HMA Construction Program Module 3 - Surface Preparation

Surface prep often doesn’t get the attention it needs. The performance of an HMA pavement under traffic is directly related to the condition of the surface on which it was placed. Surface can be subgrade, aggregate base, or existing HMA or concrete pavement. Surface prep often doesn’t get the attention it needs. “Why bother - it gets covered up anyway” It is easy to cover up problems with HMA, but rarely do the problems go away. Fix it right - First “You can pay me now, or pay me later.”

Module 3 - Objectives State the objectives of surface preparation Proper base preparation techniques Proper techniques for overlay preparation Proper materials and construction techniques for patching Proper techniques for placing leveling courses Proper milling techniques

Present the objectives. Ask: In what way do each of you get involved in surface preparation? Ask: What seems to be your biggest problem in surface preparation?

Preparation of a New Base

Base can be subgrade soil, aggregate base, or stabilized base

Subgrade must meet the requirements for moisture content, density, and smoothness. Smoothness studies show you can reduce the smoothness numbers by 1/2 with each successive HMA lift. For example, if the base has a smoothness of 40 and the final surface is to be a smoothness of 5, there would need to be 3 lifts (40 to 20 to 10 to 5) for the smoothness numbers to be achievable.

Subgrade should be able to support the weight of haul traffic. The subgrade must not distort under traffic or during paving.

(Photo shows proof rolling of aggregate base) An aggregate base course must also meet moisture content, density and smoothness requirements Ask: What are the local practices for proof rolling? Which layers? Type of equipment? Pass / Fail requirements? How do you repair failures? How long can a proof-rolled layer be left uncovered before placing the next layer? Who is responsible for maintaining a proof-rolled subgrade or base course? Other details?

The proper equipment and materials must be available to repair any problems discovered during proof-rolling. If the base is disturbed by traffic before placing the HMA, the best practice is to fix it. The pavement needs every inch of stone or HMA to be in place.

Density testing should be standard practice prior to placing the next lift. Density testing coupled with proof rolling will determine whether you are ready to pave or not.

Nuclear gauge tests are a quick and easy method to test base density, but can also be easily misused. Nuclear gauges measure relative density, and must be calibrated to the material being used.

To Prime... or Not to Prime...

Not a substitute for proper subgrade or base prep. With cutbacks, priming had been standard practice. Since cutback use has been reduced, not as common. Ask: what is local practice here? Priming used at all? On what types of base? Using what materials? Cure period? Typical application rate: 0.7 to 1.8 liters/sq meter (0.15 to 0.4 gal/sq. yd). [In some places, the term “prime” may describe a tack coat (asphalt binder on existing pavement)]

This Base Preparation Checklist is available in the Participant’s Manual.

Preparation of an Existing Surface

The next subject ...

Preparing an existing pavement for an overlay may be as simple as sweeping the surface and spraying a tack coat...

… or it may involve a myriad of other procedures: patching placing a leveling course milling the existing surface The NHI “Techniques for Pavement Rehabilitation” covers this subject in depth.

Pavement Repair Matching Techniques Materials Procedure

Pavement repair techniques must: Match the repair technique to the existing conditions. Use the proper materials and procedures to complete the repair and achieve long term performance. If the pavement failure is load related, the pavement material must be removed and replaced down to sound material.

Sometimes, pavement preparation before an overlay may involve coming back to replace someone else’s handiwork.

[This next sequence of 12 slides shows the typical steps in repairing a pavement failure. It is meant to be covered quickly.] The first step is marking the outline for the repair. The failed area should be cut back to sound pavement.

This patch would have been easier to repair if the edges were kept straight.

Cut the patch edges as vertically as possible. The type of cutting equipment used will depend on the size of the repair.

Remove the failed pavement. All loose material must be removed from the excavation. As far as “different types of equipment are used,” you may want to note that this repair was cut with a pavement saw.

You need to remove as much material as necessary to find the problem, and then fix it.

[Water seepage into excavated patch. ] Sometimes, the source of the problem may not be evident on the surface. In this case, additional drainage is required to solve the problem. New NHI course – “Pavement Subsurface Drainage”

Depending on the depth of the repair, aggregate material may be used to backfill the patch.

Compaction is crucial. The patches become a permanent part of the pavement. They must be as strong as the surrounding pavement, or problems may come up later.

Density tests may be required.

A tack coat is necessary to ensure a proper bond between the patch and the existing pavement.

If the patch is large, a paver should be used to place the HMA.

Compaction is still crucial, regardless of the size of the patch. Hand compaction is often necessary in tight corners and hard-to-reach areas. Hand compacted areas need the same attention as machine compacted areas.

[Photo of a well-performing utility cut] Properly constructed patches will perform well, whether or not the patch is part of pavement preparation prior to an overlay. Ask: What is the local practice for checking patch density? Is it standard practice? Density Requirements? Coring? Nuclear gauge? Used for HMA? Used for aggregate base? Depends on patch thickness? patch size? Pre-construction meeting item: Utility repair coordination

What about crack filling / sealing? This is an average size crack, a candidate for filling. Cracks must be cleaned and prepared properly prior to filling.

