Strengths of Recycled Aggregated Concrete

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ACI MATERIALS JOURNAL TECHNICAL PTitle no. 93-M21

Strengths of Recycled Aggregate Concrete Made Using

Field-Demolished Concrete as Aggregate

by M o stafa Tavakoli and P arviz S oroush ian

Experimental work was performed to determine the compressive. spli tt ingtensi le. and flexural strengths of recycled coarse aggregate concrete and tocompare them with those of concrete made using natural crushed stone.The propert ies of the aggregate were also compared. Thefine aggregate forrecycled and conventional concrete was 100 percent natural sand.

Two sources of recycled aggregate (crushed concrete pavements from

u.s.23 and /-75 projects in Michigan) and one source of natural aggre-gate (crushed limestone) were used. Two maximum sizes of aggregates. twolevels of water-cement ratio, and two levels of dry mixing time of coarseaggregate were selected to perform the experiments based on afullfacto-ria! design.

Test results indicate that the strength characteristics of recycled aggre-gate concrete are influenced by key factors, such as the strength of the orig-inal concrete, the ratio of coarse to fine aggregate in the original concrete,the ratio of top size of aggregate in the original concrete to that of the recy-cled aggregate, and the Los Angeles abrasion loss and water absorption ofrecycled aggregate. These factors also influence the effect of water-cementratio, aggregate top size, and dry mixing on the strength characteristics ofrecycled aggregate concrete. It is also shown that the conventional rela-tionships between splitting tensile, flexural, and compressive strengths mayhave to he modified for recycled aggregate concrete. Thefinal conclusion isthat through proper measures high-quali ty concrete materials can be pro-duced using recycled concrete aggregate. For this purpose, it is needed todetermine the propert ies of the original concrete, based on which realist icqualities can he targeted for recycled aggregate concrete.

Keywords: aggregates; compressive strength; demolition; flexural

strength; recycling; splitting tensile strength.

The depletion of the supply of quality aggregates togetherwith environmental, economic, and energy considerations areencouraging the recycling of demolished concrete structuresand pavements as aggregate in new concrete construction. Re-searchers have tried to relate the quality of recycled aggregateconcrete to the properties of the original concrete and paste,deterioration condition of the old concrete, crushing proce-dure, and the new mix composition; their findings have beenextensively reviewed and discussed by Hansen. I It is generally

accepted that the cement paste from original concrete that isadhered to the recycled aggregate plays an important role indetermining the performance of recycled aggregate concrete.The qualities of the paste and the interface zones, as well as

the paste content of the original concrete, thus influence theproperties of the recycled aggregate concrete.Most researchers have made the original concrete in the

laboratory to have control of the properties and mix propor-tions of the original concrete and to be able to study somespecific properties of recycled concrete under controlledconditions. However, this will lead to results that may differfrom those obtained when field-demolished concrete is usedto produce recycled aggregate.The main thrust of this research was to evaluate recycled

concrete from field demolition operations for use as coarseaggregate in concrete; a broad basis covering wide ranges ofexperimental design and data analysis was adopted to vali-date the conclusions at a high level of confidence.

RESEARCH SIGNIFICANCEEnvironmental and economic factors are increasingly en-

couraging higher value utilization of demolition debris. Theresearch reported herein on recycling of demolished con-crete as aggregate in new concrete addresses this critical is-sue. Realistic demolition conditions and broad ranges ofvariables are used in this research to provide a general basisfor evaluating recycled concrete as aggregate for new con-

crete and help reach consensus regarding the potentials andlimitations of recycled aggregates.

EXPERIMENTAL PROGRAMAn experimental program based on the statistical concept

of factorial design was devised to investigate the effects ofrecycled coarse aggregate source, size, dry mixing, and wa-ter-cement ratio of the new concrete on the properties of re-cycled aggregate concrete. The factorial design of ex-periments is presented in Table 1, together with the controlexperiments conducted with crushed limestone. The vari-

ACI Materials Journal, V 93, No.2, March-April 1996.Received March 14, 1994, and reviewed under Ins ti tute publicat ion policies. Copy-

r ight © 1996, Amer ican Concrete Ins ti tute. Al l r igh ts reserved, inc luding the makingof copies unless permission is obtained f rom the copyr igh t propr ie tors . Per tinen t dis-cussion will be published in the January-February 1997 ACI Mater ia ls Journa l ifreceived by October I , 1996.



