Quiz 4

Question 1

Chemical Admixture Definition

Answer 1

a liquid, or dispersible
powder, used as an ingredient in a cementitious
mixture to improve its economy and/or properties in
the plastic and/or hardened state (ACI CT-18)

Question 2

Major Reasons for Using chemical admixtures in concrete

Answer 2

1. To achieve specific properties in fresh and
   hardened concrete
2. To maintain the consistency of the mix
3. To overcome certain emergencies during
   concreting operations
4. To make the mixture more economical
5. To enable use of a wider selection of concrete
   materials (including marginal materials)

Question 3

5 Main Chemical Admixtures

Answer 3

Water Reducing 
Retarding 
Accelerating 
Specific Performance
Air-Entraining

Question 4

Water-Reducing Admixtures (WRAs) -

History

Answer 4

• Concrete with carbon black
  • Mix included chemical to disperse carbon black to
  minimize strength loss and provide uniform color
  • Found that chemical also dispersed cement
  • Interest in learning more about chemical

Question 5

Water Reducing Admixture

Answer 5

1. an admixture that either increases
   slump of freshly mixed mortar or
   concrete without increasing water
   content,

or

2. maintains slump with a reduced
   amount of water, the effect being due
   to factors other than air entrainment.
   (ACI CT-18)

Question 6

Plasticizer

Answer 6

- Slump increased while 
maintaining water content (& 
W/CM). 
• Improves placing 
characteristics without 
adversely affecting strength 
and durability

Question 7

Water Reducer

Answer 7

• Water content reduced while 
maintaining slump. 
• Reduces w/cm, leading to 
improved strength and 
durability.

Question 8

Normal Range WRA

Answer 8

• Type A, D, or E
• 5-10% water 
reduction (typical)
• Used by itself or in 
combination with 
high-range water 
reducers

Question 9

Mid-Range

Water Reducing

Answer 9

•Does not have 
specific ASTM 
designation.
• Typically meets Type 
A or D
• 6-12% water 
reduction (typical)
• Used often in 
“superflat” floors

Question 10

High-Range
Water Reducing
(HRWR)

Answer 10

Type F or G
• 12-30% water 
reduction (typical)
• Polycarboxylates are 
most common.
• Key component in 
self-consolidating 
concrete.
• Can be used across 
range of concretes

Question 11

Cement Flocculation

Answer 11

• Cement grains electrically attract other cement
  grains. The cement conglomerates and “flocs” of
  cement and trapped water are formed.
• Cement grain with
  excess, unbalanced
  surface charges
• Surface area of cement
  grains is also more limited
  à reduced hydration
  -trapped water
  -It is desirable to break up the flocs of cement to allow all of the mix water to
  lubricate cement grains, aggregates, SCMs, etc

Question 12

Electrostatic Repulsion

Answer 12

- Contributes to dispersion of all water 
reducing admixtures (normal, midrange, and high-range)

Question 13

Steric Repulsion

Answer 13

Contributes to dispersion of 
polycarboxyates only.
• More powerful mode of dispersion 
than electrostatic repulsion.
• Can extend slump life of concrete

Question 14

Normal Range Water-Reducers

Answer 14

• Consist of Lignosulfonate (Waste product of
  paper industry) and Sulfonated naphthalene formaldehyde
• anionic hydrophilic head with polar chain (+ - )
• Polar chain selectively adsorbs
  onto surface of cement particles.
• WRAs are ”surfactants”, they act on the surface
• Negatively-charged heads
  repel each other
• This mechanism is known as electrostatic repulsion and contributes to dispersion of all WRAs (Normal, mid-range, and high-range)

Question 15

Dispersive Action

Answer 15

The WRA will also adhere to aggregate particles and

will help to disperse cement, aggregate and air entrained bubbles

Question 16

HRWRs (Polycarboxylates)

Answer 16

• polar chain plus Hydrophilic side chain
• Polar chain adsorbs onto and envelopes portland cement grains
• Polycarboxylate-based HRWRs work using electrostatic repulson AND steric repulsion (sometimes referred to as steric hindrance)

Question 17

Self-Consolidated Concrete

Answer 17

• The introduction of polycarboxylates opened the door for self-consolidated concrete (SCC)
• SCC - Fresh concrete that can flow around reinforcement and consolidate within formwork under its own weight without vibration (ACI –CT18)
  • Only possible with polycarboxylates
  • Typically requires higher fines content, lower FA/CA ratio, and smaller max. size aggregate in order to avoid segregation.
  • Can also use viscosity modifying admixtures (VMAs) to prevent segregation.
  • Used in precast plants and special applications

Question 18

Mid-Range Water-Reducers

Answer 18

• Introduced in mid-1980’s (yet still does not have specific designation in ASTM C 494)
• Set time generally not sensitive to dosage rate
• Designed for enhanced finishability
• Slumps in 5-8 inch range typical
• Very common in superflat floor specifications

Question 19

Impact of WRAs on Fresh Concrete

Answer 19

• Increases slump
• Increases slump retention
• Improves flow
• Improves pumpability
• Improves finishability (MRWR)
• Improves appearance of formed 
surface
• May increase setting time

Question 20

Impact of WRAs on Hardened Concrete

Answer 20

• Increases strength
• Reduces permeability to water and ions (sulfates, chlorides, etc.)
• Increases sulfate resistance
• Increases abrasion resistance Hardened Concrete*
* When using water reducers to lower w/cm ratio

Question 21

Set Modifying Admixtures

Answer 21

• Set modifiers include accelerators and retarders
• Accelerating admixture - an admixture that causes an
  increase in the rate of hydration of the hydraulic cement and thus shortens the time of setting, increases the rate of strength development, or both.
• Retarding admixture - an admixture that causes a decrease in the rate of hydration of the hydraulic cement and lengthens the time of setting

Question 22

Hydration and Set Modifiers

Answer 22

• Set control admixtures primarily work by slowing down or speeding up the dissolution of cement grains (by strengthening or weakening the
  diffusion barrier that forms around cement grains in early hydration).

Question 23

Setting Time (Initial Set)

Answer 23

• Time from moment water is 
added until the paste ceases to 
be fluid and plastic
• Time when average worker’s foot 
leaves ¼” indentation and bleed 
water appears and disappears.

Question 24

Setting Time (Final SeT)

Answer 24

• Time from moment water is 
added for the paste to acquire a 
certain degree of hardness
• Time after which no further 
modifications to concrete surface 
are possible.