O«latE~ ; i:() rxdot Surface Treatment Training Manual

Transcription

1 O«latE~ ; i:() rxdot Surface Treatment Training Manual Research Prject Number Research Prduct Number P4 Sanjaya Senadheera and M. Vignarajah Perfrmed in Cperatin with the Texas Department f Transprtatin And the Federal Highway Administratin Center fr Multidisciplinary Research in Transprtatin Texas Tech University

2 NOTICE The United States Gvernment and the State f Texas d nt endrse prducts r manufacturers. Trade r manufacturers' names appear herein slely because they are cnsidered essential t the bject f this reprt

3 Draft Updates t TxDOT Surface Treatment Training Manual By Sanjaya Senadheera, Ph.D. and M. Vignarajah Department f Civil Engineering Texas Tech University Center fr Multidisciplinary Research in Transprtatin (TechMRT) Research Prject Number: Research Prject Title: Cnstructability Review f Surface Treatments Cnstructed n Base Curses Perfrmed in Cperatin with the Texas Department f Transprtatin and the Federal Highway Administratin March 2007

4 Disclaimers The cntents f this reprt reflect the views f the authr, wh is respnsible fr the facts and the accuracy f the data presented herein. The cntents d nt necessarily reflect the fficial view r plicies f the Federal Highway Administratin (FHWA) r the Texas Department f Transprtatin (TxDOT). This reprt des nt cnstitute a standard, specificatin, r regulatin. This reprt is nt intended fr cnstructin, bidding, r permit purpses. Page 2 f25

5 Table f Cntents 1. Table f Cntents 3 2. Chapter 2, Sectin 5: Surface Treatments (Insert n secnd page f the sectin) 4 3. Chapter 3, Sectin 2 (New Sectin Insert)- Surface Treatments n Base 6 4. Chapter 8, Sectin 4 (New Sectin Insert)- Surface Treatments n Base 16 Page 3 f25

6 Chapter 2 Guidelines fr Treatment Selectin Sectin 5 - Surface Treatments Onsert n secnd page f the sectin) The surface treatment is the primary tl in the pavement engineer's arsenal t seal the pavement layers such as the base, subbase and subgrade. Whether ne uses a surface treatment as a wearing surface, r as an undersea!, it des perfrm this functin mre effectively than any ther. When a surface treatment is used as an undersea!, it can als prlng the life f the pavement structure thrugh imprved fatigue resistance. It als can be used as a temprary wearing surface fr a limited time until the ht mix asphalt cncrete is placed. A surface treatment wearing surface is very cst effective when used under apprpriate cnditins such as lw traffic and rural settings. In such applicatins, it is imprtant t keep in mind that the surface treatment des nt prvide structural strength t the pavement. Therefre, apprpriate cnstructin techniques fr the base layer, and taking gd care f the base are very imprtant. A surface treatment wearing surface is a prven wearing curse fr flexible pavements. It is very imprtant that apprpriate prjects are selected fr surface treatment wrk. The fllwing factrs are cnsidered in selecting prjects fr surface treatment. 1. Traffic level (ADT) 2. Lcatin f highway (urban r rural) 3. Percent trucks n highway 4. Availability f funds fr cnstructin 5. Highway functinal classificatin 6. Feasible traffic cntrl plan during cnstructin 7. Characteristics f the existing pavement, if any 8. Pavement lcated in an area with mving subgrade 9. Presence f turning traffic n the highway The traffic level, usually expressed by Average Daily Traffic (ADT) f the highway sectin, is the mst widely used prject selectin criterin. This is fllwed by, pssibly in the rder f decreasing use, highway lcatin (urban r rural), percent trucks n the highway, funds available fr cnstructin and the highway functinal classificatin. Less ften used prject electin criteria include the nature f traffic cntrl plan needed, characteristics f the existing pavement (surface, etc.) and the presence fturning traffic. Successful past experience with surface treatment wearing surfaces indicate that they can be effectively used in highways with ADT values up t 5,000. It has been reprted that surface treatments have been successfully used in highways with ADT values as high as 12,000 in a few cases. Page 4 f25