Mostafa Tavakoli i s an ass is tan t profe . .sor of c ivi l engineer ing at Sharif University of

Technology, Tehran, Iran. He received his BS from Tehran University. Iran, his MS

from Wayne State University. Detroit, Michigan, and his PhD from Michigan StateUniversity, East Lansing. Michigan. His research interests include concrete materials

and design.

ACI member Parviz Soroushian is an associate professor of civil engineering at

Michigan State Universi ty. l ie received his BSfrom Tehran University and his /vlSandPhD from Cornell University. Ithaca, Nor York. He serves on a number (~ rACI,ASTM, and TRB technical committees . His research interes ts inc lude concre te materi -als and technology.

abies of the experimental design are discussed below.

Source of recycled and natural coarse aggregateTwo different sources of recycled coarse aggregate were

selected, one from the U.S. 23 job at Fenton and the otherfrom the 1-75 Detroit reconstruction project, both freewaysin Michigan. The natural aggregate was crushed limestonefrom St. John, Michigan. Table 2 presents some informationabout these two reconstruction projects.

Aggregate maximum sizeTwo maximum sizes, 0.75 and 1 in. (19.0 and 25.4 mm),

were tried for the recycled and natural coarse aggregates.

Table 1-Factorial design of experiments

Dry mixing of coarse aggregatesDry mixing of coarse aggregates for 20 min in a rotary

drum mixer (used for concrete mixing) was tried to investi-gate the possibility of partial removal of the mortar adheredto coarse recycled aggregate. For the purpose of dry mixing,coarse aggregate in saturated surface dry condition wasplaced in the drum mixer and the mixer was run for 30 minprior to the addition of other materials. These materials wereadded to the mixer while it was running. The dry mixing pro-cess and time results in the evaporation of some moisture andreduces the moisture content of the coarse aggregate. How-ever, any modification of the moisture content after the drymixing was disregarded in this investigation.

Water-cement ratioTwo levels of water-cement ratio (0.3 and 0.4) were con-

sidered to study the effects of water-cement ratio on recycledaggregate concrete performance while accounting for thewater-cement ratio of the old concrete.

PROPERTIES OF ORIGINAL CONCRETEThe size distribution of the original aggregates in the old

concrete is shown in Table 3. Table 4 shows the mix propor-

Recycled aggregate source

Crushed limestone

Recycled aggregate maximumsize

U.S. 23 aggregate 1-75 aggregate

sIZe

Dry mixing Dry mixing

None 30 min None 30 mini + t -; -

t., t t

Control (natural) aggregate

Recycled aggregate maximum

0.75 in. I in. 0.75 in. I in.VVater-~----------r-----------+-----------+-----------+-----------+------------celnent~_D_r~y_m,i_x_in~g_ r-_D_r~y_m,i_x_in~g~+-_D_r~y_1nTi_x_in~g~+-_D_r~y_nTl_ix_i.n~g~+-_ ~.-~~+- __ ~.- __ ~

ratio None 30 min None 30 min None 30 min None 30 min

Natural aggregate maximum size

*

*

0.75 in. I in.

*

0.3

0.4

* * *

* * ** Vanables considered for recycled aggregate concrete.t Variablesconsidered for control (natural) aggregate.Note: 1in. = 25.4 mm.

*

* *

Table 2-Sources of recycled aggregates

Aggregate source U.S. job at Fenton 1-75 Detroit reconstruction

Project length About 5.5 miles Ahout 2.5 miles

Original pavement 9-in.-thick and 24-ft-wide IO-in. -thick and 48-ft-wide

Type of reinforcement Steel mesh Steel mesh

Demolition method Impact hammer Impact hammer

Crusher type Jaw primary, cone secondary Jaw primary, cone secondary

Exist ing use of recycled materialsCoarse aggrega te in drainage and MDOT' open grade 5G; MDOT*

lines, s teel sold 21AA

*Michigan Department of Transportation.Note: I mile = 1.61km; J in. = 25.4 mm; J ft = 305 mm.

Table 3-Properties of original aggregates in demolished concrete

Coarse aggregate grada- MaximumSource tion", percent by weight aggregate size, in. Type of t ine aggregate

50: 4A 2U.S. 23 50: 6AA I Natural sand

50: 4A 21-75 50: lOA I Natural sand

'"MichiganDepartmentat Transportation (MOOT) gradation.Note: 1in. = 25.4 mm.



