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Building Materials > Fresh Concrete > Flashcards

Fresh Concrete Flashcards

1
Q

Concrete Composition

A
  • Water
  • Cement
  • Aggregates

Often concrete consists of a multi phase composition:

  • Admixtures (retarder, accelerator, air entraining agents)
  • Additives (fibers)
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2
Q

Aggregates

A

Definition: inert, granular and inorganic material

  • 70-80% volume fraction in the concrete mix
  • strong influence on the properties of concrete

:

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3
Q

Division of aggregate

A
  • Sand with a maximum size of 2 mm - Concrete sand
  • Fine aggregate with a maximum size of 8 mm - Concretre 2/8
  • Coarse aggregate with a maximum size of 16 mm - Concrete Splinter 8/16
  • Coarse aggregate with a maximum size up to 32 mm - Concrete Splinter 16/32
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4
Q

Specification and Qualities

A

The compressive strength of concrete is determined by the compressive strength of the aggregates.

Naturally formed gravel and sand - 80 N/mm2 and 300 N/mm2

Crushed gravel - > 200 N/mm2

Natural and crushed - frost-resistant if > 150 N/mm2

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5
Q

Deleterious Substances

A
  • reducing the strength
  • reduce the corrosion protection

Deleterious substances may either be attached to aggregates or may be present in the form of loose particles not bonded to aggregates.

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6
Q

Limitation of deleterious substances

A

Fine materials ≤ 63 μm

Measured by the sedimentation test or a sieve or screen analysis

Fine materials reduce the bond and thus reduce the concrete strength.

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7
Q

Limitation of materials which disturb the hardening

A
  • Sugar-like materials: effective in very low amounts.
  • Sulphur components: gypsum and oxidizable sulphites can have a damaging effect
  • Reactive silica: can react in humid environments, increase of volume and cracks in the concrete
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8
Q

Grading requirements

A

a) Grading curve of aggregates is found by a sieve or screen analysis.
b) A sample of aggregate is divided into fractions, each fraction contains particles of the same size
c) Result is usually represented graphically in a grading curve.
d) The ordinate represents cumulative % passing through a screen, abscissa represents the screen opening plotted in a logarithmic scale.

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9
Q

Limiting grading curves of aggregates of 32 mm according to DIN 1045-1:

A
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10
Q

Grading requirements

A
  • Maximization of the relative volume occupied by the aggregate
  • Optimization of the workability
  • Minimization of the gaps in the matrix
  • Minimization of the amount of required cement paste
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11
Q

The grain size of such an aggregate can be calculated according to the “Fuller parabola”

A 
**a = (d/D)1/2**
a = weight of aggregates with a diameter \< d
d = diameter being examined
D = diameter of the maximum grain size
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12
Q

Concrete additives

A
  • To improve the qualities of the fresh or hardened concrete
  • They must be taken into account as a part of the volume of concrete as far as it is specially controlled.
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13
Q

Concrete water

A

All naturally found, non-contaminated water suitable
• substances which can negative influence the hardening of cement paste (e.g. sugar) have to be limited
• not include substances which influence the corrosion of reinforcement (e.g. Chlorides as in sea water).

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14
Q

Manufacture of concrete

A

The concrete should be proportioned such that is can be
1. Transported
2. Placed
3. Compacted easily without segregation
The workability is determined by the consistency of concrete and it is necessary for a correcting placing of the concrete.

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15
Q

Slump test

A
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16
Q

Calculation of concrete composition

A
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17
Q

Minimum cement content

A

Concrete must contain such cement as:
• that the required compressive strength is achieved
• in case of reinforced concrete adequate protection of the
reinforcement from corrosion is ensured.

