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Important Parameters in Concrete Mix Design that govern the Performance and Durability of Structures built with Concrete

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making concrete
H.G. Sreenath, B.Sc (Engg.), M.S., MICI, MISFRC
Technical Director, M/s Chennai Civil-Tech
Research Foundation Pvt. Ltd.
Formerly, Deputy Director, Structural Engineering Research Centre, Taramani

Introduction:

The Versatility of making concrete with locallyavailable materials, ease in moulding it into any shape and size, and achieving economy in its making and use in the structures has made the concrete the 2nd largest material used on earth!.

Concrete has undergone rapid and phenomenal development in the past few years in India. Revision of IS 456 : 2000 with emphasis onthe quality assurance, and suggesting the use of Mineral and Chemical Admixtures to achieverequired compressive strength, and modification in micro-structure of the concrete in order to enhance the required durability against corrosion of embedded steel reinforcements in marine and chemical environment, includingperformance against accidental fires in r.c.c structures.

In order to produce the designed concrete mixin large quantities,(as per the mix design done including all the required parameters determined from the tests), necessitatedestablishmentsforthe setting-up of a large number of Ready Mixed Concrete(RMC) plants. The RMC’snormally produce uniform good quality concrete (if proper quality control is exercised by tests on constituent materials of concrete) to achieve durability of concrete structures. This is important in structures of national importance such as, rail/road bridges, flyovers, roads, and airport runways etc., where huge money is spent. Normally, the RMC plants are established in the suburb of metros in particular, for want of open space, and to avoid traffic congestions. The concrete mixwith conventional materials, introduction of new alternative materials, use of mineral and chemical admixtures,its production, developments in instrumentation, testing equipment have contributed to this growth and transformation.The government’s thrust on infrastructure development at faster rate, have also influenced the concrete technology development, where newer materials, alternate materials to accommodate the demanding requirements without sacrificing economy are being proposed.

On the other hand, globalisation of the Indian economy paved the way for easy availability of micro-silica and latest super-plasticizers for applicationin the country. In addition to using Moderate and High Strength concrete, M60 and higher grades of concrete, including High Performance Concrete are now becoming popular in the country with its proven utility in the construction of important structures.

Research and development on new materials to replace the conventional concrete in order to achieve the requisite properties for application in different situations is well understood.The latest development in concrete is use of “Geo-polymer concretes”,where a new binder that replaces cement in concrete mix with a Geo-polymer combination (Resin and hardener) as binder, is becoming popular.

Concrete Mix in general

Basic Concrete mixture is an intimate mixof Cement, Sand (Fine Aggregate), Coarse Aggregate, and Water in a certain proportion to obtain a semi-solid cohesive mass that can be moulded to any shape, and upon curing and hardening processpossessesphysical properties for its use in the Structures. New Generation of Concretes, need the useof Special Materials in addition to the above i.e. “ADMIXTURES”. Admixtures may be Mineral or Chemical.

The introduction of concrete chemicals in the construction industry have opened new areas in the design of concrete mixes that can be used to the advantage of achieving the required parameters in a better way to achieve economical mixes to suit the present day construction methods, without sacrificing the long time performance (Durability) of the structure.

 

 

Requirements of Good Concrete

It is expected that the good concrete mixture designed should as economical as possible,could be mixed, transported and compacted as efficiently as possible to obtain compact, dense and hardened mass,and resist the environmental threats such as, marine, chemical environment including accidental fires. Further, confirm to the strength requirement as Measured by compressive strength from design requirements, and fulfill the durability requirements to resist theenvironment in which the structure isexpected to serve.

Where big construction companies are involved, concreting is done mostly either through a batching plant commissioned at the site itself or RMC. With the adoption of IS 456: 2000 (Reaff.2015), the construction agencies have now been left with no choice but to adopt designed concrete mixes. The adoption has implied a better understanding of the role of aggregate shape (Coarse and Fine),water content in fine aggregates, grading of aggregates and above all water-cement(w/c) ratio that controls the strength and voids, even forM20-M40 grades of concretes by the average user. This in turn has improved the quality of the concrete in general.