Table 4-Properties of original (demolished) concrete

Compres- SplittingUnit sive tensile

Slu~p, Age, weight, strength, strength,Source w/c* sle* gle* g/s' * Ib/ft3t psi" psi"n. years

U.S. 23 0.483 2.016 4.234 2.1 2.5 36 139.6 7914 398

1-75 0.436 2.588 3.236 1.25 2.5 30 147.2 6432 384

*Original concrete propcrtres and mix proportions by weight: w, s, c, and g indicate weights of water, sand, cement,and gravel, respectively.

t Propert ies of concrete at c rushing (present ) time.

Note: 1 i n. = 25.4 mm; I Ib/ft3 = 16.02 kg/m ': I psi = 6895 N/m2

Table 5-Gradation of coarse aggregate

No.4, No. 40,Sieve size 2.25 in. 2.0 in. 1.5 in. 1.0in. 0.75 in. 0.5 in. 0.375 in. 0.187 in. 0.0165 in.

MOOT6A 100 95 to 100 30 to 60 o to 8actual* - - 100 100 - 30 - 0 -

MOOT 17A 100 90 to 100 50 to 75 o to 8 -Actual * - - - 100 95 70 - 8 0

95 toMOOT4A 100 100 65 to 90 IOt040 - - Oto 5 - -

MOOT6AA - - 100 95 to 100 - 30 to 60 - o to 8 -

MOOT lOA - - 100 95 to 100 - 35 to 65 - o to 8 -

* Actual and exact percentage used for recycled aggregate gradation.Note: Tota l percent passing, 1 in. = 25.4 mm.

Table 6-Gradation of natural sand

No.4 No.8 No. 16 No. 30 No. 50 No. 100Sieve size 0.375 in. 0.IS7 in. 0.093 in. 0.047 in. 0.024 in. 0.012 in. 0.006 in,

Total per-cent passing 100 98 84 71 57 20 4

Note: I in. = 25.4 mm.

t ions and the strengths of the demolished concrete. As can beseen from Table 4, the ratio of coarse to fine aggregates in theoriginal concrete mix for the U.S. 23 recycled aggregate wasabout 1.68 times that of the recycled aggregate from 1-75. Theoriginal 1-75 concrete had a higher cement factor than theU.S. 23 original concrete. Therefore, the 1-75 recycled ag-gregate had more mortar attached to the original aggregatethan the U.S. 23 aggregate of the same size.

PROPERTIES OF AGGREGATES

The recycled aggregate concrete consisted of 100 percentrecycled coarse aggregate and 100 percent natural sand. Twoaggregate gradations with two different maximum sizeswere selected (see Table 5). Table 5 also compares the gra-dations of the aggregates used in the original concrete fromthe U.S. 23 and 1-75 sources with those of the recycled ag-gregates of the two different maximum sizes. The gradationof natural sand is shown in Table 6. The specific gravity ofthe sand was 2.5.Table 7 shows the properties of the coarse aggregates

from different sources. The 1 /2hr dry mixing of aggregatewas performed to study the possibility of the release of mor-tar attached to the original aggregate in the recycled aggre-gate. Aggregates were placed in the drum mixer and themixer was run for 1 / ,hr; the aggregates were then taken outof the mixer, received, and weighed to determine the amountof very fine particles released in the dry mixing process and

passing the smallest size sieve of the corresponding grada-tion of the coarse aggregate. The amount of fines releasedfrom recycled aggregates from U.S. 23 and 1-75 sources didnot differ drastically. Size 17A U.S. 23 aggregate seemed tohave the smallest loss in the process. Natural aggregate wascrushed stone, and that led to a relatively large loss (com-pared to that of the recycled aggregates) for the natural ag-gregate. However, the L.A. abrasion loss percentage for the1-75 aggregate was about l.5 times that of the aggregatefrom U.S. 23. This may be an indication that a larger amount

of mortar was attached to the aggregate from 1-75 that had asmaller coarse-to- fine aggregate ratio and higher cement fac-tor in the original concrete when compared to aggregatefrom U.S. 23. Therefore, under the stresses caused by the im-pact loads applied by the L.A. abrasion machine, largeramounts of mortar were released from 1-75 aggregate.There was another major difference between aggregates

from the two different sources. The water absorption of 1-75 aggregate was between 1.5 and 2.25 times that of the ag-gregate from U.S. 23, depending on the size of the aggre-gates. This further supports the assumption that 1-75recycled aggregate had more mortar attached to the surfaceof the original aggregate. The aggregates were mixed in con-crete in a saturated surface dry condition.In general, aggregate from the U.S. 23 project showed

properties closer to those of natural aggregate when com-pared to aggregate from the 1-75 project.



