Optimizing cold weather concreting practices

by sadia_badhon | April 10, 2020 11:20 am

By Alicia Hearns

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For many construction companies, the time of the year is approaching when dropping temperatures play a massive role in day-to-day site operations, especially where concrete is being employed. Concrete temperature monitoring becomes more critical in cold weather as low temperatures decrease the rate of strength development affecting the durability of the structure. For this reason, accurate and consistent temperature readings of concrete elements are important so that the quality of the structure is optimized.

Understanding heat of hydration

The heat produced by concrete during curing is characterized by an exothermic reaction called heat of hydration. After this initial reaction, the rate of hydration significantly slows down. At this time, fresh concrete is usually transported and placed. The concrete then enters the dormant phase where it has not yet hardened and is workable. Due to this, the length of the dormant period depends on multiple factors, including cement type, additives such as accelerators and retarders, and additions like fly ash and slag. The amount of heat produced during hydration is largely related to the volume, composition, and fineness of the cement.

The end of this phase is characterized by the initial set where strength gain begins. As the concrete hardens, the heat generated during this phase can last for multiple hours. The temperature of the in-situ concrete then begins to stabilize, and the hydration process slows down. It is extremely important to control the temperature of concrete during the dormant and setting phases.

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Importance of temperature monitoring in cold weather

The hydration process can be drastically impacted if freshly placed concrete is exposed to temperatures that are too high or too low, compromising strength development. Therefore, the American Concrete Institute (ACI) has provided specific guidelines that outline an allowable temperature range for each pour. This range depends on the element size and ambient conditions (ACI 207, Guide to Mass Concrete, 301, Specifications for Structural Concrete, and 306, Guide to Cold Weather Concreting).

Guidelines for pouring concrete in cold weather can be found in ACI 306. ‘Cold weather’ is defined as three or more consecutive days of low temperatures, specifically ambient temperatures below 4 C (40 F) and air temperatures below 10 C (50 F) for more than any 12-hour period. To ensure proper strength development in these ambient conditions, some placement specifications include:

• concrete temperatures must be maintained higher than 4 C for 48 hours after a pour;
• all concrete surfaces must be protected within the first 24 hours after being placed to prevent freeze damage; and
• concrete temperature cannot reach freezing levels before reaching a specific strength (3.5 MPa [500 psi]) or the overall structure will have a reduced strength.

Controlling concrete curing conditions in cold weather

To ensure concrete elements are curing properly in cold weather, different techniques are used to maintain high temperatures and consistent strength gain. One technique includes the use of insulation blankets. These protective blankets cover the concrete to distribute the heat generated during the hydration process. This helps maintain consistent temperatures throughout the concrete element. The same can also be achieved using external heating systems.

The rate of strength gain can also be increased by using a mix with a lower slump (less water), adding hot water to the mix, and using additives to accelerate curing and lower the concrete’s setting time. Moreover, reducing the time between mixing and placing the concrete can minimize a drop in temperature.

Another approach for controlling heat generation is to select a specific concrete mix design for cold weather conditions. For example, type III cement can generate more heat than types I and II. Further, using finer cement will generate more heat. Similarly, using supplementary cementitious materials (SCMs), such as slag or fly ash, will reduce the amount of reactive materials in the early stages and, therefore, the amount of heat generated.

Why does it matter?

As mentioned earlier, if the ambient temperature is too low, the hydration of the cement will slow down or stop altogether until the temperature increases. As a result, there will be a considerable reduction or an end to the strength development of the concrete element. Hence, it is important to understand these guidelines and how concrete performs during cold weather so temperature and curing conditions can be controlled easily.

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Furthermore, in mass concrete elements, if proper techniques are not used to control the temperature, thermal cracking can occur. This happens when the core temperature of the element is very high due to the mass effect, while the surface temperature is lower because of ambient conditions. In these cases, the temperature differential between the core and the surface is too large, resulting in tensile stress in the concrete. If the tensile stresses outrange the tensile strength of the concrete, cracking will occur.

Cracking reduces concrete’s durability by increasing the permeability of the structure and making it easier for water, air, and chloride to penetrate the material. This can result in rebar corrosion and, in severe cases, reduce the durability and integrity of the entire concrete structure. Therefore, it is essential to monitor concrete mix, ambient temperature, and differential temperature and to adjust curing techniques appropriately to avoid thermal cracking.

Five common mistakes to avoid during cold weather concreting

To ensure proper strength gain and maintain control of the temperature of in-situ elements, it is important to keep these methods in mind when pouring concrete in cold weather. It is also advisable to familiarize oneself with these five common mistakes.

Pouring concrete on frozen ground

If concrete is poured on frozen ground, it increases the risk of cracking. This happens as the fresh concrete closest to the ground continues to cure slower than the surface. Additionally, frozen ground can settle when thawed, causing the concrete to crack.

