FAQS

Yes. It should be checked whether this leads to:

• segregation,
• bleeding,
• loss of cohesion,
• exceeding of permissible w/c ratio,
• reduction in strength,
• other important properties of concrete.

Important affects of increasing workability by adding water are:

In fresh concrete,

• segregation,
• bleeding,
• loss of cohesion,
• exceeding of permissible w/c ratio,
• tendency to develop shrinkage cracks

In hardened concrete

• Possible reduction in strength properties
• Increased permeability to aggressive agents such as moisture, oxygen, CO2, chlorides etc.
• Increased capillary porosity
• More time required to develop discontinuity of capillary pore.

Lyse’s rule on water content of cement concrete mix is:
In the range of usual concrete mixtures, produced with a given set of constituents, the water content essentially controls the consistency.

[Lyse, I. (1932). Tests on consistency and strength of concrete having constant water content. Proceed, of the ASTM, 32, Part ii, pp 629-636. Powers, T. C. (1968). Properties of fresh concrete. John Wiley & Sons. New York]

Plasticiser is generally a surfactant which generally reduces the water content of cement concrete mix for a given workability.

Superplasticiser (SP) is also a plasticiser which is generally a surfactant and which generally reduces the water content of cement concrete mix for a given workability. But, SP reduces the water requirement of concrete much more than that by plasticiser; the level of water decrease achieved by SP can not be achieved by even excessive dosages of plasticiser.

Major difference between SP and plasticiser (P) are:

• SP is more efficient in dispersing cement in water than P.
• Water content reduction by SP is much higher than P.
• Relative excessive dosage of P also cannot match the performance of SP as it would lead to severe adverse effects on properties of fresh concrete.
• Over dosage of P generally leads to bleeding and segregation.

Generally yes. However, if the original mix is deficient in fines, depending upon the SP dosage and compatibility of SP with cement, bleeding may occur, and redesigning of the mix may be necessary.

A well designed concrete mix can have a satisfactory setting time. However, incompatible pair of cement and SP, besides excessive SP dosage can have enhanced setting time in concrete.

The degree of increase of workability by SP depends upon several factors, such as

• type of cement
• type and dosage
• level of SP,
• workability of concrete before addition SP,
• concrete mixing technique,
• ambient temperature etc

However, it can be said that flowing concrete, including that having self levelling property (self compacting concrete) is possible with proper choice of SP and ingredients of concrete.

It would be difficult to achieve very high workability at reasonable SP dosage, in case, the concrete mix is too stiff before addition of SP.

In order to avoid segregation possibility of very highly workable concrete (such as pumped concrete, self compacting concrete, etc), sometimes, a viscosity modifying agent (VMA) is added which imparts cohesiveness to the mix.

Not necessarily always. A redesigning of mix proportions may be required when SP dosage is to be increased to achieve desired level of workability.

SP enhances workability by following actions/mechanisms:

• Like electric charges are created on the cement particles so that they repel each other leading to proper dispersion of cement in the concrete mix (surface active nature of SP).
• Surface tension (and viscosity) of water is reduced. This enables stabilisation of very small air bubbles within mass of the concrete. As these entrained air acts as frictionless ball bearings, workability of concrete is increased.
• Setting of cement particles is reduced when SP forms a coating, on the cement particles thereby causing less water to come in contact with cement particles. This enhances the time period over which concrete is workable and mouldable.

Surface tension of water is reduced by detergent like action (surfactant property) of SP and this makes water more mobile in the concrete mix.

The following effects of SP on cement paste may be observed:

• SP creates electric charges on the cement particles and thereby cement particles repel each other. Agglomerates of cement powder/paste are broken. Cement gets more dispersed in the mix.
• Chemical reactivity of cement is reduced as SP is adsorbed by cement and a form of coating around cement particles is generated which prevents easy access of water to the cement.

Any SP designed for Portland Cement (P-C) can be utilised for PPC and PSC also, but, dosage level may differ in each of the type of cements.

Any SP designed for Portland Cement (P-C) can be utilised advantageously in   high volume fly ash concretes.

Any SP designed for Portland Cement (P-C) can be utilised advantageously in concretes containing silica fume. It can be sated here that for obtaining maximum contribution from silica fume (also called as microsilica or condensed silica fume) to various performances of the concrete, addition of SP is almost a necessity, since SPs help to break the agglomerates of particles of commercially available silica fume into individual micro-sized particles

• SP dosage is measured by weight of cement % SP dosage = 100*(Weight of SP)/ (Weight of cement)= Weight of SP per 100 kg of cement
• When Pozzolanas such as fly ash, silica fume are added to cement, then, % SP dosage = 100* (Weight of SP)/(Weight of cement+weight of Pozzolana)

Usually, for given workability level, SP dosage increases with decrease in w/c ratio.

Lower workability of a concrete with a very low w/c ratio cannot be always made up increased SP dosage.

A minimum level of workability or wetness of concrete is required for SP to enhance the workability.  It may be too difficult to increase the workability a very stiff mix made with a very low w/c ratio. This minimum workability level depends upon the compatibility between cement and SP besides ambient conditions; it may vary from slump of 50 mm to 100 mm.

The side effects of over-dosage are one or more of the following:

• Bleeding
• Excessive retardation
• Delay in setting (increase in mould release time)
• Very low strength and/or reduced strengths
• Non-cohesive mix
• Segregation

Though both over-dosage and under-dosage of SP are harmful, under-dosage can sometimes tolerable provided the reduced workability is still sufficient for compaction.