7 Apprximately percent f radbed miles statewide have an ADT fless than 1000 and anther 14% has ADT values between 1000 and This shws that ver 50% f statewide radbed miles culd easily be ptential candidates fr surface treatments. If sufficient funds are available, mst highway agencies wuld like t use HMAC instead f surface treatments as a wearing curse. Hwever, surface treatments can be an effective ptin when cnsidering pavement perfrmance and life-cycle cst. Pavements with surface treatments as wearing surfaces are cnsidered t generate higher levels f traffic nise cmpared t ht mix asphalt. Hwever, research ahs shwn that highway nise is nt a direct functin f the pavement surface type, and several ther factrs influence it. When the highway sectin is lcated in r near a residential area, highway nise cnsideratins may be a deciding factr in prject selectin. Hwever, it shuld be nted that surface treatments can be cnstructed t generate less highway nise by prper use f materials including binder type and aggregate grade. Pavements cnstructed n 'mving' subgrades such as expansive sils, have a very high likelihd f cracking. Design f the pavement structure n such subgrades is likely t result in a very thick asphalt cncrete layer which may nt be cst effective fr rads with lw traffic levels. A surface treatment wearing curse that uses a binder with lw susceptibility t cracking can be a candidate fr such prjects. If HMAC has t be used, a surface treatment undersea! wuld be a viable ptin. Page 5 f25

8 Chapter 3 Material Selectin and Plan Preparatin Sectin 1 Cmmunicatin and Crdinatin Sectin 2 (New Sectin Insert) - Surface Treatments n Base There are sme issues that are unique t surface treatments cnstructed directly ver base when cmpared with preventive maintenance seal cats. These issues distinct t surface treatments that are relevant t material selectin and plan preparatin are discussed in this sectin. Prime Cat Almst all TxDOT districts that use surface treatments either as a wearing curse r as an undersea! typically use a prime cat. The prime cat plays a very imprtant rle in pavement structures. Its primary benefit is the facilitatin f bnd between the surface treatment and the base layer. The binders that are used in the surface treatment curses need t be strng and durable. Such binders d nt have the lw viscsity needed t penetrate the base layer and grip it, t prevent it frm de-bnding due t shear stresses exerted by traffic and due t ther factrs. A prime cat which uses a lw viscsity binder can act as an intermediary between the surface treatment binder and the base. The gripping effect f the prime cat nt the base als strengthens the base layer by prviding mre chesin t the tp f the base. The penetratin f the prime cat int base is very imprtant t get the maximum benefit frm the prime cat. The amunt f penetratin wuld depend n a number f factrs including the prime cating methd, prime cat binder, base material, base finishing technique and the prsity f the base curse. Typical penetratin f a sprayed cutback prime culd be in the range f 1/8-3/8 inches. Figure 3.1 shws a schematic f prime cat penetratin int the base layer. The picture in the inset shws the penetratin f an MC-30 prime fr a specimen prepared in a labratry. There is usually a time lag between the cmpletin f base layer cnstructin and the applicatin f surface treatment. A well-applied prime cat can prtect the base layer frm adverse weather cnditins and frm wear due t cnstructin and regular traffic until the surface treatment is applied. It can als either prevent r slw dwn the frmatin f dust n the surface that will have a serius negative impact n the bnding f binder t base. Page 6 f25

9 Figure 3.1 Schematic Shwing Prime Cat Penetratin int Base with Picture in Inset Shwing Actual Penetratin in Labratry Cmpacted Limestne Base The design f prime cat includes the fllwing elements. I. Selectin f an apprpriate prime cat methd 2. Selectin f a prime cat binder type and an applicatin rate The selectin f the prime cat type t use fr surface treatments depends n several factrs including the fllwing. 1. Past experience 2. Availability f a cntractr pl with expertise in prime cat type 3. Traffic cntrl plan during cnstructin 4. Base material Prime Cat Types At least fur different prime cat types are typically used including the fllwing. I. Spray prime with r withut bltting material 2. Wrked-in (r cut-in) prime 3. Cvered prime 4. Mixed-in prime Spray Prime Figure 3.2(a) shws a MC-30 cutback prime cat sprayed using an asphalt distributr. The typical rate f applicatin f the prime cat binder is 0.2 gallns per square yard that may be adjusted depending n the "tightness" f base finish and n whether cnstructin traffic has t be allwed quickly n the primed surface. If the prject is cnstructed Page 7 f25

10 under traffic, a bltting material such as field sand may be spread n sprayed binder minutes after the prime cat binder is sprayed [Figure 3.2(b)]. Figure 3.2 Spray Prime (a) MC-30 Cutback Prime (b) Bltting Sand Applied n Prime Wrked-in Prime Figure 3.3 shws a wrked-in prime cat applicatin where diluted emulsified asphalt is sprayed n the finished base, which is then cvered with a thin cating f fine base material dust wrking the windrw with the mtr grader. This prcess is usually repeated 2-3 times t get a ttal emulsin applicatin rate f 0.2 gallns per square yard. The emulsins cmmnly used fr this purpse are SS-1, CSS-1 h and MS-2. This leaves an asphalt-sand layer n the finished base that is apprximately 118 in. thick (see inset). Figure 3.3 Applicatin fwrked-in (cut-in) prime n limestne base Page 8 f25

