10  Rigid - General Procedure

The general rigid pavement construction procedure involves placement, consolidation, finishing, curing and jointing in rapid succession.  "Placement" involves any equipment or procedures used to place the delivered PCC on the desired surface at the desired thickness; "consolidation" involves any means used to eliminate undesirable voids; "finishing" involves any equipment or procedures used to impart desirable surface characteristics; "curing" is the maintenance of satisfactory moisture and temperature in PCC as it sets and hardens such that the desired properties can develop; and "jointing" involves all those actions used to insert purposeful discontinuities in the pavement and seal them appropriately.  This section provides a generic description of these six steps and any associated considerations.  Specifics of how they are accomplished in fixed form and slipform paving are shown in the next two sections.  More detailed information can be found in:

• American Concrete Pavement Association (ACPA).  (1995).  Construction of Portland Cement Concrete Pavements.  National Highway Institute Course No. 13133.  AASHTO/FHWA/Industry joint training.  Federal Highway Administration, Department of Transportation.  Washington, D.C.

10.1  Placement

PCC can be placed directly in the desired location by truck or truck attachments (see Figures 7.95 and 7.96), or can be fed into a placement machine for more accurate and even placement.  PCC that is moved excessively once it has been unloaded from the transport truck will tend to segregate (become less homogeneous).

Figure 7.95: Placement Over Dowel Bars in an Intersection	Figure 7.96: Placement in Front of a Rolling Screed

10.2  Screeding (Strikeoff)

During the screeding (or strikeoff) process, excess portions of the roughly placed PCC are cut off in order to bring the slab to the required elevation.  This is usually done by dragging a straightedge across the slab at the required elevation.

10.3  Consolidation

Consolidation is the process of making the freshly placed PCC into a more uniform and compact mass by eliminating undesirable air voids and causing it to move around potential obstructions (such as reinforcing steel).  Consolidation is usually accomplished using long, slender vibration rods called vibrators.  Vibrators work by rotating an eccentric weight which causes the entire vibrator to move back and forth.  This movement excites particles within the PCC mass, causing them to move closer together and better flow around obstructions. 

Vibrators can be defined by the amount of energy the impart to the surrounding PCC mass.  This energy transmission is defined by two processes.  First, the amount of energy generated by the vibrator is proportional to the size and speed of the rotating weight.  Usually, the size is fixed and the speed is variable.  Second, the energy transmitted from the vibrator to the surrounding PCC mass is related to paver speed (the faster the paver runs, the less time the vibrator has in a particular volume of PCC) and vibrator location within the PCC mass.  All of these factors together comprise and control the size and shape of an "influence zone" - the volume of PCC mass around a vibrator that receives its energy (see Figure 7.97).  This influence zone is usually conical in shape and varies in size depending on the previously mentioned factors.

Figure 7.97: Vibrator Influence Zone

Proper consolidation by vibration is critical to rigid pavement performance.  In particular:

• Too much vibration, either by allowing vibrators to operate too long in one area or by using to high a vibration rate, can result in (1) non-uniform distribution of coarse aggregate particles, (2) loss of entrained air, and (3) bleeding (water accumulation on the surface).  All of these results can greatly reduce PCC durability.
• Too little vibration, either by not allowing vibrators enough time to operate in one area or by using to low a vibration rate, can result in (1) non-uniform distribution of coarse aggregate particles, and/or (2) large air voids within the PCC mass.  Again, either result can greatly reduce PCC durability.
• Vibrator static head (amount of PCC above the vibrator) influences efficiency.  Higher static heads will help push coarse aggregate particles together behind the vibrator as it travels along.

10.4  Finishing

Finishing involves all processes and equipment used to create the final surface finish and texture of fresh PCC.  Generally, finishing can be divided into floating and texturing:

• Floating.  A flat surface is run across the PCC in order to eliminate high and low spots, embed larger aggregate particles beneath the surface, remove slight imperfections and to compact the mortar at the surface in preparation for texturing (PCA, 1988).  Floating can involve a number of different tools and may involve multiple passes over the same surface.
• Texturing.  After floating, fresh PCC is usually quite smooth.  In order to create a slip resistant surface for traffic, a rough pattern is usually imparted by dragging a broom, rough-textured item, or tined instrument across the surface.  Typically, texturing is divided into the following two categories (FHWA, 1999):
  • Microtexture (Figure 7.98).  This is achieved by dragging a section of burlap or artificial turf behind the paver.  Microtexture enhances friction between vehicle tires and the pavement surface, and enhances safety at low speeds.
  • Macrotexture (Figure 7.99).  This is generally achieved by tining the pavement surface.  Macrotexture permits water to escape from between tires and the pavement surface and enhances safety at high speeds.  Typically, an average texture depth of 0.7 mm (0.03 in) will substantially reduce both total and wet weather accident rates.  Tining practices vary by agency, but many states require transverse grooves on the order of 3 - 5 mm (0.12 - 0.20 inches) deep, 3 mm (0.12 inches) wide and spaced 12 - 20 mm (0.47 - 0.79 inches) apart (ACPA, 1995).  Sometimes the area over the future joint locations is not textured in order to provide a good sawing and sealing surface.  Some agencies consider microtexturing sufficient and do not macrotexture their rigid pavements.
     

