Application of Equivalent-Layer-Thickness Concept in a Mechanistic Rehabilitation Design Procedure

Transcription

1 TRANSPORTATON RESEARCH RECORD Application of Eqivalent-Layer-Thickness Concept in a Mechanistic Rehabilitation Design Procedre EMLE HORAK The Soth African mechanistic rehabilitation design procedre is ell established and has been verified ith the help of a fleet of heavy vehicle simlators. Srface deflection basins can be measred ith the road srface deflectometer or the National nstitte for Transport and Road Research deflectograph. Typical Soth African pavement strctres ere analyed, and varios deflection basin parameters ere calclated. The eqivalent-layer-thickness (ELT) concept as investigated for its applicability to this analysis procedre and as fond to be sefl in representing the strctral capacity of a pavement. The basis for the calclation of the EL T is the effective elastic modls of the sbgrade. Deflection points measred on the extremes of the deflection basin are sed to calclate the sbgrade effective elastic modls. Varios deflection basin parameters can be sed to calclate pavement EL T, hich can be related to typical distress determinants or directly to remaining life in a design-crve approach. Odemarks (J) eqivalent-layer-thickness (ELT) concept is often sed as a simple method of approximation in pavement strctral analysis, since it permits the conversion of a mltilayered system into a single layer ith eqivalent thickness. t is based on the principle that the eqivalent layer has the same stiffness as the original layer, so as to give the same pressre distribtion beneath the layer. This concept of classifying a pavement by means of one nmber that represents the approximate bearing capacity of that pavement has been clearly illstrated by Molenaar and Van Grp (2) and Molenaar (3). n these cases the investigated pavement strctres ere mainly three-layered strctres ith different loading conditions. Typical Soth African pavement strctres fond in the TRH4 catalog ( 4) ere analyed and converted into the EL T. n that catalog, varios pavement strctres are sggested for varios traffic classes and categories of pavement. Pavement types are generally groped together as bitmen, granlar, cemented, or concrete base (Figre 1). Most of the typical flexible pavement strctres analyed, sch as the granlarand the cemented-base pavements, have thin asphalt srfacings (3 to 5 mm). The calclated ELT vales ere then related to varios distress determinants and fatige life in order to evalate this concept as a possible aid in the mechanistic rehabilitation design procedre (5). This stdy is part of an investigation in hich varios deflec- National nstitte for Transport and Road Research, CSR, P.O. Box 395, Pretoria 1, Repblic of Soth Africa.. PAVEMENT CLASS A, B, c,. 2. PAVEMENT TYPE GRANULAR BASE BTUMNOUS BASE CEMENTED BASE CONCRETE 3. PAVEMENT STATE VERY STFF STFF FLEXBLE VERY FLEXBLE 4. PAVEMENT LAYER STATE WET, DRY, CRACKED + 5. DESGN NPUTS LAYER THCKNESS ELASTC MODUL 6. EVALUATON LNEAR ELASTC MODELS USED N MECHANSTC ANALYSS FGURE 1 Flo diagram of mechanistic pavement rehabilitation design method (5).