Table 7-Properties of natural and recycled aggregates

Source of aggregate Natural aggregate U.S. 23 aggregate 1-75 aggregate

MOOT gradation 6A 17A 6A 17A 6A 17A

Maximum aggrega te size, in. 1.0 0.75 1.0 0.75 1.0 0.75

Unit weight, Ib/ft 3 100.8 98.7 82.54 83.33 84.92 85.65

Unit weight, ssd, Ib/ft3 99.70 96.79 79.68 79.76 79.65 79.23

Bulk specific gravity, dry 2.67 2.58 2.37 2.31 2.22 2.09

Bulk specific gravity, ssd 2.69 2.63 2.45 2.41 2.37 2.26

Apparent specific gravity 2.75 2.72 2.59 2.57 2.61 2.51Voids, percent by weight 39.29 38.54 44.00 42.02 38.66 34.09

Absorption, percent by weight 1.11 1.97 3.60 4.48 6.62 8.10

Loss after 1.2 hr dry mix ing , *percent by weight

6.82 3.44 4.53 3.20 4.71 4.45

L.A. abrasion loss, percent by 22.87 22.87 28.72 26.40 42.69 41.67weight

* Percent smaller than #40 steve (smallest size in Table 5) for 17A and #4 SIeve (smallest size m Table 5) tor 6A aggre-gates after 1/2 hr dry mixing in drum mixer (percent of aggregate weight before dry mixing) .

Note: I in. = 25.4 rnm; I Ib/ft] = 16.02 kg/rrr',

Table a-Standards for testsTest ASTM standards

Sieve ana lysis o f aggregate, fine and coarse

Unit we igh t and voids in aggregate

Specific gravity and absorption of aggrega te , coarse and fine

Total moisture content of aggregate, drying method

Resistance to degradation of coarse aggrega te by abrasion in L.A. machine

Making and curing concrete in laboratory

Compressi ve strength of cylindrical concrete specimens

Spli tt ing tensile strength of cylindrical concrete specimens

Flexural strength using third-point loading

Table 9- Test specimens and equipment

C 136

C 29

C 127. C 128

C 566

C 131

C 192

C 39

C496

C 78

Rate of loading Shape and size of specimens Number of specimens

Test 10 to 12 Iblmin Cylinder, d = 3 in. , L = 6 in. 4

Splitting tensile 10 to 121b/min Cylinder, d = 6 in. , L = 12 in. 3

FlexureDisplacement control,

Prism, 4 in.* 4 in., L = 12 in. 4UIOOO = 0.012 in.lmin

Note: I m. = 25.4 mm; I Ib = 4.448 N.

TEST PROCEDURESAll test procedures conformed to ASTM standards as indi-

cated in Table 8. 2 Table 9 shows the size of the specimensand loading rates for different tests.

CONCRETE MIX DESIGNWater-cement ratio was considered at two different levels:

0.3 and 0.4. The cement used was Type I ortland cement, andthe cement contents for lower strength concrete (water-cementratio of 0.4) were 530 lb/yd'' with 0.75 in. (314 kg/rrr' with 19mm) and 510 lb/yd ' with 1 in. (303 kg/nr' with 25.4 mm) topsize aggregates. The corresponding values were 700 and 675lb/yd ' (415 and 400 kg/m ') for higher strength concrete (wa-ter-cement ratio of 0.3). Coarse (recycled or natural) aggre-gate and natural sand were used in equal volumes. Recycledconcrete contained 100 percent recycled coarse and 100 per-cent natural fine aggregates. About 0.1 percent by weight ofcement of an air-entraining agent conforming to ASTM C250 was used in all mixes.

TEST RESULTS AND ANALYSISFig. I through 6 compare the mean values of the compres-

sive, splitting tensile, and flexural strengths of the natural ag-gregate concrete to those of the recycled aggregate concretefor each level of water-cement ratio, dry mixing time, andaggregate top size.Test results for 28-day compressive, split ting tensile, and

flexural strengths are shown in Fig. 7 through 9 for all the ag-gregate sources and the two levels of water -cement ratio, drymixing time, and aggregate top size. The 95 percent confi-dence intervals are also shown in the figures. The test dataand statistical analyses of the results for each of the threestrength types are discussed in the following.