11 Cvered Prime Figure 3.4 shws a cvered (r inverted) prime applied n the finished base. This cvered prime is similar t a curse in the surface treatment where RC-250 cutback is first applied n the finished base and then cvered by spreading Grade 5 rck. This 'priming' technique is particularly useful when traffic has t be allwed n the primed surface befre the ther half f the radway is primed. This type f a prime can prvide 2-3 mnths f satisfactry service as a very temprary wearing curse under favrable traffic cnditins including little r n turning traffic r heavy traffic. Figure 3.4 Cvered (r inverted) prime using RC-250 and grade 5 rck Mixed-in Prime The furth type f prime cat is the "mixed-in prime". In this type, when the base density is achieved and the base is cmpleted up t the blue-tps, the tp 2-3 inches f base is remixed with a diluted emulsin and then re-cmpacted. There is sme ambiguity in the way terms such as "Cut-in", "Wrked-in" and "Mixed-in" are used t describe the prime cat. "Cut-in" r "Wrked-in" prime essentially means the same in which the prime cat binder, a diluted emulsified asphalt, is sprayed n the finished base and the base material windrw is wrked back and frth t create a thin sand-asphalt layer that acts as the prime cat. A mixed-in prime is ne where the tp 2-3 inches f base is remixed with diluted emulsin and then re-cmpacted. Surface Treatment Curses The third and final step in the design prcess is the design f the surface treatment itself, and this includes the fllwing elements: Research Prduct P4 Page 9 f25

12 1. Decisin n the number f surface treatment curses (including their cnstructin sequence) 2. Selectin f aggregate type and grade 3. Selectin f binder type and grade 4. Design f material applicatin rates Number f Surface Treatment Curses In many situatins, a ne-curse surface treatment is applied in late fall, which is used as a temprary wearing surface fr several mnths until the next warm weather seasn arrives when the subsequent curse( s) are applied. If a cvered prime, which is really a single curse with grade 5 aggregate, the service life ne can expect t get as a temprary wearing curse may be n mre than 3 mnths. One-curse surface treatments are als used fr underseals where an undersea! t a HMAC layer paved n it subsequently. In sme cases, the HMAC may nt be applied fr several mnths after the surface treatment curse, making it act as a temprary wearing curse. Hwever, use f a I-CST fr an extended perid is nt recmmended n a highway with significant levels f heavy traffic. A 2-CST is the mst cmmn surface treatment type used by TxDOT, and it is smetimes applied in tw stages, where the secnd curse is applied smetime, typically a few mnths, after the first curse is applied and traffic is allwed n it. A 3-CST is nt as cmmnly used by TxDOT as it did smetime back, but three districts use it n a regular basis. It is smetimes applied in multiple stages, where the secnd and third curses are applied mnths apart. In ther districts, after an initial first curse is applied and traffic is allwed n it fr several mnths, subsequent curses are applied all in ne day within hurs f each ther. This is dne when ht asphalt is used as the surface treatment binder. Aggregates in Surface Treatment Curses The aggregate rate used fr a surface treatment curse is an imprtant part f the surface treatment rate design, and it is ften the first item that is designed. The selectin f aggregate grade and rates fr a 1-CST wuld be similar t that f a preventive maintenance seal cat. The generally accepted practice is t leave sufficient rm (i.e % in plan view) between rck particles s that they can 'wiggle' and settle t the mst stable psitin when rlled. A 'Bard Test' is used by sme districts t help determine the rck rate. The bard test uses a 1 yard by 1 yard bard n which aggregate is spread t the required cverage level, and then determines the applicatin rate by using the calculatin methd utlined in the TxDOT Seal Cat and Surface Treatment Manual (2004). Typically, a crrectin has t be made t the rck rate determined using the bard test t suit field cnditins. The key is t avid ver-applicatin f stne. If 't much' rck is Research Prduct 0-5l69-P4 Page lo f25

