Strength Of Hardened Concrete Flashcards
What is characteristic strength and state it’s formula?
The strength at which 95% of samples will exceed that strength.
Characteristic strength=mean strength - n*standard deviation
Explain how fracture occurs in normal strength concrete
Micro-cracks gain enough energy, due to loading, to grow outside ITZ and join together around aggregates forming a macro-crack.
Explain micromechanically why hysteresis occurs in normal strength concrete when loaded over 40% of final strength.
Micro cracks within ITZ begin to grow as they gain energy from the excessive loading
Name the phase/zone that controls the strength of normal strength concrete and explain why it is weaker than other zones.
Inter-facial transition zone, ITZ. Higher w/c ratio, higher porosity, lower strength.
List 4 factors in order of importance that effect the strength of concrete
W/C ratio, age, aggregates, curing process
Explain how the use of aggregates affects the strength of concrete
Crushed aggregates have rough surfaces, better bond between HCP and aggregate particles. Angular shape means better interlocking and packing of particles.
Explain how curing process affects strength of concrete
Ensures water available for as long as possible, no evaporation. Maximum hydration of cement compounds producing maximum hydration products filling pores in between agg particles. Decreases porosity, increases strength.
Compare the strength of concrete cured in dry air and most air
Concrete cured in dry air for 28 days has half strength of concrete cured in moist air for 7 days + 21 days in dry air
Explain why there is reduced CH in higher strength concrete and why this helps to increase strength.
Additives (CSF, PFA etc) react with CH. CH is weaker than C-S-H, weak points within HCP paste.
Explain why there is no ITZ in higher strength concrete
Low w/c ratio of high strength concrete means no spare water to collect around aggregate. CSF fills remaining pores and micro cracks
What is the formula for design strength?
Design resistance= characteristic resistance/ material partial factor
Explain how strength and stiffness of HCP in higher strength concrete compares to aggregates and how micro cracks are affected
They are similar, micro cracks form in aggregate as well as HCP
Compare stress-strain curves of normal strength concrete and higher strength concrete
HSC is more linear but brittle, failure occurs at peak strength. Hysteresis occurs in NSC and is non linear, failure occurs beyond peak strength.
