Consideration of aggregate’s effect on concrete properties is vital to manufacturing quality precast products.
By Eric Carleton, P.E.
High-quality concrete mixes depend on a variety of factors. Attention to detail, proper proportioning and thorough quality control are all important, but at the core of the process are the materials used. Aggregate, defined by the American Concrete Institute as “granular material, such as sand, gravel, crushed stone, crushed hydraulic cement concrete, or iron blast-furnace slag, used with a cementing medium to produce either concrete or mortar,” is a vital component of every concrete mix. And with the aggregate composition in a mix constituting approximately 60% to 75% of the volume, employees must take care to ensure that the aggregate used possesses the proper characteristics for a successful end product.
The importance of aggregates in mix design
When developing a mix design, the precaster must first determine the required properties of the mix. The precaster may base initial mix proportions for conventional wet-cast concrete on the scientific method detailed in ACI 211.1-91, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete.” During this initial design phase, the precaster should consult with aggregate suppliers to ensure that the gradations, shapes and specific gravities of the course and fine aggregates selected are available. If the precast products will be installed for department of transportation work, the precaster should also confirm DOT approval of the selected aggregates.
The next step is to run trial batches of the initial mix design with the intent to modify and revise proportions to meet product specifications – including compressive strength, air entrainment, absorption, etc. – and specific production practices such as slump, early strength and surface finish.
When an issue arises during these phases, the manufacturer will often attribute it to the cement, fly ash or admixture used. However, these components are typically governed by strict QC and most times are not the source of the problem. But if the issue can be narrowed down to mixing or batching, the most likely cause will be variation within the aggregates.
When aggregate properties vary from the intended mix design, the concrete can be affected in dramatic ways, altering workability, air content, hardened properties, finish, density and more. And even though the majority of precasters understand the importance of aggregates for concrete strength and durability, failure to meet aggregate testing requirements remains an issue for some. As a result, thorough aggregate inspection and testing is necessary for a quality finished product.
Inspection is key
QC managers must inspect and verify that the aggregate used complies with the final mix design. This verification process may occur once per day, during delivery, or more frequently based on the precaster’s history with the supplier.
Visual inspection should confirm that aggregate size, shape, coarseness and color is consistent with the final mix design. Precasters should also ensure deleterious material is not introduced into the loads.
A good practice is to pick up aggregate and run it through your fingers. Then, ask yourself, “How dirty are my hands?” Too much dirt will create a weak paste layer around the aggregates in the mix. Additionally, loader operators who charge the batch plant from aggregate stockpiles should avoid contaminating it with the soil where piles are stored.
The QC manager should also verify that the aggregates furnished meet the quality requirements described in ASTM C33 / C33M, “Standard Specification for Concrete Aggregates,” and are within the acceptable limits established for gradation. The precast manufacturer must secure certification that the aggregate supplier has tested the materials and that they are in accordance with the provisions of ASTM C33 / C33M. If the precaster is an NPCA Certified Plant, it should renew certification at least annually and any time when an aggregate source is changed.
It should be noted that many precast concrete product standards include the text, “Aggregates shall conform to ASTM C33 / C33M, except that the requirements for gradation shall not apply.” This does not mean that the precaster should ignore gradations. Instead, this permits the precaster to optimize mix designs by selecting aggregate gradations and blends outside of those included in the standard.
Consider closely examining your gradations and optimizing to improve economy and performance. Establish acceptable limits for each sieve used within the gradation test. Gradation limits that are too large could significantly alter the characteristics of fresh concrete and the performance of hardened concrete.
NPCA Certified Plants should perform gradation testing at least once per 1,500 tons of fine aggregate used, once per 2,000 tons of course aggregate used or once per month, whichever comes first. The plant should conduct these tests in accordance with ASTM C136 / C136M, “Standard Test Method for Sieve Analysis of Fine and Course Aggregates.”
The best means to verify furnished aggregate gradations and to comply to minimum QC testing requirements is to invest in soil sieves, shakers, and scales, and to train staff so that testing can be conducted in house. This provides an opportunity to perform gradation testing at a greater frequency, enabling the development of valuable mix design data for better troubleshooting. It also enhances employees’ knowledge of aggregates, allowing them to better spot variations before they become a larger problem. With a sound technical background, employees can also engage in more fruitful conversations when discussing mixing and batching issues with the aggregate supplier.
Don’t let a failure to meet aggregate testing requirements prevent your plant from performing at its best. Taking the time to ensure proper inspection and testing procedures are in place will go a long way to improving your mix design and finished products.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He also is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13, Concrete Pipe.
- American Concrete Institute CT-13, “ACI Concrete Terminology”
- Additional guidance and information on this process can be found in ACI E-1(16), “Aggregates for Concrete,” ACI 221R-96, “Guide for Use of Normal Weight and Heavyweight Aggregates in Concrete,” and the Portland Cement Association’s Design and Control of Concrete Mixes.