1. Can 10,000 psi Concrete be made from 100% 3000 psi Recycled Concrete Aggregate?
Allyn Luke
New Jersey Institute of Technology
October 29, 2014
Presentation to ACI Session on Use of Recycled Materials in Concrete

2. Sustainability in the Concrete Lab
Year after year students study ACI 211 design
Every semester 3 to 4 classes make 100 cylinders
2000, 3000, 4000, 5000, 6000 psi mixes made and tested
We make almost 1 cubic yard a semester just in the basic ACI 211 design labs
4 to 6 pick-up trucks a year

3. Save Time and Money by Calling the Trash RCA
We know how strong the cylinders were
Usually, the crumbled cylinders are trashed
Why not reuse them to make more concrete?

4. Two Questons
Can we make 10,000 psi concrete from 100% recycled concrete aggregates?
Can we make 10,000 psi concrete from 100% recycled 3000 psi aggregate?

5. Expectations Probably not
Expect 3000 psi RCA would be lower than 6000 psi RCA would be lower than psi RCA would be lower than natural aggregate concrete

6. Experimental Concept Collect concretes with f’c less than 3000 psi
Collect concretes with f’c over 6000 psi
Collect WTC concrete f’c over 12,000 psi
Crush to broken cylinders to NMSA 1-inch
Sieve analysis of 1-inch natural stone
Screen RCAs
Recombine RCA to match natural gradation

7. 10,000 psi Concrete Design
Done for various student and research projects
w/cm 0.3
Include microsilica
And polycarboxolate admixture

8. Experimental Concept Cont.
Measure relative density (sg) of natural stone (2.79)
Measure relative density of RCA (2.5 to 2.6)
Replace natural stone on a volumetric basis
Mass of natural aggregate multiplied by rdRCA/rdNA
Mix natural aggregate at SSD
Soak oven dry RCA ½ hour and mix

9. 28-day Results What’s the difference?

10. Student’s Conclusion
“Although the compressive strength obtained didn’t achieve the target strength; but it so close to the target strength which proves that with the right concrete mix proportions and the right aggregate gradation, the target strength can be achieved.”

11. Testing at 2.75 years Found HS RCA cylinders cleaning out curing room
Heading for Trash
Why not test anyway
Too few specimens to form definitive conclusions
Labels on some specimens could not be read

12. Testing Results

13. Can 10,000 psi Concrete be Produced from 100% 3000 psi RCA
YES

14. Why?
Conjecture:
The load path follows the stiffest elements. In concrete the stiffest element is presumed to be the aggregate. Aggregate to aggregate contact is the ideal but can not be assumed. Therefore, it would appear that for HSC the paste is stiffer than the aggregate and the load is transferred through the paste and that the strength of the aggregate carries relatively less load.

15. Limitations More time needed to gain full strength
Study done with uncontaminated RCA
Most RCA is contaminated

16. Advantages
In the academic concrete lab RCA from broken cylinders can reduce the material costs and effort of maintaining aggregate stocks
The study shows that if RCA can be adequately cleaned it can be used like natural aggregates (absorption is higher and must be considered)

17. Questions?

18. Thank You for Your Attention
Questions?