The use of ultra-high-performance concrete is becoming more common, thanks in no small part to the fact that it can now be mixed, transported, and poured using standard ready-mix trucks. The cost is also coming down, because you don’t need to buy a concrete manufacturer’s sand or cement; you can use your own. More on that in a bit.
What we want to focus on in this article are the questions you should be asking your UHPC supplier. Why is this important now? Because as UHPC gains momentum among contractors, there has been an associated increase in myths and misconceptions as suppliers jockey for dominance. This can make it difficult to separate the hype from the reality.
We’ve always thought that honesty is the best policy, and that solid stats coming from solid testing is the best source of dependable information. Which is why we have been careful to publish information based on real-world testing. Here are the most up-to-date stats for our CT-25 UHPC mix, compared to traditional concrete:
800 – 1300 coulombs*
.40 – .44
.13 to .17
15 – 25 years
2,500 – 5,000 psi
25,000 – 30,000 psi**
400 – 700 psi
300 – 700 psi
*Using ASTM C1202 and AASHTO T277 tests. **Ambiently cured.
We don’t want to point fingers at other UHPC suppliers; there’s no need and it’s not very polite. But we do want to make sure that you ask the kinds of questions that will help you when deciding which ultra-high-performance concrete will be best for your project.
Questions you should be asking your UHPC manufacturer
Question #1: What is the water-to-cementitious material ratio of your concrete?
As you know, the water-to-cementitious ratio is the percentage of water to cementitious materials. In a recent bidding situation for a very large project, the project’s contractors specified what is normally specified for UHPC: a cementitious ratio of .25. Two manufacturers requested a waiver, asking that the target be moved to above .25, as they could not meet the requested specs. One asked for .30 and another wanted .32. We wondered: does this include or disclude water that they get from their admixtures?
In any case, they requested to be able to use more water to achieve this ratio; about double the amount of water we need for our UHPC mix. If your cement-to-water ratio rises above .25, by definition, this concrete can’t even be considered as ultra-high-performance concrete. Fortunately, in this case, the DOT did not grant the waivers.
The Cor-Tuf cementitious ratio is .17 including any free water from admixtures.
Some UHPC manufacturers also have to add more water to reach a plastic state. We don’t need to do that.
Question #2: How long does your UHPC need to be mixed before it can go to plastic?
One manufacturer’s UHPC mix must be mixed for an hour and twenty minutes before it’s ready to pour. I’ve seen this myself on two separate occasions; I was careful to time it.
The CT-25 UHPC that we offer takes 8.5 minutes to go plastic in a ready-mix truck versus the 4 to 6 minutes we see when mixing in a full-scale batch plant. This was a tall order, but we pulled it off. We were determined to make CT-25 “contractor friendly,” and mixing time was key.
Additionally, there are obvious correlations between minimized mixing times and cost savings, as we all know. Ultimately, “time is money.” In the world of UHPC and truncated production schedules, minimal mixing times become incredibly important.
If I had a nickel for every time we have been in a pre-construction meeting with a general contractor needing to reopen a bridge in 12-14 hours after a closure, I’d have a ton of nickels.
Question #3: Can you use your own sand and cement, or do you have to purchase special sand and cement from the concrete manufacturer?
Every UHPC supplier has a proprietary ingredient or method that distinguishes them from their competitors. For most UHPC producers, their “secret sauce” is on the sand/cement side, which is why they insist that you use their proprietary product pre-mixed with their sand and cement.
Our proprietary ingredients are in the dry CT-25 mixture that we provide, which you can mix with virtually any sand and cement. The nature of the mixture still achieves the particle-packing density required to meet UHPC specifications, as regulated by the Federal Highway Administration and various departments of transportation, but does so without altering established ready-mix production methods.
This was another challenge we needed to meet in our quest to develop contractor-friendly UHPC. We wanted to let concrete manufacturers use their own sand and cement, to save them time, money, and more importantly, to remove the often costly supporting infrastructure configuration requirements necessary to shift their production methods over to a new system.
Additionally, CT-25 can be delivered via bulk tanker, furthering the value proposition for ready-mix producers.
Of course, there may be sand or cement someday from some unusual geographic location that isn’t compatible with Cor-Tuf, but so far, we haven’t encountered any material that hasn’t been compatible with our product.
Question #4: What is the shipping going to cost you? If the UHPC manufacturer is shipping 3500 pounds of complete mixing material, including sand and cement, that’s a lot of shipping costs to cover over the course of a project. For every 3500 pounds of material our competitors send, including sand and cement, we only need to send 1,080 pounds of our mix. You can use your sand and your cement with our UHPC and realize the cost savings in every load you ship.
Question #5: Can you mix the concrete right on location, in a ready-mix truck? If not a ready-mix truck, how are you going to mix it? Onsite? If so, how big an area do you have for onsite mixing? How big of a cleanup will be necessary? What are the risks of spills on site? What are the ecological implications? What are your storage plans for the bagged materials?
CT-25 can be mixed in any standard ready-mix truck in 8.5 minutes and transported to the site, just like traditional concrete.
Question #6: I see this is your published concrete strength. What conditions need to be met to achieve that strength?
Additional questions: What are your concrete strengths at the various ambient temperatures? 60 degrees? 80 degrees? Thermally cured? Is accelerated thermal curing required to achieve the strength? What model do you use? What is the ultimate temperature that has to be reached? How long is the curing process? What does the regimen look like? Can it be practically replicated in the field?
As we all know, in some parts of the country, temperatures can go from 70 degrees during the day to 38 degrees at night. It’s important to know how these temperature swings are going to affect the strength of the UHPC.
It’s also quite common right now for contractors to be using accelerated thermal curing. If performed incorrectly, this method introduces its own problems as the molecules expand rapidly. It may cause faults (microfractures) in the concrete.
One client told us that the strength promised by one vendor was not achieved as expected, and when confronted, the manufacturer said that the client hadn’t paid attention to the words “accelerated curing process” buried somewhere in instructions, in the fine print. Using an accelerated curing process is not always practical on site, and it should have been made more obvious to the client that the desired strength could not be achieved without accelerated heat curing.
Our CT-25 gets stronger as it cures in ambient temperatures, with noticeable strength gains between 28 and 56 days and beyond.
We always recommend utilizing ambient curing when possible. However, we can also play the short, quick-set game, with accelerated thermal curing if a truncated timetable is required to satisfy a specific requirement. As mentioned previously, we always preface any discussions of accelerated curing with the caveat that it is a game of give and take. What can be gained in terms of early strength numbers often comes at the expense of other attributes and at an additional monetary cost.
An absolute obsession with contractor convenience.
Over the last three years, we have been absolutely determined to meet three goals:
1. The product needs to meet engineering standards for all of its applications.
2. It must meet DOT standards.
3. It must be “go-to-work-immediately” convenient for contractors. They should be able to use it without any special machinery or methods of action. As a best practice, we require our partner contractors to have our technicians onsite for three projects so that we can ensure that a satisfactory level of comfort with the product has been reached. After that time has elapsed, we will certify your crew for independent production with remote technical support at the ready should you need it.
Frankly, the effort has been intense. But we have achieved these goals, as evidenced by rigorous testing and numerous projects. We can package up the product in two-cubic-foot kits all the way up to super sacks. For our ready-mix friends, we also offer bulk tanker shipments to handle whatever project you may have on the horizon. Whatever size the mixer is, we can give you exactly what you need. And all we have to ship is the mix; you can use your own cement and sand.
Let us know if you’d like to discuss your next project. We’d be happy to help.