Fresh Concrete

1
Fresh Concrete

• Materials of Construction
• Dr. TALEB M. AL-ROUSAN

2
Introduction

• Long term properties of hardened concrete
  (strength, volume, stability, and durability) are
  seriously affected by its degree of compaction.
• Consistency or workability of fresh concrete
  should be such that concrete can be properly
  compacted, transported, placed, and finished
  easily and without segregation.

3
Concrete Composition

• Cement Paste (25- 40 of concrete volume)
• Cement (25- 50 of cement paste)
• Water
• Aggregates (60-75 of concrete volume)
• Fine (30 45 of aggregate volume)
• Coarse
• Air (2 - 8 of concrete volume)
• Mineral Admixtures
• Liquid Admixtures

4
Workability

• Represents the ability of concrete to be mixed,
  handled, transported, and placed with a minimum
  loss of homogeneity.
• Workability is expressed in terms of consistency,
  mobility, and compactibility.

5
Workability

• Workability The amount of useful internal work
  necessary to produce full compaction.
• Internal work is the energy required to overcome
  the internal friction between the individual
  particles in the concrete.
• In practice, additional energy is required to
  overcome surface friction between concrete and
  form work or the reinforcement.
• Consistence the ease of which material will
  flow.
• Consistence (in concrete) means degree of
  wetness.
• Consistency (general) degree of fluidity.

6
Workability Cont.

• To obtain good strength maximum possible density
  is vital to achieve.
• Compressive strength increase with increasing
  density (i.e. reducing air voids).
• 5 air voids can lower strength by 30.
• Voids in hardened concrete
• Bubbles of entrapped air (its volume depends on
  grading of fine aggregates and degree of
  wetting).
• Spaces left after excess water has been removed
  (its volume depends on w/c).
• Thus for any method of compaction there is an
  optimum water content of the mix at which the sum
  of volume of voids will be a minimum, and the
  density will be maximum.

7
Factors Affecting Workability

• Water content (increase workability since acting
  as lubricant).
• Aggregate type and grading
• Finer particles require more water to wet their
  larger specific surface.
• Irregular shape rougher texture of angular
  aggregates demand more water than rounded
  aggregates.
• Porosity absorption.
• Light weight aggregates lower workability.
• High ratio of volumes of coarse aggregates to
  fine aggregate can result in segregation and in a
  lower workability (harsh mix).
• Too many fines lead to higher workability

8
Factors Affecting Workability Cont.

• Aggregate/ Cement ratio
• for constant w/c , workability increase as the
  aggregate cement ratio is reduced because the
  amount of water relative to the total surface of
  solids is increased.
• Presence of admixtures air entrainment reduce
  water requirement.
• Fineness of cement the finer the cement the
  greater the water demand.
• Time
• Fresh concrete stiffens with time (different than
  setting).
• Stiffening of concrete is measured by loss of
  workability with time (Slump Loss).
• Temperature
• High temperature reduce workability and increases
  slump loss.

9
Cohesion Segregation

• Concrete should not segregate (i.e. ought to be
  cohesive).
• Segregation Separation of the constituents of a
  heterogeneous mixture so that their distribution
  is no longer uniform.
• Primary cause Difference in size of particles
  and some times different in S.G of mix
  constituents method of handling and placing.
• Controlled by Suitable grading and care in
  handling, transporting, and placing, use of air
  entrainment.
• Forms
• Coarser particles tends to separate out (happens
  in dry mixes).
• Separation of grout (occurs in wet mixes) from
  the mix.
• Improper use of vibrators (Over-vibration) cause
  segregation.

10
Bleeding

• Also known as water gain.
• Form of segregation in which some of the water in
  the mix tends to rise to the surface of freshly
  placed concrete.
• Cause Inability of solid constituents in the mix
  to hold all the water when they settle downwards.
• Expressed quantitatively as the total settlement
  (reduction in height) per unit height of
  concrete.
• As a result of bleeding, the top of every layer
  of concrete placed become too wet, and if water
  is trapped, a porous and weak layer of
  non-durable concrete will result.

