Mechanical Behavior Of Cementitious Composites With A New Fiber Hybridization

Cementitious composites has been widely used in practical engineering, but its inherent high brittleness is one of the main causes for its poor structure security. Due to the cementitious composites has obvious multi-leve characteristics, which determines its failure process is multi-level. Using the bridging effect of fiber to limit the development of the micro crack and macroscopic fracture, adding it into the cement matrix can improve the mechanical properties of cement-based composite materials greatly. In order to obtain a new type system of hybrid fiber with its mechanical properties unchanged, but the cost decreased obviously, in this paper, calcium carbonate whisker was used to partially substitute polyvinyl akohol (PVA) fiber and steel fiber. Another aim is to reduce the fiber cost in engineering. The influence of the new system to the static and dynamic mechanical properties was studied, the main work of this paper are summarized as followed:(1) Compressive performance of the new system of hybrid fiber is studied, and the test results show that after adding fiber into cement matrix, the compressive strength of the specimens increase is slight. The new system of hybrid fiber has better performance. After damage, it maintains the integrity of specimens and multiple cracks have appeared.(2) The bending capacity of the beam structure is compared by flexural test. The results show that the specimen singlely adding steel fiber has the best performance. Compared to the traditional system of hybrid fiber, the flexural evaluation index of the new system is similar to it. Such as first peak strength of tradition HyFRCC with 1.5% steel fiber+0.5% PVA fiber is 4.55 MPa, which is 4.85 MPa of the new HyFRCC with 1.5% steel fiber+0.4% PVA fiber+ 1% whisker, the difference is only 0.30 MPa, the first peak deflection, equivalent bending strength, energy absorption, peak strength, the difference are not obvious But considerating the cost synthetically, calcium carbonate whisker can be used to partially replace expensive fibers.(3) Flexural test for the plate type structure is studied. And a variety of evaluation indexes are used to analyze the results. It discoveres that the toughness of the specimen adding fiber into it has increased significantly. And specimens reinforced by steel fiber, PVA fiber and calcium carbonate whisker show the best flexural toughness at the fiber dosages of 1.5%,0.4% and 1%, respectively. Its first peak strength is 9.50 Mpa. When its displacement up to 3 mm, the residual strength and the energy absorption of the specimens are 13.95 MPa and 1.94 J, respectively. When its displacement up to 10 mm,the corresponding index are 11.60 MPa and 9.30 J, respectively, the peak strength is 15.35 MPa, and the equivalent bending strength is 12.40 Mpa. Its multiple cracking phenomenon is obvious, too.(4) Improving the method of falling ball impact recommended in ACI 544, impact flexural of the beam specimens are studied. The impact bending performance of the system are analysed adopting ultrasonic detection technology, crack width testing technology, dynamic elastic modulus detection technology, etc. The results show that though adding fiber into cement matrix slightly changed the first crack of the specimen, but it has a significant effect on its impact number to failure. Statistics found that the ultrasonic wave velocity, the arithmetic square root of maximum crack width have a linear function relation to the impact times. And the fitting degree is higher. The slope of the function represents the internal damage accumulation rate of the specimens. The fitting results show that adding whisker into the cementitious composites can slow the internal damage accumulation rate.