About "Summer Concrete"—Concrete Placement During the Summer Months
With global warming on the rise, it has become increasingly common for outdoor temperatures during the summer to exceed body temperature, making working conditions at construction sites even more grueling. When conducting construction work during such summers, it is important to plan for measures to prevent heatstroke among workers and ensure adequate rest breaks; however, it is also essential to pay close attention to various factors related to ready-mix concrete placement when developing the construction plan.
Generally, when construction takes place during periods when the daily average temperature exceeds 25°C, the concrete must be treated as “summer concrete.” During the summer, when temperatures are high and solar radiation is intense, the temperature of the concrete inevitably rises. High concrete temperatures can lead to problems such as a loss of slump during transport, the formation of cold joints and cracks, and reduced strength and durability. For this reason, JASS5 and the Standard Specifications stipulate that the concrete temperature at the time of unloading ready-mix concrete must be 35°C or lower. However, these guidelines are phrased as “in principle” or “as a standard,” making the wording somewhat ambiguous.
Incidentally, according to JASS5 and the Standard Specifications, “shall not” is the most binding expression, followed by “shall, in principle,” “shall, as a standard,” and finally “is desirable.” The fact that the language remains this vague, combined with the rise in concrete temperature accompanying the increase in actual outdoor temperatures, has become a problem throughout the country in recent years. There have even been cases where ready-mix concrete was rejected because its temperature exceeded 35°C at the time of unloading. So, is ready-mix concrete really unusable once it exceeds 35°C? Conversely, how do ready-mix concrete suppliers manage to keep the temperature below 35°C?
Research on “summer concrete” is progressing
Research on concrete temperature issues has been conducted in various regions and by various organizations; in particular, research conducted in Osaka, Nara, and Aichi has been presented in papers at the ZENNAMA Ready-Mix Concrete Technology Conference. A common conclusion drawn from these studies is that concrete performance remains unchanged even when the concrete temperature is between 35 and 38°C, compared to when it is 35°C or lower.
While it is true that slump may decrease to some extent, this does not pose a major problem as long as it remains within the normal transport time range. Rather, this issue can be resolved by specifying a higher slump (to improve workability) in order to shorten the concrete placement time, thereby preventing the formation of cold joints. Furthermore, regarding strength, it has been confirmed that there is virtually no change up to 38°C, and the specified strength is achieved.
Is it really okay not to take any measures?
That said, this does not mean that no measures need to be taken. Both JASS5 and the Technical Guidelines require that measures be taken. Therefore, once concrete placement during the summer has been scheduled, the contractor and the ready-mix concrete plant should consult with each other. Effective measures include using a delayed-action, high-performance air-entraining (AE) admixture; lowering the temperature by spraying water on the aggregate; and adjusting the placement schedule. Additionally, covering the mixer truck’s drum with a cover and spraying water on the cover is also highly effective. Provided these measures are taken, temperatures exceeding 35°C could arguably be considered acceptable.
In addition, research findings presented at the aforementioned ZENNAMA Ready-Mix Concrete Technology Conference indicated that “opening the hopper cover of a mixer truck can help suppress temperature rise.” On sunny days, it would likely be effective to take measures such as installing a mesh sheet to prevent foreign matter from entering even when the cover is open.
What have ready-mix concrete plants been doing up until now?
So, how have ready-mix concrete plants handled this situation up to now, given the current circumstances? Of course, measures such as spraying water on aggregates, covering drums, and applying heat-reflective paint to mixer trucks are commonly taken. Additionally, since cement is hot immediately after delivery, using cement that arrived several days earlier rather than the freshest batch is also an effective practice that is widely implemented.
Finally, during acceptance testing, we use “negative thermometers” that are set so they do not display the actual temperature. If you ask the manufacturer, you can obtain thermometers set to “−2°C” or “−3°C.” Conversely, this means that ready-mix concrete that registered 34°C during acceptance testing was actually 36°C or 37°C, and therefore, there were no issues with its strength or other properties.
Is a concrete temperature of up to 38°C acceptable?
Revisions are currently underway for the provisions regarding concrete temperature during hot weather in both JASS5 and the Standard Specifications, and it is expected that these will be updated sometime next fiscal year. Although it is still unclear exactly what the new provisions will entail, given the ongoing research mentioned above and the actual rise in ambient temperatures, it seems likely that concrete temperatures up to 38°C will be permitted. If that happens, ready-mix concrete plants will be able to conduct acceptance tests with confidence—without resorting to deceptive practices such as using thermometers that read below the actual temperature—and contractors will no longer have to worry about issues such as insufficient strength.