13 applied, rck particles may nt be seated in a stable manner, and result in rck lss due t lack f embedment. When multiple-curse surface treatments are designed, the aggregate rates and grades used fr the upper curses are generally selected based n experience. Figure 3.5 illustrates tw cmmnly used aggregate cmbinatins fr 2-CST. Of the tw grade cmbinatins shwn, Figure 3.5(a) which has grade 3 rck verlaid by grade 4 rck is the mst cmmnly used cmbinatin. Figure 3.5(b) shws grade 4 rck verlaid by grade 3 rck. This is typically used in a highway with lw ADT. (a) (b) Figure 3.5 Aggregate Grade Cmbinatins Used in 2-CST (a) Grade 4 n Grade 3 (b) Grade 3 n Grade 4. Figure 3.6 illustrates three cmmnly used aggregate cmbinatins fr 3-CST. Figure 3.6(a) has grade 5 n grade 4 n grade 3. Figure 3.6(b) has grade 4 n grade 4 n grade 3 rck. Figure 3.6(c) has grade 4 n grade 3 n grade 5 rck. The grade 5 rck in the bttm curse is frm a cvered prime. Binders in Surface Treatment Curses A variety f binder types and grades are used by districts in surface treatment curses. These binders can be classified int binders used in warm weather and cl weather. The selectin f a surface treatment has t be dne with sme care. If the designer can determine with sme certainty that cnstructin cnditins, primarily the ambient temperature, are ging t be either warm r cl, it wuld be acceptable t specify a particular binder in the plans. If nt, a better apprach wuld be t prvide a chice f binder fr the cntractr, and the selectin f the binder t use can be dne by the cntractr in cnsultatin with the Engineer. Use f warm weather asphalts in cler weather is likely t lead t aggregate lss. Research Prduct P4 Page 11 f 25

14 The cmmnly used warm weather asphalts in surface treatment cnstructin are listed belw. Mdified asphalt binders such as AC-20 5TR are the mst cmmnly used warm weather ST binders. AC-20 5TR AC-15P AC-5 AC-lO+Latex and AC-5+Latex AC-15 XP CRS-2P Figure 3.6 Aggregate Grade Cmbinatins Used in 3-CST (a) Grade 5 n Grade 4 n Grade 3 (b) Grade 4 n Grade 4 n Grade 3 (c) Grade 4 n Grade 3 n Grade 5. The cmmnly used cl weather asphalts used by districts in surface treatment cnstructin are listed belw. The emulsin grade CRS-IP is the mst cmmnly used cl weather ST binder, fllwed by AC-5 and AC-12 5TR. Research Prduct 0-5 I 69-P4 Page 12 f25

15 CRS-lP AC-5 AC-12 5TR AC lo+latex AC 5+Latex MC-2400 MC-3000 Design f Binder Applicatin Rate The design f binder rates fr a multiple-curse surface treatment is dne based n a rate design fr a single curse treatment such as fr a seal cat. Districts d seal cat binder rate design in several ways. The mst cmmnly used methd is based n the aggregate type & grade, aggregate rate, percent embedment desired, and the traffic level (ADT and percent trucks). The prcedure t perfrm the binder rate design is utlined in the TxDOT Seal Cat and Surface Treatment Manual f2004. Binder rate determined using this prcedure is used in the preparatin f plans. Sme districts determine the binder rate based n experience, but this apprach is nly recmmended under the fllwing cnditins. 1. The designer has sufficient experience in seal cat wrk 2. The designs are cnducted by a team frmed with cntinuus prcess imprvement in mind. In either case, the design has t be cnducted by TxDOT prfessinals wh have experience with the fllwing cnditins that are typically applicable t design f material applicatin rates. 1. Binder type and grade 2. Aggregate type, surce and grade 3. Traffic level (ADT and percent trucks) 4. Type base material and base finish specified Once the cntract is awarded, the binder rate specified in the plans has t be adjusted by an experienced surface treatment prfessinal by cnsidering the fllwing factrs in additin t thse indicated abve. 1. Cntractr experience with the material 2. Climatic cnditins during cnstructin 3. Cnditin f the surface n which the surface treatment is applied The binder rate design in seal cats is clsely related t the desired percent embedment f aggregate int the binder. Typically, a 30% embedment is used fr high traffic vlume Page 13 f25

