摘要
为了探究玻璃纤维增强复合材料(GFRP)筋在模拟和真实混凝土环境中性能衰退的规律及机理,设置了碱溶液(AS)和混凝土包裹GFRP筋后置于自来水中(CS)2种侵蚀环境,采用短梁剪切法分析了GFRP筋力学性能的衰退规律,并借助扫描电子显微镜、差示扫描量热法分析了GFRP筋的微观结构和玻璃化转变温度(Tg).结果表明:随着温度的升高,GFRP筋层间剪切强度衰退的速率加快;GFRP筋在60 ℃的AS中老化183 d后层间剪切强度保留了48.6%,Tg降低了9.2%,部分纤维与树脂脱黏,树脂出现孔洞;相同条件下CS中GFRP筋的层间剪切强度保留了61.4%,Tg降低了3.4%,纤维产生浅坑.基于Arrhenius方程建立了北京地区GFRP筋的性能预测模型.
玻璃纤维增强复合材料(GFRP)筋具有耐腐蚀、重量轻等特点,是腐蚀环境中钢筋的有效替代品之
在上述研究中,侵蚀环境多采用人工溶液,但人工溶液模拟的混凝土环境与真实混凝土环境对GFRP筋力学性能的影响差异较
基于此,本文通过测试不同温度(25、40、60 ℃)和老化龄期(15、30、60、90、183 d)GFRP筋的层间剪切强度保留率,分析其力学性能衰退的规律,利用SEM、DSC等方法揭示GFRP筋的降解机理,以得到GFRP筋在模拟和真实混凝土环境中的长期力学性能预测模型.
试验选用山东斯福特实业有限公司生产的GFRP筋,直径(d)为16 mm,主要原材料为玻璃纤维和乙烯基树脂,其中纤维含量(质量分数)约80%.将GFRP筋截至长度80 mm,共制作104个试件.不同侵蚀环境中的试件如

图1 不同老化环境中的试件
Fig.1 Specimens in different aging environments
依据CAN/CSA S807‑19《Specification for fibre‑reinforced polymers》规定,在1 L去离子水中加入118.5 g Ca(OH)2、4.2 g KOH和0.9 g NaOH,测得溶液pH值约为13,用来模拟混凝土的孔溶液,该环境记为AS.另外,用厚度为20 mm的混凝土包裹GFRP筋后浸入自来水中,此环境记为CS.每种侵蚀环境均设置25、40、60 ℃ 3种温度,采用恒温水浴槽控制试验水温.为保证GFRP筋的老化温度达到设计值,部分试件在GFRP筋表面安装贴片式热电偶温度传感器,采用多通道测温仪对其温度进行定期监测.
按照ASTM D4475‑02(2016)《Test method for apparent horizontal shear strength of pultruded reinforced plastic rods by the short‑beam method》规定,设置跨距为48 mm,采用WAW‑100D型电液伺服万能试验机进行短梁剪切试验(见
(1) |

图2 短梁剪切试验
Fig.2 Short‑beam shear test
为分析GFRP筋性能衰退的机理,采用Gemini Sigma 300型SEM观测GFRP筋横、纵截面微观结构的变化,并利用Perkinelmer DSC 4000型DSC测试GFRP筋的玻璃化转变温度(Tg).

图3 GFRP筋的表面形态
Fig.3 Surface morphologies of GFRP bars
试验加载过程中试件两端出现裂缝,裂缝沿轴向逐渐增大,直至贯穿破坏.试验测得未加速老化GFRP筋试件的层间剪切强度为46.93 MPa. 不同侵蚀环境下GFRP筋层间剪切强度保留率(Y)的对比如

图4 不同侵蚀环境下GFRP筋的层间剪切强度保留率对比
Fig.4 Comparison of Y of GFRP bars in different aggressive environments
(1)2种侵蚀环境中GFRP筋层间剪切强度保留率的衰退速率均随着温度的升高而增大,GFRP筋在25、40、60 ℃的CS环境中加速老化183 d后,层间剪切强度相较于初始强度值分别衰退了17.7%、27.6%和38.6%.主要原因为温度升高可以加速GFRP筋的水解反应,降低其力学性能.
(2)GFRP筋老化前期的强度衰退较快.同样以CS环境为例,0~90 d GFRP筋在25、40、60 ℃下的层间剪切强度相较于初始强度值分别衰退了11.4%、20.9%和27.8%,90~183 d该数据为6.3%、6.7%和10.8%.在老化前期,水分子迅速扩散至筋材,削弱了树脂与纤维的界面黏结能力,导致层间剪切强度降低显著.在老化后期,随着筋材的饱和,水分子破坏GFRP筋界面相的能力下降,材料强度降低速率放缓.
(3)在25 ℃下老化的前30 d及40 ℃下老化的前15 d,GFRP筋在CS环境中层间剪切强度的衰退较快,可能是因为CS环境中试件在养护过程中对GFRP筋造成了损伤.在其他老化条件下,GFRP筋在AS环境中层间剪切强度的衰退速率均快于CS环境.以60 ℃下加速老化183 d为例,AS和CS环境中GFRP筋层间剪切强度的保留率分别为48.6%、61.4%.CS环境中GFRP筋的损伤低于AS环境中,主要原因是混凝土有效阻止了O
将60 ℃下2种环境中加速老化183 d的试件与未加速老化试件进行对比,其微观结构如

