土木专业英文翻译

外文翻译

Discuss the construction temperature and crack of the

concrete lightly

By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Edum-Fotwe ,The American Society of Civil Engineers

The summary Through observation live for many years, through consulting the monograph about stress within the concrete, explain to concrete temperature reason , on-the-spot concrete control and measure , prevention of crack of temperature that crack produce.

Keyword: Concrete;Temperature stress;Crack ;Control

1. The concrete occupies the important position in modern engineering construction. But today, the crack of the concrete is comparatively general, the cracks are nearly omnipresent in the science of bridge building. Though we take various kinds of measures in constructing, careful, but the crack still occurs now and then. Tracing it to its cause, it is one of them incompletely that our change to concrete temperature stress pays attention to. In the large volume concrete, temperature stress and temperature control are significant. This is mainly because of the reason of two respects. First of all, concrete often appear the temperature crack in not constructing, influence the globality and durability of the structure. Secondly, in the course of operating, the temperature change has remarkable influence that can't be ignored on the stress state of the structure. We meet to construct temperature crack in mainly, so only to origin cause of formation and treatment measure, concrete of crack make a discussion in constructing this text.

Reason of a crack

Have many kinds of reasons to produce the crack in the concrete, it is mainly

the changes of temperature and humidity, fragility and disparity of the concrete, and the structure is unreasonable, the raw materials is not up to standard (if the alkali aggregate react), the template is out of shape, the foundation does not subside etc. evenly . The cement emits a large amount of heat of hydration when the concrete is hardenned, inside temperature is rising constantly, cause the stress of drawing on the surface. In the course of lowering the temperature , is it congeal foundation pay restrain to mix always later stage, will present the stress of drawing within the concrete . Reducing of temperature can surface cause heavy stress of drawing very in concrete too. When these draw the stress and go beyond resisting the ability of splitting of concrete , namely will present the crack . A lot of inside humidity of concrete change very light or change relatively slow, surface humidity might change heavy the violent change takes placing. Such as maintaining thoroughly, when getting wetter when not doing,contract surface there aren't deformation doing, often cause the crack too. The concrete is a kind of fragility material , tensile strength is about 1/10 of the compression strength, is it carry on one's shoulder or back limit when draw out of shape to have *104 only , is it carry on one's shoulder or back limit location when stretch out of shape to there is *104 to add for a long time to add a short time. Because raw materials even, water dust than unstable, transport and build phenomenon of emanating of course, its tensile strength is not even in the same concrete, a lot of resist the ability of drawing very low, it is apt to present the weak position of the crack. Among armored concrete , draw stress to undertake by reinforcing bar mainly , concrete bear stress of keeping just. Or reinforcing bar mix if edge position gone to to congeal present the stress of drawing in the structure in plain concrete, must rely on the concrete oneself to bear . Require to avoid the stress of drawing or only very small stress of drawing appears of the the general design. But the concrete is cooled from maximum temperature to the steady temperature of operating period in constructing, often cause sizable to draw the stress within the

concrete. The temperature stress can exceed other outsides and load the stresses caused sometimes, know change law , temperature of stress for carry on reasonable structural design and construct extremely important.

Analysis of 2 temperature stresses

Can be divided into following three stages according to the forming process of the temperature stress:

(1)It is early: Build concrete is it is it over basically to send out heat to cement to begin , generally one one day by oneself. Two characteristics at this stage, first, the cement emits a large amount of heat of hydration, second , mix and congeal the changing sharply of elastic model quantity. Because of the change of elastic model quantity , form the remaining stress in the concrete in this period.

(2)Middle period: Up till the concrete is cooled until stability temperature from cement send out heat function basically when expiring, in this period, the temperature stress is mainly because the cooling of the concrete and external temperature change cause, these stresses and remnants stresses that is formed in early days are superposed , mix and congeal the elastic mould amount that goes to and does not change much during this period.

(3)Later period: Operation period after the complete cooling of concrete. Temperature stress whether external temperature change cause mainly, these stresses and first two kinds of remnants stresseses are changed and added .

Can be divided into two kinds according to the reason why the temperature stress causes:

(1)Spontaneous stress: There are not any restraint or totally static structure at the border, if inside temperature is non-linear distribution, temperature stress appearing because structure restrains from each other. For example, the body of mound of the bridge, the physical dimension is relatively large, surface temperature is low when the concrete is cooled, inside temperature is high, present the stress of drawing on the

surface, present the stress of pressing in the middle.