Compressive strengthTables 10 and II show the significance of different vari-

ables and their interactions in determining the 28-day com-pressive strength of concrete. A key observation in thesetables is that various parameters (i. e., aggregate source, ag-



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10000

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I800 _ ,

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I5/ I o,

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1I CI l 600s: 6000 I l z

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200g 2000l

o ~ __ ~ ~ ~ -L__~

o 2000 4000 6000 8000 10000

o "---------------___J--- _ _I

o 200 400 600 800 1000

Compressive Strength. US23 Recycled ,,5:

Fig. I-Mean value compressive strengths of natural versusU.S. 23 recycled aggregate concretes (1 psi = 6895 Nlm2)

Splitting Tensile Strength. 115 Recycled PSI

Fig. 4--Mean value splitting tensile strengths of natural ver-sus 1-75 recycled aggregate concretes (1 psi = 6895 Nlm2)

1000e

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~s: 6000

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o L_ __ ~ ~ -L _L ___

o 2000 4000 6000 8000 10000o L . . . . . . . - - - ' - - - - - ' - - - - - ~ - - - ' - - - _ _ ,o 200 400 600 800 1000

Comoressive Strength. 115 ReCYCled.psi FlexuralStrength. US23 psi

Fig. 2-Mean value compressive strengths of natural versus1-75 recycled aggregate concretes (1 psi = 6895 Nlm2) Fig. 5-Mean valuejlexural strengths of natural versus U.S. 23

recycled aggregate concretes (1 psi = 6895 Nlm2)

Splitting Tensile Strength. US23 Recycled PSI

C ii 800c,

16'il; 800

~ a.CI l 600

a5

s 2 '5Ci i

a;600s Z

f- 400 s:

co 2':§ ~" " 400-n (J)

(J)

200 Ci5x~u, 200

ig. 3-Mean value splitting tensile strengths of natural versusU.S. 23 recycled aggregate concretes (1 psi = 6895 Nlm2)

Fig. 6-Mean valuejlexural strengths of natural versus 1-75recycled aggregate concretes (1 psi = 6895 Nlm2)



3

2

1

a 0 ~ 0 ~ 0 "! 0 ~• " " , II , II II: : : i i i : : : i i i : : : i i i : : : i i i : : : i i i : : : i i i : : : i i i : : : i i ia a 0 0 0 a 0 a

"l "l o t: o t: "l "l 'I; ":• " • • II n • •o o o o o o o

3: 3: 3: 3: 3: 3: 3: 3:

~ o n ~ o n Ii Ii. . ~

": ": (J)

'" •, II • « '" « '"J)

'" '" (J) « «« « « «

US23 O rig in al C o n cre te

1<95Original C oncrete

Compressive Strength, ksi10~--------------------------------~9 _ _ .

8

7

6

5

4

• Natural

D Recycled US23

e z 1Recycled 1-75

AS = Aggregate Size, inWC = Water - Cement RatioOM = Dry Mixing Time, hr

Fig. 7-Compressive strength of concrete at 28 days (means and 95 percent confidence intervals; 1 in. = 25.4 mm, 1 ksi =6.895 MPa)

Tensile Strength, psi1,000,----------------------------~900 _._ - .

800700600500400

300200

100a "!

": : i i i0

"l "l ": ": "l <'l ": ":• , • • • , ,"o o o

3: o 3: o 3: o3: o

3: 3 : 3 : 3:

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1 :] 5 O ri gi na l Conc re te

• Natural

[] Recycled - US23

IZ 1Recycled - 1-75

AS = Aggregate Size, in

WC = Water - Cement RatioOM = Dry Mixing Time, hr

Fig. 8-Splitting tensile strength of concrete at 28 days (means and 95 percent confidence intervals; 1 in. = 25.4 mm, 1 ksi= 6.895 MPa)



Flexural Strength, psi1,100,- _--- - - -1 ,000900800

700600500400300 I

200100

0 "1 "1» n:; :;0 0

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l IDNaturalD Recycled US23

r z aRecycled - 1 - = 7 5

AS = Aggregate Size, in

WC = Water· Cement RatioDM = Dry Mixing Time, hr

Fig. 9~Flexural strength of concrete at 28 days (means and 95 percent confidence intervals; I in. = 25.4 mm, I ksi = 6.895MPa)