2 7 TRANSPORTATON RESEARCH RECORD 127 TABLE 1 SUMMARY OF DEFLECTON BASN PARAMETERS Porometer Formla. Maximm deflection 8 2. Radis of crvotre R. r2 2 8 ( 8 8 r Spreodability s r 127 mm ( ( )/SJ space 8 35mm 4. Are o A. 6[ + 2 ( 1 /8) + 2(82/ / 8] 5 Shope factors F = (8-82)/81 F 2 = ( ) Sr face c rvotre index SC. Bo - Or ; r = 35 or 5 mm 7. Bose crvatre index SC = 8s1- s1s 8 Bose domoge index SD = 835-8s1 9 Deflection rotio Or. 8r/ 8 o i here Sr 8o/2 1. Bending index B = 8 /o i here o =deflection bosin length 11. Slope of deflection SD = ton- 1 (8 -Srl/r ; here r 61mm 12. Tangen slope ST = (8o-8rl/r; here r = distance to inflection point 13 Radis of inflence R = R/8 here R is the distance from 8 to here basin is tangent o horiontal TYPCAL DEFLECTON BASN a or R Deflection basin length Applied load Deflected / shope of road srface Positive crvatre tion basin parameters ere calclated for these pavement strctres (6) and related to typical distress determinants. n Table 1 the most significant of these deflection basin parameters are iisted ith their formias reiaied io the road srface deflectometer (RSD). The RSD is a modernied, atomated Benkelman beam (7) hich is sed ith the accelerated testing facilities, i.e., the heavy-vehicle simlators (HVSs). The National nstitte for Transport and Road Research (NTRR) deflectograph has also been modernied and can measre the deflections on the deflection basin, ths enabling the calclation of the deflection basin parameters as listed in Table 1. An attempt is made in this paper to sho ho the mechanistic design procedre (5, 8) can be enhanced ith the aid of these nondestrctive deflection measring devices. These measrements can then also be sed to calclate the EL T ith a minimm of information. The prpose is to establish typical design crves hereby flexible pavements can be analyed ithot the need for detailed calclations typical of fndamental linear elastic analysis. Sch an approach can greatly enhance the Soth African mechanistic rehabilitation design procedre, hich is ell established and has been verified ith the HVS testing program (8, 9). MECHANSTC DESGN PROCEDURE The mechanistic pavement rehabilitation design procedre (5) is smmaried in the flo diagram of Figre 1. The pavement classes identified as A, B, or C in step 1 can be broadly defined as freeays, major highays, or lightly trafficked roads, respectively ( 4). Pavement types (step 2) in the present stdy are restricted to granlar, bitminos, and cemented bases, since the focs is on flexible pavements. Pavement behavior states (step 3) are defined as very stiff, stiff, flexible, and very flexible (5). From data on deflection basins measred nder accelerated tests ( 6), the ranges of

3 Horak 71 TABLE 2 BEHAVOR STATES DEFNED BY DEFLECTON BASN PARAMETERS 5 FLEXBLE PAVEMENTS Deflection basin parameter ranges Behavior state Max. deft. SD SC BD (mm) ( x 1-6 ) (mm) (mm) Very stiff <,2 < 5 <,1 <,1 Stiff,2 -,4 5-4,1 -,2,1 -,1 Flexible,4 -,6 4-75,2 -,4,1 -,15 Very >,6 flexible >75 >,4 >,15 BC (mm) <,1,1-,5,5 -,8 >,8 oo Ē ::.. 1 f- LU...J... LU D 5 deflection basin parameters in each behavior state, as shon in Table 2, can be sed to define the behavior state accrately. Previosly, this distinction of behavior states as based on maximm deflection only, bt it is no sggested that at least to of the other deflection basin parameters listed in Table 2 be sed for this prpose. Steps 4, 5, and 6 of Figre 1 form part of the normal mechanistic design procedre hereby the design inpts in terms of moistre condition, state of cracking of treated materials, layer thicknesses and elastic modli are needed before the typical linear elastic models can be sed for the analysis. A design-crve approach is sggested, here ith the se of relations established throgh prior analyses of the typical pavement types in the flexible state, steps 4, 5, and 6, can be greatly simplified. This design-crve approach ill also render sperflos detailed knoledge of the design inpts (as shon in step 