Impact of course aggregate on mix design of concrete

Coarse aggregate is considered as one of the vital elements of concrete and captures the major volume in the mix as well as impacts the concrete mix design significantly. Its different characteristics like strength, maximum size, shape, and water absorption affect water demand, the quantity of cement and fine aggregate in concrete mixture.

It is found that high maximum coarse aggregate size can reduce water demand in the mixture as such aggregate contains lower surface area with regards to small coarse aggregate size.

While considering shape, rounded type aggregate offers reasonable mix design for normal strength concrete.

For high strength concrete, angular coarse aggregate is essential. The scope of segregation is reduced when coarser aggregate is graded perfectly for concrete mix design.

While considering strength, greater aggregate strength generates higher concrete strength.

1. Maximum Aggregate Size: The maximum size of coarse aggregate is another vital factor that checks water demand to attain definite workability. It also determines the quantity of fine aggregate content required for developing cohesive mix.

For a specified weight, if the maximum size of aggregate is increased, the surface area of coarse aggregates is reduced and conversely. When the maximum size of coarse aggregate is decreased, the surface area of coarse aggregate is raised. If the surface area is large, the water demand will be increased to coat the particles and produce workability.

For smaller maximum size of coarse aggregate, larger fine aggregate content is required to coat particles and retain cohesiveness of concrete mix. Therefore, for similar workability, 40mm down aggregate contains lower water/cement ratio, thus strength is raised with regards to 20mm down aggregate. Due to its lower water demand, higher maximum size of coarse aggregate can reduce the cement consumption.

Maximum size of aggregate is confined with clear cover and minimum distance among the reinforcement bars. Maximum size of coarse aggregate is lower than clear cover or minimum distance among the reinforcement bars. As a result, the aggregates can get through the reinforcement in congested areas, to form dense and uniform concrete.

2. Grading of Coarse Aggregate: Grading means the establishment of the particle-size distribution for aggregate. It influences the amount of cement and water requirements, workability, pumpability, and stability of concrete. The grading of coarse aggregate is crucial to acquire cohesive and dense concrete. The voids due to larger coarse aggregate particles are filled with smaller coarse aggregate particles.

If the grading of coarse aggregate is perfect, the scope of segregation is reduced, specifically for higher workability and the compatibility of concrete in enhanced. The coarse aggregate grading limits are provided in ASTM C33/ C33M and IS 383 – 1970 – table 2, Clause 4.1 and 4.2 for single size aggregate as well as graded aggregate.

3. Shape of Coarse Aggregate: The shape of coarse aggregates range from round, angular, or irregular. Rounded aggregates contain lowest water demand because of lower surface area, and also contain lowest mortar paste requirement.

These properties facilitate to produce rounded aggregate to give in the most reasonable mixes for concrete grades up to M35. However, for concrete grades of M40 and above the scope of bond failure would slant the balance in support of angular aggregate containing larger surface area.

Flaky and elongated coarse aggregate particles raise the water demand as well as the susceptibility of segregation. The flexural strength of concrete is minimized due to flakiness and elongation. Specifications provided by Ministry of Surface Transport confine the combined flakiness and elongation to 30% by weight of coarse aggregates.

4. Strength of Coarse Aggregate: Material strength of coarse aggregate is determined by crushing strength of rock, aggregate crushing value, aggregate impact value, aggregate abrasion value. The IS limits for above tests range from Aggregate Crushing value, Aggregate Impact value, and Aggregate abrasion value.

5. Aggregate Absorption: The purpose of aggregate absorption is to employ a correction factor for aggregates in dry condition and find out water demand for concrete in saturated surface dry condition. Aggregate can consume water up to 2 % by weight when remains in bone dry state. But occasionally, the aggregate absorption remains as high as 5%.

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Published By
Rajib Dey
www.constructioncost.co
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How to get rid of common errors in concrete floor slab construction

To get rid of common errors in concrete floor slab construction, various steps like perfect base preparation, mix design, placement, finishing, and curing should be taken.

Typical concrete floor slab thickness in residential construction should be 4 inches. In case the concrete has to sustain extreme loads irregularly, the thickness should be five to six inches; as for instance, motor homes or garbage trucks.