(2)Restrain the stress: All of the structure ones or it restrain external one some border,can't out of shape and stress not cause not free. Such as case roof beam roof concrete and guardrail concrete.

This two kinds of temperature stresses draw back stresses caused to act on with the doing of concrete together frequently. It is a more complicated job to want to analyse the distribution , size of the temperature stress accurately according to known temperature. In case of great majority , need to rely on the model test or the number value to calculate. To is it make temperature stress have sizable limp to creep concrete, at the stress accounting temperature, must consider the influence that creep , calculate concretly that no longer states thinly here.

Control and preventing the measure of the crack of 3 temperature

For prevent crack , lighten temperature stress can from control temperature and is it is it set about to restrain terms from two to improve.

The measure of controlling temperature is as follows:

(1)Is it improve aggregate grade mix , is it do rigid concrete to spend , mix mixture to adopt, is it guide angry pharmaceutical or plastification pharmaceutical ,etc. measure in order to reduce cement consumption of concrete to add;

(2)Add water or the water to cool the broken stone in order to reduce the temperature of building of the concrete while mixing and shutting the concrete;

(3)Reduce the thickness of building while building the concrete on hot day, utilize and build the aspect to dispel the heat;

(4)Bury the water pipe underground in the concrete, enter the cold water to lower the temperature openly;

(5)Stipulate rational form removal time, the temperature keeps warm the surface while lowering suddenly, in case that the rapid temperature gradient takes place in

the concrete surface;

(6)The concrete with medium and long-term and exposed construction builds a piece of surface or thin wall structure, take the measure of keeping warm in cold season;

The measure of improving condition of restraining is:

(1)Divide and sew and divide one rationally ;

(2)Prevent the foundation from rising and falling too big;

(3)Rational arrangement construction process, prevent the too big discrepancy in elevation and side from exposing for a long time;

In addition, improve the performance of the concrete and improve and resist the ability of splitting, strengthen maintenance , prevent the surface from being done and contracted , especially guarantee the quality of the concrete is very important to preventing the crack, should pay special attention to avoiding producing and running through the crack , the globality resumed its structure after appearing is very difficult, so should rely mainly on preventing the emergence of the running through crack while constructing.

In construction of concrete , for raise turnover rate of template , demand concrete form removal as soon as possible that build newly often. Should consider form removal time properly when concrete temperature is higher than the temperature, so as not to cause the superficial early crack of concrete. Building the early form removal newly, cause very large stress of drawing on the surface, the phenomenon that " temperature is assaulted " appears. Build initial stage in concrete, because heat of hydration is sent out, the surface causes sizable to draw the stress, surface temperature is also higher than temperature at this moment, remove the template at this moment , surface temperature is lowered suddenly, must cause temperature gradient , thus add and draw the stress on the surface , change and add with the heat of hydration stress, in addition, the concrete dries and contracts , the

superficial stress of drawing reaches very great number value, have danger of causing the crack, but cover a light-duty heat insulator with on the surface in time after removing the template , for instance foam sponge ,etc., for prevent concrete surface from produce the too big stress of drawing, have remarkable results.

Add muscle influence to large volume temperature stress of concrete very light , because large volume concrete include muscle to be rate very much low. Just have influence on the general armored concrete. On terms that temperature is not very high and the stress is less than limit of surrendering, every performance of the steel is steady, and have nothing to do with stress state , time and temperature. Line bloated coefficient of steel and concrete line bloated coefficient difference very light, take place little internal stress very only between the two while changing in temperature. Because elastic mould amount of steel concrete elastic mould 7~15 of quantity, reach as interior concrete stress tensile strength and when fracturing, the stress of the reinforcing bar will not exceed 10000kg/cm2. . So is it is it prevent tiny appearance difficulty very much of crack from to make use of reinforcing bar to want among concrete. But the crack in the structure generally becomes numerous, the interval is little, the width and depth are smaller after adding the muscle. And if diameter of reinforcing bar detailed and when interval dense, to improve concrete resist result of person who split better. Concrete and surface , armored concrete of structure can take place detailed and shallow crack often, among them the great majority belong to and do and draw back the crack. Though this kind of crack is generally all relatively light, it stills have certain influence on the intensity and durability of the structure.

In order to guarantee concrete project quality , prevent fracturing , improve the durability of the concrete, use the admixture to reduce one of the measures that fractures correctly. Whether is it reduce water is it split pharmaceutical to defend , I summarize his main function in practice to use.