The solid content of SP does affect the dosage in cement concrete mix; it actually determines the contribution of SP to workability. Higher solid content of a liquid SP means lower dosage required and vice versa.

SP is incorporated in cement concrete mix in many ways:

• Adding into the mixer machine after all the otheringredients are mixed in machine,
• SP mixed with a portion of mix water and this mixture is poured into the mixer machine after the other ingredients are mixed,
• Adding SP just before unloading the concrete mix on the site.

However in all cases, sufficient mixing time must be allowed for proper distribution of SP into the concrete mass.

While concrete mixing, SP is added either with mixing water or separately after a lapse of some time after adding water to the mix. The latter method allows more efficient use of SP since it avoids adsorption of SP by fast setting C3A portion of cement.

C3A component of chemical constituent of cement affects considerably the efficiency of SP.

• The re-dosaging of SP if properly planned, may allow concrete to regain the workability lost without affecting the other properties such as strength or rate of strength gain.
• Improperly planned re-dosage or inadequate mixingafter re-dosaging would affect adversely properties of concrete both in fresh and hardened stages.

• Marsh Cone Flow Time tests are useful to study various parameters affecting the effectiveness of SP to impart workability/flowability in a given cement.
• Flow-table method of ASTM C230
• Mini-slump cone test developed by Kantro

• Marsh Cone Flow Time tests are useful to select a SP for given cement. The SP with minimum dosage to achieve required flow time is the most suitable SP for the given SP. Another additional parameter may be to select a SP which requires the minimum dosage to be used to maintain the required flow time after the desired period.

• Marsh Cone is conical funnel made of either stainless steel or glass having top diameter of 150 mm reducing to about 8mm at the bottom over a vertical length of about 75 mm. It is preferable to have a cylindrical stem length of about 25 mm at the bottom.

• About 5 kg of cement is usually used for mixing of paste for test in Marsh Cone Flow test.

• 200 ml or 400 ml of fresh paste is allowed to flow in Marsh Cone flow test.

• Time required in seconds to flow 200 ml or 400 ml of fresh paste is measured in Marsh Cone flow test.

• A cement can be considered to be compatible with SP when following conditions are satisfied:

  • Not excessive SP dosage is required for paste to achieve required flow time in MCFT. Not excessive SP dosage is required for concrete mix to achieve required workability.
  • SP dosage is required to achieve required flow time in MCFT does not adversely affect the setting or strength gain properties of paste or concrete
  • MCFT time of paste is maintained over desired working period. Required workability level is maintained over the desired working period.

• Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates, water, admixtures and cement. The aggregates are sand and gravel or crushed stone.

  Chemical admixtures, are a critical part of the formulation of concrete. Admixtures can alter the chemical reaction of cement and water.

  Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.

• Admixtures enhance the properties of concrete or mortar in the plastic state and improve durability in the hardened state. Admixtures increase the efficiency of cementitious materials and/or improve the economy of the concrete mix.

• Admixtures are defined in following categories:

  • Air Entraining Agents
  • Water Reducers
  • Mid-Range Water Reducers
  • Superplasticizers or High-Range Water Reducers
  • Corrosion Inhibitors
  • Set Retarders
  • Set Accelerators
  • Chloride
  • Non-Chloride

  Many admixtures are defined by ASTM C-494.

• An air-entraining agent is a liquid chemical admixture used to create bubbles that will provide adequate freeze-thaw protection in concrete by generating 5 to 7 percent of the volume with air of properly sized and properly spaced bubbles. Air-entraining admixtures are surfactants, soaps or detergents, primarily used to generate microscopic size bubbles that render hardened concrete resistant to the effects of freezing and thawing. It is also used to improve the workability/place ability of fresh concrete

• Superplasticizers are liquid chemical admixtures used in concrete because of its ability to perform as a highly effective wetting agent. Superplasticizers increase water reduction to achieve low water/cement ratios, increase strength gain, increases slump for ease of placement and creates a denser and more durable concrete.

• Slump, air content, unit weight and compressive strength tests are the most common tests.

  Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can’t flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won’t stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.

  Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing.

  Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.

  Unit weight measures the weight of a known volume of fresh concrete.

  Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500 psi below the design strength and the average of three consecutive tests equals or exceeds the design strength then the concrete is acceptable. If the strength tests don’t meet these criteria, steps must be taken to raise the average.

• Ingredients of cement concrete mix which contribute most to workability are : cement and water.

• Yes. It should be checked whether this leads to:

  • segregation,
  • bleeding,
  • loss of cohesion,
  • exceeding of permissible w/c ratio,
  • reduction in strength,
  • other important properties of concrete

• Types of SP are :

  (a) Based on effect of setting of cement paste

  • Normal setting
  • Retarding type
  • Accelerating type

  (b) Based on ASTM classification

  • ASTM Type F superplasticisers (Added to keep concrete flowable for a short period of time; the concrete may lose slump quickly)
  • ASTM Type G superplasticisers(Delays setting, but concrete remains flowable for a longer period which could delay finishing)

  (c) Based on chemical composition

  • SNFC
  • SMFC
  • Modified Lignosulphonates Carboxylic acid based formulations (Polycarboxylate ethers, PCE)
  • Various other chemicals

  (d) Based on surfactant nature of SP

  • Anionic
  • Cationic
  • Non-ionic

• Adsorbed SP molecules convey a net electric charge to the cement particle surfaces and this charge is measured as zeta potential. Like charges on particle surfaces induce repulsion between neighbouring
  cement particles and thus contributing to increased dispersion.

Higher the molecular weight, generally, higher is theefficiency of SP

• The general range of molecular weight of SP is 8 to 20 depending upon the type of SP