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Glynn Holleran, M App Sci. B App Sci (hons), MRACI, CC,ACS, AAPT Vice President VSS Asphalt Technologies USA. Back to VSS Technology Papers Library INTRODUCTION: Pavements fail for many reasons from poor construction, poor design, poor materials. However one very important cause of failure lies in the depths of the pavement. Drainage, according to AASHTO accounts for 60% of all pavement failures. (1). This covers all sorts of pavements. In light pavements such as parking lots, truck parks and grounds it is a far greater problem. This paper discusses pavements and how they fail due to water problems, their design for moisture allowance, drainage principles and designs and rehabilitation of failed areas. The role of pavement preservation in preventing problems from surface water is discussed. 1. PAVEMENT STRUCTURE AND FAILURE Pavements are multiplayer structures as shown in figure 1.
Such bases are not usually free draining and some soil types will swell with water saturation. (2). Dense graded granular bases may become saturated. Sub-bases under old concrete slabs can, under such conditions, rocking or loading of the slabs can pump water up through joints and cracks. If overlaid with hot mix this can cause a pressure build up and stripping and potholing. The pavement becoming wet will reduce its stiffness. This occurs for several reasons:
Figures 2 and 3 show such effects. Figure 3 shows that the pavement is placed in both tension and compression by wheel load. This flexing is made more severe by less stiff pavements.
Figure 3 shows the effect of pavement stiffness on spreading load on the subgrade. Clearly as pavements are layered structures with strain being concentrated at the base of layers (3). The magnitude of the generated stresses decreases layer to layer but the duration of the stress pulse increases with depth. The stress reduction effect of any layer depends on its thickness and stiffness.( pavement design book). Strains will be increased all through the pavement in situations where the layers are less stiff due to water. This may lead to two effects: a) Fatigue failure
b) Subgrade Rutting
Saturation of the subgrade and granular base leads to a reduction in load bearing strength and the potential for deformation. In instances where the overlay is in good condition it will not rut and may collapse after cracking. In such an instance tearing of the surface and heaving may occur. This will give rise to fatigued areas that begin to pothole as more water can enter from the top.
c) Potholing and Collapse This may occur by two different mechanisms. In the first the underlying base will flex causing the overlay to crack. This will allow more water to penetrate the surface creating further stiffness reductions and water build up. Fines are pumped upward through the crack reducing the load bearing capability of the base. This increases fatigue cracking and leads to pick out of fractured parts of the overlay as it disintegrates. The formation of the pothole is the causing collapse of the overlay into a hole. In the second poor surface drainage and traffic effects can remove fines from the surface of the hot mix reducing structural strength. The process proceeds by cracking, water ingress and pothole formation as above.
Such effects are exacerbated in low temperature conditions where the HMA is stiffer and freeze thaw may occur.
2. SOURCES OF WATER DAMAGE PROBLEMS Water can cause damage if it is able to get into the pavement. Sources of water are:
Once the water is in the pavement draining it is difficult. When sub grade soils are likely to swell or hold water this situation can be worse. 3. PAVEMENT DESIGN FOR MOISTURE: The moisture regime has a profound effect on the pavement. Factors that have to be assessed for design are:
The sensitivity of the subgrade to water needs to be assessed as well. 4. BASE AND SUBGRADE DRAINAGE There are two types of drainage. a) Surface b) Subsurface a) SURFACE: Most of the water that gets into the pavement comes from the surface. As shown in the figure above the main sources are from shoulders; through pavement cracks or holes; seepage from higher points. Permeability of the base is another issue. Permeability of granular materials depends on grain size, distribution and density of the compacted pavement. For drainage the slope of the base and width need to be considered. A free draining base is preferred with adequate sub grade drainage via pipes. However sometimes this is not economically viable. In such an instance a treated high density base should be used. This may be achieved with asphalt emulsion. The base can then be constructed in a trench. However a trench can trap water at the interface of pavement and base course and allow moisture from the sides. This will be made worse under loading where the sides may open up. Surface drainage thus is a main design requirement.
