Potential Rehabilitation and Replacement Techniques to Improve Michigan Roadways
Alexander Lamb
Marlin Stewart
Alfredo Gutierrez
Lou Farkas
Nashwan Fatteh
Jeff Hanselman
Kevin Wiewiura
Introduction
Michigan Roadways are rated a “D” by the national survey of interstates and state controlled roads. This is the second worst rating next to the letter “F”. As a result of this poor road condition, system and network efficiency are diminished greatly. This is especially true from an operations and maintenance stand point. Effectively, the longer a road remains in disrepair, the more it costs to rehabilitate, where the maximum possible cost would be the reconstruction cost. This should be avoided and as a result this paper will seek to assess current rehabilitation methods and compare them with proposed methods on a cost per unit life analysis. Should reconstruction be required, alternative methods will be assessed on the same basis of cost per unit life.
1.0 Problems with Existing Roadways
If you live in Michigan you are an expert at the drawbacks of winter and concrete. We have all spent our lives trying to maneuver around the bright orange cones we call summer. We all know that it is the harsh winters we have here that are the cause of our problems. Simply stated, our roads are cracked, it rains and water gets in those cracks. Then, since it is Michigan the weather changes and it gets cold, causing the water to freeze. Every seven year old child learns that water expands as it freezes, causing the concrete to move. The constant repetition of this event is what is causing havoc on our streets, roads, and freeways. Over time the freeze-thaw cycles have created large gaping holes in our roadways. Some of these holes are even as big as our cars. Until better methods of building these roadways are developed, our poor Michigan streets will fall victim to this constant and never ending cycle.
Some methods are being utilized today to try to stop or at least slow the effects of this freeze-thaw cycle. Modern concrete uses freeze thaw resistant aggregates to reduce the effects the weather has on the concrete itself. Also used is an air-entraining additive to limit or eliminate freeze thaw damage. Air-entraining additives are used to stabilize entrapped air in the concrete during mixing in the form of small bubbles known as entrained air. Freezing and thawing damage in concrete occurs because of excessive pressure. Air in the concrete acts as a pressure relief and helps to allow ice formation without excessive pressure. The air in concrete must be well distributed for freeze thaw resistance. Simply stated, air bubbles are put into the concrete to allow for some of the pressure created by the expanding ice to be absorbed, resulting in minimal damage to the actual slab of concrete itself.
Another problem, which our roads have to endure, deal with Alkali Silica Reaction (ASR). This involves the problem that occurs when sand (silica) combines with water (alkali hydroxide) and a gel is formed. This gel causes intense pressure on the concrete resulting in a rupture of the surface. Typical ASR problems include random map cracking. ASR cracking usually appears in the presence of a frequent supply of moisture. These areas of frequent moisture include the joints and free edges on pavements or in piers or columns, which are subject to a wicking action. A petrogaphic examination of the area can conclusively identify ASR. Using certain supplements in the concrete mixture can also control this. Some of these additives, used in the proper proportions, can significantly reduce the swelling that is produced by ASR. Some of these include fly ash, silica fume, and ground granulated blast-furnace slag.
Drainage is very important in the construction of our roadways. Proper drainage can stop the effects of an extremely cold winter on our roads. Water has the power to actually lift up entire sections of concrete. This lifting or “heaving” is a problem that can be avoided by having proper drainage. For frost heave to take place you need road construction material that absorbs water and water for it to absorb, so if you remove one component with good drainage, this problem won’t occur. Subgrade drainage is also an issue. The subgrade is the material that the road is built on. This material needs to be strong enough to maintain the weight of the concrete. A test called the California Bearing Ratio (CBR) is used to test the materials. This test determines the resistance of the subgrade. It will allow us to see just how strong the ground we built the road on actually is. With good drainage the strength of the subgrade can be maintained or even increased, while the reverse is also true, bad drainage can weaken an existing subgrade.
Over the years we have come up with new ways to make concrete stronger and last longer. It is only with the future research and development of the production, preparation, and installation of concrete that our roadways can be improved. New technologies, materials and applications will allow us to enjoy longer lasting, more efficient roadways in the future.
