Ultra-high performance engineered cementitious composites (UHP-ECC) : mechanical behavior of material and structural members

Abstract

Engineered cementitious composites (ECC), designed based on the micro-mechanical theory, is a special class of high-tensile-ductility concrete with a tensile rupture strain larger than 3% and a fiber volume fraction of no more than 2%. ECC exhibits a multiple cracking feature during the strain-hardening period with the crack width less than 100 µm. However, the high tensile ductility was usually achieved compromising the strength and stiffness of cementitious materials in case of conventional ECCs. On the other hand, ultra-high performance concrete (UHPC) demonstrates a compressive strength higher than 150 MPa but only with a strain capacity (up to the peak stress) of approximately 0.5%. Hence, this research project aims to develop a new category of ECC, which is expected to retain the high tensile strain, strain hardening and multiple microcracking characteristics of conventional ECCs, while achieving high tensile and compressive strengths in the meantime. This type of ECC is termed as UHP-ECC in the present research. The UHP-ECC is targeted to achieve a tensile strain capacity from 6 to 10%, a tensile strength from 16 to 20 MPa, and a compressive strength larger than 100 MPa. It still exhibits the intrinsic multiple micro-cracking behavior under tensile loading and its residual crack width should be no more than 100 µm. The present research is consisted of two parts, i.e., the mechanical behavior of UHP-ECC material, and mechanical behavior of UHP-ECC structural members. The first part includes five chapters from Chapter 3 to Chapter 7 and the second part includes two chapters from Chapter 8 to Chapter 9. The framework of this thesis is illustrated in Fig. 1. [Fig.1 Framework of the thesis : see article file for the details of the abstract] First of all, in chapter 3, a mix proportion was developed for UHP-ECC and the basic mechanical properties of UHP-ECC were investigated. Then in chapter 4, instead of a constant fiber volume of 2%, which was often used to manufacture ECC in previous researches, different fiber volumes Vf and fiber aspect ratios Lf/df were used to investigate how they influenced the tensile properties of UHP-ECC, i.e., the initial cracking stress σtc, the peak tensile stress σtu, the tensile strain capacity εtu, the strain energy gse and the crack properties (crack width Cw and number of cracks Cn). The fiber reinforcing index VfLf/df instead of Vf alone was deployed as a parameter to control and design the performance UHP-ECC. In chapter 5, the strain rate sensitivity of UHP-ECC was investigated. Owing to the superior static tensile behavior of UHP-ECC, it is also expected to maintain a high tensile resistance even at higher strain rates. It is noted that the direct tensile behavior of UHP-ECC was only investigated under static loading in previous studies. Thus the effect of strain rates ranging from 0.0001 s-1, which represents a pseudo static loading rate, to 0.05 s-1, which is generally considered to be in the range of seismic loading rates, on the uniaxial tensile behavior of UHP-ECC was studied. The specimens with different fiber aspect ratios exhibited different failure modes, from fiber fracture of UHP-ECC-900 (Lf/df = 900) to fiber pull-out of UHP-ECC-500 (Lf/df= 500). The increase of fiber aspect ratio was found to have a significantly positive influence on the tensile properties of UHP-ECC in terms of the mechanical properties (σtc, σtu, εtu and gse) and the crack properties (Cn and Cw) at all the strain rates. Larger Lf/df helped to reserve higher portion of εtu and gse, which is favorable for structural applications of UHP-ECCs in seismic regions. While the Lf/df showed less impact on σtc and σtu. The elastic modulus Et of UHP-ECCs kept almost constant at different strain rates with the value of 40-50 GPa, while the strain-hardening modulus Esh decreased steadily with the increase of strain rates. Hence, at higher strain rates, the tensile stress-strain model of UHP-ECC is recommended to be adjusted or the stress increase in the strain-hardening period to be simply ignored as the strain rate increases for a conservative design. The SEM observations of the fracture surface were also conducted to interpret the corresponding failure modes of UHP-ECCs with different fiber aspect ratios. In chapters 6 and 7, the limestone powder (LP) and the recycled fine powder (RFP) were utilized as the supplementary cementitious materials to replace the cement. In UHP-ECC, the ratio of silica sand/binder needs to be reduced in order to obtain a better fiber dispersion and thus a higher tensile property, which adversely increased the amount of cement or cementitious