Articulo neville

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be controlled. The same applies to the effects of the creep of satisfied by a number of practical gradings. This is right and concrete columns on the cladding whose deformational charproper because at this stage, when the contractor has not yet acteristics are likely to differ from one another. Clearly, an been chosen, it is not known which source of aggregate will allowance for creep ofconcrete is necessary, but how much be used. Thus the detailed properties of the materials to be creep will there be? used are unknown. The same applies to the plant or the methMost engineers are familiar with cases of unexpected od of transportation, compaction, curing, and other site acshrinkage cracking which may well be due to the shrinkage tivities. characteristics of the aggregate or of the mixtureused. At the With the move to performance-type specifications, the sitdesign stage, these features are unpredictable, and specificauation has improved somewhat in that at least certain behavtion limits are generally very broad. ioral characteristics of concrete are ensured. Nevertheless, My contention is that the inmany other properties of confluence of these various pacrete are unknown andunpre“...the structural designer and the rameters is so great that the dictable at the structural design concrete technologist are perceived as designer must be fully familiar stage. Is it surprising then that, with them. One cannot simply in many cases, problems arise people involved in quite separate use a handbook of physical during the life of the structure? activities...” constants. At this stage, Iwould like to forestall the criticism that all this is inevitable because, unlike automobiles and aircraft, Structural design concrete structures are nearly always of a one-off variety and and durability are built in situ in the open, in variable weather, often under The same applies in the case of durability, which is by far the difficult conditions, using a labor force that is not static and,major problem with concrete structures in many parts of the let us be frank, less skilled than their factory-based counterworld. Numerous cases of inadequate durability of concrete part. Yes, this is so, but need the consequences continue forstructures built in the 1960s and the 1970s were traced to the ever? selection of concrete mixtures on the basis of strength alone. In what follows I propose,first, to show the close interdeIn fact, because of the changes in the cement properties, the pendence of structural design and the properties of concrete, same strength as previously specified could now be achieved and then to offer some pointers toward ameliorating the at a higher water-cement ratio. In consequence, for a given present situation. 28-day strength, the concrete was more permeablethan the concrete of the 1950s. Deformational characteristics of concrete The factors affecting the durability of concrete are extrinThe structural engineer is well-educated and trained in strucsic as well as intrinsic, so that to take them properly into actural analysis and design. He or she can derive the appropricount the designer must have a good knowledge of chemical ate equations, or useexisting analytical solutions. But these and physical phenomena of the interaction between concrete equations and solutions involve parameters describing mateand the environment. There exist other examples of the relarial properties. In the case of concrete, compressive strength tion between the behavior of concrete and the performance is the obvious one and it is generally most easily dealt with. ofconcrete structures in service. One example is the influBut for the strength of a structure, we require also the deforence of shape of the structure on durability; slab jetties in mational characteristics of the concrete: its modulus of elasseawater are less liable to reinforcement corrosion than beam ticity, including the fact that the material is not truly elastic, and deck construction....
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