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. Field Correlation Testing /

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Nuclear vs. Pavement Core Density Tests L. , /' ' / /

INTRODUCTION --.- -.. .. ,

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During 1972 the Materials Division of the Alaska Department of Highways performed correlation test ing on eight pro,jects t o determine the accuracy of nuclear Pavement density tes t ing equipment and procedures. asphalt pavement placement i n thicknesses of 1% to 2 inches, w i t h nuclear tes t ing and coring done a f t e r f inal compaction of the pavement. A to ta l of 107 cores were taken and tested i n the laboratory. Nuclear densities were determined a t the same locations. S t a t i s t i c a l analysis was performed on the data t o determine correlations between the two t e s t s and t o determine the re la t ive accuracy o f the two t e s t methods.

All of these projects involved new

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' TEST PROCEDURES AND EQUIPMENT I

General nuclear density t e s t procedures were i n accordance

Determination of In-Place Density of Plant Mix Asphalt Pavement by Use of a Nuclear Density Gauge." (Copy attached.) Field control density t e s t s were run i n accordance w i t h Section "E"

taken. In a d d i t i o n , on some projects one minute. " a i r gap" counts were run, and densit ies were determined'from the a i r gap t o backscatter r a t i o calibration curves fo r the parti cul a r gauge used.

- bedding layer was used t o f i l l the surface voids because of the re la t ively rough surface texture o f the pavement. All other projects had re1 a t i vely smooth , t i g h t , surfaces , so no bedding layer was used.

tables supplied w ' th the gauges used.

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i -- / .w i th proposed t e s t method T-18 - "Standard Method o f Test f o r

of this procedure, except t h a t more than two readings were qenerally

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On the Portage Glacier Road project, a sand or "native fines" t

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I A l l densi t ies were determined from manufacturers calibration --- - t

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Four dif ferent nuclear gauges were used in ' this study. One gauge was a Campbell Pacific Nuclear Model B R Mark 11. The other three were manufactured by Troxler, designated as models 2401 and 2451. The Campbell Pacif ic unit was provided with a special backscatter position for testing asphalt pavement t o assure t h a t the depth of denslty Influence would not b e greater than approximately 1% inches. The Troxler units are designed for

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relat ively shallow backscatter density influence. Manfuacturers instructions accompanyinq the Troxler units indicate tha t rouqhly 90% of the normal density influence of this gauge will be w i t h i n the t op inch o f the underlying so i l .

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A laboratory check on the depth of influence o f one of the Troxler units was made by test ing the density of a 14 inch thick asphalt concrete t e s t specimen, b o t h w i t h the sDecimen placed on a concrete f loor and w i t h the specimen suspended on a i r t o simulate a void beneath the pavement. The indicated density decreased by only about five pcf from the simulated concrete subgrade position t o the zero density subgrade position. i n density of the underlying base course will have a negligible e f fec t on indicated pavement density.

TEST RESULTS

This indicates tha t variations

Appended data sheets A - 1 through A-11 present the resul ts of a l l nuclear t e s t s and a l l core density t e s t s run under this study. Backscatter (BS) nuclear densities shown i n columns 1 t o 4 are densit ies from individual one minute counts a t the same location. The gauge was rotated 900 between t e s t s above the core location when nuclear densities were taken before corinq. Densities were taken a t a distance of approximately one foot awai f r o m the core hole when run a f t e r coring. - -

- Figures 1 through 9 show graphi cal ly the re1 ationshi ps between

. a l l nuclear and core densities a t the same locations, on a per project basis. I f correlation between the two t e s t methods were perfect, a l l data would f a l l on the 45’ or 1:l sloped l ine Dlotted on these figures. On the contrary, however, inspection of a l l data plots indicates tha t there i s l i t t l e o r no s ignif icant corre- la t ion between nuclear and core densities taken a t the same location. S t a t i s t i c a l analysis fo r correlation of .the data was made fo r

tha t correlations were very low. Therefore, unless an extremely large number of cores and comparative nuclear densities were r u n on the same project , the relationship between nuclear and core densities a t any given location apparently cannot re l iably be determined.

