【正文】 strain, ?cs, being divided into two ponents, endogenous shrinkage, ?cse, (which is assumed to develop relatively rapidly and increases with concrete strength) and drying shrinkage, ?csd (which develops more slowly, but decreases with concrete strength). At any time t (in days) after pouring, the endogenous shrinkage is given by ?cse = ?*cse ( ) (2) where ?*cse is the final endogenous shrinkage and may be taken as ?*cse 610)503( ????? cf , where cf? is in MPa. The basic drying shrinkage *csd? is given by 66* 1025010)81 1 0 0( ?? ?????? cc s d f? (3) and at any time t (in days) after the mencement of drying, the drying shrinkage may be taken as *1 csdcsd k?? ? (4) The variable 1k is given by )7/( htt tkkk ?? (5) where htek 0 0 ??? and 5k is equal to for an arid environment, for a temperate environment and for a tropical/coastal environment. For an interior environment, k5 may be taken as . The value of k1 given by Equation 5 has the same general shape as that given in Figure in AS3600, except that shrinkage develops more rapidly at early ages and the reduction in drying shrinkage with increasing values of th is not as great. The final shrinkage at any time is therefore the sum of the endogenous shrinkage (Equation 2) and the drying shrinkage (Equation 4). For example, for specimens in an interior environment with hypothetical thicknesses th = 100 mm and th = 400 mm, the shrinkage strains predicted by the above model are given in Table 1. Table 1 Design shrinkage strains predicted by proposed model for an interior environment. Electronic Journal of Structural Engineering, 1 ( 2021) 19 ht cf? *cse? (x 106) *csd? (x 106) Strain at 28 days (x 106) Strain at 10000 days (x 106) cse? csd? cs? cse? csd? cs? 100 25 25 900 23 449 472 25 885 910 50 100 700 94 349 443 100 690 790 75 175 500 164 249 413 175 493 668 100 250 300 235 150 385 250 296 546 400 25 25 900 23 114 137 25 543 568 50 100 700 94 88 182 100 422 522 75 175 500 164 63 227 175 303 478 100 250 300 235 38 273 250 182 432 Shrinkage in Unrestrained and Unreinforced Concrete (Gilbert, 1988) [7] Drying shrinkage is greatest at the surfaces exposed to drying and decreases towards the interior of a concrete member. In , the shrinkage strains through the thickness of a plain concrete slab, drying on both the top and bottom surfaces, are shown. The slab is unloaded and unrestrained. The mean shrinkage strain, ?cs in Fig. 1, is the average contraction. The nonlinear strain labelled ??cs is that portion of the shrinkage strain that causes internal stresses to develop. These selfequilibrating stresses (called eigenstresses) produce the elastic and creep strains required to restore patibility (ie. to ensure that plane sections remain plane). These stresses occur in all concrete structures and are tensile near the drying surfaces and pressive in the interior of the member. Because the shrinkageinduced stresses develop gradually with time, they are relieved by creep. Nevertheless, the tensile stresses near the drying surfaces often overe the tensile strength of the immature concrete and result in surface cracking, soon after the mencement of drying. Moist curing delays the mencement of drying and may provide the concrete time to develop sufficient tensile strength to avoid unsightly surface cracking. Fig. 1 Strain ponents caused by shrinkage in a plain concrete slab. The elastic plus creep strains caused by the eigenstresses are equal and opposite to ??cs and are shown in Fig. 1b. The total strain distribution, obtained by summing the elastic, creep and shrinkage strain ponents, is linear (Fig. 1c) thus satisfying patibility. If the drying conditions are the same at both the top and bottom surfaces, the total strain is uniform over the depth of the slab and equal to the mean shrinkage strain, ?cs . It is this quantity that is usually of significance in the analysis of concrete structures. Electronic Journal of Structural Engineering, 1 ( 2021) 20 If drying occurs at a different rate from the top and bottom surfaces, the total strain distribution bees inclined and a warping of the member results. 4. Control of deflection The control of deflections may be achieved by limiting the calculated deflection to an acceptably small value. Two alternative general approaches for deflection calculation are specified in AS3600 (1), namely ‘deflection by refined calculation’ (Clause for beams and Clause for slabs) and ‘deflection by simplified calculation’ (Clause for beams and Clause for slabs). The former is not specified in detail but allowance should be made for cracking and tension stiffening, the shrinkage and creep properties of the concrete, the expected load history and, for slabs, the twoway action of the slab. 呵呵 The longterm or timedependent behaviour of a beam or slab under sustained service loads can be determined using a variety of analytical procedures (Gilbert, 1988) [7], including the AgeAdjusted Effective Modulus Method (AEMM), described in detail by Gilbert and Mickleborough (1997) [12]. The use of the AEMM to determine the instantaneous and timedependent deformation of the critical crosssections in a beam or slab and then integrating the curvatures to obtain deflection, is a refined calculation method and is remended. Using the AEMM, the strain and curvature on individual crosssections at any time can be calculated, as can the stress in the concrete and bonded reinforcement or tendons. The routine use of the AEMM in the design of concrete structures for the serviceability limit states is strongly encouraged. 5. Control of flexural cracking In AS36001994, the control of flexural cracking is deemed to be satisfac