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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

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
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
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
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


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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