Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-068
Date: October 2005

Achieving A High Level of Smoothness in Concrete Pavements Without Sacrificing Long-Term Performance

CHAPTER 3. DESIGN AND CONSTRUCTION FACTORS THAT AFFECT INITIAL SMOOTHNESS

INTRODUCTION

Factors that can influence the initial smoothness of a PCC pavement surface generally can be separated into the following categories:

The effect of each factor on pavement smoothness is described separately in the following sections.

PAVEMENT DESIGN FACTORS

Design features such as base type and base width, horizontal alignment, and embedded items in the pavement (dowels, steel reinforcement) can have an impact on the initial smoothness of a PCC pavement. The effect of each factor is described separately.

Base/Subbase

One of the most important design considerations for PCC smoothness is the provision of a stable and smooth track line.(16) The track line is the path that wheeled tracks of the slipform paving machine will follow while paving. Providing an even and stable track line is essential for constructing a smooth concrete pavement. Irregularities in the track lines cause the profile pan of the paving machine to continuously adjust its position relative to the machine frame and can cause bumps or dips on the pavement surface.

The simplest way to provide a stable and even track line is to design the base layer 1 m (3.3 ft) beyond the edges of the PCC slab.(17) For concrete overlay projects, special provisions may be necessary to stabilize the track lines as part of the preoverlay preparation. Subbase material stability is another important consideration. Materials stabilized with cement or asphalt and dense-graded granular materials create firm support for construction equipment. Unstabilized permeable layers have caused some placement and performance problems. An important balance must be met between the degree of drainage and the stability of an unstabilized subbase layer.

Horizontal Alignment

It is more difficult to construct a smooth surface for PCC pavements along horizontal curves than those on tangents because of the transitions for superelevation. Generally, roughness is more prevalent in transitions and superelevated portions of a horizontal curve than on tangents. In the transition sections, the profile pan must adjust to meet the varied cross slope requirements of the 18 curve. As with an uneven track line, the constant adjustments of the paving machine can adversely affect the smoothness of the pavement.

As the horizontal curvature increases, the potential for roughness within the curve increases. When the degree of curvature exceeds 6 degrees (or the radius of curvature falls below 300 m (984 ft)), the contractor must focus increased attention to the machine operation and the stringline-staking interval. It has been suggested that when the curvature exceeds 7 degrees, it is virtually impossible to construct the surface to the same specified tolerance desirable for a tangent section because of the significant corrective adjustments necessary by the equipment.(17,18) Pavement smoothness on horizontal curve sections can be improved by reducing the distance between staking rods.

Embedded Items

Plain jointed concrete pavements carrying truck traffic are usually equipped with dowel bars. There are two procedures for incorporating dowel bars into a PCC pavement: inserting the dowel bar into the plastic concrete during construction or placing dowel baskets on the base before placing the concrete. Steel reinforcements are used during construction of jointed reinforced concrete pavements and continuously reinforced concrete pavements. The use of embedded items such as dowel bars and reinforcements can affect the smoothness of the pavement. According to ACPA, four main conditions can cause roughness because of the use of dowel bars and reinforcement.(17) These conditions are lack of consolidation, reinforcement ripple, springback, and damming:

For local roads, additional embedded items may include utility boxouts, cast-in-place fixtures, traffic signal handholds, and drainage structures. These items may affect the pavement smoothness because they require extra handwork vibration and finishing efforts to blend them 19 into the surrounding pavement surface. Ideally, in-pavement objects should be in position before placing the concrete to minimize any handwork.

CONCRETE MIX DESIGN

Concrete mix design can have a significant effect on the smoothness of concrete pavements. (See references 17, 19, 20, and 21.) The ACPA publication Constructing Smooth Concrete Pavements states: “The concrete mixture should be proportioned to assure proper consolidation without excessive vibration. This is achieved through optimization programs that develop mixtures containing well-graded aggregates. These mixtures are not harsh and unworkable, and they flow easily when vibrated, and consolidate well around embedded fixtures and reinforcement.”(17)

When evaluating concrete mix design and quality, many elements need to be considered. These elements include uniformity, workability, finishability, strength, durability, economics, and time required for initial set. The elements having a direct impact on the as-constructed pavement smoothness are uniformity, workability, finishability, and time required for initial set. Strength and durability will influence the roughness progression over time. The following sections describe the ways in which these factors related to mix design could influence pavement smoothness.

