Gap-graded Aggregate

Concrete, Materials

In brief, gap-graded aggregate has one or more intermediate particle-size fractions missing or present only in a very small amount. As a result, on a cumulative percent-passing curve, the missing range appears as a horizontal or nearly horizontal section.

In contrast, the term continuously graded describes a blend with particles distributed through the intermediate sizes. For this reason, gap grading is also called discontinuous, skip, or jump grading.

Quick distinction: however, gap-graded aggregate is not automatically no-fines or pervious concrete. In fact, a gap-graded blend may contain both coarse aggregate and sand while omitting only a band of intermediate sizes.

How to recognize a gap in sieve data

First, aggregate grading is determined by sieve analysis. Next, the cumulative percentage passing each sieve is plotted against sieve opening. Therefore, if the percent passing does not change over several adjacent sieves, no particles were retained between those sieve sizes.

Illustrative sieve analysis curves showing a horizontal plateau for missing intermediate sizes in gap-graded aggregate
Illustrative grading curves. The plateau from 4.75 to 1.18 mm indicates that those intermediate fractions are absent; it is not a specification envelope.

Worked gap-grading example

Sieve opening Cumulative percent passing Percent in interval above the next smaller sieve
20 mm 100% 100 – 70 = 30%
10 mm 70% 70 – 28 = 42%
4.75 mm 28% 28 – 28 = 0%
2.36 mm 28% 28 – 28 = 0%
1.18 mm 28% 28 – 18 = 10%
0.60 mm 18% 18 – 10 = 8%
0.30 mm 10% 10 – 4 = 6%
0.15 mm and pan 4% 4%

In this example, the 4.75, 2.36, and 1.18 mm sieves all show 28% passing. Therefore, the sample contains no particles between 4.75 and 1.18 mm. As a check, the interval percentages add to 100%: 30 + 42 + 0 + 0 + 10 + 8 + 6 + 4 = 100.

For the full laboratory method, see ASTM C136 dry sieve analysis. However, fineness modulus is a separate index; it cannot show the location of a grading gap by itself. For the same reason, see how to calculate fineness modulus.

Gap-graded vs continuous, uniform, and open grading

Grading type Particle-size pattern Typical curve Important note
Continuous or well graded Sizes distributed through the range Progressive slope without a deliberate plateau In practice, this often supports stable packing, but performance still depends on shape, texture, paste, and proportions.
Gap graded One or more intermediate fractions omitted Horizontal/nearly horizontal segment However, the gap can be intentional or can result from blending available stockpiles; segregation sensitivity must be checked.
Uniform or single sized Most particles near one size Steep change over a narrow range As a result, it leaves larger interconnected voids than a broad continuous blend.
Open graded or no-fines Very little small aggregate/fines Low fine-end passing values Generally, it is used for intentional void structure in some materials; however, it is not synonymous with every gap-graded blend.

Why gap grading is used

In short, aggregate particles of a given size form voids. As a result, much smaller particles may fit into those voids without much interference from intermediate particles. Consequently, this packing idea led to intentional gap-graded blends, sometimes using a coarse fraction and a fine fraction while omitting the middle range.

Accordingly, depending on the materials and target concrete, a gap-graded blend may reduce the required proportion of fine aggregate, lower total particle surface area, or help an exposed-aggregate finish. In addition, it can simplify stockpile combinations. However, these are possible mix-design benefits, not guaranteed savings.

Effects on fresh and hardened concrete

Property Possible effect What to verify
Workability For example, some low-workability mixtures can achieve the target consistency with less fine aggregate; other blends feel harsh or become unstable. Trial batches using the actual aggregate shape, moisture, paste, and admixtures
Segregation However, missing intermediate sizes can make coarse particles and mortar separate during handling or vibration, especially at higher workability. Transport, discharge, placement, vibration, finish, and hardened uniformity
Pumpability As a result, an unstable or mortar-deficient blend can block, separate, or create pressure variation. Pump-line size/route, mortar volume, grading, viscosity, and a representative pump trial
Strength Still, no automatic gain or loss follows from the word gap graded. Specified strength, density, air, consolidation, curing, and trial results
Shrinkage and paste demand In some cases, lower fine surface area may reduce paste demand in an optimized blend, but poor packing can do the opposite. Actual void content, total paste, water-cementitious ratio, and shrinkage testing where important
Batch consistency Meanwhile, small stockpile or moisture changes can move the blend away from the trial proportions. Separate stockpile control, combined grading, moisture correction, and routine testing
Do not judge compliance from the label alone. For example, ASTM C33 and other aggregate specifications contain material and grading requirements, while project specifications may set additional combined-grading limits. Therefore, a gap-graded blend is not automatically acceptable or unacceptable; check the adopted specification and approved mixture proportions.

When a gap-graded blend needs extra care

  • For example, high-slump or high-flow concrete where segregation resistance is critical.
  • In addition, pumped concrete, long lines, reducers, or congested reinforcement.
  • Also, slip-form or paving work that is sensitive to edge slump and mixture stability.
  • Meanwhile, mixtures with variable sand grading or moisture.
  • Finally, projects attempting to reduce paste or cement without measuring voids and trial performance.

Trial-batch checklist

  1. First, run sieve analyses on each stockpile and calculate the combined percent passing.
  2. Next, plot the combined curve and identify the exact missing size interval.
  3. Then, check the adopted aggregate and project specifications.
  4. After that, measure fresh consistency, air, density/yield, temperature, segregation, and finishability as applicable.
  5. In addition, use the actual transport, placing, consolidation, and pumping method in a representative trial when those operations are critical.
  6. Also, test hardened properties required by the project, not strength alone.
  7. Finally, set stockpile, moisture-correction, and grading-control limits before production.

Legacy CEW figures

Examples of gap-graded aggregate curves with omitted intermediate sizes
Typical gap gradings retained from the original CEW article.
Historical comparison of workability and strength for gap-graded and continuously graded concrete mixtures
Historical comparison retained from the original article. It illustrates one material set and should not be generalized to every mixture.

For the broader role of size, shape, texture, moisture, and mineralogy, see how aggregate properties affect concrete.

References