Allowing concrete to freeze

Concrete should be kept warm, around 10 C (50 F), to cure properly. Fresh concrete can freeze at –4 C (25 F), so it is important to keep it warm until it has reached the specified strength targets.

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Using cold tools

It is equally important to keep the tools and building materials warm. If forms or tools are too cold, it could alter the concrete coming into contact with them.

Sealing concrete when it is too cold

Concrete sealers make the material more resistant to weather exposure and other outside elements. If concrete is being placed in cold weather, it is advised to get a sealer that works well in extreme weather conditions. Sealing typically should not be done if the temperature is below 10 C.

Misjudging daylight

During colder months, the amount of daylight lessens. It is essential to use time wisely as daylight not only gives more light, making work easier, it also results in warmer temperatures.

Measuring concrete temperature and strength in cold weather

The most common method for monitoring the strength of in-situ concrete is the use of field-cured cylinders. This practice has remained generally unchanged since the early 19^th century. The samples are casted and cured according to ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, and tested for compressive strength by a third-party lab at various ages. Usually, if the slab has reached 75 per cent of its designed strength, engineers will give the ‘go ahead’ to their team to move on to the next steps in the construction process.

However, when pouring in cold weather, ACI 306 specifically recommends not using this method as field-cured cylinders “can cause confusion and unnecessary delays in construction.” This is largely because it makes it difficult to maintain the cylinders in the same conditions as the in-situ concrete. It is, therefore, recommended that other in-place testing methods, or maturity testing, be used for monitoring concrete strength. When these methods are correlated to standard-cured cylinders, they can be used to determine the concrete strength accurately. Below are some examples of in-place testing methods.

Rebound hammer or Schmidt hammer

This is covered in ASTM C805, Standard Test Method for Rebound Number of Hardened Concrete. A spring release mechanism is used to activate a hammer which impacts a plunger to drive into the surface of the concrete. The rebound distance from the hammer to the surface of the concrete is given a value from 10 to 100. This measurement is then correlated to the concrete’s strength.

Pros

It is relatively easy to use and can be done directly onsite.

Cons

A pre-calibration using cored samples is required for accurate measurements. Test results can be skewed by surface conditions and the presence of large aggregates or rebar below the testing location.

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Penetration resistance test

This is outlined in ASTM C803, Standard Test Method for Penetration Resistance of Hardened Concrete. To complete a penetration resistance test, a device drives a small pin or probe into the surface of the concrete. The force used to penetrate the surface, and the depth of the hole, is correlated to the strength of the in-place concrete.

Pros

It is relatively easy to use and can be performed directly onsite.

Cons

Data is significantly affected by surface conditions, as well as the type of form and the aggregates used. It requires pre-calibration using multiple concrete samples for accurate strength measurements.

Ultrasonic pulse velocity

This is covered in ASTM C597, Standard Test Method for Pulse Velocity Through Concrete. This technique determines the velocity of a pulse of vibrational energy through a slab. The ease at which this energy makes its way through the slab provides measurements regarding the concrete’s elasticity, resistance to deformation or stress, and density. This data is then correlated to the slab’s strength.

Pros

This is a non-destructive testing technique which can also be used to detect flaws within the concrete such as cracks and honeycombing.

Cons

This technique is very highly influenced by the presence of reinforcements, aggregates, and the moisture in the concrete element. It also requires calibration with multiple samples for accurate testing.

Pullout test

The main principle behind this test, as per ASTM C900, Standard Test Method for Pullout Strength of Hardened Concrete, is to pull the concrete using a metal rod that is cast-in-place or post-installed in the concrete. The pulled conical shape, in combination with the force required to pull the concrete, is correlated to compressive strength.

Pros

It is easy to use and can be performed on both new and old structures.

Cons

This test involves crushing or damaging the concrete. A large number of test samples are needed at different locations of the slab for accurate results.

Cast-in-place cylinders

This is covered in ASTM C873, Standard Test Method for Compressive Strength of Concrete Cylinders Cast in Place in Cylindrical Molds. Cylinder moulds are placed in the location of the pour. Fresh concrete is poured into these moulds which remain in the slab. Once hardened, these specimens are removed and compressed for strength.

Pros

It is considered more accurate than field-cured specimens because the concrete is subject to the same curing conditions of the in-place slab.

Cons

This is a destructive technique that requires damaging the structural integrity of the slab. The locations of the holes need to be repaired afterward. A lab must be used to obtain strength data.

Drilled core

This is outlined in ASTM C42, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. A core drill is used to extract hardened concrete from the slab. These samples are then compressed in a machine to monitor the strength of the in-situ concrete.

Pros

These samples are considered more accurate than field-cured specimens because the concrete that is tested for strength has been subjected to the actual thermal history and curing conditions of the in-place slab.