Mix Design and Proportions

The concrete mix design can affect the smoothness achieved during pavement construction. The concrete mix design influences the workability and slump of concrete, which have a direct impact on the ease of concrete placement and finishing. The concrete mix design will influence how well the slab can be extruded by the paver into its final configuration.

Harsh concrete mixes slow down the paving operation and require extra effort for finishing. The equipment needs to work harder to spread and extrude harsh concrete mixes. The pavement will require more finishing, resulting in a rougher surface. A harsh and unworkable mixture can override other attempts by a contractor to attain a smooth surface.

A quality concrete mix must have the proper blend of good quality aggregates, appropriate water to cement ratio, and proper air entrainment system for durability. Materials in the mix must be uniform in nature. In addition, to achieve better smoothness, concrete mixes should contain enough fines to relieve harshness. Some contractors have suggested that concrete mixes with a slump between 50 to 65 mm (2.0 to 2.6 inches) work best to construct smooth pavements.(19)

In the 1990s, distresses appeared in some newly constructed concrete airfield pavements. After investigating these pavements, the Air Force Civil Engineering Support Agency concluded that in general, the concrete designed with well-graded combined aggregates performed better than that with gap-graded or poorly graded aggregates. Consequently, the U.S. Air Force has specified that Shilstone-type mix design should be used for rigid airfield pavement construction.(22)

The use of well-graded aggregates can reduce water demand and improve concrete workability, finishability, and strength. In the Shilstone-type mix design procedure, the gradation of the 20 combined aggregate is evaluated graphically to see whether the gradation is acceptable.(23,24) This procedure is illustrated in figure 11. The percentage retained for each reporting sieve size (Y-axis) is plotted versus the considered sieve size (X-axis). The resulting line should have a relatively smooth transition between coarse and fine aggregates. The dotted lines in the figure represent the maximum and minimum percent retained on each sieve size that is used as a guide by the U.S. Air Force.(22) Figure 11 shows two lines representing satisfactory and unsatisfactory combined aggregate gradations.

Figure 12 shows the plot of workability factor versus coarseness factor, another criterion that is evaluated in the Shilstone mix design procedure.(24) The coarseness factor for a combined aggregate gradation is determined by dividing the amount retained above the 9.5-mm (0.4-inch) sieve by the amount retained above the No. 8 sieve, and multiplying the ratio by 100. The workability factor is the percentage of combined aggregate finer than the No. 8 sieve. The plot has five zones; the unique characteristics of each are:

The unlabeled zone on top of Zone V is referred to as the trend bar. It is the transition zone between Zone V and the other zones on top of the trend bar.

Figure 11. Percent of combined aggregate retained.(22)

Percent of combined aggregate retained. This figure shows a plot of percent retained versus American Society for Testing and Materials (ASTM) sieve size. The X-axis of the plot shows the ASTM sieve size, while the Y-axis shows the percent retained. Two trapezoids are shown by dotted lines in the figure. The four points that define the first trapezoid are: sieve size 19 millimeters (0.75 inch) and percent retained 0, sieve size 12.5 millimeters (0.5 inch) and percent retained 8, sieve size number 30 and percent retained 8, sieve size 50 and percent retained 0. The four points that define the second trapezoid are: sieve size 25 millimeters (1 inch) and percent retained 0, sieve size 19 millimeters (0.75 inch) and percent retained 18, sieve size number 50 and percent retained 18, and sieve size number 100 and percent retained 0. There are two solid lines in the plot. One solid line falls inside the larger trapezoid, but is outside the smaller trapezoid, and is labeled as satisfactory. A portion of the other line falls inside the smaller trapezoid, and it is labeled as unsatisfactory.

Figure 12. Workability factor versus coarseness factor chart.(24)

Workability factor versus coarseness factor chart. The X-axis of this figure shows coarseness factor that shows values between 0 and 100. The Y-axis shows workability factor that shows values between 45 and 100. Solid lines divide this chart into five regions, and these regions are numbered from 1 to 5. There is an unlabeled zone above zone 5.

Aggregate

Aggregates play a major role in affecting the workability of a concrete mixture and thus affect pavement smoothness. Aggregate factors that affect pavement smoothness include gradation, particle shape, stockpiling and handling of aggregate, and moisture in the aggregate.

