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Prefabricated Manufacturing: How to Apply Steam Curing in Prefabricated Structural Component Manufacturing?

Strength gaining rate increases when concrete curing temperature is kept at elevated levels. Steam curing is applied on concrete components in order to have high early strength. Atmospheric steam curing is applied in 4 intervals on prefabricated concrete construction components in order to obtain higher strengths in the first days and acceptable strengths at later days: 

  1. Waiting time
  2. Controlled temperature rising time
  3. Time during which maximum curing temperature is kept stable
  4. Controlled cooling time

Each interval is closely related to the future strength of concrete. Especially waiting time has a significant impact. If a waiting time shorter than the initial setting time is applied on fresh concrete, concrete strength would be lesser than desired. 

What is Steam Curing? What is it good for?

Hydration speed increases as a result of high temperature curing of concrete and high strength is obtained early on. In line with this purpose, some methods were developed for steam curing of concrete.

Prefabricated concrete construction components which commonly receive steam curing are precast concrete components which were cast in a different place than the place they will be used in the structure after being cured. Concrete blocks, concrete pipes, prestressed reinforced concrete beams and concrete wall panels may be listed as examples.

Main reasons to apply steam curing on concrete may be summarized as follows:

  1. Concrete gains strength in shorter times compared to the curing method under normal conditions. Thus, concrete components can be used at an earlier time.
  2. Shorter time for the cast concrete to gain the desired strength allows for shorter times for the cast to be removed.
  3. Removed casts can be used in other concrete castings, the job can be completed faster, and economy can be obtained thanks to this.
steam-curing-application

Image 1. Steam Curing Application

What are Steam Curing Methods? 

There are two different steam curing methods in concrete: 

(a) Steam curing with atmospheric pressure 

(b) Steam curing with high pressure. 

The most applied of these two methods is the “steam curing with atmospheric pressure.” The steam in the curing medium in this method is found in the atmospheric pressure. 

The process of curing concrete includes curing rooms prepared to keep the steam input in our steam tunnels called tubes. 

Accelerated curing methods in prefabricated products are:

  • Injecting heated air/water steam in the curing medium of concrete,
  • Placing pipes around the concrete to circulate hot water or steam.
what-is-the-function-of-prefabricated-manufacturing-steam-curing

Image 2. Steam Curing Application

How to do a Typical Steam Curing Application?

When compared to strengths of concrete cured under normal temperatures, the high curing temperatures enable concrete to have higher early strengths, however, strengths in later days would be lower.

In order to enable high early strength in concrete as well as preventing lower strength levels at later times, steam curing on concrete is applied at different intervals and at different temperatures. 

Four intervals make up the total time of steam curing:

  1. “Waiting time” (This interval is between the time during which material mixing process is completed and concrete component is prepared, and the time at which steam curing is applied.): 
  • The length of waiting time normally ranges between 3 to 5 hours. During this time, fresh concrete is kept at ambient temperature (20 C°). 
  1. “Controlled temperature rising time”:
  • Temperature rising usually ranges between 10 ºC and 20 ºC per hour. Thermal cracks can be formed on concrete if temperature is risen at a rate over 20 C°/hour. 
  1. “Time during which maximum curing temperature is kept stable”:
  • It is recommended not to apply early curing processes at over 65 °C in order to prevent the final strength of concrete significantly decrease. Concrete components are kept at this stable temperature after they reach the maximum temperature.
  1. “Controlled cooling time”:
  • At the end of steam curing application at maximum temperature, the ambient temperature is decreased in a controlled manner. A typical curing circuit applied for atmospheric pressure steam curing is shown in Figure 1. 

Figure 1. Typical Steam Curing Circuit

Image 3. Steam Curing Application

What are Considerations in Steam Curing Applications?

In steam curing with atmospheric pressure, it is recommended to keep the total time, except the waiting time, below 18 hours.

  • In the curing process, the temperature of the concrete should not be increased before initial setting. Otherwise, the air, water and aggregate inside the fresh concrete would experience thermal expansion since tensile strength of fresh concrete is incredibly low, and the void inside the structure of the concrete would increase. 
  • Covering materials such as tarpaulin should be used over the product in facilities where steam curing is conducted with heated air to prevent humidity on the surface of the concrete to be lost. 
  • If steam curing is conducted through a fossil fuel heater, CO2 gas inside the steam reacts with the free CaO inside the concrete leading to carbonation and white spots form over the surface of the concrete. Therefore, consideration should be given to the choice of heater to be used in the facility. 

According to TS EN 13369 “Precast Concrete Products – General Rules” standard, below conditions should be provided in order to prevent micro cracks and durability faults in the concrete:

A proper pre-heating should be applied when the highest average temperature of thermal process, i.e., T exceeds 40 ºC.

When T exceeds 40 ºC, the temperature difference between joint parts of the components should be limited to 20 ºC during heating and cooling stages.

The highest average temperature T for thermal process during all heating and cooling process should be limited to the values presented in TS EN 13369 Standard Chart 3. However, higher temperature values can be used if concrete durability is proven to be obtained under the indicated environmental impacts through long-term experiences. 

 Chart 1. TS EN 13369 Accelerated Hydration Conditions

What is the Importance of Steam Curing?

Steam curing is applied on manufactured concrete components in order to obtain the strength expected of concrete in shorter times and to be able to use concrete earlier. Steam curing consists of 4 stages and the length of each interval, maximum curing temperature to be applied on the concrete and total curing time are closely related to  28-day and later strengths of the concrete.

The determination of the length of intervals and the level of maximum curing temperature depends on the properties and amounts of the material used in concrete in order to obtain the optimum results. Therefore, these values should be very well determined beforehand.

Since the main reason for using steam curing with atmospheric pressure is to enable faster gain of strength in concrete component, thus enabling economy, generally steam curing time, especially waiting time, is not kept long enough in practice. However, waiting time has great impacts on the future performance of the concrete. Waiting time should be at least as long as the setting time of the concrete.

References:

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