La noción de campo aplicada al mundo de la muerte

3.1 In tro d u c tio n

In Chapter 1 a tte n tio n was drawn to problems r e la te d to the r e s ­ is ta n c e to deform ation of H.R.A. su rfa cin g s and to c e r t a in inad­ equ acies of the " re c ip e " method of s p e c if ic a tio n fo r such m ix tu re s,

in p a r t i c u l a r an i n a b i l i t y to c a t e r fo r the d if f e r in g binder requirem ents of d i f f e r e n t fin e aggregate sands. Moreover, w ith the in tro d u c tio n of the M arshall Test in to B.S. 594 (1973), to perm it the d eterm in atio n of an "optimum b in d er c o n te n t" f o r the a c tu a l m a te ria ls (sand, f i l l e r , b in d er) to be used, i t was hoped th a t the r e s u ltin g m ixtures would have improved Engineering p ro p e rtie s and in p a r t i c u l a r an in creased re s is ta n c e to d e fo r­ m ation under heavy t r a f f ic .-

The M arshall Test i s , of co u rse, only one of many m echanical t e s t procedures, which could be a p p lied to t h i s same end.

The m o tiv atio n fo r and o v e ra ll aim of the p re sen t in v e s t ig a tio n was, th e re f o r e , to determine the a b i l i t y of th re e m echanical

t e s t procedures to :

(a) a s s i s t in the s e le c tio n of s u ita b le m ixture p ro p o rtio n s, and in p a r t i c u l a r the binder co n ten t re q u ire d to o b ta in the b e s t compromise between d u r a b ility and maximum re s is ta n c e to d e fo r­ m ation, fo r a given s e t of c o n s titu e n t m a te ria ls

(b) p re d ic t/d e te rm in e the p r o p e rtie s of m ixtures having good re s is ta n c e to deform ation, but in p a r t i c u l a r to e s t a b l i s h a r e l ­ a tio n s h ip between M arshall param eters and re s is ta n c e to deform ation, fo r a range of H.R.A. wearing course m ix tu res.

3.2 Mechanical Test Procedures Employed

Three m echanical t e s t .procedures, namely, the M arshall t e s t , In d ir e c t T ensile t e s t and Wheel-Tracking t e s t are considered in t h i s in v e s tig a tio n , each fo r the follow ing reason s:

(a) M arshall t e s t : t h i s in c lu s io n was of g r e a te s t importance in view of i t s re c e n t in c lu s io n in B.S. 594 and a lso because th e re i s l i t t l e re p o rte d inform ation reg ard in g i t s a p p lic a tio n to H.R.A. m ix tu res.

(b) I n d i r e c t - t e n s i l e t e s t : in v e s tig a tio n of t h i s t e s t method was suggested by the C o lla b o ra tin g E stablishm ent, ESSO Petroleum Company Lim ited, in the lig h t of prom ising work on the c o n tin e n t of Europe.

(c) W heel-tracking t e s t : t h i s t e s t method was included due to the d e sire to c o n sid er a t e s t method which sim ulated the p ra c ­ t i c a l loading c o n d itio n , the r e s u l t s of which could be used as a " y a rd stic k " to judge a m ixture*s a b i l i t y to r e s i s t deform ation under t r a f f i c .

3.3 H.R.A. M ixtures Considered •

A s e r ie s of H.R.A. wearing course m ixtures covering the range of Stone Contents perm itted by B.S. 5 9 4 ^ ^ were in v e s tig a te d . For each Stone Content, m ixtures were te s te d over a narrow range of binder c o n te n t, n o t, however, too narrow as to preclude the d eterm in atio n of an "optimum binder c o n ten t" fo r the p a r t i c u l a r Stone Content under c o n sid e ra tio n . The Stone Contents and range of binder co n ten ts considered, are summarised in ta b le 13.

RANGE OF MIXTURE COMPOSITIONS CONSIDERED IN MAIN INVESTIGATION 9 (IN VS V (IN VS 66

M arshall and I n d i r e c t - t e n s i l e t e s t s were c a r rie d out over the whole range of Stone C ontents, while Wheel-Tracking t e s t s were

lim ite d to m ixtures c o n tain in g 307o stone and above. This was p rim a rily done in ord er to reduce the time involved in m anufacture and te s t i n g of specimens, but a lso because i t was considered

t h a t in p r a c tis e only such m ixtures would be considered f o r use under heavy t r a f f i c .