,

several o f the projects, which indicated tha t sca t t e r was so great __

To determine the reasons fo r the lack o f core t o nuclear density correlat ion, i t i s necessary t o examine the variables involved i n the two t e s t methods which will af fect the actual or indicated f i e l d densities. This include the following:

Variables i n Laboratory Core Density Determinations: -. ,

a) Testing vari ables , i ncl udi ng wei g h i ng accuracy , etc. b) Sample Pososity and continuity of internal voids

- Variables i n Field Nuclear Density Determinations:

Surface roughness and trueness Random nature o f radioacti ve source

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Accuracy of cal i bration curves Chemical ef fects of d i f ferent aggregates . .

L -

! -.. - - - - - - - _ _

' Density Variables i n actual pavement a f t e r f i e l d compaction:

a ) Gradation and asphalt content variations b Actual compactive e f fo r t received by a speci f ic location c] Subgrade support variations during compaction

- - - - _

Review o f the principles behind nuclear density testing i n ...- the Backscatter mode indicates some reasons f o r lack of agreement

.between nuclear and core densities run a t the same location. The . first i s the surface roughness problem. Nuclear Gauges include

- even amplify this fac tor because the uppermost soi l layer has the greatest influence on indicated density. cedures, however, eliminate the ef fects of the surface voids of the cores, since the densit ies are determined from dry and submerged

i n Table 1 were determined without wax coating. the remaining projects were wax coated and tested i n accordance w i t h AASHO Test Method T-166-70. cause a lack o f correlation i s the smaller depth of influence of the nuclear method as compared t o the core method. The dens i ty o f the upper one h a l f inch of soi l has roughly a 50% influence on the indicated nuclear density, b u t only a 25% influence on the density of a two inch thick core. Density variations of several pounds per cubic foot can be expected t o occur between di f ferent

. \ the effects of surface voids on the indicated density, and may . .* -.._ - . Core density t e s t pro-

Core densities fo r the f i r s t f ive projects l i s t ed

# ------

't . core weights. Density cores f o r I

A second factor which would -\- .

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--- . -- - - -- - depths w i t h i n an asphalt pavement due t o minor differences i n

. . . gradation, asphalt content, and compactive e f fo r t . variations i n indicated density between core and nuclear tests a t the same point should be expected.

DISCUSSION

Consequently,

Since a l l pavements i n this study were tested a f t e r compaction was completed and the pavement had cooled, the actual pavement

a result of the above variables. will add fur ther randomness t o the indicated density resul ts . Tables 1 and 2 summarize the resul ts of correlation test ing on a l l projects. Table 2 presents density averages and standard deviations fo r the core and nuclear density data from each project. For a l l core t e s t s the overall average s t a n d a r d deviation (SD) was 2.02 pcf. For a l l nuclear t e s t s the average was 2.25pcf. However, for the f ive projects where nuclear densities were run before coring, the average nuclear density SD was 1.66 pcf while the SD of the cores was 1.98 pcf. The nuclear densit ies used in this SD analysis were generally determined from the average of three one minute counts L

deviations a b o u t 20% higher than the backscatter t o standard count ratio method. ._

density can be expected t o vary randomly from point t o ooint as . . ' 1 ,

In addition, the test ing variables

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A i r-gap/backscatter density determinations showed standard

1 _--- 6 The test data indicates t h a t the test ing accuracy of the

nuclear method i s similar t o tha t of the coring method, b u t tha t

used were i n e r ro r by d i f ferent amounts.

The poss ibi l i ty of calibration errors affecting the i ndi cated densities of the various gauges was investigated by nuclear equip- ment calibration checks run on the concrete f loor o f the Anchorage laboratory and on a large concrete standardization block a t the Sta te Materials Laboratory. These checks, made by a ser ies of f ive one-minute counts w i t h a l l guages on the same location, showed indicated density averages fo r the different gauges t o range from 142.1 to 149.6 pcf. The one minute indicated densities

I . the manufacturers calibration curves for the different gauges )

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' used t o obtain the five count averages had a range o f 1.3 t o 3.0

The apparent cal ibrat ion curve error , i n pcf, was determined for each gauge by comparing the indicated standardization check density from that gauge w i t h the average o f a l l gauges checked.

