What is ‘Mass Concrete’?
Why Is It Important to Monitor Mass Concrete Temperature?
How Do Temperature Differentials Affect Mass Concrete?
What’s the Best Way to Monitor Temperature Differentials?
Why Are Temperature Differentials in Mass Concrete So Important in Winter?
Why Are Temperature Differentials in Mass Concrete So Important in Summer?

 In the unique climatic conditions of Australia and New Zealand, monitoring peak temperatures and temperature differentials in mass concrete is crucial for the durability and structural integrity of concrete structures. When it comes to mass concrete structures, temperature differentials need to be considered due to the risk of a large difference between the relatively hot internal temperature and cool surface temperature. If a too-large temperature differential occurs, the surface of mass concrete will start cracking. For this reason, the CCANZ and CCAA identify that limiting the temperature rise in concrete is important to prevent cracking and improve mass concrete structures’ durability and the length of its service life.

What is ‘Mass Concrete’?

Mass concrete isn’t defined by any specific measurements. According to the American Concrete Institute (ACI), mass concrete is “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimise cracking.” Some examples of mass concrete include dams, large bridge piers and columns, mat slabs and foundations. In Australia and New Zealand, mass concrete that requires thermal monitoring can sometimes be classified as a concrete element that has at least one side greater than 500 or 600 mm.

It’s important to note that smaller structures may also be categorised as mass concrete depending on several factors, such as type and quantity of cement, volume to surface ratio of the concrete, weather conditions, concrete placing temperatures, degree of restraints to volume changes, and the effect of thermal cracking on function, durability, and appearance.

Why is it Important to Monitor Mass Concrete Temperature?

In Australia and New Zealand, where mass concrete projects range from major infrastructure developments to large scale private developments, the necessity of pouring mass concrete within the optimal ambient temperature range of 10-16 °C is critical. The diverse climates of these regions, from the tropical north Queensland to the cooler climates of New Zealand’s Christchurch, present unique challenges in maintaining optimal concrete temperatures. The hydration process of cement in concrete generates heat, and in large concrete masses, this can lead to significant internal temperature rises. If not properly managed, temperature differentials between the core and the surface can cause thermal cracking. This is particularly crucial in these regions, where temperature variations can be pronounced.

The curing temperature of concrete significantly influences its strength development. In the warmer Australian climate, especially in the northern regions, concrete can cure more rapidly, potentially leading to lower final strength if the initial temperatures are not carefully controlled. Conversely, in the cooler climates prevalent in much of New Zealand, ensuring that the concrete does not cure too slowly or unevenly is essential to achieve the desired strength in a timely manner and durability. Moreover, both countries have strict construction standards and specifications to ensure the longevity and safety of structures. Adhering to these standards often requires reliable and accurate temperature monitoring to maintain compliance and ensure quality control throughout the construction process.

Peak temperature control in concrete is critical as it is closely linked to the durability of the final structure with issues such as delayed ettringite formation (DEF). As mentioned in the CCAA Guide to Concrete Construction:

“Delayed ettringite formation (DEF) may result in the expansion and cracking of concrete associated with the delayed formation of the mineral ettringite which is a normal product of early cement hydration. DEF is generally a result of high early temperatures in concrete (typically an early curing temperature above 70°C to 80°C) which prevents the normal formation of ettringite. The likelihood of this occurring is increased where thicker sections of concrete are cast (mass concrete), where concrete contains low levels or no SCM and contains higher levels of Portland Cement.”

During the design of concrete elements with respect to peak temperatures and temperature differentials, designers utilise CIRIA C766, AS5100.5, CIA Z Series, government specifications and published papers as guides.

Consistent monitoring of concrete temperature is a critical practice in these regions to mitigate such risks, ensuring that structures not only meet immediate functional requirements but also stand the test of time in varying environmental conditions.

How Do Temperature Differentials Affect Mass Concrete?

As concrete hydrates and hardens, it generates heat. When constructing mass concrete structures, significant tensile stresses and strains are likely to develop due to the difference between the structure’s core temperature and surface temperature (this difference is known as the temperature differential). These stresses occur because the volume of the warmer concrete expands while the cooler concrete contracts.

The likelihood of thermal cracking increases as the inner core of the mass concrete continues to heat (hydration) while the outer surface of the concrete cools (heat dissipation). This is exacerbated by extreme cold weather, which quickly cools the outer, exposed surface of the concrete, but does not reach the inner core.

According to ACI 301-16: Specifications for Structural Concrete and CCAA, the maximum concrete temperature differential should not exceed 19-20°C during curing. In most situations, this approach is very conservative, but in other cases, it can be an overestimation of the allowable temperature differential.

Without monitoring temperature differentials in mass concrete, contractors and project managers could run into serious problems such as cracking, reduced service life, project delays and noncompliance. These problems can make it hard to have a sustainable concrete construction.

What’s the Best Way To Monitor Temperature Differentials?

On construction sites, temperature differentials are normally measured using thermocouples or data loggers. Unfortunately, using these tools to collect data and subsequently analysing the data on a computer becomes quite time-consuming, which negatively impacts a project’s schedule.