b) SUBSURFACE: Poorly drained subgrades can cause a layer of water to form at the base/subgrade interface forming a mud slurry under traffic that will fill voids in the base material clogging and reducing particle interlock and friction. The sensitivity of the sub grade to changes in moisture content depends on its composition. For sandy soils small changes give little change. For silty soils little change in volume occurs but significant changes in strength can occur. For clay both the volume and strength will change for the worse with water. Especially as it approaches optimum moisture content. The subgrade must be properly shaped and sloped when constructed to ensure drainage. Volume changes are not a problem if compaction has been achieved in the original construction but stiffness reduction is always a problem as it contributes directly to cracking. Freezing will cause heave of a wet pavement creating surface fracture. Subgrade drains are used to drain the soil. This may be agricultural perforated drain pipes ( perforations down ) and backfilled so the fill acts as a filter. This is very important where water may collect and freeze or for soft sub grades.
Design of Subgrade drainage: Aims: There are two sources of water. a) Ground water b) Surface water Ground water is below the water table. It is the aim of the drainage to lower the water table. Infiltration is the largest source. Its movement depends on permeability of the soil, the hydrostatic head. Design Of Subsurface Systems: The design is aimed at lowering water table, eliminating active springs, seepage and other water sources and to collect discharge from other drainage systems. It must have adequate capacity and maintain this during the life of the pavement.
It is preferable to consider water flow and drainage during design and construction phase. The Asphalt Institute have an excellent manual on this MS-15. However most paving contractors are faced with other people's errors. How do they go about helping a customer with a problem? 5. HOW TO CORRECT PROBLEMS AND REHABILITATE Water moves and collects under pavements. If cracking or potholing is not extensive and as most water enters from the surface the first option is to seal the pavement surface. Selected dig outs for the worst areas of cracking and potholes and installation of full depth patches can be used. A slurry seal over the top can then be used as a pavement sealer. Paving and sealing the shoulder will assist in preventing further water ingress. This means allowing the pavement to dry out and sealing the surface and edges with an asphalt based sealing method such as chip sealing. For worse situations a subsurface drainage system can be installed, after repair, by trenching and installation of drop inlets at low spots. In some cases the area may be overlaid and levels changed to facilitate drainage to low spots. These drop inlets are connected via pipes to the main drain system. If frost and freeze thaw is a problem this drainage system must keep the top metre dry. In flat areas longitudinal drains are only needed along the lower edge of the paved area. In hilly areas or pavements with a number of sloped areas and low spots lateral drains spaced to prevent pressure build up are required. Drop inlets in low spots with pipe connection to the lowest spot. For elevated curves collector drains should be placed along the lower side of the curves connected to lateral drains from the pavement. Water will always go to the lowest spot. They are the last parts of the pavement to drain and can force free moisture up through cracks. Standing water will result and create a hydrostatic head.creating debonding. Where it is likely that such collection will occur lateral drains through the shoulder should be installed. If the pavement has been widened and no provision has been made for drainage right through the old and new pavement collection at edges can occur and even collapse of the surface forming a basin this will cause surface damage by loss of fines and potholing. A method of preventing this is to make the widened are with open graded hot mix as a drainage layer.
Side drains must be kept clear so progressive overlays must not cover or raise the area in front of them. So use slurry or chip seal or raise such drains before overlay. Rehabilitation Strategies: It is important to distinguish between treatments to prevent water ingress and stop extra damage and instances where the drainage itself must be rehabilitated. Preventing Damage Maintenance: As most water comes from above a sealed pavement is a good pavement. Cracks must be dealt with promptly along with any potholes. Surface sealing may be all that is required for moderate damage. This means a positive maintenance and inspection program.
Rehabilitation: When structures break down they must be repaired in a timely fashion. This may mean reconstruction in part or full. Before large scale reconstruction is required it may be possible to carry out smaller scale repairs like digging out the area failed and replacing the pavement with full depth hot mix patching.
In many instances where drainage is not the cause of the fatigue or cracking failure, eg age cracking an asphalt rubber SAMI with overlay or slurry ( cape seal) may be cost effectively applied. (10,11).
It is important that drainage problems are solved before any rehabilitation is done. Geotextiles too may be used to improve drainage, as well as installation of surface drains, altering cross slope and may other methods. The reader is directed to the excellent Asphalt Institute publication on this subject (4).
6.0 CONCLUSION Drainage is a key factor in successful pavements. The designer, contractor and customer need to be conversant in its basics and effects. 7.0 REFERENCES
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