In the mid-1970s more than 65 people died in two tragic construction accidents. In both cases, cold weather had delayed the hardening of concrete elements and early stripping of formwork resulted in failure of the concrete and reinforcing steel. These tragedies led to the recognition that concrete maturity, originally developed in the 1940s, could be used to evaluate the hardening process in concrete elements in cold weather.
The American Concrete Institute (ACI) and ASTM define concrete maturity as a method to estimate strength development in construction based on the assumption that samples of a given concrete mixture attain equal strengths if they attain equal values of the maturity index regardless of temperature changes. Today, concrete maturity provides real-time data that enables the end-user to make better-informed and instantaneous decisions.
Within the last three decades, through the efforts of concrete contractors, the Federal Highway Administration, and state DOTs, concrete maturity has resurfaced as a quality assurance method for fast-track pavement construction, specifically in determining when the pavement can be opened to traffic. Concrete maturity found a foothold where a concrete manufacturer and contractor could use maturity to assure the purchasing agency that the concrete mix would attain the traffic-opening strength at the specified time. This concept has been repeated for years with few failures.
That brings us to today; the objective of this article is to determine if the same concrete maturity used for short-term strength development, cold weather precautions, and fast-track construction can be extended to long-term strength estimation and as a supplement to concrete cylinders.
The maturity method establishes a relationship between strength, temperature, and time that can be used to estimate strength development (ASTM C 1074-11). That relationship builds on the idea that the maturity, or strength development, of a given concrete mixture will be equal no matter the ambient temperature during the time over which it gains strength. Assuming these temperatures are not so extreme as to be detrimental to the hydration process of cement, the mix cured in a hotter environment will more quickly reach the same maturity (and therefore, theoretically, the same strength) as that achieved by samples cured in a standard cure environment and much more quickly than samples cured in a colder environment, as illustrated above.
The basis of concrete maturity is an empirical relationship between the compressive strength of the concrete and the maturity index. These relationships are obtained by testing for compressive strength on laboratory specimens whose temperature history has been measured, recorded up to the time of testing, and can be compared at a later point to the concrete maturity on the jobsite.
Implementing concrete maturity
Concrete maturity implementation can be cumbersome and costly, but despite the needed initial investment of time and resources, concrete maturity is an invaluable method for strength estimation on a jobsite. While concrete maturity has the potential of giving the end-user immediate real-time data of a particular concrete mix’s strength development, the process to get to the prediction takes some legwork. The user must conduct the following three steps for each concrete mix design:
- Develop the strength-maturity relationship for the mix
- Place maturity loggers into the forms prior to concrete placement
- Use the appropriate model to determine the maturity index
Let’s look at each of these in detail.
Developing maturity curves
strength-maturity relationship is developed with laboratory compressive strength tests on the concrete mixture to be used. A maturity curve is specific to a single concrete mix design. When a concrete producer has a change in a material’s source or concrete mix design, a new maturity curve must be developed.
Part of the laboratory work for the concrete maturity curve is to determine the datum temperature or equivalent energy of the concrete. This includes casting, curing, and breaking grout samples that represent the concrete mixture to be used. The grout samples are cured at three different temperatures and broken intermittently over 32 days. This practice allows us to determine the minimum temperature or energy needed for cement hydration.
In most cases, though, the datum temperature and equivalent energy are assumed by the end-user. While this practice is usually considered a conservative approach to maturity, it can result in wasting time and money on the jobsite. And, in some instances, especially in long-term applications of maturity, assuming the wrong maturity data can risk the safety of workers.
As an example, consider two concrete mixes. One, a classic curb and gutter concrete mix with a low total cementitious materials content and the other a fast-track pavement mix with a much higher total cementitious content and high admixture content for rapid strength gain. The fast-track concrete with higher cementitious content would be more likely to continue hydrating at lower temperatures and thus the datum temperature and activation energy will be lower for the fast-track mix than for the curb and gutter concrete.
But if instead of determining the datum temperature and activation energy, we assume a value it could be too low or too high. If too low, the result will be a set of calculated concrete maturity values that are higher than the actual value which could lead us to pull our forms too early. If a higher value is assumed, it would take a longer time to reach the target maturity, which is conservative, but also wastes time and money waiting for the strength gain that has actually already been achieved. Since the objective of concrete maturity is to expedite construction safely, assuming datum temperatures and activation energies results in reduced reliability for both short- and long-term strength prediction.