Fortunately, contractors and engineers can take advantage of advanced technology that uses the maturity method to monitor concrete temperatures. One such technology is Giatec’s SmartRock®, which has now been re-engineered with innovative dual-temperature capabilities. This trusted wireless sensor, used on over 10,000 sites worldwide, is installed on the rebar before a mass concrete pour and enables the measurement of concrete temperatures in two locations simultaneously.

These easy-to-install sensors offer precise real-time temperature readings for mass concrete pours at the surface and centre of the slab, which are immediately sent to the user’s mobile device.

For greater control over the monitoring and analysis of mass concrete element temperatures, project owners and contractors can complement SmartRock with SmartHub™, a remote monitoring system that allows users to access SmartRock data anytime, anywhere (even off-site).

Why Are Temperature Differentials in Mass Concrete so Important in Winter?

In ACI 306: Guide to Cold Weather Concreting cold weather concreting is defined as “a period when for more than three successive days the average daily air temperature drops below 5°C and stays below 10°C for more than one-half of any 24 hour period.” In many regions, such as parts of New Zealand and Australia, these conditions are prevalent for a significant part of the year.

In addition to slowing down the curing process, cold weather also causes the water in concrete to freeze and expand, cracking and weakening the concrete. In some cases, the concrete may even be rendered useless. However, it can still be successfully poured and placed in winter as long as the right precautions are taken to heat it properly.

The first step to ensuring adequate strength gain of mass concrete in cold weather is to gather accurate data. With SmartRock’s dual-temperature functionality, users are equipped with the data they need to inform them when their mass concrete reaches critical temperatures and how much it needs to be heated.

Here are a few tips on how to control the temperature of your concrete in cold weather: 

• Keep the concrete in a dry and heated area until it’s ready to use. – ACI 306, CCANZ TM35
• Install internal electric heating: This requires using embedded coils and insulated electrical resistors. – AS 3600:2018 Concrete Structures
• Optimise your concrete mix: Using low-heat cement; aggregate substitutes such as fly ash, limestone, or slag; and a low water-to-cementitious materials ratio are all good ways to optimise your concrete mix for heat retention in cold weather. – AS 3600:2018 & NZS 3101 Concrete Structures Standard
• Use insulation: Layering helps maintain the heat that is generated from the concrete. Insulation methods like heating blankets or forms allow you to control temperature differentials between the core and the surface of your slab. – ACI 306, CCANZ TM35
• Cool your mass concrete pour before placement: This can be done using chilled water, chipped or shaved ice, or liquid nitrogen. – ACI 306
• Cool the concrete after placement: Use embedded non-corrosive cooling pipes prior to concrete placement. This removes heat by circulating cool water from a nearby source. – ACI 306, CCANZ TM35
• Remove standing water: Bleed water needs to evaporate or be removed by a squeegee or vacuum. – ACI 306, CCANZ TM35

Why Are Temperature Differentials in Mass Concrete so Important in Summer?

The American Concrete Institute (ACI) defines hot weather concreting as conditions that include high ambient temperatures, low humidity, wind speed, or solar radiation, which could impair the quality of freshly mixed or hardened concrete. In the context of placing mass concrete during hot weather, the significance of managing temperature differentials becomes critical.

Studies have shown that the long-term durability of many concrete mixes can be compromised if the maximum temperature after placement exceeds the range of 70°C. Exceeding this limit can lead to a complex set of problems affecting the concrete’s strength and durability, as outlined in standards like NZS 3101 and AS 3600 Furthermore, it has been observed that such temperatures interfere with the formation of ettringite in the initial stage and subsequently its formation in the later stages is promoted, which causes an expansive reaction and subsequent cracking. Additionally, high temperature issues are of concern, especially in mass concrete pours, where the core temperature can be very high due to the mass effect, while the surface temperature is lower. This causes a temperature gradient between the surface and the core, if this differential in temperature is too large it will cause thermal cracking.

To effectively manage temperature differentials in hot weather, consider these strategies:

• Pre-cool the ingredients, including using chilled mixing water, ice, or cooled aggregates. – CCANZ TM35.
• Modify the concrete mix to reduce heat generation, such as using low-heat cement or supplementary cementitious materials. – NZS 3101 and AS 3600
• Protect the concrete from direct sunlight and wind using sunshades and windbreaks. ACI 305R-16.
• Apply evaporation retardants to reduce moisture loss from the concrete surface. – CCANZ TM35
• Schedule pouring during cooler periods, like early morning or night. – ACI 305R-16

To ensure the optimal curing and strength development of mass concrete in hot weather conditions, the primary step involves precise and reliable temperature data. This is where the innovative SmartRock’s dual-temperature functionality becomes a game-changer for live temperature monitoring. It provides users with vital information about the exact temperatures their mass concrete is achieving in real time. This granular data is not only useful for identifying when the concrete reaches temperatures that may be detrimental; it is instrumental in guiding the implementation of appropriate and effective cooling measures. With precise temperature insights at their disposal, onsite teams can better manage the cooling process. This ensures that the concrete not only develops strength within the predetermined thresholds thereby ensuring its structural integrity, even under the challenging conditions posed by hot weather. Embracing this data-driven approach is not just beneficial but essential for the success of concrete construction projects in warmer climates, as it ensures adherence to quality and durability standards crucial in mass concrete structures.

This article was written in part by our distribution partner Giatec.