In this presentation, we will be talking about:
• Some background information on concrete strength monitoring in the field.
• Concrete maturity and why the maturity method it is one of the most popular methods for the real-time measurement of concrete strength
• Implementing the maturity method in the field
• And we’ll talk about some cases where the maturity method was used to increase the efficiency of construction projects
So, why this topic is important. Let’s start with some historical background. Concrete structures could collapse for different reasons. But, in mid-70s, there were two incidents where lack of information on the in-place strength of concrete resulted in major collapses during construction. The first one was in 1973, where a progressive collapse killed 14 workers. The investigation by the National Bureau of Standards and the Occupational Safety and Health Administration concluded that the removal of formwork before concrete was strong enough led to this incident. Later in 1978, there was another major failure in a cooling tower that killed 51 workers who were on scaffolding attached to the partially completed shell. Again, the investigations determined that there was not sufficient concrete strength to support the construction loads. So, the community was convinced that there was an urgent need to develop new methods for the in-place strength measurement of concrete.
There are of course several other critical construction operations that are dependent on knowing the in-place strength of concrete. So, to optimize the timing for these operations such as post-tensioning, curing (for example if you want to stop heating concrete in the winter time), saw cutting, and also deciding when to open a concrete road to traffic, we need to have accurate information on the strength of concrete as it is being cured on the job site.
So, how do we determine concrete strength in the field? Well, when concrete is poured in, there are generally two sets of samples taken from the pour on the job site. The first set is cured under standard curing conditions (for example in a lab) and these samples are tested for acceptance purpose. So, essentially, this is to determine if the strength of concrete that is delivered is the same as that ordered. The second set of specimens are left on the job site near the concrete elements so they are cured under more-or-less similar conditions. Then, at different times, one of these field-cured specimens is sent to a lab for strength testing and the results are sent back to the job site to make a decision if the in-place strength of concrete is sufficient for operations such as formwork removal. There is also another variation to this method where we can have cast-in-place specimens. For this, plastic cylindrical moulds are embedded in the formwork so as concrete is poured, those are filled and then cured under similar conditions as for the concrete elements. Then, they will be taken out and sent to a lab for testing.
Field-cured specimens have several limitations. First of all, the curing condition of the specimens is not truly representative of the curing condition for the actual concrete element as the specimens are generally sitting on the side with minimum protection. So, they could easily undergo lower or higher curing temperature values compared to that for the actual concrete element. In addition, there is an inherent delay in results as the specimens have to be collected and sent to a lab for testing and reporting. Also, usually one specimen is sent to lab. So, this would only provide limited information and if the specimen does not break well, this could result in inaccurate information. And of course, the results from these specimens do not differentiate between local variations of strength in different locations on the same structure.
So, there have been other methods developed for the real-time estimation of concrete strength. Some of these methods that are standardized by ASTM include, Windsor probe (where a metal rod is fired into concrete and its penetration depth is correlated with strength), rebound hammer (or Schmidt hammer), pull-out test (which is slightly destructive as a rod is embedded in fresh concrete and the force to take it out from hardened concrete is correlated with strength). But, among these methods, concrete maturity has been widely adopted as it is relatively easier and more accurate.
So, what is concrete maturity? Well, basically, the maturity method uses the temperature history of concrete to estimate its strength. And in this presentation, we will show you how this is done.
The good thing is that this method is standardized by ASTM and other standard bodies worldwide, and more importantly, it is adopted by building codes and DOT specifications as a mean and method of estimating the in-place strength of concrete. So, engineers are actually allowed to specify and use this method instead of breaking field-cured specimens, which is a huge advantage of the maturity method.
In the maturity method we have a concept that is called maturity index. The maturity index (which is a function of concrete temperature) is correlated with the strength of concrete. And this relationship is unique to each concrete mixture.
To clarify this concept, imagine that we have the same concrete mix poured in two different conditions where it will experience two different curing temperatures. If we calculate the maturity index in these two cases, and see that it reaches the SAME values whether at earlier or later times, the concrete strength would be the same regardless of its temperature history.
So, how do we calculate the maturity index and how is that correlated with strength?
There are three different methods of calculating the maturity index. The first two which are standardized by ASTM C1074, include the Nurse-Saul equation (or temperature-time factor), and the equivalent age based on the Arrhenius equation. The third one is called weighted maturity and is based on a Dutch standard which is adopted in some European countries. The maturity index whether it is represented by the temperature-time factor, or equivalent age, or weighted maturity is then correlated with strength using the same general equation that we will describe in the next few slides. It should be noted that we only provide a summary of these methods in the current webinar and this is just to give you a general idea, so you should review the full details by standard specifications before using these methods in the field.