5) that is reqired for a fndamental linear elastic analysis. nstead, the evalation phase follos from the measrement phase of the deflection basins. The prpose is therefore to make se of the ELT to simplify this analysis procedre by determining EL T directly from measred deflection basin parameters. DETERMNATON OF ELT The ELT, as developed frther by Molenaar and Van Grp (2), is calclated as follos: - 1 [ Ei(l EL T = a L h.1 E, (l here _ vj)] vf) a =.9 for flexible pavements, h; thickness of layer i (m), E; elastic modls of layer i (N/m 2 ), Es = elastic modls of sbgrade (N/m 2 ), v; = Poisson ratio of layer i, vs = Poisson ratio of sbgrade layer ith vale eqal to.35, and L = nmber of layers. The general formla for the EL T is related to the effective elastic modls of the sbgrade. This means that the latter has to be determined before the ELT can be determined. Molenaar and Van Grp (2) established a relationship beteen deflection measred at 2 m and sbgrade elastic modli as measred nder the falling-eight deflectometer (FWD) for 2 SUBGRAOE MO OULU S MPo FGURE 2 Relation of sbgrade elastic modls to deflection in for- and five-layered flexible pavements. bitmen-base pavements. This same procedre can be sed to determine the sbgrade effective elastic modls for the typical Soth African flexible pavements analyed (6). Relationships ere also established beteen deflection measred at offsets of 915, 61, and 5 mm and sbgrade elastic modli. n Figre 2, these relations are shon for for- or fivelayered flexible pavement strctres. By ths measring deflections ith the RSD or even the NTRR deflectograph at those offsets on the deflection basin, it is possible to determine the sbgrade modls. t is recommended that at least three of these relations (as illstrated in Figre 2) be sed to arrive at an average vale of sbgrade effective elastic modls. By measring deflections ith the RSD or even the NTRR deflectograph at those offsets on the deflection basin, it is possible to determine the sbgrade modls. t is recommended that at least three of these relations be sed to arrive at an average vale of sbgrade effective elastic modli. Three sbgrade effective elastic modli (5, 7, and 15 MPa), hich are typical of the sbgrades of Soth African pavements, ere selected as a basis to determine the eqivalent thickness for the typical flexible mltilayered pavements that ere analyed. n Figre 3, hoever, it is shon that the deflection basin parameter F 1 (shape factor, see Table 1), if related to ELT, cannot be sed to discern sch a difference in sbgrade effective elastic modls. n Figre 4, on the other hand, it is shon ho the deflection basin parameter SD (slope of deflection, see Table 1) can be sed to discern the difference in effective elastic modli of the sbgrade for mltilayered flexible pavement strctres. Nevertheless, ith the deflection basin parameter SC (srface crvatre index, see Table 1) it as possible to sbdivide these flexible pavements into to separate sets of relations. Those ith granlar bases had a greater sensitivity to ELT than bitmen-base pavements. This is illstrated in Figre 5,

4 72 TRANSPORTATON RESEARCH RECORD BTUMEN BASE GRANULAR BASES - L;:: - Lt Q.. ct J: (/),5,2 MEASURED Fl ---- E 5 = 5,7,15 MPa,1... _. _ ,1,2, EQUVALENT LAYER THCKNESS ( ELTl (ml FGURE 3 Relation of eqivalent layer thickness to deflection basin shape factor. Ē E " " Ē ::.. tj 1 Es = 5 M Pa \-"\ Es= 7 MP a l=:l=::::::l=11tt-:--- E 5 =!5 MPa E 5 = MPa 5 2 MEASURED SO V ---- i.= 1..J "- a 5 "- Q....J (/) c-----L-----:-----=-----:..,1,2, EQUVALENT LAYER THCKNESS lelt (m FGURE 4 Relation of eqivalent layer thickness to slope of deflection. in hich the three sbgrade effective elastic modli of the to base types in the flexible behavior state are shon. The sbgrade effective elastic 1no<lls l:an therefore be determined from Figre 2. Using that sbgrade effective elastic modls, it is ths possible to se the measred deflection basin parameters in Figres 3, 4, or 5 to calclate the ELT. ANALYTCAL PROCEDURE n the mechanistic design procedre (5), permanent deformation is related to sbgrade vertical strain (e"), and fatige cracking in the bitmen bases is related to the maximm horiontal asphalt strain (eha). As stated earlier, the prpose is to se a design-crve approach in order to simplify the analysis procedre. This is achieved by relating the previosly determined EL T directly to these strains. 