- May 26, 2020
Pervious Concrete Pavement and its Uses Transport Engineering
PerviousConcrete Pavement and its Uses: Transport Engineering
PerviousConcrete Pavement and its Uses: Transport Engineering
Perviousconcrete pavement refers to the kind of pavement which has thecapability to store stormwater and allow its infiltration into thesubgrade soil, and can also support particular traffic load andspeed. Pervious concrete (PC) is a compound material that comprisesof coarse aggregate, water, and Portland cement. It differs from thetypical concrete because it lacks fine elements in the originalcombination [Figure 2], with the fines introduced only during theprocess of compaction (Henderson& Tighe, 2011).The aggregate component normally comprises of a single size and islinked together at its areas of contact by a paste created by thewater and cement. Hence, a concrete with highly interconnected voidsresults, which allows rapid water percolation. Unlike the typicalconcrete that has a void ratio of 3 to 5 percent, the void ratio ofpervious concrete ranges between 15 and 40 percent depending on theapplication. Furthermore, when compared to the typical concrete,pervious concrete’s compressive strength is evidenced to be lower,but with a higher degree of permeability, and has a relatively lowunit weight, which is roughly 70 percent that of the conventionalconcrete (Chenetal.,2013).
Forthe efficient functioning of the pervious concrete pavement, Wangetal.(2010) asserted that itmust be constructed in a way that it fulfills the structural andhydrological conditions. If designed and constructed appropriately,pervious pavements serve as the best management practice assubstitute low impact development choice for urban regions andtransport networks. Presently, most successful pervious pavementinstallations occur on car parking lots and other commercial sectionswith medium to low speed and flimsy traffic load. During the lasttwenty years, considerable achievement has been realized concerningthe application of pervious pavements to structural design,hydrological design, water quality, and surface clogging. The presentsituation is confronted by the introduction of the concept ofpervious pavements in the urban roads construction, with keeninterests to integrate it in highways with higher speeds and loadstaking central stage. This study provides an exposition regarding theprofitable use of pervious concrete pavement design and performancewith respect to the transport networks engineering (Delatte,2014).The research details the present practice and design procedures,examines the application of pervious pavement for the highway,street, and urban environments, and provides the advantages anddisadvantages of such applications.
Perviousconcrete [Figure 1], also referred to as the non-fines, is verycritical in the control, management, and treatment of rainwaterrunoff. When applied in pavement, the concrete can efficientlyharness and store runoff due to stormwater, and permit the infiltratewater into the underground well. PC has little to no fine aggregate(sand), and water and cementitious materials are lowly concentrated.In the hardened concrete, about 15 to 25% of void is acquired, andthat facilitates fast drainage of water. Specifically, the rate offlow of water through the pervious concrete falls between 2 to 18gal/min/ft2(Ajamu, Jimoh & Oluremi, 2012).
Justlike the typical concrete, the constituents of pervious concrete arewater, cementitious materials, coarse aggregates, and admixtures.However, it lacks the fines, which are major components of theconventional concrete. With respect to cementitious materials,blended cements are primarily utilized to bind pervious concrete.During binding, coarse aggregate is maintained at a narrow gradation,and the sizes of aggregate considered fall between 7, 8, 67, and 89as provided by the ASTM C33 specifications (Kevern,Schaefer & Wang, 2011).For the parking lots and pedestrian uses, the coarse aggregate (89)has often been applied in pavements. Generally, pervious concrete haslimited cement paste the aggregate depend on the contact points forstrength. Consequently, the harder or coarser aggregates, such asgranite, are essential in generating higher compressive strength ascompared to softer ones like limestone. Mainly, because of theaugmented handling and placement, aggregate ranging between 3/4”and 3/8”. If larger aggregates outside the stated margin areutilized, the void spaces generated will be equally large, but thedisadvantage that follows is that there will be quite a rough surface(Kevernetal.,2011).
Besides,to successfully prepare pervious concrete mixture, water control isvery essential, as the w-cement (w/c) ratio used determines thestrength and void structure of the concrete. The most ideal w/cmvalues for deriving the most desirable concrete ranges from 0.27 to0.34. Water is required for the hydration of the cement and thecreation of a workable structure. Cement hydration is needed tofacilitate the formation of linkages with the aggregate, and thatgives the concrete its strength properties (Kim& Lee, 2010).Therefore, concrete strength has a direct relationship with thepermeability and the w/c ratio is a pervious concrete pavement. Asthe process of cement hydration goes on, the volume of solid in thecompartment increases, and that occurs in spaces earlier occupied bythe cement prior to hydration. The rise in the volume of solids isindicative of a fall in porosity. Permeability impacts strength,which in itself is an outcome of linkage. Certainly, establishingbonds in assortments which have high water-cement proportionsconstrains because of the separations between the respectiveparticles. At high w/c ratio, the pervious concrete blend has a highpermeability. Nonetheless, a high porosity signifies weaker bonds andreduced compressive strengths, and these are the majorcharacteristics of pervious concrete cement (Delatte,2014).