  WSDOT Macrotexturing

  WSDOT typically uses transverse tining for rigid pavement macrotexturing.

Figure 7.98 (top): Texturing Using a Piece of Artificial Turf Figure 7:99 (right): Tine Texturing

10.5 Curing

Curing refers to the maintenance of satisfactory moisture and temperature within a PCC mass as it sets and hardens such that the desired properties of strength, durability and density can develop (PCA, 1988).  The desired properties of strength, durability and density are related to the extent of hydration within the PCC mass; the more complete the hydration, the better a PCC's properties.  The extent and rate of hydration depend on two critical construction-controlled parameters: moisture and temperature.  This subsection covers:

• Moisture considerations for curing
• Temperature considerations for curing
• Curing methods

10.5.1  Moisture

Hydration requires portland cement and water.  The extent of hydration is controlled by the limiting ingredient, which is usually portland cement.  However, if any substantial portion of water is lost to evaporation, hydration may be limited by a lack of water, causing it to slow or virtually stop.  Thus, inadequate moisture will inhibit hydration, which results in a weaker, less durable PCC.  Rapid moisture loss will also cause excessive shrinking and cracking.  Therefore, a high relative humidity around a hydrating PCC mass will ensure an adequate water supply for hydration and limit shrinkage cracking.  Generally, some method of curing is specified in order to maintain the relative humidity within the hydrating PCC at an adequate level.

10.5.2  Temperature

Hydration rate is also dependent upon temperature.  Higher temperatures speed up hydration's chemical reactions, while lower temperatures slow them down.  Therefore, temperature will affect PCC strength gain.  Often, minimum ambient temperatures for PCC construction are specified to ensure an adequate hydration rate and thus, strength gain.

Maturity
Since hydration progresses over time, and the rate of this progression is dependent on temperature, it should be possible to estimate the extent of hydration by tracking time and temperature.  "Maturity" is the term used to describe this concept.  Most maturity measures are expressed as a function of the product of curing time and temperature (see Figure 7.100).  For example, the Nurse-Saul expression is:

Figure 7.100: Compressive Strength vs. Maturity

Often, maturity is correlated to PCC strength gain by laboratory testing prior to PCC placement.  A non-destructive maturity measurement can then be used to estimate strength and avoid destructive strength tests during construction.  ASTM C 1074 defines the maturity method as "...a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of maturity index."  The maturity method is useful because it can provide strength estimates of in-place PCC subject to actual environmental temperatures rather than relying solely on controlled-environment laboratory tests.  There are also a number of significant limitations when using maturity to estimate strength (Mindess and Young, 1981):

• The maturity method requires establishment of strength-maturity relationship in the laboratory prior to any field measurements.  Because different PCC mixes mature at different rates, maturity meters are typically calibrated to actual compressive strength using laboratory test cylinders.  Thus, any change in mix proportions from the laboratory design used for calibration will require a new calibration.
• Other characteristics affecting PCC strength.  Items such as moisture content, portland cement chemical composition and fineness, and construction practices (e.g., consolidation, finishing, air content) are not accounted for.
• Maturity only accounts for ambient temperature.  In large concrete volumes, the heat of hydration contributes significantly to the PCC mass temperature, and thus, strength gain.  In typical PCC pavements, which are relatively thin, this heat is quickly lost to the environment and can be ignored.
• Maturity functions are not accurate at low maturities.  This is probably because the point at which time should be measured from is poorly defined.  Probably, the best time is not the time of mixing or casting, but rather the time that the PCC actually begins to gain strength.
• Maturity does correlate well with strength when there are large temperature variations during curing.  Typically, a low initial curing temperature followed by a high temperature will lead to higher strengths, while the opposite (high followed by low) leads to lower strengths.

In sum, the maturity method is not a physical law, but rather a convenient way to estimate strength gain.  In PCC pavement applications, maturity meters (see Figures 7.101 and 7.102) can be used to estimate the appropriate time for form removal, joint cutting or opening a pavement to traffic, but should not be entirely substituted for basic laboratory strength tests.

