WHAT ARE PROPERTIES OF FRESH CONCRETE?

Definition

Fresh concrete is defined as concrete at the state when its components are fully mixed but its strength has not yet developed. This period corresponds to the cement hydration stages. The properties of fresh concrete directly influence the handling, placing and consolidation, as well as the properties of hardened concrete.

Workability

a) Definition

Workability is a general term to describe the properties of fresh concrete.  Workability is often defined as the amount of mechanical work required for full compaction of the concrete without segregation.  This is a useful definition because the final strength of the concrete is largely influenced by the degree of compaction. A small increase in void content due to insufficient compaction could lead to a large decease in strength.  The primary characteristics of workability are consistency (or fluidity) and cohesiveness. Consistency is used to measure the ease of flow of fresh concrete. And cohesiveness is used to describe the ability of fresh concrete to hold all ingredients together without segregation and excessive bleeding.

 b) Factors affecting workability

Water content: Except for the absorption by particle surfaces, water must fill the spaces among particles. Additional water "lubricates" the particles by separating them with a water film. Increasing the amount of water will increase the fluidity and make concrete easy to be compacted. Indeed, the total water content is the most important parameter governing consistency. But, too much water reduces cohesiveness, leading to segregation and bleeding. With increasing water content, concrete strength is also reduced.

Aggregate mix proportion:  For a fixed w/c ratio, an increase in the aggregate/cement ratio will decrease the fluidity. (Note that less cement implies less water, as w/c is fixed.) Generally speaking, a higher fine aggregate/coarse aggregate ratio leads to a higher cohesiveness.

Maximum aggregate size: For a given w/c ratio, as the maximum size of aggregate increases, the fluidity increases. This is generally due to the overall reduction in surface area of the aggregates.

Aggregate properties: The shape and texture of aggregate particles can also affect the workability.       As a general rule, the more nearly spherical and smoother the particles, the more workable the concrete.

Cement: Increased fineness will reduce fluidity at a given w/c ratio, but increase cohesiveness. Under the same w/c ratio, the higher the cement content, the better the workability (as the total water content increases).

Admixtures: Air entraining agent and superplasticizers can improve the workability.

Temperature and time: As temperature increases, the workability decreases. Also, workability decreases with time. These effects are related to the progression of chemical reaction.

c) Segregation and bleeding

Segregation (separation): Segregation means separation of the components of fresh concrete, resulting in a non-uniform mix. More specifically, this implies some separation of the coarse aggregate from mortar.

Bleeding (water concentration): Bleeding means the concentration of water at certain portions of the concrete. The locations with increased water concentration are concrete surface, bottom of large aggregate and bottom of reinforcing steel. Bleed water trapped under aggregates or steel lead to the formation of weak and porous zones, within which micro cracks can easily form and propagate.

Measurement of workability

 a) Slump test (BS 1881: 102, ASTM C143):  Three different kinds of possible slumps exist, true slump, shear slump, and collapse slump. Conventionally, when shear or collapse slump occur, the test is considered invalid. However, due to recent development of self-compact concrete, the term of collapse slump has to be used with caution.

 b) Compaction factor test (BS 1881: Part 103):  The compacting test was developed in Great Britain in 1947.The upper hopper is completely filled with concrete, which is then successively dropped into the lower hopper and then into the cylindrical mould. The excess of concrete is struck off, and the compacting factor is defined as the weight ratio of the concrete in the cylinder, MP, to the same concrete fully compacted in the cylinder (filled in four layers and tamped or vibrated), mf (i.e., compacting factor = MP/mf). For the normal range of concrete, the compacting factor lies between 0.8 to 0.92 (values less than 0.7 or higher than 0.98 is regarded as unsuitable). This test is good for very dry mixes.  Three limitations: (i) not suitable for field application; (ii) not consistent; (iii) Mixes can stick to the sides of the hoppers.

c) Vebe test (BS 1881: Part 104):  The Vebe consistometer was developed in1940 and is probably the most suitable test for determining differences in consistency of very dry mixes. This test method is widely used in Europe and is described in BS 1881: Part 104. It is, however, only applicable to concrete with a maximum size of aggregate of less than 40 mm. For the test, a standard cone is cast. The mould is removed, and a transparent disk is placed on the top of the cone. Then it is vibrated at a controlled frequency and amplitude until the lower surface of the disk is completely covered with grout. The time in seconds for this to occur is the Vebe time. The test is probably most suitable for concrete with Vebe times of 5 to 30s. The only difficulty is that mortar may not wet the disc in a uniform manner, and it may be difficult to pick out the end point of the test.

d) Ball-penetration test

 A measure of consistency may also be determined by ball penetration (ASTM C360). Essentially, this test consists of placing a 30-lb metal cylindrical weight, 6" in diameter and 4-5/8" in height, having a hemi-spherically shaped bottom, on the smooth surface of fresh concrete and determining the depth to which it will sink when released slowly. During penetration the handle attached to the weight slides freely through a hole in the center of the stirrup which rests on large bearing areas set far enough away from the ball to avoid disturbance when penetration occurs. The depth of penetration is obtained from the scale reading penetration of the handle, using the top edge of the independent stirrup as the line of reference. Penetration is measured to the nearest 1/4", and each reported value should be the average of at least three penetration tests. The depth of concrete to be tested should not be less than 8". This test is quickly made and is less prone to personal errors.  The ratio of slump to penetration is usually between 1.3 and 2.0.

