By Paul Ramsburg
Editor’s Note: This is the first 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 to read the second article.
A foundational part of what we do in the precast industry is design concrete mixes that meet the desired engineered properties of the products we manufacture. Product strength, durability, aesthetics, workability and cost-effectiveness are all considerations we must take into account during the process. Given this, there is a need in the industry for better understanding about precast concrete mix design by everyone, no matter what their role is in the company. Frank Bowen from Piedmont Precast and I hope to encourage new interest in the science of concrete, as well as offer some reminders to industry veterans who have worked around the batch plant for decades. In this year-long series, we will cover what we feel everyone in the industry should know about concrete mix design.
Water as a mix constituent
Water is a basic building block for concrete. If you are thinking of water as an inert material that is just sprayed into the mix until a slump forms, then you are thinking about it all wrong. Water is a critical material which must be dosed accurately in order to gain a desired outcome after undergoing a chemical reaction with the cement and other mix constituents.
As a rule of thumb, any potable water is acceptable for use in concrete. That does not entirely exclude “gray water,” known as process water, nor does it mean that all potable water meets specifications on limits of solids and compounds. NPCA-certified plants are required to ensure that the water used meets the criteria set out in relevant specifications. Impurities in water can interfere with the setting of cement and can adversely affect concrete’s strength and durability. Many possible constituents present in water may actively participate in the chemical reactions and affect concrete’s set time and strength development. Limits are specified for mix water with constituents such as total alkalis and chloride sulfate. It’s important to understand what is in your water and how it affects your mix in particular.
Many production facilities are limiting their use of fresh water in batching concrete. This may be due to a limited supply of fresh water in their area, an effort to limit waste-process water or the decision to be responsible consumers. Whatever the reason, it is possible to successfully batch quality concrete with recycled water that has been reclaimed from production processes. Testing should be done on concrete using recycled water at various percentages of total batch water to ensure it doesn’t negatively affect set time, strength and durability.
Approximately 100 years ago, American Concrete Institute’s first president Duff Abrams first published his concept on the effects of water-cement ratio on concrete’s strength and durability. Now, when I say w/c ratio, I mean water-cementitious material ratio. This means when you calculate w/c ratio include all cementitious and pozzolanic powders. It’s been known for 100 years that w/c ratio is the biggest factor on both 1-day and 28-day strength. You can increase compressive strength and reduce segregation in self-consolidating concrete mixes by lowering the w/c ratio. In addition, w/c ratio is easy to calculate. The equation is in the name: you simply divide the weight of water by the total weight of cementitious material. What ratio you target depends on what you want from the mix. Typically, high strength mixes (6,000-to-10,000 psi) have a low w/c ratio such as 0.38 and lower strength mixes (5,000 psi) may have a ratio of up to 0.45. Low w/c ratio mixes are denser, and therefore exhibit greater durability than high ratio mixes.
Concrete mixes with lower water content will need to have a higher water-reducing admixture added to achieve high slump or flowability. These mixes are typically described as sticky. The end product may be more difficult to finish, but it is often good for SCC mixes. These mixes are more resistant to segregation and excessive bleed water because they have a high viscosity.
Don’t forget about aggregate
Another source of water is introduced in concrete through aggregate. When you batch aggregate, it contains some amount of moisture, usually about 1% from stone and 3-to-7% from sand. This must be accounted for in the w/c ratio calculation. If this concept is new to you, there are several resources available on the subject on precast.org. It also bears mentioning that until you get trustworthy results from moisture probes, they must be calibrated on a regular basis. NPCA certified plants are required to validate probe calibration with aggregate moisture testing a minimum of once per week.
Looking forward to the year ahead
As we discuss precast concrete mix design over this series of articles, I hope many of you will be inspired to learn more, to experiment and to get your hands dirty. In the next article, we will learn more about aggregate, but for now remember when it comes to water there is a great need for accuracy. Water is not just water, it’s an important mix constituent.
Paul Ramsburg has worked in the prestressed concrete industry since 1988 and is currently a technical sales specialist at Sika Corp.