Step 5: Install the test strip
Test strip placement is the perfect time to make sure the paver you’re going to use on the job is operating properly. Spot check for proper thickness as paving progresses.
If the project is too wide to place in a single pass, place at least two loads on one pass. Then back up and do the second pass. About 12 inches of the first pass won’t be rolled. When the adjacent RCC is placed, those 12 inches are rolled with the subsequent pass to produce a homogeneous pavement.
The intersection of the two passes is a longitudinal joint. There are three types with RCC: fresh, cold, and horizontal.
A joint is considered fresh if RCC is placed against previously placed RCC within 60 minutes and the joint remains moist. If not correctly constructed, cracks will occur.
Density is critical for cold joints, which are any joints that don’t qualify as fresh joints. Most contractors use edging shoes within 10 degrees of vertical to meet density requirements. Cold joints that don’t should be cut using a full-depth saw beginning at least 6 inches from the exposed edge.
Remove loose and foreign material before placing fresh mixture against a compacted cold joint. Wet the vertical face immediately before placing the adjacent lane.
Horizontal joints are lifts of more than single-pass maximum of 10 inches. Place subsequent lifts within 60 minutes to ensure adequate bonding and create a monolithic slab. If that deadline can’t be met, the engineer can require that supplementary bonding materials be utilized. Keep the bottom lift’s surface moist and apply the materials right before placing the subsequent lift.
A test strip also ensures the project achieves proper elevation.
RCC is often used on local streets and roads, where it’s placed between concrete curbs. If it’s the finished surface, pavement and curb elevations must match. If the RCC will receive an asphalt wearing surface, the amount of roll down must be predictable for the thickness of the asphalt surface to be uniform.
Predictable roll down is also important at joints. If the roll down is underestimated, a lack of compaction can occur at the joint and lead to future durability issues. If the pavement doesn’t roll down as much as anticipated, there won’t be uniform elevation between placements.
Finally, curing is critical. Due to the large areas quickly being placed, the proper equipment must be used to cure the surface before it has a chance to dry out. A test strip enables the contractor to dial in the required curing compound application rate to properly cover the pavement’s surface.
Next page: Step 6
Step 6: Quality control/assurance
As previously noted, pavement strength and durability are affected by both mix design and density. Thus, verifying moisture content is absolutely critical.
Density and moisture content are monitored via the nuclear gage calibrated to the modified proctor of the approved mix. Wet density should be used because the cement’s hydrogen ions can provide misleading nuclear density results.
Once the wet density and moisture content are known, the dry density and percentage of maximum dry density can be calculated. One note: Density gauges can be excellent indicators, but they’re prone to inaccuracies. Consequently, many concrete producers calibrate hand-held moisture meters and use burn backs to provide a mixture at the proper moisture content.
As the test strip progresses, routinely test density directly behind the paver and after each rolling pass. Use this data to develop a targeted rolling pattern. Then density-test to verify proper compaction throughout the project.
Most specifications require in-place strength be verified. Do this by sawing specimens from the pavement. The test strip is an excellent opportunity to verify that the mixture and construction techniques produced results similar to those determined during the mix development stage.
Remember that temperature substantially impacts strength gain. Cylinders tested during the mix development phase were cured at 73° F. If the pavement is placed in warmer conditions, it will gain strength faster than the lab-cured specimens. Conversely, a colder placement environment will slow the rate of strength gain.
During the test strip, the relationship between lab-cured specimens and in-place cored cylinders or sawed beams can be developed.
A lab-cured ASTM C1435 cylinder should be made. Nuclear density testing can be performed on the pavement made with the same load of RCC sampled and tested. Cores taken from the pavement can be tested in compression or split tensile. Sawed beams can be tested for flexural strength.
By weighing and measuring each core and sawed beam, the density of the in-place pavement can be calculated and compared to the nuclear gage density results. The quality control team can then analyze all of the data to fully understand the pavement’s in-place strength.
The construction team can also implement maturity methods based on forecasted weather. This information will be valuable when they’re working long hours to complete a project before cold weather sets in.
Written by Christopher Tull of CRT Concrete Consulting LLC, this article originally appeared in the May 2013 issue of Concrete Construction and was updated by Chris Carwie of A.G. Peltz Group LLC. E-mail ccarwie@agpeltz.com or chris@crtconcreteconsulting.com.