(1)There is pore Dao of a large number of mao in the concrete , produce

capillary tension in the capillary after water is evaporated, make concrete is it contract out of shape to do. Increasing the thin aperture of hair can reduce the capillary surface tension , but will make the intensity of concrete reduce . This surface tension theory has already been confirmed in the world as far back as the sixties.

(2)Water dust than influence important factor that concrete shrink, is it reduce water is it split pharmaceutical can make concrete water consumption reduce by 25% to defend to use.

(3)Cement consumption important factor, concrete of person who shrink too, is it add and subtract water is it split concrete reducible 15% of the cement consumption on terms that keep the intensity of concrete of pharmaceutical to defend to mix, its volume is supplemented by increasing aggregate consumption.

(4)Reduce water is it split pharmaceutical can improve consistency of grout , reduce concrete secrete ink to defend, reduce and sink and draw back deforming.

(5)Improve glueing the strength of forming of the grout and aggregate, the concrete improved resists the performance of splitting.

(6)Concrete is it produce stress of drawing to restrain from while shrinking, crack when drawing the stress and is greater than concrete tensile strength can produce. Reduce water is it split pharmaceutical effective concrete tensile strength of improvement very to defend , improve resisting the performance of splitting of concrete by a wide margin.

(7)It can make the concrete density good to add the admixture to mix , can improve resisting carbonization of concrete effectively , reduce carbonization to shrink.

(8)Is it reduce water is it split slow coagulation time proper concrete under pharmaceutical to defend , on the basis of preventing the fast water of cement from sending out heat effectively to mix, prevent the plasticity shrink that brings because

the cement is not congealed for a long time from increasing.

(9)Mix admixture concrete and getting easy and kind , surface easy to feel flat , form little membrane, reduce the moisture to evaporate, reduce drily and shrink. A lot of admixture all have the functions of slow coagulation , increasing and apt , improvement plasticity, the experiment that we should carry on in this respect more in the project practice is compared with and studied, than lean against not improving terms more simple,may getting simple and more direct, economy.

Early maintenance of 4 concrete

Practice has proved , the common crack of concrete , most is the surface crack of different depth, main reason its whether temperature gradient cause cold temperature of area lower too easy to form crack suddenly. So say the warm - keeping of the concrete is especially important to preventing the early crack of surface.

From the viewpoint of temperature stress, should reach and require to keep warm followingly:

1)Prevent concrete internal and external temperature poor and concrete surface gradient from , prevent the surface crack.

2)Prevent concrete from to be ultra and cold , should is it is it make the minimum temperature is not lower than the steady temperature of concrete service time construction time in concrete to try to try one's best.

3)Prevent the old concrete subcooling , in order to reduce the restraint among the old and new concrete.

The early maintenance of the concrete, the main purpose lies in keeping the suitable warm and humid condition , in order to get the result of two respects, on the other hand make the concrete avoid the invasion and attack of the unfavorable and warm , humidity out of shape, the ones that prevent from harmfully are cold to contract and do to contract. On one hand make cement water function go on

smoothly , is in the hope of reaching the intensity designed and resisting the ability of splitting.

The suitable warm damp condition is interrelated. Mix warm - keeping measure paid to congeal often protect wet results too. Analyse , water concrete include moisture can meet demand , cement of water have enough and to spare newly theoretically. But because the reason of evaporating etc. often causes losses of the moisture, thus postpone or hinder water of the cement from, the surface concrete receives this kind of adverse effect easiestly and directly. Key period when maintained in initial a few days after so the concrete is built, should pay attention to conscientiously in constructing.

Conclusions

Construction temperature and relation of crack in concrete the above carry on preliminary discussion of theory and practice, though the academia has different theories to origin cause of formation and computing technology of the concrete crack, but to concrete prevention and improving the measure suggestion to relatively unify , application in practice result fine too at the same time, concrete to is it observe , compare more more by us to want in constructing, analyse more , summarize more after going wrong , combine many kinds of prevention and deal with the measure, the crack of the concrete can be avoided.

2.Quality control of waterproof concrete construction

Combined with experience, from formwork design, fabrication and installation, assembing reinforoement, pouring and curing of concrete and other aspects construction technology of fair-faced concrete is introduced as well as quality control measures and standards in order to reduce engineering cost to acquire satisfied economic and social benefits.

