Advanced Analysis of Hydration Heat in Mass Concrete

1. Introduction

The increase in the scale of concrete structures and the rapid construction of large quantities have frequently led to the generation of significant cracks in structures due to temperature-induced stress caused by cement hydration heat. Previously, such phenomena were considered to occur mainly in concrete structures with particularly large dimensions. However, it is not uncommon for cracks resulting from hydration heat to occur in relatively smaller structures, depending on factors such as materials used and construction conditions. Therefore, it is necessary to define conditions that may lead to temperature cracks caused by hydration heat as mass concrete and conduct a review of the temperature stress and cracks induced by cement hydration.

This presentation aims to explain the actual results of hydration heat analysis conducted on mass concrete structures, including the spread foundation/direct foundations of bridges, coping of bridge piers, and anchorages of prestressed concrete bridges. Additionally, a brief description will be provided regarding various considerations made during the interpretation process and the derivation of results.

2. Fundamental Theory

When cement fully reacts with water, it generates heat through exothermic chemical reactions, producing approximately 120 kcal/kg. As the thermal conductivity of concrete is relatively low, hydration heat in mass concrete tends to raise the internal temperature. Typically, the magnitude of this temperature increase is assessed using methods such as cement hydration heat measurement with a micro-calorimeter or concrete adiabatic temperature rise tests. A micro-calorimeter is an instrument used to measure the heat generated when cement and water react under controlled temperature conditions. While this method allows for comparative experiments with small samples for each mix, it is criticized for neglecting the sample's temperature, making it less suitable for predicting the actual temperature rise in mass concrete.

In contrast, the adiabatic temperature rise test is widely used for predicting the temperature rise in mass concrete. This test measures the temperature increase that occurs while maintaining the insulation state of the target concrete. Concrete adiabatic temperature rise is typically represented as an exponential function, indicating the insulation characteristics with parameters such as maximum temperature rise (K) and reaction rate (r).

The hydration reaction of cement is significantly influenced by the surrounding temperature, and as the ambient temperature increases, the hydration reaction rate accelerates. Therefore, as the casting temperature of concrete rises, the initial temperature increase of the concrete also tends to be larger. Generally, based on adiabatic temperature rise experiments conducted by varying the casting temperature for concrete using Portland cement, the results show that the maximum temperature rise (K) tends to decrease with higher casting temperatures. However, the reaction rate (r) indicates an increase proportional to temperature. This phenomenon suggests that with higher casting temperatures, the initial hydration reaction becomes more active, leading to a faster temperature rise. Yet, it is explained that the thickening of the crystals around cement particles at the initial stage hinders the complete hydration reaction of cement particles.

Given that casting temperatures vary in actual construction, the accumulation and review of data on this matter are essential. Additionally, it is crucial to consider casting temperatures meticulously in practical construction, as higher initial temperature rise rates can result in significant differences in the coefficient of thermal expansion, leading to an increased probability of early temperature-induced cracking.

Hydration heat analysis involves two distinct processes, unlike conventional structural analysis: temperature analysis and stress analysis. In the temperature analysis, input data related to the thermal properties of concrete and soil, thermal boundary conditions, and heat sources are required. In the stress analysis, structural boundary conditions, similar to those in typical structural analysis, are necessary. The determination of the occurrence of temperature-induced cracks in such hydration heat analysis is based on comparing the obtained stress values from the analysis with the concrete tensile strength at that moment. Therefore, dynamic mechanical properties of concrete, such as tensile strength and elastic modulus according to age, serve as fundamental variables in hydration heat analysis.

The introduction of the temperature crack index establishes criteria for evaluating cracks in structures based on their importance. The temperature crack index is calculated by dividing the concrete tensile strength according to age, as shown in the above equation, by the maximum temperature stress occurring within the component. This index is used as a specific indicator in the concrete standard specifications to represent the risk of temperature-induced cracks. A smaller crack index implies a higher likelihood of crack occurrence, an increase in the number of cracks, and a tendency for wider crack widths.

