Hot Weather Concreting: How the Maturity Method Technique Helps

Hello everyone. Thanks for joining me on to this webinar where we’ll be talking about hot weather, concreting and how the Maturity Technique helps concrete professionals. We are approaching the season of the year when concrete professionals will be placing concrete under high air temperatures, hence the relevance of today’s topic. 

My name is Oluseun, and I work with the Engineering Solutions Department at Giatec. I have a Masters Degree in Civil Engineering with a focus on intelligent structures and structural mechanics and in the last eight years I’ve been involved in detailed engineering designs, nondestructive testing, and concrete construction quality control. 

I would like to remind the attendees of today’s webinar that if you have questions or comments, please feel free to put them in the comment section provided and we would make sure to get back to you. Thank you. 

Before we dive right into this topic, I will also like to introduce Giatec Scientific to some of us that are not very familiar with the company. Giatec Scientific is a research driven engineering company that develops innovative products for the construction industry. Its vision is to revolutionize the construction industry and in the last 10 years, Giatec has been at the forefront of advanced technology innovations for the industry. 

For today’s webinar we will be taking a look at a number of subtopics that ranges from understanding what hot weather concreting is, some of the effects of hot weather condition on our concrete properties, then how we can monitor concrete properties in hot weather conditions. Then what exactly is maturity and where does it fit in all of this? We will take a look at some of the critical operations that we carry out in hot weather conditions, and then what are the ways we can successfully place our fresh concrete in hot weather condition. Then finally, we’ll talk about ways we can cool fresh concrete based on standard guidelines and available techniques out there. 

According to ACI 305 hot weather concreting could involve one or a combination of these conditions that can accelerate the rate of moisture loss and rate of cement hydration, which consequently could impair the quality of freshly mixed and hardened concrete. These factors are very critical when we’re placing our concrete on the very high temperatures, because at the end of the day they have a couple of effects on freshly placed or hardened concrete. 

Chapter four of the ACI 305 gives detailed explanations on the effect of hot weather on concrete properties. For example, figure 4.2 of the standard gives a very good overview of how these factors could affect the rate of moisture laws or evaporation from the surface of our fresh concrete. 

But take, for example, if we have air temperature of up to 80 degrees Fahrenheit and we have a relative humidity of about 50%, that could lead us to having a concrete temperature of about 90 degrees Fahrenheit. Now when we trace that to a situation where the wind velocity on site is very high, then we end up having a rapid or very high rate of evaporation and this is quite undesirable for freshly placed concrete. We’ll look at some of the effects this factors and why it is super important for us to be mindful of these factors when we’re placing concrete in hot weather conditions. 

So, what are some of the effects of hot weather conditions on concrete on fresh concrete properties to start with? There is a possibility for us to have increased rate of slump, loss in our fresh concrete. At high temperatures, of course, then there is also a possibility to have reduced work ability which at the end of the day will result in some handling difficulty for the people placing or working on the concrete on the job site. Then with high temperature comes increased water demand before our fresh concrete. And all of this could contribute to having a situation where we would have difficulties with being able to finish the surface of our concrete. And finally, there is also every possibility to have plastic shrinkage occurring on freshly placed concrete. Starting with the Rita slump loss for constant mixing time, the amount of water required to produce a required slump in concrete increases with high temperature. In other words, in high temperatures we would need more water content to achieve desired slopes. Let’s take, for example, Concrete truck leaves the packing plant with the concrete slump of about 7 inches. Depending on the distance of the budget, plan to the job site, among other factors, that’s long reduced by one or two inches, for example. Now to gain the walkability of that concrete, more water would need to be added to the concrete clearly from this chart at very high air temperatures. Which of course could also lead to high temperature concrete. Temperature under the water required tends to be very high. To achieve walkability and slope, then it also made mention of the fact that the surface of our concrete could become more difficult to finish. Increased handling difficulty is a common challenge that handlers have to deal with at high temperatures, and this consequently reduces their ability to finish the surface of concrete or fresh concrete very well, as desired. We also made mention of the increased water demand for fresh concrete. The water content required to achieve desired slumps, walkability of fresh concrete often increases at elevated and urban temperatures. And this causes a decrease in the rate of setting, especially if we don’t pay attention to the cementitious material. Ideally, when water content increases, we also have to adjust the cementitious material proportionately in order for us to achieve desired strength and durability in due time. 

