No More Nukes
Permeable interlocking concrete pavements (PICP) require open-graded, crushed aggregate subbases and bases for water storage and infiltration into the soil subgrade. For vehicular areas, ICPI recommends using a subbase of ASTM No. 2, 3 or 4 aggregate sizes. This layer is topped with a 4 in. (100 mm) base of ASTM No. 57 or similar-sized aggregate. A 2 in. (50 mm) thick bedding layer of smaller aggregate (typically ASTM No. 8) and concrete pavers are placed on the base layer and compacted. The joints are filled with permeable aggregate and the pavers are compacted again.
As with all pavements, PICP subbase and base layers must be compacted. Lift thickness should be no thicker than 6 in. (150 mm). Maximum lift thickness can increase to 12 in. (300 mm) if a 10-ton roller compactor seats it (Figure 2). A large, reversible plate compactor can be used as well, and will be required to compact next to curbs, foundations and utility structures, as well in corners, i.e., places where roller compactors cannot reach. Large means 13,500 pound-force (60 kN) and equipped with a compaction indicator (Figure 1). These machines weigh around 900 lbs (~400 kg), so they require moving assistance with a forklift or forks attached to a Bobcat-type equipment.
A core question is determining the level of compaction. The compaction indicator on a plate compactor tells the operator when the machine is finished compacting. That is a good start. An effective way to determine compaction levels is by measuring deflection of the compacted stone. This is done with lightweight deflectometer or LWD. Figure 3 illustrates the device that uses a shaft-guided, 22 lb (10 kg) weight that when allowed to fall, hits a plate. The impact force simulates an instantaneous pressure from a car tire passing over the pavement. LWD instrumentation records the movement of the aggregate surface in millimeters, calculates the stiffness and provides GPS coordinates.
An LWD is useful for checking post-compaction deflection of compacted, open-graded aggregate subbase and base for PICP. An LWD can also be used to test deflection on compacted, dense-graded aggregate and subgrade soils. For these materials, LWDs are replacing nuclear density measurements. Initial acceptance by the Indiana and Minnesota Departments of Transportation likely has initiated the start of acceptance by other DOTs in the near future. Indiana and Minnesota DOTs developed quality control/quality assurance test specifications after their research and by the National Cooperative Highway Research Council in their 2014 Synthesis 456, Non-nuclear Methods for Compaction Control of Unbound Materials.
Open-graded aggregates can be difficult to test for compacted density using a nuclear density gauge. ‘Nuke’ testing must be done in backscatter mode where the gauge probe is not inserted into the material. See Figure 4. Gamma rays are shot into the material and some bounce back to the bottom of the device and measured. This test method must be used because the probe cannot penetrate compacted, open-graded aggregates. The method can produce high variability measurements. Moreover, the nuclear gauge operator and device must be certified since the latter contains radioactive material. The LWD has none of these restrictions.
A draft national standard, ASCE PICP design, construction and maintenance guide, is in its final stages and will likely be approved and published later this year. It includes a guide construction specification with a test method using an LWD. Testing follows ASTM E2835 Standard Test Method for Measuring Deflections Using a Portable Impulse Plate Load Test Device. The specification provides guidance on the minimum number of tests including those close to adjacent curbs, pavements and buildings. The specification includes limits for variation in measured deflection as well as a maximum allowable deflection. These are currently drafted at 0.05 mm and 0.5 mm, respectively based on in-situ tests. The test objective is to have minimum variability in stiffness across the pavement subbase and base, thereby minimizing settlement and callbacks, as well as the rate of rutting from vehicle tires.
An existing ASTM test method with acceptance criteria to help contractors, engineers and project owners verify the level of compaction on a project through measuring deflection. The LWD can measure deflection in maximum 12 in. (300 mm) thick lifts, so that provides another reason (besides time and money savings) to use equipment that can compact 12 in. (300 mm) of subbase aggregate all once. The LWD costs about $8,000, about as much as a high-end nuclear density gauge without the extra user time for certifications.
As the permeable pavement market grows, the LWD will be the means for checking if open-graded base compaction achieved consistent results. As a tool for assessing compaction of soils and dense-graded bases, it can be immediately applied to regular interlocking concrete pavement construction as well as to other pavements and subgrade soils.