汪青杰,張延年,劉旭峰,徐 馳

凍融時(shí)間對(duì)早期混凝土抗壓性能的影響試驗(yàn)

汪青杰a,張延年b,劉旭峰b,徐 馳b

(沈陽(yáng)建筑大學(xué)a.理學(xué)院,b.土木工程學(xué)院,遼寧沈陽(yáng) 110168)

為研究?jī)鋈谇梆B(yǎng)護(hù)時(shí)間和凍融時(shí)間對(duì)早期混凝土抗壓性能的影響,采用混凝土凍融試驗(yàn)機(jī)凍融循環(huán)和冬季室內(nèi)外自然凍融循環(huán),對(duì)不同凍融前養(yǎng)護(hù)時(shí)間和冷凍時(shí)間的混凝土立方體試塊進(jìn)行抗壓性能試驗(yàn).凍融循環(huán)后混凝土表面不再光滑,表面剝落情況越嚴(yán)重,混凝土疏松、起皮,甚至脫皮,空洞現(xiàn)象也隨之增多.齡期越早、凍融時(shí)間越長(zhǎng)表面破壞越嚴(yán)重.混凝土凍融試驗(yàn)機(jī)和室內(nèi)外自然凍融受凍試塊的凍融時(shí)間與質(zhì)量損失率的關(guān)系曲線大致相同,受凍時(shí)間為6 h的試塊均比受凍時(shí)間為12 h的質(zhì)量損失大,且凍融前養(yǎng)護(hù)3 d和7 d的質(zhì)量損失較大,7 d后質(zhì)量損失較小,斜率趨于平緩.混凝土凍融試驗(yàn)機(jī)和冬季室內(nèi)外自然凍融受凍12 h的試塊比受凍6 h的抗壓強(qiáng)度低,抗壓強(qiáng)度損失率要大.

混凝土;凍害;凍融循環(huán);立方體試塊;齡期;質(zhì)量損失;抗壓強(qiáng)度

混凝土常溫施工季節(jié)較短,大多數(shù)工程在初冬時(shí)期還未完工,有些在采用簡(jiǎn)單保護(hù)措施的情況下可以施工,而有些則需要在第二年的春天才能繼續(xù)施工.由于施工條件和氣候的影響,使還處于施工期的混凝土構(gòu)件常會(huì)出現(xiàn)受凍的問題.若不及時(shí)發(fā)現(xiàn),不僅會(huì)影響工程質(zhì)量,嚴(yán)重的還會(huì)造成不可挽回的巨大損失.

近些年來(lái),有許多圍繞混凝土的凍融破壞的試驗(yàn)研究,主要集中于凍融機(jī)理、混凝土的材料及外加劑等方面[14].國(guó)內(nèi)外大量的試驗(yàn)研究主要是針對(duì)混凝土的材料組成對(duì)其抗凍性的影響[59].由于試驗(yàn)設(shè)備缺乏和試驗(yàn)難度大等原因,對(duì)凍融循環(huán)后混凝土力學(xué)性能的研究資料很少[10].而在實(shí)際應(yīng)用中,混凝土的力學(xué)性能是最需要關(guān)心的,因?yàn)閺?qiáng)度的損失直接關(guān)系到建筑物的使用性能及安全.全面了解經(jīng)受設(shè)計(jì)凍融循環(huán)次數(shù)后的剩余強(qiáng)度,十分必要.

在初冬施工,凍融前養(yǎng)護(hù)時(shí)間和凍融時(shí)間往往影響混凝土的力學(xué)性能,本文采用混凝土凍融試驗(yàn)機(jī)凍融循環(huán)和冬季室內(nèi)外自然凍融循環(huán),對(duì)不同凍融前養(yǎng)護(hù)時(shí)間和冷凍時(shí)間的混凝土立方體試塊進(jìn)行抗壓性能試驗(yàn)研究.

1 試驗(yàn)概況

設(shè)計(jì)100 mm×100 mm×100 mm的混凝土立方體試塊,試驗(yàn)采用混凝土凍融試驗(yàn)機(jī)凍融循環(huán)和冬季室內(nèi)外自然凍融循環(huán).混凝土凍融試驗(yàn)機(jī)凍融循環(huán)試驗(yàn)的受凍溫度定為-20℃,融化溫度為室溫20℃,分組見表1;冬季室內(nèi)外自然凍融循環(huán)試驗(yàn)為冬季室外平均溫度低于-10℃開始,分組見表2.經(jīng)20次凍融循環(huán)后,計(jì)算質(zhì)量損失平均值、質(zhì)量損失率、抗壓強(qiáng)度平均值和抗壓強(qiáng)度損失率.

