Applications of thickness and density gauges

The development of gamma radiometry techniques did not really begin until radioisotope sources became widely available with the advent of nuclear reactors after World War 2. Malhotra reported Smith and Whiffin as the first users of gamma radiometry on concrete in 1952. They made direct transmission measurements using a ⁶⁰Co source inserted in a hole in a concrete block and a Geiger-Muller tube detector external to the block. The apparatus allowed measurements of variations in density with depth in order to evaluate the effectiveness of an experimental surface vibrating machine.

In his 1976 survey, Malhotra reported gamma radiometry had been used for measuring the in situ density of structural concrete members, the thickness of concrete slabs, and the density variations in drilled cores from concrete road slabs. With the possible exception of its application in Eastern Europe for monitoring density in precast concrete units, radiometry was still an experimental non-destructive testing tool for concrete at that time. Density monitoring applications increased in the highway industry after a 1972 report by Clear and Hay showed the importance of consolidation in increasing the resistance of concrete to penetration by chloride ions. A number of U.S. state and Canadian province highway agencies began using commercially available nuclear gauges to evaluate the density achieved in bridge deck overlays, particularly overlays employing low slump, low water-cement ratio mixes. In 1979, the American Association of State Highway and Transportation Officials (AASHTO) adopted a standard method, T 271, for the Density of Plastic and Hardened Portland Cement Concrete in Place by Nuclear Methods, and, in 1984, the American Society for Testing and Materials (ASTM) followed with a slightly different version, Test Method C1040. Most recently, Whiting, et al. showed the strong influence of consolidation on several critical properties of concrete including strength, bond to reinforcing steel, and resistance to chloride ion penetration. They also evaluated several existing nuclear (gamma radiometric) gauges and strongly recommended their use for monitoring consolidation during construction. They pointed out the value of density monitoring in evaluating the quality of concrete construction itself, rather than just the quality of the materials being delivered to the job site. Currently no procedures are in standard use to measure the in-place quality of concrete immediately after placement; that quality is not assessed until measurements such as strength, penetration resistance, and/or smoothness can be made after the concrete has hardened.

Gamma radiometry is also being used extensively for monitoring the density of roller compacted concrete. Densification is critical to strength development in these mixtures of cement (and pozzolans), aggregates and a minimal amount of water. After placement the concrete is compacted by rollers, much the same as asphalt concrete pavements.

Commerica1ly available nuclear gauges have become standard tools for insuring the concrete is adequately compacted.

Gamma radiometry has found limited application in composition determinations on PCC. When radioisotope sources emit low energy (below 60 keV) gamma rays, photoelectric absorption is the predominant attenuation mechanism, rather than Compton scattering. Since the absorption per atom increases as the fourth power of the atomic number Z, it is most sensitive to the highest Z element present in a sample. Noting that calcium in portland cement is the highest Z element present in significant quantities in PCC (in mixtures containing non-calcareous aggregates), Berry used ²⁴¹Am (60 keV gammas) in a prototype backscatter device

for measuring the cement content of fresh concrete. Mitchell refined the technique, and the resulting instrument was reported to measure cement contents to within ± 8% (at a 95%

confidence level) for siliceous aggregate mixtures and to within ±11% for calcareous aggregate mixtures. Because of the sensitivity of photoelectric absorption to Z, the cement content procedure required calibration on a series of mixtures of different cement contents for a given aggregate source. This sensitivity to aggregate composition remains a barrier to further application of the technique.

A short lived but interesting application of gamma radiometry is in pavement thickness determinations. As Equation 46 shows gamma ray absorption is a function of the thickness of a specimen. Therefore, a source could be placed beneath a PCC pavement, and, if a detector is positioned directly over the source, the count recorded by the detector would be a function of the pavement thickness. Researchers placed thumbtack-shaped ⁴⁶Sc sources on a pavement sub-base before a PCC pavement was placed. The sources were difficult to locate after the concrete was placed, however, and the technique was abandoned albeit with a recommendation that it deserved further research.

Tayabji and Whiting reported on two field tests using commercially available nuclear density gauges on PCC pavement. A typical data collection effort is shown in Fig. 14.3, where measurements are being taken during slipform paving operations from a platform that was part of a dowel inserter. Readings were taken in the direct transmission mode, with a 10 mCi

137Cs source inserted 8 in. (200 mm) into the pavement. The technician made a 15 sec. count at every third stop of the inserter unit, i.e. at about 42 ft (13-m) intervals.

Measurements were made at eight locations in each of eight lots. The average consolidation in the eight lots ranged from 98.9 to 100.2% of the rodded unit weight, with standard deviations within the lots ranging from 0.5 to 1.3%.

Density monitoring is critical on RCC projects since high density is needed to develop adequate flexural strength. On a pavement project such as the one shown in the figure, the concrete behind the paver typically has a density of 95% of the laboratory maximum, but will reach 98% after additional roller compaction.

Ozyildirim cautions that static gauges are not suitable for exact determinations of degree of consolidation in the field, because variations in component proportions or air content within acceptable ranges can cause variations in the maximum density attainable.

Fig. 14.4 is a photograph of the consolidation monitoring device (CMD) in use over a newly placed, conventional PCC pavement. The CMD is a non-contact backscatter density gauge, shown here with the source/detector unit mounted on a track on the back of a slip form paver. The unit rides back and forth transversely while the paver moves forward, thus monitoring the density of a significant portion of the pavement. The CMD uses a 500 mCi

137Cs source and a 1-3/4 in. dia. × 4 in. long (45 × 102 mm) sodium iodide scintillation crystal. Results indicate that the device is capable of duplicating core density measurements within a ± 2-1/4 lbs/ft³ (± 36 kg/m³) range at a 95% confidence level The CMD appears to be effective for tasks such as establishing proper vibrator operation, alerting vibrator malfunctions, and detecting significant changes in mixture composition, i.e. too little or too much air entrainment.

FIG. 14.3. Static nuclear density gauges in use during construction of conventional PCC pavement (Photo courtesy of FWHA).

FIG. 14.4. Dynamic nuclear density gauge (consolidation monitoring device) in use during construction of conventional PCC pavement (Photo courtesy of FWHA).