Table 10-Significance of different variables andtheir interactions from statistical analysis of all

three aggregate sources in determining 28-daystrength

Compressive Splitting tensileVariable strength strength Flexural strength

I * * *AS - * *wlc * * *DM * t -

I*AS * * :j :

I*WC * * *I*DM * * *AS*wlc * t *AS*DM - * *wlc*DM - * -

I*AS*w/c * * *I*AS*DM j- * -

I *w!c*DM * :j : tAS*wlc*DM * - *I *AS*w!c*DM * * -

Note: I:::: source of aggregate; AS:::: aggregate Size; ~t'k = water-cement rauo:DM = dry mixing time.

" 'Significant at 98 percent level of confidence.t Signif ican t a t 95 to 98 percent leve l of conf idence.~ Signif ican t a t 90 to 95 percent leve l of conf idence.

gregate top size, water-cement ratio, and dry mixing time)generally show strong interactions, Therefore, the effect ofeach of these variables would be different at different levelsof other variables.

Table 11-Significance of different variables andtheir interactions of 28-day compressive strength

AS :j :

Aggregate source Natural U.S. 23 1-75

wlc

DM

AS*wlc

AS*DM

w!C*DM

AS *w/c*DM

*

*

* *

* * *

*

* *AS:::: aggregate Size; I 'v ic : :: :water-cement rauo; DM ::::dry nuxing time.* Significant at 98 percent level of confidence.t Signil ican t a t 95 to 98 percent level of conf idence.:j :Significant at 90 to 95 level of confidence.

Fig. 7 indicates that the U.S. 23 recycled aggregate con-crete generally had higher compressive strength than boththe control and I-75 recycled aggregate concretes. This wasparticularly true when the water-cement ratio was low andthe resulting strength was larger than the strength of the I-75original concrete. The better performance of the U,S. 23 re-cycled aggregate was generally confirmed statistically at a 95percent level of confidence. Dry mixing does not change thestrength of U.S. 23 recycled concrete significantly, but it seemsto be weakening concretes made using natural or I-75 recycled

aggregates, However, the statistical level of confidence forthe effect of dry mixing is different for different values of othervariables. For identical water-cement ratios, the I-75 recycledaggregate seems to perform more poorly than natural aggre-gate when the strength of control concrete is higher than that of

A'-'" 11."_.1._ • • :_1_ 1_ ..



the original concrete in the 1-75 project, and better when thestrength of control concrete is lower than that of the original con-crete. This conforms well to the findings of others. I Dry mixingincreases the gap between the strengths of control and 1-75recy-cled aggregate concretes since dry mixing weakens controlconcrete more at the higher water-cement ratio and weakens 1-75aggregate concrete more at the lower water-cement ratio.The fact that recycled aggregate concrete may behave dif-

ferently has also been observed by other researchers. They

found that the strength of recycled aggregate concrete couldbe lower than that of corresponding control concrete madewith natural aggregate at the same water-cement ratio whenthe strength of such control concrete exceeded the strengthof the original concrete from which the recycled aggregate isoriginated.' The weaker bond between the old mortar and theactual stone particles in the recycled aggregate seems tocause this behavior. Compared to control concrete havinglower strength than the original concrete, however, recycledaggregate concrete performs well. This is confirmed in thisresearch work even when the 28-day strength control con-crete is compared with the long-term (over 30 years) strengthof original concrete.The effect of aggregate size on compressive strength is not

statistically significant for natural aggregate and U.S. 23 re-cycled aggregate concretes due to the fact that there is not asignificant difference between the two maximum aggregatesizes. However, even at this level of difference between thetwo aggregate top sizes, size plays a significant role for the1-75 recycled aggregate concrete at low water-cement ratioand with dry mixing. This can be attributed to the mortarpaste adhered to the stone particles in the recycled aggregate.Previous research has shown that the size of recycled coarse

aggregate is a critical factor in determining the amount ofmortar attached to the stone particles, with the larger sizehaving less adhered mortar. I Therefore, the smaller size(17A) recycled aggregate loses a larger percentage of ad-hered mortar than the bigger size (6A) recycled aggregate.This leads to a larger difference in the actual stone particlesize between the two sizes of recycled aggregates. On theother hand, the U.S. 23 recycled aggregate had a largeramount of coarse stone particles in the original concrete thanthe 1-75 recycled aggregate, leading to less adhered mortarand closer actual stone particle top sizes for the two differentaggregate maximum sizes, and a behavior that is closer tonatural aggregate in terms of aggregate top size insignificanteffect on the compressive strength of the U.S. 23 recycledand control concretes.The lower amount of coarse to fine aggregates and the

larger cement factor in the original 1-75 concrete mix leadsto a higher amount of mortar attached to the actual stoneparticles in the 1-75 recycled aggregate. This, in turn, pro-duces higher L.A. abrasion loss and water absorption forthe 1-75 recycled aggregate when compared to the U.S. 23recycled aggregate.