Having looked at some of the effects of hot weather conditions on fresh concrete, what are some of the effects of hot weather condition on hardening concrete as well? We have the possibility of hardening concrete to experience drying shrinkage. There is also the possibility to have some cracking on our concrete data thermal stresses. They would also have reduced durability as a result of the cracking to occur in our concrete. Then we have instances of cold joints on some of the sections of our concrete. Take two differential hardening. Then there is also the probability for the strength gain at ladder age for concrete on the high temperature to be bit lower than what we would experience for samples that are completed or are being cured at a steady relatively lower temperature. 

Take a look at some of these effects one after the other. Starting with drying shrinkage due to rough rapid evaporation of the moisture content from the surface of our concrete. 

There is a possibility to have different cracks on the surface of the concrete and the rapid evaporation is usually caused by high wind speed, high air temperature or low relative humidity. Now when we have drying shrinkage cracking on a concrete surface, it grossly affects the long-term durability of our concrete. Which means through these cracks we could have harmful external agents making their way into our concrete and this could be very very undesirable, especially for reinforced concrete structural elements. I also make mention of the fact that there is high probability for some cracking to occur. Especially when we consider relatively large concrete sections or sections otherwise known as mass concrete elements, when we place mass concrete elements where there is a very high level of temperature according to the heat of hydration from the very center of that mass element. 

Concrete is commonly known to be very good in compression, but poor in tension. But the thermal stresses that mass elements are subjected to tend to cause some sort of expansion from the very central interior of the concrete, and contraction will be going on at the surface due to the temperature differential between these two regions. When we have a huge temperature differential between the core and the surface of our concrete, then such a concrete would be subjected to very high thermal stresses leading to cracks and again this will be very, very undesirable for long term durability goals on our concrete. 

Then we could have cold joints at different sections of our concrete data rapid hydration, which could cause differential hardening of the concrete section. The rapid  hydration rather occurs when, based on the factors that we mentioned earlier; high wind speed, lowering of humidity, whatever fresh concrete we bring to an existing concrete structure or concrete of the same mix poured at different times of the pouring days, we could have situations where a part of that section hydrates very rapidly, causing a rapid hardening of the section, whereas other sections are yet to hard in those, causing some weakness in terms of bonding and those when we have cold joints. This also could allow for the increase of harmful external agents, and for the perspective of durability, this is absolutely undesirable for concrete. 

Then lastly, we made mention of the probability for our concrete to rapidly gain strength at a very early age on the high temperature. Rapid hydration occurs due to high air temperature in our concrete, and concrete from a given mix under elevated temperatures tends to gain strength much faster at a very early age, but this strength gain tends to lower at the later age. While if we have a lower relatively lower temperature condition to cure those same samples from that same mix then we end up having a concrete section with higher strength at later age. This phenomenon is often referred to as the crossover effect, whereby concrete that are cured under relatively lower temperature gains more strength at that latter age compared to those that are cured or subjected to very high air temperatures. 

But what are some of the ways that we utilized to monitor the strength of our concrete in hot weather conditions? Various methods are utilized to cure and test for the strength of concrete. Concrete samples are killed either on the field or in the laboratory to test for their strength. In some cases, majority meters can also be put in place on the actual structural elements to track the majority of that concrete element based on the concrete temperature data. It is noteworthy; however, that when we have field cured samples taken from the same mix, these samples would rarely have a full representation of what the actual temperature condition of the outside structure element is and this is because of two reasons: one, the samples that are taken off are of smaller volume and the environment where these samples are placed are often protected, say under shade on the job site or in the truck. The standard also provides guidelines on laboratory curing techniques, but this condition rarely represents the fields current condition, nor the outside temperature conditions that the actual structural concrete element is subjected to. And what this leads to is a variation in and some sort of discrepancies due to certain limitations, such as inaccurate temperature measurements based on the conditions that these samples are subjected to. We could also have delayed results, especially when we’re looking at the perspective of samples that are taken to a controlled temperature environment in the laboratories. Then we have limited information about what exactly is the temperature condition that our actual concrete element is subjected to, and this gives us low visibility as to understanding the local variations that are occurring to our structural element. 

Having said all this, what are some of the critical operations that we carry out when we test for the strength of our concrete, for example? This could be the removal of formworks, it could also involve the post tensioning of cables in our pity slabs or decks. We could also be opening concrete pavement to traffic, and in some cases, the focus of professionals are to have a very good understanding of the temperature difference of their mass elements. I have made mention of how thermal stresses can really cause huge problems for mass concrete elements, so this is quite critical in some instances; to monitor the mass concrete temperature differential between the core and the surface of our concrete. 