In the manner o u tlin e d above i t was hoped to determine how the m ixture p ro p e rtie s r e s u ltin g from a given combination of c o n s t i t ­ uent m a te ria ls v a rie d as b in d er co n ten t was a lte r e d over a f a i r l y narrow range which included the b in d er co n ten ts lik e ly to be used in p r a c ti s e . From such r e s u l t s i t should a lso be p o ssib le to e s t a b l i s h c o r r e la tio n s between d i f f e r e n t mixture p r o p e r tie s , f o r the same lim ite d range of b in d er c o n te n t. This approach i s considered by the author to improve upon those used in s im ila r in v e s tig a tio n s re p o rte d elsew here, in which r e s u l t s , f o r what may be considered "in d iv id u a l" m ixtures, of varying type and com­ p o s itio n have been brought to g e th e r. This has re s u lte d in a very wide range of m ixture p ro p e r tie s being considered w ith m ixtures varying from very r e s i s t a n t to very s u sc e p tib le to d e f­ orm ation, being used to e s ta b l i s h c o r r e la tio n s between the v ario u s t e s t s co n sid ered . Giving c o n sid e ra tio n to the s c a t t e r of r e s ­ u l t s in what maybe termed the p r a c t i c a l range, t h i s tends to in d ic a te th a t previous work i s saying, the t e s t methods considered are capable of t e l l i n g "chalk from cheese" but l i t t l e e l s e .

R esu lts of t h i s n atu re are u s e f u l, in t h a t they i d e n tif y the presence of such c o r r e la tio n s , but u n t i l s im ila r r e la tio n s h ip s can be id e n tif ie d fo r the ranges of b in d er con ten t of the o rd er of those used in t h i s work, l i t t l e p r a c t i c a l value can be o btained

due to the high s c a t t e r of r e s u l t s .

3.4 Comparison of Laboratory and Road R esults

I f any degree of confidence is to be placed upon la b o ra to ry t e s t r e s u l t s , i t i s e s s e n t i a l th a t such r e s u l t s are c o rre la te d w ith a c tu a l road perform ance, in o rd er to answer such q u estio n s a s: (a) Is the "optimum b in d er c o n ten t" determ ined in the la b o ra to ry equal to the b inder co n ten t g iving "optimum" performance under road co n d itio n s?

(b) What lim its should be s e t on the values of t e s t param eters measured in ord er to ensure the pro d u ctio n of m ixtures w ith the d e sire d degree of re s is ta n c e to deform ation under given t r a f f i c and c lim a tic conditions?

In an attem pt to e s t a b l is h some c o r r e la tio n between la b o ra to ry and road r e s u l t s , the c o n s titu e n t m a te ria ls fo r the m ixtures in v e s tig a te d were the same as some of those used by the T.R.R.L.

(39) in a f u l l - s c a l e road t r i a l on the A33 W inchester by-pass.

Throughout the S h e ffie ld in v e s tig a tio n , m ixtures were m anufactured using the same coarse aggreg ate, f i l l e r and binder as used a t W inchester and two of the fin e aggregate sands. The sands used in t h i s in v e s tig a tio n were chosen follow ing d isc u ssio n s a t T.R.R.L. in e a r ly 1978, when i t appeared th a t m ixtures made w ith these m a te ria ls and la id a t W inchester, were showing signs of providing meaningful d ata more quickly than s e c tio n s c o n ta in ­ ing o th e r sands. I t should be pointed out th a t in ord er to confirm any re la tio n s h ip s in d ic a te d , i t was o r ig i n a l l y intended to u t i l i s e th ree sands from the A33 experim ent, but in the event

lack of time prevented t h i s .

The H.R.A. m ixtures la id a t W inchester contained 307o sto n e, so to f a c i l i t a t e the c o r r e l a ti o n , la b o ra to ry t e s t s on 307. stone m ixtures were conducted over a range of binder c o n ten ts which

included those found on a n a ly s is in the se c tio n s on the A33.

3.5 Statement of Aims and O bjectives

The o v e ra ll aims of the in v e s tig a tio n have been r e fe rre d to p re ­ v io u s ly . To recap, they a re :

(a) To a sse ss the a b i l i t y of the th re e t e s t methods co n sid ered , to a s s i s t in the s e le c tio n of an "optimum" mixture com position, reg ard in g d u r a b i li t y and, in p a r t i c u l a r , re s is ta n c e to deform ation. (b) To determine the p ro p e rtie s of H.R.A. m ixtures having good r e s is ta n c e to deform ation, b u t, in p a r t i c u l a r , to e s t a b l i s h a re la tio n s h ip between M arshall param eters and re s is ta n c e to d e f­ orm ation.

In ord er to achieve th ese o v e ra ll aims the attain m en t of a number of secondary o b je c tiv e s was a n tic ip a te d during the course of the in v e s tig a tio n , namely:

(a ) The development of s u ita b le specimen manufacture and t e s t i n g p ro c e d u res.

(b) A c r i t i c a l a p p ra is a l of specimen manufacture and t e s t i n g procedures, along w ith the a sso c ia te d a n a ly s is of t e s t r e s u l t s employed.

(c) Formulation of methods of s e le c tin g "optimum b in d er c o n te n t s ," from t e s t r e s u l t s .

(d) Assessment of the v a lid ity of extending the M arshall mix d esig n (as per B .S. 594) to s to n e - f ille d m ix tu res.

(e) C o rre la tio n of M arshall t e s t and In d ire c t T en sile t e s t re s u l t s over the range of stone c o n ten ts c o n sid ered .

( f ) C o rre la tio n of M arshall t e s t and Wheel Tracking t e s t r e s ­ u l t s fo r the h ig h -sto n e co n ten t m ix tu res.

CHAPTER 4