It is generally recognized that consistent gradation of aggregate is essential for producing concrete mixes with better smoothness. Variability in gradation can cause a change in water demand, resulting in inconsistent workability of the mix from batch to batch. An inconsistent concrete mix affects the paving operation directly because the paver must make constant adjustments, resulting in a rougher pavement. Poor aggregate gradation decreases mix workability and often causes segregation of the concrete mix.

Aggregate particle shape can also influence the workability characteristics of concrete mixtures, such as consolidation, flow, and the finishability. Natural (rounded) aggregates produce more workable concrete mixes, while angular manufactured aggregates can produce harsh mixes, and thus result in rougher surfaces.

Attention to aggregate stockpiling and handling is also important for producing good quality concrete mixes. Improper stockpiling and handling of aggregate can cause segregation in the concrete mix, which makes it impossible to produce uniform concrete. In addition, slump, yield, and air content can also be affected.

Moisture content of the aggregate plays an important role in obtaining a uniform and quality concrete mix. Uncontrolled aggregate moisture can lead to a loss of concrete uniformity. Variable moisture content in the aggregate can produce concrete mixes with variable slump. If not accounted for in the mix design, excessive moisture in the aggregate will likely reduce concrete strength and increase the shrinkage cracking potential because of the high water-to-cement ratio.

Admixtures

Admixtures in concrete mixes have a direct impact on concrete quality and properties and thus can influence pavement smoothness. Experience has shown that different types of fly ash and the use of water reducing and air-entraining agents affect the workability of concrete mixes, and hence the smoothness of the pavement. Fly ash in concrete can improve the workability of the mix because of the smaller size and spherical shape of fly ash particles. Although fly ash usually slows the early strength gain of concrete, it can ultimately increase the long-term concrete strength.

An air-entraining agent improves the durability of the concrete and has a positive effect on pavement smoothness because it increases the slump (workability) of the concrete. In general, air content in the 5 to 7 percent range helps improve the workability. Concrete mixes with low air content tend to bleed, whereas mixes with high air content will make the concrete mix sticky and unworkable.

The use of a water reducer in the concrete mix also has a positive impact on pavement smoothness because it will result in a lower water-to-cement ratio and better mix consistency. The use of retarder admixtures is also beneficial to the concrete mix because the retarders can keep the mix workable in hot and/or windy weather or when long-distance transportation is required.

Influence of Ride Specification

Since the mid-1980s, most State DOTs have developed and implemented a ride quality specification for concrete pavements. This study investigated whether modifications made to concrete mix designs improved smoothness yet had a detrimental influence on other concrete properties such as strength and durability. A literature search was conducted, then State DOT personnel and industry organization personnel were contacted to explore whether changes in concrete mix designs have been necessary after the implementation of a ride specification.

The literature search did not identify any documents that directly address the issues of pavement smoothness related to concrete mix design. The following organizations were asked for their input on this issue: Florida DOT (FDOT), Colorado DOT (CDOT), Virginia DOT (VDOT), Iowa DOT, Northeast Chapter of ACPA, and Western Pennsylvania ACPA. Most respondents said they believed that no modifications have been required in the concrete mix design to achieve higher smoothness. They also reported that no pavement performance problems have been encountered because of the implementation of a smoothness specification. However, it was mentioned that better mix designs with a more uniform aggregate gradation might improve the constructability of the concrete mix, and hence result in higher smoothness.

A representative from FDOT indicated that Florida has implemented a smoothness specification for concrete pavement construction. After FDOT personnel approve the mix design for a project, it cannot be changed without the approval from the FDOT engineers. No major performance problems have been reported on recently constructed PCC projects.

A CDOT representative indicated that as long as the concrete mix design meets the strength specification, it should not require any modifications for smoothness specification. It was suggested that smoothness is mostly affected by the paving equipment and the construction process.

A representative from VDOT indicated that Virginia has implemented a ride quality specification for concrete pavement construction. In the past, contractors have not consistently achieved the specified degree of smoothness (in terms of IRI) during construction. However, improved pavement designs and concrete placement practices have resulted in more projects meeting or exceeding the smoothness criteria. In Virginia, contractors generally have the flexibility of proposing the concrete mix design for paving operations. The proposed mix design requires approval from VDOT personnel. The State utilizes performance-based specifications for its concrete mixes, using strength and air content as the acceptance criteria.