Typically, cracks less than 10 mm wide are too small to be filled effectively. The sealant does not enter the crack. The amount of reflective cracking in an overlay may be reduced by using a surface treatment, depending on the cause of the cracking. The surface treatment can be a chip seal or a slurry seal. Surface treatments used as crack fillers can be more economical that crack sealing.

This crack needs to be treated with a patch. Ask: What is the local practice for filling cracks prior to an overlay? Standard practice? Only cracks wider than _______ ? Typical crack sealing procedures? Surface treatments?

After patching and sealing, and prior to applying the tack coat, the surface must be properly cleaned. Typically, a power broom or street sweeper is used. Any foreign material (dried mud, spilled asphalt, etc.) must be removed. Cleaning is typically done immediately prior to placing the tack.

While the surface is still clean, place the tack coat. The tack coat ensures a bond between the existing pavement and the overlay. Slippage cracking can occur if a good bond is not formed. Diluted slow-set asphalt emulsions are typically used. Distributor must be working properly for even application. Ask: What is the local material used for tack coat? Comment on poor quality of spray nozzles.

Some surface will show through a properly placed tack coat. Asphalt emulsions are applied brown, and then turn black after they break (the water separates from the asphalt). The water from the emulsion must be allowed to evaporate before placing the HMA. Typical tack coat application rate: 0.2 to 0.3 liters/sq meter (0.04 to 0.06 gal/sq. yd) of residual asphalt. Too little or too much – both are bad.

Make sure the tack coat goes where it is supposed to go….

This Existing Surface Preparation Checklist is contained in the Appendix.

Leveling can be used to correct low spots and cross section deficiencies. Other terms include scratch course, wedge, and level. The extent of leveling course is marked off, and then placed with a paver or spread with a grader or by hand. (This leveling was paver placed.) Compaction of leveling courses is best achieved with a rubber-tired roller, rather than steel wheel rollers. Density is difficult to measure in variable depth leveling courses.

Even small areas need to be leveled prior to overlay, as in this hand placed leveling.

Ask: what are local practices for leveling courses? Be careful with the maximum aggregate size in the HMA and leveling course thickness… The center was thinner than the edges–the white aggregate pieces were being crushed by the screed. Ask: what are local practices for leveling courses? Types of applications Equipment used for placement Equipment used for compaction Density checked?

Concrete pavement surface preparation is similar to other existing surfaces. Severely distressed slabs need to be removed and replaced. The source of the distress needs to be identified and corrected. Surface distresses need to be repaired. Poorly sealed joints need to be cleaned and resealed. The surface must be properly cleaned and tack coated prior to placing the overlay. Problems with reflective cracking in asphalt overlays of concrete have led to several techniques to prevent or control the reflective cracking.

[Don’t dwell on these techniques; some may have been used locally, others have not; this is mainly for information only.] The 2 types of slab fracturing techniques–rubblizing and cracking and seating Rubblizing destroys the slabs, turning them into a strong, high quality aggregate base. Reflection cracking occurs at joint, and rubblizing removes the joints.

An “after” photo of rubblizing.

After the slabs are rubblized, the material is compacted, any soft spots are repaired, and the overlay is placed. Paving on this surface will be very similar to paving on an aggregate base.

Cracking and seating breaks the slabs into smaller pieces. This machine is called a “guillotine” breaker. The breaker mass is dropped from a given height onto the concrete, fracturing the slabs into smaller pieces.

[Photo of “after” cracking and seating operation.] The slabs are not destroyed, but left in 300 to 450 mm sized pieces.

After the pavement is cracked, a heavy roller “seats” or compacts the pavement. These rollers must weigh between 35 and 50 tonnes to effectively seat the concrete pieces back onto the base or subgrade. Again, any soft spots that are discovered must be repaired prior to placement of the overlay.

(A quip about sawing HMA pavement may be appropriate here.) Sawcut and sealing is not used to prevent the reflective cracking, but control it. The concrete beneath the overlay is not treated, but sawcuts are placed in the HMA directly over the pcc joints. The asphalt overlay is marked directly above the concrete joints. The sawcuts are then made, and then cleaned and sealed.

The sawcuts help control the crack in the overlay, keeping it straight and making it easier to maintain.

Preparing a Milled Surface

Milling can be used in lieu of leveling to remove the high spots from an existing surface. Milling is frequently used when maintaining the surface profile is necessary, such as in curb and gutter situations.

Milling can be varied in width from 0.15 m to over 4 m The largest milling machines can remove to depths over 200 mm. The RAP removed from the surface can be saved for future recycling.

Proper pavement repairs when milling and overlaying are even more important, since milling removes some of the pavement structure. Underlying pavement problems may be uncovered during milling. As this photo shows, milling often leaves a very dirty and dusty surface. Multiple sweepings are often necessary to make sure all of the dust and dirt is removed.

Traffic control may be more critical in repair operations. Closures may be short Many locations, with frequent equipment moves

Extreme caution must be stressed to prevent accidents in these situations. Even though traffic control may be more difficult due to frequent moves or the number of locations, it must not be compromised.

Module 3 - Review Objectives State the objectives of surface preparation Proper base preparation techniques Proper techniques for overlay preparation Proper materials and construction techniques for patching Proper techniques for placing leveling courses Proper milling techniques

Ask for specific responses to these questions. Use questions that will help draw out responses, such as: What do you remember? How is it related to what you know? How will you use this information in the future?