HW6 Flashcards
Why do we add sand to concrete mixes?
Adding sand reduces the cost (by reducing the amount of cement paste required), increases elastic modulus of the concrete, and improves dimensional stability.
What is the significance of the fineness modulus?
Fineness modulus is an indication of the mean size of the sand particles
Is this sand suitable for making concrete? (FM=1.75)
No, this sand is too fine for making concrete (FM should be between 2.00-4.00)
Why do FAs follow a certain grading limit?
Coarser sands produce unworkable concrete, and fine sands increase water
requirement.
Why are coarse aggregates graded?
Well-graded coarse aggregates reduce the amount of paste required.
g) What do we mean by the maximum coarse aggregate size?
The sieve size on which 15% or more of the particles are retained
What considerations control the choice of the maximum aggregate size in a
concrete mix
The MSA is often chosen as less than 1/5 of the narrowest formwork dimension,
and less than ¾ of the maximum clear distance between reinforcing bars. For high
strength mixtures, the MSA is limited to 19mm, as large aggregates tend to
produce more microcracks in the ITZ.
How does the aggregate shape influence the properties of the concrete mix?
Angular aggregates create a less workable concrete compared to rounded
aggregates. Flat or flaky aggregates are generally weaker than more spherical
particles.
Basalts are generally i) fine-grained or ii) glassy. Please explain why.
i) Basalts form closer to the surface of the earth, and the quicker
cooling causes fine-grained crystals.
ii) Rapidly cooled magma produces non-crystalline, glassy matter
Many of the properties of the aggregate are significantly affected by its porosity.
List 3 properties that are affected by the porosity of the aggregate.
Density, moisture absorption, strength, hardness, stiffness, soundness
Why are aggregates with lower porosity desired for concrete mixes?
Less porous aggregates tend to be stronger and stiffer
List the compounds in the clinker grains
C3S, C2S, C3A, C4AF
Do the compounds you listed in a) develop strength at the same time or do some
compounds contribute to early strength development?
No, C3A contributes to early strength development, C3S and C4AF have medium
early strength, and C2S has low early strength
Why is gypsum always interground with cement clinker during the final stages of
Portland cement manufacture?
Gypsum delays the reaction of C3A which enables the concrete to be workable for
some amount of time after water is added
What is the difference between type II and type III cement in regards to i) heat
evolution, ii) strength development, iii) chemical composition.
Type III has i) higher early heat, ii) higher early strength, and iii) higher content
of C3A. Type II limits the amount of C3A to 8%, and the sum of C3A and C3S is
limited to 58%
