13  Rigid - Joints

Joints are common to all rigid pavements and both rigid pavement construction methods.  Even CRCP uses longitudinal joints and periodic transverse joints.  Joints can be formed in two ways.  Contraction joints are most often sawed in after PCC placement.  Others such as expansion, isolation and construction joints, are created by formwork before the PCC is placed.  Each one of these methods of joint construction has its own method and set of considerations.

13.1  Sawed Joints

Most sawed joints are contraction joints.   This section covers:

• Saw blades
• Sawing equipment
• A general sawing procedure

13.1.1  Saw Blades

	Figure 7.142: Saw Blade

Sawed joints are formed using hard-tipped rotary saws, which can use either diamond or other types of abrasive blades (see Figure 7.142).

13.1.1.1  Diamond Blades

Most joint sawing is done using industrial diamonds as the primary abrasion element.  Diamond tipped blades will generate high amounts of heat when cutting and generally must be cooled by water to prevent the saw blade metal from overheating and melting.  Water also prevents dust formation during sawing.

13.1.1.2  Abrasive Blades

Dry sawing uses abrasive blades that are usually made from fiber reinforced silicone carbide or carborundum since these types of blades do not require water for cooling (ACPA, 1995).  These abrasive blades have less cutting ability than diamond blades and are usually used on PCC with softer aggregate (ACPA, 1995).  When cutting, these blades will wear down over time making it important to periodically check the sawed depth and saw blade diameter to ensure adequate joint depth (ACPA, 1995).  Additionally, unless water is used to prevent it, these types of blades will generate sawing dust. 

	Figure 7.143: Small Saw

13.1.2  Saw Equipment

Equipment used for sawing can range from small to quite large.  A typical categorization is (ACPA, 1995):

• Small saws.  6 - 13 kW in power and most commonly used for dry sawing applications.  Lightweight saws (see Figure 7.143) used for early sawcuts are categorized as small saws.
• Medium saws (see Figure 7.144).  15 - 28 kW in power and most commonly used for wet sawing applications. 
• Large saws.  50 - 55 kW in power and most commonly used for wet sawing applications.  These saws are often used for longitudinal joints where their lack of maneuverability due to size is not critical.
• Span saws (see Figure 7.145).  50 - 150 kW in power and are most commonly used for wet sawing applications.  These are usually multi-bladed and used in high production jobs.

Figure 7.144: Medium Saws	Figure 7.145: Span Saw

13.1.3  Procedure

Joint sawing is generally accomplished in two passes.  First, an initial thin cut is made to control shrinkage cracking.  The timing of this joint cut is critical in order to avoid permanent slab damage.  Later on, a second, wider cut is made over the first cut in order to make the joint wide enough to accommodate joint filler material.  Some joint cutting considerations are:

• Sawing window.  This prevents permanent slab damage due to uncontrolled shrinkage cracking, loading an inadequately set slab, or spalling the cut joint of an inadequately set slab.
• Proper joint location.  Contraction joints must be cut over the internal dowel bar locations.  If these locations are not marked, joints may not be cut at the dowel midpoints, which can result in excessive slab and dowel bar stresses.
• Joint cleaning.  Joint cutting will result in a fair amount of debris generation.  After sawing, this debris should be washed out to prevent it from contributing to faulting or joint stresses.  Before sealing, joints should be abrasively cleaned to provide a good textured surface with which the joint sealant can bond.  Finally, the residue from the abrasive cleaning should be blown out to provide a clean bonding surface. 

13.2  Other Joints

Expansion, isolation and construction joints are created by formwork before the PCC is placed.  Since these joints are designed to completely separate adjacent masses of PCC, they are usually made by inserting a small non-PCC piece of material such as a strip of wood.

13.2.1  Construction Joints

Construction joints, sometimes called "headers" are usually made to separate successive construction activities; they do not serve any design purpose.  However, with proper planning, construction joints can often be made to coincide with other planned joints, such as a transverse contraction joint.  For instance, in slipform paving a construction joint is made at the end of the day as a transverse piece of formwork used to shape the last slab.  If enough PCC is available at the end of the day, the construction joint can be placed at a planned transverse contraction joint.  Construction joint considerations include:

• Ensure adequate PCC is available to finish the last slab of the day.   The construction joint is set before all the PCC is placed.  If truck delivery is stopped to soon, the temptation may be to use the lower-quality PCC that has been pushed in front of the screed to fill the remaining volume.  This low-quality PCC may contain little portland cement, excess water, low air and/or segregated aggregate. 
• Ensure proper consolidation.  Typically, the paver does not traverse the construction joint.  Therefore, the construction joint is not consolidated by the paver vibrators and it should be manually consolidated.
• Dowel placement should not segregate or cause air voids in the adjacent PCC.  Sometimes, dowel bars are placed after the PCC has been placed up to the header.  If dowel bars are pounded in by hammer, the resultant vibrations may cause air pockets or segregated aggregate.