Concrete-Properties

Question 1

Primary steps to concrete production (5)

Answer 1

1. Batching - weighing out ingredients
2. Mixing - central mix, shrink mix, truck mix
3. Transporting/Placement - mixer truck, dump truck, bucket,
   buggy, conveyor, pump, chute, tremie
4. Finishing - compaction, strikeoff, floating, troweling, texturing
5. Curing - sprinkling, wet covering, ponding, waterproof
   membrane, steam
   a. Prevent loss of moisture
   b. Supply additional moisture

Question 2

Optimum mixing time depends on many factors (7)

Answer 2

1. Type of mixer
2. Condition of mixer
3. Speed of rotation
4. Size of the charge
5. Nature of ingredients
6. Mix design/proportioning
7. Ambient temperature

Question 3

types of mixers (3):

Answer 3

1. Drum Mixers – a sloping horizontal drum with fixed blades attached to the inner face. Uses gravity to induce mixing. Not very effective for low workability mixes.
2. Pan Mixers – consists of a flat pan within which a series of rotating blades mixes the concrete. Good for low workability
   mixes and FRC.
3. Continuous Mixers – a long trough containing a spiral blade. Conveyors continuously feed ingredients and the concrete is mixed as it passes along the trough. Allows continuous production of concrete.

Question 4

ready mixed concrete

Answer 4

• Central Mixed – Completely mixed at plant. Truck mixer only used to agitate concrete during conveyance.
• Transit Mixed – Partially or completely mixed while being transported to the site.
• Shrink Mixed – Partially mixed at the plant to reduce overall volume, and mixing is completed in the truck mixer.
• Truck Mixed – Completely mixed within the truck mixer. Water can be added later, if aggregate is dry

Question 5

segregation of concrete can lead to

Answer 5

the formation of rock pockets and honey combing

free falls can cause segregation

Question 6

effect of vibration

Answer 6

reduces air content and makes aggregates uniform

(consolidation)

Question 7

ideal fresh concrete

Answer 7

-easily mixed and transported
-uniform throughout a given batch
-capable of flowing such that it completely fills the forms
-compactable without the need for excessive energy
-resistant to segregation during placing and consolidation
-capable of being finished properly

Question 8

define workability

Answer 8

the amount of mechanical work required to produce
full compaction of the concrete without segregation.

Question 9

factors affecting workability

Answer 9

segregation and bleeding

Question 10

define segregation

Answer 10

Segregation refers to the separation of the components of fresh concrete, resulting in a nonuniform mix.

Question 11

factors of segregation

Answer 11

-large aggregate size
-high specific gravity of coarse aggregate relative to fine
-decreased amount of fines
-variations in particle shape away from smooth and rounded
-mixes that are either too wet or dry

Question 12

define bleeding

Answer 12

refers to the upward movement of water after concrete is consolidate but before it has set

Generally caused by aggregates settling within the fresh concrete and being unable to hold all the mixing water

Question 13

impact of bleeding

Answer 13

-distinct, localized channels that later act to increase the permeability of the hardened concrete

-The upper layer of the concrete may become rich in cement paste which has a very high W/C ratio.

-Both of these factors lead to weakness, porosity and reduced durability.

-Water pockets may form beneath inclusions in the concrete, leaving weak zones and reducing bond

Question 14

plastic shrinkage

Answer 14

If bleed water evaporates more quickly than bleeding rates, plasticshrinkage cracking can form
Problematic in hot, dry and/or windy weather.

Question 15

laitance

Answer 15

A scum of fine particles or dissolved salts may be carried to the surface, resulting in a weak and nondurable layer (laitance).
Laitance at the top of a lift can prevent proper bonding.
Laitance at the final surface can result in dusting

Question 16

slump test

Answer 16

The slump test provides a rough measure of consistency - not an absolute measure of workability!!!

Question 17

when cant you use the slump test

Answer 17

High slump concrete
Low slump concrete
Fiber reinforced concrete
Certain admixtures

Question 18

In air-entrained concrete, there are two types of air ….

Answer 18

Entrained Air & Entrapped Air

Question 19

3 tests for air content

Answer 19

the gravimetric, volumetric and pressure methods

Question 20

gravimetric method

Answer 20

Compares the unit weight of concrete containing air with the calculated unit weight of air-free concrete

Question 21

volumetric method

Answer 21

Based upon a comparison of the volume of concrete containing air with the volume of the same concrete after the air has been removed

Requires a significant amount of physical effort to remove the air from the concrete

Question 22

Pressure method

Answer 22

The most common method for measuring the air content of fresh concrete in the field.
Based upon a measurement of the change in volume of the concrete when subjected to a given pressure.
This change in volume is assumed to be entirely caused by compression of the air.

