Influences And Control Of Raw Materials On Crack Of Mass Concrete Of Foundation

Nowadays, to control the crack of the mass concrete of foundation has always been an important subject for the scholars and project units. The research on this subject should be carried on from two aspects: first, take some concrete technological measures while constructing to avoid concrete surface crack and shrinkage crack caused by a too big difference between internal and external temperature of the concrete. Second, take strict control of the raw materials and improve the concrete's resistance to splitting and deformation by choosing different components, and optimal grade proportioning .In fact, much attention has been paid to the first aspect and many achievement have been made. Yet the second aspect is often ignored by people. So this dissertation is trying to make a research and discussion on the second aspect, namely the Influences and Control of Raw Materials on the Crack of Mass Concrete of Foundation. Relative search has been made on the internet about the masters' and doctors' dissertation from 1997-2002 with in WanFang database. The range of search is architecture science and irrigation engineering , and 26 have been found to be relative to mass concrete foundation. But few has carried on systematic research from exactly the same aspect. It is obvious that this dissertation has certain innovation.The following problems are mainly discussed in this dissertation.1. First, make an analysis and a summary of the micro and macroscopic cracks in mass concrete of foundation. Second, study the basic forms of cracks and the reasons why cracks emerge. Four main reasons have been put forward including the materials used, the construction factors, the environment for use and external force of structure. Finally, the reason of the emergence of surface crack, deep crack and run through crack have been explored and, with the reference to literature both at home and abroad , eight factors have been found to contribute to their appearance . They are the heat of hydration of cement, internal and external constrained force, external temperature change, internal and external temperature difference, shrinkage deformation of concrete, weak resistance to tensility, quality of raw materials andconstruction method and control in advance.2.The choice of cement and aggregate in mass concrete of foundation. First, the make-up and technological characteristics of different kinds of cement, including Portland cement, ordinary cement, volcanic ash cement and fly ash cement, are analysed with their impact on the heat of hydration of mass concrete.Great mixing amount fly ash cement and low heat portland cement are especially suggested besides slag cement, volcanic ash cement and fly ash cement in project use.For the maximum amount of cement in concrete, it is better around 320 kg/m3 and be no more than 350kg/m3. Or, both the pump sending obstruction and the heat of hydration of the concrete will be increased.On the choice of coarse aggregate, the ones with low coefficient of hot expansion are proposed, such as limestone, basalt, diabase, granite and other aggregate with high limiting draw intension. Otherwise, to control the crack of mass concrete of foundation will became more difficult; Concerning the foot-path of the grain in aggregate, broken rock and a lower water-cement ration should be chosen in order to improve the adaptability to different limiting draw tension. Meanwhile, the content of needle particles should not exceed 10%, strictly.On the choice of fine aggregate, compared to the sand with a modulus of 2.12 and an averaged footpath of 0.336mm, the sand with a modulus of 2.79 and an averaged footpath of 0.381mm can reduce the water consumption by 20~25kg per cubic meters of concrete. Thus, not only the increase in temperature of the concrete can be reduced, the degree of shrinkage can also be lowered.Briefly, in the construction of mass concrete of foundation, sand of medium size is suggested for both a minor consumption of cement and water and an easier construction of pumped concrete. The particles with a footpath of 0.315mm should be no less than 15% and had better within 20%. Obviously the particle gradation has to be accorded to our country's norm.3. Admixture used in mass concrete of foundation. According to the results of survey and a large number of project instances, I suggest the use of wooden calcium, FDN, MC-5, JM-II, FNC of high efficient water-reducing admixture in theconstruction of mass concrete of foundation. These admixture can reduce cement heat of hydration and the peak value of the heat of hydration notably, and make the concrete more pumpable as well as more resistant to splitting. Expansion agent should be UEA, CSA and CAS. According to relative theory and practical experience of the project, a variety of expansion agents are named and precautions are proposed.When UEA is adopted, the usage of slay portland cement is a good choice to produce a better limiting expanding rate, but not a negative one. In construction, UEA expansion agent and JM-II high efficient water-reducing admixture, CAS expansion agent and MS-5 high efficient water-reducing admixture are two best matched couples giving the best intensity and pumpability, which has been proved both by experience and real construction.4. Fly ash used in mass concrete of foundation. The fineness and water requirement are two important indexes affecting the intensity of concrete. With careful exploration, it is supposed that the mixing amount of fly ash can be brought up to 50%. We set that: mixing amount higher than 40% is called a great mixing amount and the match ration and the check before acceptance should be based on the intensity of 60d's or 90d's concrete.In order to improve the early intensity of fly ash concrete to meet the demends of construction, different measurement can be taken, such as adding DG-1 high efficient activator of fly ash, adding high efficient water-reducing admixture or rubbing the fly ash.5. Different ways of adding admixture have various impacts on the features of concrete. When high-efficient water-reducing admixture is used, post-mixing method and stagnant water method should be adopted.Adding at the same time is the choice when water-reducing admixture of slow-coagulation or high-efficient water-reducing admixture of slow-coagulation is used. There is no restriction for the method used with wooden calcium water-reducing agent.Proved by a large number of project practice and relative theories, a larger quantity of fly ash than cement should be used. This can improve the early intensity ofconcrete, yet reduce the consumption and the hydration heat of the cement at the same time, when the technology of "Sanchan" is used to weaken the limiting action of fly ash to the expansion rate of the concrete, the amount of expansion agent can be increased by 0.6%~1.2% to meet the demands when the mixing amount of fly ash is more than 40%.6. Among the methods of controlling cracks, the increase of in-place concrete temperature is extremely unfavorable to reducing the internal maximum temperature. Through studying the instances of projects both at home and abroad and data from experiments, cast in-place concrete temperature within 30 °C is found to be OK.Moreover, in order to reduce both the external and internal constrained force, and the incidence of surface cracks and shrinkage cracks as well, we can set up a band of area to be cast later than other areas or an area to be especially expansion strengthened to weaken the constrained force.According to a large number of project instance results, when strengthened bands are used, micro-expanding concrete (with a UEA amount with in 10%~~12%), While the strengthened area itself using macro-expanding concrete (with a UEA amount within 14%~15%). When the concrete is cast on the base rock or concrete, a slipping layer twice asphalted and with an asphalt felt can be adopted to reduce external constrained force and the incidence of run-through cracks. This method is proved to produce better results.In order to reduce the internal constrained force, control the difference between internal and external temperatures and avoid the emergence of surface crack, using different heat-insulation layers for summer and winter time are found to be the best on the basis of the original heat-insulation method. In addition, to lower the possibility of concrete fracture, the increase of construction reinforcement and secondary tamped concrete are suggested to improve the mass concrete resistance to limiting draw.Finally, for the structure not allowing the emergence of cracks, the control of temperature difference should be based on the internal temperature of the concrete and the temperature of the upmost concrete under the coverings.For the structures that cracks are allowed but should be kept within a certaindegree, or should not develop in depth, the control of temperature difference can based on the difference between the temperature in the concrete center and the temperature of 50mm place under the surface.