18
Q

Describe Concrete admixtures

A
  • Concrete admixtures are powder-like materials or fluids which are added in small amounts (≤50 g(ml) /kg of cement). These values being valid for plain and for
  • reinforced concrete.
  • For prestressed concrete the value of ≤ 20 g/kg cement is allowed.
  • They change some properties of fresh concrete and/or physical effects.
  • The admixtures must not damage the concrete and must not increase the risk of corrosion of the reinforcement.
  • Admixtures are classified by their effects on the concrete.
19
Q

Name some important admixtures and their perfomance

A
  • Concrete workability agent: Reduction of water demand and/or improvement of the workability
  • Flow agent: Reduction of water demand and/or improvement of the workability, for the production of concrete with flowable consistency
  • Air entraining agent: By producing uniformly distributed, closed air voids, the freeze-thaw-resistance increases
  • Water proofing agent: Reduction of the capillary intake
  • Retarders: Delay of the setting time of a mix
  • Accelerators: Increases the rate of setting and hardening of the cement paste
  • Grouting agent: Improvement of the fluidity, reduction of water demand, reduction of the rate of setting or aiming at adequate swelling of the grouting mortar
  • Stabilizers: Prevention of water separation (Bleeding)
20
Q

Plasticizers. Which other names they have and why? Which is the main function?

A
  • Plasticizers, also called workability aids, increase the fluidity or workability
  • They are also known as water-reducers since they can produce a concrete with the same workability at a lower water/cement ratio,
  • The plasticizing action of plasticizers is due to the absorption of the molecules of the admixture onto the surface of the cement grains.
  • Plasticizers increases the concrete workability without increasing the w/c ration
21
Q

Schematic of dispersing action of plasticizing admixtures

A

Schematic of dispersing action of plasticizing admixtures:

a) flocculated particles
b) dispersed particles after admixture addition

The plasticizing action of plasticizers is due to the absorption of the molecules of the admixture onto the surface of the cement grains and early hydration products, each admixture molecule being orientated with an ionic group outwards. The cement particles then mutually repel each other, thereby breaking up any flocs

22
Q

Describe Air entraining agents

A
  • Organic materials which, when added to the mix water, entrain a controlled quantity of air in the form of microscopic bubbles (d ~ 0.1mm) in the cement paste component of the concrete
  • They are stable during the placing, compaction, setting and hardening of the concrete
  • Entrained air should not be confused with entrapped air which is normally present as the result of incomplete compaction of the concrete and is characterized by much larger bubble diameters
  • Entrained Air provided freeze-thaw resistance to the concrete
23
Q

Effect of air entrainment on the durability factor of concrete - a measure of the freezy-thaw resistance (diagram)

A

Entrained air volumes of only about 4 - 7 % by volume of the concrete are required to provide effective protection. A high factor indicates a high durability.

24
Q

Define accelartor and retarder.

A
  • Accelerator: is used to increase the hardening of the cement paste, thus enhancing the early strength.
  • Retarder: A retarder is used to decrease the setting time of the cement paste allowing a longer time of concrete workability.
25
Q

Typical effects (diagram) of calcium chloride admixture on

a) setting times and
b) early strength of concrete

A
  • Calcium chloride (CaCl2) has historically been very popular as it is readily available and very effective.
  • Figure a shows, that it accelerates both the initial and final set, and Figure b shows that a 2 % addition by weight of cement can result in very significant early strength increases.
  • This effect diminishes with time, and the long term strength is similar to that of non-accelerated concrete.
26
Q

Behavior of concrete after placing

A
  • Between the time of placing and the final set, a period that can last several hours, concrete is in a plastic, semi-fluid state
  • The constituent materials are still relatively free to move
  • The denser aggregate and cement particles tend to settle and the mix water has a tendency to migrate upwards
27
Q

Effects of movement on freshly placed concrete

A

a) Bleeding
b) Plastic settlement cracking
c) Plastic shrinkage cracking

28
Q

Describe bleeding and the problems.

A
  • Description: Process of upward migration or upward displacement of water. Reduction of the original concrete volume.
  • Problems of bleeding process:
    • Surface laitance: cement paste at or just below the top surface of the concrete becomes water rich and therefore hydrates to a weak structure
    • Local weakening: of the transition zone between the paste and the aggregate and hence an overall loss of concrete strength.
29
Q

Describe Plastic settlement

A

The overall settlement of the concrete will result in greater movement in the fresh concrete near the top surface of a deep pour. If there is any local restraint to this movement from, say, horizontal reinforcing bar, then plastic settlement cracking can occur, in which vertical cracks form along the line of the bars, penetrating from the surface to the bars.

