【正文】 aints, but also often leads to cracks. Shortterm loading at the time of ultimate tensile deformation only ( ~ ) 104, the limit of longterm loading spaces when the only stretch deformation ( ~ ) 104. Unevenly as a result of raw materials, watercement ratio of instability, and transportation and pouring in the process of segregation phenomena, in the same piece of concrete in the tensile strength is uneven, there are many low tensile capacity, easy to fracture in a weak position. In reinforced concrete, the tensile stress is mainly borne 第 107 頁(yè) by the steel, concrete is subject to pressive stress. In plain concrete or reinforced coagulation on the edge of the site if the structure of the tensile stress appears to be relying on its own concrete mitment. 2. thermal stress analysis According to the formation of thermal stress can be divided into the following three stages: (1) early: Since the beginning of pouring concrete to cement the end of exothermic basic general about 30 days. Two characteristics of this stage, first, to release a large number of cement hydration heat, and the other is the elastic modulus of coagulation dramatic changes. Modulus of elasticity as a result of changes within this period in the formation of residual stress in the concrete. (2) mediumterm: from the basic role of cement heat until the end of the stable temperature of the concrete cooling time, during this period, the temperature stress was mainly due to cooling and the outside concrete caused by temperature changes, stress and early formation of these residual stress superposition phase, during which the coagulation of the elastic modulus changed little. (3) late: concrete pletely cooling period after the operation. Thermal stress is mainly caused by external temperature changes, the stress and the first twophase superposition of residual stress. Under the thermal stress caused by the reasons can be divided into two categories: (1) selfstress: there is no constraint on the border or pletely static structure, if the internal temperature distribution is nonlinear, due to structural constraints arising from their cotemperature stress. For example, the conversion layer of its relatively larger size, concrete cooling surface temperature low, the internal temperatures are high, the surface tensile stress, pressive stress appears in the middle. (2) bound by stress: the structure of all or part of the boundary bound by the outside world can not be freeform deformation caused by stress. Roof box, such as concrete and concrete barrier. Both the temperature and the concrete stress is often caused by shrinkage of the bined effect of stress. Known to be in accordance with an accurate analysis of the temperature of the thermal stress distribution, size is a more plex task. In most cases, the need to rely on model test or numerical calculation. Creep of concrete so that the temperature there is considerable stress relaxation, thermal stress calculation, we must consider the effects of creep, is no longer calculated in detail here. 第 108 頁(yè) 3. temperature control and measures to prevent the cracks In order to prevent cracking, reduce the thermal stress can control the conditions of temperature and to improve the binding of two aspects. Temperature control measures are as follows: (1) used to improve the aggregate gradation, dry hard concrete, mixed mixture, add airentraining agent or plasticizer, etc. measures to reduce the amount of cement concrete。 (3) reasonable arrangements for the construction process, to avoid excessive side elevation and longterm exposure。后期在降溫過(guò)程中，由于受到基礎(chǔ)或老混凝上的約束，又會(huì)在混凝土內(nèi)部出現(xiàn)拉應(yīng)力。一般設(shè)計(jì)中均要求不出現(xiàn)拉應(yīng)力或者只出現(xiàn)很小的拉應(yīng)力。溫度應(yīng)力主要是外界氣溫變化所引起，這些應(yīng)力與前兩種的殘余應(yīng)力相迭加。混凝土的徐變使溫度應(yīng)力有相當(dāng)大的松馳，計(jì)算溫度應(yīng)力時(shí)