16 rads and a 40% embedment is recmmended fr lw traffic rads. The design percent embedment depends n the fllwing factrs. 1. Binder type and grade 2. Aggregate type and grade 3. Traffic level (bth ADT and percent trucks) Generally, a higher percent embedment is used when a larger rck size is used, because mre binder is needed t retain the aggregate. Since the asphalt binder is the mst expensive item in a surface treatment, the strategy is t use the lwest quantity (i.e. applicatin rate) fbinder withut allwing rck lss. If t much binder is used, the surface treatment may experience flushing and/r bleeding with time, creating maintenance prblems. Even thugh seal cat binder rate design can be used t guide the binder rate design fr multiple-curse surface treatments, the fllwing key adjustments need t be made t accunt fr the design and cnstructin sequence f the surface treatment. 1. If successive curses are applied quickly ne after the ther, allw fr drain-dwn f binder (i.e. use a lighter rate fr lwer curse, and heavier fr the upper curse). This may be achieved by designing a rate similar t that f a seal cat by cnsidering the existing surface t be highly textured (carse). 2. If upper curse is applied mnths later, use a heavier rate at the bttm and lighter at the tp. 3. The adjustment f the rate, either an increase r a decrease frm a single carse rate design, is typically arund 0.05 gallns per square yard. Finish Quality as an Acceptance Criterin One f the mst imprtant tpics in surface treatment specificatins is the use f finish quality as an acceptance criterin in a cntract. A finish quality can be specified n the fllwing surfaces. 1. Finished base layer 2. Primed base 3. Finished surface treatment befre traffic is allwed n it It can be used either as a jb acceptance criterin, r t intrduce bnus/penalty clauses t a cntract. The quality is typically specified using IRI per unit length f pavement (typically a mile r 1110 f a mile) fr each wheel path. Preliminary finish quality specificatins written fr finished base used IRI values in the range 120 t 125 per mile. Recent studies cnducted by several districts n prjects cnstructed by gd experienced cntractrs suggest that IRI values as lw as the fllwing can be achieved. Hwever, the specificatin value fr IRI has t be determined based n the capabilities f the average cntractr pl in the district. Page 14 f25

17 60-90 inches per mile n finished base inches per mile n cvered (r inverted) prime r first curse f surface treatment Page 15 f25

18 Chapter 8 Seal Cat/Surface Treatment Applicatin Prcess Sectin 1-Overview... Sectin 2 - Weather.... Sectin 3 Traffic Cntrl... Sectin 4 (New Sectin Insert)- Surface Treatments n Base Even the mst effective design may nt ensure a satisfactry surface treatment due t the strng influence cnstructin practices have n perfrmance. Similar t the preventive maintenance seal cat peratins, the surface treatment prcess is nt that cmplicated. Hwever, they bth cnsist f systems whse satisfactry functining depends heavily n the cnditins under which they are cnstructed. The designers f bth these systems are cnstrained by nt knwing the field cnditins fr which t design fr. This puts a tremendus burden n the field prject manager r chief inspectr t make critical adjustments and decisins in the field. Mst practitiners call seal cat and surface treatment wrk mre f an "art" than a science fr this reasn. Hwever, there is mre t the science f surface treatments and seal cats than it appears t be. The surface treatment (ST) cnstructin prcess can be divided int the fllwing five parts, and the next sectin will be cvered in that sequence. Cnstructin Management Base Finishing Prime Cat Surface Treatment Applicatin Quality Cntrl Cnstructin Management The cnstructin management prcess plays a very imprtant rle in the perfrmance f surface treatment. Surface treatments are generally a part f a larger cntract that invlves ther "high dllar value" cnstructin elements. This can smetimes lead t a situatin where the surface treatment may nt get the "attentin" it deserves frm the prime cntractr. In sme cntracts, the surface treatment prtin f the wrk in a larger cntract may be subcntracted t a specialist surface treatment cntractr. This is a very desirable arrangement frm the TxDOT standpint because f the value f cntractr experience in cnstructing a quality surface treatment. Hwever, the final quality f the prduct will depend greatly n the subcntractr's expertise and experience with the cnditins. The specificatin writer can intrduce language t ensure that whever des the surface treatment wrk will have persnnel wh can ensure gd decisin making and quality wrkmanship. Research Prduct 0-5l69-P4 Page 16 f25

19 The cntractr-related factrs that strngly influence surface treatment quality, in n particular rder, include the fllwing. 1. Cntractr's expertise and experience under jb cnditins 2. Cntractr unfamiliarity with materials used (the binder, in particular). 3. Cntractr r subcntractr nt mbilizing n time 4. Cntractr's jb site manager's experience 5. Skill fkey equipment peratrs (e.g. Base finishing skills f mtr grader peratr) 6. Cntractr's wrk lad during the cnstructin perid (e.g. multiple cncurrent cntracts handled by the same cntractr will result in use f a sub-standard wrkfrce fr sme prjects) 7. Pr cnditin f cntractr's equipment 8. Cntractrs' inability t adhere t established asphalt rates 9. Wrk-zne prperty access management issues 10. Wrk-zne safety issues 11. Finish quality f the surface treatment Base Finishing The quality f base finish is critical t the bnding f surface treatment t base. In this sectin, several aspects related t base finishing are discussed. The base materials mst cmmnly used with surface treatments by TxDOT districts are listed belw in the rder f decreasing use. Limestne Caliche Irn Ore Gravel Gravel Fly Ash Stabilized Base Cement Treated Base Asphalt Stabilized Base Bth the pneumatic rller and the steel wheel rller are used t finish the base. The pneumatic rller is used first, fllwed by the steel-wheel rller. The kneading actin f the pneumatic rller helps the initial rlling t even-ut the bladed surface. The steelwheel rller helps t get an even and less rcky surface befre the prime cat is applied. There are three cmmnly used base finishing methds. 1. Slush rlling 2. Blade and rll 3. Trimming Page 17 f25