Splitting tensile strengthFactorial analysis of variance of the splitting tensile

strength test results was indicative of strong interactions be-tween different variables (see Tables 10 and 12). Fig. 8 indi-cates that splitting tensile strength may be affected

ril 1 Qa~

Table 12-Significance of different variables andtheir interactions of 28-day splitting tensilestrength

Aggregate source Natural U.S. 23

AS

wlc *DM :j :

AS*wlc ~:

AS*DM *wlc*DM

AS*wlc*DM * t

1-75

*

*

Note: AS = aggregate size; ~vk = water-cement rauo; DM = dry mixingtime.

* 'Significant at 98 percent level of confidence.'i' Signif icant a t 95 to 9Hpercent leve l o f confidence.:f Signif icant a t 90 to 95 percent leve l o f confidence.

differently from compressive strength by the variables con-sidered. At a 95 percent level of confidence, aggregate sizedoes not affect the splitting tensile strength of recycled ag-gregate concrete, but an increased top size of natural aggre-

gate leads to a higher splitting tensile strength except in thecase of dry mixing at low water-cement ratio. Dry mixingdoes not significantly change splitting tensile strength forany of the aggregates. The two recycled aggregates producedsplitting tensile strengths that were either higher than or sta-tistically comparable to that obtained using natural aggregate.For stone or gravel concrete, tests show that the splitting

tensile strength can be approximated by 6(fc.')1/2 psi,0.50(fc,')1/2 MPa, where I e ' is the 28-day compressivestrength in psi, MPa. 4 Table 13 compares this relationshipwith the splitting tensile strengths obtained in the present re-search. The established relationship seems to work better forrecycled aggregate concrete with lower water-cement ratios.The empirical relationship seems to overestimate the split-ting tensile strength for recycled aggregate concrete of high-er water-cement ratio and for natural aggregate used in thisinvestigation at all water-cement ratios.

Flexural strengthResults of the factorial analysis of variance of the flexur-

al strength test results are presented in Tables 10 and 14, inwhich strong interactions between different variables areobserved again. As shown in Fig. 9, the U.S. 23 recycledaggregate of smaller top size generally leads to higherstrength than the 1-75 recycled aggregate. Compared to thenatural aggregate, the U.S. 23 recycled aggregate per-formed better at the higher water-cement ratio, but it per-formed worse at the lower water-cement ratio except whendry mixing was applied.ACI Code 318 5 expresses the flexural strength (modulus

of rupture) I. by the following equation

L> 7.5 f+c')1/2 . F" .VI PSI, .Ic III pSI

j,r=0.62 (F(.,)1/2 MP F" MPVI a,jc III a

Table 13compares the values obtained using this equationand the values obtained from the tests. The relationship isvery conservative for natural aggregate concrete, especially

ion



Table 13-Splitting tensile and flexural strength versus compressive strength for concretes withaggregates from different sources

Natural aggregate U.S. 23 recycled aggregate 1-75 recycled aggregate

Aggre~ate size, Water-cement Dry mixing Splitting tensile Flexural Splitting tensile Flexural Splitting tensile Flexuralm. ratio t ime, hr strength* strength+ strength* strength+ strength* strength+

0.75 0.3 0.0 -27.9 +52.6 -6.7 -25.4 -9.0 -27.3

0.75 0.3 0.5 -32.7 +54.3 -2.4 -21.9 -4.5 -23.6

0.75 0.4 0.0 -31.4 +14.7 -37.4 -49.9 -11.7 -29.3

0.75 0.4 0.5 -17.2 +16.0 -16.2 -32.9 -28.4 -42.7

1.0 0.3 0.0 -14.2 +37.5 -1.9 -21.5 -2.7 -22.1

1.0 0.3 0.5 -15.6 +24.4 -12.1 -29.6 -11.0 -28.9

1.0 0.4 0.0 -13.9 +13.1 -19.9 -36.0 -28.6 -42.9

1.0 0.4 0.5 -13.4 +13.4 -23.0 -38.5 -22.9 -38.3

*Measured-6 ifc.') 1/2, percent.

