2.1 The Mechanical T esting of Bituminous Paving M ixtures
2 .1 .1 In tr o d u c tio n :
The follow ing s e c tio n s review in g en eral terms the a p p lic a tio n of Mechanical te s t in g procedures to Bituminous Paving m ixtures w ith an emphasis on those used to asse ss r e s is ta n c e to deform ation.
Since the e a r ly p a rt of t h i s cen tu ry many such procedures have been developed in many c o u n trie s , n o tab ly the U.S.A. When con s id e rin g the wide d iv e r s ity of the a v a ila b le methods, i t i s con v e n ien t to make a d i s t i n c t i o n , based upon the n atu re of the t e s t i t s e l f , and regard these methods as being e i th e r
Em pirical methods, Fundamental methods or Sim ulative methods.
2 .1 .2 Em pirical Test Methods: 2 .1 .2 .1 General
An Em pirical t e s t i s one in which some a r b i t r a r y p ro p e rty of a compacted paving mixture is determ ined, ty p ic a lly the maximum
load su sta in e d under given loading c o n d itio n s . Such t e s t methods now form the b a s is of many w idely used Mix Design procedures in which t e s t r e s u l t s are considered along w ith the d e n sity and void co n ten t of compacted specimens, in order to s e le c t a m ixture com p o s itio n which w il l perform adequately from the p o in t of view of r e s is ta n c e to deform ation and d u r a b i l it y , y ie ld in g a m ixture w ith :
( i i ) S u f fic ie n t s t a b i l i t y to w ithstand t r a f f i c loading w ith out undue d i s t o r t i o n .
( i i i ) S u ffic ie n t a i r voids to allow fo r s l i g h t a d d itio n a l
compaction under t r a f f i c w ithout bleeding or lo ss of s t a b i l i t y , y et low enough to exclude a i r and m o istu re.
(iv ) S u f fic ie n t w o rk a b ility to perm it easy placement and compaction to the re q u ire d d e n sity .
In most cases an "optimum binder co n ten t" fo r a given aggregate g ra d a tio n i s s e le c te d and i s used d i r e c t l y or a c ts as a guide to the binder co n ten t to be used in p r a c t i s e .
In such t e s t s , i t i s the a r b i t r a r y p ro p erty measured which i s
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used to a sse ss a m ix tu re 's re s is ta n c e to deform ation under t r a f f i c , o fte n termed " s t a b i l i t y " . This p ro p e rty i s not a measure of any i n t r i n s i c m ixture p ro p e rty , i t i s , th e r e fo re , e s s e n t i a l th a t a c o r r e la tio n between la b o ra to ry and road performance be e s ta b lis h e d . This done, i t i s then p o ssib le to define design c r i t e r i a in terms of la b o ra to ry param eters, to be applied depending upon t r a f f i c and c lim a tic c o n d itio n s .
2 .1 .2 .2 Common Em pirical Test Methods M arshall t e , t < l ><7><17>(23>
Hveem S tabilom eter and Cohesiometer t e s t ^ ^ ^ 7^ 24^ Hubbard - F ield t e s t ^ 1™ 25*'^26>
Duriez t e s t . <14><27>
2 .1 .2 .3 Advantages of Em pirical Test Methods
( i ) Equipment and procedures are u s u a lly sim ple.
com positions to be assessed quick ly .
( i i i ) Provided they are used w ith in the c o n s tr a in ts of proven la b o ra to ry - road c o r r e la tio n s , adequate, economic m ixtures can be produced.
( iv ) T heir s im p lic ity and speed of o p e ra tio n means they can e a s i l y be adopted f o r o n -s ite q u a lity c o n tro l.
2 .1 .2 .4 Disadvantages of Em pirical Test Methods
( i ) I t is n ecessary to e s ta b l i s h a c o r r e la tio n between la b o ra to ry and road performance before r e s u l t s can be of any p r a c t i c a l u s e .
( i i ) Many t e s t s are only ap p lic ab le to a lim ited range of mix tu re types and w e ll defined c o n d itio n s of t r a f f i c and c lim a te . ( i i i ) R esu lts obtained w ith d i f f e r e n t Em pirical methods do not agree w ith each o th e r to any g re a t e x te n t.
