Different types of concrete admixtures

Instantly before or during mixing concrete, the admixture should be added to the batch of concrete to make the quality of concrete manageability, acceleration, or retardation of setting time better. Now-a-days, several concrete mixes comprise of one or more concrete admixtures that will aid in reducing the cost of pouring method as well as growing the productivity, The cost of these admixtures will differ on the basis of the quantity and type of admixture to be applied. All of these will be added to the cubic yard/meter cost of concrete.

Given below, the details about the different types of concrete admixtures :-

Concrete Admixtures: Set-Retarding: The purpose of set retarding concrete admixtures is to defer the chemical reaction that occurs when the setting process is initiated for concrete. These types of concrete admixtures are generally applied to minimize the impact of high temperatures that can expedite the initial setting of concrete.

Set retarding admixtures are mostly found in concrete pavement construction. They provide sufficient time for finishing concrete pavements, lessen extra costs to arrange a new concrete batch plant on the job site and facilitate removing cold joints in concrete. Retarders are also useful for withstanding cracking due to form deflection that can happen when horizontal slabs are arranged in sections. Most retarders also perform as water reducers and may entail some air in concrete.

Concrete Admixtures - Air-Entrainment: With air entrained concrete, the freeze-thaw strength of concrete is raised significantly. This type of admixture develops a more executable concrete as compared to non-entrained concrete and at the same time the bleeding and segregation of fresh concrete is minimized. Besides, resistance strength of concrete against extreme frost action or freeze/thaw cycles is considerably improved. This admixture provides the following advantages:

• Greater resistance against cycles of wetting and drying
• Superior degree of workability
• Superior degree of stability

The entrained air bubbles function as a physical buffer against the cracking resulting from the stresses owing to water volume augmentation in freezing temperatures. Air entrained admixtures are well suited with almost all the concrete admixtures. Normally, for each one percent of entrained air, compressive strength will be decreased by about five percent.

Water-Reducing Concrete Admixtures: Water-reducing admixtures belong to chemical products which can be added to concrete for producing a required slump at a lower water-cement ratio than what it is generally designed. The purpose of water-reducing admixtures is to retain certain concrete strength with lower cement content. Lower cement contents lead to lesser CO2 secretions and energy consumption per volume of concrete created.

This type of admixture facilitates to enhance the properties of concrete as well as set concrete under tough situations. Water reducers are mainly utilized in bridge decks, low-slump concrete overlays, and patching concrete. Now-a-days, mid-range water reducers are gaining popularity because of the improvements in admixture technology.

Concrete Admixtures - Accelerating: Accelerating concrete admixtures are applied to accelerate the rate of concrete strength formation as well as minimize the setting time of concrete. Calcium chloride is the example of common accelerator component though it may develop the scope of erosion in steel reinforcement. Accelerating admixtures are suitable for altering the properties of concrete in cold weather.

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Some vital instructions to make your boundary walls stronger

Substandard foundations, poor design, scarcity of expansion joints and piers (support pillars) generally lead to cracks in masonry boundary walls.

If a subsisting wall is inclined, unsteady or contains severe cracks, then proper remedies should be taken immediately.

Given below, some vital guidelines while setting up a new boundary wall:

1. The design of the wall: Besides, adhering to local by-laws concerning height restrictions and set-backs on street corners, proper engineering principles should also be applied to the design of any freestanding wall.

Both the thickness of the wall and the spacing of piers control the maximum height of the wall over ground level.

2. Foundations: While creating the design of the foundations, soil conditions and slopes should be considered. Steel reinforcement of the foundations and of the wall itself are vital components. A general rule of thumb is that the foundation should be broader and deeper so that the support of the wall becomes more stable.

3. Retaining walls: When the boundary wall functions as a retaining wall and is susceptible to water and soil thrust, then the foundation footing should be expanded more beneath the upper side of the wall to make sure that the weight of the chosen soil further firm ups the foundation structure.

4. Adequate expansion joints and piers: If expansion joints are not set up perfectly without adequate piers, the walls can be cracked easily.

If there are cavities in the piers of freestanding walls (along with hollow units), they should be filled with concrete instantly.

5. Drainage: If the boundary wall is located on a slope, then sufficient weep holes should be installed to dispose of accumulated storm water.