When most contractors / engineers think about concrete mix design, the first thing that comes to their mind is “number of cement bags”. In the olden days, when most concrete operations werecarried out on site, cement was purchased in bags. The mix could be completely inferior for one’s application, and could even be inferior concrete. The main factors,hence, to be addressed in concrete application are;What slump?, What strength?,Needs entrained air?, Which admixture is most suitable for the chosen cement ? What size of aggregate is best? Should fly ash or any other mineral admixture be incorporated mix to improve the micro-structure of concrete?.The right concrete mix design can solve these problems. What you really want in a concrete mix is the one that is easy to place, strong enough to meet the needs of the application. This can best be achieved by taking all the parameters of constituents of concrete that influence the final mix, and its performance designed for.

Indian Standard on Concrete (IS: 456;2000 (reaff.2016), has suggested nominal mixes for lower grades of concrete. These suggested proportions are only for guidance and may not work out to be economical. Hence, designed mixes as per IS:10262: 2009(Reaff.2014) on, “Guidelines for concrete Mix Design Proportioning” needs to be followed. The table below shows, “why concrete mix needs to be designed.

The various parameters to be considered in the design of concrete mix in the green and hardened stagesto achieve enhanced Performance, hence durability are discussed briefly.

Concrete mixture in the green stage

Workability

This parameter is very important, as it depends on the water-cement (W/C)ratio. Excessive (W/C) may result in more voids and poor quality concrete.The cohesiveness of concrete is another parameter, which mainly depends on composition of the constituents of concrete mix.

It is well proven that cement along with pozzolanas (mineral admixtures) can increase the fines to give a cohesive mix. Since fines can give more paste, the dosage of workability admixture (High Range Water Reducer admixture preferred) is a necessity and advantageous. The cohesiveness of concrete, when designed properly will enable easy placement, and degree of compactness that could be achieved by using external means (Vibrators-removes air from the mix )controls the strength of the hardened concrete and its performance.

Concrete mixture in the Hardened stage

Compressive strength

It is one of the most important properties of concrete and influences many properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement (W/C) ratio of the mix. The other factors affecting the strength of concrete at a given age and cured at a prescribed temperature is the workability factor, which has been already discussed. Mixing of chemical admixtures also reduces the (W/C) ratio to the extent of 15-20%, when High Range Water Reducing (HRWR) admixtures are used that increases the strength of the concrete without increasing the cement content. The slump is also increased to our requirement.

Role of Chemical and Mineral Admixtures in Concrete Mixes for enhancing Rheological Properties

What are Admixtures, and types?

Admixtures are ingredients other than water, aggregates, hydraulic cement, and fibers that are added to the concrete batch immediately before or during mixing. Admixtures vary widely in chemical composition, and many perform more than one function.

  • Two basic types of admixtures are available: chemical and mineral. Mix designs made with mineral admixtures combined with chemical admixtures, improves properties of green and hardened concrete.
  • Properties of green concrete, such as, retarding the delayed setting to help transportation, facilitating easy pumping with reduced (W/C), and improving the microstructure of concrete that results in improved durability, of hardened concrete are attracting the construction industries. It is to be noted that use of mineral admixtures in mass concrete is vogue in our country in the construction of dams, with main purpose to reduce the heat of hydration during concrete hardening process.

However, due to increased awareness of the possible durability problems in the coastal areas and chemical industries, the architects and designers in India always propose buildings with concrete incorporating chemical admixtures along with;

  • Specified thickness of cover, minimum cement content, and max. Permissible (W/C), as given in IS 456:2000 (Reaff.2016)

It is estimated that 80% of concrete produced in North America these days contains one or more types of admixtures. According to a survey by the National Ready Mix Concrete Association (NRMCA), 39% of all ready-mixed concrete producers use fly ash (mineral admixture), and at least 70% of produced concrete contains a water-reducing admixture.

