Achieving A High Level of Smoothness in Concrete Pavements Without Sacrificing Long-Term Performance
CHAPTER 8. RECOMMENDATIONS AND GUIDELINES
This chapter presents recommendations and guidelines for design features, PCC material properties, and construction procedures for constructing pavements having high initial smoothness while ensuring good long-term performance. Recommendations and guidelines for smoothness testing and use of profile data to troubleshoot problems are also presented in this chapter.
VALUE OF BUILDING SMOOTH PAVEMENTS
The roughness progression plots for JPC pavements in the LTPP program showed a parallel pattern in roughness progression. This finding indicates that pavements that are built smoother will provide a longer service life. These pavements will also provide road users with a better ride quality.
Construction of nondoweled pavements in freezing regions is not recommended. Nondoweled pavements in freezing regions have shown poor performance from a roughness point of view.
Some nondoweled pavements have shown an excessive amount of upward slab curvature. The upward slab curvature in these pavements has been increasing over time. These pavements have a high amount of faulting and are thus showing poor performance from a roughness point of view. In addition, many of these pavements are showing other distress. Factors associated with higher amounts of slab curvature over time in nondoweled pavements are high values of freezing index, CTE of PCC, and PCC elastic modulus. Higher values of the following factors were associated with lower curvature: mean annual temperature, annual precipitation, number of wet days per year, and slab thickness. To prevent an increase in upward slab curvature over time, it is recommended that dowels are used for all pavements constructed in freezing areas, as well as for projects that utilize PCC having a high CTE or a high modulus (greater than 35,000 MPa (5.08 million psi)).
The provision of dowels in pavements has served its intended function by preventing faulting. If there is any reason to believe there is even the slightest possibility for faulting to occur, dowels are recommended. Providing dowels will ensure a long lasting smooth pavement.
Doweled pavements with a joint spacing of 4.8 m (15.7 ft) or less seem to perform better than those having a higher joint spacing. It is recommended that SHAs utilizing a joint spacing much greater than 4.8 m (15.7 ft) investigate whether using 4.8 m (15.7 ft) or lesser spaced joints will give better performance.
PCC MATERIAL PROPERTIES
For both doweled and nondoweled pavements, using PCC with higher split tensile strength (which results in a higher flexural strength) appears to be beneficial for long-term performance from a roughness point of view.
In nondoweled pavements, PCC having high elastic modulus values (greater than 35,000 MPa (5.08 million psi)) or PCC having a high ratio (greater than 8,000) between elastic modulus of concrete and split tensile strength appear to be showing high rates of increase of roughness. These trends were not seen for doweled pavements.
Evidence suggests that higher values of coarse-to-fine aggregate ratio in concrete results in pavements that maintain their smoothness over longer periods.
A survey of State DOT personnel and concrete industry personnel showed no evidence suggesting that contractors have been adjusting their mix designs to achieve higher smoothness. The general consensus was that no modifications have been required in the concrete mix design to achieve higher smoothness. In addition, no pavement performance problems were thought to have been encountered because of the implementation of a smoothness specification.
An overview of the construction procedures needed to construct a smooth pavement were outlined in this report. More detailed descriptions are presented in ACPA publication Constructing Smooth Concrete Pavements(17) and FHWA publication PCC Pavement Smoothness Characteristics and Best Practices for Construction.(25) Adherence to these procedures is necessary to construct a smooth pavement.
Analysis of data from five projects indicated that the smoothest pavement was constructed in the project where the tie bars were attached to chairs fixed to the base before paving, whereas in all other projects the tie bars were inserted by the paver. The IRI of this project was 0.80 m/km (51 inches/mi). The IRI of the other four projects where the tie bars were inserted by paver were 1.11, 1.44, 0.95, and 1.07 m/km (70, 91, 60, and 68 inches/mi). Although fixing the tie bars to the base before paving may be more costly, these results indicate that doing so may achieve a smoother pavement.
MEASUREMENT OF SMOOTHNESS
The surface of the pavement must be clean when performing profile measurements. Residue from saw-cutting operation that is present adjacent to the transverse joints can appear as small humps on the measured profile, and these can affect the smoothness indices computed from the profile data.
The data collected by a profiler do not measure the shape of the joint accurately. Lightweight profilers from different manufacturers are using different methods to filter data. As a result, joints are being measured differently. Some profilers eliminate most of the effect of a joint by filtering, which flattens out the profile at the joint. There is a possibility that smoothness indices obtained by devices manufactured by different manufacturers are different because of differences in the way the joints are measured.