图5 GFRP筋的微观结构
Fig.5 Microstructure of GFRP bars
(1)GFRP筋由纤维、树脂及其界面相组成,试件老化与其关系紧
(2)在AS环境中老化试件的纤维表面附有沉淀物,在CS环境中老化试件的纤维表面有受侵蚀产生的浅坑.在AS环境中存在大量的O
Si‑O‑Si+O | (2) |
Si‑O‑Na+H2O→Si‑OH+NaOH | (3) |
基于宏观力学性能试验结果及微观结构观测,现将GFRP筋的退化机理分析如下:
(1)树脂基体在水分子和O
(2)O
(3)树脂基体中酯键的水解反应同样会降低GFRP筋的性能,本次试验GFRP筋采用乙烯基树脂,其酯键的数量较少,因此仅观测到少量的树脂损伤.
依据ASTM D3418《Standard test method for transition temperatures and enthalpies of fusion and crystallization of polymers by differential scanning calorimetry》,利用DSC测试GFRP筋在60 ℃下2种环境中老化183 d时的Tg.本次试验对GFRP筋样品进行了2次升、降温,2次升温过程中测得的Tg分别记为Tg1和Tg2,结果如

图6 不同侵蚀环境中GFRP筋的玻璃转化温度对比
Fig.6 Comparison of Tg of GFRP bars in different aggressive environments
目前,常用的GFRP筋长期力学性能预测模型大多基于Arrhenius方程提
(4) |
式中:为退化时间,d;为退化常数,1/d;为材料的活化能,J/mol;为理想气体常数,J/(mol·K).
对
(5) |
(6) |
结合试验数据,在预测GFRP筋的长期性能时以下3种模型应用最为普遍,如式(
Model 1:
(7) |
Model 2:
(8) |
Model 3:
(9) |
式中:a、b、是拟合参数;是FRP筋在无穷长老化时间时的层间剪切强度保留率,%.
模型1最早由Litherland
GFRP筋在2种加速老化环境中的长期力学性能预测模型建立步骤如下:
(1)将试验数据按

图7 基于模型3的GFRP筋长期力学性能试验数据的拟合结果
Fig.7 Fitting results of long‑term mechanical property test data for GFRP bars based on model 3
(2)将、带入

图8 不同环境中GFRP筋的耐久性预测模型Arrhenius线
Fig.8 Arrhenius line for durability prediction model of GFRP bars in different environments
(3)利用Arrhenius方程得出不同老化条件下的时间转换因子(TSF).
在不同温度下GFRP筋退化至相同强度保留率所需时间的关系用TSF表
(10) |
(11) |
(12) |
以北京地区为例,利用
Environment | 13.2 ℃ | 25 ℃ | 40 ℃ | 60 ℃ |
---|---|---|---|---|
AS | 1.00 | 1.72 | 3.25 | 6.91 |
CS | 1.00 | 1.73 | 3.29 | 7.06 |
(4)利用

图9 温度为13.2 ℃时GFRP筋的长期力学模型主曲线
Fig.9 Master curves of long‑term mechanical property of GFRP bars at 13.2 ℃
温度为13.2 ℃时,若GFRP筋在AS和CS 环境下层间剪切强度保留率相同则需满足下列等式关系:
(13) |
(14) |
式(
(1)在碱溶液(AS)和混凝土包裹玻璃纤维增强复合材料(GFRP)筋后置于自来水中(CS)2种环境中,GFRP筋层间剪切强度的衰退速率均随着温度的升高而加快,在老化前期的层间剪切强度衰退速率快于后期.GFRP筋在AS环境中的衰退速率普遍快于CS环境中(25 ℃及40 ℃初始老化阶段除外).
(2)加速老化183 d后GFRP筋的微观结构发生了变化.AS环境中老化GFRP筋的纤维表面有沉积物,少量纤维与树脂脱黏,此现象由树脂基体吸水后的膨胀程度各异引起,且渗透水压亦会使界面相产生破坏.树脂产生了少量孔洞,说明树脂受到了侵蚀.在CS环境中老化GFRP筋的纤维表面产生了浅坑,老化受损较轻.
(3)与普通试样相比,GFRP筋在60 ℃下AS和CS环境中老化后的玻璃化转变温度分别下降了9.2%、3.4%,说明树脂发生了不可逆水解反应,且在AS环境中GFRP筋的反应程度较大,与微观组织的观测结果一致.
(4)建立了北京地区GFRP筋长期力学模型主曲线,利用该曲线可以预测GFRP筋在2种环境中服役时间的层间剪切强度保留率,并得到2种环境中GFRP筋老化程度相同时的老化时间关系.
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