As we mentioned, all of the maturity methods use concrete temperature as the main parameter to calculate the maturity index. So, we first need to obtain the temperature history of concrete. The most popular method of measuring concrete maturity index is based on the Nurse-Saul equation or temperature-time factor. In this method, we define a threshold temperature that is called datum temperature. Datum temperature is essentially the temperature below which we are assuming that concrete does not develop any strength. Then, to calculate the maturity index at any time, we simply calculate the area underneath the temperature curve above the datum temperature as determined by the Nurse-Saul equation.
In this equation, M is the maturity index defined as temperature-time factor, Ta is the average concrete temperature during time interval delta-T, T0 is the datum temperature, and delta-t is the time interval. The datum temperature is the only parameter other than the concrete temperature in this equation. ASTM provides a test method to measure the datum temperature for a particular concrete mix. But, traditionally, the datum temperature can be assumed to be 14 degrees F or -10 degrees C. ASTM also suggests that for a mix with cement type I and within a certain temperature variations, the datum temperature can be taken as 32 degrees F or 0 degree C. Please note that in this equation, if the concrete temperature drops below the datum temperature, we do not include the negative value in the sum. So, we simply eliminate that interval from the equation.
If you plot the temperature-time factor values determined at different times along with the corresponding strength values at these times, you will get a plot like this. This is essentially the maturity-strength correlation that follows a logarithmic relationship. This is the simplest maturity method, and of course has some limitations. For example, it assumes that the rate of concrete strength development has a linear relationship with its temperature. So, some researchers have developed other variations of the Nurse-Saul equation to account for its limitations. And if you are interested to know the details you can find these in the literature.
The second method of measuring maturity index, is based on the Arrhenius equation that describes the effect of temperature on the rate of a chemical reaction. This method, unlike the Nurse-Saul method, allows for taking into account the effect of temperature on the rate of strength development by introducing the concept of equivalent age. As it is shown in this equation, the actual age of concrete is converted to its equivalent age in terms of strength gain at a specified temperature. This method does not have the main limitations of the Nurse-Saul method as it allows for a non-linear relationship between the initial rate of strength development and curing temperature.
In this equation, t-sub-e is the equivalent age at a specified temperature representing the maturity index, Q is the activation energy divided by gas constant. T-sub-a is average concrete temperature during time interval delta-t, T-sub-s is the specified temperature, and delta-t is the time interval.
In addition to the concrete temperature that is measured, Q and T-sub-s should be determined for a specific concrete mixture. ASTM provides a procedure for this, but it also suggests using a value of 5000 K for Q if cement type I is used. The specified temperature is traditionally taken as 20 degrees C or 293 degrees Kelvin, but in the US, a value of 23 degrees C or 296 degrees Kelvin is used for specified temperature.
If you plot the equivalent age values at various times along with the corresponding concrete strength values at those times, you will get a curve like this that similarly to the temperature-time factor method can be fitted with a logarithmic equation.
The third method of calculating concrete maturity index is called weighted maturity method. This method is standardized by NEN 5970 which is a Dutch standard and is adopted in some European countries. Similar to the Nurse-Saul method, we use a datum temperature. But, the area underneath the temperature curve is calculated using horizontal stripes. The average temperature of the stripe is called hardening temperature and its length is called hardening time. So, the first part of the weighted maturity equation essentially calculates the area underneath the temperature curve which is the same value as that obtained from the Nurse-Saul equation. But, in addition to that, a factor of C to the n-sub-k is introduced in this equation to account for the effect of temperature on the strength development of cement.
In this equation, M is the weighted maturity, t-sub-k is the hardening time, T-sub-k is the hardening temperature, C is the C-value of cement that considers the influence of cement on maturity and is provided by the cement manufacturer, and n-sub-k is the parameter that considers the sensitivity of the used cement to temperature and is based on the hardening temperature, T-sub-k.
Similar to the other two maturity methods, if you plot the weighted maturity values at different times along with the corresponding concrete strength values, a logarithmic relationship can be obtained. In the Dutch standard, the maturity curve that is used in the field is actually obtained by shifting the values in this curve down by a safety margin. So, this makes the weighted maturity method more conservative. Ok, now that we described all the three methods of calculating concrete maturity index, let’s see how a concrete mixture is calibrated for maturity-strength relationship.
The calibration procedure involves casting 15 cylindrical specimens in the lab from the mixture that is going to be used in the field. 15 cylinders are cast and tested for compressive strength at 5 different times. Here we are showing 1, 3, 7, 14, and 28 days as an example. But, if you are interested say in the first 3 days of in-place concrete strength, then you can select 5 different times during the first 3 days from casting the cylinders to test their strength. In parallel, you also need to cast two specimens and their monitor their core temperature over time. ASTM specifies to measure temperature in these specimens once every half-an-hour during the first 48 hours and then once every hour. When you obtained the temperature history of concrete, you can use any of the maturity methods that were described in the previous slides to calculate the concrete maturity index corresponding to the times at which concrete strength was tested.