Cons

This is also a destructive technique that requires damaging the structural integrity of the slab. The results can be influenced by the direction of the extraction (e.g. columns or slabs). The locations of the cores need to be repaired afterward. A lab must be used to obtain strength data.

Tests like the rebound hammer and penetration resistance technique, while easy to perform, are considered less accurate than other testing methods, as they do not examine the centre of the concrete element, only the curing conditions directly below the surface of the slab. This is especially important during cold weather as temperature differentials can affect the durability of the structure. Practices such as the ultrasonic pulse velocity method and the pullout test are more difficult to perform as their calibration process is lengthy, requiring a large number of sample specimens to obtain accurate data.

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As destructive testing techniques, the drilled core and cast-in-place cylinder methods need third-party labs to perform break tests to get the data. As a result, more time is needed in the project schedule when using either of these methods.

The maturity method

The maturity method, per ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method, is based on the principle that concrete strength is directly related to its hydration temperature history. Though this methodology has been used since the 1970s, it has only recently gained popularity on jobsites with the use of wireless sensors. The method is defined as “a technique for estimating concrete strength that is based on the assumption that samples of a given concrete mix attain equal strengths if they attain equal values of the maturity index.” In other words, maturity is a value that represents the progression of concrete curing. As a result, a mix calibration is required to implement this technique in the project.

The goal of the calibration is to determine a relationship between maturity and strength for a specific mix. A maturity calibration only needs to be completed once for a specific mix and its properties. This maturity-strength relationship is developed in the lab, using cylinder break tests, and then correlated to the strength of the in-place concrete on the jobsite. Though it may seem daunting, the maturity calibration can be done in the following five easy steps:

1. Make a minimum of 17 cylinders­­—two for temperature monitoring and the rest of the specimens for testing compressive strength breaks. All cylinders must be cured together in a moist environment according to ASTM C511, Standard Specification for Mixing Rooms, Moist Cabinets, Moist Rooms, and Water Storage Tanks Used in the Testing of Hydraulic Cements and Concretes.
2. Select a minimum of five break times. For example one, three, seven, 14, and 28 days. For each day, obtain the compressive strength of two cylinders. It is advisable to break a third cylinder if the results vary more than 10 per cent from the average. It is also important to note the time of the breaks.
3. At the time of the breaks, it is critical to obtain data from the two cylinders that were used for temperature monitoring and make an average of these values. Input these values into either the Nurse-Saul Equation or the Arrhenius Method according to ASTM C1074 to obtain a maturity value.
4. At this point, there will be a set of at least five data points each with a strength associated to a maturity value. Plotting the data points allows one to obtain a curve with a logarithm equation. The formula for calculating the maturity of concrete is: Strength=a+b LOG (maturity).
5. It is important to validate the calibration curve by making a couple of additional cylinders on the next pour. Compare the calculated strength obtained from the maturity calculation to the compressive strength that was achieved in the lab. A difference of up to 10 per cent is acceptable.

As a non-destructive testing technique, concrete maturity allows one to estimate the early age and compressive strength of in-place concrete. This is done using a sensor. Wireless sensors are placed within the concrete formwork and secured on the rebar before pouring. The goal of these sensors is to measure the temperature of the slab in real-time and correlate this data to the concrete’s strength based on the mix’s maturity calibration. A physical connection to the sensor is not needed to obtain this information. Instead, data is uploaded to any smart device within an app using a wireless connection where it is updated every 15 minutes.

Data collected by these sensors is considered more accurate and reliable as they are fully embedded in the formwork, meaning they are subject to the same curing conditions as the in-situ concrete element. Equipped with real-time results, contractors can improve the heating process, decrease energy costs, and save time in their project schedule by knowing when to move on to subsequent construction operations, such as formwork removal or post-tensioning.

The main limitation of wireless sensors is the maximum allowable distance between the sensor and the surface of the concrete. Since concrete can block wireless signals, the sensors usually need to be placed within a certain distance from the surface to ensure connectivity.

Selecting the method of strength testing

Monitoring the temperature and strength of the concrete during cold weather becomes much more difficult as a result of ambient conditions and the need for extra equipment onsite to monitor the in-situ elements. The decision in choosing a testing method may simply come down to what the individual knows and is used to. However, the accuracy of these tests, and the time they take to obtain strength data, are significant factors that must be taken into consideration. When pouring concrete in cold weather, the technique chosen not only determines how easy it will be to obtain strength values, but also play a part in whether the project will stay on schedule. This method will also ensure the strength, quality, and durability of the structure is optimized.

[7]Alicia Hearns is a content marketing specialist with Giatec Scientific Inc., a global company providing smart concrete testing technologies and real-time data collection. She can be reached via e-mail at alicia.hearns@giatec.ca.

Source URL: https://www.constructioncanada.net/optimizing-cold-weather-concreting-practices/

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