- T h i s difference, shown as the Gauge Calibration Error i n Table 1, was used t o a d j u s t the average nuclear density fo r each project. As shown by the l a s t column of Ta':.:e 1, the adjusted nuclear t o core density differences were considerably reduced, showing tha t nuclear calibration curve errors are very signif icant . Based on the adjusted data, the agreement between nuclear and core densities fo r a l l projects averaged -0,9%, w i t h a maximum difference

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O f -1.6%.

To check f o r correlations between core and indicated nuclear densities a t the same locations, the differences between the two results were analyzed by determining the standard deviations of the difference between each p a i r of t e s t s from the average o f a l l

shown i n Table 2 , ar2 essent ia l ly as large as the overall standard deviations of the core and nuclear test resul ts , again indicating l i t t l e or no correlation between the two sets o f data.

. . differences for the same project. These standard deviations,

I -

/ REQUIRED TESTING FREQUENCIES , /-

difference i s extremely important when calibration of the nuclear

nuclear t o core density differences, the number of t e s t s (N)

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- The determination of the actual average nuclear t o core density-- --- - 1 \ . gauge f o r the pavement mixture and surface texture of a part icular

project is required. Based on the standard deviations (SD) of the

required t o achieve a 95% confidence level can be calculated by use

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.-i t Using a three pcf range, the number o f t e s t s required f o r , correlation, based on the SD's of the f ive projects analyzed i n

-- - - - Table 2, ranged from 3 t o 10, w i t h a mean of 6 tes ts . A 3 pcf - . range would assure t ha t the maximum error i n the determination

I -_ . ._

of the average difference between core and nuclear densit ies would not exceed ltl% more often than one project i n twenty, and would not exceed +0.5% more than one time i n three. In order t o reduce the range t o 1.5 pcf, equivalent t o assuring accuracy of the core t o nuclear adjustment factor w i t h i n -+0.5%, the required number o f correlation cores must be increased by a factor o f four.

of core or nuclear densities needed t o determine the average density of a pavement w i t h 95% confidence t h a t the indicated

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The above formula can also be used t o calculate the number -

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t . average i s w i t h i n a given range of the true average. For accuracy 1 . w i t h i n k1%, the number o f cores required ranged from 2 t o 12, while t h 4 - : number of nuclear t e s t s ranged from 1 t o 27. Again, t o reduce the

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range t o +0.5% the number o f t e s t s must be increased by a factor of

SUMMARY AND CONCLUSIONS: 1 i

.- .. Correlation test ing between nuclear backscatter and a i r gap t e s t methods And laboratory determined densities on 6 inch cores, taken from asphalt concrete pavements on eight projects, showed the

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Both core and nuclear t e s t methods appear t o measure random variations i n the density o f an asphaltlpavement.

No re1 i able correlations were found between the indicated densities a t a given p o i n t when determined by core and nuclear methods. This i s probably because the range o f actual pavement densities was very similar t o the range of deviations w i t h i n bo th the core and nuclear t e s t methods.

The average standard deviations were 2.02 pcf for a l l cores, and 2.25 pcf for a l l nuclear tes ts . . requiredto determine the density of a pavement w i t h a 95% probability a t an accuracy level o f +1%, would be seven t e s t s

. f o r the core method and nine t e s t s for the nuclear method.

The average number of t e s t s

For the five projects on which nuclear densities were run above the core removal location before coring, the core and nuclear standard deviations were 1.98 and 1.66 pcf, respectively. The standard deviation from the average difference between core and nuclear densit ies a t the same po in t was 1.93 pcf. Therefore, an average o f seven and a maximum of ten correlation se t s o f core and nuclear t e s t s would be required t o determine the average correction factor for a given project. t o adjust the nuclear average t o agree w i t h the core average w i t h an accuracy level o f +1%.