In many States, an initial sawcut is made on the pavement, a joint reservoir is formed, and then the joint is sealed. Profile measurements should not be obtained when the joint reservoir is formed and the joint has not yet been sealed. At that point, data can yield high roughness values if the joint reservoir appears in the profile data.
On transverse tined surfaces, when three repeat runs were obtained with lightweight profilers, the difference between the maximum and minimum IRI of the three runs over an approximate distance of 161 m (528 ft) typically ranged from 0.03 to 0.06 m/km (1.9 to 3.8 inches/mi). However, in longitudinally tined surfaces, this value was much higher and ranged from 0.06 to 0.09 m/km (3.8 to 5.7 inches/mi). Because of the interaction between the laser sensor and the longitudinal tining, the IRI obtained on longitudinally tined surfaces is less repeatable than that obtained on transverse tined surfaces.
The short-interval IRI repeatability over 15-m (9-ft) segment lengths for transverse tined surfaces showed that the average difference in IRI between the maximum and minimum IRI obtained from three repeat runs ranged from 0.09 to 0.11 m/km (5.7 to 7 inches/mi). These results for a longitudinal tined surface ranged from 0.12 to 0.25 m/km (7.6 to 15.6 inches/mi). However, when the IRI over the entire section was computed, the repeat runs showed very close IRI values, due to compensation effects that cause the IRI differences occurring within short intervals to cancel out. These results indicate that implementing an IRI-based specification that relies on short-interval IRI (e.g., 15 m (49 ft)) is not practical for pavements that have longitudinal tining.
Certifying profilers will ensure that consistent measurements are obtained between devices. Profilers used to measure smoothness of PCC pavements should be certified on PCC sections. There are differences in the way profilers treat joints as well as differences in the way tining is treated. Hence, certifying profilers on asphalt surfaces and using the profilers on a PCC section may not necessarily result in comparable data being collected on the PCC surface by the different profilers.
Testing at five projects over a 3-month period indicated that negligible built-in slab curling was present on the pavement. Curvature of the slabs changed little over the 3-month period, and no noticeable effect of slab curvature affecting the IRI was noted.
A study performed by collecting data at various periods over a 3-month interval at five projects showed somewhat variable results in changes that occur in IRI over the first 3 months after paving. In general, it appears that smoothness measurements can be performed at any time within the first few months after a project is paved. In some projects, very little change in IRI was noted (within ±5percent for a wheel path). In some projects, changes up to ±10 percent along a wheel path were noted. It is unclear whether these changes were occurring because of changes in pavement profile or whether they were related to either equipment effects or lateral wander during profiling.
That study also found that little change in RN occurred over a 3-month period from that obtained immediately after paving. For most cases, the RN measured at different times within a 3-month period varied from that obtained immediately after paving by about ±5 percent.
A study involving two profilers from two different manufacturers showed no evidence to suggest IRI or RN computed from data collected under the following two conditions would be different: (1) 3-mm (0.12-inch)-wide initial sawcut was present on the pavement and (2) joint reservoir sawed and sealed.
USE OF PROFILE DATA FOR CONSTRUCTING SMOOTH PAVEMENTS
Usually smoothness indices like IRI are computed for each lane over a 161 m (528 ft) length for construction acceptance. The IRI for the overall section does not provide any information on how IRI varies within the section or where rough spots within the section are located. A roughness profile of the section can be used to investigate how roughness varies within the section and where rough spots within the section are located. If rough spots are detected within the section, their location could be correlated to the construction process to obtain information on a specific construction event that occurred at the location that resulted in a high roughness value. Overlaid roughness profiles of the wheel paths of the inside and outside lane can be used to see how roughness varies across the pavement. Using this approach may help detect problems in the construction process that result in rough spots.
Advanced profile analysis techniques, such as PSD plots, can be used to identify whether a roughness associated with a specific wavelength is present in the new pavements. A wavelength that significantly contributes to the roughness will appear as a spike in the PSD plots. If such spikes are detected, the cause for the prominent wavelength to occur in the profile can be investigated. Repetitive wavelengths in the profile can be caused by equipment that is used for paving, finishing process, or stringline effects.
Using roughness profiles as well as PSD plots on profile data collected at the start of a paving project will indicate whether there are any features in the profile that are contributing to a high roughness. If such features are identified, they can then be corrected at the start of the project.
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