The Executive Director and Chief Operating Officer of the Northeast Chapter of ACPA stated that concrete mixes with more uniform combined aggregate gradation, such as those proposed by Shilstone, have improved workability and other properties that promote better pavement 24 smoothness. Using this type of mix design has these advantages over traditional concrete mix designs:

The Director of Paving Services for Western Pennsylvania ACPA agreed that using more uniform aggregate gradation in concrete mix design might reduce shrinkage and therefore slab warping. The better aggregate gradation will also improve workability (not slump) of the concrete mix. However, the impact of these improvements on pavement smoothness has not been studied and therefore cannot be determined with much confidence.

Although not directly related to the use of ride specification for concrete pavement construction, input from Iowa DOT indicated that this State has adopted concrete mix design procedures based on the Shilstone approach since 2000. Some past mix designs might have been produced with a high sand content or with gap-graded aggregate. By using more uniformly graded aggregates in the mix design, the concrete mixture is more workable, requiring less water in the mix, and resulting in reduced shrinkage. The mixtures also require less vibration energy to achieve the desired consolidation. For example, the combined aggregate gradation of the concrete mix for a concrete pavement with poor performance is shown in figure 13, which indicates that the aggregate gradation is in the unsatisfactory category (i.e., percentage retained for some sieve sizes fall within the lower band). When the coarseness factor and the workability factor of the combined aggregate for this concrete mix are plotted, the point falls within Zone I shown in figure 12. This zone is associated with gap-graded mixes, which have a high potential for segregation. Similar plots were also observed on several other concrete projects exhibiting poor performance.

Figure 13. Aggregate gradation for a concrete project with poor performance.

Aggregate gradation for a concrete project with poor performance. The Y-axis of the chart shows the percent retained, with values ranging from 0 to 20. The X-axis shows the sieve size, with values ranging from number 200 to one-and-one-half inch. Two gradation bands represented by two trapezoids are shown in the chart. The four points that define the first trapezoid are: sieve size number 50 and percent retained 0, sieve size number 30 and percent retained 8, sieve size one-half inch and percent retained 8, sieve size three-quarters inch and percent retained 0. The four points that define the second trapezoid are: sieve size number 200 and percent retained 0, sieve size number 100 and percent retained 18, sieve size 1 inch and percent retained 18, and sieve size one-and-one-half inch and percent retained 0. The values of percent retained for individual sieve sizes of a combined aggregate used for a portland cement concrete (PCC) mix is plotted on this figure, and straight lines connect the data points. Two of the straight lines, one connecting percent retained values for number 16 and number 8 sieves, and other connecting percent retained values for number 4 and number 8 sieves, fall within the smaller trapezoid.

CONSTRUCTION OPERATIONS

The key construction factors that can influence pavement smoothness are:

Stringline Setup and Maintenance

A stringline is used to provide an accurate reference for elevation and alignment control of the subgrade surface, placing the base layer, and concrete paving. The stringline is the primary guidance system for a slipform paver operation. The paver's elevation sensing wand rides beneath the string, and the alignment-sensing wand rides against the inside of the stringline. Accurate elevation of the stringline is essential because the sensors controlling the profile pan adjust based on the location of the stringline. Therefore, close attention to the stringline setup and maintenance is required to achieve a smooth pavement surface. Neither the elevation sensing wand nor the alignment sensing wand should measurably deflect the stringline.(17)

The interval between stringline stakes is also important. On tangent sections, a maximum staking interval of 7.6 m (25 ft) typically produces excellent results. Decreasing this interval in horizontal and vertical curves is usually necessary to construct a smooth pavement.(17) Guidelines for staking intervals on horizontal and vertical curves are presented in reference 17.

Associated stringline factors such as stringline material, staking interval, splices, and repositioning frequency can affect the pavement smoothness. Acceptable stringline materials include wire, cable, woven nylon, or polyethylene rope.(17) Air temperature and relative humidity variations during the day can cause sag in the stringline between the stakes.(17) More tension in the stringline allows less sag to occur, even with substantial changes in weather conditions. The staking system normally includes hand winches placed at intervals not more than 300 m (984 ft), allowing the line to be tightened to prevent sagging between stakes.

