Well-Graded Aggregates vs. Gap-Graded Aggregates

June 19, 2019 Jay Shilstone

 

Well-Graded Aggregates vs. Gap-Graded Aggregates 
June 11, 2012 

Over the past decade a major “discussion” has arisen over the desirability of the use of well-graded aggregates (WGA) vs. gap-graded aggregates (GGA) in concrete. The discussion has centered on two areas: 1) the technical benefits of WGA vs. GGA and 2) the use of specifications to promote the use of WGA. 

This discussion began back in the late 1980s when my father, Jim Shilstone, and others, such as Jerry Holland, discovered that many current concrete mixes were gap-graded and that adding pea gravel to the mix would improve workability and strength, while reducing water demand and permeability. While many of the benefits reported were anecdotal, this concept was supported by almost all research, codes and standards which required the use of “well graded aggregate”.  

Re-discovery of the benefits of WGA led to a desire to create a specification for assuring that well-graded aggregate mixes were used in all projects. This led to the development of the 8-18 spec (no less than 8% and no more than 18% retained on each sieve), the Coarseness Factor Chart optimum zone (a trapezoidal zone on the Coarseness Factor Chart, developed in 1976 by my father) and the 0.45 power chart (often used for asphalt mix designs).  However, concrete producers who had to meet these specifications often had to add a 3rd or 4th aggregate to make their current aggregates work, which was a major burden on the industry in the U.S. 

Magazine articles began to appear that seemed to contradict the notion that WGAs outperformed GGAs.  In 2005 the article “Effects of Aggregate Grading on Drying Shrinkage of Florida Concretes”, by McCall, King  & Whisonant, appeared in Concrete International. The NRMCA published a 2-part research study on “Effect of Continuous (Well-Graded) Combined Aggregate Grading on Concrete Performance”. Both publications demonstrated that the specifications currently being used to promote the use of WGAs did not always result in concrete mixtures that out-performed GGAs.  While the publications’ primary conclusion was that the specifications currently in use did not guarantee better concrete, many readers concluded that the publications were trying to state that WGAs did not produce better concrete than GGAs. 

There are three primary difficulties in establishing the benefits of WGAs vs GGAs. The first is that aggregate shape and texture are not quantifiable with regard to establishing their impact on desirable aggregate gradings.  The Concrete International article compared gap-graded mixes against well-graded mixes that used a highly angular intermediate aggregate to fill the gap. These angular intermediate particles increased water demand to a point that it offset the benefit of the well-graded aggregate blend. The NRMCA study intentionally did the same thing in that they stated in the study that they wanted to avoid influencing the study with regard to particle shape. The problem was that the coarse aggregates studied were crushed, so the intermediates were crushed as well. The more angular the intermediate particle, the fewer of those particles should be used in the concrete mix. 

The second problem in this discussion is that there are no true definitions for “well-graded aggregate” and “gap-graded aggregate”. Early work by Weymouth and Ferret indicate that gap graded mixes are the result of 2 aggregate materials where the smaller aggregate is 1/10th the size of the larger aggregate. This is a totally different scenario than where there are multiple aggregate sizes and the difference between succeeding sizes is ¼ or 1/6. While the 8-18 spec and the 0.45 power spec evaluate a full range of sieves, neither takes into consideration particle shape and texture. The Coarseness Factor chart only utilizes the 3/8” and #8 sieves, and so can’t truly be said to determine whether a mix is gap-graded or not. It is primarily an indicator for mixes using standard ASTM materials and cannot be adequately used with non-standard materials, such as the fine sand found in Florida. 

The third difficulty in establishing the benefits of WGAs is that many of the beneficial properties that WGAs impart cannot be measured. The American Society of  Concrete Contractors, ASCC, has worked with the NRMCA on their “Prescriptive to Performance”, P2P, committee for at least 5 years, but has reached an impasse based on the industry’s inability to measure things like ”curling potential” or pumpability and using those criteria to develop concrete mixtures. Until recently there was no measure of segregation potential, a characteristic well-graded aggregate tend to mitigate. Even now there is only a measure of static segregation potential and not the potential for segregation found in a flowing concrete mixture. 

My opinion is a combination of the following statements: 

  • For a given set of materials there is an optimum combination of materials that will result in the best concrete performance based on a single characteristic. For example, the optimum combination for strength may not be the best combination for workability. 

  • With today’s production materials, the combination of materials that minimizes water demand will probably result in a broader quantity of particle sizes (WGA) and not a smaller quantity of particle sizes (GGA). 

  • The more angular the particle, the less of that size particle should be used in a concrete mix. 

  • An interlocking group of particle sizes, separated by a suitable quantity of “lubricant” (such as paste or mortar), will result in less shrinkage, cracking and permeability. 

  • While well-graded aggregate specifications may not result in the “best” combination of materials, the resulting mixture will typically be better than the gap-graded mixtures the industry produced in the 1970s – 1990s. Hence, combined aggregate grading specifications are a “good” thing. However, concrete producers should have a mechanism for developing better mixtures than the specs require and getting those mixtures approved. 

 

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