Whole-life Cost Analysis of Concrete Block Paving.pdf

7/30/2019 Whole-life Cost Analysis of Concrete Block Paving.pdf

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byAnthony N.S. Beaty, Royal Mil i ta ry Col lege , Canada.

o f the major advantages claimed fo r a wel l -cons t ruc ted ,concre te block pavement i s low maintenance cos t overlong l i f e . In order to t ake in to account the d i f f e r e n tc os t s assoc ia ted with a l t e rna t ive pavement sur fac ings ,i s necessary to c a l c u l a t e discounted , whole - l i fe c o s t s in whichcos t s throughout the ana lys i s per iod a re accounted fo r .cos t s fo r concre te block pavements a re no t read i lyand a plea i s made fo r the co l l ec t ion o f the requiredThe elements o f c o s t to be taken in to account a re di scussedequa t ions developed to compare block paving with a bi tuminousover a fo r ty year per iod . Values of discounted , wholee cos t s a re ca l cu la t ed fo r t h ree discount r a t e s , us ing t yp i ca l

t d a t a . The advantages o f a low maintenance pavement a re shownbe grea te s t a t low va lues o f the discount r a t e . Comparison ofcos t s o f a l t e rna t ive sur fac ings , on the bas i s o f i n i t i a lcos t s a lone , does not r e f l e c t the economic bene f i t s ofmaintenance cos t s and re -usab i l i t y of paver s .

ng th e advantages a t t r i b u t e d to in te r lock ing concre te blockwhen compared to othe r forms of sur fac ing , a re lowcos t s over a long l i f e and the a b i l i t y to re -use paverspavement re ins ta tement fol lowing access to bur ied s e r v i c e s . I fsu r f ac ings a re considered only on th e b a s i s o f i n i t i a li on c os t s , the longterm economic b e n e f i t s , accruing fromadvantages noted , w i l l not be taken in to account . It i s arguedt a l t e rna t ive sur fac ings should , more proper ly , be compared onbas i s of discounted whole - l i f e cos t s , in which th e e f f e c t s ofmaintenance and r ehab i l i t a t ion cos t s a re inc luded . Thism of economic ana lys i s i s only poss ib le if r e a l c o s t da ta a refo r a l l elements o f c os t including annual maintenance.the presen t t ime, the cos t s of annual maintenance o f d i f f e r e n tnot we l l documented and th i s paper i s , in p a r t , aa fo r the co l l ec t ion and disseminat ion of the necessary c os t

PAVING APPLICATIONSo f concre te block paving may be grouped according toof e i t h e r engineer ing or a rc h i t e c t u r a l aspec t s .a re apprec ia ted by a r c h i t e c t s and landscape a rc h i t e c t s fo rhuman sca le and fo r th e dive r s i ty of shape and co lour whichy o f f e r . In t yp i ca l a r c h i t e c tu r a l app l i ca t ions , it i s aes the t i cra the r than economics which govern the choice o f mater ia l s .app l i ca t ions may inc lude road pavements, p a r t i c u l a r l y ino f urban renewal , however, in most engineer ing app l i ca t ions ,