Question 23

steps to time of set

Answer 23

Setting refers to the onset of rigidity in fresh concrete.
Hardening is the development of useful and measurable strength.
Both are gradual changes controlled by the continuing hydration of the cement

Question 24

why is time of set important

Answer 24

1. To help regulate mixing and transit times.
2. To gauge the effectiveness of various set-controlling
   admixtures.
3. To help plan the scheduling of casting and finishing
   operations.
4. As a QC test to monitor the accelerating/retarding effects of additives.

Question 25

how to measure time of set?

Answer 25

measure penetration resistance at different times

Question 26

define curing

Answer 26

Affects both mechanical properties and durability characteristics and reduces volume changes due to shrinkage

Question 27

two components of curing

Answer 27

1. Prevents loss of moisture through evaporation 2. Supplies additional water for continued hydration Excessive evaporation from concrete surface must be avoided since they can cause plastic shrinkage cracking

Question 28

water cement ratio ideal

Answer 28

0.42

Question 29

how is water lost in concrete curing

Answer 29

Water is lost due to evaporation or by absorption by the aggregates, formwork and/or subgrade Once internal relative humidity drops below ~80%, the hydration reaction stops … as does strength development

Question 30

curing of SCMs

Answer 30

Slags or pozzolans require even longer curing periods to ensure that there is enough water present for the pozzolanic reaction to take place.

Question 31

factors affecting curing (3)

Answer 31

– Duration of Curing Once moist curing stops, the rate of strength gain slows down (as water is lost from the concrete) and eventually stops - Interrupted curing can be restarted with a corresponding increase in strength. Ultimate strength will not reach the same level as continuously cured concrete -Temperature affects the rate of strength development and thus the length of curing necessary. At early ages, cement hydrates more rapidly, resulting in increased early strength. However, high temperatures produce a nonuniform distribution of hydration products. These leave weak zones in the hcp that govern strength.

Question 32

concrete should always be tested in

Answer 32

fully saturated condition

Question 33

water curing

Answer 33

Continuous supply of water to the concrete surface Can be accomplished by ponding, spraying, fogging, sprinkling, or saturated coverings. Must maintain water in concrete as a continuous liquid system

Question 34

sealed curing

Answer 34

Sealing of the concrete surface to prevent evaporation Can be done using waterproof paper, plastic sheeting or membrane forming curing compounds Sprayed curing compounds consist of solid materials dissolved in a solvent that form a membrane as the solvent evaporates. Colored pigments are added to reflect heat and clearly indicate coated areas

Question 35

what should not be used with sealed curing

Answer 35

Curing compounds should never be used between lifts or bonded overlays, or when the surface is to be painted. Curing compounds should not be used during the fall on pavements to be exposed to deicing salts Self-desiccation is a common problem with sealed curing procedures

Question 36

define plastic shrinkage cracking

Answer 36

A phenomenon induced when the rate of evaporation of free water from the concrete surface becomes excessive. As the concrete begins to stiffen and set, shrinkage is restrained and tensile stresses develop. Under extreme conditions (hot, dry, windy weather), can lead to severe cracking of the concrete surface if left unprotected.

Question 37

factors of plastic shrinkage cracking factors

Answer 37

* Concrete and air temperature * Relative humidity * Wind velocity

Question 38

hot weather effects on fresh concrete (7)

Answer 38

Accelerated slump loss Increased rate of setting Difficulty in controlling air content Increased loss of moisture during curing Increased risk of plastic shrinkage cracking Higher concrete temperatures Greater thermal variation within concrete

Question 39

precautions when hot weather concreting (10)

Answer 39

Cool concrete materials Cool/protect handling equipment Cool/moisten forms Increase relative humidity of air Avoid hottest periods of the day Reduce time between mixing and curing Protect from wind Implement proper curing methods Use set retarding admixture Use low heat of hydration cement

Question 40

cold weather on concrete

Answer 40

Accelerated slump loss Decreased rate of setting Increased risk of plastic shrinkage cracking* Extreme damage if concrete allowed to freeze

Question 41

precautions for cold weather concreting

Answer 41

Heat concrete materials Insulate forms Avoid coldest periods of the day Reduce time between mixing and curing Protect from wind Use shelters/heaters to keep concrete warm Use air entraining admixture Use set accelerating admixture Use high early strength cement

Question 42

concrete is a ________ material

Answer 42

brittle

Question 43

ultimate strength is a function of:

Answer 43

paste and aggregates

Question 44

Increasing water content=

Answer 44

higher W/C ratio lower strength