【正文】 isturb the flow considerably and reduce the effective length by the width of the piers plus about for each pier. Streamlining the piers or flaring the spillway entrance minimizes the flow disturbance. If the crosssectional area of the reservoir just upstream from the spillway is less than five times the area of flow over the spillway, the approach velocity with increase the discharge a noticeable amount. The effect of approach velocity can be accounted for by the equation 2320g2VhQ ???????? ?? LC? where 0V is the approach velocity. PROPERTIES OF CONCRETE The characteristics of concrete should be considered in relation to the quality for any given construction purpose. The closest practicable approach to perfection in every property of the concrete would result in poor economy under many conditions, and the most desirable structure is that in which the concrete has been designed with the correct emphasis on each of the various properties of the concrete, and not solely with a view to obtaining, say, the maximum possible strength. Although the attainment of the maximum strength should not be the sole criterion in design, the measurement of the crushing strength of concrete cubes or cylinders provides a means of maintaining a uniform standard of quality, and, in fact, is the usual way of doing so. Since the other properties of any particular mix of concrete are related to the crushing strength in some manner, it is possible that as a single control test it is still the most convenient and informative. The testing of the hardened concrete in prefabricated units presents no difficulty, since plete units can be selected and broken if necessary in the process of testing. Samples can be taken from some parts of a finished structure by cutting cores, but at consider one cost and with a possible weakening of the structure. It is customs, therefore, to estimate the properties of the concrete in the structure on the oasis of the tests made on specimens mounded from the fresh concrete as it is placed. These specimens are pacted and cured in a standard manner given in BS 1881 in 1970 as in these two respects it is impossible to simulate exactly the conditions in the structure. Since the crushing structure is also affected by the size and shape of a specimen or part of a structure, it follows that the crushing strength of a cube is not necessary the same as that of the mass of exactly the same concrete. Crushing strength Concrete can be made having a strength in pression of up to about 80N/ 2mm ,or even 3 more depending mainly on the relative proportions of water and cement, that is, the water/cement ratio, and the degree of paction. Crushing strengths of between 20 and 50 N/ 2mm at 28 days are normally obtained on the site with reasonably good supervision, for mixes roughly equivalent to 1:2:4 of cement: sand: coarse aggregate. In some types of precast concrete such as railway sleepers, strengths ranging from 40 to 65 N/ 2mm at 28 days are obtained with rich mixes having a low water/cement ratio. The crushing strength of concrete is influenced by a number of factors in addition to the water/cement ratio and the degree of paction. The more important factors are Type of cement and its quality. Both the rate of strength gain and the ultimate strength may be affected. Type and surface texture of aggregate. There is considerable evidence to suggest that some aggregates produce concrete of greater pressive and tensile strengths than obtained with smooth river gravels. Efficiency of curing. A loss in strength of up to about 40 per cent may result from premature drying out. Curing is therefore of considerable, importance both in the field and in the making of tests. The method of curing concrete test cubes given in BS 1881 should, for this reason, be strictly adhered to. Temperature In general, the rate of hardening of concrete is increased by an increase temperature. At freezing temperatures the crushing strength may remain low for some time. Age Under normal conditions increase in strength with age, the rate of increase depending on the type of cement with age. For instance, high alumina cement produces concrete with a crushing strength at 21 hours equal to that of normal Portland cement concrete at 28 days. Hardening continues but at a much slower rate for a number of years. The above refers to the static ultimate load. When subjected to repeated loads concrete fails at a load smaller than the ultimate static load, a fatigue effect. A number of investigators have established that after several million cycles of loading, the fatigue strength in pression is 5060 per cent of the ultimate static strength. Tensile and flexural strength The tensile strength of concrete varies from oneeighth of the pressive strength at early ages to about one twentieth later, and is not usually taken into account in the design of reinforced concrete structures. The tensile strength is, however, of considerable importance in resisting cracking due to changes in moisture content or temperature. Tensile strength tests are used for concrete roads and airfields. The measurement of the strength of concrete in direct tension is difficult and is rarely attempted. Two more practical methods of assessing tensile strength are available. One gives a measure of the tensile strength in bending, usually termed the flexural strength. BS 1881:1970 gives details concerning the making and curing of flexure test specimens, and of the method test. The standard size of specimen is 150 mm 150mm 750mm long for aggregate of maximum size 40mm . If the largest nominal size of the aggregate is 20 mm , specimens 100mm 100mm 4 750mm long