The splices in the stringline need to be clean and tight. Stakes for securing the stringline should be long enough to be able to firmly embed the stake below the subgrade surface. The above-grade stake length must be adequate to allow adjustment of the stringline to the desired height above the subgrade profile.

Care must be taken to avoid tripping over or disturbing the stringline. If the stringline is disturbed, it must be checked immediately and repositioned to avoid bumps or dips in the pavement. Regular visual inspection of the stringline is recommended. If problems are detected during this inspection, surveying equipment should be used to check the grade of the stakes and stringline.(25) Stringline setup should be kept to a minimum during a project. A reduced number of stringline setups can lead to better smoothness control.

In many paving operations, stringline guidance is setup on both sides of the paver. Advantages offered by using dual stringlines include the ability to see problems before paving, control of yield loss, and the assurance of greater accuracy of the finished grade.(26) ACPA also reports the use of dual stringlines can be beneficial for wide sections with a uniform cross slope. In these cases, small deviations in a single stringline can propagate into large variations in the surface elevation on the other side of the paving machine.(17)

Grade Preparation

Base course placement is considered a very important element affecting concrete pavement smoothness. Good uniformity and stability of the base course is essential for achieving smooth pavements. Because concrete pavements are usually built in one layer, there is only one opportunity to average out roughness from the base surface. It is critical that the base surface be as true to grade as possible before placing the concrete.

For a granular base layer, automated fine-grading equipment guided by a stringline will create the best base surface possible. Fine-grading equipment is easily capable of meeting specifications within a tolerance of ±12 mm (±0.5 inch), when controlled by a stringline. Close control is also required for constructing stabilized base to facilitate pavement surface 27 smoothness. However, instead of using fine-grading equipment, an operator can use skill and experience, which often contributes significantly to meeting surface tolerances for stabilized base materials.

Concrete Consistency

The importance of consistent concrete in ensuring a smooth concrete pavement must not be underestimated. Good batch-to-batch consistency of the concrete mixture improves the quality of the finished pavement because it affects how paving equipment performs. The main goal is to avoid alternating wet and dry concrete batches, which would induce constant equipment adjustment and make it difficult to produce a smooth pavement surface.(19)

Stationary (ready-mix) plants, onsite batching and mixing plants, and truck mixing operations are all capable of producing concrete with consistent properties. However, to achieve consistency between batches, it is important to follow quality control procedures during batching, mixing, hauling, placing, and finishing. A regularly scheduled review of each phase in the operation can identify problems.

Another important item for consideration in producing consistent concrete mixes is sound moisture management. A concrete mixture is sensitive to increases in moisture, especially from free moisture in the fine aggregate. Normally, the moisture content of the sand has more impact on the concrete mix than the moisture content of the coarse aggregate. A mix adjustment to account for the free water in the sand may be required to maintain the water-to-cement ratio and workability.

According to ACPA, the following are important considerations that can help control batch-to-batch variations in concrete consistency:(17)

Concrete Delivery

Once the concrete has been mixed, uniform delivery of the concrete to the job site has a direct impact on the smoothness of the finished concrete pavement.(27) A slowdown or stoppage in the paving process due to a lack of concrete results in bumps or dips on the pavement surface. Consistent delivery of concrete to the paving project is essential for keeping a steady operation. Keeping a consistent delivery schedule is particularly challenging in urban areas; it requires careful evaluation to predetermine whether traffic delays will hamper concrete delivery.

Feeding concrete to the paving machine consistently requires an adequate number of delivery trucks. The delivery of concrete dictates the slipform paver speed. The entire cycle of mixing, discharging, traveling, and depositing concrete must be coordinated for the mixing plant capacity, hauling distance, and paving machine capability.

In a study performed in Argentina, pavement smoothness measurements were obtained on eight concrete pavement sections using an inertial profiler.(28) IRI computed from the profile data was used to judge the smoothness of the pavement. The variables evaluated in this study included the type of paver, the rate of concrete placement, concrete delivery, and concrete plant type. One conclusion from the study was that concrete production and delivery had an effect on concrete pavement smoothness. The pavement sections with shorter concrete delivery distances showed lower IRI.

