Volumetric batching can be advantageous for precast manufacturers.
By Eric Carleton, P.E.
When baking a cake or making pancakes, do you measure your ingredients by volume or weight? Or, in other words, in cups or grams? In many ways, batching baking ingredients is like batching raw materials for producing concrete. And like baking, concrete can be batched by either volume or weight. In the precast industry, the most common method is to batch by weight.
This method considers the specific gravities of all concrete components (cement, water, fine and coarse aggregate, admixtures and air) along with the corresponding weights of each to develop a perfect volume of one cubic yard. Initial quantities are based on design guides such as American Concrete Institute 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete,” and refined through trial batches.
Batching through the ages
Despite its prevalence today, batching by weight was not used until recent history when weighing technology and production advancements began enabling easier application of the practice. In the past, the production of concrete mixtures of cementitious materials, sand, rock and water were successfully batched using simple volumetric formulas and equipment. Before weigh-up hoppers, specific gravity calculations and moisture probes, Romans proportioned concrete mixes by filling a box of a known volume with the raw material, and then added the raw material to a mixing vessel. These recipes were typically 1 part cementitious binder, 2 parts sand, and 3 parts rock (or shells), or a slight variation of these proportions. But, as with modern concrete, Roman builders needed to revise the mix design based on the raw materials locally available to optimize the design for the specific building they were constructing.
In the modern era, continuous mixing machines (typically employing a screw-type auger) were available for use in the early 1900s. However, it was in the early 1960s that Harold Zimmerman and Irl Daffin moved this technology forward by combining continuous mixing with improved precision of the volumetric controls to deliver the materials to the auger. Ingeniously, they also found a way to mount this machine on a truck to bring individual mix capacity to the job site.
As the use of volumetric batching systems continued to grow, so did the need for uniform technical guidance for the designer and producer. In 1971, ASTM International published ASTM C685, “Standard Specification for Concrete Made by Volumetric Batching and Continuous-Mixing,” which was based on many provisions already described within ASTM C94, “Standard Specification for Ready-Mix Concrete.” As described within its scope, ASTM C685 provided a standard minimum for material quality and production tolerances necessary to produce consistent concrete when batched by volume and mixed in a continuous mixer. The latest edition of ASTM C685 was published in 2014.
Similarly, ACI acknowledged the rapid increase in use and application of this concrete technology by publishing ACI 304.6R, “Guide for the Use of Volumetric-Measuring and Continuous-Mixing Concrete Equipment,” in 1991. This document expanded on volumetric batching applications, publishing recommendations for equipment calibrations and concrete testing. The latest version of ACI 304.6R was published in 2009.
Batching by volume would not have expanded in use if it did not provide some unique benefits over batching by weight.
Variable batching quantity
By introducing dry components into a high-speed auger system, volumetric batching and continuous mixing can produce small quantities of concrete with little-to-no waste. This can be advantageous for precasters who cast small specialty items or use forms with small volumes. The military has recognized the benefit of a self-contained mobile concrete batching system for producing small quantities to fill in damaged airfields or other concrete installations in remote locations. However, these mixing systems are not limited simply to small quantities. As the name suggests, these systems can operate on a continuous basis and, given an adequate raw material feeding system, can produce hundreds of cubic yards of concrete in one day. Consequently, there is little waste and no need for mud pans to clean out any extra mix or to dispose of excess concrete if something unexpectedly slows down operations.
Mix design flexibility
In addition to being able to vary batch quantities easily, modern mixer units can quickly produce variable mix designs. Batching operations can have a continuous pour of 4,000-psi concrete, and within a few seconds continue to pour a mix of 6,000-psi concrete. Similarly, volumetric batching systems can be used to produce self-consolidating concrete, fiber-reinforced concrete or low-slump concrete. Volumetric mixing system operators are advised to work closely with their admixture supplier to ensure the rapid mixing time of the auger system is compatible with the minimum requirements of each admixture.
Mixing of all the raw materials occurs within the auger system. The auger, which is compact and easily accessible, is very easy to clean and wash out compared to other mixing systems. This can be particularly advantageous when producing pigmented concrete because this system can be cleaned faster and more thoroughly than larger mixers that are used in batching-by-weight applications.
Quality, the proof is in the testing
For a long time, there was a bias that volumetric and continuous batching methods produced inconsistent or poor-quality concrete when compared with weight-batched methods. This would be true if the volumetric method used for the comparison was the manually-filled, box-batching method used by the Romans. However, today’s modern volumetric batching equipment can produce consistent, high-quality concrete that meets the most stringent of precast concrete product standards.
In any precast concrete batching operation, the key to batching consistent and high-quality concrete is to employ best management practices for raw material and finished product testing. It is imperative that volumetric batching operators have full knowledge and control of the properties of the aggregates used in the mix. This includes bulk densities, specific gravities and absorption capacities, as well as the ability to properly calibrate the mixer gates (or counters) to provide the exact volumes required by the mix design. A critical piece of information for any concrete mixing operation is the actual moisture content of both the fine and coarse aggregate at the time of batching.
When you hear the term “bulking up” you might think of a body builder increasing caloric intake to gain more muscle. It’s a similar concept with respect to fine aggregates – the volume of dry sand will increase due to absorption of moisture. That is, a given volume of sand will take up more space once it begins to absorb water.
When fine aggregate is exposed to moisture, the surface tension of the water in the pores will push the fine particles away from each other. This leads to a general increase in volume for a given mass of fine aggregate. Interestingly, this surface tension effect will continue to push particles apart between an aggregate moisture content range of 4% to 6%, after which the volume will begin to decrease, as shown in Figure 1. This increase is inconsequential with a weight-based batching method because the weight hasn’t changed. But with volumetric batching methods, this substantial volume increase due to bulking must be accounted for. Consequently, fine aggregates used in volumetric batching need to have their bulking factor tested. This must be known by the batching operator, along with the current aggregate moisture content, to make the appropriate adjustments to increase the fine aggregate volume batched, as needed, to meet the actual volume design proportions.
Fine aggregate bulking can be substantial. And although it may vary depending on the materials source and gradation, bulking generally increases with the fineness of the aggregate
If there are any changes in the original mix design or raw materials, then additional material property testing shall be completed and modifications made to the batching quantities. For volumetric mixing, ensure proper proportions are being met in the batching calibrations – either the gates or counters – which control the volume of materials sent to the mixing auger. To provide mix consistency assurance, ACI 304.6R recommends a complete calibration of batching equipment:
- for all new equipment prior to initial use
- when test data indicates the concrete is not meeting specified performance levels
- when specified by the purchaser or engineer
- when major mechanical repairs are performed on the volumetric measuring and continuous mixing unit
- when a change is made in materials or mixture proportions for which previous calibration data are unavailable
Concrete can be batched volumetrically or by weight. It is up to individual precasters to determine which method will best meet their production needs. Both methods require a thorough quality control program to be successful, as detailed in the NPCA Quality Control Manual for Precast Concrete Plants and described in ASTM standards, ACI practices and guides, and manufacturer recommendations.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13 on Concrete Pipe.