**Abstract:**
This paper discusses the technical measures implemented during the construction of a large foundation slab for the 30th-floor Zhejiang Xiaoshan International Hotel under high summer temperatures. By carefully controlling the concrete temperature, the project successfully minimized the temperature difference between the internal and external parts of the structure, preventing shrinkage cracks. Additionally, the slump loss due to a 35 km transportation distance was reduced, and the setting time was delayed to ensure smooth pumping and high-quality pouring. The results achieved demonstrate the effectiveness of these strategies in managing mass concrete placement in extreme conditions.
**Keywords:** Mass concrete, Pumping, Commercial concrete
**1. Project Overview and Characteristics**
The Xiaoshan International Hotel is a four-star Sino-foreign joint venture hotel completed in 1995, located in the northwestern corner of Xiaoshan city. It has a total building area of 42,500 square meters, with the main building reaching 28 floors and a total height of 107 meters. The structure includes two underground levels and three to four above-ground floors. The basement is supported by 104 bored piles, with a foundation pit depth of 8.7 meters and a floor thickness of 2.6 meters. The designed concrete strength is C30, with a total volume of 3,500 cubic meters (of which 2,700 m³ is for the main building). All the concrete was pumped on-site, with a slump of 12 ± 2 cm, requiring continuous pouring without any construction joints.
The project faced several challenges: the concrete had to be transported 35 km from Hangzhou to the site, often encountering traffic congestion, leading to delivery times of 1.25 to 1.5 hours. The pouring took place in early August, during a period of sustained high temperatures, with maximum temperatures reaching 39°C. Moreover, the foundation slab was large, measuring 33 meters in length and width, with embedded beams and dense reinforcement, making the construction technically demanding. In addition to meeting strength and durability requirements, the key challenge was ensuring the pumpability of the concrete, controlling its internal and external temperature differences, and preventing harmful cracks.
**2. Construction Technology and Measures**
Mass concrete is prone to cracking due to thermal stress caused by hydration heat, temperature changes, and shrinkage. To address this, the project focused on reducing the cement content, controlling the temperature of the concrete, and implementing insulation and cooling measures to minimize temperature differentials and ensure smooth pumping.
**2.1 Reducing Cement Content to Control Hydration Heat**
Cement was selected as slag Portland cement #425, which has lower early hydration heat compared to ordinary Portland cement. Fly ash was added at 75 kg per cubic meter to improve workability and reduce cement usage by 50 kg. High-quality retarders were used to control slump loss and extend setting time. The late strength of fly ash concrete was also utilized, allowing the design strength to be reduced from 28-day C30 to 60-day C30, thus lowering cement content by 50 kg/m³ and reducing internal temperature by 5–6°C. The final slump was set to 18 ± 2 cm, with cement content below 370 kg/m³, resulting in a temperature reduction of 16–18°C.
**2.2 Cooling Raw Materials to Control Exit Temperature**
Gravel was continuously watered to lower its temperature from 56°C to 29°C, while river water was used to cool sand. Ice was added to the mixing water, reducing its temperature from 31°C to 24°C. A total of 75 tons of ice was used, resulting in an exit temperature of 32.8°C, with an average measured value of 33.2°C and a delivery temperature of 34.6°C.
**2.3 Ensuring Continuous Concrete Supply and Controlling Pouring Temperature**
Two mixing stations were arranged to provide a steady supply of concrete, with 18 mixers and two mobile pumps operating continuously. The foundation pit was covered and cooled to reduce the pouring temperature. Pipes were wrapped with sacks and cooled regularly. A layered pouring method was adopted to allow proper cooling between layers, avoiding cold joints and surface cracks.
**2.4 Enhancing Moisture Retention and Insulation**
Once the concrete surface was firm enough, it was covered with plastic film and grass bags to retain moisture and reduce heat loss. This helped maintain a stable temperature gradient and prevented surface cracking.
**2.5 Monitoring Concrete Temperature for Real-Time Adjustments**
A total of 25 temperature monitoring points were installed in the main building, with measurements taken every 2 hours for one week. The maximum internal temperature occurred 3–4 days after pouring, with an internal-to-surface temperature difference of only 15°C, well within the code requirement of 25°C.
**3. Results and Conclusions**
The concrete strength met the required standards, and the use of fly ash and retarders improved workability and reduced slump loss. No harmful cracks were observed, and the concrete remained smooth and compact. These measures ensured successful pumping and high-quality construction under challenging environmental conditions.
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