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Cement pastes of water to cement ratio (w/c) of 0.45 with and without nanosilica are hydrated under two conditions, room condition (20 °C with 0.1 MPa pressure) and an oil well condition (80 °C with 10 MPa pressure) for 7 days. For the cement pastes with nanosilica, 1% and 3% of cements weights were replaced by nanosilica. The composition of the hardened cement pastes is investigated using X-ray diffraction (XRD). Nuclear magnetic resonance (NMR) experiments are used to quantify the silicate polymerization in hydrated cement paste. Microstructural phases are identified according to the corresponding mechanical property using nanoindentation. The results showed that under room curing conditions, hardened cement paste with 1% nanosilica has the highest level of calcium silicate hydrate (C–S–H) polymerization. However, under high temperature and pressure curing conditions, hardened cement paste with 3% nanosilica has the highest level of C–S–H polymerization. A new relatively stiff microstructural phase is observed in cement pastes incorporating nanosilica and cured under elevated pressure and temperature conditions. The significance of curing conditions and nanosilica content on the polymerization and stiffness of hydrated cement pastes are discussed.

Experimental investigation is conducted to assess structurally the effectiveness of patch repair in axially loaded columns. Two patch repair materials are selected with high and low modulus of elasticity. The concrete columns were patch repaired under loaded and unloaded conditions. The patch repair is structurally effective for concrete columns repaired in an unloaded state. While for concrete columns repaired in a loaded state, the patch repair is structurally effective only when additional loading is applied. The load distribution between the patch repair, concrete core and steel reinforcement depends on the modulus of elasticity and areas of these components in the composite section at the repaired zone. For patch repair to be structurally effective, it is recommended to relieve the loads before the patch repair is applied either partially or totally if constructionally possible.