20 Slush Rlling Slush rlling has been cmmnly used in base finishing. Hwever, this base finishing technique varies amng districts depending n the amunt f water used. Sme districts use little water, whereas thers use a lt f water. Slush rlling with excessive water can create an undesirable base in the pavement. Using excessive amunts f water, as sme districts have dne, can weaken the base significantly, first by trapping the water in the base, and then by altering the gradatin f the base due t the pumping f fines t the tp. Furthermre, excessive slush rlling can buildup a layer f fines n the tp f base that will hinder the penetratin f prime cat and the prime can de-bnd frm the base easily. Therefre, it is recmmended that base finishing be dne using a technique that des nt invlve use f excessive water. A light sprinkling f water may be used in case f a dry finished base that has dust n the surface. Blade and Rll The blade-and-rll technique invlves the blading f the base t achieve the desired finish quality and then rlling it. Rlling is typically dne first by a pneumatic rller fllwed by a medium t heavy weight steel wheel rller. Base Lay-dwn Machine The base lay-dwn machine is a relatively new piece f equipment in the pavement cnstructin market. It is smewhat similar t an asphalt cncrete paving machine, and it is capable flaying dwn bth flexible base and emulsified asphalt base (Figure 11). The base material, either flexible r stabilized, is mixed in a pugmill away frm the jbsite and trucked t the prject. Experience with a limited number f prjects have shwn that it can prvide benefits by way fbetter finish fthe base and better cntrl f base misture cntent because it is mixed in a pugmill. The thickness f base that culd be laid-dwn in ne pass wuld depend n the efficiency with which the material culd be cmpacted. Equipment specificatins suggest that thicknesses as high as 7 inches culd be laid dwn in ne pass. The equipment has a hpper with a cnveyr taking the base material t the screed area. Trucks bring the material frm the pugmill and it is laded in t the hpper f the laydwn machine. The equipment can mve at speeds in the range f feet per minute (r 0.2 miles per hur). The material is laid dwn and cmpacted by tw bars that scillate vertically. Page 18 f25

21 Figure 11 Base Lay-dwn Machine (a) Hpper and its Cnveyr (b) Unlading Base Material Transprted frm Pugmill t Hpper (c) Lay-dwn f Emulsified Asphalt Base in Prgress (d) Rlling flaid-dwn Base Applicatin f Prime Cat One f the mst critical elements f surface treatment cnstructin is the priming f base. The prime cat helps the surface treatment binder t adhere t the base curse by penetrating and sticking t the base. Fur types f cmmnly used prime cating techniques were described in the Design sectin f this manual. Several cnditins may be identified as "ptimum" fr a base t have befre a prime cat is applied. Under these cnditins, the base shuld be 1. Reasnably smth, 2. Reasnably prus 3. Nt dusty 4. Structurally strng The base shuld nt have standing dust when the prime is applied, and the finished base may be brmed t remve the dust. Hwever, brming has t be dne carefully nt t disturb the base layer particles. Page 19 f25

22 These "ptimum' cnditins fr the base may nt necessarily be "cmpatible" with each ther. Fr example, a "reasnably smth" finished base is required t achieve a desirable ride quality in the finished surface treatment. Hwever, an verly smth base can prevent the prime cat binder frm penetrating int the base and achieve a gd bnd between the base and the prime. Therefre, sme prsity (fine r small pres) is needed fr this bnd t be develped. The desirable pre size is determined by the prime cat binder and its wettability f the base material. ln many instances, slush rlling is used t btain a smth finished base surface. Hwever, unless care is taken t cntrl the "slushing" water cntent, excess water can weaken the base significantly by making its density lwer. Timing f Prime Cat Applicatin The timing f the prime cat applicatin is f great significance in achieving a gd bnd with the base. The misture cntent in the base has t be "just right" fr the prime t penetrate int the base. The 2004 TxDOT Standard Specificatin Item 247.4E stipulates "Cure the finished sectin until the misture cntent is at least 2 percentage pints belw ptimum r as directed befre applying the next successive curse r prime cat." Therefre, base must be allwed t dry t sme extend after finishing befre the prime cat is applied. Hwever, t dry a base can generate a fine dust cating that inhibits the bnding f the prime cat t the base. This can result in freckling f the binder that leaves uncated pen spts n the base where surface treatment binder may nt bnd well (Figure 12). Therefre, 'skeeting', which invlves light sprinkling f water n an verly dry surface t make it mre suitable t apply the prime cat binder, is dne. Figure 12 Freckling f Prime Cat Binder n a Dry Base It is als undesirable t apply the prime cat binder when the base is t wet. Shaded areas, which are cmmn in east Texas, dry slwer than nn-shaded areas. If the prime cat is applied under these cnditins, it may nt effectively stick t the base and eventually delaminate. Figure 13(a) shws the remval f prime cat frm such a Page 20 f25