The factors of influening waterproof- concrete quality are very many

Any links does not pay attention to the water-proof concrete of field loss hinders

the water function without exception jointly with degree.Engineering construction in the basement adopts secondary form board fabrication and installation, reinforced bar fabrication and bind, concrete stirring and mixing system and transport, concrete concrete covibration beat with a stick, construction joint practice, concrete curing and dismantle model and beingready for backfill and so on aspects.These are very critical to quality method to ensure that water-proof concrete self water-proof, and the way of practice has wan out.

Method being under construction

2.1 Fabrication and Installation

According to the concrett of closely knit , demand of reason why to form board since the water-proof also concrete have made and have assembled corresponding rise is special , be to require that not leaving out thick fluid , firm closely knit block of wood deformation , water absorption Character should be small and ought to give priority to select and using bamboo slab rubber form board or the steel form.. Strict control form board room gap size, necessary exceeding 2 mms uses foam rubber or plastic to squeeze a crack in , porous form board nonutility without exception to board face Be ready for wall post at the same time rotting the prevention and cure job Adopt the cement mortar pouring same ,indicia in before the root segment sticking the foam rubber or plastic strip , the bottom puts on a cement mortar , concrete a concrete, first 5 cm ~ 10 cm. Since water-proof, concrete structure wall thickness is mostly more infertile .Be to ensure that component geometry dimension , Chang adopt the inside and outside bolt to pull the measure meeting attention to, responds to on play receive bolt centre interpose stop water iron plate, to prevent water from forming pilotage passage along bolt leakage.

2.2 Assembing reinforoement

Water-proof concrete structure has demanding as follows to the reinforced

bar

1) reinforced bar should adopt twisted steel as far as possible , increases by hold wrap a force composing in reply a water ability

2) reinforced bars connect should try one's best to adopt to solder connection , stop using and being needless to bind connection to the full

3) when binding a reinforced bar, the iron wire head responds to inner bending.

4) strict control reinforced bars protective layer thickness.

Minimal thickness of water-proof concrete reinforced bar protective layer is not smaller than 25 mms , the protective layer welcoming water surface especially inadmissibility to disappoint error,

. The iron wire and reinforced bar that application buries in advance within mortar piece while

using mortar heel block as protective layer, are bound

solid .When the cavalry puts up the fixed reinforced bar if adopt a reinforced bar, Ying Jia also solders water iron plate or fixation just goes ahead, to strengthen water-proof effect in the

heel block.This project uses new materials nylon to have fixed there is an effect's had guarded against reinforced bar protective layer deviation piece big mass common failings.

The concrete stirring and mixing makes and transports

Since the water-proof concrete requires that higher closely knit , reason why stir and mix system also need to have the fairly good homogeneity , should be ready for burning as follows almost for this purpose

1) ensures that mixing time , mixing at every time are secondary jump into a expect the general ejection of compact block of wood less than 2 mins.

2) should use the apposition agent , the solution queen who should manufacture certain thickness from apposition agent adds the mixer inner, the dried powder or

high concentration solution will add an agent extra not to adds the mixer inner directly ,prevent from mixing is uneven but part

concentrates, both lose the apposition agent effect, and affect concrete mass.

3) responds to the assured source of life degree having a spot test on the admeasurement concrete at the regular intervals collapsing in the process being under construction , construction is middle

if Yu rains or other cause, respond to the ratio determining whose water ratio, and adjusting the composition being under construction in time when change happened in sandstone moisture content.

4) project uses the commodity concrete , has boundary have raised a concrete stirring mass and of all kinds effect apposition agent adulterating falls when amounts , the water ash having controlled a concrete strictly collapsing.

5) concretes concrete adopt a pump to have given handicraft , effective avoiding a concrete producing the phenomenon isolating Mi Shui and leaving out thick fluid in the

process of transportation.

2.4 Matters needing attention in being under construction

1) construction school assignment soft and floury is divided .Water-proof concreting should stratify strictly being in progress, and a continuous construction iscompletedThe front and back and high and low connect between the tier should subjugate within the cement initial setting

time，For this purpose ,with handling a worker dividing into several, at the same time each other, school assignment group faces or it is all right for each other, carry on the back .

2)Achieve strictly fixed point determines the amounts of the components of a substance material down According to the vehicle capacities stratifying concrete altitude and the means of transport, the quantify carrying out fixed point strictly is

able to go down one important ring expecting that this is to improve water-proof concreting mass.

