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1. Quality of Concrete in affected by:

  • Chemical composition of Portland Cement
  • Hydration and development of the microstructure
  • Admixtures
  • Aggregate characteristics
  • Placement
  • Consolidation
  • Curing

2. Alternatives to conventional concrete

  • Lightweight, high-strength, polymer concrete, fiber-reinforced concrete, and roller compacted concrete

Proportioning of Concrete Mixes

  • Properties concerned with in the plastic state

– Workability

  • Finishing characteristics
  • Properties concerned in the solid state

– Strength

  • Modulus of elasticity

– Durability

  • Porosity
  • Strength is generally the controlling design factor

fc’ = concrete strength

  • PCA quality requirements of properly proportioned concrete mixtures
  1. Acceptable workability of freshly mixed concrete
  2. Durability, strength and uniform appearance of hardened concrete
  3. Economy
  • What is Mix Design? Determine the proportions of cement, water, fine & coarse aggregates, and the use of admixtures
  • Mix design methods:

– Arbitrary Volume Method (1:2:3 cement : sand : coarse aggregate proportion )
– Weight Method
– Absolute-Volume Method
Note: The absolute volume method is the most accurate method

Basic steps for weight and absolute volume methods

Details covered in lab
1. Strength Requirements
Three quantities must be known:
1.The specified compressive strength, fc’
2.The variability or standard deviation, s
3.The allowable risk of making concrete with an unacceptable strength
fcr’ = fc’ + 1.34.s,
For mixes with a large standard deviation in strength use
fcr’ = fc + 2.33.s – 3.45
Note: The required fcr’ is determined as the large values obtained from the above equations
2. Water-Cement (W/C) Ratio Requirements
· Historical data, usually 3 trial batches are made
· Check for the exposure conditions
3. Coarse Aggregate Requirements
· Most economical mix contains large-dense graded aggregate
· Round aggregates require less water than angular
· Maximum allowable size is limited by the dimensions of the structure and the type of construction equipment
· Fineness modulus
– Dependent on the coarse aggregate size and quantity of cement paste
– Low fineness modulus is desired for mixes with low cement content to promote workability
4. Air entrainment Requirements
· Used whenever concrete is exposed to freeze-thaw conditions and de-icing salts
· Used for workability in some situation

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Mild Exposure, Moderate Exposure, Severe Exposure

· Air content decreases with increasing maximum aggregate size
5. Workability Requirements
· The ease of placing, consolidating, the finishing freshly mixed concrete
Slump test
. Water Content Requirements
· Dependent on the maximum size, shape of the aggregates, and the use of air entrained admixture
7. Cement Content Requirements
· Cement = weight of water / water cement ratio
· PCA recommends a minimum content of 334 kg/cu.m
· No less than 385 kg/cu.m for under water applications
8. Admixture Requirements
· To add a specific quality for the concrete, their quantities should be considered in the mix proportion
9. Fine Aggregate Requirements
· Absolute volume mix method component weight and specific gravity determine volumes of water, aggregate, and cement
· Bulk SSD specific gravity is used for weight-volume conversions
10. Moisture Corrections
· Adjust the weight of water and aggregates to account for the existing moisture content of the aggregates
11. Trial Mixes
· Used to check mix design
· 3 cylinders are made and cured for 28 days
· Tested for slump and compressive strength
· Adjust the mixture if necessary

Mixing Concrete for Small Jobs

· Jobs requiring less than one cubic meter of concrete
· Multiply required total weight or volume of concrete mix by the ratio to obtain the total weight of finished component

Mixing and Handling of Fresh Concrete

Batching:
Measuring and introducing the concrete ingredients into the mixer. Can be done by either weight or volume
· Batching by weight is more accurate
· Batching by volume is more common when continuous mixers are used or when mixed by hand
Mixing:
Can be performed on-site or in ready-mix plants.
· Central Mixed Concrete – Completely mixed in an RMP
· Shrink Mixed Concrete – Partially mixed in an RMP
· Truck Mixed Concrete – Mixed entirely inside the truck
Placing(Vibration):
Required to consolidate the concrete by releasing excess air voids created during pumping
· Manual – by ramming and tamping the concrete
· Internal vibrators – a weight is rotated at high speeds inside a spud to cause vibration.
· Others – external vibrators, vibrating tables, surface vibrators, electric hammers, and vibratory rollers.
>>> Too much vibration causes the cement to separate
from the aggregates
Curing:
Maintaining satisfactory moisture and temperature (above 50 degrees F) in the concrete for a period of time. This allows the concrete to gain strength.

Depends on temperature and time (maturity)
· Affects durability, water tightness, abrasion resistance, volume stability, resistance to freeze and thaw, & resistance to de-icing chemicals

Workability

1. Water – Improves workability in the field, but it
decreases the hardened concrete’s strength and quality
2. Air – Can change with both mixing and handling.
Tests are required to ensure limit has not been exceeded
– Pressure Method
Based on Boyle’s law, which relates pressure to volume.
Not good for lightweight aggregates, since they contain air voids which can be compressed.
Most widely used.
– Volumetric Method
A displacement method using water. Agitate sample with equal amount of water in a cylinder so the water displaces the air. The changes in volume of water corresponds to the total volume of air.
Accuracy of test depends on the time duration of agitation.
– Gravimetric Method
Comparison of the unit weight of the freshly mixed concrete to the maximum theoretical unit weight (determined using the mix proportions)
– Chace Air Indicator
A displacement method using alcohol.
This test is not precise and unable to give repeatable results

Curing Concrete

Gain of strength:

Curing Time
% Strength Obtained
No Time Allowed 50%
3 days 60%
7 days 80%

Curing also provides durability, water tightness, abrasion resistance, volume stability, resistance to freeze-thaw and resistance to de-icing chemicals
3 ways for curing:
1. Maintaining the presence of water by immersion,
ponding, spraying, fogging, or wet coverings
2. Preventing loss of mixing water by sealing surface
– Tactics used are plastic sheets, membrane-
forming compounds, and leaving the forms in place
3. Steam curing, insulating blankets, and other various
heating techniques to heat and add additional water
Curing period, dependent upon factors:
· Type of cement
· Mixture proportions
· Required strength
· Weather conditions
· Size, shape of structure
· Future exposure conditions
· Method of curing

Gopal Mishra

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