Changes in Roadway Design -Continued

To many motorists, notwithstanding their experience of occasional areas of reconstruction as they travel from one end of the state to the other, I-94 seems a finished construction.  In some respects that is so – its route will not change in any appreciable way, if at all, in the foreseeable future.  Nor, most likely, will roadway width.  To most of us it seems a finished construction project with some ongoing maintenance.

In fact, as engineering works go, it is undergoing constant change.  First constructed to a useful life of twenty years, the road proper in many places outlasted that span.  Its replacement sections are designed for a useful life of thirty years.  That requires making projections of vehicle sizes, speeds, load bearing capacities and estimated trips over the road for a span of three decades in advance of the replacement construction.  Re-evaluation of soil types, sub-soil moisture levels, construction materials and cost must also be made in order to satisfy the design requirements at the most economical levels. 

When driving on I-94 today, one is driving on one of three types of road. 

Continuously Reinforced Concrete Pavement (CRCP) – Concrete pavement with continuous steel reinforcing running both transverse (across) and longitudinal (in the direction of) to the roadway.  CRCP has no contraction joints.  The advantage to this type of roadway is that it eliminates joint deterioration that can occur from moisture penetration in roads having contraction joints.  It is also thought to provide a better (smoother/quieter) ride because it is “jointless”.  Finally, it is thought to provide a better surface on which to apply overlays, such as asphalt, when the pavement surface begins to deteriorate.  The practicality of this type is based largely on the cost of reinforcing steel – the higher the cost the less practical, or economical, CRCP becomes.

Jointed Plain Concrete Pavement (JPCP) – Non-reinforced concrete pavements are constructed. A contraction joint is cut into the surface approximately every fifteen feet, thereby creating concrete “panels”.  The purpose of the joint is to control where cracking of the pavement will occur.  The advantage of such a design is that it minimizes the use of reinforcing steel, thus resulting in a cost savings.  The disadvantage is that where cracking occurs at the joint, as it is designed to do, a point of weakness is created.  The joint, and the crack in the concrete beneath it, are thus exposed to water, which can cause deterioration of the concrete over time.  To minimize the negative effects of water on the concrete, and on the soils beneath the pavement; joints are treated with sealing materials and drainage systems are installed to remove the water from beneath the pavement. 

When this type of roadway was conceived, it was thought that the aggregates in the concrete would cause uneven cracking from top to bottom through the concrete, and that those irregularities and interlock between the aggregates would keep the adjacent sections from “faulting” and creating a rough driving surface.  In order to minimize the stress on the concrete, joints were skewed to the roadway at a 6:1 ratio.  The thought was that one tire hitting the joint at a time would exert less pressure and stress on the joint than if both (or, in the case of trucks, all) tires on an axle were exerting the pressure all at once.

Today’s motorist will oftentimes notice two sets of three strips spanning the contraction joints in the driving lane of I-94.  These strips indicate that the driving lane has been retrofitted with dowel bars (steel bars approximately 1 ½ inches in diameter by 18 inches in length), a method used to extend the life of the pavement and improve the ride quality.  This retrofit is aimed at correcting or preventing “faulting”, a condition that occurs when the aggregate interlock in the contraction joint, on this type of pavement, no longer serves its intended purpose.  The retrofit involves cutting a rectangular opening or “slot” into the pavement spanning the contraction joint, inserting the reinforcing bar into the slot and filling the slot with concrete. Following the dowel bar retrofit, the driving lane is “ground” to remove the faulting.  This retrofit is one good example of the fact that the interstate highway is a continual work in progress. 

One may also note that where this retrofit exists, the “tining” (i.e., the tightly spaced shallow grooves in the road surface aimed at providing a surface texture that improves skid resistance) runs longitudinal to the road, while the tining in the passing lane runs transverse to the road.  Originally the tining, (which is “raked” into the surface of the freshly laid concrete by an operation that follows the slip form machine that lays the pavement) was transverse to the road in both lanes.  Grinding is performed longitudinal to the road because that is the most efficient way to re-establish the texture in the lane (i.e., the grinding machine can make a single pass from beginning to end of the retrofitted lane rather than back and forth across a single lane from beginning to end of the project).

Driving lanes only are retrofitted because they carry the great bulk of traffic and, consequently, that is where faulting is most likely to occur, and because retrofitting the passing lane is thought to be not cost effective.  This type of retrofitting began in the 1990s.

Overlayed CRCP or JPCP -  Road sections to which an asphalt overlay has been provided to extend the life of that particular length of road.

Today contraction joints in new I-94 construction contain dowel bars just as the retrofitted older sections of road do, except that the dowels are positioned at the location of the construction joints before the concrete is laid and nothing on the road surface indicates that they exist to inhibit faulting of the road sections.

Rumble strip designs of which several are represented along the length of I-94, are based on continuous studies aimed at finding the one that is most effective at alerting the inattentive or dozing driver.