This factor i s needed

When tested i n s i gni f i can t l y nuclear gauges

the same position as a standardization check,

used i n this study. T h i s indicates t ha t the different densities were indicated by the different

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manufacturers calibration curves used for determining densities i n this study are not rel iable and tha t recalibration of a l l

. gauges on the same se t of standards should be performed on a . I . regul a r basis.

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! . 6) Air Gap Testing w i t h one surface and one a i r gap count genetally produced standard deviation's approximately 20% hiqher t h a n normal backscatter testing using the average of three one- minute counts. Therefore the use of the air-gap method i s not recommended.

2 . . - - - ._ - -------:7) Direct transmission nuclear testing was t r i ed only a t one location and was not tested further due t o the d i f f icul ty and disturbance o f d r i l l i n g o r d r i v i n g a hole through the pavement for positioning o f the source rod.

indicated tha t different resul ts should be expected, since surface voids are not included i n core densities b u t are a fac tor i n nuclear backscatter tes t ing , and *also since the density o f the upper one-half inch of pavement

- has a much higher influence on the indicated nuclear density than i t does on the indicated core density.

9) Because the standard devi ations are very 'simi 1 a r between the nuclear and core methods, the resul ts o f nuclear test ing are considered t o be as "accurate" an indicator of density as the

- core method, assuming tha t the nuclear calibration curves are correct for the aggregates used on the part icular project.

between average nuclear and core densities or f o r determining the density average and range by the core method alone, a m i n i m u m of ten cores should be taken f o r 95% confidence tha t

- the average density has been determined w i t h i n an accuracy of *I%.

. . - . . -.- - . -- 8) Review of the nature of both core and nuclear density t e s t s

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STANDARD FlETMOD OF TEST FOR DETERMINATION OF IN-PLACE DENSITY OF PLANT MIX ASPIULT PAVEMENT

BY USE OF A NUCLEAR DENSITY GAUGE I .

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2. A r e f e r e n c e s t a n d a r d o r block f o r purposes o f checking

3. . determine d e n s i t y .

.. the o p e r a t i o n of t h e nuc lea r device.

C a l i b r a t i o n curves as supp l i ed by t h e manufacturer t o

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-..' 4. ' A supply o f l i g h t o i l , d i e s e l f u e l , o r o t h e r m a t e r i a l . s u i t a b l e f o r c l e a n i n g a s p h a l t from t h e bottom of t h e gauge . a f t e r t e s t i n g .

. I . , . .. . C. STANDARDIZATION PROCEDURE: .. . -

. _ 1. S t a n d a r d i z a t i o n o f Equipment

--: - *-

&-e. Before u s i n g t h e n u c l e a r gauge, ch.eck t h e opera.t ion

~ w i t h t h e gauge p laced on t h e manufac turer ' s d e n s i t y r e f e r e n c e s tandard . da i ly , and more f r e q u e n t l y i f e r r a t i c count r a t e s

. are n o t i c e d o r i t i s f e l t t h a t f i e l d measurements : ' .:are ques t ionable ,and v e r i f i c a t i o n of gauge o p e r a t i o n

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. -., Checks a r e t o be r u n a t l e a s t . . *. - - I . . --- .

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4 ~ I b o After t h e gauge has been turned on and pe rmi t t ed t o

- . reference ' s tandard.

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warm up on a "count" mode according t o t h e manufac turer ' s instructions, perform a minimum of f i v e one-minute s t a n d a r d count checks, i n t h e d e n s i t y mode, on t h e

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/ : c. According t o laws of random s t a t i s t i c s , 1 9 out of /

. 20 s t a n d a r d count checks (95%) will f a l l x i t h i n the

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1. F i e l d Densi ty Control by Laboratory Design Maximum Densi ty

range o f : (SC) 2 1.96 ym where (SC) is t h e t rue average Standard Count, i n counts p e r niinute.

