Fresh Concrete

Question 1

Concrete Composition

Answer 1

• Water
• Cement
• Aggregates

Often concrete consists of a multi phase composition:

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

Question 2

Aggregates

Answer 2

Definition: inert, granular and inorganic material

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

:

Question 3

Division of aggregate

Answer 3

• 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

Question 4

Specification and Qualities

Answer 4

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

Question 5

Deleterious Substances

Answer 5

• 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.

Question 6

Limitation of deleterious substances

Answer 6

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.

Question 7

Limitation of materials which disturb the hardening

Answer 7

• 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

Question 8

Grading requirements

Answer 8

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.

Question 9

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

Answer 9

Question 10

Grading requirements

Answer 10

• 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

Question 11

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

Answer 11

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

Question 12

Concrete additives

Answer 12

• 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.

Question 13

Concrete water

Answer 13

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).

Question 14

Manufacture of concrete

Answer 14

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.

Question 15

Slump test

Answer 15

Question 16

Calculation of concrete composition

Answer 16

Question 17

Minimum cement content

Answer 17

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.

Question 18

Describe Concrete admixtures

Answer 18

• 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.

Question 19

Name some important admixtures and their perfomance

Answer 19

• 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)

Question 20

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

Answer 20

• 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

Question 21

Schematic of dispersing action of plasticizing admixtures

Answer 21

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

Question 22

Describe Air entraining agents

Answer 22

• 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

Question 23

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

Answer 23

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.

Question 24

Define accelartor and retarder.

Answer 24

• 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.

Question 25

Typical effects (diagram) of calcium chloride admixture on a) setting times and b) early strength of concrete

Answer 25

* 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.

Question 26

Behavior of concrete after placing

Answer 26

* 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

Question 27

Effects of movement on freshly placed concrete

Answer 27

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

Question 28

Describe bleeding and the problems.

Answer 28

* 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.

Question 29

Describe Plastic settlement

Answer 29

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.

Question 30

Describe Plastic shrinkage

Answer 30

* 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)

Question 31

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

Answer 31

* 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

Question 32

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

Answer 32

Question 33

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

Answer 33

Question 34

Describe Curing of concrete and its importance.

Answer 34

* 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.

Question 35

Describe internal and external curing of concrete.

Answer 35

* 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)

Question 36

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

Answer 36

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.

Question 37

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?

Answer 37

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

Question 38

Effects of Concreting in cold weather

Answer 38

* 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.

Question 39

Protective measures for concreting in cold weather

Answer 39

* 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

Question 40

Concreting in hot weather

Answer 40

* 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

Question 42

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

Answer 42