3) insist that you go down material opening the door or use string to expect that under barrel (chute)Be to prevent a cement paste from parting from aggregate for , to expect that liberty should not exceed 1.5 ms now and then highly under water-proof concrete。The water-proof structure reinforced bar is concentrated Should concrete altitude when higher , respond to according to the material stipulating that you go down material opening the door on form board or using string of barrels (chute) to fall, prevent a cement paste from parting from aggregate for , avoid at the same time also the cement paste or the concrete

accumulating the place and cement paste alternating in the reinforced bar loosely splashing sticky Yu form board with outsideNow that material is difficult under bringing about a later, the surface affecting structure glossy , becomes in the future leakage hidden trouble.

4) concretes vibrating beat one important ring being to ensure that the concrete is closely knit , defend against to seep with a stick. Also, should adopt fixed point vibrating , handle meticulously on basis expecting that under determining the amounts of the components of a substance homogeneously in fixed point .Pay attention to high and low tier of linking up

Now that needing to prevent from miss and vibrating , not vibrating enough , being going to prevent a fault from vibrating again, Need to forbid using prying the method coming to aid to vibrate moving a reinforced bar or shaking a reinforced bar especially, such separates to making the front already concrete the reinforced bar and concrete shaking the reality part , causes serious consequences.

5) being ready for the assured source of life degree taking over thick fluid measure , adjusting in time to collapse seriously .Average water-proof concrete structure construction, the construction joint being under construction respectively ,

composing namely on erecting wall sells be bottom board and vertical wall at reduced prices.Should irrigate thick a layer of 5 cm ~ 10 cms surname indicia cement mortars in seam place first linking up closely knitting tier, in order to ensuring that the seam place vibrates when concrete vertical wall.Simultaneous , improve step by step with concrete an altitude'.Should fall off step by step with the water yield , diminish collapsing to fall degree prevent from concrete insist to be short but the cement paste is partial to much phenomenon to aggregate appearing on upper part.

2.5 The method of Construction joint

The construction joint is one of water-proof architectural weak link. The construction joint is one of water-proof architectural weak link. Should stop staying or not staying to the full. equipment or props up conditional restricting of model, may then behind the agreement

collecting a design organ, on the wall stipulating that lienable level of location construction joint , sort keep vertical strokes getting along in all above 200 bottom board .Adopt new handicraft expansibility in the project rubber water just brings handling the construction joint method , substituting method in common use , anxiously expecting the dyadic construction joint of mouth , at any rate dyadic construction joint , flat mouth construction joint such as and so on .Water proofing is in no lienable construction joint of perpendicularity direction like the concrete , rubber stops in having set up the construction joint irrigating belt , perpendicularity directionthe day afer tomorrow between the main building and the skirt building interposing expansibility in seam location water belt or steel plate stops water belt.weak link of water-proof structure, respond to specially-assigned person conscientiously , handle meticulously . Ensure mass to be under construction in construction. Adopt to stop having not to miss and not soldering time water steel plate, adopt rubber water belt just should be solid without fail reliably in receiving long time will

connect using special use glue to paste. When only think that concreting accepts

2.6 Dismantling model and curing of concrete

The water-proof concreting queen maintains goodness and badness working, not only affecting

the intensity arriving at a concrete, moreover the impervious function affecting to the concrete. Evidence , water-proof concrete early phase dehydration or curing process lack for water , will reduce whose impervious function by a wide margin. Water-proof concrete curing

responds to the block of wood less than 14 ds.

The water-proof concrete dismantles a model demanding to be unable to compare high , dismantle model time unsuitable untimely, respond to when dismantling a model handle meticulously , prevent an iron rod from using to pry, stiffly, that the sledgehammer is knocked suddenly

and forcibly pries , uses a concrete meet with destroy

2.7 Do good well at backfill

Water-proof architectural backfill is a final working procedure , it be sometimes ignored , this is a very big faults in fact under the ground.When the water-proof structure job is over queen responds to the construction carrying out backfill immediately.Most important ,we should take backfill seriously self mass; Secondly, backfill varia and ponding should clear up the pit bottom in the front , respond to densification with layers when backfill. Water-proof structure vicinity layer pounds true backfill , self is a reliable and economical as well water proofing drapery under the ground. In project basement engineering construction Shanghai,since organizing water proofing down to earth strictly with all above seven passes, the concrete is under construction , effect is comparatively ideal.The percolating water phenomenon does not appear.