1,3 2 EQUVALENT LAYER THCKNESS ELT \ml FGURE 5 Relation of eqivalent layer thickness to srface crvatre index. n Figre 6 it is shon ho it is possible to relate EL T directly to sbgrade vertical strain ( ev,) for the varios sbgrade spport conditions for flexible pavements. When the ELT is related to maximm horiontal asphalt strain (eha), it is applicable only to flexible bitmen-base pavements. The relations for the varios sbgrade spport conditions are illstrated in Figre 7. This means that the granlar-base pavements, hich have relatively thin asphalt srfacings (on average 3-5 mm in the dry regions of Soth Africa), cannot be analyed in this fashion. This emphasies the fact that the ELT concept as developed to calclate compressive strain in the sbgrade and does not necessarily give a good indication of horiontal strain in the asphalt layers (1). The ELT determined earlier sing Figres 3, 4, or 5 is no sed in Figres 6 and 7 to determine the sb grade vertical strain ( evs) or the maximm horiontai asphait strain (eha). Remaining life can be determined for sbgrade vertical strain ( ev,) by sing the typical fatige life crves of the Soth African mechanistic design procedre (5, 9). n Figre 8 these crves are shon for the typical road categories according to the NTRR design traffic classes ( 4). The sbgrade vertical strain (ev,), as determined from Figre 6, can therefore directly be sed as inpt ith Figre 8 to calclate the remaining life. All that is needed is to establish the road category. n the calclation of the remaining life of bitmen bases, Figre 9 can be sed as in the Soth African mechanistic design procedre (5, 9). n this case, hoever, it is necessary to kno the stiffness of the base layer. Stiffness can be determined by sing typical tablated vales (5) or by sing the typical nomograph approach to deter-

5 Horak 73 :: - (/) ::l.. > ".! F LEXBLE PAV EMEN TS 1\ i t----t----t- \l-lot ,.._ " " \. \ \ \ 5 \. \ " soo i-T----Jf l <( er 1- Vl :1_ <( \jj \ E 5 15 MPo E 5 7MPa._...,3 2 EQUVALENT LAYER THCKNESS!EL Tl!ml ,._... FGURE 6 Relation of sbgrade vertical strain to eqivalent layer thickness for flexible pavements. mine stiffness modls vales of bitminos mixes (2, 11). By sing the previosly determined maximm horiontal strain (EHA) as inpt, the eqivalent traffic can be determined for the related bitmen base stiffness. The recommended shift factors shon in Table 3 shold be applied to the calclated remaining fatige life. ELT can, hoever, be related directly to fatige life (3) as calclated above. t as shon previosly ho deflection basin parameters can be sed to identify the pavement behavior state accrately. ELT can be sed to indicate hether a pavement strctre ith cemented sbbases or bases is in the flexible behavior state. According to the definition given by Freeme (5), sch flexible pavements ith the cemented layers are in the cracked phase, exhibiting eqivalent granlar behavior. n Figre 1, ELT for the precracked life of pavements ith cemented sbbases and bases is shon in terms of standard 8-kN axle repetitions. A distinction can be made on the basis of the variance of the elastic modls of the sb grade. Hoever, an EL T vale of at least 1. 