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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
- Lightweight, high-strength, polymer concrete, fiber-reinforced concrete, and roller compacted concrete
Proportioning of Concrete Mixes
- Properties concerned with in the plastic state
- Finishing characteristics
- Properties concerned in the solid state
- Modulus of elasticity
- Porosity
- Strength is generally the controlling design factor
- PCA quality requirements of properly proportioned concrete mixtures
- Acceptable workability of freshly mixed concrete
- Durability, strength and uniform appearance of hardened concrete
- Economy
- What is Mix Design? Determine the proportions of cement, water, fine & coarse aggregates, and the use of admixtures
- Mix design methods:
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 situationMild 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 necessaryMixing 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 componentMixing 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 chemicalsWorkability
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 resultsCuring Concrete
Gain of strength:
Curing Time |
% Strength Obtained |
| No Time Allowed | 50% |
| 3 days | 60% |
| 7 days | 80% |
