Abstract: In this paper, under the condition of high temperature in summer, the large foundation slab of Zhejiang Xiaoshan International Hotel (30th floor) pumped commercial concrete through strict control of concrete temperature, reducing the temperature difference between inside and outside, preventing shrinkage cracks and reducing the slump when the distance is 35km Degree of loss, delaying the setting time, to ensure smooth pumping and pouring quality of a series of technical measures taken and the results achieved.
Keywords: Mass concrete pumping commodity concrete
1 project overview and characteristics
Xiaoshan International Hotel is a Sino-foreign joint venture four-star high-class hotel completed in 1995. It is located in the northwestern corner of downtown Xiaoshan, with a building area of 42500m.2. The main building is 28th floor, with an inner cylinder frame reinforced concrete structure with a total height of 107m. 3 to 4 floors and 2 underground floors. The basement of the main building is supported by 104 1000 bored piles. The excavation depth of the foundation pit is 8.7m, the concrete floor thickness is 2.6m, the designed concrete strength C30, and the total concrete 3500m.3 (of which the main building Floor 2700m.3), all used to pump concrete, slump 12 ± 2cm, requiring a continuous pouring, leaving no construction joints.
The project is characterized by: ① concrete transportation distance, from Hangzhou mixing station to Xiaoshan construction site up to 35km, and the urban traffic congestion, serious road congestion, the relatively normal passage of circumstances, the concrete arrived at the scene takes about 1.25 ~ 1.5h; ② basic concrete pouring according to the construction schedule and schedule requirements in the first ten days of August, when hot summer, and the year there once a hundred years of sustained up to two months of sustained high temperature, the maximum temperature of 39 ℃; ③ large size structure, the main building The basis of the length and width of the 33m, 2.6m thick, and embedded with hidden beams, intensive steel, high construction technology requirements. According to these characteristics, in addition to meeting the requirements of concrete strength and durability, the key is to ensure the pumpability of concrete, control the maximum temperature rise of concrete and the temperature difference between inside and outside, and prevent the structure from harmful cracking.
2 construction technology and measures
Massive concrete is less likely to crack due to external loads and the combined effect of hydration heat released during hydration of concrete and temperature changes caused by pouring temperature and concrete shrinkage, resulting in thermal stress and shrinkage Stress, is the main factor leading to cracks in the structure. Therefore, the main use of cement to reduce the amount of control hydration heat, reduce the temperature of the concrete to control the placement of pouring temperature and take measures to maintain the insulation and other comprehensive measures to limit the maximum temperature inside and outside the concrete temperature and temperature control, and to ensure cracks under high temperature Smooth pumping and pouring.
2.1 limit the amount of cement to reduce heat within the concrete hydration
(1) Select the cement. Hangzhou cement plant hydration heat # 425 slag Portland cement used. Its early heat of hydration compared with ordinary Portland cement of the same age, 3d heat of hydration can be about 30% lower.
(2) Add finely ground fly ash. In each cubic meter of concrete mixed with 75kg of fly ash to improve the cohesion and pumpability of concrete, but also to save 50kg of cement. According to the test data show that, for every cubic meter of concrete cement dosage increase or decrease 10kg, the hydration heat caused concrete temperature rise and drop of 1 ~ 1.2 ℃, so that the internal concrete temperature can be reduced by 5 ~ 6 ℃.
(3) Selection of high quality additives. In order to achieve the requirements of reducing water and retarding slump and making the loss of slump small, the EA-2 type retarding and water-reducing agent whose production effect in Shanghai is obviously superior to that of wood calcium is finally selected by comparison, which can reduce 10% The corresponding reduction in the amount of cement, reducing the heat of concrete hydration.
(4) Make full use of concrete late strength. Practice has proved that high-strength fly ash concrete late high strength, 60d intensity within a certain range than about 29d can increase about 20%. At the same time by "Fly Ash Concrete Application Technical Specification (GBJ146-90)", underground engineering should be 60d age strength requirements. In order to further control the temperature rise, reduce the temperature stress, according to the actual load bearing structure, with the consent of the design unit, the original design of concrete 28d age C30 to 60d age C30 (ie 28d age C25 instead of design strength), so Can reduce the amount of cement per cubic meter of concrete 50kg, corresponding to the concrete temperature decreased 5 ~ 6 ℃.
(5) Considering the above factors, considering the slump loss caused by high temperature and long distance transportation, the slump of take-out machine is 18 ± 2cm and the dosage of cement is controlled below 370kg / m.3. As a result of reducing the amount of cement can reduce the concrete temperature 16 ~ 18 ℃.