2.0 Current Methods for Rehabilitation
Rehabilitation of highways plays a key role in how roads give users a comfortable and smooth ride. The U.S. highway system is the most expensive public works problem that has ever existed. Some states have repair and maintenance budgets that approach a billion dollars a year. In the 1980’s the U.S government made repair standards on roads. This article is concerned with the current rehabilitation methods that have been in practice for the roads of southeastern Michigan.
Usually an engineer goes out to the site and performs certain tests on the roads to make sure it should be rehabilitated. This could consist of inspecting the base and sub base of the surface. Milling or taking a core sample of the road does this. One this is done the engineer can determine what type of rehabilitation method to be used. To most adequately address the types of rehabilitation methods currently used in Michigan, two categories are established: those for concrete, and those for asphalt.
2.1.0 Concrete Rehabilitation
There are 4 basic rehabilitation methods that are used for concrete: Joint and Crack Sealing, Partial Depth Repairs, Full Depth Repairs, and Slab Replacement. These methods serve to extend the life of existing concrete (rigid) pavements as low cost alternatives to complete replacement through reconstruction.
2.1.1 Joint and Crack Sealing
The main practice of sealing cracks is to saw out the crack to ensure a smooth and geometric surface. Then a sand or air blaster is used to clean the inside of the crack to ensure that the sealant will adhere to the surface. The joint is then sealed with either silicone or hot pour sealant (Asphalt Cement). This serves to alleviate stress build up at the joint from beam action in rigid pavements. An example of this is shown in Figure 2.1.1.
Figure 2.1.1: Joint and Crack Sealing
2.1.2 Partial Depth Repair
This type of repair is for more deteriorated concrete. The area needed for milling is marked out and can be milled up to half the concrete slap above the top of the dowel bars. Then the surface area is cleaned by air or sand blasting. Then grout is applied to make the concrete adhere to the surface. This type of repair is shown below in Figures 2.1.2.1 and 2.1.2.2.
Figure 2.1.2.1: Pavement before repair Figure 3.1.2.2: Pavement after repair
2.1.3 Full Depth Repair
This procedure requires that the total slab be removed. A saw is used to cut out the portion of concrete and then is lifted out using pins. New dowels have to be put into the concrete and coated with grout. An important factor in having a successful repair is having the dowels aligned properly and straight. This is shown in Figures 2.1.3.1 and 2.1.3.2. Note that saw cutting is important and should be cut as closely to perpendicular to the direction of intended stress transfer as possible so that fatigue related issues are minimized.
Figure 2.1.3.1: Pavement during repair process Figure 2.1.3.2: Pavement after repair
2.1.4 Slab Replacement
This repair is the same as full depth replacement, but on a larger scale. The whole slab of concrete is removed. It follows the same steps as type c repairs. Sometimes it is recommended that a joint be placed in the middle of the repair length, to prevent the likelihood of cracking. This rehabilitation method is show below in
2.1.4.1.
Figure 2.1.4.1: Slab replacement
2.1.5 Cold Patch Asphalt
This is a short-term rehabilitation method that is the cheapest to use and consequently the most used method. Cold patch asphalt does not need to be kept heated and can be placed in any large cracks or potholes that need to be filled. It is very convenient because it does not need any large rolling machine. The only problem is that it only may last a year before the bonded material wears off and becomes rubble. To properly install it, the hole has to be cleaned and then laid down in a hole preferably with vertical walls. This will ensure its maximum longevity. One problem with cold patch applications is that they are typically applied to potholes, which have circular profiles. This serves to increase the rate of degradation of the existing pavement surface due to stress risers around the patched section. This is shown below in Figure 2.1.5.1. Stress risers effectively increase the stress in the areas adjacent to the opening, which cause that area to fail first. Then a cascading effect happens where the pavement surface fails incrementally, outwards from the opening. Stress riser phenomena only occur in rigid pavements due to the transfer of tensile loads.