11
Bleeding Cont.

• Bleeding on the top surface cause a weak wearing
  surface. This can be avoided by
• Delaying finishing untill bleeding water
  evaporates
• Using wood floats
• Avoiding overworking the surface
• If evaporation of water from the surface of
  concrete is faster than the bleeding rate,
  plastic shrinkage cracking may result.
• Some of bleeding water become trapped under
  aggregate particles or reinforcement, thus
  creating zones of poor bond. This water leaves
  behind voids which are oriented in same
  direction which may increase permeability of
  concrete.
• Bleeding may increase frost damage.

12
Bleeding Cont.

• Bleeding is not always harmful, when water
  evaporate the water cement ratio decrease which
  result in strength increase.
• Tendency of bleeding depends largely on cement
  properties (bleeding is lower with finer cement,
  high alkali, high C3A, or when NaCl is added).
• Lower bleeding can be achieved with rich mixes,
  addition of Pozzolan, and air entrainment.

13
Workability Tests

• There is no direct test that measures workability
  as defined.
• Slump test.
• Compaction factor test.
• Vebe test.
• Flow table test.
• Ball penetration test.

14
Slump Test / ASTM 143- 90a

• Mold is a frustum of a cone (12 in high, 8 in D
  base, and 4 in D opening).
• Mold placed on a level smooth surface with small
  opening up.
• The inside of the mold and base are moistened
  with water to reduce influence of the variation
  of surface friction.
• Mold is filled with concrete in three layers.
• Each layer is tamped 25 times with a steel rod
  (5/8 in D).
• Top surface is struck off by screeding.

15
Slump Test Cont.

• Mold must be firmly held against its base during
  the entire operation (facilitated by handles and
  foot-rests).
• Clean the area around the base of the mold from
  concrete which may be dropped during filling.
• Immediately after filling, the cone is slowly
  lifted.
• Unsupported concrete will slump.
• The decrease in the height of the center of the
  slumped concrete is called slump and is measured
  to the nearest 1/4 in.

16
Slump Test Cont.

• True slump concrete slump evenly all around (See
  Figure 5.2 a) (0 125 mm).
• Shear Slump One half of the cone slides down an
  inclined plane (Test should be repeated). It
  indicates lack of cohesion of the mix. ( up to
  150 mm)
• Collapse Slump (Test should be repeated).(150
  250 mm)
• Advantages check variation in materials being
  fed into the mixer for a daily or hourly bases.
• For example increase in slump may indicate
  increase in water or change in grading of
  aggregates such as deficiency in sand.
• Very simple test which made it widely spread.

17
The Slump Test - ASTM C 143 Measures Consistency
(b) Normal slump or collapse slump (harsh or
extremely wet mix) (c) Shear Slump Concrete
lacks plasticity and cohesion
18
                      
19
Notes on the Slump Test

• Recommended slump values depend on types of
  construction.
• Can be used to test for the uniformity of the
  concrete delivered to the project.
• Influenced by changes in ambient temperature.
• Values lower than requirements can be used if the
  concrete can be placed within the forms.
• Workability degrees very low (0 25mm) low (25
  50mm) medium (50 100mm) high (100 175mm).

20
Compaction Factor

• Best test available that uses an inverse approach
  to the workability definition (the amount of work
  necessary to produce full compaction).
• Compaction factor Approach determining the
  degree of compaction achieved by a standard
  amount of work.
• Measured by density ratio Ratio by the density
  actually achieved in the test to the density of
  the same concrete fully compacted.
• Tests developed in UK (BS).

21
Compaction factor Cont.

• Apparatus (See Fig 5.3 in Text)
• Two hoppers in a shape of a frustum of a cone
  with hinged doors at the bottom (one above the
  other with the lower being smaller).
• One cylinder.
• The upper hopper is filled with concrete gently
  with no compaction.
• The bottom door of the upper hopper is opened and
  the concrete falls into the lower hopper.
• Since the lower hopper is smaller therefore it
  will be filled to overflowing.
• The bottom door of the lower hopper is released
  and the concrete falls into the cylinder.
• Excess concrete is cut.

22
Compaction Factor Apparatus
                       
23
Compaction factor Cont.

• Net mass of the concrete in the known volume of
  the cylinder is determined.
• Calculate the density of the concrete in the
  cylinder.
• Compaction Factor Ratio of the density of
  concrete in the cylinder to the density of the
  fully compacted concrete.
• Fully compacted concrete density is found by
  filling the cylinder with concrete at four (4)
  layers each tamped or vibrated. Density is found
  by dividing concrete mass in the cylinder to its
  known volume.