When the wall also functions as a retaining wall, there should proper arrangement of sub-soil drainage. NHBRC (Part 3. 3:24) indicates that weep holes should be set up in all retaining walls at a height not greater than 300mm over the lower ground level at centres not surpassing1.5 metres. Weep holes should be created with a 50mm plastic pipe covered on the non-exposed end with a geofabric.

6. Damp: Boundary walls are generally freestanding as these walls do not usually build section of a structure (like a house), where the connected walls are self-bracing. Since the freestanding walls are not supported by a structure of other walls, boundary walls are normally constructed exclusive of a damp proof course (DPC), plastic membrane so as to make the bond stronger among the freestanding wall and its foundation.

A DPC can rupture the bond among wall and foundation that leads to instability. Without DPC, mounting damp is frequently found on plastered and painted boundary walls. To get rid of this issue, the harder (less porous) bricks should be used for the lower courses (first 150mm) and make sure that the plaster does not expand to the level of the soil, else the plaster will function as a wick and allow water going from the ground to wick upwards.

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Some vital characteristics of concrete in fresh and hardened state

STRENGTH:

Concrete has good strength against compression but poor strength against tension and bending as a result the concrete can’t be demolished easily but with little force, it can be separated into pieces or leads to bending cracks.

Compressive strength is mainly based on the amount of cement utilized, but it is also influenced by the ratio of water to cement along with proper mixing and placing, and the capacity and extent of hydration and curing.

Both tensile strength and flexural bending strength will be raised with inclusion of steel or fiber reinforcement.

Desired compressive strength is based on an analysis of the loads that will be applied and the soil conditions at the project site.

DURABILITY

Concrete that will be walked or driven on should have good resistance capacity against abrasion in order that it doesn’t corrode.

Concrete that is uncovered to the exterior of a building should contain strong resistance capacity against weather in order that it doesn’t weaken from frequent freezing and thawing.

The stability of concrete uncovered to frequent freeze-thaw cycles is considerably raised with air entrainment.

Concrete in which steel reinforcement is implanted should control extra moisture absorption with the purpose of safeguarding the metal from corrosion.

VOLUME STABILITY:

As a porous material, the concrete is widened and compacted with variations in temperature and moisture content. Preliminary shrinkage occurs to cement-based products like concrete, concrete masonry and stucco since the cement hydrates and additional mixing water evaporates.

Additional shrinkage in concrete leads to cracking and consequently facilitate the moisture to infiltrate, and a vicious cycle of corrosion will start.

Shrinkage cracking is controlled moderately with steel or fiber reinforcement, and the position and weather resistance of shrinkage cracks can also be controlled via control joints which segregate the concrete into smaller panels or sections.

The shrinkage cracking may also be affected with the mix design and ingredient proportions.

WORKABILITY:

The workability of fresh concrete becomes superior when it is formed, compacted, and finished to its final shape and texture with nominal attempt and devoid of segregation of the materials.

Due to poor workability, the concrete fails to flow smoothly into forms and correctly envelop reinforcing steel and embedded items, and it becomes complicated to compact and finish.

Every mix should be perfect for its proposed application, maintaining a balance between desired fluidity, strength, and economy.

Workability is associated with the stability and cohesiveness of the mix, and is influenced by the cement content, aggregates, water content, and admixtures.

CONSISTENCY:

Consistency is an attribute of workability concerning the flow characteristics of fresh concrete.

It is the evidence of the fluidity or wetness of a mix and is calculated with the slump test. Fresh concrete is set in a metal cone and after detaching the cone, the concrete slumps a certain amount based on fluidity. A wet, soft mix slumps in a greater extent as compared to a drier, stiffer one.

A high-slump mix leads to extra bleeding, shrinkage, cracking, and dusting of the hardened concrete surface.

A specific range of consistency exists that is ideal for each type of work. Workability is highest in concrete having medium consistency with a slump among 3 and 6 in. The functionality of both very dry (low-slump) and very wet (high-slump) mixes is reduced.

COHESIVENESS:

Cohesiveness refers to the component of workability to identify whether a mix is harsh, sticky, or plastic.

A harsh mix does not have plasticity and the elements may have a propensity to detach.