To arrange the base, the ground level should be dug according to the perfect depth to provide for the slab thickness. Take out all organic material and large hard objects like stones and tree roots to a depth of minimum 4 inches. If it is required to develop the grade, apply gravel or sandy soil, and compress the final base with a vibratory plate or equivalent device.

The edge is built up with any straight material that can be fixed into position. If case of constant non-availability of straight lumber, plastic or metal forms should be used. Fix a string line with grade stakes or batter boards to provide a square, level reference prior to place the formwork.

For the concrete mix, it should satisfy the compressive strength requirements (normally 3000 pounds per square inch) devoid of measures that lead to extreme shrinkage. The shrinkage and cracking are increased because of the existence of water, a plasticizer should be used to attain required slump.

Also provide fibers to check plastic shrinkage cracking. Greater strength and entrained air are necessary for exterior slabs susceptible to freezing weather or deicing chemicals.

It is suggested not to add water at the jobsite more than 1 to 2 gallons per cubic yard. In case, extra slump is essential, take advice from the mixer truck driver concerning the quantity of water to be included devoid of taking the concrete out of specification.

Spread the concrete accross the slab area as close to its final position as possible, and then rake it into exact location. Hardens low-slump mixes manually with a vibrator or apply a vibratory screed. Complete with the least force and strokes of the float required to attain a smooth surface.

Develop control joints no farther apart than 24 to 30 times the slab thickness and at no time exceeding 15 feet along both the width and length of the slab by pushing a 1-inch deep grooving tool into the surface.

To keep Joint spacing more than 15 feet, different types of load transfer devices should be used which range from dowels or dowel plates. For slabs with long joint spacing or no joints, steel reinforcement will be suitable.

It will raise the chances for random cracking, but will maintain cracks firmly to keep up superior structural performance.

The curing process should be initiated when the finished surface can counter damage. The concrete should not be enabled to freeze or dry out. Arrange a curing compound over the surface, or apply proper moist curing.

In case of freezing, the slab should be wrapped with an insulator, like insulating blankets or a 4-inch-thick layer of straw that is weighted down so that it can’t blow away. Unless the concrete attains a strength of minimum 500 psi, put the insulator in place. It normally happens within a few days.

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7 kinds of Construction failings in Reinforced Concrete Structures

There can be happen several concrete defects in Reinforced Concrete structures which lead the concrete structures weak and make the building defective.

Concrete is a popular building material which often known for its toughness; Concrete is made up with three basic things which are: water, aggregate (rock, sand or gravel) and Portland cement. It is known as a very versatile and reliable material though some construction faults and imprudence can lead to the growth of defects in a concrete structure. These failings can be seen as per poor construction practices, poor quality control or for poor structural design and detailing.

In this article we will discuss about some known defects in concrete structures and the types are written here:

1. Honeycomb and Rock Pockets: This kind of defect can be seen on the concrete structure where gaps are left for the failure of cement mortar to pour spaces around and among coarse aggregates. It happens when poor quality control is mixed during mixing; transporting; or laying of concrete, under or over-compression of concrete, lack of space between bars and low cement content or improper mix design. This kind of defect may reduce durability and make the concrete weak; but if they are minor can be repaired by cement mortar within 24 hours or it can’t be repaired.

2. Poor Formwork Installation: This error includes misalignment, movement, and loss of support, failure of forms which can be lead to cracking and structural failure. The loss of support during construction can increase settlement cracks; while insufficient formwork support and premature removal of formwork are main reasons of loss of support in the construction. These errors can be repaired with surface grinding to maintain the prop of the structure if the error is minor; for major errors, it shall be repaired by removing the concrete in defective area and then building that portion again.

3. Concrete Dimensional Errors: These errors happen when there is poor entering of a structural member or for deviation from the specifications.

4. Finishing Errors: They include over-finishing of the concrete surface or addition of more water or cement to the surface while finishing of the concrete which makes the concrete permeable and makes concrete less durable.

5. Shrinkage Cracks: It happens due to the evaporation of water from the concrete mixture; the intensity of this problem is depended on some reasons like the amount of water in concrete, weather conditions and curing regime.