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any aes the t i c appeal i s usual ly of secondary importance compared tocos t . Concrete pavers are f requent ly used fo r pavements support ingt r a f f i c and heavy, i n d u s t r i a l loads on roads and in por t s ,con ta iner t e rmina ls , heavy equipment maintenance yards anda i r p o r t s . In these engineer ing appl ica t ions , cos t comparison oft echn ica l a l te rna t ives i s usual ly the deciding fac tor . Thus, inthese app l ica t ions it i s important t ha t the method of ecomomicana lys i s used, r e f l e c t the fu l l spectrum of cos t s and benef i t s fo reach a l t e rna t ive des ign .COST EVALUATIONWhere concre te pavers a re se lec ted pr imar i ly for t h e i ra rc h i t e c t u r a l o r aes the t i c fea tu res , economic c r i t e r i a are ofl e s s e r importance and comparat ive economic ana lys i s o f a l te rna t ivesmay be considered unimportant . However, where mater ia l se lec t iondepends on the ab i l i t y of the pavement to meet func t iona l ,engineer ing performance c r i t e r i a , the comparison of the cos t s o fa l t e r n a t i v e sur fac ings i s l i k e l y to be dec is ive . In such cases ,economic ana lys is of t echn ica l ly acceptable pavement a l t e rna t ivesshould be an essen t ia l s tep in the se lec t ion process . This hasbeen recognized, fo r example, in South Africa[ll where cos tana lys i s i s an i n t eg ra l p a r t of the des ign o f s t ruc tu ra l , concreteb lock pavements. In o rde r to compare the cos t s of pavement designa l t e r n a t i v e s , a common ana lys i s per iod must be se lec ted and a l lcos t s occurr ing dur ing t ha t per iod should be taken in to account .In prac t i ce , t h i s i s seldom done, due in p a r t to the d i f f i c u l t i e sin pred ic t ing fu tu re cos t s and in pa r t to p o l i t i c a l aspec ts of thef inancing of publ ic sec to r road pro jec t s , in which there i sconsiderable pressure t o se l ec t the pavement having the lowestin i t i a l cons t ruc t ion c o s t . In the case o f block paving the absenceo f r e l i ab l e , long-term maintenance cos t da ta i s a fu r ther obs tac le .As d i f f e r en t pavement surfac ings have d i f f e r en t lengths o f servicel i f e and requ i re d i f f e r en t l eve l s of maintenance expenditure , anaccura te comparison o f the t o t a l cos t s o f a l t e rna t ive s can only bemade through the use o f discounted , whole- l i fe (or l i fe -cyc le ) cos tana lys i s .WHOLE-LIFE COSTSUnder the combined e f f ec t s of c l imate and t r a f f i c , every pavementbegins to d e t e r i o r a t e from the moment it i s completed. This maybe expressed as a progress ive dec l ine in the r id ing qua l i ty tosome te rminal value a t the end of the pavement 's service l i f e .Moreover, in ca lcula t ing the design l i f e of a pavement in terms ofthe t r a f f i c it wi l l car ry , it i s assumed t h a t regular rout inemaintenance, and in the case of aspha l t pavements, occasiona lsur face d ress ing to mainta in skid r e s i s t ance , wi l l be requi red .Thus, in add i t ion to the i n i t i a l construct ion cos t , there wil l bea se r i e s of maintenance cos t s incurred which must be taken in toaccount . In whole- l i fe cos t ana lys i s , a l l cos ts are consideredover the ana lys is period . I t i s assumed t ha t the a l t e rna t ivepavement sur fac ings being compared wi l l each procure the samebene f i t s and t ha t the d i f fe ren t sur faces w i l l not r esu l t ins ig n i f i c an t dif fe rences in vehic le opera t ing cos t s . The purposeo f whole- l i fe cos t ana lys is i s to show which form of const ruc t ion

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produce the lowest discounted t o t a l c os t over the l i f e o f theThis wil l not neces sa r i l y be the pavement s t r u c tu r ethe lowest i n i t i a l cons t ruc t ion cos t . The main economica f f ec t ing the c os t a re : - ana lys i s per iod , pavement se rv ice, discount ra t e , cons t ruc t ion cos t s , maintenance cos t s andva lue .AND SERVICE LIFEwhole- l i fe cos t ana lys i s it i s necessary to e s t a b l i s h anper iod , with in which a l l cos t s w i l l be taken in to

It i s convenient to t ake the ana lys i s per iod equa l to thel i f e of the pavement. However, pavements const ruc ted withsur fac ing ma te r i a l s a re designed fo r d i f f e r e n t serv iceFor example, in the U.K., design l i ve s o f twenty years fo rsu r fac ings and for ty years fo r concre te roads a reused. [2 ) Twenty years i s a lso recommended fo r roadswith concre te pavers . [3J In o rde r to compare the cos t s ofpavement des igns , a common ana lys i s per iod must beThere are two main approaches: -( i) to se l ec t an ana lys i s per iod equal to the l onges t designl i f e . For example, taking for ty years as the des ign l i f e ofa concre te road and twenty years fo r a bituminous pavement, anana ly s i s period of for ty years would convenient ly encompasstwo, twenty year , bituminous des ign l i v e s .( i i ) to se lec t an ana lys i s per iod equal to the s h o r t e s t l i f eand to take in to account the res idua l value , a t the end of theana lys i s per iod, fo r pavements having longer l i v e s .f i r s t approach has been app l i ed to footways in the U.K.[4) usingl i v e s of f i f t y years fo r concre te f lags and twenty-f ivefo r bituminous cons t ruc t ion . This approach i s usefu l fo r lowof the discount r a t e .