Construction Equipment

The use of proper paving equipment is also a requirement for constructing smooth concrete pavements. Attention must be paid to function, weight, and size in selecting the most appropriate equipment for the job.(29)

On many mainline paving projects, contractors use a paving train that includes a placer/spreader in front of the slipform paver. A paving train is required when constructing a continuously reinforced concrete pavement. A placer/spreader is beneficial for attaining smoothness. A placer/spreader provides a consistent amount of concrete in front of the paver and allows the paver to maintain a constant speed.

Clean and well-maintained equipment is also key to achieving an efficient and quality paving operation, which in turn affects pavement smoothness. Not only does this refer to paving equipment, but also to mixing and delivery equipment. A very high correlation has been established between equipment condition and pavement smoothness. Loose parts or connections result in poor paver response to changes in trackline and base conditions and can result in poor pavement smoothness. Poorly maintained equipment is also more prone to breakdown, resulting in unexpected stops.

All other things being equal, a heavier paver generally produces a smoother pavement because it is less affected by surges of concrete coming into the paver. In a study conducted in Argentina, eight concrete pavement sections were compared on three highway and airport facilities that 29 were constructed using slipform pavers with different weights. One of the study's conclusions is that pavement sections constructed with the heavier pavers had lower IRI values.(28)

A heavier paver is generally stiffer and more resistant to lifting/buoyancy forces from the concrete. The increasing trend of using end result strength specifications is resulting in stiffer concrete mixes. It is therefore important that the weight of the paver is adequate for placing a stiffer concrete mix. Paver size may also have an effect on concrete pavement smoothness. A larger paver of the same capacity will provide a smoother pavement because it can handle concrete more easily and respond more precisely.

Slipform Paver Operation

A slipform paver spreads and consolidates the concrete as it moves forward. Many factors can affect the operation of the slipform equipment and influence the pavement smoothness. Controlling concrete consistency (mix control) and delivery will result in a steady operation and a smoother surface. The paver cannot produce adequate results if it must stop often or push a large pile of concrete. Set out below are the essential factors that must be followed for constructing smooth pavements. (See references 17, 28, 30, and 31.)

Proper Paver Setup

All slipform paving equipment contains molding components, including a profile pan and side forms. The base or subbase is at the bottom of the mold. These elements confine the concrete and create the mold for the pavement shape. A slipform paver also contains tools that help fill the forms and create a uniform shape. These tools include the auger spreader, spreader plow, strikeoff, and/or tamper bar. Setting up the paver is an important factor affecting the smoothness of the finished pavement surface. The proper steps in setting up the paving equipment must be followed. Attention also must be given to ensuring effective adjustments during the paving operation.

Adequate Paver Travel Speed

A constant paver speed is essential for producing a smooth pavement. There are no set standards for the proper paver speed. The paver speed is proper when it can be supplied with a constant supply of concrete without having to slow down or stop during the paving process. It is essential to avoid stopping the paver during the paving operation to reduce the bumps or dips in the pavement surface.

Vibration of Concrete

In slipform paving, the mold is forced through concrete that remains static on the grade. However, vibrators mounted to the slipform machine are essential to fluidize the concrete and make it easier to mold. The slipform paver passes over the fluidized concrete, with its mass keeping the pan and side forms steady to confine and shape the material. In a steady paving operation, vibration of the concrete as it passes through the paver influences the surface and the resulting smoothness. Too much vibration causes pumping and fluffing at the rear of the pan and brings excess grout to the surface. Insufficient vibration creates drag-out and surface voids. Vibration of concrete is not a cure for other problems in the concrete mix. Vibrators may identify and exacerbate a concrete mixture problem. Adjusting the frequency of vibrators will not overcome poor equipment set up, poor alignment, or poor mixtures. When operating at a very high frequency, vibrators may cause undesirable results such as loss of air entrainment or vibrator trails

Concrete Head

Maintaining a consistent and adequate head of concrete in front of the paver can improve the smoothness of a pavement. Maintaining a consistent head of concrete will ensure consistency in the speed of paving, vibration, and consolidation. A consistent head of concrete evens the pressure on the paver so that when required, adjustments to the paver can be made more accurately. If the head of concrete gets too high, it might create a pressure surge under the paving machine that can cause the concrete behind the paver's profile pan to rise up and create a bump. If not enough concrete is placed in front of the paver, the concrete head may run out or the grout box may run empty, creating low spots or voids on the pavement surface.