But where do those the maturity methods fit in with all of this? The ASTM C1074 defines the maturity method as a technique for estimating concrete strength based on the assumption that for some samples of a given concrete mix, when those samples achieve the same maturity index, they are bound to have equal strength. The relationship between maturity and strength is quite unique, and we have to note that the maturity index in itself is a function of concrete temperature. Then, we relate this with concrete strength for specific concrete mixtures, and when we build this relationship, we have a better understanding of what maturity and strength for concrete mix looks like. But we need to understand that this relationship is specific to one mix design. Whenever a mixed proportion changes in the concrete, this relationship has to be established. It is also important to note that if we have concrete of the same mix design placed in hot weather condition versus colder weather seasons, the concrete samples would develop maturity at different rates. We take a look at how some of these relate so you can get a better understanding of what I’m trying to drive out here. 

Across North America, there are various standards that gives guidelines on how to utilize this technique. While the ASTM C1074 provides standard guidelines on this method, the standard also makes references to this technique. There are various maturity questions that could be utilized in the field. We have the temperature time factor or that TTF as it’s called. We also have the equivalent age method and the weighted majority calculation method. I have to point out though, that the weighted maturity method is quite common across northern Europe. But in all, the maturity index is primarily dependent on the temperature history of our concrete. 

First, the schtick of today’s webinar will be focusing our attention on the temperature time factor equation, and the reason for this is because it’s quite common here across North America. It is very conservative and very easy to implement. 

Out of the equation, the parameter that we probably are not very familiar with, or we might not have is the different temperature, but the standard provides a constant value for this. The ASTM C1074 provides for the fact that when we are utilizing cement types, one and carrying on with a maximum temperature of no more than 40 degrees Celsius, We can utilize the daytime temperature to be zero degrees Celsius or 32 degrees Fahrenheit. Now what exactly is the daytime temperature as indicated from this equation? That’s the T note. I’ll take you through an example, assuming this is a typical concrete temperature profile. The term temperature, T note, is the temperature below which concrete is assumed to struggle with gaining strength. While the standard gives guidelines on how we can manually calculate the temperature, I can say that it is safe to use the constant provided of say 32 or 0 degrees Celsius. That’s two degrees Fahrenheit or zero degrees Celsius in hot weather seasons because our current temperature would not drop below this temperature level. 

Let’s take an instance on our temperature chart for concrete. Assuming my time T, this time T could range from anytime from after the concrete is poured till we are satisfied with the strength gain at say 28 days for example. My primary interest is to calculate the area under this chart, and this is where the majority equation comes into play. Like I said, time T could be 24 hours after pouring it, three days, seven days, 14, 28 and so on. But primarily the information of the average temperature TA is what we measure from either cylindrical samples that are taken from the job site or the actual structural element temperature. 

Let’s take a look at a real example where concrete samples were taken from the same concrete mix at the same time. The samples were cured in all the three curing conditions that are mentioned earlier. The green line represents the on-site temperature measurement and the red line represents the temperature of samples they were taking and cured in the field, whereas the blue line is that of the samples that are taken to the laboratory. Clearly, both the field and lab cured samples had relatively lower temperature during this entire curing period. These low temperature readings are a result of the curing conditions that these samples were subjected to. For example, field cure samples like I mentioned earlier are taken from the trucks and kept sometimes under shade or in the on-site offices or trucks, whereas the lab cured samples are also kept under controlled room temperature, and this does not necessarily represent the actual curing temperature that our concrete structural components are exposed to on the job site. It is noteworthy, however, that when we have a system in place that can adequately measure the temperature of on site concrete, this provides us with a true representation and a better understanding of the temperature conditions that our concrete elements are subjected to. 

Let’s take a look at how these various temperature profile translates to the maturity index. I’ve mentioned earlier that maturity index is primarily a function of concrete temperature time history. Clearly, based on the variation of the temperature profiles we had based on the different caring conditions, the on-site concrete component has a higher maturity index when compared to both samples that are cured in the field kept somewhere and those that are taken in the laboratory. How does this translate to strength? What are the implications this has on our strength on a concrete strength result? Let’s take a look. 