Conclusions

Construction temperature and relation of crack in concrete the above carry on preliminary discussion of theory and practice, though the academia has different theories to origin cause of formation and computing technology of the concrete crack, but to concrete prevention and improving the measure suggestion to relatively unify , application in practice result fine too at the same time, concrete to is it observe , compare more more by us to want in constructing, analyse more , summarize more after going wrong , combine many kinds of prevention and deal with the measure, the crack of the concrete can be avoided.

浅论混凝土的施工过程中温度与裂缝控制

By G. K. Kululanga, W. Kuotcha, R. McCaffer, Member, ASCE, and F. Edum-Fotw美国土木工程师学会

摘要本文通过多年的现场观察，通过查阅有关混凝土内部应力方面的专著，对混凝土温度裂缝产生的原因、现场混凝土温度的控制和预防裂缝的措施进行等进行阐述。

关键词：混凝土；温度应力；裂缝；控制

1 混凝土在现代工程建设中占有重要地位。而在今天，混凝土的裂缝较为普遍，在桥梁工程中裂缝几乎无所不在。尽管我们在施工中采取各种措施，小心谨慎，但裂缝仍然时有出现。究其原因，我们对混凝土温度应力的变化注意不够是其中之一。

在大体积混凝土中，温度应力及温度控制具有重要意义。这主要是由于两方面的原因。首先，在施工中混凝土常常出现温度裂缝，影响到结构的整体性和耐久性。其次，在运转过程中，温度变化对结构的应力状态具有显著的不容忽视的影响。我们遇到的主要是施工中的温度裂缝，因此本文仅对施工中混凝土裂缝的成因和处理措施做一探讨。

1.1 裂缝的原因

混凝土中产生裂缝有多种原因，主要是温度和湿度的变化，混凝土的脆性和不均匀性，以及结构不合理，原材料不合格（如碱骨料反应），模板变形，基础不均匀沉降等。

混凝土硬化期间水泥放出大量水化热，内部温度不断上升，在表面引起拉应力。后期在降温过程中，由于受到基础或老混凝上的约束，又会在混凝土内部出现拉应力。气温的降低也会在混凝土表面引起很大的拉应力。当这些拉应力超出混凝土的抗裂能力时，即会出现裂缝。许多混凝土的内部湿度变化很小或变化较慢，但表面湿度可能变化较大或发生剧烈变化。如养护不周、时干时湿，表面干缩形变受到内部混凝土的约束，也往往导致裂缝。混凝土是一种脆

性材料，抗拉强度是抗压强度的1／10左右，短期加荷时的极限拉伸变形只有（0.6～1.0）×104，长期加荷时的极限位伸变形也只有（1.2～2.0）×104.由于原材料不均匀，水灰比不稳定，及运输和浇筑过程中的离析现象，在同一块混凝土中其抗拉强度又是不均匀的，存在着许多抗拉能力很低，易于出现裂缝的薄弱部位。在钢筋混凝土中，拉应力主要是由钢筋承担，混凝土只是承受压应力。在素混凝土内或钢筋混凝上的边缘部位如果结构内出现了拉应力，则须依靠混凝土自身承担。一般设计中均要求不出现拉应力或者只出现很小的拉应力。但是在施工中混凝土由最高温度冷却到运转时期的稳定温度，往往在混凝土内部引起相当大的拉应力。有时温度应力可超过其它外荷载所引起的应力，因此掌握温度应力的变化规律对于进行合理的结构设计和施工极为重要。

1.2 温度应力的分析

根据温度应力的形成过程可分为以下三个阶段：

（1）早期：自浇筑混凝土开始至水泥放热基本结束，一般约30天。这个阶段的两个特征，一是水泥放出大量的水化热，二是混凝上弹性模量的急剧变化。由于弹性模量的变化，这一时期在混凝土内形成残余应力。

（2）中期：自水泥放热作用基本结束时起至混凝土冷却到稳定温度时止，这个时期中，温度应力主要是由于混凝土的冷却及外界气温变化所引起，这些应力与早期形成的残余应力相叠加，在此期间混凝上的弹性模量变化不大。

（3）晚期：混凝土完全冷却以后的运转时期。温度应力主要是外界气温变化所引起，这些应力与前两种的残余应力相迭加。

根据温度应力引起的原因可分为两类：

（1）自生应力：边界上没有任何约束或完全静止的结构，如果内部温度是非线性分布的，由于结构本身互相约束而出现的温度应力。例如，桥梁墩身，结构尺寸相对较大，混凝土冷却时表面温度低，内部温度高，在表面出现拉应力，在中间出现压应力。