Once you obtain the 5 pairs of strength and maturity index values, you can plot them in the so-called maturity calibration curve. So, the way it works is that you measure the maturity index of the same concrete mixture in the field, and then you can use this curve to estimate the real-time strength of concrete.
You can also simply fit a logarithmic function to this curve and calculate the a and b parameters from curve-fitting. So, this equation can be used in the field instead of the plot to estimate the in-place concrete strength based on its maturity index as calculated knowing the temperature history of concrete in the field. Please note that you may need to verify this calibration in the field in order to assure that concrete being placed in the field has not deviated from the original mix design.
Although the maturity method offers a convenient way of measuring concrete strength, it has some limitations as well.
The Nurse-Saul method, in particular, is limited as it assumes a linear relationship between the initial strength gain and concrete temperature. This is not valid when curing temperatures vary over a wide range.
Another limitation is that we are assuming the concrete mixture does not change in the field. So, if we get inconsistent concrete pours, the maturity method can not be accurately applied to estimate concrete strength.
Also, the general logarithmic relationship should be used with caution as it may not provide the best fit. So, some researchers including Nick Carino have shown that a hyperbolic equation provides a better curve-fitting.
It should also be noted that the values recommended by ASTM for datum temperature and activation energy are general recommendations when cement type I is used. So, for critical jobs and when high accuracy is needed, these values should be determined experimentally for the concrete mixture.
Another limitation of the maturity methods is that high temperature values at early-age will result in inaccurate predictions of long-term strength. So, one of the challenges in the maturity method is to compensate for the so-called “cross-over” effect.
Last but not least, the maturity concept assumes that adequate curing is provided so the changes in the humidity conditions on the job site are not really accounted for. And, in general, field-related issues such as those from improper concrete placement and consolidation, and variations in air content which could affect concrete strength can not necessarily be picked up by the maturity method.
In the field, the majority of uses for Maturity Monitoring is In-situ strength for post-tensioned decks. Maturity Monitoring is used almost exclusively for determining the strength of the stacks. It can also be used for the in-situ strength for form-jumping and cycling, generally to save time for the contractor. For example if the concrete needs to reach a strength of 500 psi in order to jump the forms to the next level, the needed maturity can be determined to minimize that cycle time.
A little less common, more unique use is in mass concrete, large elements, for thermal cracking. For example, a maximum temperature differential post-specified of 35 degrees in an element can leverage some ACI documents which an engineer can use to increase the allowed temperature depending on the capacity of the element and the allowable internal stresses.
There are 2 primary types of systems used for determining maturity. The first is an external data logger, that’s where you have wires running to a data logger, that’s sort of how the original systems were all setup. Down in the bottom right there, there is a unit that has 4 different channels. Typically all 4 channels would be used in-case any damage occurred to a wire during stripping or re-shoring. If the contractor cuts a wire, or pulls one out, you would lose that channel and lose the data. So to ensure redundancy you can use all 4. In order to access that data, you would go to that central unit and either see it on the screen or have to download it.
Another system is an individual logger. More recently, it’s become easier to integrate all those components into 1 small unit that can be embedded in the concrete. Each sensor then has to be accessed to get to the data, but it can be a little bit easier to spread out the sensors across the job site and also from a protection stand point, there’s a little less interference.
That one at the bottom middle there, that black end is the sensor, and all the data and reading is done there and stored there. That’s buried in the concrete, the wires just there to access that. On the left is the special unit that has to plug into it to download that data. The one on the right is a newer method. In the top image, that tail is the actual temperature sensor, the black head is where the data is stored and transmits. It’s buried in the concrete, usually close to the surface to increase the signal and it can be downloaded via Bluetooth, with a smartphone. So it’s a little bit easier to access. And these are just a few examples, there’s quite a few different maturity systems out there that are available.
For preparation you need to generate a calibration curve. STM mentions 28 day strengths, however if a mix takes longer than 3 or 4 days to reach the target strength you can bet that the mix is going to be substituted out for something more efficient. But that is performed in the lab.
There’s some question whether you use a ready-mix truck and sample it that way or produce a mix in the lab. In our opinion, it’s definitely best to go the ready-mix route, to more accurately model what’s actually happening in the field and what you’ll be getting. You start say getting 45 minutes away from the lab with the mix, you might want to start considering a lab mix, but in my opinion you’d still be better off casting the specimens, instrumenting in the field and transporting at later date than doing it in the lab.