In India the main mineral admixture fly ash causing pollution and disposal problems is available from thermal power plants. The awareness of advantageous properties of fly ash and its use in concrete to achieve economy and enhanced durability of structures is increasing.

High strength (HSC), High performance Concrete (HPC) and Self Consolidation Concrete (SCC) is being used extensively in multi-storied concrete structures, particularly in the coastal cities, to realize the advantages of economy and durability.High Strength concrete (HSC) andHigh Performance Concrete (HPC) can benefit from the three most commonly used mineral admixtures produced as waste or by product.  Fly ash, silica fume, and Ground Granulated Blast-furnace Slag are examples. These materials have become necessary constituents in the production of HSC, HPC and SSC in addition to the conventional basic materials. It is to be noted that they are to be designed using required normal/Special Chemical admixtures and tested in the laboratory before large scale applications.

Chemical Admixtures

These are added to concrete in very small amounts mainly for the entrainment of air, reduction of water or cement content, plasticization of fresh concrete mixtures, or control of setting time. Seven types of chemical admixtures are specified in ASTM C 494, and AASHTO M 194, depending on their purpose or purposes in PCC. Air entraining admixtures are specified in ASTM C 260 and AASHTO M 154. General and physical requirements for each type of admixture are included in the specifications. Indian Standard codes; 456, 9103, 6925 discuss general use, specifications, and tests for chemical admixtures.

Mineral Admixtures

Fly ash, Silica Fume [SF], and GGBS are mineral admixtures usually added to concrete in larger amounts to enhance the workability of fresh concrete and to improve resistance of concrete to thermal cracking, alkali-aggregate expansion, and sulfate attack; and to enable a reduction in cement content.

Mineral admixtures are discussed in IS: 456, 3812, and 12089. Mineral admixtures that are popular are: Fly Ash, Silica Fume, and Ground Granulated Blast Furnace Slag.

Fly ash

The substitution rate of fly ash for Portland cement will vary depending upon the chemical composition of both the fly ash and the Portland cement. The rate of substitution typically specified is a minimum of 1 to 1 ½ pounds of fly ash to 1 pound of cement. It should be noted that the amount of fine aggregate will have to be reduced to accommodate the additional volume of fly ash. This is due to fly ash being lighter than the cement.

The amount of substitution is also dependent on the chemical composition of the fly ash and the Portland cement. Initially, maximum substitution permitted was in the range of 15 to 25 percent.

Effects of fly ash, especially Class F, on fresh and hardened concrete properties has been extensively studied by many researchers in different laboratories, the two properties of fly ash that are of most of concern are the carbon content and the fineness. Both of these properties will affect the air content and water demand of the concrete.

The finer the material the higher the water demands due to the increase in surface area. The finer material requires more air-entraining agent to five the mix the desired air content. The important thing to remember is uniformity. If fly ash is uniform in size, the mix design can be adjusted to give a good uniform mix. The carbon content, which is indicated by the loss of ignition, also affects the air entraining agents and reduces the entrained air for a given amount of air-entraining agent. An additional amount of air-entraining agent will need to be added to get the desired air content. The carbon content will also affect water demand since the carbon will absorb water. Again uniformity is important since the differences from non-fly ash concrete can be adjusted in the mix design.

Fresh concrete workability

Use of fly ash increases the absolute volume of cementitious materials (cement plus fly ash) compared to non-fly-ash concrete; therefore, the paste volume is increased, leading to a reduction in aggregate particle interference and enhancement in concrete workability.

Use of Fly Ash in Concrete – Precautions

  • Special precautions may be necessary to ensure that the proper amount of entrained air is present
  • All fly ashes may not have sufficient pozzolanic activity to provide good results in concrete
  • Suitable fly ashes are not always available near the construction site, and transportation costs may nullify any cost advantage and,

Mix proportions might have to be modified for any change in the fly ash composition, since the cement-fly ash reaction is influenced by the properties of the cement, it is importantto test and approve each fly ash source, but also to investigate the properties of the specific fly ash-cement combination to be used for each project.