orde r to compare cos t s occurr ing a t d i f f e r e n t t imes , it i sto se l ec t a discount ra t e to express fu tu re cos t s ino f t h e i r presen t worth. The discount ra t e i s the minimumof re tu rn on an investment . The choice of andiscount ra te i s important and the s en s i t i v i t y of theto changes in the d iscount r a t e , should be checked.ra t e s , the e f f e c t of longterm cos t savings i sand the con t r ibu t ions beyond approximately t h i r t y yearsneg l ig ib le .

wi l l normally be poss ib le to determine i n i t i a l construct ionaccura te ly , on the bas i s o f e i the r pr ices b id fo r the job orpublished un i t cos t s . As d i f fe ren t sur fac ing mater ia l srequ i re di f f e ren t th icknesses of roadbase and sub-baseeach des ign should be costed per u n i t of su r face a rea fo rl aye r . It i s not necessary to include i tems which may bewhichever surfac ing i s se l ec t ed , fo r example, kerbs anddra inage , e t c .

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COSTSi f fe ren t pavement surfacings requ i re d i f f e r en t rout ineI t should be assumed tha t preven ta t ive maintenancebe carr ied out when required on the bas i s of engineeringjudgement. In the case o f bituminous sur fac ings , allowance shoulde made fo r f i l l i ng cracks as soon as the occur , so as to avoidi n f i l t r a t i on and fur ther de te r iora t ion . Every f ive to sevendress ing wi l l be required t o re s to re skiddinges i s t ance . Strengthening over lays should be budgeted fo r whenu t t ing in the wheel pa ths reaches 10mm so as to preserve thet r u c tu r a l i n t e g r i t y o f the exis t ing pavement. The t ime taken toeach th i s s t a t e o f ru t t i n g wil l depend on the t r a f f i c using theroad. Maintenance cos ts fo r well constructed concrete blockwil l genera l ly be low and wil l of ten .be associa ted withfea tu re s , fo r example, it may be necessary to replace.mal l numbers o f blocks physical ly damaged by con ta iner handlingopera t ions o r by s lewing of t racked veh ic le s . Concrete blockaving i s of ten se l ec t ed where severe d i f f e r en t i a l se t t lement i sexpected. This w i l l requi re l i f t i ng the pavers , res tor ing theroadbase and r e lay ing the pavers . It has been suggested lS ) tha t at yp ica l allowance fo r t h i s a t U.K. por ts would be o f the order of0.04 per square metre , per annum (1988 p r i ce s ) .

SALVAGE VALUEThe sa lvage value o f a pavement a t the end o f the ana lys is periodi s the value ass igned to the pavement mater ia ls fo r re -use , mostprobably in ano the r pavement s t ruc tu re . In the case of concreteblock paving, undamaged pavers which can be l i f t e d , cleaned andr e l a i d wi l l have a sa lvage value. The salvage value i s consideredto be a negat ive co s t and w i l l be discounted to year zero .OTHER COST FACTORSIn some cases d i f fe rences in the t ime taken to cons t ruc t thepavement and open it to t r a f f i c w i l l give r i s e to costd i f f e r en t i a l s . An example o f th i s occurred in the const ruc t ion ofth e concrete block taxiways a t the Dal las /For t Worth a i rpor t wherethe use o f pavers reduced, by two hours per night , the period ofad jacen t runway c losure . This was est imated l61 to have savedUS$37,130 per day in reduced a i r l i n e de lays , or a t o t a l of morethan four mil l ion US do l la r s over the f u l l cons t ruc t ion period. Asecond example o f t h i s nature occurred during the construct ion ofan ore process ing p l a n t in South Afr ica 171 where concrete blockswere used to pave roads , s torage area s , and genera l open spaces.The f as t - t r ack cons t ruc t ion programme required use o f the pavementby s t e e l fabr ica tors as soon as poss ib le a f t e r construct ion wascomplete. I f in s i t u concrete s labs had been spec i f ied thecons t ruc t ion programme would have been delayed due to curingper iods and aspha l t would have suf fe red considerable damage byheavy cons t ruc t ion equipment. This represen ts a c o s t advantage toconcre te block paving which should be included in the whole- l i fecos t comparison.