1 m is reqired for a sbgrade modls of 7 MPa to have any significant precracked life for the cemented layers. Althogh the mechanistic rehabilitation procedre (5) makes provision for the application of crack groth factors, this does not change the fact that the major portion of the strctral life of typical Soth African pavement strctres ith cemented layers is in the cracked phase. n flexible pavement strctres, typical fatige life crves, sch as those shon in Figre 11, can be sed. n this figre the sbgrade spport condition applies for a sbgrade effective elastic modls(,) of7 MPa. The relation for sbgrade vertical strain (Evs) holds good for all flexible pavements, bt the relation for maximm horiontal asphalt strain ( EHA) holds only for flexible bitmen-base pavements. BTUMEN BASES f ,3 2 EQUVALENT LAYER THCKNESS ELT l lml FGURE 7 Relation of eqivalent layer thickness to maximm horiontal asphalt strain for bitmen-base pavements. CONCLUSONS AND RECOMMENDATONS Deflection basin parameters, as measred ith the RSD or the NTRR deflectograph, can be sed to enhance and simplify the Soth African mechanistic design procedre. The mechanistic design procedre can be simplified by sing a design-crve approach, achieved in this instance by sing the EL T concept. The deflections, as measred on the reverse crvatre of the deflection basin, correlate ell ith sbgrade effective elastic modls. The sbgrade effective elastic modls determined sing sch relations forms the basis for calclation of the EL T. t also has intrinsic significance in terms of identifying statistically niform sections of sbgrade spport conditions in a typical initial assessment of a pavement length. The EL T concept is applicable to the approximation of strctral capacity of flexible pavements. Deflection basin parameters correlate ell ith a vale sch as ELT in general, as calclated for flexible pavements. For some deflection basin parameters it is even possible to discern beteen granlarand bitmen-base pavements. Using sch measred deflection basin parameters, it is possible to determine ELT for the previosly determined sbgrade effective elastic modli. EL T correlates ell ith sb grade vertical strain (Ev,) for flexible pavement strctres and can be sed to discern the effect of variance of sbgrade elastic modli. On the other hand, clear relations beteen maximm horiontal asphalt strain ( EHA) and ELT do not exist for granlar-base pavements as they do for bitmen-base pavements. This is de to the thin asphalt layer typically sed on granlar-base pavements

6 74 TRANSPORTATON RESEARCH RECORD DESGN TRAFFC CLASS EO El E2 E3 E4 _ <1 LGHTLY TRAFFCKED ROADS CJ n 111 " ljj - MAJOR ROADS B> Cl FREEWAYS A> <..? CD ::i 1 2 _J - 11 ljj > EQUVALENT TRAFFC, E8 FGURE 8 Recommended sbgrade vertical strain criteria for different road categories.. : oo ::i " - _J :l..j 1 :>oo DESGN TRAFFC CLASS -Ea El EZ E3 E4-1 W - :: 3: : 5 - (/).J< WO _J 3" : ; 1-11 : EQUVALENT TRAFFC,EBO (Use eqivalency coeff1cienr n41! 18 FGURE 9 Recommended fatige life criteria for thick bitmen bases. in Soth Africa. There are other design crves (6) hereby the maximm horiontal asphalt strain ( EHA) can be calclated by sing deflection basin parameters. These distress determinant strains can be calclated from previosly determined ELT vales. The vale of EL T can be sed in a mechanistic design or an analytical procedre to establish the strctral life of a TABLE 3 SHFT FACTORS FOR BTUMNOUS BASES A (Lightly trafficked roads) Road category B (Major roads) c (Freeays) flexible pavement ith regard to the distress determinants (Evs and eha). Use is made of the typical fatige-life relations of these to distress determinants. A design-crve approach can also be folloed hereby ELT, as determined from deflection basin measrements, is directly related to remaining life. The precrack life of cemented sbbase::s and base::s can ais be:: determined sing sch a design crve. Overlays are important rehabilitation options. Althogh the EL T concept does not allo for the analysis of individal layers, the design-crve approach lends itself ideally to se ith overlay-design crves developed for these types of pavements (6). Apart from that, the concept of the ELT can be sed in the initial assessment of rehabilitation investigations. ACKNOWLEDGMENT The athor thanks the Chief Director of the National nstitte for Transport and Road Research for permission to pblish this paper.