For example, i f t h e average Standard Count, a s l i s t e d by .. t h e manufacturer , i s 1 0 , 0 0 0 ( t h e square roo t being 100) 9 5 % of a l l checks on t h e s tandard count would be w i t h i n t h e range of 9 , 8 0 4 t o 1 0 , 1 9 6 . I f ' c o u n t s a r e e r r a t i c and ' checks exceed t h i s range more f r e q u e n t l y than 1 time i n

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a, ' -Cal ibrat ion curves r e l a t i n g d e n s i t y w i t h b a c k s c a t t e r . - count/s tandard count r a t i o s a r e supp l i ed by t h e manu-

facgurer . a t t h e s t a r t of paving o p e r a t i o n s t o v e r i f y t h e accuracy

, o f t h e c a l i b r a t i o n curves , when used on t h e s p e c i f i c . - job mixture.

such as t h a t r e s u l t i n g from a change i n aggregate source , w i l l r e q u i r e a recheck on c o r r e l z t i o n . C o r r e - l a t i o n t e s t i n g i s necessary because nuc lea r d e n s i t y measurements will vary s l i g h t l y depending on aggregate

C o r r e l a t i o n t e s t i n g s h a l l be performed -- -

Any major change i n t h e job mixture ,

*

. . composition and on s u r f a c e t e x t u r e .

:bo A minimum o f f o u r c o r r e l a t i o n t e s t s p e r p r o j e c t s h a l l be run , by comparing n u c l e a r t e s t r e s u l t s w i t h d e n s i t i e s

t e s t s i t e s . For each c o r r e l a t i o n t e s t , a s e r i e s of f o u r one-minute counts s h a l l be made i n t h e b a c k s c a t t e r -

. -*- between counts . Care s h a l l be taken t o a s s u r e t h a t

. 'a smooth l e v e l a r e a i s s e l e c t e d f o r c o r r e l a t i o n . t e s t i n g so t h a t no "rocking" occurs which would i n d i c a t e s u r - face voids beneath t h e gauge.

. I . determined from samples cut from t h e i d e n t i c a l n u c l e a r

* * A.C. pavement p o s i t i o n , wi th t h e gauge r o t a t e d 90° --

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D e n s i t i e s s h a l l be determined f o r each of t h e f o u r counts and averaged t o determine t h e nuc lea r d e n s i t y a t each t e s t s i t e . Pavement c o r e s o r sawed blocks - s h a l l be obta ined from d i r e c t l y beneath t h e nuc lea r gauge p o s i t i o n a t each t e s t s i t e , and t h e core d e n s i t i e s determined. The r e s u l t a n t core and nuc lea r densities s h a l l be used t o modify t h e f a c t o r y c a l i b r a t i o n curve by the method of b e s t f i t , assuming a l i n e a r r e l a t i o n - s h i p on a semi logar i thmic p l o t of t h e d a t a .

2, ' F i e l d Density Contro l Based on Contro l S t r i p Densi ty ..

a. -.*

l a e n c a l l e d f o r by t h e c o n t r a c t s p e c i f i c a t i o n s , t h e c o n t r a c t o r s h a l l c o n s t r u c t Dens i ty Contro l T e s t S t r i p s

I . .

. ,

. . . . .2:c ---, -.--- ,--.L,. .._I-.. . . . .. . .* . . _.. I - . , . . . . . .... - . - , - . .. , .e.-. , .- . ... . . ._ .__.-

_I-&==-- . .

- - -.--.. - _ . _ F . . . . .

' .

* a t t h e beginning of work on each pavement course,/ and whenever t h e r e i s a change i n aggregate source.

.The t e s t s t r i p will be r o l l e d with t h e s p e c i f i e d s e r i e s o f r o l l e r s , w i t h compaction continued u n t i l - n o apprec i - a b l e i n c r e a s e i n d e n s i t y can be obta ined by a d d i t i o n a l r o l l e r coverages. for c o n s t r u c t i o n of the c o n t r o l s t r i p a r e t o be t h e same as f o r o t h e r pavement a r e a s , and s i n c e d e n s i t y of o t h e r a r e a s w i l l be r e l a t e d t o t h e c o n t r o l s t r i p , use of t h e manufac turer ' s c a l i b r a t i o n curves w i l l be accep tab le and no pavement co res w i l l be r equ i red .

