Effects in concrete mix design

Concrete Mix design is complete science and it is based on a lot of research. Popular Mix Design method is highlighted in ACI 2011 committee report. It is a systematic approach in which user starts with defined slump and strength and some other parameters. End result gives quantities of different constituents.

Performance-based methods are alsoavailable in which engineer based on his experience selects initial proportions and modifies as needed. Both systematic and performance-based methods have certainadvantages and disadvantages which will be covered in later posts.

Though mix design is a comprehensive process still it is very useful for engineers to know certain mix design concrete thumb rules which will make his life easier. Here is the list,

By adding 1 litre of water in 1 cubic meter of concrete mix

Increase slump of about 25 mm is expected.It will decrease compressive strength of about 1.5 to 2.0 N/mm²Increase shrinkage potential of about 10%Waste as much as ¼ bag of cement

Effect of increasing concrete mix temperature by 1 celsius

About 4 liters of water per cubic metermaintains equal slumpAir content decreases about 1%Compressive strength decreases about 1.0 to 1.5 N/mm²

Effect of air content on concrete mix 

If air content increases 1%, it will result incompressive strength decrease of about 5%If air content decreases 1%, then it will cause yield to decrease about 0.03 cubic meter per 1 cubic meter of concrete mix.If air content decreases 1%, then slump willdecrease about 12.5 mmIf air content decreases 1%, it will result in durability decreases of about 10%.

Sources of crack in concrete

We all know cracking is one of the major problems in concrete and it should be considered while mix design, but for that, we have to know the sources which can cause cracking in concrete. Here is a list of sources of cracking in concrete.

Sources Of Cracking In Concrete :

Use of low-grade materials:

One of the major sources is the use of lower quality materials which include both concreteand steel in reinforced concrete structures.

Shrinkage:

Shrinkage can cause cracking if not controlled during mix design and curing stage. Shrinkage can become critical in high strength concretebecause of low water/cement ratio and also because of use of Mineral Admixtures.

Quality and Type Of Aggregate:

The quality of aggregate used in concretedetermines the overall strength of concrete. If the aggregate is of poor quality it will not make a proper bond with cement.

Overloading of structure:

Overloading of structure especially at younger age is a common source of cracking. This can happen if formwork is removed before time or more construction load is present.

Mistakes at design stage in office:

If there are errors at design stage than it is obvious that problems will occur at site.concrete cracking is one of them.

Improper Curing:

Another major cause of concrete cracking. If curing is not done appropriately for given time span than one should expect cracking.

Early Formwork Removal:

If formwork is removed before concrete has achieved strength, there will be cracking.

Use of Congested Reinforcement in Lean Concrete:

If you use heavy reinforcement in average quality concrete than stress distributionbetween steel and concrete can become non-linear causing cracking.

Mistakes at Site or during erection:

Proper and trained labour and workmanship are necessary for any site work. Lack of it during concreting can cause cracking.

ROLE OF SITE ENGINEER

ROLE OF CONSTRUCTION SITE ENGINEER

Role of Construction Site Engineer depends on the type of work involved and experience of site engineer in a construction project.

The duties and responsibilities of a construction site engineer are typically as follows, many of these will be delegated to other engineers on the site according to their experience and ability:

# Setting out the works in accordance with the drawings and specification

# Liaising with the project planning engineer regarding construction programmes

# Checking materials and work in progress for compliance with the specified requirements

# Observance of safety requirements

# Resolving technical issues with employer’s representatives, suppliers, subcontractors and statutory authorities

# Quality control in accordance with CSIs/procedures method statements, quality plans and inspection and test plans, all prepared by the project management team and by subcontractors

# Liaising with company or project purchasing department to ensure that purchase orders adequately define the specified requirements

# Supervising and counselling junior or trainee engineers

# Measurement and valuation (in collaboration with the project quantity surveyor where appropriate)

# Providing data in respect of variation orders and site instructions.

# Preparing record drawings, technical reports, site diary

# Job review of subordinate staff

IS CODES FOR RCC STRUCTURAL DESIGN

This article describes the basic codes for RCC structural design as per Indian standard codes. The structural design of reinforced concrete structures should be carried so as to conform to the Indian codes for reinforced concrete design, published by Bureau of Indian standards, New Delhi.

Purpose of design codes:

National building codes have been formulated in different countries to lay down guidelines for the design and construction of structures. The codes have been evolved from the collective wisdom of expert structural engineers, gained over the years. These codes are periodically revised to bring them in line with current research, and often current trends.

Following are the functions of design codes:

Firstly, the design codes ensure adequate structural safety, by specifying certain essential minimum reinforcement for design.

Secondly, they render the task of the designer relatively simple, often the result of sophisticate analysis is made in the form of a simple formula or chart.

Thirdly, the codes ensure a measure of consistency among different designers.

Finally, they have some legal validity in that they protect the structural designer from any liability due to structural failures that are caused by inadequate supervision and/or faulty material and construction.

Following are the design codes in India:

(i) IS456: 2000 – plain and reinforced concrete – code of practice (fourth revision)

(ii) Loading standard codes

The loads to be considered for structural design are specified in the following loading standards:

IS 875 (Part 1 to 5) : 1987 – code of practice for design loads (other than earthquake) for buildings and structures (second revision).

Part – 1: Dead loads

Part – 2: Imposed (Live) loads

Part – 3: Wind loads

Part – 4: Snow loads

Part – 5: Special loads and load combinations

IS 1893: 2002 – criteria for earthquake resistant design of structure (fourth revision).

IS 13920: 1993 – ductile detailing of reinforced concrete structures subject to seismic forces.

Design Handbooks:

The bureau of Indian Standards have also published the following handbooks which serve as useful supplement to the 1978 version of the codes. Although the handbooks need to be updated to bring them in line with the recently revised (2000 version) of the code, many of the provisions continue to be valid (especially with regard to structural design provisions).

SP 16 – 1980 – Design Aids (for Reinforced Concrete) to IS456: 1978

SP 24: 1983 – Explanatory handbook on IS 456: 1978

SP34: 1987 – Handbook on Concrete Reinforced and Detailing.

LIST OF IS CODES FOR REINFORCEMENT

1.  IS:432 – Mild steel & medium tensile steel bars and hard drawn steel wires for concrete reinforcement : Part-II -Hard drawn steel wire.

2. IS:1786 -  Specification for High strength deformed steel bars and wires for concrete reinforcement.

3.  IS:2502 -  Code of practice for bending & fixing of bars for concrete reinforcement.

4.  IS:2751 -  Recommended practice for welding of mild steel plain  & deformed bars for reinforced construction.

5.  IS:5525 -  Recommendation for detailing of reinforcement in reinforced concrete works.

6.  IS:9077 -  Code of practice for corrosion protection of steel reinforcement in RB & RCC construction.

7.  SP:34 – Handbook on concrete reinforcement detailing.