23 lcatin due t cnstructin traffic. Figure 13(b) shws the "pick-up" f prime cat and surface treatment by cnstructin traffic when the prime cat is applied t wet. Figure 13 (a) Prime Peeled-ff at Shaded Area Due t Cnstructin Traffic (b) Failed Surface Treatment Due t Prime Applicatin When Base is T Wet The "wrked-in" prime cat layer is strng enugh t run traffic fr several days when applied under the 'right' circumstances which are given belw: Prime applied under apprpriate base misture cnditin D nt allw traffic n the primed surface fr at least ne day Allw reduced-speed traffic fr the first cuple f days, particularly at the intersectin appraches When a "wrked-in" prime is used and if traffic is allwed n the primed base fr several days, it is recmmended that there are drainage paths fr strm water. Applicatin f Surface Treatment Curses The first decisin that a TxDOT prject manager r inspectr has t make with regard t surface treatment applicatin is whether the primed pavement is ready fr surface treatment applicatin. The researchers asked the districts hw lng they wuld wait after the priming befre applying the surface treatment. The respnses received t that questin varied frm same day t 10 days. This time lag between the prime and the surface treatment depends n the fllwing factrs: Type and grade f binder (i.e. prvide time fr cutback vlatiles t evaprate and fr emulsins t cure and penetrate) Type f base (allw prime t penetrate) Cntractr's cnstructin schedule Wrk-zne management Page 21 f25

24 Hwever, the fllwing general practices are recmmended. Wait at least 3 days when the prime cat binder is an emulsin Wait at least 7 days if the prime cat binder is a cutback. One f the mst ften discussed issues in surface treatment and seal cat cnstructin is the timing when aggregate is applied n the binder. Fr bth ht asphalt and emulsin, it is strngly recmmended that aggregate be applied "as-sn-as-pssible" after the binder is sprayed. Anther imprtant issue is the timing between successive curses f a multiple curse surface treatment. Recmmended duratins between first and secnd curses fr different surface treatment binders are listed belw. 1. When ht asphalt is used as the binder in the first curse, the timing f the secnd curse depends entirely n factrs related t cnstructin scheduling. The secnd curse culd ptentially be applied within hurs after the curse. Sme practitiners prefer t allw traffic fr at least a cuple f weeks befre applying the secnd curse. 2. When emulsified asphalt has been used fr the first curse, a minimum curing perid fr the emulsin f at least a few days (2-7 days) is recmmended. Typically, emulsified asphalt is used fr the first curse when cnstructin is dne late in the fall r during winter. In such instances, the secnd curse is nt typically applied until the next asphalt seasn, which culd be several mnths later. 3. In the case f cutback asphalts, the delay between first and secnd curses depends n the type f"first" curse used. When a 'cvered (r 'inverted') prime, which can be cnsidered as a first curse in a ST is used, the RC-250 cutback asphalt used as the binder wuld require a minimum f 7 days fr the vlatiles t escape the rapid curing cutback. Fr cutback asphalts such as MC r MC-3000, a minimum f90 days is recmmended befre the secnd curse is applied. Inspectin f Surface Treatment Cnstructin 1. The use f an effective methd t finish the base is f utmst imprtance. The inspectr has t ensure that the cntractr uses an apprpriate base finishing methd that des nt cmprmising the quality f the base, while ensuring that the finished base surface is cmpatible with the prime cating methd. It is als imprtant t cnsider the cnstructability f the activity, particularly if the cntractr has the expertise t d the base finishing wrk as stipulated in the plans. The finish quality f the base has t be checked by the inspectr, because it dictates the final ride quality f the surface treatment. It wuld be much easier t ask the cntractr t refinish the base rather than having t ask Page 22 f25