Figure 2.1.5.1: Stress risers around circular opening
2.2.0 Asphalt Rehabilitation
The effects of problems in asphalt are repaired primary by 3 methods: cold patch, chip seals, and re-surfacing. These methods are used to extend the life of existing asphalt (flexible) pavements so as to avoid full replacement. The cold patch method is the same material and process as used in rigid pavements so will not be discussed further in this section
2.2.1 Chip Seals
Chip sealing an existing roadway is a method of resurfacing an existing roadway to improve various qualities and its overall longevity. Chip sealing is more of a means of preventative maintenance, but is also used to improve the frictional characteristics of roadways. They are applied in 2 layers; an asphalt cement layer, then a layer of crushed stone. This method is shown below in Figure 2.2.1.1.
Figure 2.2.1.1: Chip seal
2.2.2 Resurfacing
Resurfacing is the process of grinding down the surface of the asphalt so that it can be replaced. Typically 2-4 inch depths are selected for resurfacing because of the depths at which layers (courses) of asphalt are applied (base and wearing courses are always present). Figure 2.2.2.1 shows the wearing course being applied to the base layer of an asphalt roadway in Michigan. The wearing course is the layer that is resurfaced.
Figure 2.2.2.1: Application of wearing course
3.0 Current Methods for Reconstruction
Current methods to reconstruct a pavement structure incorporate empirical data from decades of testing and measuring response. Design methodologies in use by the Michigan Department of Transportation (MDOT) for flexible and rigid pavement designs will be discussed.
3.1.0 Concrete Reconstruction
The Michigan Department of Transportation uses a modified version of the AASHTO pavement guidelines for all of their rigid pavement designs. They currently use two methods for the design: Jointed Reinforced Concrete Pavement (JRCP) and Continuous Reinforced Concrete Pavement (CRCP). Both of these methods use reinforcing steel and load transfer dowels to supplement the tensile deficiencies in concrete. These methods help ensure the pavement slab is allowed to act a beam through its predetermined life span.
3.1.1 Jointed Reinforced Concrete Pavement (JRCP)
Almost since its inception as it exists today, concrete has had reinforcing steel added to it. This reinforcing steel is used to improve the tensile characteristics of rigid pavements. Designs for this pavement type call of A36 – 36ksi steel grid to be placed along the bottom of the section (shown in Figure 3.1.1.1). Sections are connected with load transfer dowels to help facilitate load transfer between adjoining slabs. Tie bars to keep them from separating connect adjacent slabs. Issues with this type of pavement are thermal expansion and contraction, and contraction of the concrete after during the curing process. This expansion and contraction creates stresses, and with all the steel tying everything together, the stress builds up because deformation is limited.
Figure 3.1.1.1: JRCP slab
3.1.2 Continuous Reinforced Concrete Pavement (CRCP)
These pavements are structured in much the same way as JRCP; however saw cut joints are minimized. These pavements respond to expansion and contraction much worse than JRCP pavements, but are much more durable because typical failure locations are at the joints.
3.2.0 Asphalt Reconstruction - Marshal Stability Method
MDOT uses the tried and true Marshal Stability Method for the design of their flexible pavements. This method developed a method for testing the stability of asphalt mixes through an unconfined compression test. This test measures the amount of flow of the viscoelastic material with respect to time. It considers what is referred to as %VMA (Voids in Mineral Asphalt) and tries to minimize this number, but also seeks to stabilize the asphalt matrix with this.
4.0 Potential Methods for Rehabilitation
Potential rehabilitation techniques analyzed, seek to improve the condition of the road vs. cost ratio. Improving this aspect compared to existing repair methods, allows for MDOT to maintain a higher level of quality in their roadways.
4.1.0 Concrete Rehabilitation
There are 2 primary methods for rehabilitation of rigid pavements not currently used by MDOT. These methods are: preventative saw cuts and square cuts around replacement sections. Negative aspects of these methods are that they require more labor work in implementation as compared to those currently used. However positives are that they last much longer and do not reduce the life of the existing pavement.