AsphaltPavement Problem Statement
Effectsof Construction Deficiencies on Design for Asphalt Pavements
Concerns have been expressed in the building and constructionindustry that a declining standard of design quality has contributedefficiently to decline construction efficiency. Inadequate designshave had major impacts on projects, causing delays, reworks, andvariations, contributing to an increase in the project time and cost.Thus, it is important during construction that materials, mixtures,and the contrasted product meet the specification requirements. Thereexists the notion that inspectors and other oversight personnel allthey do is ensure that specification requirements are met.Nevertheless, this is not the case since there are many items tocontrol during construction, and there is hardly a project whereeverything goes according to plan. Those involved in oversight of theproject must understand materials, mix designs, and construction tosee to it that specification requirements are met. In addition, it issignificant to understand how deficiencies in pavement designrequirements will impact on performance to ensure appropriate stepsare taken. This paper intends to be informative to the technical andmanagerial teams, provide overviews of dynamic problems and solutionsin construction deficiency on design.
Someof the answers the essay will be seeking to give will includequestions on changes in thickness and how they affect performance,and lack of positive control of drainage and its effects onperformance, and how plasticity index affects performance. Technicaland managerial teams should keep in mind the construction of modernairfield pavements must ensure products in use conforms to the designassumptions and concepts if proper performance is to be achieved.Below are some critical design issues that may be compromised duringconstruction.
FundamentalAirfield Flexible Pavement Design Concept
Aclassical airfield asphalt concrete pavement will consist of arelatively thin asphalt concrete surface on relatively thick granularbase and sub-base layers resting on the natural sub-grade (Brown,2004). This asphalt concrete layer is designed to provide a smooth,all weather operational surfaces that limit water penetration intothe underlying layers. Notably, the pressure exerted by aircrafttires affects the quality of material to be used near the surfacestill the amount of load applied by the tires affects the quality ofmaterial used at a deeper depth. The applied surface load is usuallydistributed through each succeeding layer, which in turn spreads theload over increasingly wider areas thereby reducing the appliedstress.
Thus,the basic concept behind flexible pavements designs is providingsufficient total pavement thickness to reduce applied stress on thesub-grade to a level appropriate for the sub-grade strength andanticipated load repetitions. This is the set fundamental conceptapproach behind most designs, with the sub-grade strength being themajor parameter governing pavement thickness required for any load.Therefore the different layers must be sufficient and strong enoughto withstand the amounts of stress intensity to which it will besubjected these would mean that any changes in layer thickness orquality of material in the layers might impact negatively on thepavement’s life (Mallick and El-Korchi, 2013).
Thetechnical and managerial teams should be well informed that thesub-grade strength has a major impact on the required design airfieldpavement thickness. A deficiency will result into the pavement’spremature shear rutting before it even reaches the intended lifeperiod. Thus, the thickness in various layers may be controlled bydetermining the difference in the surface elevations between the topand the bottom of each layer, or simply by measuring the layerthickness (Delatte, 2014).
Accordingto Anderson et al (2001), fatigue cracking in the asphalt concretesurface develop from a complex interaction of support conditions,mixture properties, loads, and thickness of the asphalt concretelayer. If there is a deficiency in thickness during construction, itwill likely result into an increased strain at the bottom of theasphalt mixture, consequently, leading to premature cracking of thesurface. However, this premature cracking may be as a result of otherthings besides inadequate thickness, which include, blocked drainageresulting in saturated sub-grade or base materials with defectionshigher than those anticipated, overloads, and variability duringconstruction that result in undesirable mixture properties such as alow asphalt content or an overheated asphalt content. Thesevariations will eventually cause fatigue cracking especially were athin layer of hot mix asphalt is used.
Thedifferent layers in an airfield pavement structure call for differentand specified degree of compaction to be attained duringconstruction. The aim of the compaction is to see to it that the mostdesirable engineering properties of pavement materials improve withimproved compaction and higher density such as strength increase andpermeability decrease. If this is not adhered to and an asphaltconcrete layer is inadequately compacted during construction, it mayhave inadequate shear strength which may result to failure when it isloaded. The permeability of an asphalt concrete increases rapiditywith an decrease in compacted density, thus, a poorly compactedasphalt layer increased permeability may allow penetration oraccumulation of water that would likely lead to stripping the asphaltconcrete and weakening the underlying layers. For cases where thelower mixture compaction does not result in excessive permeability,it will suffer increased oxidation and the earlier cracking problems(White, 2002).
Secondly,compaction is carried out for the sake of identifying all materialsin the pavement structure and sub-grade to avoid furtherdensification under traffic. The huge loads and hire tire pressure ofaircrafts make them highly effective compaction devices. Materialsmay have sufficient shear strength to resist shear deformation,should be denser under traffic, one will still have rutting andsurface distortion simply because of the decreases in materialvolumes. Therefore failure to get to the specified compaction in anypavement layer is a major potential for construction defects that mayconsequently significantly shorten the life of an airfield pavement(Shoenberger and DeMoss, 2005).
Accordingto Kandhal (1992), water has advance impacts on flexible airfieldpavements, above all, water on the surface can pose safety problemsdue to loss of skid resistance and hydroplaning hazards foraircrafts. Secondly, water allowed to accumulate near a pavement mayattract bird life, which in its turn is a bad aircraft safety hazard,still water in the pavement structure will usually cause a decreasein the strength and stiffness of most engineering materials, soilsand aggregates near saturation develop excess pore water pressureunder traffic loads consequently reducing the shear strength. Stillmore increases or decrease may cause the volume changes or collapseof some soils, while water and moisture in asphalt concreteespecially where it is combined with repeated traffic loads, it leadsto stripping in the asphalt concrete.
Dealingwith the water menace, designers and technicians incorporate verypermeable drainage layers to rapidly remove subsurface water from thestructure. These drainage layers are typically an open-gradedaggregate that may or may not be stabilized with the help of Portlandcement or asphalt cement. The unstable layers are typically placed ontop of the base course and can cause some construction difficultiesas grading and compaction is redone to correct surface rutting ormovement in the open-grade materials under construction traffic(Kandhal, 1992).
Compliancewith Material Specification
Managersand technicians should be well aware that failure to comply withmaterial specifications is an area construction can have particularlyadverse impacts on the performance of an airfield pavement. Pavementsare comprised largely of natural materials that are inherentlyvariable. As such, meeting tight airfield pavements specifications onaggregate, asphalt concrete, and other pavement components, this isnot an easy task. Therefore, a strong quality-control system by themanagement and technicians will ensure that materials placed arewithin specifications (Shi et al, 2009).
Considerableefforts are put across in designing airfield, however, should thedesign objective not be achieved during construction, and then theairfield pavement may fail or require major maintenance prior to theend of its expected design life. Thus, this paper has outlined someof the areas where construction deviations, sometimes only minordeviations, can have serious repercussions on pavement performance.In the long run, a well build and designed airfield pavement willgenerally outlive a well-designed but poorly designed pavement, thus,control during construction is very important. Declining standards ofdesign quality contribute significantly to similar declines inconstruction efficiency, the aim of this essay way to assist managersand technicians to plan effectively before starting a project.
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