試驗(yàn)所用水泥采用遼寧山水工源水泥有限公司制造的礦渣硅酸鹽水泥,標(biāo)號(hào)為P·O32.5級(jí);天然碎石粒徑5～25 mm;普通河砂,細(xì)度模數(shù)2.9;水采用自來(lái)水;坍落度為70～90 mm.

表1 混凝土凍融試驗(yàn)機(jī)分組Table1 Grouping of concrete freeze-thaw test machine

表2 冬季室內(nèi)外自然凍融分組Table 2 Grouping of the indoor and outdoor natural freeze-thaw in winter

2 試驗(yàn)結(jié)果

2.1 試驗(yàn)現(xiàn)象分析

圖1為試塊凍融循環(huán)前后外部現(xiàn)象對(duì)比照片,凍融循環(huán)后,試塊表面出現(xiàn)不同程度的裂縫,在混凝土表面的砂漿與粗骨料交界處均可看見產(chǎn)生輕微的裂縫,并出現(xiàn)裂紋,開始疏松、起皮,甚至脫皮,空洞增多.部分骨料和砂漿也隨凍融次數(shù)的增加而發(fā)生脫落.試塊兩端棱角處凍融破壞較為嚴(yán)重.

圖1 試塊凍融循環(huán)前后外部現(xiàn)象對(duì)比Fig.1 External phenomena contrast of test block before and after freeze-thaw cycle

2.2 試驗(yàn)結(jié)果分析

表3為混凝土凍融試驗(yàn)機(jī)試驗(yàn)結(jié)果,受凍時(shí)間為6 h的混凝土試塊,質(zhì)量損失均大于受凍12 h的試塊,但抗壓強(qiáng)度損失均比12 h的小,受凍時(shí)間越短,對(duì)混凝土抗壓強(qiáng)度損失越小.表4為冬季室內(nèi)外自然凍融試驗(yàn)結(jié)果,受凍時(shí)間為6 h的混凝土試塊質(zhì)量損失均大于受凍12 h的試塊;由于自然環(huán)境溫度波動(dòng)造成影響,總體上看抗壓強(qiáng)度損失均比12 h的小,受凍時(shí)間越短,對(duì)混凝土抗壓強(qiáng)度損失越小.

表3 混凝土凍融試驗(yàn)機(jī)試驗(yàn)結(jié)果Table 3 Test results of concrete freeze-thaw test machine

表4 冬季室內(nèi)外自然凍融試驗(yàn)結(jié)果Table 4 The experimental results of the indoor and outdoor natural freeze-thaw in winter

2.2.1 凍融時(shí)間對(duì)質(zhì)量的影響

圖2為凍融時(shí)間與質(zhì)量損失率的關(guān)系,無(wú)論是混凝土凍融試驗(yàn)機(jī)還是室內(nèi)外自然凍融受凍試塊,曲線大致相同,受凍時(shí)間為6 h的試塊均比受凍時(shí)間為12 h的質(zhì)量損失大,且齡期在3 d、7 d質(zhì)量損失較大,斜率較大;7 d后質(zhì)量損失較小,斜率趨于平緩.這是因?yàn)槭軆? h、融化18 h的試塊和受凍融時(shí)間12 h的試塊,溫度回升時(shí)間不同.根據(jù)P.C.Powers的水壓破壞理論,混凝土就是在凍結(jié)產(chǎn)生水壓、融解吸水使飽和度不斷提高的反復(fù)凍融作用下遭受破壞的.當(dāng)溫度回升時(shí),首先應(yīng)該是混凝土內(nèi)部的融解,因?yàn)榛炷羶?nèi)部融點(diǎn)要比外部的冰層融點(diǎn)低.如果溫度回升過程較慢,則混凝土內(nèi)部的大多數(shù)冰都融解后,外部的冰層才剛剛開始融化,此時(shí)毛細(xì)管就會(huì)被內(nèi)部融水所回填而飽和,融解過程中從外界吸水作用就很微小.但是,如果溫度回升很快,在混凝土內(nèi)部的大多數(shù)冰都未融解之前,外部的冰層已經(jīng)融化或者開始融化,這樣就會(huì)在混凝土內(nèi)部產(chǎn)生負(fù)壓,就會(huì)有充分的外界水可供吸收,從而使混凝土的飽和度有較顯著提高,為下一次凍結(jié)產(chǎn)生更劇烈的破壞提供了條件[78].在反復(fù)凍融循環(huán)后,受凍6 h 的,在室溫(20士2)℃融化時(shí)間為18 h,即相當(dāng)于溫度回升較快,就會(huì)使混凝土質(zhì)量大大減小.說(shuō)明受凍時(shí)間的長(zhǎng)短與質(zhì)量損失成反比,受凍時(shí)間越短,混凝土試塊質(zhì)量損失的影響越大.