24
Compaction factor Cont.

• High compaction factor indicate higher
  workability that is equivalent to high slump
  values.
• More sensitive than slump at low workability.
• Not very appropriate for dry mixes.
• Due to its large size the compaction factor
  apparatus is not convenient at site.
• Workability degrees very low (0.75) low (0.85)
  medium (0.92) high (0.95).

25
Vebe Test

• BS 1881 Part 104 1983
• See Fig. 5.4 in text for Apparatus diagram.
• Standard slump cone is placed in a cylinder
  (D9.5 in , H8 in).
• Slump cone is filled in a standard manner.
• Cone removed
• A disc-shape rider (2.75 kg) is placed on top of
  the concrete.
• Compaction is achieved using vibrating table.
• Compaction is complete when the transparent rider
  is covered with concrete and all cavities on the
  surface are disappeared (judged visually).

26
Vebe Test

• The time required for complete compaction is
  known as Vebe seconds and its considered as
  measure of workability.
• Dry concrete or low workability concrete required
  more time.
• Vebe is good workability specially for very dry
  mixes.
• The method has the advantage of being similar to
  the work done in site (i.e. treatment of concrete
  during test is closely related to method of
  placing in practice).

27
Vebe Apparatus
28
Flow Table

• BS 1881 Part 105 1984
• Apparatus shown in (Fig. 5.5 in Text).
• Wooden board covered by a steel plate with total
  mass (16 kg).
• This board is hinged along one side to a base
  board. Each board being square with (700 mm)
  side.
• The upper board can be lifted up to a stop so
  that the free edge rises (40mm).
• The table top is moistened and a frustum of a
  cone of concrete is placed using mold with (8 in
  high 8 in bottom diameter 5 in top diameter)
  that is lightly tamped with wooden tamper at two
  layers.
• Remove excess concrete before lifting the mold.
• Remove mold after 30 sec.

29
Flow Table Cont.

• The table top is lifted and allowed to drop for
  15 times for the specified distance (40 mm). Each
  cycle should take 4 sec.
• In consequence, the concrete spread.
• Measure the max. spread parallel to the two edge.
• The average of the two values represent the
  flow.
• A value of 400 mm indicates Medium workability
  concrete.
• A value of 500 mm indicates High workability
  concrete.
• Concrete should appear uniform and cohesive
  otherwise it is considered inappropriate.

30
Flow Table
31
Ball Penetration Test

• ASTM C360 -92
• Simple field test
• Determines the depth to which a (6 in) diameter
  metal hemisphere (30 Lb) will sink under its own
  weight into fresh concrete.
• Known also as Kelly Ball (See fig. 5.6).
• As slump test, Kelly ball test is used for
  routine checking.
• Simpler than slump test and quicker to perform.
• Can be applied to concrete in wheelbarrow or in
  forms.
• Depth of concrete being tested should gt (8 in)
  and least lateral dimension gt (18 in).

32
Kelly Ball
33
Comparison of Tests

• There is no unique relation between the results
  of the various tests.
• Fig. 5.8 for pattern of relation between
  workability tests.

34
General Pattern of Relations between Workability
Tests
35
Density (Unit mass or Unit Weight in Air) of
fresh Concrete

• ASTM C138 -92
• Density is obtained by weighing the compacted
  fresh concrete (by rodding or vibrating) in a
  standard container of known volume and mass.
• If the density is know, it becomes easy to find
  the volume of concrete using the mass of the
  ingredients.
• When the ingredients are expressed as quantities
  in one batch, then we can calculate the yield of
  concrete per batch.
• Yield volume of concrete in a given batch

36
Density of Fresh Concrete (r)

• Let mass per batch of
• W Water mass per batch
• C Cement mass per batch
• Af Fine aggregate mass per batch
• Ac Coarse aggregate mass per batch
• Then the volume of compacted concrete obtained
  from one batch (yield) is
• V (C Af Ac W) / r
• Also the cement content (mass of cement per unit
  volume of concrete ) is
• C/ V r - (Af Ac W) / V

37
Volume of Air Voids

• Volume of air voids in concrete (Va)
• Va Vconc (Vc Vw Vagg)