30
Q

Describe Plastic shrinkage

A
  • Bleed water arriving at an unprotected concrete surface will be subject to evaporation; if the rate of evaporation is greater than the rate of arrival of water at the surface, then there will be a net reduction in water content of the surface concrete, and plastic shrinkage will occur.
  • The restraint of the mass of concrete will cause tensile strains to be set up in the near surface region, and as the concrete has near zero tensile strength, plastic shrinkage cracking may result.
  • Different cracking pattern than plastic settlement (more regural)
31
Q

Which are the Influences on the capillary porosity at time t?

A
  • w/c ratio: Nothing else influences the porosity as much as the initial water/cement ratio. If there is more water within the system the gaps between the cement grains are increasing
  • Hydration degree: As long as the hydration continues the reaction products are growing into the pores closing them or at least reducing their size
  • Air pore entrained: Organic admixtures can produce artificial pores
32
Q

Effect of water/cement ratio and degree of hydration on the connectivity of capillary pores in hydrated cement paste (diagram)

A
33
Q

Describe The volume ratio in paste matrix as a function of the w/c-value after complete hydration

A
34
Q

Describe Curing of concrete and its importance.

A
  • The strength development, the durability as well as the shrinkage and creep properties of concrete depend to a great extent on the curing of concrete.
  • Concrete has to be protected from destructive influences, such as extreme cold or heat, drying, strong rain or flowing water, strong vibration etc.
  • Curing should prevent evaporation and reduce shrinkage.
  • It is necessary precaution for all types of pores, independent of the initial w/c ratio.
  • Can be subdivided into internal and external methods.
35
Q

Describe internal and external curing of concrete.

A
  • External Internal
    • Water curing: supply of additional moisture and prevention of moisture loss (water ponding or spraying)
    • Sealed curing: prevention of moisture loss (plastic sheeting)
  • Internal
    • Water entrainment: water reservoirs (saturated light weight aggregates) release water during hydration
    • Sealing: prevention of moisture loss (water-soluble polymers)
36
Q

Effect of discontinuous curing on the strength development of concrete (assuming no shrinkage damage) - diagram

A

On rewetting, there is a rapid strength loss caused by the absorption of water, followed by a recovery such that, in the long term, provided shrinkage damage has not occurred, the strength approaches the value that would be obtained with continuous curing.

37
Q

Important facts of concrete:

a. What is the most important for structures?:
b. Which is the Highest influence on durability:
c. What is The porosity of capillary pores and how is it determined?

A

a. DURABILITY and COMPRESSIVE STRENGTH
b. POROSITY
c. porosity is the WATER-CEMENT RATIO and it is determined by: HYDRATION DEGREE

CURING

38
Q

Effects of Concreting in cold weather

A
  • At low temperature the hydration and the strength development will stop
  • If the water in fresh concrete freezes, the development of ice crystals (expansion!) will lead to a permanent defect in the structure and to a reduction of the strength
  • DIN 1045 specifies: At air temperatures between + 5°C and -3°C, the temperature of concrete during casting should not fall below +5°C (or +10°C if cement content ≤ 240 kg/m3)
  • At air temperatures below -3°C, the temperature of the concrete should be at least + 10°C. Afterwards the concrete should be kept at + 10°C for 3 days.
39
Q

Protective measures for concreting in cold weather

A
  • Use of fast hardening cement with a high hydration heat
  • Warming of aggregates and of mixing water (at the same time the temperature of fresh concrete should not exceed +30°C)
  • External heat insulation and/or heating of the compound unit.
  • steam curing -> t > 30°C -> acceleration of hardening reduction of strength
40
Q

Concreting in hot weather

A
  • Increased rate of water evaporation from the fresh mix.
  • The increased temperature of fresh concrete leads to an increase of the early strength but to a decrease of the final strength of concrete.
  • T should be smaller than about 30°C to 35°C. The actual temperature of the fresh concrete will be higher due to hydration of cement and sunshine.
  • To reduce damaging effects one should
    • use ice instead of water (ice must melt in the mixer)
    • cast in evening
    • use cement with low hydration heat
  • To reduce the rate of evaporation (prevent drying out) the concrete must be cured very carefully.
    • protect concrete from sun (to prevent cracking during cold nights)
    • seal concrete by plastic foils and keep the foils wet
41
Q
A
42
Q

Effects of curing temperature on compressive strength of concrete (diagram)

A

Building Materials (12 decks)

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