Finally,just like in the typical concrete, Putman& Neptune (2011) clarified that chemicaladmixtures are also utilized in pervious concrete to attain specificmixture characteristics as desired. Mainly, hydration stabilizers andretarders have been frequently applied to counter the rapid settingcommon to most pervious concretes. Under the free-thaw atmospheres,the admixture used for pervious concrete relates to air-entraining.However, as these mixtures are added, special consideration isrecommended to ensure that there is a state of equilibrium betweenthe paste materials, strength, void, and workability. In other words,to develop a pervious concrete configuration with maximum porosityand compressive strength, the quantity of water, the concentration ofcement, the nature and type of aggregate, and compaction must not bedisregarded.
Accordingto Delatte(2014), thereare two leading consideration areas used to determine the thicknessof pervious concrete on the transport networks. These include thehydrological designs aimed to meet the environmental needs, and thestructural designs which work to endure the expected traffic loadsand speed. These design consideration areas are as detailed below:
Inassessing the capabilities of the hydrological design of a perviouspavement, the method is to verify whether the features of thepervious concrete are efficient to infiltrate, store, and release thedesired water inflow. The information needed in the analysis ofhydrology includes the levels of precipitation intensity, thepervious concrete pavement’s thickness and permeability features,the cross slopes and geometrics, and the characteristics of theunderlying base and subgrade materials with respect to permeability.Several methods of hydrology designs occur, and include the Rationalemethod as well as the Natural Resources Conservation Service Curveapproach (Kevernetal.,2011).To avoid excess surface runoff, the hydrological design of perviousconcrete pavements focuses on two probable situations. The firstcondition relates to the pervious concrete pavement’s lowpermeability which is inefficient in capturing the initial flush of arainfall occurrence. The second consideration is based on the poorretention present in the structure of pervious concrete due to thepresence of slab and subbase. Normally, the pervious concretepavement’s thickness is determined while basing on the structuralneeds and scrutinized to ascertain its appropriateness to fulfill thehydrological requirements of the road network site. If the thicknessis ruled out as unreliable, adjustments can be done to the underlyingbase material or the pervious concrete itself (Delatte,2014).
Thedesigning of pervious concrete pavement can take virtually anyaccepted procedure such as those of the StreetPave and AmericanAssociation of State Highway and Transportation. Particularly, aninitiative termed as PerviousPave has been formulated by the AmericanConcrete Pavement Association, which is utilized to establishpervious pavements’ hydrological and structural designs.Irrespective of the procedure employed, several factors are vital andmust be considered while designing the pervious concrete pavements(Vancura,MacDonald & Khazanovich, 2011).
Subgradeand subbase:Often, the foundation pillar in pervious pavements’ designcomprises of a complex modulus of Subgrade reaction, which explainsthe impacts of the subbase and subgrade. Mainly, a subbase that isopen-graded is introduced under the pervious concrete pavements tofacilitate water drainage and storage. However, in cases of expansivesoils and frost, direct treatment is applicable (Vancuraetal.,2011).
Concreteflexural strength:It is an essential input in the concrete pavements structural modelwhich should not be overwritten. Comprehensive strengths andempirical relationships are used in the estimation of flexuralstrengths to be applied in the design. That is because the process oftesting so as to ascertain the flexural strengths is confronted bysignificant variations (Thorpe& Zhuge, 2010).
Trafficloading application:The projected traffic to be accommodated by a pervious concretepavement is measured in terms of equivalent 80kN single-axle loadrepetitions, and that is usually calculated directly based on theassumption of the distribution of truck and traffic. It is criticalto mention that most of the pervious concrete pavements are utilizedin low truck-traffic situations. At present, there is no universalthickness for pervious concrete pavements, but a majority of perviouspavements serving as parking lots are built 6 inches thick.Conversely, low-volume street pervious pavements are constructedwithin the thickness range of 6 and 12 inches (Henderson& Tighe, 2011).
Dueto the distinct material features, pervious concrete pavement hasseveral special installation or construction requirements, which areas detailed below:
Often,fixed-form construction is applied to place pervious concrete.Specifically, small-scale constructions make use of vibrating screedor straightedge in the placement of pervious concrete. However,A-frame, and low-frequency vibrating creed are utilized inlarge-scale projects. For consolidation, the utilization of a steelroller to spin the concrete takes central stage. Because of the lowwater content of pervious concrete, coupled with its porouscharacteristics, the process of delivery and placement must be rapid(Delatte,2014).
Arguably,the finishing procedure for pavements made up of pervious concretedoes not take place in a similar version as the conventionalpavements. Notably, the last surface finish is attained as part ofthe course of consolidation, and that leaves an open surface. Theconstructors should avoid the usual procedures for finishing, such asthe introduction of trowels and bull floats (Kevernetal.,2011).
Inorder to contain random crack occurrence, jointing must be performedon pervious concrete pavements. The joints, while making use ofroller-jointer, are done to a depth range of a quarter to a third ofthe thickness of the slab (Vancuraetal.,2011).
Followingthe jointing of concrete, the result must be completely cured, andthat is possible through the inclusion of thick (0.15mm) plasticsheeting on top of every exposed surface. The application of thesheeting must occur within 20 minutes after concrete discharge, afterwhich it should be left intact for at least 7 days (Ajamuetal.,2012).