Setting of concrete

 a) Definition: Setting is defined as the onset of rigidity in fresh concrete. It is different from hardening, which describes the development of useful and measurable strength. Setting precedes hardening although both are controlled by the continuing hydration of the cement.

 b) Abnormal setting

 False setting: If concrete stiffens rapidly in a short time right after mixing but restores its fluidity by remixing, and then set normally, the phenomenon is called false setting. The main reason causing the false setting is crystallization of gypsum. In the process of cement production, gypsum is added into blinker through inter-grinding. During grinding, the temperature can rise to about 120^oC, thus causing the following reaction:

CSH₂ →CSH_1/2

The CSH_1/2 is called plaster. During mixing, when water is added, the plaster will re-hydrate to gypsum and form a crystalline matrix that provides ‘stiffness’ to the mix. However, due to the small amount of plaster in the mix, very little strength will actually develop. Fluidity can be easily restored by further mixing to break up the matrix structure.

Flash setting: Flash setting is caused by the formation of large quantities of monosulfoaluminate or other calcium aluminate hydrates due to quick reactivities of C₃A. This is a rapid set with the development of strength and thus is more severe than false setting. However, as we mentioned before, flash setting can be eliminated by the addition of3-5% gypsum into cement.  Thixotropic set is due to the presence of abnormally high surface charges on the cement particles. It can be taken care of by additional mixing. As the hydration reaction progresses with time, the concrete becomes less flowable, and the slump value will naturally decrease. However, if the slump value decreases at an abnormally fast rate, the phenomenon is called “slump loss”. It is often due to the use of abnormal setting cement, the unusually long time taken in the mixing and placing operations, or the high temperature of the mix (e.g., when concrete is placed under hot weather, or when ingredients have been stored under high temperature). In the last case, ice chips can be used to replace part of water to lower the temperature.

Placing, Compacting and Curing

Concrete should be placed as close to its final position as possible. To minimize segregation, it should not be moved over too long a distance. After concrete is placed in the formwork, it has to be compacted to remove entrapped air. Compaction can be carried out by hand rodding or tamping, or by the use of mechanical vibrators.  For concrete to develop strength, the chemical reactions need to proceed continuously. Curing refers to procedures for the maintaining of a proper environment for the hydration reactions to proceed. It is therefore very important for the production of strong, durable and watertight concrete. In concrete curing, the critical thing is to provide sufficient water to the concrete, so the chemical reaction will not stop. Moist curing is provided by water spraying, ponding or covering the concrete surface with wet sand, plastic sheets, burlaps or mats. Curing compounds, which can be sprayed onto the concrete surface to form a thin continuous sheet, are also commonly used. Loss of water to the surrounding should be minimized. If concrete is cast on soil subgrade, the subgrade should be wetted to prevent water absorption. In exposed areas (such as a slope), windbreaks and sunshades are often built to reduce water evaporation. For Portland cement concrete, a minimum period of 7 days of moist curing is generally recommended. Under normal curing (at room temperature), it takes one week for concrete to reach about 70% of its long-term strength. Strength development can be accelerated with a higher curing temperature. In the fabrication of pre-cast concrete components, steam curing is often employed, and the 7-day strength under normal curing can be achieved in one day. The mold can then be re-used, leading to more rapid turnover. If curing is carried out at a higher temperature, the hydration products form faster, but they do not form as uniformly. As a result, the long-term strength is reduced. This is something we need to worry about when we are casting under hot weather. The concrete may need to be cooled down by the use of chilled water or crushed ice. In large concrete structures, cooling of the interior (e.g., by circulation of water in embedded pipes) is important, not only to prevent the reduction of concrete strength, but also to avoid thermal cracking as a result of non-uniform heating/cooling of the structure.  After concrete is cast, if surface water evaporation is not prevented, plastic shrinkage may occur. It is the reduction of concrete volume due to the loss of water. It occurs if the rate of water loss (due to evaporation) exceeds the rate of bleeding. As concrete is still at the plastic state (not completely stiffened), a small amount of volume reduction is still possible, and this is accompanied by the downward movement of material. If this downward movement is restraint, by steel reinforcements or large aggregates, cracks will form as long as the low concrete strength is exceeded. Plastic shrinkage cracks often run perpendicular to the concrete surface, above the steel reinforcements. Their presence can affect the durability of the structure, as they allow corrosive agents to reach the steel easily. If care is taken to cover the concrete surface and reduce other water loss (such as absorption by formwork or subgrade), plastic shrinkage cracking can be avoided. If noticed at an early stage, they can be removed by re-vibration