Generally, Fly ash is frequently used in mass concrete as a cement replacement to reduce the heat of hydration, which in turn reduces peak temperatures, temperature gradients, and the likelihood of thermal cracking. Generally, mass concrete only requires a low compressive strength, so development of strength is not a controlling factor in selecting mix proportions. The following are the advantages of fly ash;

  • Fly ash reduces permeability and chloride diffusivity and increases resistivity
  • Beneficial material in concrete that is exposed to chlorides such as bridge decks, Structures located in marine and structures located in chemical environment
  • Fly ash also binds up the alkalis in the concrete and, thereby, reduces the potential for alkali aggregate reactivity and,

The addition of fly ash to concrete enhances the strength gain at later ages, making it beneficial, when high-strength concrete is specified at ages of 56 or 90 days.

Silica fume

Silica fume, also known as condensed silica fume and micro-silica, is a very fine pozzolanic material produced as a by-product in the production of silicon or Ferro-silicon alloys. Silica Fume consists of very fine vitreous particles with a surface area ranging from 13,000 to 30,000 m2/ kg, when measured by nitrogen absorption techniques, with particles approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material (ACI Comm. 226 1987b; Luther 1990).

Silica Fume is used in concrete to improve its properties. It has been found that Silica Fume improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.

The silica fume content of concrete generally ranges from 5 to 10 percent of the total cementitious materials content. The use of silica fume can be specified using ASTM C 1240 (AASHTO M 307).

For most applications where durability is a concern, the use of silica fume will reduce the permeability of the concrete, thereby slowing the rate of penetration of aggressive chemicals such as deicing salts. The use of silica fume in concrete can reduce in rapid chloride permeability values of less than 500 when tested in accordance with ASTM C 1202 (AASHTO T 277).

The use of silica fume improves the early age strength development of concrete and is particularly beneficial in achieving high release strengths in precast, prestressed concrete beams. Use of silica fume often allows a reduction in the total amount of cementitious materials. At later ages, Concretes made with silica fume can achieve compressive strengths in excess of 117 M Pa.

Mix Design

Silica Fume has been used as an addition to concrete up to 15 percent by weight of cement, although the normal proportion is 7 to 10 percent. With an addition of 15 percent, the potential exists for very strong, brittle concrete. It increases the water demand in a concrete mix; however, dosage rates of less than 5 percent will not typically require a water reducer. High replacement rates will require the use of a high range water reducer.

Effects on Air Entrainment and Air-void System of Fresh Concrete

The dosage of air-entraining agent needed to maintain the required air content when using Silica Fume is slightly higher than that for conventional concrete because of high surface area and the presence of carbon. This dosage is increased with increasing amounts of Silica Fume content in concrete.

Effects on Water Requirements of Fresh Concrete

Silica Fume added to concrete by itself increases water demands, often requiring one additional pound of water for every pound of added Silica Fume. This problem can be easily compensated for by using HRWR (Admixtures and ground slag 1990).

Effects on Consistency and Bleeding of Fresh Concrete

Concrete incorporating more than 10% Silica Fume becomes sticky; in order to enhance workability, the initial slump should be increased. It has been found that Silica Fume reducesbleeding because of its effect on rheological properties.

Effects on Strength of Hardened Concrete

Silica Fume has been successfully used to produce very high-strength, low-permeability, and chemically resistant concrete. Addition of Silica Fume by itself, with other factors being constant, increases the concrete strength.Incorporation of Silica Fume into a mixture with HRWR also enables the use of a lower water-to-cementitious materials ratio that may not have been possible otherwise. The modulus of rupture of Silica Fume concrete is usually either about the same as or somewhat higher than that of conventional concrete at the same level of compressive strength.

Effects on Freeze-thaw Durability of Hardened Concrete

Air-void stability of concrete incorporating Silica Fume studied by Pigeon, Aitkin, and LaPlante in 1987, and1989, indicated that the use of Silica Fume has no significant influence on the production and stability of the air-void system. Freeze-thaw testing (ASTM C 666) on Silica Fume concrete showed acceptable results; the average durability factor was greater than 99%.