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T DATA cos t comparison fo r a p a r t i c u l a r pro jec t i s seldom madeHowever, un i t cos t s fo r concre te s l abs , a spha l t andblock paving a re usua l ly ob ta inab le to permi t a comparisoni n i t i a l cons t ruc t ion cos t s . The b ig var iab le between count r i e sto be the cos t of b i t uminous paving mate r i a l s . Forin the U.K. , concre te block paving and hot ro l l ed aspha l tappear to be comparable in i n i t i a l cons t ruc t ion cos t s ,in Canada, th e i n i t i a l cons t ruc t ion cos t o f concre te blocktends to be approximately twice t h a t of comparable aspha l t .c o s t da t a l8 ] have shown t h a t i n s t a l l ed concre te blockin th e United Sta t e s , has h i s t o r i c a l l y c os t almost threethe comparable c os t in Europe, Aus t ra l i a and South Afr ica .o f approximately US$14 per square metre fo r the l a t t e r andp e r square metre fo r the United Sta tes a re i nd i ca t ive , bu tt be t r ea t ed with caut ion as they a re unadjusted fo r t ime.accura te l o c a l pro jec t cos t s a re an essen t i a l pre requ i s i t emeaningful c o s t comparisons. Very few da ta on maintenance c os t sconcre te block paving appear to be ava i l ab le , but t he genera lo f publ i shed s ta tements i nd ica te s t h a t fo r a we l l pavement these a re considered to be neg l ig ib le . In theit has been found l91 t h a t a block pavementa l i f e o f fo r ty years and t h a t one f u l l - s c a l e repa i r , requi r ingof f ive to ten per cen t o f the blocks , w i l l be requi redt h a t l i f e - t i m e . Using these da ta , Rolling(:> 181 ca lcu la tedfo r an e i g h t per cent d i scount r a te and a fo r ty year ana lys i sa concre te block pavement would have a c os t advantage overomparable bi tuminous pavement, if the t r a f f i c exceeded onee qu i va l e n t s tandard ax le s . Typica l maintenance c os t s fo rconcre te s lab su r f ac ings a re b e t t e r known as theset y p i c a l l y been s tud ied by bitumen and cement t rade

It may be observed t ha t the rou t ine p reven ta t iverequ i red by bi tuminous and concre te s lab pavements i sc a r r i e d out when required but i s e i the r omit ted ora s a c o s t saving measure. This can be taken in to accountthe whole - l i f e c os t ca lcu la t ions as it l eads to reducedl i f e and higher r e h a b i l i t a t i o n cos t s . Thus the c os to f a pavement surfac ing which su f fe r s l e a s t from neglec tmaintenance can be quan t i f i ed .ANALYSISt he re w i l l be a degree of uncer t a in ty assoc ia ted with many ofva lues used in the cos t comparison, it i s use fu l to determines e n s i t i v e th e r e s u l t i s to var i a t ions in the i npu t va lues . For

the d i s c oun t r a te used w i l l be a be s t es t ima te , bu t thew i l l be a f fec t ed by the va lue chosen. The ca lcu la t ionsbe repea ted fo r higher and lower values in order toth e s en s i t i v i t y of the r e s u l t obta ined to var i a t ion indi scount r a t e . A ll othe r var i ab le s can be t r ea t ed s imi l a r ly .T COMPARISONorde r to demons t ra te the ana lys i s o f whole - l i fe cos t s , cons iderpavement des ign to suppor t f ive mil l ion s tandard ax les over

with a Cal i fo rn ia Bearing Rat io o f f ive . This would

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in the t yp ica l designs shown in Figure 1. The anali s taken to be for ty years .

