Experimental study on fracture properties of alkali-activated concrete

Abstract

Portland cement concrete (PCC) is the most widely used construction material in the world. However, the increasing emphasis on the sustainable development has highlighted the inherently adverse effects of Portland cement (PC) production on the environment and motivated researchers to explore new cementitious materials as partial or complete alternatives to PC. Alkali-activated cement has been recently widely regarded as a potential alternative to PC. Similar to PCC, alkali-activated concrete (AAC) also exhibits a brittle nature. However, little has been understood on its fracture properties, which are important for the safe applications of AAC in practice. In addition, the dynamic properties of AAC have been rarely explored although AAC also has great potential for use in structures frequently subjected to dynamic load. Against the above background, this thesis is concerned with the fracture properties of AAC, with strong focuses on its static fracture properties and dynamic flexural properties. This thesis consists mainly of the following four parts: (1) the fracture properties of alkali-activated slag concrete and mortar (AASC and AASM); (2) the fracture properties of steel fiber-reinforced AASC and AASM; (3) the fracture properties of alkali-activated slag/fly ash concrete (AASFC); (4) the dynamic flexural properties ofAASC and AASFC subjected to impact load. In the first part of this thesis, an experimental study was conducted on the fracture properties of AASC/AASM and PCC/PCM with similar compressive strengths (i.e., 30 MPa, 50 MPa and 70 MPa) through three-point bending (TPB) tests. The test results indicated that the fracture energy GF of AASC was always higher than that of PCC given the same compressive strength, while GF of AASM became lower than that of PCM when the compressive strengths were 50 and 70 MPa. In addition, the characteristic length lch values of AASC and AASM were all smaller than those of PC, implying that the former was more brittle given the same compressive strength. Micro-structural observations were carried out to explain the above differences. The tension softening curves of AAS and PC concrete and mortar were obtained by inverse analysis. Then, the initiation, cohesion and unstable fracture toughness of AAS and PC concrete and mortar were calculated and compared according to the double-K fracture criterion. The consistence of the cohesion fracture toughness calculated by experimental and analytical approach demonstrated the correctness of the obtained softening curves. In the second part of this thesis, deformed steel fiber was added into the matrix to improve the ductility and fracture toughness of AASC and AASM. Three fiber volume contents, 1.0%, 1.5% and 2.0% were selected to reinforce the plain AASC and AASM. TPB tests were also conducted and the enhancement of fracture energy and fracture toughness with fiber incorporation was critically analyzed and discussed. Aset of tri-linear tension softening curves were proposed through inverse analysis and validated through comparing the predicted load-displacement curves with the experimental ones. The third part of this thesis presented an experimental study on the fracture properties of AASFC in which the combination of ground granulated blast furnace slag (GGBFS) and fly ash (FA) was used as the binder materials. The alkali concentration, the modulus, the water/binder ratio and the slag/FA mass ratio were selected as parameters. The test results on the splitting tensile strength, the elastic modulus and the fracture energy of AASFC were compared with predictions by the existing empirical formulae for PCC. A model was proposed to predict the characteristic length of AASFC since the existing formula for PCC leads to an underestimation. The variations of initiation, cohesion and unstable fracture toughness of AASFC with different testing parameters were discussed in details. The bilinear tension softening curves of AASFC were obtained by inverse analysis. The kink point coordinates of the bilinear softening curve of AASFC were fixed by regression analysis. Then, the consistence of the cohesion toughness of AASFC calculated using experimental and analytical approach in turn demonstrated the correctness of theobtained softening curves. The final part of this thesis presented an experimental study on the dynamic flexural properties of AASC and AASFC with different material parameters and under four different rates of high strain loadings. The dynamic increase factor (DIF) of flexural strength of AAC was found to be quasi-static strength dependent. An empirical formula was proposed to predict the DIF of the dynamic flexural strength of AAC. The enhancement of dynamic energy of AAC was not directly related to its quasi-static strength while almost increased linearly with strain rate. The low temperature effect on the dynamic flexural properties of AASC and PCC was also investigated and discussed. In summary, this thesis has provided a systematic study for the first time on the fracture properties and the dynamic flexural properties of AACs. The differences of the fracture properties between AASC and PCC were mainly attributed to their different micro-structures. The former exhibited more brittleness than the latter given the same compressive strength. A combined use of slag and FA to form AASFC was proven to be an effective solution and the room temperature cured AASFC might exhibit more ductile behavior than PCC. In addition, fiber incorporation could also significantly enhance the mechanical properties of AASC. The models proposed for predicting the tension softening behavior of AACs with different material parameters and the DIF of the flexural strength of AACs can be used to analyze the static and dynamic mechanical performance of AAC structures. All the above understandings achieved through this project have laid a solid foundation for the economic and safe structural applications of AAC in engineering practice.