The Constructor

Fresh Concrete

Gopal Mishra
Civil Engineering

Reading time: 1 minute

1. Quality of Concrete in affected by: 2. Alternatives to conventional concrete

Proportioning of Concrete Mixes

- Workability - Strength - Durability fc' = concrete strength
  1. Acceptable workability of freshly mixed concrete
  2. Durability, strength and uniform appearance of hardened concrete
  3. Economy
- 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 路 Historical data, usually 3 trial batches are made 路 Check for the exposure conditions 3. Coarse Aggregate Requirements 路 Most economical mix contains large-dense graded aggregate 路 Round aggregates require less water than angular 路 Maximum allowable size is limited by the dimensions of the structure and the type of construction equipment 路 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 路 Used whenever concrete is exposed to freeze-thaw conditions and de-icing salts 路 Used for workability in some situation
Mild Exposure, Moderate Exposure, Severe Exposure
路 Air content decreases with increasing maximum aggregate size 5. Workability Requirements 路 The ease of placing, consolidating, the finishing freshly mixed concrete Slump test . Water Content Requirements 路 Dependent on the maximum size, shape of the aggregates, and the use of air entrained admixture 7. Cement Content Requirements 路 Cement = weight of water / water cement ratio 路 PCA recommends a minimum content of 334 kg/cu.m 路 No less than 385 kg/cu.m for under water applications 8. Admixture Requirements 路 To add a specific quality for the concrete, their quantities should be considered in the mix proportion 9. Fine Aggregate Requirements 路 Absolute volume mix method component weight and specific gravity determine volumes of water, aggregate, and cement 路 Bulk SSD specific gravity is used for weight-volume conversions 10. Moisture Corrections 路 Adjust the weight of water and aggregates to account for the existing moisture content of the aggregates 11. Trial Mixes 路 Used to check mix design 路 3 cylinders are made and cured for 28 days 路 Tested for slump and compressive strength 路 Adjust the mixture if necessary

Mixing Concrete for Small Jobs

路 Jobs requiring less than one cubic meter of concrete 路 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 路 Batching by weight is more accurate 路 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. 路 Central Mixed Concrete - Completely mixed in an RMP 路 Shrink Mixed Concrete - Partially mixed in an RMP 路 Truck Mixed Concrete - Mixed entirely inside the truck Placing(Vibration): Required to consolidate the concrete by releasing excess air voids created during pumping 路 Manual - by ramming and tamping the concrete 路 Internal vibrators - a weight is rotated at high speeds inside a spud to cause vibration. 路 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) 路 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: 路 Type of cement 路 Mixture proportions 路 Required strength 路 Weather conditions 路 Size, shape of structure 路 Future exposure conditions 路 Method of curing
Exit mobile version