Fresh Concrete

Materials of Construction

Dr. TALEB M. AL-ROUSAN

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

            

          

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).

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).

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.

Compaction Factor Apparatus

           

            

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.

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).

Vebe Test

BS 1881: Part 104: 1983 See Fig. 5.4 in text for Apparatus diagram. Standard slump cone is placed in a cylinder (D=9.5 in ,

H=8 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).

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).

Vebe Apparatus

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.

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.

Flow Table

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 > (8 in) and

least lateral dimension > (18 in).

Kelly Ball

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.

General Pattern of Relations between Workability Tests

Vebe time -

sec

Compaction factor

Compaction factor

Vebe time -

sec

Slump -mm

Slump -mm

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

Density of Fresh Concrete (

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) / ] Also the cement content (mass of cement per

unit volume of concrete ) is C/ V = [ (Af + Ac + W) / V ]

Volume of Air Voids

Volume of air voids in concrete (Va)Va = Vconc – (Vc + Vw + Vagg)