Harshness occurs due to extra or shortage of mixing water (high- or low-slump mixes), a shortage of cement (lean mixes), or fine aggregate particles.

Harshness may also occur because of extra rough, angular, flat, or elongated aggregate particles. Harsh mixes are improved with air entrainment or by enhancing the fine aggregate or cement content, but modifications should be done to the overall mix to sustain the exact proportion of all materials.

A sticky mix may contain higher cement content (fat mixes) or large amounts of rock dust, fine sand, or similar fine materials (over-sanded mixes). Sticky mixes are not detached easily, but as they need a lot of water to attain nominal workability, sticky mixes may frequently contribute to extra shrinkage cracking.

A plastic mix is cohesive without becoming either sticky or harsh, and the materials can be easily detached until the concrete is managed incorrectly.

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Types of steel bars for construction

Generally, the following four types of steel bars are utilized in concrete structure : Types of Steel Reinforcement used in Concrete Structures -

1. Hot Rolled Deformed Bars: It is extensively used in regular RCC structures. Hot rolling is performed in the mills by providing deformations on the surface i.e. ribs with the purpose of developing bond with concrete.

The stress - strain curve presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. It comprises of typical tensile yield strength of 60,000 psi.

2. Mild Steel Plain bars: These belong to plain bars and do not contain ribs on them. These are suitable for small projects where cost is a vital factor. Since the plain bars can’t be secured properly with concrete therefore hooks are arranged at the ends. In this type of steel, stress - strain curve also presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. Normal tensile yield strength is 40,000 psi.

3. Cold Worked Steel Reinforcement: Cold worked reinforcement is provided if hot rolled steel bar encounters process of cold working. Cold working is done by twisting or drawing the bars at room temperature. It properly reduces the Plastic Stage in the Stress-Strain curve, even though it provides more control on the size and tolerances of bars. As there is no plastic stage, ductility becomes lower as compared to Hot Rolled bars. It is mostly effective for the projects where low tolerances and straightness are vital factors.

The stress – strain curve does not present a separate yield point since the plastic stage is completely discarded. Yield point is obtained by drawing a line parallel to the Tangent Modulus at 0.2% strain. Yield stress stands for the point where this line overlaps the stress – strain curve.

It is defined as 0.2% proof stress. If yield stress is obtained at 0.1% strain it is defined as 0.1% proof stress. Normal tensile yield strength is 60,000 psi.

4. Prestressing Steel: Prestressing steel is applied in the form of bars or tendons which are constructed with several strands. The application of tendons / strands is very common since these can be easily placed in different profiles. Prestressing strands are built up with various wires (usually with 2, 3 or 7 wire strands). Normal seven wire strand comprises of six wires spun around the seventh wire with marginally a greater diameter, consequently a helical strand is created.

These wires are cold drawn and contain very high tensile ultimate strength (normally 250,000 - 270,000 psi). Due to extreme tensile strength, the concrete is prestressed properly even after experiencing short term and long term losses. These are applied as prestressed concrete in bridges or prestressed slabs in buildings. Prestressing steel also comes as non-bonded strands encased in PVC sheath. It is applied in Post-Tensioning of members. Prestressing strands can also be applied as Low Relaxation Strands which show low relaxation losses after prestressing. These are normally applied in prestressing members with large distances./p>

Because of cold drawing method, plastic stage in this type of steel is removed. Thus stress – strain curve does not present a separate yield point. Yield point is obtained at 0.1% or 0.2% proof stress. The design of prestressed concrete is not based on yield stress rather it relies on the ultimate strength; therefore the property of interest in this type of steel is the ultimate strength.

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How temperature plays an important role on concrete

The concrete is extended with the rise in temperature and contracted with reduction in temperature. The range in difference in temperature differs from areas to areas, seasons to seasons and day to day.

The crack may happen in concrete because of contraction integrated with the impact of shrinkage.

Sometimes, big and injurious stress may produce owing to deformation that takes place for temperature variation.

The coefficient of thermal expansion of contraction is dependent on the type and quantity of cement, aggregate, proportionate humidity and sizes of section.