( iv ) Loading c o n d itio n s o fte n bear no resemblance to those found in p r a c t i s e .
( v) In most cases i t i s not p o ssib le to analyse the s tr e s s e s a c tin g during t e s t i n g .
2 .1 .3 Fundamental Test Methods: 2 .1 .3 .1 General
A Fundamental t e s t s e ts out to measure some i n t r i n s i c (fundam ental) p ro p erty of the m ixture under t e s t . Unlike Em pirical methods, they a re , w ith c e r t a i n ex ce p tio n s, u n su ita b le fo r Mix Design a p p li c a tio n s and tend, th e re fo re , to be considered as re se a rc h t o o ls .
C ertain t e s t s of t h i s type were designed s p e c if i c a lly fo r the d eterm in atio n of those p ro p e rtie s which would allow the problem of re s is ta n c e to deform ation to be ta ck led from a more t h e o r e t i c a l
( r a t io n a l ) approach. Others developed more re c e n tly allow the d eterm in atio n of those p r o p e rtie s re q u ired fo r computer programs used in m u lti- la y e r e l a s t i c ( v is c o e la s tic ) methods of pavement design.
2 .1 .3 .2 Common Fundamental Test Methods: T r ia x ia l t e s t < D (2 8 )(2 9 )(3 0 )(3 1 )
(1 )(3 2 ) Unconfined Compression t e s t
In d ire c t-T e n s ile t e s t ^ 33^ 3^ U niaxial Creep t e s t ^ 35^ 36^ 37^ Various rep eated loading t e s t s :
Dynamic Modulus t e s t ^ 35^ 39^ 40^ Fatigue t e s t (38)(40)
/ 30 \
Dynamic Creep t e s t .
2 .1 .3 .3 Advantages of Fundamental Test Methods:
( i ) Loading systems employed perm it the c a lc u la tio n of s tr e s s e s a c tin g during te s t in g .
( i i ) S tre sse s more c lo s e ly resem bling those found in p r a c tis e can be reproduced.
( i i i ) R esu lts obtained perm it a more th e o r e t ic a l approach to the problem of re s is ta n c e to deform ation, and a com puter-aided approach to Pavement Design.
2 .1 .3 .4 Disadvantages of Fundamental Test Methods:
( i ) T esting equipment i s u s u a lly complex, expensive and re q u ire s tra in e d o p e ra tiv e s .
( i i ) Time req u ired fo r t e s t i n g i s o fte n c o n sid e ra b le .
( i i i ) Most procedures are not s u ite d to Mix Design in the sense described p rev io u sly and, th e r e fo re , usage i s mainly confined
to re se a rc h r a th e r than p r a c t ic a l a p p lic a tio n s .
2 .1 .4 Sim ulative Test Methods: 2 .1 .4 .1 General
Test methods re f e r r e d to p re v io u sly are considered to have 2 major drawbacks. F i r s t l y , in many cases the loading c o n d itio n s and the s tr e s s e s produced w ith in the specimen f a i l to sim ulate the p r a c t i c a l s it u a ti o n . Secondly, even i f i t i s p o ssib le to re p ro duce such c o n d itio n s , d i f f i c u l t i e s a r is e when try in g to apply r e s u l t s obtained fo r t e s t specimens to the behaviour of the same m ixtures in a layered pavement system.
As the name su g g ests, sim u lativ e t e s t s s e t out to overcome such shortcomings by attem pting to sim ulate the c o n d itio n s experienced in the p r a c t i c a l road s it u a ti o n .
2 .1 .4 .2 Common Sim ulative Test Methods: Laboratory Wheel-Tracking t e s t ^ ^ ^ ^ ^ ^ M iniature Test T racks(****)
Large-Scale Test Tracks
F u ll Scale Road T r i a l s / 46X 48X 49)
2 .1 .4 .3 Advantages of Sim ulative Test Methods:
( i ) Simulate to varying degrees, the loading c o n d itio n s , s t r e s s e s , compaction, environment and layered s tru c tu re found in p r a c t i s e .