. *

Since equipment and m a t e r i a l s used *

I

. b, .

To provide d a t a on t h e i n c r e a s e of d e n s i t y dur ing con- t r o l s t r i p compaction o p e r a t i o n s , nuc lea r d e n s i t y checks w i l l be taken a f t e r each r o l l e r coverage a t two l o c a t i o n s , s e l e c t e d s o t h a t a s i n g l e r o l l e r p a s s w i l l

. f i n e d as a p n l i c a t i o n of one r o l l e r pass over t h e e n t i r e surface a r e a of the c o n t r o l s t r i p . w i l l be made u s i n g 1 / 2 m i n u t e counts . should be marked and t h e same gauge p o s i t i o n used f o r d e n s i t y checks a f t e r each r o l l e r coverage. Ro l l ing and

, d e n s i t y checks w i l l cont inue u n t i l t h e d e n s i t y i n c r e a s e becomes less than 1 pound p e r cubic f o o t p e r r o l l e r

. not cover both l o c a t i o n s . A r o l l e r coverage i s de- I

. Densi ty checks

e . The gauge 1ocati:ns

,

-- -~~ -_ ' ._ pass, f o r two consecut ive r o l l e r passes . ---

' .. 'c. . After -compaction has been completed, t h e mean d e n s i t y of t h e c o n t r o l s t r i p w i l l be determined from t e n ran-

become t h e s t andard o r " cont ro l s t r i p dens i ty" f o r . subsequent pavement r e p r e s e n t i n g t h e same l i f t and

- . _ _ * . domly l o c a t e d one-minute d e n s i t y counts . Th i s w i l l - .

I source b -

- _ . . . . If .

-. -- ---___ ' E, FIELD CONTROL DENSITY TESTING: - - I ) - . . .

. . . . . .

-.----- -_ - - . _- - . ' . '1, Procedure

. 4- .. -c -

: \ - . -

%:-- -- ..... C - Surface c h a r a c t e r i s t i c s should be s i m i l a r t o those

. --., - e x i s t i n g on t h e c o n t r o l s t r i p o r d e n s i t y c a l i b r a t i o n

s u r f a c e i r r e g u l a r i t i e s , and excess ive s u r f a c e v o i d s , -. 2 t e s t s e c t i o n . S e l e c t a s u r f a c e f r e e of loose m a t e r i a l , \

--. -. . . . and a t l e a s t 1 2 inches away from pavement edges. Make "certain t h a t t h e bottom o f t h e gauge is fre.e of excessia-s

- . . . . a s p h a l t o r o t h e r adherent m a t e r i a l . " , - b. In t h e event t h a t excess ive s u r f a c e voids o r roughness

are ev iden t and a d e n s i t y i s r e q u i r e d , n a t i v e f i n e s f rom ' the aggregate source may be spread i n t o t h e s u r - face voids, avoid ing any excess ive s u r f a c e bui ldup.

of uneveness.

I . d, Obtain a one-minute r ead ing , r o t a t e t h e gauge 90°, and o b t a i n another one-minute reading. age of t h e two readings t o determine t h e i n - p l a c e

4

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i i - 2. :i i . . c. Seat t h e gauge t o avoid rocking o r o t h e r i n d i c a t i o n s

1 . . %

'7; Use t h e ave r - t - ; I : t

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ._.-_ . .,-*;e ..._. . . ....... . . . . . . . .. - .-- - - ? ' .-. , .. - . . . -

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- . r ' . I ' . Alaska Test Method T-18 . 1 : * I

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I density from the appropriate calibration curve, using 'the count ratio as determined below.

Do not leave the gauge on the hot asphalt pavement 1 ;

e, * . --. - _-____ unless testing is in progress. 1 I ;-* -?*

~ 1 . 2. Calculations ---.....- * *

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- 1 .- - - _ . . - - . a. Calculate the count ratio for density as follows:

I

* . - . - . - *

---- . . Where (FC) = average counts per minute from field on pavement. - - - - _ _

mode. (SC) = Standard counts per minute on density __

-- I . .

b, Calculate the relative density as follows: . .

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