+ Measured-7.5 U;:)1/2, percent.

Note: I psi = 6895 N/m2

Table 14-Significance of different variables andtheir interactions of 28-day flexural strength

Aggregate source U.S. 23 Natural 1-75

AS t :j : tw/c :j: :j: :j:

DM :j : t :j :

AS*w/c :j : ~ :j :+AS*DM :j : - -

w/C*DM :j: - -

AS *w/c*DM :j : - -

AS = Aggregate size; wlc = water-cement ratio; DM = dry rmxmg time.* Signif ican t a t 90 to 95 percent level of conf idence.t Signif ican t a t 95 to 98 percent level of conf idence.:j: Significant at 98 percent level of confidence.

at the lower water-cement ratio. For recycled aggregate con-crete, the test values are less than the ACI values, and the dif-ference is larger at a higher water-cement ratio.

CONCLUSIONS1. If the compressive strength of the original concrete that

is being recycled is higher than that of the control concrete,then the recycled aggregate concrete can also be made tohave higher compressive strength than the control concrete.

2. Increased L.A. abrasion loss and water absorption ca-pacity of recycled aggregates, which partly reflect the in-

creased amount of mortar adhered to original stoneaggregate, generally lead to reduced compressive strength ofrecycled aggregate concrete.

3. Splitting tensile and flexural strengths of recycled ag-gregate concrete can be higher or lower than those of the nat-ural aggregate concrete, depending on water-cement ratioand dry mixing period.

4. Effects of dry mixing and recycled aggregate top size onthe strength of recycled aggregate concrete depend on the ra-tio of the top size of the original stone particles in the originalconcrete to the top size of the recycled aggregate, the coarse-

to-fine aggregate ratio in the original concrete, the cementcontent of the original concrete, and the water-cement ratioof the recycled aggregate concrete.

S. Conventional relationships established between split-ti t il d fl l t th d th i

strength of natural aggregate concrete are generally uncon-servative in application to recycled aggregate concrete.

6. The qualities of original concrete seem to restrict the

qualities achievable in recycled aggregate concrete. Howev-er, the complex effects and interactions of various variablesmake it difficult to come up with specific predictions regard-ing the behavior of recycled aggregate in concrete withoutconducting tests under applicable circumstances.

7. As far as strengths are concerned, the basic trends in be-havior of field-demolished concrete aggregate are not signif-icantly different from those of the laboratory-made recycledconcrete aggregate. The major difference between the twocases is that many different variables such as the mix propor-tions and the aggregate gradation in the original concrete areinvolved in the field-demolished concrete that cannot bechanged during the recycling process. In demolishing thelaboratory-made concrete, the properties of the original con-crete can be controlled.

ACKNOWLEDGMENTThe authors wish to thank David Smiley of the Michigan Department of

Transportation, Jack Kzeski of Interstate Highway Construction, Mark

Johnson of Ajax Paving, Mike Gleeson of Angela Iafrate , Gerald J. McCarthy

of the Mich igan Concrete Paving Association, and Siavash Ravanbakhsh of

the Department of Civil and Environmental Engineering at Michigan State

University fo r their technical assistance. The support o f the Department ofCiv il Engineering at Sharif University of Technology, Tehran, Iran, and the

Department of Civil and Environmental Engineering at Michigan State Uni-

versity is gratefully acknowledged.

REFERENCES1. "Recycling of Demolished Concrete and Masonry," T. C Hansen, ed.,

Report 6, International Union of Testing and Research Laboratories for

Materials and Struc tu res, 1992 , 316 pp.

2. "Concrete and Aggregates," 1993 Annual Book of ASTM Standards,

Section 4, V. 04.02, American Society for Testing and Materials, Philadelphia.

3. Hansen, T. C, and Narud, H., "Streng th of Recycled Concre te Made

from Crushed Concrete Coarse Aggregate," Concrete International-Designand Construction, V. 5, No. I, Jan. 1983, pp. 79-83.

4. Leet, K. , Reinforced Concrete Design, Second Edition, McGraw-Hill,

Inc., 1991.

5. ACI Committee 318, "Building Code Requirements for Reinforced

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Strengths of Recycled Aggregated Concrete