Installing maturity meters
The second step in implementing a maturity program is to attach a concrete maturity logger or meter in the formwork where the concrete in question is to be placed. The temperature history of the in-situ concrete, for which strength is to be estimated, is recorded from the time of concrete placement to the time when we want to estimate the strength. The recorded temperature history is used to calculate the maturity index of the field concrete. There are several different types of maturity loggers and meters that will be covered in a follow-up article.
The final step in estimating the strength
of the field concrete is to use the appropriate model to calculate the
maturity index and the strength-maturity relationship. This step can be another source of error when using concrete maturity to predict long-term strength. There are two models used: the temperature-time factor and the equivalent-age. While changing from one model to the other can be as simple as changing the selection
in a drop-down menu, most maturity loggers default to the temperature-time factor. But for long-term strength prediction the maturity curves should be based on the equivalent-age method, which is better suited to predicting strength development of concrete after 14 days. For strength development within 14 days, the temperature-time method (most widely used method) is recommended.
Advantages and limitations
Concrete maturity is often used in critical construction activities such as removing formwork and reshoring; post-tensioning of tendons; termination of cold-weather protection; and opening roads to traffic. We have identified the advantages of using concrete maturity for short-term and long-term concrete strength prediction by conducting a survey with concrete and construction entities that have used maturity methods.
From the survey, we found that end-users prefer concrete maturity as an estimation technique because it is an inexpensive, repeatable, and easy-to-implement process. And while the traditional method of casting cylinders creates a snapshot of the concrete strength in given sections of the concrete, cylinders provide only limited information from a larger concrete pour. Maturity loggers and meters are relatively easy to use and maintain, and downloading the data takes no more than five minutes in the most extreme cases. End-users can include maturity loggers in multiple locations to create a more detailed account of in-situ concrete strength development, monitor critical areas of construction, and take advantage of early strength gains to move the project schedule forward.
Despite the potential of concrete maturity as a nondestructive prediction method, there are limitations that must be recognized. As listed in ASTM C 1074-11, here are the top three limitations to implementing and employing concrete maturity:
- Concrete must be maintained in a condition that permits cement hydration.
- Concrete maturity does not take into account the effects of early-age temperature on long-term strength (see sidebar by Ken Hover).
- Concrete maturity is recommended by ASTM, ACI, and others as an estimation method only. Concrete maturity must be supplemented by other indications of the potential strength of the concrete mixture. This practice is often dismissed when it comes to long-term strength determination. But concrete maturity was never intended to be a stand-alone prediction method for the entirety of the concrete elements in question on a jobsite.
One of the toughest roadblocks to overcome in implementing a maturity program is the cost of developing the concrete maturity curves. The maturity loggers are inexpensive; but developing the maturity curves is a costly and involved process that must be done months before starting the project. Ultimately, the cost of developing the concrete maturity curves is a burden that hangs between the concrete manufacturer and the general contractor.
Another roadblock with concrete maturity is that it is not concrete strength. And while there are plenty of case studies to prove the effectiveness of concrete maturity over the short term, it’s less common to estimate long-term concrete strength with maturity. Whether you believe that concrete cylinders are accurate representations or not, as an industry there is more confidence in concrete cylinders and beams than in concrete maturity when evaluating long-term strength.
The next step
The road the industry must go down to adopt concrete maturity as a long-term strength prediction method includes several questions and activities.
- Are we using the right prediction method for long-term qualification of strength?
- Should we use concrete curing boxes?
- How many data loggers are needed per volume of concrete?
- Can we assume a single datum temperature or activation energy for all concrete mixes?
- Do the swings in concrete constituents allowed by ASTM C 94 invalidate the established maturity curves?
Currently, the authors, working with concrete manufacturers, universities, and contractors in Colorado have started a pilot program on active jobsites that includes concrete maturity methods and devices. These results will be compared to the theories at the foundation of this strength prediction method. Ultimately, the hope is that the pilot programs will validate the established practices and suggest revisions to some of those practices if they are not feasible in the field. Details of this work will be published in a follow-up article where we will attempt to show whether concrete maturity is an effective and realistic means to supplement concrete cylinders on a jobsite.
Jon Belkowitz is director of research and development, Whitney Belkowitz is CEO and president, and David Harris, P.E.. is principal engineer all with Intelligent Concrete in Elbert, Colo. For more information visit intelligent-concrete.com.