As far as recalibrating, that’s probably the most difficult question. Realistically you have to use some engineering judgment. Anytime there is a material change, especially cement type, definitely re-calibrate then.
Also whenever using any add mixtures that might affect the rate of strength gain. For example many contractors like to use 2%-3% non-chloride accelerator in the mix during the winter and then in the spring they want to take it out and go back to an un-modified mix. That would be a good time to re-calibrate for that mix in case the rate of strength gain is a bit lower.
In the bottom right is an example of a calibration graph.
For installation you want to place the sensors at critical locations. Now the definition of critical is going to change depending on what you want to monitor. For a post-tension deck that might be at the thinnest section, it might be the last place, or perhaps a corner that’s a little bit hard to protect, you’re always going to be monitoring at a couple of different locations to try and identify the lowest value of strength.For a mass placement, critical location would be at the center of mass and at the surface.
As far as when to turn on and begin logging, it mostly depends on the system. The older systems would start calculating maturity as soon as you started logging from that location. Some of the newer sensors you can adjust the maturity after the fact. So with those you can turn on as soon as it’s installed.
You can see in the graph on the right an example of a mass placement. This one had a bottom logger, which can be seen in the blue line where the concrete hit first. The orange line is the surface. Where it jumps and takes a big dip is when the protection blankets were removed temporarily which had quite a big effect.
For determining how many sensors, you always want to plan for some loss of equipment as the construction environment can often be harsh and sensors can be damaged.
Once the sensors have been installed and the concrete has been placed, it’s time to start collecting the data and make it useful. Typically the on-site personal have a copy of that calibration curve to reference and make decisions based on that.
One of the big advantages of the maturity method is the ability to forecast strengths. For example if the maturity shows that another 200 or 300 TTF maturity is needed in order to reach the target strength, by assuming a future temperature for the concrete and knowing what the current temperature is you can determine a fairly accurate estimation.
One general tip would be installation timing. A lot is based on access. For a large mass placement, especially if you need to place the sensors at a specific location such as the center of mass, you might only have access 2 or 3 days ahead of time which will need to be planned out in advance.Some precautions can be taken to help protect the sensors as well. In the top right you can see a few sensors on the top bar that could be stepped on and damaged. Most of the time the sensors are rotated slightly to the side to provide more protection.
Another important tip is having strong communication with producers if you are using maturity. If for some reason different mixes were provided and used unknowingly, the rate of hydration and strength gain may be very different which would result in incorrect estimations. Most importantly you cannot ignore the real world with maturity. Maturity just uses temperature to determine the strength. In some cases you may use a Schmidt Hammer or a rebound hammer as backup to verify and confirm that the concrete in place is close to what’s being read for maturity.
Speaking more on benefits. Field cures are a hassle. Those cylinders could be lost or damaged upon pickup. Also the multiple trips between the calibrated lab and the project are not ideal. The ability to use maturity is a much more efficient method. Also the early completion time of projects is a major benefit. You can really optimize your deck cycling and form jumping, even saving a few hours, especially on a high-rise project with many stories. A few hours per deck can equate to major cost savings.
One example project where maturity was used was McCormick Hotel, which is a 41 story post-tensioned concrete floor plate, which is rather large at about 22,000 square feet per floor. Every placement, 3 loggers were installed. Either the contractor or the technician could check the data anytime, typically the next morning. They likely saved almost a full day on the post-tensioning cycle by using the maturity method. Multiplying that out by 82 deck placements, that’s almost 16 weeks over the project.
1571 Maple Avenue was a project where maturity was used for form-jumping. It’s a 13 story residential building with a couple concrete cores. 2 loggers were placed in every core placement. This helped shorten the critical path by allowing forms to be jumped every 4 days or so which again was a huge time saver.
Last case study which is a bit unique. Massive foundation elements required monitoring for thermal cracking. The engineer specified the max differential at 35 degrees. With the use of ACI207.2R, there are equations that determine the thermal stresses and temperature of cracking based on the differentials and the strength of the concrete. By using those equations and knowing the strength of the material in place, the differential limit was able to be relaxed slightly. Maturity sensors were used both in monitoring the temperatures, knowing what that differential is and then knowing what the in-place maturity is and the strength of the element to allow them to protect them for a shorter period of time.
The top left is an example of what the engineer came up with to monitor the center of mass as well as a corner to try and capture the worst case behavior. The graph below was generated from ACI207. As the in-place strength changes, the concrete is able to withstand higher cracking stresses from the thermal differentials. So as the concrete gains strength less protection is needed. On the left picture you can see some of the Bluetooth sensors attached to the rebar. Those will end up relatively close to the form so it’s a couple inches from the concrete where the data can be read from a smartphone.