6. Poor Reinforcement Placement: Defects during reinforcement installation can cause serious concrete deterioration; also insufficient chair bars and lack of tying of reinforcement would cause rebar movement which may cause to insufficient concrete cover and reduction in effect depth of the concrete section. As a result, the durability of the concrete structure is compromised and the structure would be exposed to chemical attacks.

7. Bugholes: Bugholes or surface voids are small regular or irregular cavities made due to the entrapment of air bubbles in the surface during placement and consolidation. They commonly found in vertical cast-in-place concrete like walls and columns. Both the size and number of bugholes vary and depend on form-facing material and condition, release-agent type and application thickness, concrete mix characteristics and placement and consolidation practices.

Source www.theconstructor.org



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Published By
Rajib Dey
www.constructioncost.co
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Different grades of Ordinary Portland cement

Ordinary Portland Cement(OPC) is extensively utilized in the construction sectors as primary component for developing concrete, mortar, stucco, and non-specialty grouts. Ordinary Portland Cement is graded on the basis of its strength. The grade specifies the compressive strength of the mortar cube that will be achieved once 28 days of setting is completed.

Grades of Ordinary Portland Cement - The following grades of OPC are available:

1. OPC 33 Grade Cement: This grade of cement is suited for general construction under normal ecological condition.

Compressive Strength of OPC 33 – The average compressive strength of minimum three mortar cubes with a face area of 50 sq.cm is considered at the time of verifying the compressive strength. These mortar cubes comprise of one part of cement and three parts of standard sand.

Compressive Strength of OPC 33:

a) 72 +/- 1 hour = Not under 16 N/mm2
b) 168 +/- 2 hours = Not under 22 N/mm2
c) 672 +/- 4 hours = Not under 33 N/mm2

IS Code – IS 269 : 1989 for Ordinary Portland Cement, 33 Grade.

2. OPC 43 Grade Cement: Now-a-days, this grade of cement is gaining popularity in the construction sectors. OPC 43 is suitable for general RCC construction where the grade of concrete is up to M30. It can also be applied for the construction of various precast items like blocks, tiles, asbestos products like sheets and pipes, and for non-structural works like plastering, flooring etc.

Compressive Strength of OPC 43:

a) 72 +/- 1 hour = Not under 23 N/mm2
b) 168 +/- 2 hours = Not under 33 N/mm2
c) 672 +/- 4 hours = Not under 43 N/mm2

IS Code – IS 8112: 1989 for 43 Grade Ordinary Portland Cement.

3. OPC 53 Grade Cement: OPC 53 is useful while requiring concrete with greater strength at very reasonable cement content. OPC 53 facilitates 8 to 10% saving of cement in concrete mix design, for concrete M20 and over.

This cement grade is very effective for specialized works like pre-stressed concrete components, several precast items like paving blocks, building blocks etc, runways, concrete roads, bridges, and other RCC works where the grade of concrete remains M25 and over.

Compressive Strength of OPC 53:

a) 72 +/- 1 hour = Not under 27 N/mm2
b) 168 +/- 2 hours = Not under 37 N/mm2
c) 672 +/- 4 hours = Not under 53 N/mm2

IS Code – IS 12269 : 1987 for Specification for 53 grade ordinary portland cement.

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Importance of pre-pour cards for concrete placement

Pre-pour checklists are very crucial for proper concrete placement. There should be an authentic verification and checking method to make sure that your next concrete placement is accurate. Without a pre/post pour checklist, huge is money is lost due to inappropriate lighting, incorrect mix design, slump, spacing, manpower etc.

In a pre-pour checklist, the following items should be included :-

1. Fill out as soon as the pour is already finished.
2. Fill it out 5 minutes prior to the pour
3. Left blank
4. Don’t have/use one.

Then it is required to re-assess what a good pre-pour checklist can achieve.

To make a pour card effective, the perfect process should be developed while running through the checklist. A perfect pre-pour process will achieve all the steps and the: Who, What, When, Where, How”.

Who: has verified, examined, checked, and figured: yardage, elevations, sleeves, anchor-bolts, embeds, reinforcing, elevations, mix design etc.

What: is being placed, size of pour, manpower required (footing, wall, column, SS, SOG, S.O.M.D)

When: is the pour occuring (Date, Time, truck spacing)?