We clearly could see that for the on-site monitored strength, it attained the threshold of 4300 PSI ahead of the lab cured samples signals by almost a day. Now, relative to the field-cured samples, which attained the threshold of 4300 PSI about half a day after actual components had reached the threshold, we could clearly see the variation in terms of strength. So, while we’re waiting for the result from the laboratory, actual structural component had already attained the required strength. And if we could have a knowledge of this information ahead of time, this would mean a big deal in terms of reducing our project timelines and reducing costs. It is noteworthy, however, that on the conservative side, the results of the laboratory cured samples are quite acceptable. 

So, what are some of the ways we can successfully place concrete in hot weather conditions? We can have concrete mixers include set retarders so we can delete the hydration process and therefore reduce the concrete setting times. We can also have some portions of our cement content substituted with pozzolanic materials such as fly ash and silica films which are quite common nowadays. Then we can cool some of our mix ingredients or the entire fresh concrete to reduce the temperature. We could also moisten the subgrade or say the formworks of where we will be placing the concrete prior to placement. When we moisten the subgrade, this relates to instances of concrete pavement, this can also really contribute to reducing the eventual temperature that our concrete will experience on this site. Then we can have a system in place to adequately provide for machinery and manpower so that we can place, finish and cure concrete in due time. It is quite common because in hot weather seasons we do not have the luxury of time, so it is super important for us to plan ahead of time so that we can have adequacy in terms of the machinery and the manpower that will be needed to complete the placement exercise. Nowadays, we have maturity meters, such as the SmartRock® sensors that can be fully embedded in the on-site concrete to track the concrete temperature data. With the knowledge of the temperature, we can have an idea of what the maturity index of that concrete is and relate that to the strength gain. Lastly, it is also possible to take advantage of the lower temperatures outside the peak hours of the day. It is common for concrete professionals during summer seasons, for example, to pour concrete early in the morning when temperature levels are very low to take advantage of that period before we have temperature rise during the day. I believe all of these techniques can really help in successfully carrying out placement of fresh concrete in hot weather conditions. We will take a look at how we can cool mix ingredients or the entire fresh concrete to reduce the concrete temperature during hot weather seasons. 

What are some of the ways we can cool fresh concrete in hot water? We can have some of the water content substituted with crossties. This process lowers the concrete temperature dramatically. Ice blocks are crushed to find grains before we add them to the fresh concrete trucks. And when ice melts, it absorbs heat, lowering the temperature of our concrete in the process. ACI 305 recommends that the ice that we’re adding to our concrete mix must completely melt on or before the concrete mixing process is completed.  We can also store cement materials in multiple silos to allow for the cement to cool down. For example, if we have limited number of silos and we end up adding new cement contents to existing cement are yet to cool down, this would not allow for the cement to cool down properly and we end up having cement deliver to batching plants at very high temperatures. Now this is undesirable and if we can reduce the temperature of cement before they are brought to the batching plant for mixing, it will also greatly help reduce eventual temperature experience for our concrete. Then it is also a common practice to store our aggregates on in some protected environment. We can store them with mist or under shade to prevent them from reaching high temperatures when directly exposed to the sunlight during summer seasons or hot weather conditions. Water sprinklers or just water systems can also be installed in storage facilities from time to time reduce the temperature of aggregate. When we lower the temperature of aggregate it cools down the entire concrete mixture eventually without changing the water content of our mix. And finally, nowadays we also have common practices where liquid nitrogen are being introduced into the entire concrete mix or the cement paste, or to the aggregate. When we add liquid nitrogen to aggregate, for example, this aggregates enters into the concrete mix and act as ice cubes, thereby reducing the eventual temperature generated from concrete such that when we take this concrete, when they arrive on site, we would have reduced drastically the temperature of concrete before being placed. And this would really help prevent some of the effects that we mentioned at the beginning of this presentation. This process of introducing liquid nitrogen works effectively because liquid nitrogen itself has a very low temperature at about negative 196 Fahrenheit. So, this cools down the whole mix more effectively without changing our water content at all. But I have to point out though, that this technique can be quite expensive, at least for now, until better techniques are developed, and this becomes much cheaper and more affordable for everybody. 

I think it’s a great point for us to wrap it up for today. I really want to appreciate everyone for spending the time to attend to this webinar. Again, don’t forget to put your questions or comment in the comment section. And you can also contact us directly at Giatec if you have any inquiries about our product and services. And until next time please stay safe. Thank you.