck.
Limitations: On-site velocity calibration is necessary to get the best results.
Impact Echo (IE)
For detailed description of Impact Echo in this application, please refer to Impact Echo - Pavements.
Interpretation: The best use of impact echo on a bridge deck is for overlay thickness. It is necessary to look for sites in between rebar to get the best depth measurement of the deck. Sites affected by the rebar will be readily in the data as a very shallow measurement (at the depth of the rebar).
Advantages: This method provides thickness with no coring.
Limitations: The user must make sure thickness measurements are made with interference of rebar.
Spectral Analysis of Surface Waves (SASW) and Ultrasonic Surface Waves Methods
For detailed description of these methods in this application, please refer to Spectral Analysis - Pavements.
Interpretation: In bridge deck applications these methods are used to measure modulus just like in pavement applications. The purpose is to get a quality measurement of modulus without coring the new deck.
Advantages: This method gives concrete quality without coring.
Limitations: This method requires road and bridge closures.
Spectral Analysis of Surface Waves (SASW) and Impact Echo (IE) Combined
Ultrasonic Seismic (SASW) and Impact Echo (IE), particularly when applied together in an integrated instrument, such as the Portable Seismic Property Analyzer (PSPA), are high-frequency, acoustic (seismic) geophysical methods. Concrete condition assessment by integrated ultrasonic methods has recently been applied to a number of engineering problems associated with both new and aging concrete structures. Recent work on evaluating concrete integrity, particularly, looking for the formation of corrosion-induced delamination in early, moderate, and late stages of development, has demonstrated that these integrated methods show considerable promise in terms of current and potential capability.
See Spectral Analysis of Surface Waves and Ultra Sonic Surface Wave Methods for further comments on this system.