2.2 with raw materials cooling control concrete exit temperature
According to the principle that the total heat of concrete before mixing is equal to the total heat of concrete after mixing, the exit temperature of concrete can be obtained, which shows that the temperature of the exit of concrete is proportional to the temperature of raw materials. The yard of gravel continuous watering, the temperature dropped from 56 ℃ before watering to 29 ℃ after watering, and can pre-absorb enough water to reduce the loss of concrete slump; ② Yellow sand at the water from the Qiantang River pier , Use the water to cool the river to cool. ③ Although the amount of water in concrete is relatively small, it has the largest specific heat, so ice is added into the three reservoirs for mixing concrete to reduce the water temperature from 31 ℃ to 24 ℃. 75t ice is used in total. In this way, the calculated temperature T of the machine is 32.8 ° C., the average measured value of 37 times is 33.2 ° C., and the measured temperature delivered to the scene is 34.60 ° C., so that the temperature of the mold is greatly reduced.
2.3 to maintain a continuous supply of concrete supply control of the pouring temperature
(1) In order to work closely with the construction schedule and ensure a continuous and uniform supply of concrete, after careful calculation and preparation, two mixing stations of Nanxing Bridge and Liubao are arranged with simultaneous agitation. 18 6m.3 mixers and two mobile pumps , In the three days and four nights has always maintained a stable supply intensity, basically to do a pump ranging from the mixer, mixer truck ranging from pump, there is no one as a result of waiting each other to cause blocking the phenomenon.
(2) The excavation depth of the foundation pit is 8.7m. The highest measured temperature in the pit reaches 62 ℃. To avoid the direct sun exposure and the overheating, which results in the pouring difficulties, the roof of the pit is covered and the ventilating and cooling facilities are installed , So that pouring pit temperature significantly reduced, close to the natural temperature, not only to control the maximum temperature rise, but also to improve the working conditions of workers, to be pouring.
(3) In order not to overheat the concrete conveying pipe, wrap it around the outer wall of the pipe with sacks, and cover it with grass bag and repeat the watering and cooling.
(4) consider the level of concrete Laying pipe demolition and demolition too often, the construction organization difficult to implement, so take a slope layered pouring, staggered open and pouring time between the layers to facilitate the lower concrete cooling, but between the upper and lower layers Strict control shall not exceed the initial setting time of concrete, construction may not be "cold seam." Due to the pumping of concrete slurry more, after pouring open flat with a straight ruler. About 1 ~ 2h intervals, with crab pressure twice, so as to avoid surface water cracks.
2.4 strengthen concrete moisturizing insulation
Concrete wiping pressure, when people step on the above no obvious footprints, then covered with plastic film tightly, not to breathe air leaks, moisture evaporation away and take away the heat. And in the film covered with two layers of grass bag moisturizing insulation to reduce the thermal dispersion of concrete surface, extend the cooling time, reduce the temperature difference between inside and outside the concrete. The measured surface temperature of concrete within 3 days between 48 ~ 55 ℃, and rarely found on the concrete surface cracks.
2.5 through the monitoring of concrete temperature to grasp the dynamic changes
(1) The ultimate goal of temperature monitoring is to grasp the actual maximum temperature rise inside the concrete and the temperature gradient from the center of the concrete to the surface, ensuring that the temperature difference between the inside and the surface required by the code is less than 25 ℃ and the cooling rate.
(2) temperature is directly related to the quality of the entire foundation of the key. In order to objectively reflect the concrete temperature condition, the 7 items of raw material temperature, exit temperature, mold temperature, natural temperature, cover curing temperature, concrete internal temperature, shed temperature and other items are tested to facilitate timely adjustment of temperature control measures.
(3) The temperature of the concrete in the main building is arranged with 25 measuring points according to the different plane parts and depths. It is responsible for continuously measuring the temperature for one week and measuring once every 2h, which is larger than the regulation of measuring twice every 8h .
3 effects and conclusions
(1) The concrete strength according to "concrete strength test and assessment standards (GBJ107-87)" was tested, is qualified.
(2) Due to the adoption of "double-blending technology" (retarders and ground fly ash), the setting time is retarded, the slump loss is reduced, and the workability and pumpability of concrete are improved. Making the concrete at high temperature, long-distance delivery conditions can still be successfully pump, nor occurred blocking pump.
(3) The concrete exit temperature and the mold temperature were measured 37 times in total, the temperature of the raw material was tested 20 times, the temperature inside and outside the concrete was continuously measured for one week, and the highest temperature in the concrete center appeared between 3 ~ 4 days after pouring. Internal and external temperature difference is only 15 ℃, and lower than the standard requirements shall not exceed 25 ℃ requirements.
(4) No harmful cracks (only a few water-collecting cracks on the surface) were found after strict inspection by the relevant units and in recent years. Compact and smooth concrete, no cellular Ma face.
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