Aggregate shape’s effect on concrete quality.
By Frank Bowen
Editor’s Note: This is the second article in a year-long series that explores the science of concrete to provide a better understanding of mix design. The series will be collaboratively written by Paul Ramsburg, technical sales specialist at Sika Corp., and Frank Bowen, quality control manager at Piedmont Precast. Click here for the first article in the series or here to read the third article.
Precasters are economically limited to readily available aggregates from their local quarries and stock yards. Understanding the characteristics of these materials allows producers to sharpen their mix designs. By focusing on the physical properties of concrete aggregates, precasters can achieve improvements in concrete workability and paste-aggregate bond.
This starts with collecting some key information about the available aggregates. Producers should first examine the shape of their aggregates and (possibly using a microscope) determine the surface texture quality. This visual inspection allows the producer to gain insight into the aggregate’s ability to bond with the paste. Next, it is essential to review a current copy of the gradation analysis for all fine and coarse aggregates. Lastly, determine the total volume percent used by coarse aggregates by gathering the results of testing the dry rodded unit weight known as bulk density. DRUW is determined by compacting dry aggregate into a container of a known specific volume as described in ASTM C29, “Standard Test Method for Bulk Density (“Unit Weight”) and Voids in Aggregate.” The weight of the aggregate is measured and divided by the volume of the measure used to yield the DRUW. Results are expressed as mass per volume. For example, a given crushed limestone could weigh 93 lbs./ft.3. By knowing the DRUW of a coarse aggregate, the maximum nominal aggregate size and the fineness modulus of the fine aggregate, the weight of the coarse aggregate needed per cubic yard of concrete mix can be estimated.
Aggregate shape plays a key role in the function of freshly mixed concrete workability. According to American Concrete Institute, there are five categories of shape that describe concrete aggregates: angular, rounded, flaky, elongated and flaky/elongated. Aggregates with a round shape, such as sand and gravel from beaches or rivers, typically have a low percentage of void spacing and a low surface-to-volume ratio. Rounded aggregates demand less cement paste to produce a workable mix due to a reduction in frictional resistance. However, rounded aggregates are not typically considered suitable for high-strength concrete designs because of their poor interlocking behavior and weaker bond strengths.
Crushed stone, on the other hand, has better bond characteristics with the cement paste, which can help develop higher strengths. Crushed stone is less likely to segregate during handling and placing, but may require an increased use of admixture dosages to achieve the desired slump or spread. When using angular aggregates, it is also important to know that more air may be entrapped in the fresh concrete during mixing and placing. This can be resolved through attentive and careful placing and proper consolidation.
Aggregates with angular shapes typically have a higher percentage of void spacing between them and produce less workable concrete than rounded aggregate. Water demand is higher because of higher friction resistance and greater surface area-to- volume ratios, resulting in the need for more cement to maintain the desired water-cement ratio. Angular aggregates are preferred for manufacturing high-strength concrete because of an improved bond between the aggregate and paste.
The types of aggregates considered unsuitable for concrete mixing are flaky, elongated or a combination of both. Flaky aggregates tend to be aligned in one plane, thereby causing issues with concrete durability. Aggregates are considered flaky when the smallest dimension of the aggregate is less than the 60% of its mean dimension. That is, when the thickness of the aggregate is compared with its length and width. Slate is an example of a flaky aggregate. When the length of aggregate is greater than 180% of its mean dimension (thickness and width combined), then it is considered elongated. Aggregate is deemed flaky and elongated when it satisfies both the previously mentioned conditions. Elongated or flaky particles exceeding 15% should be considered unsuitable for concrete use.
When considering the combined gradation for the aggregates, a compromise between workability and economy is necessary to produce a suitable mix. Typically, the benefits of using crushed aggregates outweigh the advantages of rounded aggregates when considering bond and strength. The needed amount of cement paste is dependent upon the amount of aggregate voiding that must be filled and the surface area that must be covered. With poorly graded aggregates, the voiding is the greatest. The more these voids are filled, the less workable the concrete becomes. Proper grading of coarse aggregates is important to achieve dense and interconnected packing. The voids left by larger particles are filled by smaller particles, thereby reducing the possibility of segregation and improving the ability to compact the concrete. Once all of this is addressed carefully, it is then time to test batch and determine conformance to the concrete’s required physical specifications.
For further reading on the effects of aggregate shape and gradation, ACI 238 provides an in-depth review of workability and rheology of fresh concrete. In addition, when testing aggregates, refer to ASTM C33, ASTM C136 and ASTM C29.
Frank Bowen, a 2013 Master Precaster graduate, received his M.B.A. from Middle Tennessee State University through the Concrete Industry Management graduate program in 2014 and is the director of quality assurance at Piedmont Precast in Atlanta, Ga.