sur fac inghe cos t s cons idered a re the i n i t i a l construct ion cos t , IAt an",>maintenance co s t , MA and a per iodic surface dress ing costI t i s assumed t h a t maintenance wil l be requi red each year ana..!$the funds wi l l be ava i lab le and cons idered spent a t therl!oof each yea r . Surface dress ing wi l l be car r ied outthe seventh and four teenth years . The cos t o f surfacewhen ca r r i ed out , i s considered to inc lude t ha t year ' scos t s . Construc t ion funds a re cons idered spent onep r i o r to the en t ry in to service of the road. At the end ofyears se rv ice the bituminous pavement wi l l be replaced by aew one and the cyc le o f cos t s from year zero to year twenty wi l le repea ted from year twenty-one to year fo r ty . These cash flowsre i l l u s t r a t ed in Figure 2. The discounted cos t s wi l l be asfo r a di scount ra t e o f 'a' per annum:-

(1)

WLC,is the discounted whole l i f e cos t of the f i r s t hot ro l ledsur fac ing from construct ion to the end o f i t s twenty yearThe t o t a l whole- l i fe cos t s fo r the two aspha l t pavementstherefo re be : -(2)

discount r a t e s o f f ive , ten and f i f t een per cen t , theol lowing express ions can be der ived: -

a0.050.100.15WLCASWLCA10WLCA15

= 1.359IA += 1 . 135IA += 1 . 053IA +15.283MA + 1.653D8.783MA + 0.776D6.046MA + 0.51 7D

3(a)3 (b)3(c)

ubs t i tu t ing the fo l lowing va lues in u.s do l l a r s per square metre,IA = 18.4 , MA = 0.45, and D = 2.05, one obta ins the whole- l i fe cos t sshown in Table 1 .Concrete block pavingThe cos t s cons idered a re the i n i t i a l construct ion cos t I B, and anannual maintenance c o s t MB The pavement, with rout inemaintenance i s considered to l a s t fo r the f u l l for ty years of theana lys i s per iod and to have a sa lvage value , S, a t the end of tha tper iod. This i s t r ea ted as a cos t reduct ion a t the end of thefo r ty years . The cash f lows are a l so shown in Figure 2. In t h i scase the discounted , whole- l i fe cos t s w i l l be given by thefo l lowing express ion : -

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(4)

g di scount r a te s of f ive , te n and f i f t een per cen t , asfo l lowing express ions can be der ived : -

a0.050.100.15===

IBIBIB

+++

17 159MB - O. 135S9.779MB - 0.020S6.642MB - 0.003S5(a)5 (b)5(c)

the fol lowing values in U.S. do l l a r s per square m et re ,23.60, MB = 0.131 and S = 5.90 , one obta ins the whole - l i f ets shown in Table 1 .

Discount Rate Asphal t Pavers0.05 35.27 25.050.10 26.42 . 24.760.15 23.16 24.45

I n i t i a l Costs 18.40 23.60Table 1 Whole- l i fe cos t s in U.S. do l l a r s per square metre

can be seen t h a t the d i scounted c os t approach re su l t s i n ad i f fe rence between i n i t i a l cons t ruc t ion cos t and wholec os t f o r pavements which requ i re more maintenance over t h e i rl i v e s . For the low maintenance c os t s assumed fo r theblock pavement, whole - l i fe cos t s were no t apprec iab lythan i n i t i a l cons t ruc t ion cos t s and were i n sens i t i ve to ther a t e in the range f ive to f i f t e en per c e n t . At a di scountf ive percen t discounted maintenance and rehab i l i t a t ionfo r the a spha l t surfac ing were o f the same order of magnitudei n i t i a l cons t ruc t ion cos t s , bu t were s ig n i f i c a n t l y reduced asdiscount ra t e increased to f i f t e en per cen t . Clear ly , wholee cos t ana lys i s can only be use fu l if c os t s can be accura te ly

Whole- l i fe c os t ana lys i s shows t ha t pavements havingmaintenance over a long l i f e become increasingly a t t ra c t i v e a tdiscount r a t e s .