7 Horak 75 f:: >= Cl. Ct: REFERENCES rob.::= Evs FOR - FLEX BL E - PAVEMENTS TRH41N!TRR TR AFFC CL A SSES - E E3...J x < >-E2 < 1 6 < >- -El EO.,,! :!!.. ;. - 1 s 7 MPo ;, EHA F O R_ 1 BTUMEN! BASE : i PAVEMENTS 1 5 P EQUVALENT LAYER THCKNESS ELT lml FGURE 1 nitiation of cracking of cemented bases and sbbases in terms of eqivalent layer thickness. J. N. Odemark. lnvestigatio 11s on the Elastic Propertie. of Soils and tlte Design of Pm1eme11ts A ccording 1 the Theory of Elasticity. Statcns Vaeginslitre, Stockholm, Seden A. A. A. Molenaar and Van Grp. Optimiatio11 of the 711ick11ess Design of Asphalt Co11crete. Proc., 1th A tralian Road Research Board Confe rence. Vol. 1 No. 2, 198, pp A. A. A. Molenaar. Str11c111ral Performance and Design of Flexible Road Constrctioris and Asphalt Concrete O verlays. D. Tech. Sc. thcsi, TechnischcHogeschool. Delft, The Netherlands, Stn1ct11ral Design of.l11terrba11 and R11ml.Road Pavements. TRl-14, National Jnstitte for T {ansport and Road R esearch, CSR, Pr - toria, Soth Africa, C. R. Freeme. Ev<1l11ntio11 of Pavement Belravr for Ma1or Rehabilitation of Roads. NlTRR Technical Report RP/19/83, CSR, Pretoria, Repblic of Soth Africa, E. Horak. The Use of Srface Dcfiec1ion Basin Measrements in tbc Mccba11istic Analy: is of Flexible Pavement. Proc., 6th /memational C.Onfere11ce 11 the Strctral Design of A sphalt Pavements, Ann Arb r, Mich. Vol. l 1987, pp Horak E. Metisrement and Data Processing of Deflection Basins. NrrRR Technical Report RP/23, CSR, Pretoria, Repblic of Soth Africa, <V </) f::: - Q. : _J x < <V Cl : < Cl 1- </ 1 4 " < : Q MPo 7 MPo 15MPo- r--c ,,_.. - Es. 1 3,2 1, 5, 11 : 1 11 l i!! i : j i!!!!! 11! 1.. i EQUVALENT LAYER THCKNESS ELTJ(m) FGURE 11 Pavement life for maximm horiontal asphalt strain and sbgrade vertical strain criteria in terms of eqivalent thickness. 8. C. R. Freeme, M. De Beere, and A. W. Viljoen. The Behavior and Mechanistic Design of Asphalt Pavements. Proc., 6th nternational Conference on the Strctral Desigri of Asphalt Pavements, Ann Arbor, Mich., Vol. 1, 1987, pp C. R. Freeme, J. H. Maree, and A. W. Viljoen. Mechanistic Design of Asphalt Pa ve menr and Verification of Designs Using the Heavy Vehicle Simlator. Proc., 5th Jmem(ltional Conference 11 tlie Strcral Design of Asvhalt Pavem ent., Pretoria, Repblic of Soth Africa, Vol. 1, CSR Reprint RR362, 1982, pp G. J. Jordaan. An Assessment of the Pavement Arialytical and Rehabilitation Desigri Methods: A Method Based on the FWD Measrements arid the Eqivalent Layer Concept (The Netherlands). NTRR Technical Note T45/85, CSR, Pretoria, Repblic of Soth Africa, F. Bonnare, G. Gest, A. Gravios, and. Uge. A Ne Method of Predicting the Stiffness of Asphalt Paving Mixtres. Proc., Association of Asphalt Paving Techriologists, Dallas, Texas, Vol. 46, 1977, pp Pblication of this paper sponsored by Committee on Flexible Pavement Design.