Abstract: This paper presents a detailed discussion on the technical measures implemented for pumping large-volume concrete at the foundation slab of Zhejiang Xiaoshan International Hotel, which has 30 floors. Given the high ambient temperatures during summer, strict temperature control was applied to the commercial concrete in order to minimize the internal and external temperature differences, prevent shrinkage cracks, and reduce slump loss over a 35 km delivery distance. The measures taken also helped delay the setting time, ensuring smooth pumping and high-quality pouring. The results achieved demonstrate the effectiveness of these strategies under challenging conditions.
Keywords: Mass concrete, Pumping, Commercial concrete
1. Project Overview and Characteristics
The Xiaoshan International Hotel is a Sino-foreign joint venture four-star hotel completed in 1995, located in the northwestern corner of Xiaoshan city center. It has a total building area of 42,500 square meters, with the main structure consisting of 28 floors. The building features an inner cylinder frame reinforced concrete system, standing at a total height of 107 meters. There are three to four above-ground floors and two underground levels. The basement is supported by 104 bored piles with a diameter of 1 meter. The foundation pit was excavated to a depth of 8.7 meters, with a floor thickness of 2.6 meters. The designed concrete strength is C30, and the total volume of concrete used is 3,500 cubic meters, with 2,700 cubic meters used for the main building. All the concrete was pumped, with a slump of 12 ± 2 cm, and required continuous pouring without any construction joints.
The project presented several challenges: (1) the long transportation distance from the mixing plant in Hangzhou to the site, up to 35 km, with frequent traffic congestion causing delays of about 1.25 to 1.5 hours; (2) the need to pour the foundation during the first ten days of August, when summer temperatures reached as high as 39°C, with sustained heat lasting for two months; (3) the large-scale structure with a 33m x 33m foundation slab, 2.6m thick, containing embedded beams and dense reinforcement, requiring advanced construction techniques. In addition to meeting strength and durability requirements, the key challenge was ensuring the pumpability of the concrete, controlling its temperature rise, and preventing harmful cracking due to thermal and shrinkage stresses.
2. Construction Technology and Measures
Mass concrete is prone to cracking due to the combined effects of hydration heat, temperature changes, and shrinkage. To mitigate this, the project focused on reducing the amount of cement used, lowering the hydration heat, controlling the pouring temperature, and implementing insulation and cooling measures.
2.1 Reduce Cement Content to Minimize Hydration Heat
(1) A slag Portland cement of grade 425 was selected, which has lower early hydration heat compared to ordinary Portland cement, reducing the heat by approximately 30% at 3 days.
(2) Fly ash was added at 75 kg per cubic meter of concrete, improving workability and reducing cement usage by 50 kg. Each 10 kg reduction in cement resulted in a temperature drop of 1–1.2°C.
(3) A retarding and water-reducing agent (EA-2 type) was chosen for its superior performance in reducing slump loss and water content.
(4) High-strength fly ash concrete was utilized, with 60-day strength increasing by about 20% compared to 28-day strength. This allowed the use of a lower design strength (C25 instead of C30) at 28 days, reducing cement usage by 50 kg/m³.
(5) The slump was set to 18 ± 2 cm, with cement content controlled below 370 kg/m³, resulting in a temperature decrease of 16–18°C.
2.2 Cool Raw Materials to Control Concrete Exit Temperature
Gravel was continuously watered, reducing its temperature from 56°C to 29°C. River water was used to cool sand, and ice was added to the mixing water, lowering 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 Ensure Continuous Supply and Control Pouring Temperature
Two mixing stations were arranged to ensure a steady supply of concrete, with 18 mixers and two mobile pumps operating continuously. The foundation pit was covered and cooled to reduce the temperature to near natural levels, making it easier to pour. The conveying pipes were wrapped with sacks and watered regularly to avoid overheating. A layered pouring method was adopted to allow proper cooling between layers, avoiding cold joints.
2.4 Enhance Moisture Retention and Insulation
After the concrete surface showed no footprints, it was covered with plastic film and grass bags to retain moisture and reduce heat loss. The surface temperature remained between 48–55°C within the first three days, with no visible cracks observed.
2.5 Monitor Concrete Temperature for Real-Time Control
Temperature monitoring was conducted at 25 points across the foundation, measuring internal and external temperatures every 2 hours for a week. The maximum temperature rise occurred 3–4 days after pouring, with an internal-to-surface temperature difference of only 15°C, well below the 25°C limit.
3. Results and Conclusions
(1) The concrete strength met the standards specified in "Concrete Strength Test and Evaluation Standards (GBJ107-87)."
(2) The use of double-blending technology (fly ash and retarders) improved workability and reduced slump loss, allowing successful pumping even under high-temperature conditions.
(3) Temperature measurements confirmed that the internal-to-surface temperature difference was within acceptable limits.
(4) No harmful cracks were found after thorough inspection, and the concrete was compact and smooth, with no honeycombing or voids.
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