（2）约束应力：结构的全部或部分边界受到外界的约束，不能自由变形而引起的应力。如箱梁顶板混凝土和护栏混凝土。

这两种温度应力往往和混凝土的干缩所引起的应力共同作用。

要想根据已知的温度准确分析出温度应力的分布、大小是一项比较复杂的工作。在大多数情况下，需要依靠模型试验或数值计算。混凝土的徐变使温度应力有相当大的松驰，计算温度应力时，必须考虑徐变的影响，具体计算这里就不再细述。

1.3 温度的控制和防止裂缝的措施

为了防止裂缝，减轻温度应力可以从控制温度和改善约束条件两个方面着手。

控制温度的措施如下：

（1）采用改善骨料级配，用干硬性混凝土，掺混合料，加引气剂或塑化剂等措施以减少混凝土中的水泥用量；

（2）拌合混凝土时加水或用水将碎石冷却以降低混凝土的浇筑温度；

（3）热天浇筑混凝土时减少浇筑厚度，利用浇筑层面散热；

（4）在混凝土中埋设水管，通入冷水降温；

（5）规定合理的拆模时间，气温骤降时进行表面保温，以免混凝土表面发生急剧的温度梯度；

（6）施工中长期暴露的混凝土浇筑块表面或薄壁结构，在寒冷季节采取保温措施；

改善约束条件的措施是：

（1）合理地分缝分块；

（2）避免基础过大起伏；

（3）合理的安排施工工序，避免过大的高差和侧面长期暴露；

此外，改善混凝土的性能，提高抗裂能力，加强养护，防止表面干缩，特别是保证混凝土的质量对防止裂缝是十分重要，应特别注意避免产生贯穿裂缝，出现后要恢复其结构的整体性是十分困难的，因此施工中应以预防贯穿性裂缝的发生为主。

在混凝土的施工中，为了提高模板的周转率，往往要求新浇筑的混凝土尽

早拆模。当混凝土温度高于气温时应适当考虑拆模时间，以免引起混凝土表面的早期裂缝。新浇筑早期拆模，在表面引起很大的拉应力，出现“温度冲击”现象。在混凝土浇筑初期，由于水化热的散发，表面引起相当大的拉应力，此时表面温度亦较气温为高，此时拆除模板，表面温度骤降，必然引起温度梯度，从而在表面附加一拉应力，与水化热应力迭加，再加上混凝土干缩，表面的拉应力达到很大的数值，就有导致裂缝的危险，但如果在拆除模板后及时在表面覆盖一轻型保温材料，如泡沫海棉等，对于防止混凝土表面产生过大的拉应力，具有显著的效果。

加筋对大体积混凝土的温度应力影响很小，因为大体积混凝土的含筋率极低。只是对一般钢筋混凝土有影响。在温度不太高及应力低于屈服极限的条件下，钢的各项性能是稳定的，而与应力状态、时间及温度无关。钢的线胀系数与混凝土线胀系数相差很小，在温度变化时两者间只发生很小的内应力。由于钢的弹性模量为混凝土弹性模量的7~15倍，当内混凝土应力达到抗拉强度而开裂时，钢筋的应力将不超过100~200kg／cm2..因此，在混凝土中想要利用钢筋来防止细小裂缝的出现很困难。但加筋后结构内的裂缝一般就变得数目多、间距小、宽度与深度较小了。而且如果钢筋的直径细而间距密时，对提高混凝土抗裂性的效果较好。混凝土和钢筋混凝土结构的表面常常会发生细而浅的裂缝，其中大多数属于干缩裂缝。虽然这种裂缝一般都较浅，但它对结构的强度和耐久性仍有一定的影响。

为保证混凝土工程质量，防止开裂，提高混凝土的耐久性，正确使用外加剂也是减少开裂的措施之一。例如使用减水防裂剂，笔者在实践中总结出其主要作用为：

（1）混凝土中存在大量毛细孔道，水蒸发后毛细管中产生毛细管张力，使混凝土干缩变形。增大毛细孔径可降低毛细管表面张力，但会使混凝土强度降低。这个表面张力理论早在六十年代就已被国际上所确认。

（2）水灰比是影响混凝土收缩的重要因素，使用减水防裂剂可使混凝土用水量减少25％。