Figure 7.101: Maturity Meter	Figure 7.102: Measuring Maturity

10.5.3  Curing Methods

Generally, curing is accomplished by one of two methods (Mindess and Young, 1981):

1. Water curing.  Methods that prevent moisture loss and supply additional water to the PCC surface.  These methods usually involve ponding water on top of a slab, continuously spraying a slab with a fine mist or covering a slab with a water-retaining material such as burlap.  These methods are labor intensive and are generally not used on PCC pavements any more.
2. Sealed curing.  Methods that prevent moisture loss but do not supply any additional water.  These methods usually involve placing a waterproof covering over a slab (such as plastic) or using a liquid membrane-forming chemical compound.  Curing compounds are typically formed using resins, waxes or synthetic rubbers with a dissolved volatile solvent.  Once the solvent evaporates, the curing compound forms a near-impermeable membrane over the PCC.  Pigments are often added to curing compounds in order to reduce (white pigment) or increase (dark pigment) heat absorption.  Additionally, pigments allow workers to see where the curing compound has been applied, which helps to ensure complete coverage.

10.6  Joints

All PCC pavement types use all types of joints, however, CRCP uses longitudinal reinforcing steel in order to limit the number of transverse contraction joints.  This subsection discusses the basics of transverse contraction joint construction including:

• Joint location
• Saw cutting timing
• Saw cutting depth
• Joint sealing 

10.6.1  Location

Typical joint locations are covered in Module 2, Section 6: Rigid Pavement Types, and are not repeated here.  However, it is important to note that joint locations should be indicated on the construction plans and planned in advance (see Figure 7.103).  Intersection joint locations can be quite complex and should be marked out on the base in advance (see Figure 7.104).

Figure 7.103: Joint Layout on Base Material	Figure 7.104: Joint Layout in an Intersection

10.6.2  Saw Cutting Timing

The timing of contraction joint sawing depends upon two key factors:

• Shrinkage cracking.  Since contraction joints are used to control shrinkage cracking, they should be sawed before slab shrinkage stresses become great enough to cause uncontrolled cracking.  See figure 7.105.
• PCC support strength and joint raveling.  Sawing must be delayed until the PCC is strong enough to both support the sawing equipment and to prevent raveling during the sawing operations.  See Figure 7.106.

Figure 7.105: Shrinkage Crack Possibly Due to Late Sawing	Figure 7.106: Joint Raveling due to Early Sawing

Thus, as the PCC hydrates and strengthens, there is a short window of time in which sawing can occur as illustrated by Figure 7.107. 

Figure 7.107: Saw Cutting Window

10.6.3  Saw Cutting Depth

Transverse contraction joints are usually cut to a depth of 1/4 - 1/3 of the total slab depth to ensure cracking occurs at the joint (see Figure 7.108).  For example, a 250 mm (10 in.) thick slab would require a joint depth between 63 and 83 mm (2.5 and 3.3 inches).   In no case should the sawcut be less than 1/4 of the slab depth.  The FHWA (1990) recommends that transverse joints be cut in succession rather than skip sawed (e.g., initially cutting only one out of every 5 or 6 joints then going back later and cutting the rest) because skip sawing can result in a wide range of crack widths that form beneath the sawed joints.  These varied crack widths may cause excessive sealant stresses in the initially sawed joints initially. 

Figure 7.108: Contraction Joint Showing Sawcut Depth

10.6.4  Joint Sealing

Once a joint is cut or otherwise made, it needs to be sealed to minimize water and incompressible material entry.  Sealants may also reduce dowel bar corrosion by reducing entrance of de-icing chemicals (ACPA, 2001a).  Joint sealants used today are typically one of three types (ACPA, 2001a):

• Hot-pour liquid sealants.  These sealants are heated up to decrease their viscosity and then poured.  Joints are ready for traffic as soon as the sealant has cooled.  About 25 percent of roadway agencies use hot-pour sealants in transverse contraction joints.  Most hot-pour sealants are used in longitudinal joints and low-traffic PCC pavements.  Figure 7.109 shows joints filled with hot-pour sealant.
• Compression seals.  These are preformed rubber compounds placed into a joint under compression.  After they are placed, they form a seal by pushing against each side of the joint and are immediately ready for traffic.  Compression seals, commonly called neoprene seals after their primary constituent, are used by about 21 percent of roadway agencies in transverse contraction joints.
• Silicone sealants.  These sealants are silicone polymer compounds that are poured into joints at ambient temperatures.  It generally takes about 30 minutes for them to harden and make the joint ready for traffic.  About 52 percent of roadway agencies use silicone sealants in transverse contraction joints.

Figure 7.109: Joints Sealed with Hot-Pour Liquid Sealant on a Freeway On-Ramp
(normally, joints should coincide with lane divisions as they do near the horizon of this photograph)

10.7  Summary

This section has provided an overview of the basic elements of rigid pavement surface course construction: placement, consolidation, finishing, curing and jointing.  These basic elements are common to both fixed form and slipform paving; the differences are in the equipment and methods.