Effects on Permeability of Hardened Concrete

It has been shown by several researchers that addition of Silica Fume to concrete reduces its permeability. Rapid chloride permeability testing (AASHTO 277) conducted on Silica Fume concrete showed that addition of Silica Fume (8% Silica Fume) significantly reduces the chloride permeability. This reduction is primarily the result of the increased density of the matrix due to the presence of Silica Fume.

Effects on Alkali Silica Reactivity of Hardened Concrete

Silica Fume, like other pozzolans, can reduce ASR and prevent deleterious expansion due to ASR.

Availability and Handling

Silica Fume is available in two conditions: dry and wet. Dry silica can be provided as produced or densified with or without dry admixtures and can be stored in silos and hoppers. Silica Fume slurry with low or high dosages of chemical admixtures are available.

Use of Combination of Mineral Admixtures in Concrete

Ground Granulated Blast Furnace slag (GGBFS), which has been dried and ground to a fine powder. Iron ore, limestone, and coke are fed into the blast furnace, where they reach a temperature of 15000 C and the raw material reduced to molten iron and blast furnace slag. These are tapped off from the blast furnace and separated for processing. Molten iron is sent to the steel producing facility and slag (GGBS) comes as waste. It is the Glassy granular material formed when molten blast furnace slag is rapidly chilled as by immersion in water. The permeability of concrete depends on its porosity and pore-size distribution. Pores in concrete, normally containing calcium hydroxide, are then, in part, filled with calcium silicate hydrates. The use of GGBF slag in hydraulic structures is well documented. The permeability of mature concrete containing GGBF slag is greatly reduced when compared with ordinary concrete. As GGBS content is increased, permeability decreases.

The following tests are carried out for ascertaining the durability of slag cement concrete.

  • Water absorption test
  • Macro cell corrosion test
  • Chloride penetration test.

With today’s requirements for High Performance Concrete (HPC), mix proportions containing cementitious materials in addition to Portland cement are used more commonly.

According to a survey by the Portland Cement Association published at least 60-75 percent of ready mixed concrete contains other cementitious materials, often referred to as mineral admixtures or supplementary cementitious materials.

The benefits of using these materials, either separately or in various combinations in concrete mix designs results in:

  • Higher early strength,
  • Higher later age strength,
  • Reduced permeability,
  • Control of alkali-aggregate reactivity,

Lower heat of hydration, and reduced costs.

Mineral admixtures are used in addition to the normal amount of Portland cement or as a substitute for a portion of the cement depending on the required or specified properties of the concrete.

It can be concluded that each of the three mineral admixtures discussed above can be used individually or in combination with Portland cement to achieve the desired characteristics of the hardened concrete. Since admixtures, both mineral and chemical can affect the properties of both the fresh and hardened concrete, they should always be tested by trial mixes prior to the start of production. This will ensure that the desired characteristics are achieved and that no undesirable properties are present. It is to be noted that the mineral and chemical admixtures are necessary, in addition to the conventional concrete constituents in the design HSC, HPC and SSC concretes to obtain a suitable concrete micro-structure for enhancing the durability of structure.

In addition to the normal and specified tests, special tests to evaluate the chloride permeability as per AASHTO-T277 and Chloride Diffusion through the concrete using Diffusion Cells are important, when HPC mix is to be finalized. Incidentally, the diffusion cell test can be used to evaluate the service life of marine concrete structures (corrosion of reinforcements) too!. The initiation period in the corrosion process is determined by Fick’s second law of Diffusion using Diffusion cells. The diffusion cell and some of the results obtained as trials for normal mixes are shown (the tests were carried out in SERC, when the author was in Concrete Construction Laboratory of SERC, Chennai). Determination of “Diffusion Coefficient” for concrete, using a Concrete slices in Diffusion Cells is shown in Fig.1

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