圖2 凍融時(shí)間與質(zhì)量損失率的關(guān)系Fig.2 The relationship between freeze-thaw time and mass loss rate

2.2.2 凍融時(shí)間對(duì)抗壓強(qiáng)度的影響

圖3為凍融時(shí)間與抗壓強(qiáng)度損失的關(guān)系,混凝土凍融試驗(yàn)機(jī)受凍12 h的試塊比受凍6 h的抗壓強(qiáng)度小,強(qiáng)度損失率高;由于冬季室外溫度波動(dòng)影響,使凍融前養(yǎng)護(hù)7 d的試塊出現(xiàn)波動(dòng),但總體上受凍12 h的試塊比受凍6 h的抗壓強(qiáng)度要小,強(qiáng)度損失率高.嚴(yán)格的凍融溫度控制的變化規(guī)律明顯,而自然環(huán)境凍融的試塊變化規(guī)律雖然存在小的波動(dòng),但二者總體規(guī)律是一致的:受凍時(shí)間越長(zhǎng),抗壓強(qiáng)度越小,而強(qiáng)度并沒有隨著質(zhì)量的降低而減小,反而增大.

圖3 凍融時(shí)間與抗壓強(qiáng)度損失的關(guān)系Fig.3 The relationship between freeze-thaw time and compressive strength loss

3 結(jié) 論

通過混凝土立方體受凍的試驗(yàn)研究,主要得出以下結(jié)論:

(1)凍融循環(huán)后混凝土表面不再光滑,剝落情況嚴(yán)重,混凝土疏松、起皮,甚至脫皮,空洞現(xiàn)象也隨之增多.齡期越早、凍融時(shí)間越長(zhǎng)表面破壞越嚴(yán)重.

(2)混凝土凍融試驗(yàn)機(jī)和室內(nèi)外自然凍融受凍試塊的凍融時(shí)間與質(zhì)量損失率的關(guān)系曲線大致相同,受凍時(shí)間為6 h的試塊均比受凍時(shí)間為12 h的質(zhì)量損失大,且凍融前養(yǎng)護(hù)3～7 d的質(zhì)量損失較大;7 d后質(zhì)量損失較小,斜率趨于平緩.

(3)混凝土凍融試驗(yàn)機(jī)凍融的混凝土強(qiáng)度變化規(guī)律明顯,而自然環(huán)境凍融的試塊變化規(guī)律雖然存在小的波動(dòng),但二者總體規(guī)律是一致的:受凍時(shí)間越長(zhǎng),抗壓強(qiáng)度越小,而強(qiáng)度并沒有隨著質(zhì)量的降低而減小,反而增大.

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【責(zé)任編輯:祝 穎】

Experimental Research on Compressive Property of Crumb Rubber Concrete Based on Freezing and Thawing Time

Wang Qingjiea,Zhang Yannianb,Liu Xufengb,Xu Chi b
(a.School of Science,b.School of Civil Engineering,Shenyang Jianzhu University,Shenyang 110168,China)

In order to research compression behavior of concrete cubes based on the time of freezing and thawing,the compression tests of concrete cube test blocks in two kinds of freeze-thaw-cycle condition of concrete freeze-thaw test machine and natural state in winter are made.The results show that after the freeze-thaw cycles,the concrete surface is no longer smooth,the surface flakes seriously,and the concrete is loose,peeling,or even scaling,the number of concrete hollows is also increased.Erosion damage of concrete is more serious with the decrease of concrete maintenance time before freezing and thawing and the increase of the time of freezing and thawing.The relation curves of freeze-thaw time and mass loss of two kinds of freeze-thaw-cycle condition are roughly the same. The loss of quality of the test block which got frozen for 6 h was more than that got frozen for 12 h. And the quality loss of the blocks conserved for 3 d or 7 d before freeze-thaw was larger than after 7 d, the slope was leveling off.The compressive strength of the test block that was frozen in concrete freeze-thaw testing machine and the natural freeze-thaw indoor and outdoor in winter for 12 h is lower than that for 6 h;the loss rate of compressive strength is larger.

concrete;freeze injury;freeze-thaw cycle;cube test blocks;age;quality loss; compressive strength

2095-5456(2014)01-0061-05

TU 528

A

20131025

質(zhì)檢公益性行業(yè)科研專項(xiàng)項(xiàng)目(20141012103).

汪青杰(1978),女,遼寧葫蘆島人,沈陽(yáng)建筑大學(xué)講師,碩士;張延年(1976),男,遼寧葫蘆島人,沈陽(yáng)建筑大學(xué)教授,博士.