The Research On Countermeasures To Prevent Cracking In Mass Concrete

No detrimental cracking is allowed in some important structures according to theChinese standards and criteria. Cracks in structures may result in leakage and causeand accelerate the spalling of concrete protective cover, corrosion of reinforcing steel,and carbonization of concrete, affecting the durability of the structures. Especiallysuch marine structures as docks used for shipbuilding and ship-repair, which arenormally located in a seawater environment, similar to a tidal zone, damage to theconcrete caused by frost-thawing may be accelerated in freezing regions, hencerequiring special attention to the frost resistance of concrete. The object of thisresearch task is limited to the cracks resulted from deformation of mass concretestructure of docks, i.e. cracking in concrete caused by temperature variation andvolume change (mainly shrinkage strain) of concrete.The present research has focused on the cracking problem in close combination withengineering practice and, in view of the characteristics of the concrete structure ofdocks and the technology of concrete construction for the docks, has worked outoperable technical measures to prevent detrimental cracks in dock structures on thebasis of the analyses of the performance and cracking mechanism of concrete. Theoperable technical measures were also summarized and theorized to provide referencefor designs and construction practice.To control thermal cracking in concrete, it is necessary to carry out theoreticalcalculations first. The thermal field and thermal stress in concrete were firstcalculated with the parameters carefully selected according to the properties of theraw ingredients in the concrete and taking into integral consideration of the influenceof environmental conditions to ensure the accuracy of the calculations. Measuresagainst cracking of concrete should mainly start with the following aspects. One is totake every measure to minimize the temperature difference inside and outside theconcrete;another is to reduce the restrictions of ambient conditions upon thedeformation of concrete;and last is to improve the anti-cracking property of theconcrete itself.Thermal stress in mass concrete has a very complex process of generation,development and transformation. It is thus more difficult to prevent thermal cracksthan to prevent cracking caused by external loading. In the meantime, thermal cracksin concrete are not uncontrollable and they can be absolutely reduced and evenavoided if some technical measures are taken. However, cracks in concrete can bereduced and prevented only when comprehensive technical measures are formulatedaccording to the emphases and difficulties in cracking control and only when themanagement of construction is strengthened, the technological level is improved andthe anti-cracking measures are fulfilled item by item.The direction of development of the technology for controlling cracks in concrete ofdocks lies first in the development and application of new technique, new technologyand new materials to research and develop high performance concrete (HPC) andlow-shrinkage and low-hydration-heat concrete, thus improving the performance ofconcrete. Secondly, mathematical simulation of cracks in concrete should be made byusing computers to better understand and master the regular patterns of variation ofthe thermal field and stress of concrete. During concrete construction, advancedtesting and measuring techniques should be employed to realize informatizedconstruction.