material. LP was found to act as a filler, while RFP was found to act as both a filler and pozzolanic material. These two materials could largely reduce the amount of cement usage to achieve the eco-friendliness of UHP-ECC. In chapters 6, the effect of LP on the hydration, micro-structure and mechanical properties (compressive and tensile properties) of UHP-ECC with different levels of replacement ratios (12.5%, 25% and 50% by volume) were investigated. The addition of LP was found to significantly reduce the hydration heat release of UHP-ECC matrix. The UHP-ECC with the 12.5% LP replacement ratio had the best tensile performance in terms of tensile strength and tensile strain energy, which was also supported by the single fiber pullout test results. Moreover, it was interestingly found that the UHP-ECC with 50% LP replacement ratios had almost the same tensile properties although they led to apparently different compressive strengths. In chapters 7, the use of recycled fine powder (RFP) to replace the cement was found to lead to a similar phenomenon in chapter 6. However, in addition of the filler effect, the RFP could also promote the secondary hydration effect, which increased the mechanical properties of UHP-ECC at different curing ages (3d, 7d and 28d), i.e., the compressive and tensile strengths, up to the 25% RFP replacement ratio. Additionally, the UHP-ECCs with the RFP replacement ratios of 25% and 50% were featured with high early-strength at 3d while exhibiting no significant variation in their strain capacities during the curing ages from 3d to 28d. The SEM observation showed that most of the PE fibers in RFP-added UHP-ECC were pulled out from the matrix but with a better adhesion property that led to the higher tensile strength of RFP-added UHP-ECCs. In chapter 8, the structural behaviors of steel reinforced UHP-ECC beams under bending were experimentally explored with comparison to the ordinary reinforced concrete (RC) beams. Two series of beams, with and without stirrups, were prepared to study the flexural and shear performance of UHP-ECC beams. The UHP-ECC with the tensile strength of 16 MPa and the tensile strain capacity around 8% was used in the tests. The longitudinal reinforcement ratios of the UHP-ECC and RC beams were 0.69%, 1.86% and 2.94%, respectively. The crack propagation and the failure mode of UHP-ECC and RC beams were monitored by the digital image correlation (DIC) method. Additionally, the load-displacement relationships, the ductility index values and the strain values of steel bars and UHP-ECC in the beams were obtained. It was found that the UHP-ECC beams showed superior mechanical performance in terms of both the load-carrying capacity and the ductility at the ultimate limit state; while the crack width and deflection at the serviceability limit state of UHP-ECC beams are much smaller than those of RC beams. The un-reinforced UHP-ECC beam achieved the similar load-carrying capacity with the conventional RC beam with the reinforcement ratio of 1.86%, demonstrating the potential feasibility of utilizing UHP-ECC to partially replace the steel reinforcement in structural members. Finally, in chapter 9, a series of reinforced UHP-ECC columns (with the section area of 120 mm × 120 mm) were tested under large eccentric compressive loading (the eccentricity is 80 mm). The test results showed that the peak load of a plain UHP-ECC column could approach about 50% of that of an RC column with the reinforcement ratio of 4.3% under the above eccentricity provided that the compressive strength of concrete is 60 MPa. Parameters analyses showed that the load-carrying capacity of UHP-ECC-100 (i.e., with the compressive strength of UHP-ECC at 100 MPa) was similar to that of the column RC14 (i.e., with 4Φ14 steel bars in the four corners of column) with reinforcement ratio of 4.3%, while the load-carrying capacity of UHP-ECC-120 exceeded that of RC14. The column RC14 corresponds to a reinforcement ratio of 4.3%, which is close to the upper limit (5%) specified in the codes for both seismic and non-seismic area, implying that UHP-ECC has the potential to partially replace the steel bars in columns. In summary, this thesis provides a comprehensive study on the mechanical properties of UHP-ECC material as well as its structural members. The static and dynamic properties of the UHP-ECC were well understood through the study and alternatives were found to make the UHP-ECC more eco-friendly. The experimental tests on the structural performance of UHP-ECC beams and columns demonstrated the feasibility of utilizing UHP-ECC to fully replace the steel reinforcement in structural concrete members. The findings from the thesis has laid down a solid foundation for a widespread engineering application of UHP-ECC in future.