( i i ) Allow assessm ent of re s is ta n c e to deform ation under r e a l i s t i c c o n d itio n s, thereby allow ing m ixtures to be "ranked" more r e a l i s t i c a l l y than by o th e r methods.
( i i i ) L arge-scale t e s t tra c k s perm it the behaviour of m ixtures under c o n tro lle d t r a f f i c co n d itio n s to be in v e s tig a te d .
under r e a l environm ental and t r a f f i c c o n d itio n s to be observed.
2 .1 .4 .4 Disadvantages of Sim ulative Test Methods:
( i ) They are in g en eral more expensive to c o n stru c t and operate than "conventional" t e s t methods.
( i i ) Require more time to c a rry out and in the case of Large- Scale t e s t tra c k s and F u ll-S c a le Road T r ia ls t h i s time can be p r o h i b i t i v e .
( i i i ) In F u ll-S c a le Road T r ia ls and Large-Scale t e s t tr a c k s , environm ental f a c to rs are v a r i a b l e .
( iv ) R esults from Laboratory Wheel-Tracking t e s t s and M iniature t e s t tra c k s need to be c o rr e la te d w ith a c tu a l road performance before the r e s u l t s are of p r a c ti c a l use.
( v) Such methods cannot reasonably be ap p lied to Mix Design in the conv en tio n al s e n s e .
2.2 Test Methods Used in the Current I n v e s tig a tio n ; 2 .2 .1 In tr o d u c tio n :
The follow ing s e c tio n s r e f e r in d e t a i l to v ario u s asp e c ts of the t e s t methods used in the c u rre n t in v e s tig a tio n . These methods are name l y :
M arshall t e s t
In d ire c t-T e n s ile t e s t and Wheel-Tracking t e s t .
I t should be noted th a t many of the previous a p p lic a tio n s have not been d i r e c t l y r e la te d to H.R.A., however, in o rd er to draw upon the experience gained w ith o th e r m a te r ia ls , the au th o r f e l t i t necessary to re p o rt upon a p p lic a tio n s to Bituminous m ixtures in g e n eral; w ith s p e c ific re fe re n ce to H.R.A. whenever p o s sib le ^
2 .2 .2 The M arshall Test Method: 2 .2 .2 .1 O utline of Test Method
M ixtures are made up a t s e v e ra l b in d er co n ten ts and c y l i n d r i c a l specimens 101.6 mm d ia x approx 63.5 mm high (4 in d ia x 2.5 in ) are compacted in s t e e l moulds using a standard drop-hammer. (See l a t e r ) .
D eterm inations are then made to f a c i l i t a t e the c a lc u la tio n of the d e n sity and void co n ten t of specimens.
The M arshall t e s t i t s e l f is a type of compression t e s t , conducted a t 60°C (140°F). Load i s ap p lied to the curved su rface of the specimen a t a co n stan t ra te of s t r a i n of 50.8 mm/min (2 in /m in ), and the maximum load su stain ed along w ith the deform ation a t max imum load are recorded as M arshall S t a b i l i t y (N, lb f or k g f) and
M arshall Flow (mm or 0.01 in ) r e s p e c tiv e ly .
From the r e s u l t s o b tain ed , an "optimum b inder c o n te n t" i s d e te r mined, the manner in which t h i s i s done depends upon the type of mix and i t s a p p lic a tio n .
(7 )(1 7 )(2 3 ) N.B. A more d e ta ile d d e s c rip tio n can be found elsew here.
2 .2 .2 .2 H is to r ic a l Background:
The M arshall t e s t procedure was o r i g i n a l ly developed in America by Bruce M arshall of the M iss is sip p i S tate Highways Department.
During the 1940*s, the U.S. Corps of Engineers adopted t h i s method of t e s t i n g and developed around i t the M arshall Mix Design p ro cedure, v a r ia tio n s on which are w idely used today. The work c a r r ie d out to t h i s end was tw o -fo ld :
( i ) the development of a compaction procedure, which r e s u lte d