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EGB270 (completed)
Concrete Mix Design
| Question | Answer |
|---|---|
| Ideal Concrete (Appropriate cement type) | Low C_3_A, MgO, free lime |
| Ideal Concrete (Resistance to weathering & chemicals) | low w/c ratio, proper curing, suitable admixtures, alkali resistant aggregates (Controlled proportions) |
| Ideal Concrete (Strength) | Good quality paste; low w/c ratio and air content; sound aggregates, grading & vibration (Controlled placement/curing) |
| Ideal Concrete (Economy) | large max. agg size; efficient grading; min slump; use of admixtures (Controlled handling) |
| Ideal Concrete (Resistance to wear/deterioration) | low w/c ratio, proper curing, dense & homogeneous concrete, good surface texture (Controlled material/quality) |
| Workability | – quantified in terms of the amount of mechanical work or energy required to place the concrete and produce full compaction without segregation; and – typically measured with the slump test |
| Slump Test | Concrete is filled in the slump cone in layers and then the cone is removed to measure the reduction in the height of the sample |
| Segregation | Separation of the components of fresh concrete so that they are no longer uniformly distributed |
| Segregation Process: | – Coarse aggregate separates out from the concrete: when Concrete too wet or too dry – Water separates out of concrete (bleeding) |
| Effects of Segregation: | – Impossible to achieve full compaction; and – Strength and durability are affected |
| Bleeding | - Upward movement of water after concrete is placed and compacted but prior to setting; - Layer of water forms on the surface of freshly placed concrete |
| Effects of Bleeding: | – Concrete near surface is weakened; – Fines are carried to the top, reducing abrasion resistance; and resulting in – Weak zones |
| Causes of Segregation | • Too many large particles of aggregate with density that is either too high or too low; • Not enough fines; • Mix is too wet or too dry; and/or • Over vibration |
| Control of Segregation | • Coarse aggregate: – Reduce max aggregate size; – Use more sand or finer sand; and/or – Use mineral / air entraining admixtures • Bleeding: – Use mineral / air entraining admixtures; and/or – Reduce water content while maintaining workability |
| ACI Concrete Mix Design Process | This process describes methods for selecting proportions for hydraulic cement concrete made with and without the cementitious materials and admixtures |
| ACI Concrete Mix Design Process is used only for... | normal or high density aggregates (light-weight aggregates are excluded) |
| ACI Concrete Mix Design Process is applicable to usual... | cast in place construction |
| For the ACI Concrete Mix Design Process, hydraulic cements includes... | OPC and blended cement |
| The ACI Concrete Mix Design Process provides a first approximation of proportions intended to be checked by trial batches in the lab or field and adjusted, as necessary, to produce... | the desired characteristics of the concrete |
| Required Material Information for ACI Mix Design method: | • Sieve analysis data of fine and coarse aggregates; • Dry rodded unit weight of coarse aggregates; • Bulk specific gravity of materials; and • Absorption capacity and Moisture content of aggregates |
| Step 2: Choice of Max Aggregate Size (ACI Mix Design method) (Considerations) | • Larger Max Agg. Size (MAS): – Less void space; and – Minimise water requirement ∴ reduce amount of cement required |
| Step 2: Choice of Max Aggregate Size (ACI Mix Design method) (Selecting an appropriate MAS) | – Limited by dimensions of structural elements, space between reinforcement and concrete cover |
| Step 2: Choice of Max Aggregate Size (ACI Mix Design method): A smaller MAS... | – For a given w/c ratio, can achieve higher strength; and – Local availability may limit agg. sizes to 19mm – 25mm |
| Step 8: Adjust for Moisture Content | This method gives quantities of aggregate in SSD condition, adjustment has to be made to the water and stock content if aggregates are wet or dry |
| Step 8: Adjust for Moisture Content (additional info) | - Add extra water if the stock is below SSD condition; and - Deduct water if the stock is above SSD condition - Similarly, reduce stock weight of aggregates if they are dry and vice versa |
| Step 9: Trial Batch Adjustments (Low Strength) | Strength too low (workability ok): – Reduce w/c ratio: • Increase cement, reduce agg |
| Step 9: Trial Batch Adjustments (Low/high Slump) | Slump too low / high (strength ok): – Increase / decrease water and cement content: • ±2 kg/m^3 ➝ ±10mm slump – Increase / decrease dosage of plasticiser |
| Step 9: Trial Batch Adjustments (Segregation) | – Increase fine agg, reduce coarse agg; or – Replace coarse sand with a finer sand |
| Concrete Mix Design Summary (Basic Considerations) | – Cost; – Strength; – durability; – Workability; and – Segregation |
| Concrete Mix Design Summary (ACI Mix Design method) | 1)Decide slump; 2) Determine max agg size; 3) Estimate water & air; 4) Select w/c ratio; 5) Calculate amount of cement; 6) Estimate coarse agg amount; 7) Calculate amount of fine agg; 8) Adjust for moisture content; and 9) Trial batch adjustments |
Created by:
Asher - S
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