Intro To Hydration & Mix Design

Question 1

What is the main binding phase in concrete?

Answer 1

Calcium silicate hydrate, C-S-H.

C: CaO
S: SiO2
H: H2O

Question 2

Describe the process of cement hydration in chemical terms.

Answer 2

C3A most violent, controlled by addition of gypsum
C3A + 26H + 3CŠH2—>C3A.3CŠH32
C3A + 10H +CŠH2—>C3A.CŠH12

Calcium silicates most important
2C3S + 6H—>C3S2H3 + 3CH
2C2S + 4H—>C3S2H3 + CH

Question 3

Describe where the products of the reaction of cement and water form

Answer 3

Outer product C-S-H forms between adjacent particles where water was. Inner product forms in spaces left by cement grains.

Question 4

What 3 properties of concrete are decided during mix design?

Answer 4

28 day strength, durability, workability.

Question 5

In concrete mix design what are the 4 most important properties affecting strength?

Answer 5

W/C ratio (lower is stronger), age (older is stronger), crushed or uncrushed aggregate (crushed is stronger), air entrainment (without is stronger)

Question 6

Describe the properties of workability of concrete

Answer 6

Fluidity-transport properties
Compatibility-ability to settle around reinforcements
Cohesion-ability to not settle into component parts

Question 7

Describe how w/c ratio affects strength

Answer 7

Higher water content, increased capillary porosity, porosity left behind when water used up or evaporated, strength decreases

Question 8

Describe how age affects the strength of concrete

Answer 8

Hydration happens over time, more hydration=more hydration filling capillary pores, reducing porosity, increasing strength.

Question 9

Why is it important fine aggregates have a fraction smaller than 600 microns?

Answer 9

Allows electrostatic forces to overcome gravity forces keeping mix from separating into layers of components. Cohesion.