Workability over time and mechanical properties of ultra-high performance concrete.
MetadataShow full item record
Ultra-high performance concrete (UHPC) is characterized by high compressive and tensile strength along with excellent durability due to a densely packed matrix with low porosity. It has a low water-to-binder ratio (w/b) that can be compensated by the supplementation of superplasticizers (SP) to improve its workability. This thesis presents a literary study on UHPC and a laboratory report in which a total of 22 mix designs were batched and tested with respect to their mobility and stability over time. Compressive, tensile, and flexural strength were determined in the concretes hardened state, in addition to the modulus of elasticity, permeability, and porosity. For each new mix design, a material component was altered, either in quality of quantity, to isolate its effect on the material ́s properties. A total of five binders, two fillers, four aggregates, two superplasticizers, and a shrinkage-reducing admixture were utilized in the mix designs. The main focus of this thesis was to understand how alterations to the mix design affect the workability over time, mechanical properties, and durability of UHPC. This was achieved though a range of tests performed both in its fresh and hardened states. The results show that UHPC ́s have a similar density to that of a normal strength concrete. The mix designs examined exhibited good stability with no sign of water separation, only a tendency for paste separation when larger-sized aggregates were used. Improving the concrete ́s mobility over time can lead to a reduction in its mechanical properties. A higher SP dosage increases mobility over time and, in most instances, improves the compressive strength as well. A higher w/b ratio indicates higher mobility but decrease in compressive strength and durability due to a higher capillary pore structure. A higher initial slump flow usually preserves the mobility over longer periods compared to a concrete with a low initial slump flow. The use of smaller particle-sized granular constituents of under 1mm in diameter can on an overall improve the material ́s properties both in its fresh and hardened state. The tensile strengths for the mix designs qualified as UHPC; however, there were unexpected large variations registered in specimens of the same design that had under- gone the same curing regime, whether this is a result of uncertainties in the method of measurement, or actual variations in the tensile strength was uncertain. Water permeability tests were performed and showed that the mix designs had low capillary porosity with a very gradual ingress of water, providing the material with superior resistance to chemical attack such as chlorides.
Master's thesis in Structural engineering