Proceduresare in place as brought forth by the American Concrete Institute toaid in the inspection and testing of pervious concrete pavementsduring the process of construction. Density and thickness are the keyqualities used in the testing of concrete, with various methods suchas ASTM C1688, ASTM C1747, and ASTM C1754 already conceived tofacilitate the process (Delatte,2014).
Perviousconcrete pavements have voids that are likely to be filled by sand,dirt, and other debris over time, and that may interfere with itsporosity, adversely impacting the functionality of the structure. Toprevent such incidences, maintenance procedures are executed throughpressure washing and power vacuuming (Ajamuetal.,2012).
Regardingperformance of pervious concrete pavements, indicators such aspermeability, presence or absence of the structural and surfacedistress, and the resistance to freeze-thaw are examined (Ajamuetal.,2012).On that regard, several pros and cons of the pervious concretepavements have been established, and are as discussed below:
Thereare many cons attributed to pervious concrete pavements, which arerelated to the high permeability of pervious concrete, its low unitweight, high heat insulating values, and less shrinkage tendencies.Hence, Thorpe& Zhuge (2010) argued that perviousconcrete pavements are efficient in the control of the rainwaterrunoffs, and that reduces the expenditures on storm sewers and curbs.Second, they system ensures facilitates decreased contamination inthe waterways because it permits rapid percolation or infiltration,processes that are also crucial in refilling the water table. Also,pervious concrete pavements, due to their porosity and texture,ensure that the noise emissions as a result of tire-pavementfrictions are reduced. It also gets rid of water stagnation on theroad networks, thus limiting the destruction of the channels throughcorrosion and pollution (Wangetal.2010).
Althougha number of advantages related to the use of pervious concretepavements exist, several cons have also emerged. First, the strengthof bonds between the pervious concrete particles is weaker than inthe typical concrete, and that makes the use of this type of pavementunsuitable for heavy vehicle traffic regions or highways. Besides, ifthe pavements are not regularly maintained, clogging occurs as aresult of the blockade of the voids by dirt particles itseffectiveness in water drainage is, therefore, short-term in theabsence of maintenance. Finally, since its use in the United Statesis fairly new, there are few to no expert engineers/contractors toforesee its successful installation (Thorpe& Zhuge, 2010).
WherePervious Concrete Pavements are used
Thequalities of pervious concrete determine the areas where thepavements can be used in a road network. Because the compressivestrength and unit weigh of pervious concrete are lower than that oftypical concrete, pervious pavements are not used in highways wherethere are many trucks and high loads to support. However, thesepavements are very effective low traffic roads and car parking lotsin the urban areas. Furthermore, in such regions, effective drainagesystems are required. Hence, pervious concrete pavements areinstalled because they are highly porous and would encourage rapidinfiltrations whole eliminating surface runoffs (Delatte,2014).
TypicalRoadway Concrete and Pervious Concrete Pavements, Comparison
Thetypical concrete pathways constitute the busy highways because theconventional concrete has a high unit weight, which is 70 percentmore than that of pervious concrete. It can, therefore, accommodatethe heavy loads as speedy movements of the high-traffic roads.Furthermore, the fact that it has a higher compressive strength alsoenables the typical concrete to support such loads and constanttraffic flows in highways. However, conventional concrete, because ithas fines, displays the least permeability or porosity, meaning thatsuch pavements encourage surface runoffs and erosion of roads.Nevertheless, roads made up of typical concrete pavements are usuallyreinforced with other drainage systems such as the curbs and sewers,and a lot of costs are incurred (Putman& Neptune, 2011).Conversely, pervious concrete has low unit weigh and compressivestrength. Therefore, roads with pervious concrete pavements cannotaccommodate heavy loads and high traffic. They are thus used asparking lots and also as transport avenues in low traffic areas.However, the high porosity levels of pervious concrete facilitaterapid stormwater infiltration, and such roads or parking lots do notrequire the installation of other drainage systems for watermanagement (Delatte,2014).
Fromthe above considerations, it is patent that the field or transportengineering is considering the use of pervious concrete pavements inthe construction of road networks. Although pervious concrete is notas strong as the typical concrete, it works as an alternative whenapplied in low traffic and low impact sections of the road channels.In as much as the strength is sacrificed in the favor for porosityduring the processing of pervious concrete, it does not happen to theextent that the structure is rendered functionless. To develop apervious concrete configuration with maximum porosity and compressivestrength, the quantity of water, the concentration of cement, thenature and type of aggregate, and compaction are all areas of stress.Although pervious concrete pavement is very successful as a tool forcontrolling water drainage through infiltration, its worstdisadvantage is related to its inability to support highway orheavy-traffic operations, which remain the busiest areas of any roadconstruction.
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Figure1: Pervious Concrete Section:http://www.construction.basf.us/images/interface/spotlight_pervious_image_2.jpg
Figure2: Comparison of Typical Concrete and Pervious Concrete Pavements:https://sourceable.net/wp-content/uploads/2014/07/pervious-concrete.jpg