1、 1 OVERFLOW SPILLWAY An overflow spillway is a section of dam designed to permit water to pass over its crest. Overflow spillways are widely used on gravity, arch, and buttress dams. Some earth dams have a concrete gravity section designed to serve as a spillway. The design of the spillway for tow d
2、ams is not usually critical, and a variety of simple crest patterns are used. In the case of large dams it is important that the overflowing water be guided smoothly over the crest with a minimum of turbulence. If the overflowing water breaks contact with the spillway surface, a vacuum will form at
3、the point of separation and cavitations may occur. Cavitations plus the vibration from the alternates making and breaking of contact between the water and the face of the dam may result in serious structural damage. Cavities filled with vapor, air, and other gases will form in a liquid whenever the
4、absolute pressure of the liquid is close to the vapor pressure. This phenomenon, cavitations, is likely to occur where high velocities cause reduced pressure. Such conditions may arise if the walls of a passage are so sharply curved as to cause separation of flow from the boundary. The cavity, on mo
5、ving downstream, may enter a region where the absolute is much higher. This causes the vapor in the cavity to condense and return to liquid with a resulting implosion, or collapse, extremely high pressure result. Some of the implosive activity will occur at the surfaces of the passage and in the cre
6、vices and pores of the boundary material. Under a continual bombardment of these implosions, the surface undergoes fatigue failure and small particles are broken away, giving the surface a spongy appearance. This damaging action of cavitations is called pitting. The ideal spillway would take the for
7、m of the underside of the napped of a sharp-crested weir when the flow rate corresponds to the maximum design capacity of the spillway. More exact profiles may be found in more extensive treatments of the subject. The reverse curve on the downstream face of the spillway should be smooth and gradual；
8、 A radius of about one-fourth of the spillway height has proved satisfactory. Structural design of an ogee spillway is essentially the same as the design of a concrete gravity section. The pressure exerted on the crest of the spillway by the flowing water and the drag forces caused by fluid friction
9、 are usually small in comparison with the other forces acting on the section. The change in momentum of the flow in the vicinity of the reverse curve may, however, create a force which must be considered. The requirements of the ogee shape usually necessitate a thicker section than the adjacent no o
10、verflow sections. A saving of concrete can be effected by providing a projecting corbel on the upstream face to control the flow in outlet conduits through the dam, a corbel will interfere with gate operation. The discharge of an overflow spillway is given by the weir equation 23CQ Lh Where Q=discha
11、rge, or sec/3m tcoefficienC L=coefficient h=head on the spillway (vertical distance from the crest of the spillway to the reservoir level), 2 m The coefficient C varies with the design and head. Experimental models are often used to determine spillway coefficient. End contractions on a spillway redu
12、ce the effective length below the actual length L. Square-cornered piers disturb the flow considerably and reduce the effective length by the width of the piers plus about 0.2h for each pier. Streamlining the piers or flaring the spillway entrance minimizes the flow disturbance. If the cross-section
13、al 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 veloc
14、ity. 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
15、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 desi
16、gn, 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
17、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 complete units can be selected and broken if necessary in the process of testing. Samples can be taken from some parts
18、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
19、specimens are compacted 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
20、 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 compression of up to about 80N/ 2mm ,or even 3 more depending mainly on the relative proportions of water and cement, that is, the wate
21、r/cement ratio, and the degree of compaction. 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 sleep
22、ers, 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 compaction. The more important factors are Type of cemen
23、t 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 compressive and tensile strengths than obtained with smooth river gravels. Ef
24、ficiency 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
25、 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 ag
26、e. 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
27、 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 compression is 50-60 per cent of the ultimate static strength. Tensile and flexural strength The tensile
28、 strength of concrete varies from one-eighth of the compressive 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 chang
29、es 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
30、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 150mm 150mm 750mm long for aggregate of maximum size 40mm . If the largest nominal size of the aggregate is 20mm , specimens 100mm 100mm