In high temperature, the concrete is influenced by the following factors :-

1. The elimination of evaporable water
2. The elimination of combined water
3. Modification in cement paste
4. Disruption (of beam) from discrepancy of expansion and resultant thermal stresses
5. Modification of aggregate
6. Alteration of bond among aggregate and paste

Cycles of temperature can provide a gradual impact on the curtailment of strength and even long-lasting curing can’t improve the loss.

Tensile strength of concrete is mostly affected with temperature.
If the heating is adequately fast, high stresses can be added and therefore, failure and instability may occur.

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Some useful tip on placing of Reinforcement

Reinforcement should be perfectly arranged and sufficiently supported prior to placing of concrete as well as safeguarded against displacement within allowable tolerances which are given below.

The tolerances, given below, should be maintained while placing reinforcement unless otherwise mentioned by the engineer:

(a) Tolerances for depth d, and minimum concrete cover in flexural members, walls and compression members should be as follow :-

Tolerances for Placing Reinforcement

Tolerance for d - Tolerance for Minimum Concrete Cover

d ≤ 200 mm ±10 mm –10 mm
d > 200 mm ±13 mm –13 mm

(b) Regardless the provision of (a) above, tolerance for the clear distance to formed soffits should be minus 6 mm and tolerance for cover should not be in excess of minus 1/3 the minimum concrete cover stated in the design drawings or specifications.

(c) Tolerance for longitudinal location of bends and ends of reinforcement should be ± 50 mm, excluding at discontinuous ends of brackets and corbels, where tolerance should be ± 13 mm and at discontinuous ends of other members, where tolerance should be ±25 mm. The tolerance for concrete cover should also be used in discontinuous ends of members.

Welded wire reinforcement (with wire size not more than MW30 or MD30) applied in slabs not over and above 3 m in span should be allowed to be curved from a point adjacent to the top of slab over the support to a point near the bottom of slab at midspan, in case such reinforcement is either continuous over, or securely anchored at support.

Welding of crossing bars should not be allowed for assembly of reinforcement unless the concerned engineer approve.

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Why reinforcement is provided in a column

Concrete is strong at bearing compressive stress. Plain concrete can sustain compressive loads capably. But it is always recommended to utilize the R.С.С. columns in place of plain concrete columns in modern day structures for the following reasons :-

Temperature stresses are formed in Concrete because of differences in weather. Cracks may happen for this stress. To get rid of the issue and reduce cracks, some steel is provided at the face of the concrete.

Columns specifically slender columns are prone to lateral loads and moments. Sometimes, tensile stress may also build up particularly in columns at exterior boundary of the building. Steel can deal with this tensile stress.

While making the design of RCC structures, the reinforcement is provided in beams to tie them securely with beams.

Reinforcement is provided so that the size of the columns is not increased.

Reinforcement steel improves the ductility of the member so that the structure gets the ability to withstand earthquake in a superior way.

In R.C.C. columns, less area is required with regards to a plain concrete column It is found that steel can bear load m-times that of concrete of the similar area. To deal with a specific load, the section of an R.C.C. column will be much finer as compared to that of plain concrete. By applying R.С.С. columns, huge space is saved since the size of the column will be less.

A minimum area of steel should be arranged in the column if any case it is necessary for bearing load or not. It is performed to withstand tensile stresses which occur because of eccentricity of loads.

Two types of reinforcements are arranged in a R.C.C. column.

a. Longitudinal reinforcement.
b. Transverse reinforcement.

Longitudinal Reinforcement: The longitudinal reinforcement comprises of steel bars which are arranged longitudinally in a column. It is also known as main steel. The properties of longitudinal reinforcement are given below:

i. To distribute the compressive loads along with concrete, consequently minimizing the size of the column on the whole and parting more usable area.
ii. To withstand tensile stresses which are formed because of any moment or accidental eccentricity.
iii. To yield ductility to the column.
iv. To lessen the impact of creep and shrinkage because of continuous constant loading applied for a long time.

Transverse Reinforcement: The transverse reinforcement is arranged along the lateral direction of the column in the shape of ties spirals which cover the main steel. The function of transverse steel are as following -

i. To retain the longitudinal bars in exact place.
ii. To resist buckling of the main longitudinal bars.
iii. To avoid diagonal tension that happens because of transverse shear formed due to any moment or load.
iv. To yield ductility to the column.
v. To resist longitudinal splitting or bulging from concrete by enclosing it in the core.



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