25 the whle surface treatment t be re-dne. Traditinally, mst inspectrs cnduct a "seat f the pants" test t ensure the quality f surface finish. Hwever, it is nw pssible t use the Internatinal Rughness Index (IRI) calculated using the prfiler measurements t check the finish quality f the finished (r primed) base. This prvides inspectrs with a mre bjective technique t evaluate the quality f surface finish. 2. The prime cating prcess als invlves several key steps that require inspectr invlvement. It is critically imprtant t check the readiness f the finished base fr prime cat. The attainment f the right misture cntent f the base is f utmst imprtance. Mst experienced inspectrs may be able t check the misture cntent using simple techniques such as by lking the base surface and by digging int the base at several lcatins. Hwever, it is recmmended that a misture cntent measurement be made t verify the state. The new 2004 TxDOT specificatin recmmends a misture cntent f base ptimum misture cntent (OMC) minus 2% t be ptimum befre prime cating. Having t much water in the base may prevent effective bnding f the prime, and can trap t much misture in the base making it weaker (lw mdulus) that it was designed fr. A base that is t dry generally creates a thin cating f dust that will als reduce the effectiveness fbnding. In such a situatin, a light spray f water (skeeting) can help alleviate the situatin. The ther key items in the inspectin f prime cating prcess are the use f an apprpriate prime cat binder rate and the prime cating technique itself. The inspectr must als ensure that the cntractr takes all pssible precautins t prtect the prime cat after it is cmpleted. Wearing fthe prime can prevent the surface treatment binder frm bnding t the base. It is apprpriate t use f cututs in the windrw t prevent water-lgging f the primed base (when base material windrws are used in the priming prcess). It is imprtant t minimize bth cnstructin and regular traffic n the primed base t reduce wear f prime cat. 3. Once the prime cat is applied n base, the next step is t apply the first curse f the surface treatment. The inspectr has t ensure that the cntractr applies the surface treatment curse at the mst ptimum time. If traffic has t be put n the primed base, the surface treatment must be applied as sn as pssible t prevent ptential damage t the primed base. Hwever, the inspectr must ensure that the cntractr allws sufficient Page 23 f25

26 time fr the prime t cure befre the surface treatment is applied. If emulsified asphalt is used fr the prime, it must be cured apprpriately (all the water is evaprated). When cutback asphalt is used, all the vlatiles must be evaprated frm it, and therwise, the surface treatment will be weakened nce applied. The surface treatment binder must be applied at the prper rate and temperature. The inspectr must check the nzzle angles and the spray bar height t ensure prper binder rate applicatin. The shts must be marked, and the distributr must be peridically strapped t ensure that the cmputer readings are accurate. Aggregate (rck) must be applied as sn as pssible fr bth ht asphalt and emulsified asphalt. The inspectr must check the aggregate stckpiles fr dust, and debris, and ensure that aggregate gradatin is apprpriate. In the case f precated aggregate, prper precating cverage must be ensured. The inspectr must mark rck lands, and ensure that the lader peratr lads trucks cnsistently s that prper applicatin rate can be checked using rck lands. Rlling f the aggregate is very imprtant t ensure that aggregate particles are seated n the pavement in their mst stable psitin (lwest center f gravity). This will ensure that aggregates will maintain a satisfactry bnd with the binder. The inspectr must pay particular attentin t the rller cverage f the surface treatment and if the specified number f rller passes is used. When surface treatment is dne in the mrning and later afternn, particular attentin must be made t the binder temperature, and the number f rller passes culd be increased in such situatins t ensure that design embedment is achieved. Lse rck n the pavement can cause numerus prblems t the traveling public as well as cnstructin persnnel. It is a safety hazard, and it may als cntribute t windshield damage. Having lse rck n the primed base can damage the prime (and the base), and timely remval f lse rck is very imprtant. One way t reduce damage t prime due t lse rck is t sht the surface treatment binder a little wide, and then t verlap it with the adjining binder sht. 4. Under current TxDOT specificatins, the acceptance f the jb after the cmpletin f surface treatment cnstructin is a key milestne. Once accepted, the radway becmes the client's (TxDOT) respnsibility, and therefre, the inspectr must ensure that the jb is accepted in gd cnditin. The inspectr must ensure that quality f cmpleted cnstructin prject is checked as per specificatins. An imprtant part f this prcess is checking the cmpleted structure fr Page 24 f25

27 any damage during cnstructin. The inspectr must ensure that such damage is repaired by the cntractr as stipulated in the cntract prir t the acceptance f the jb by TxDOT. Page 25 f25