3. Heat of Hydration Analysis Method

3.1 Analysis Flow

3.2 Analysis Conditions

4. Heat of Hydration Analysis Method Result

4.1 Modeling and Construction Stage

4.2 Tempreture and Stress Contour

Classify	Tempretue Contour in construction stage (℃)	Stress Contour in construction stage (MPa)
1st stage pouring
3rd stage pouring
6th stage pouring
10th stage pouring
14th stage pouring
20th stage pouring

4.3 Time History Graph

Tempreture History	Stress History	Cracking Index History

4.4 Conclusion

Through the implementation of temperature crack reduction measures (using a ternary cement and applying an appropriate casting height), a hydration heat analysis was conducted at each casting stage, ensuring a minimum temperature crack index of 1.20 or higher. This approach satisfied the target crack index specified in the concrete standard specifications under the section "Limiting Crack Formation."

It is important to note that the current state of hydration heat analysis techniques is not yet perfectly established. Results may vary depending on the materials of the cement, casting conditions, and curing methods. Therefore, when dealing with hydration heat analysis issues during concrete casting, it is crucial to refer to the analysis conditions and results mentioned above, proactively adapt to the actual site conditions, and resolve issues through continuous monitoring and management during construction.

5. Key Points in Hydration Heat Analysis

☞ Element Size Determination: The area under consideration is typically set at a basic size of 0.5m, expanding up to 1m or 2m for other sections and the ground. The reviewed area is generally within 1m from the outer edge, and the thickness direction elements are modeled with a minimum of two elements, each at least half the casting thickness.

☞ Symmetry Conditions: Apply symmetry conditions wherever possible to reduce analysis time (1/2, 1/4 symmetry conditions).

☞ Boundary Conditions: All boundaries, except for symmetry planes and ground surfaces, are considered with fixed temperature and convective boundaries.

☞ Initial Temperature: Set the initial temperature to 20 degrees Celsius during the construction stages.

☞ Hydration Time: Input elapsed time for hydration analysis based on curing days (7, 12, 24, 36, 48, 60, 72, 84, 96, 120, 144, 168 days).

☞ Consideration of Self-Weight: A slight difference in results may occur when considering self-weight in the hydration heat analysis (an increase of approximately 0.05~0.1 in the crack index). The results without considering self-weight are on the conservative side.

☞ Crack Index Verification: Verify a crack index of 0.7 or higher for all surfaces excluding the base under the planned casting height. If 0.7 is not achieved, reduce the casting height for each construction stage.

☞ Inner Surface Crack Index: After achieving a crack index of 0.7 or higher on the outer surface, examine the inner surface within 1m from the outer surface for a crack index of 1.2 or higher. Select the line with results between 1.2 and 1.5, ignoring the minimum value (selecting the line with the intersection of the same temperature).

☞ Stress and Crack Index History: Output stress history (including tensile strength - principal stress: P1), crack index history, and temperature history in graphs for the selected line. Reassign node numbers for review nodes, input output nodes from MCT files, and perform the analysis again.

☞ Temperature History for Critical Nodes: As the temperature values for nodes with crack indices of 1.2 or higher may be somewhat lower, output temperature values for intermediate nodes (locations of maximum temperature) for each construction stage.

☞ Final Crack Index Calculation: If displaying the crack index for each construction stage, calculate the crack index separately by considering the tensile strength (there may be some difference between the output crack index and the crack index calculated as [tensile strength / induced stress]).

[References]

⦁MIDAS Civil Tutorial : Hydration Heat Analysis for Each Construction Stage

About the Editor

TJ

Structural Engineer More than 10 years

I have been working in structural design at an engineering firm in South Korea for over 10 years, developing my skills through a variety of projects over this long yet brief period.

I am delighted to share my practical experience and concise, yet useful know-how, hoping it will contribute to our collective advancement through technology accumulation.

 

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