Precast concrete is a high performance building material and it’s no secret that good quality concrete is the fundamental component of any high quality precast concrete product.Producers are often judged on the quality of their products based strictly on appearance.
On the other hand, this area represents an opportunity to improve quality with minimum investment, and turn that advantage into customer satisfaction, retention and ultimately higher margins for both.The Problem – Finishing, Workability and Costs
Despite advances in concrete technology, over 60% of precasters experience challenges at some point with the workability and finishing of their product, leading to high re-work costs and scrap.
A reality at many plants is the need for rework to correct these bugholes, not to mention minimizing the growing scrap pile of precast elements rejected for quality issues.This represents significant losses to the producers’ bottom line and can be reduced significantly.
The truth is many factors affect the finished quality of concrete, and in many cases, significant improvements in quality and cost reduction can result from an analysis of the mix workability.What are the basics for a good product finishing?
This video from NPCA gives good insight to the fundamentals of mixing, placing, finishing and curing of concrete for all types of concrete:NPCA Video – Concrete Fundamentals
Where precast is concerned, PCI MNL 116 lists mix proportioning as the primary factor in the production of high quality precast concrete:“Much of the skill, knowledge, and technique of producing quality precast concrete
elements center around the proper proportioning of the concrete mix. Before a concrete mix can be properly proportioned, several factors must be known such as the finish, size and shape of the units to be cast. The method of consolidation should be known to help determine the required workability of the mix. The maximum size of the coarse aggregate should be established. The required compressive strength affects the amount of cement to be used as well as the maximum water allowed.”The Accepted Methods of Selecting Mix ProportioningInformation is found in the following publications:
- Portland Concrete Association:
- Design and Control of Concrete Mixtures
- American Concrete Institute:
- Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete (ACI 211.1)
- Standard Practice for Selecting Proportions for Lightweight Concrete (ACI 211.2)
- Standard Practice for Selecting Proportions for No-Slump Concrete (ACI 211.3)
- Specifications for Structural Concrete for Buildings (ACI 301)
While there are excellent sources of information to guide the starting point for proportioning of mixes for structural precast, most focus on strength, durability and cost-effectiveness. However, workability, which is a primary factor affecting the finishing of the product, still requires further guidelines.Where do producers turn for advice?
Workability is a key factor to eliminate bugholes. Researchers at Oklahoma State University (OSU) recognized that there was a severe lack of information on proportioning concrete mixes for workability and set out to research and develop reliable techniques of proportioning the aggregate fraction of mixes that optimizes finishing, minimizes segregation, and improves compaction for both wet cast and dry cast mixes.In conducting the research, several popular methods of proportioning were evaluated empirically, namely the Shilstone Coarseness Chart, Individual Percent Retained Chart, Power 45 Chart, and 60% coarse aggregate and 40% fine aggregate.The research paid close attention to the combined % retained gradation (both coarse and fine aggregates) but also examined the effects of varying of the coarse (#8 to #30) and fine (#30 to #200) sand sizes to optimize cohesiveness, finishing and consolidation.After 4 years of design and development, four thousand mix designs and over eight hundred trial batches, the ‘Tarantula Curve’ was developed by the researchers at OSU as a tool for proportioning mixes, along with established gradation limits for normal (flowable) mixes as well as dry-cast mixes.The information shared by OSU is very concise and easy to follow and has helped many precast producers who were experiencing problems with workability and finishing.
As such, when troubleshooting mix designs, we frequently encourage producers to refer to the Tarantula Curve method with great success.The method and much more supporting information can be found here: Optimized Graded