Most pavement engineers learn about materials on the job. Civil engineering programs may cover portland cement, but usually not rapid-hardening cement. That’s not surprising given that portland cement’s been one of the most widely used construction materials for more than a century – but it does lengthen the learning curve.
It also doesn’t prepare future public agency engineers for the real world. Producers are devoting more capacity to portland limestone cement (PLC, or Type 1L) because it’s promoted as being more environmentally friendly than ordinary portland cement (OPC). Virtually every state allows or uses PLC. The federal government will eventually tie project funding to concrete’s global warming potential (GWP). As a result, OPC is becoming increasingly difficult to source and will eventually be replaced by PLC.
I’ve watched the market evolve over five decades as a county engineer, producer, and heavy/highway contractor. I can help you identify the best long-term value for rapid-strength concrete, which I define as a mix that gains design strength in three to four hours.
Why Shrinkage Rate and Mix Consistency Matter
You’ve probably heard this, but I’ll say it again because it’s the root cause of concrete failure: Water increases shrinkage, which increases cracking, which causes deterioration, which requires maintenance. Water causes cracking as the mix dries. This is called shrinkage cracking because the concrete mix mass “shrinks” as the water leaves the system.
Cracking lets water and road salts penetrate the pavement surface. In freeze/thaw environments, expansion and contraction gradually enlarge and lengthen cracks. Eventually, water and road salt reach the reinforcement, which starts to corrode. Over time, structural failure leads to potential safety issues.
Unfortunately, you can’t make concrete without water. Portland and portland limestone mixes use about 55% of their mix water to start the reaction that binds aggregate and cement powder. I call the remaining 45% “convenience water” because it provides time for the mix to be transported and placed. The rest bleeds to the surface as the mix cures, where it carves tiny channels that make the concrete porous. If finishers don’t wait for that water to dissipate, they work it back into the surface, promoting further surface shrinkage that causes scaling.
Increasing cement content increases water requirements – which increases shrinkage. You can make rapid-strength concrete with Type I and Type II portland cement, but it requires considerably more cement. It also requires high doses of admixtures and accelerators, which is a problem for reasons I’ll get into shortly.
Finer cement powder promotes early strength gain, but increases water requirements – which increases shrinkage. Type III OPC is basically Type I and/or II portland cement that’s ground to a finer powder. The smaller particles increase the total surface area that the water in the mix must saturate. Over the last decade or so, Blaine fineness of portland cement and portland limestone cement has been increased to further promote early strength gain. That works, but the powder’s smaller particle size increases water requirements – which increases shrinkage.
Anything added to a mix comprises consistency, which can lead to deterioration and failure. The market is filled with additives, admixtures, and supplementary cementitious materials (SCMs) that combat portland cement’s shrinkage, speed strength gain, and/or lower carbon footprint. However, adding anything to a mix potentially compromises its consistency, which also can compromise the concrete’s long-term durability.
Calcium sulfoaluminate (CSA) cement can be added to promote early strength gain and minimize shrinkage, but the portland cement in the concrete negates the CSA’s benefits.
A side note about PLC. ASTM C595 allows 5% to 15% of the portland cement in a mix to be replaced with limestone, which is said to lower carbon footprint by 10% on average. That makes PLC a blended cement, which like all blended cements is vulnerable to mix inconsistency. In addition, each state has developed its own standard governing limestone sourcing and quality. As a result, transportation departments are reporting a higher level of inconsistency than expected.
How Rapid Set® Differs
Rapid Set® is the brand name for CTS Cement Manufacturing’s belitic calcium sulfoaluminate (BCSA) cement. It is not portland cement and doesn’t hydrate like portland cement. The water/cement ratios are similar, but CSA uses almost all the water to quickly form an interlocking network of crystals called ettringite. Only 2% of the mix water leaves the system.
The result is much less permeable concrete that’s much less vulnerable to water and road salt. Per ASTM C157, shrinkage at 28 days is 200 microstrains compared to 600 to 700 microstrains for portland cement.
Rapid Set® is unique in the rapid-strength marketplace because its patented formula promotes rapid strengthening and very low shrinkage without additives, admixtures, or portland cement. That’s why we call it a “standalone” cement. It’s mixed, placed, cured, and finished like portland cement using the same equipment and same testing procedures (e.g., slump and strength). It just gains strength much more quickly, usually reaching opening strength in less than two hours – plenty of time for a crew to remove and replace slabs and make other common repairs overnight.
Click here to request access to a free webinar where I explain all this in greater detail and answer questions from DOT engineers.
How States Specify and Use Rapid Set®
We know Rapid Set® lowers maintenance needs because we’ve monitored placements since introducing the product in the 1970s. It’s the primary component of dozens of cements, concrete, mortars, and grouts developed for specific applications. State transportation departments have consumed more than 2 million tons for bridge and highway panel replacement, partial- and full-depth pavement rehabilitation, and emergency repairs. All but four allow Rapid Set® products.
Remember: Rapid Set® isn’t portland cement, so you can’t use ASTM C150 (Standard Specification for Portland Cement) to write a specification. That’s why ASTM C1600 (Standard Specification for Rapid Hardening Hydraulic Cement) was introduced in 2009. We recommend a prescriptive rapid-strength concrete specification based on ASTM C1600 cement and adding maximum allowed shrinkage to your other concrete performance requirements (e.g., freeze-thaw, permeability, etc.).
Given how detrimental shrinkage is to concrete’s long-term durability, states have begun doing just that. If you’d like an example, click here, select “Other type of information” from the How Can We Help You? dropdown menu, and write in the message box: “Shrinkage specification question for Matt Ross.”