Simulation Analysis of Embedded Steel Pipe Construction in Massive Concrete Base Slabs

Literature Overview

The paper by Li Hui, Cai Wenming, and Du Yongfeng, published in Journal of System Simulation in 2013 (Vol. 25, No. 2, pp. 361-366), presents a finite element simulation study of temperature control measures for massive concrete base slabs. The research was funded by the Gansu Provincial Science and Technology Support Program (JK2010-15) and conducted at Lanzhou University of Technology. The study compares two construction scenarios: layered pouring without embedded pipes and layered pouring with vertically embedded steel pipes arranged in a specific pattern.

Thermal Cracking Mechanism in Massive Concrete

Massive concrete structures are susceptible to thermal cracking due to the temperature differentials generated by cement hydration heat. The fundamental mechanisms include:

1. Autogenous shrinkage: Internal volume change due to hydration reactions, independent of temperature.
2. Thermal gradient cracking: Temperature differentials between the core (hot) and surface (cool) create tensile stresses in the cooler region.
3. Restrained cooling cracking: As the concrete cools from peak temperature, restrained contraction generates tensile stresses that can exceed the tensile strength of young concrete.

The critical period for cracking is typically during the first 7-14 days after placement, when hydration heat generation is at its peak and the concrete's tensile strength is still developing.

Construction Methods Compared

Layered Pouring Without Embedded Pipes

The baseline scenario involves placing the concrete in multiple horizontal layers, allowing each layer to partially cool before the next layer is placed. This method reduces the peak temperature by interrupting the continuous hydration heat accumulation, but the thermal gradient between layers can still generate significant inter-layer stresses.

Layered Pouring with Embedded Vertical Steel Pipes

The enhanced method incorporates vertically embedded steel pipes before the first layer of concrete is placed. These pipes serve as internal cooling conduits, with cooling water circulated through them to extract heat from the concrete core. The steel pipes are arranged in a plum blossom pattern (梅花形布置), which provides uniform thermal coverage across the slab area.

Finite Element Simulation Approach

ANSYS Model Configuration

The simulation models the time-dependent temperature field evolution during the construction period, incorporating:

• Cement hydration heat generation rate as a function of time and temperature.
• Layer-by-layer placement schedule with specified intervals.
• Cooling water circulation through embedded pipes with defined flow rate and inlet temperature.
• Surface heat exchange with ambient air conditions.

Temperature Field Comparison Results

The simulation results demonstrate the effectiveness of embedded steel pipes:

The plum blossom arrangement of vertical pipes ensures that no point in the concrete slab is more than a critical distance from a cooling pipe. This distance is determined by the thermal diffusion length, which depends on the concrete's thermal diffusivity and the duration of peak heat generation.

Temperature Stress Analysis

The temperature stress field is derived from the temperature field through the thermoelastic constitutive relationship, accounting for:

• Concrete elastic modulus development with age.
• Creep and shrinkage effects at elevated temperatures.
• Restraint conditions from formwork and previously placed layers.

The maximum tensile stresses occur at the slab surface during the cooling phase, where the temperature differential between the still-warm core and the rapidly cooling surface creates a bending-type stress pattern. The embedded pipes reduce this differential by maintaining a more uniform internal temperature distribution.

Engineering Practice Considerations

Pipe Arrangement Optimization

The plum blossom pattern (hexagonal arrangement) provides optimal coverage with minimum pipe density. The spacing between pipes should be calculated based on:

• Maximum allowable temperature gradient (typically 20-25°C/m for crack control).
• Concrete thermal properties and hydration heat characteristics.
• Cooling water flow rate and temperature control capability.

Typical spacing ranges from 2.0 to 3.5 meters depending on slab thickness and cement type. For slabs exceeding 2.5 meters in thickness, closer spacing (2.0-2.5 m) is recommended.

Construction Sequence

1. Install and fix embedded steel pipes in the plum blossom pattern, ensuring vertical alignment and secure anchorage.
2. Verify pipe integrity through pressure testing before concrete placement.
3. Place first concrete layer, ensuring proper encapsulation of pipe bases.
4. Initiate cooling water circulation once the first layer reaches initial set.
5. Continue layered placement according to the thermal schedule, monitoring temperatures at multiple depths.
6. Maintain cooling circulation until concrete temperature drops below 40°C and the temperature gradient is below 20°C/m.

Monitoring and Control

Real-time temperature monitoring is essential for adaptive control of the cooling process. Thermocouples should be installed at multiple depths and locations to capture the three-dimensional temperature distribution. The cooling water flow rate and inlet temperature can be adjusted based on measured temperatures to maintain the target thermal profile.

Study Insights and Reflections

This study effectively demonstrates the value of computational simulation in optimizing construction methods for massive concrete structures. The quantitative comparison between scenarios provides clear evidence for the effectiveness of embedded steel pipe cooling, which can inform design decisions regarding crack control measures.

The plum blossom arrangement is an elegant solution that maximizes thermal coverage efficiency. The hexagonal geometry ensures that the maximum distance from any point to the nearest pipe is minimized, which is critical for controlling the thermal gradient. This arrangement also provides structural benefits, as the embedded pipes can serve as reinforcement for the concrete slab, particularly in regions subject to high tensile stresses.

One consideration not fully addressed in the study is the interaction between the embedded pipes and the concrete during the thermal cycling. The differential thermal expansion between steel (α = 12 × 10⁻⁶/°C) and concrete (α = 10-14 × 10⁻⁶/°C) can generate interface stresses at the pipe-concrete boundary. While these stresses are generally manageable, they should be considered in the design of pipe anchorage and connection details.

The study also highlights an important principle in massive concrete construction: prevention is more effective than treatment. By controlling the temperature field during construction, the need for post-placement crack repair is eliminated, resulting in a more durable and aesthetically acceptable structure. This philosophy aligns with modern construction practices that emphasize process control over corrective measures.

The simulation methodology presented can be extended to other cooling techniques, such as internal cooling tubes with different geometries, external surface cooling systems, or hybrid approaches combining multiple methods. The ANSYS-based framework provides a versatile tool for evaluating alternative construction strategies under various environmental and material conditions.