Where: (Placement location, pump truck setup, truck route, wash out bin)

How: is it being placed (pump, screeds, equipment, lighting, safety, etc.)?

The pre/post pour cards facilitate a simple check and verifications. While a detailed assessment and paying special attention to detail and passing through all the steps to checkoff is accomplished, it will save your significant time and money and re-work as well as allow you planning and performing a effective pour.

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www.constructioncost.co
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Concrete Mix Design & Concrete Calculator – A useful app for civil engineers

Concrete Calculator is a free calculator that can be used for the following purposes :-

1. Measure cement, sand and aggregate quantity in concrete.
2. Measure the number of premix bags necessary for your project.

3. There is option to settle your own size and rate of premix bags.
4. Measure the volume of concrete necessary for slabs, walls, footings and columns.
5. Work out the weight of materials essential for making the calculated volume of concrete.

The purpose of concrete mix design method is to make proportion of the materials of concrete (cement, sand, and aggregate) inexpensively to attain superior strength and stability on the basis of the materials obtainable at a construction site.

The nominal concrete mix proportions adhering to the code may contain a greater amount of cement with regard to the actual amount necessary when it is designed on the basis of actual design parameters, consequently the cement requirement may be low for the equivalent grade of concrete for a specified site.

The proportions originating from concrete mix design are examined for their strength through compressive strength test on concrete cubes and cylinders.

This concrete calculator is specifically designed for professional Civil Engineers, Concrete Technologists, Civil Engineering Students and DIY(Do It Yourself) enthusiasts similarly.

The user interface is very simple and results are provided specifying the amount of ingredients necessary in kilograms. The design steps are also provided in order that s the user will be able to simply validate the calculations.

To download, click on the following link play.google.com

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www.constructioncost.co
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Common methods for concrete construction

In this civil engineering article, you will get detail information on the most common methods for developing a concrete structure.

Designing the concrete mix: The most vital part in the method is to find out the components that will form the concrete and their ratios. Several types of variables should be taken into consideration at the time of designing which range from cement type, aggregate size and type, amount of water, and mineral and chemical admixtures.

A good mix design can also lead to improper or substandard quality concrete when it is accomplished inappropriately. Based on the type of project, it is decided who will take responsibility for designing the mix. For large, publicly funded projects, a licensed civil engineer should be liable for the final design.

For residential projects like foundations and driveways it is the private contractor, who will prepare the mix design. For do-it-yourself projects it is of course the homeowner who should take responsibility to design his own mix.

To produce an exact mix design, initially, detect the properties that the fresh and solidified concrete should have and then move backwards to opt for the most inexpensive mix design that provides these properties.

Here, focus should be given on the following factors :

Supported Loads: Concrete is formed with different types of strengths, so this is considered as the gateway of the mix design. As the cost of concrete scales rather narrowly with its strength, one does not like to make the concrete stronger than it should have been.

However, if the application provides support to considerably small loads, it is generally not a good idea to indicate weak concrete since weak concrete does not have good stability. For low load applications the quality of the concrete is settled with other factors like resistance to freezing or wear resistance.

Workability: The necessary workability is mainly based on how the concrete will be arranged. Concrete can be poured, pumped, and even sprayed into place, and it will impact the desired workability. Various other factors like the shape of the molds, the rebar spacing, and the accessible equipment at the site for solidifying the fresh concrete once it is placed should also be taken into consideration.

Workability is generally settled with the slump, the tendency for the fresh concrete is to expand under its own weight when it is arranged onto a flat surface.

Environmental conditions: When the concrete is uncovered to severe conditions, then this may perfectly ascertain the required concrete quality in spite of the applied loads. In cold-weather locations the concrete should have the strong resistance capacity against freezing. Besides, it must have the ability to resist the corrosive effects of salt. Underground applications should have the capability to withstand the penetration of moisture and aggressive species from the soil. For almost any type of conditions or mode of attack, the most effective way for defense is to retain the w/c low.

Surface wear: For some applications the physical loads can erode the concrete rather than breaking it. For roads, parking garages, driveways, and industrial floors, the longevity of the structure depends on the hardness and wear resistance of the top layer of concrete.



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