Comparison of a l t e rna t ive pavement surfac ings on the bas i sof i n i t i a l cons t ruc t ion cos t s does no t t ake in to account longterm economic benef i t s of low-maintenance pavements such asthose surfaced with i n t e r lock ing concre te pavers . Whole- l i fe c os t ana lys i s t akes in to account a l l cos t s over

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a given per iod o f t ime and a l lows comparison o f a l t e rna t ivepavement s t ruc tu re s on the bas i s of discounted t o t a l cos t s . P a r t i c u l a r l y fo r low values o f the discount r a te , theranking of pavement s t ruc tu res on the bas i s o f whole - l i fecos t s can d i f f e r from t h a t based on i n i t i a l cons t ruc t ioncos t s . Meaningful whole - l i f e cos t analyses requ i re accura tees t ima tes o f a l l cos t s over the f u l l ana lys i s per iod . The bene f i t s accruing from the low maintenance required bya wel l -cons t ruc ted i n t e r lock ing concre te block pavement canbe taken in to account through the use o f whole - l i f e cos tcomparisons with a l t e r n a t i v e sur fac ings . There i s a need fo r da ta concerning the ac tua l maintenancerequirements and assoc ia ted cos t s of i n - se rv i ce concre te blockpavements

REFERENCES1 . Committee o f Urban Transpor ta t ion Aut ho r i t i e s , St ruc tu ra l des ignof segmental block pavements fo r South Afr ica , Urban Transpor t a t ionGuide l ines , UTG2, 1987, pp.113-1252. Powell W.D. e t al., The s t ru c t u r a l des ign o f bi tuminous roads ,Transpor t and Road Research Laboratory, LR 1132, 19843. B r i t i sh Standards In s t i t u t i o n , Guide fo r s t r u c t u r a l design o fpavements cons t ruc ted with c lay or concre te block pavers , BS7533,19924 . Pearson B ., Flags bea t blacktop in whole - l i f e cos t ing o f footwayc ons t ruc t i on , Highways, May 1990, pp.25-275. Knapton J., A review o f concre te block pavements a t por t s in theUK,Proceedings ,Paving, Rome,6. King P.M.pavements ,3rd .1988,and

In t e rna t iona lpp.380-392Smi th D.R. ,Conference

In te r lock ingon Concrete Blockconcre te a i r f i e ld

Concre te In te rna t iona l , December 1991, pp.24-277. Michau P.S t .C . and Wilson D.E. , The choice o f pre c a s t concre teblock paving fo r l a rge i n d u s t r i a l pro jec t s in remote reg ions ofSouth Afr ica , Proceedings , 3rd . In t e rna t iona l Conference onConcrete Block Paving, Rome, 1988, pp.405-4128 . Rol l ings R .S . , Concre te block pavements, Technica l repor tGL-83-3, U.S. Army Waterways Experiment Sta t ion , 19839 . Kellersman G.H., Urban block paving in the Nether lands ,Proceedings , 1 s t . In t e rna t iona l Conference on Concrete BlockPaving, Newcast le-upon-Tyne, 1980, pp.93-101Author ' s a dd re s s : - Department o f Ci v i l Engineer ing, Royal Mil i ta ryCol lege , Kingston, Onta r io , K7K 5LO, Canada Fax 1 613 545 3481

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mm I aversI mmHot rolled asphalt 80110 bedding sand 30lean concrete cement-boundroadbase 180 roadbase 180sub-base 225 sub-base 225

capping layer 350 capping layer 350subgrade subgrade

(a) Bituminous Surfacing (b) Concrete pavers

Figure 1 - Pavement designs for cost comparison

Year(a) Bituminous surfacing

Year

(b) Concrete block PavementFigure 2 - Cash flows for cost analysis

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Whole-life Cost Analysis of Concrete Block Paving.pdf