Combined Aggregate Grading Why all the fuss? What are we talking about? Claims in balance. 1

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1 8 th Annual Concrete Pavement Workshop Utah ACPA, LTAP, ACI Combined Aggregate Grading Why all the fuss? What are we talking about? 1. Do well graded aggregate concrete mixes out-perform gap graded mixes? 2. Do current combined grading specifications result in better concrete? Claims in balance SSI, Home Depot, Walmart FHWA, ACPA, DOTs Shilstone CI Article NRMCA 1

2 Do well graded aggregate concrete mixes out-perform gap graded mixes? C Graded Mix Proportion coarse agg ACI

3 Coarseness Factor Chart 1990 s s Aggregate Distribution HAYSTACK - Optimum 3

4 Workability Zones WATER DEMAND RESULTS OF GAP GRADING 4

5 Colorado Bridge Deck Wisconsin Univ/DOT Research Optimized Aggregate Conclusion Compressive strength - plus 10 to 20% Air entraining agent - minus 20 to 30% Potential water demand - minus 20 to 30% Potential high spacing factor for air voids Reduced segregation and higher density following extended vibration Wisconsin DOT/Univ Effects of particle distribution on segregation 5

6 .45 Power Chart Benefits attributed to well graded aggregate mixes Increased strength Decreased shrinkage & cracking Decreased permeability Better workability, pumpability Decreased curling in slabs Less blistering & delamination Decreased air loss Better ride on pavements Decreased segregation library.modot.mo.gov/rdt/ reports/ri98035/rdt05001.pdf 6

7 8/18 Grading Spec Concrete producers respond. Concrete International March,

8 D340%20AGR%20report%20phaseA2.pdf NRMCA Findings 1. Water demand: Similar (67%) and higher (33%) of the cases 2. Bleeding water amount: Similar (75%) and higher (25%) of the cases 3. Strength: Similar (67%) and lower (22%) of the cases 4. Shrinkage: Similar (92%) and higher (8%) of the cases 5. Finishability: Better FR with higher coarseness factor and lower workability factor (about 68/33) and similar FR with intermediate coarseness factor and workability factor (about 60/35) 6. Segregation: Lower segregation with intermediate coarseness factor and workability factor (about 60/35) and similar or higher segregation with higher coarseness factor and lower workability factor (about 68/33) NRMCA Statements..none of these empirical approaches have been scientifically tested in the laboratory to see if it leads to maximum aggregate packing density or lower cementitious paste for a given amount of workability. The intermediate coarse aggregate (No. 8) was obtained from the same quarry as the larger coarse aggregates (No. 467, No. 57). This was done in order to keep the particle shape consistent and discount the influence that a different particle shape can have on test results. 8

9 Mischaracterization of well graded aggregates Problems with current well graded aggregate specs Don t t take into consideration particle shape and texture May require the use of 3, 4 or 5 aggregates (and bins) Some particle sizes may not be locally available May inhibit research into viable alternatives 8/18 Grading Spec 9

10 NRMCA Conclusions Based on the results of this study, it can be concluded that there is no assurance that a concrete specification that includes a requirement for WG through compliance with CF and/or 8-18 charts will lead to reduced mixing water content or lower shrinkage as is typically the goal with these controls on aggregate grading. NRMCA Conclusions The above conclusion does not mean that aggregate grading is unimportant for concrete performance. In summary CF and 8-18 charts are potential concrete mixture optimization tools. These should not be invoked as requirements in project specifications. NRMCA Acknowledgements The authors would like to thank Dr. Cesar Constantino and Mr. James Cook of Titan America, FL, and Mr. Roger Tate of Heidelberger Technology Center, GA for participating in the round robin test program 10

11 Do current combined grading specs result in better concrete? What have combined grading specs done for the industry? Diverted emphasis from just strength, slump and air content Forced the industry to recognize that changes in standards have affected concrete quality Focused attention on contractor needs Recognized the need for reduced water in concrete mixes Blind Men & the Elephant 11

12 Should combined aggregate grading be specified by the designer? No but until we have a better way to specify our needs, combined grading specs should continue