Communication

Effective communication measures between machine operators and between the paving operation and the concrete batch plant must be implemented. Constant communication between the plant and the paving operation allows the slipform operator to match the paver speed to accommodate concrete delivery.

Finishing, Texturing, Curing, and Headers

Finishing

If the paver has been operated properly, only minimal hand finishing is required.(25) In many cases, the finished profile of a pavement is worsened by hand finishing. Hand finishing the pavement surface using bullfloats is only necessary when the surface contains voids or imperfections. Some contractors overuse mechanical longitudinal floats directly behind the slipform equipment. In general, it is best to limit hand and mechanical finishing. If longitudinal floating is the only method to produce an acceptably closed surface, some corrections are needed to the concrete mixture and/or paving equipment.(17)

Checking the surface behind the paver with a 3- to 7.6-m (9.8- to 25-ft)-wide hand-operated straightedge is a recommended procedure to finish the concrete.(17) Successive straightedging should overlap by one half the length of the straightedge to ensure that high spots are removed and low spots are filled.(17) Experienced finishers can use the straightedge to remove or reduce noticeable bumps by using a scraping motion.(17) In some cases, the profile of the finished pavement shows surface waves that were likely induced or augmented by the improper use of a straightedge.

Texturing

The surface texturing operation does not usually affect pavement smoothness. However, transverse tining equipment has been known to cause surface roughness when rakes do not run over the surface evenly.

Curing

In a recent study, profiles were measured on eight test sections on four newly constructed concrete pavements located along I-70 and I-135 in Kansas.(32) The pavements, with concrete slab thicknesses of 275 and 312 mm (10.8 and 12.3 inches), were placed on top of a stabilized drainable base that was on a lime-treated subgrade. The effect of four different parameters on as-constructed pavement smoothness, as represented by IRI, was assessed. The four parameters were lane (driving and passing), wheel path (left and right), curing condition (single or double curing compound applications), and time of paving (morning or afternoon). Various statistical techniques were employed to analyze the IRI data. Results of the study showed that application of curing compound was the most significant factor affecting the IRI value, with lower IRI being obtained for the double application of the curing compound.

Headers

Headers (transverse construction joints formed at the end of a day's work) are one of the most significant contributors to concrete pavement roughness. Most pavers leave a dip in the slab when they lose the head of the concrete. Headers typically require hand finishing, which makes controlling tolerances harder.

Two possible solutions have proven effective in minimizing ride problems at a header. The first method is to avoid forming headers and use a cut back method to create the joint. In this method, the paving operation continues until all the concrete is used. The following morning, a transverse sawcut is made 1.6 m (5.2 ft) or more from the end of the hardened concrete slab. The end material is removed, and dowels are grouted into holes drilled into the smooth face. The second method is to control the elevation of the header very carefully to properly align and finish the header to correct grade. Then, the paver is pulled off slowly to permit the concrete to set in place.

Dowel Baskets and Reinforcement

Dowel baskets and steel reinforcement in a pavement can adversely affect pavement smoothness. Pinning the basket assemblies securely to the grade is essential so that they can withstand the pressure applied by the concrete during placing.(33) The paver must be heavy enough not to ride up over basket assemblies. Proper dumping or placement techniques and adequate vibration frequency and depth can also minimize dowel-related pavement roughness. The use of an automated dowel bar inserter for dowel placement may help prevent dowel-related pavement roughness by eliminating the movement of the dowel baskets due to the extreme pressure. Other problems related to smoothness that can occur with dowel bars and reinforcements were discussed previously in the section dealing with embedded items.

Vertical Grades and Curves

There are no known differences between paving uphill and downhill.(17) It is more difficult to construct smooth pavements on grades exceeding 3 percent than on flatter grades. According to ACPA, the following adjustments may be necessary during paving operations on steep grades:(17)

Skilled and Motivated Crew

Regardless of the equipment and processes, constructing a smooth pavement requires experienced and motivated personnel. Crew training is vital, especially in subjects that directly affect smoothness. Stringline personnel need math skills and a keen eye, and operators need to understand what equipment activities affect pavement smoothness. In a study conducted in Argentina, the IRI values were reported to have decreased chronologically for paving projects.(28) The authors concluded that one reason for the improvement in pavement smoothness was the increased experience and knowledge the crew gained from performing previous jobs.

 


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration