摘要
活性粉末混凝土(Reactive Powder Concrete,RPC)是一种具有超高强度、韧性的新型水泥基复合材料,它与普通的混凝土相比具有明显的在强度、韧性、耐久性等方面的优势,特别是其抗拉强度和韧性远高于普通混凝土。
RPC抗弯拉强度跟材料的配合比、养护条件、钢纤维掺量等有密切关系,其中影响最显著的是钢纤维掺量。钢纤维对RPC基体的增强、增韧作用主要是通过钢纤维与基体之间的应力传递实现的,因此界面粘结性能的好坏对钢纤维增强、增韧的效果又起着至关重要的作用。
本文先对钢纤维与RPC基体的粘结性能进行了试验研究,深入分析了钢纤维间距、钢纤维分布对粘结性能的影响、采用力学计算模型分析钢纤维与基体间的界面应力传递以及通过试验测量荷载—滑移全曲线,并以此分析钢纤维对RPC材料的增强、增韧作用及拔出过程中的粘结耗能情况。在对钢纤维与RPC基体粘结性能的试验基础上进一步研究了钢纤维掺量、骨料的粒径、养护制度对RPC材料抗弯拉强度的影响,得出了以下结论:
(1):通过钢纤维和RPC基体材料的粘结拉拔试验,分析了钢纤维的长径比、纤维间距、不同的纤维分布对粘结性能的影响。从试验结果中发现钢纤维在不同的纤维间距下与基体的极限粘结强度明显不同,相同的拔出纤维根数下随着钢纤维间距的增大极限粘结强度呈现先增大而后趋于稳定的趋势,同时发现纤维分布对粘结强度影响不显著;推导了纤维拔出过程的力学计算模型,并采用该力学计算模型结合本实验的试验结果分析了纤维拔出过程中界面的应力分布及传递;采用粘结性能试验结果拟合了纤维间距与钢纤维粘结强度之间的关系,分析发现当纤维间距很小时粘结强度随着纤维间距的减小呈指数形式衰减;分析了钢纤维粘结滑移曲线特征、纤维间距对纤维拔出过程耗能能力的影响。
(2):试验发现在RPC浆体中钢纤维能充分均匀分布的情况下RPC抗弯拉强度随着钢纤维掺量的增加而增加,2%钢纤维体积掺量时纤维增强效率最佳。热水养护下由于能够充分发挥RPC原材料的硅灰等火山灰质材料的火山灰活性,所以85℃热水养护48小时的RPC试件抗弯拉强度均略高于28天标准养护的RPC试件的抗弯拉强度。试验发现当RPC中细骨料的粒径较小时对RPC的抗弯拉强度不产生明显的影响,但是当骨料粒径较大时会对浆体中的纤维分散产生比较明显的影响,试验中发现骨料粒径过大时会有纤维成团现象从而显著降低了RPC材料的抗弯拉强度。同时采用了控制位移加载的方式测量了RPC矩形小梁的荷载—挠度全曲线,并对其韧性进行了评价。
(3):本文分析和总结了关于纤维增强复合材料的增强理论,在此基础上并基于纤维均匀分布的假设及结合钢纤维与RPC基体粘结强度试验结果推导了RPC抗弯拉强度计算公式。将理论计算结果与本文抗弯拉试验结果进行了对比,发现当钢纤维分布较为均匀时理论计算结果跟试验结果具有比较好的近似性,当钢纤维出现成团等不均匀分布现象时理论计算结果跟试验值具有很大的差异。
Reactive Powder Concrete(RPC) is a cementitious material with ultra-high performance, such as ultra-high strength、ultra-high ductility, it obviously have higher strength、higher ductility and higher durability、especially much higher tensile strength and ductility compared to normal strength concrete(NSC).
There are many factors which will affect the bending tensile strength of RPC, such as mix proportion、curing condition、fiber volume fraction, while fiber volume fraction play a major role in determining the bending tensile strength of RPC. The reinforcing and toughening effect of steel fiber in the matrix of RPC mainly through the stress transfer taking place across the fiber/matrix interface.
This paper based on the fiber pull-out experiment to study the bond slip characteristics between steel fiber and the matrix of RPC. Several parameters were taken into account in this test such as fiber diameter, different fiber distribution and different fiber distance. The influences of fiber distribution and fiber distance on the pullout curves and pullout works are studied. The two-cylinder model for the single-fiber pull-out test was used to analyze the stress transfer between fiber/matrix interface. Then, the influence of bending tensile strength (such as steel fiber volume fraction、curing regimes、fine aggregate diameter )of RPC was systematically analyzed. From the fiber pull-out experiment and flexural test we get the following main conclusions:
(1): By means of the fiber pull-out experiment, we found that the effect of fiber distance play a major role in determining the adhesive property and stress transfer of steel fiber/RPC matrix interface. With the increasing of fiber distance, maximum bond strength of fiber/matrix was also increasing firstly and then becoming stable. Mechanical calculation model for the fiber pull-out was established, and also by the use of the model, adhesive property and stress transfer of steel fiber/RPC matrix interface were systematically analyzed. By use of the experimental results, we create fit curves, which can indicate the relationship between the fiber distance and adhesive strength。
(2):The increase of steel fiber volume fraction can enhance the bending tensile strength of RPC under uniform distribution of fibers, when the steel fiber volume fraction increases from 0% to 3%, the bending tensile strength increases from 9.0MPa to26.9MPa.But from the economical point of view ,2% steel fiber volume fraction provide the best reinforcing efficiency. RPC is normally heat treated at an early age for 24~48 hours at 80~90℃,because of such treatment can accelerate the pozzolanic reaction of silica fume with Ca(OH)2.This explains the bending tensile strength of the specimens under hot water curing condition for 48 hours at 85℃is a little higher than the specimens under standard curing condition for 28 days. It was found that aggregate diameter will affect the distribution of steel fibers, when the aggregate diameter increases to 5mm, the bending tensile strength will drops significantly.
(3): This paper summarizes the theories of fiber reinforced composites, by using of these theories and based on the assumption of uniformly distributed fibres、perfect bonding between the fiber and matrix, a computational formula of flexural strength of RPC was deduced. The flexural strengths estimated based on the computational formula was then compared with the measured flexural strengths. It can be seen that there is little difference in flexural strength between the measured and the predicted values when fibers are uniformly distributed in matrix, nonetheless there is noticeable diffenence while fibers are badly distributed in matrix.
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