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properties
Properties of Materials ch 7
| Question | Answer |
|---|---|
| PCC - your responsibility as an engineer | Proportioning PCC constituents, production, and quality control of fresh and hardened concrete. |
| Proportioning means | Percentage by weight of each of the basic constituents': Portland cement, coarse aggregate, fine aggregate, and water. Use of additives or admixtures are recommended based of the type of job required, and it WILL affect concrete proportions. |
| Quality of PCC is controlled by... | chemical composition of PC, hydration, development of micro-cracks, admixtures, aggregate characteristics. |
| Quality of PCC is also affected by... | placement, consolidation, curing Note: PCC performance is largely affected by methods of mixing, transporting, placing, and curing of PCC in the field. |
| Types of PCC | traditional, lightweight, high strength (high performance concrete), polymer, fiber reinforced, roller compacted. |
| why we use concrete in most engineering structures | available material, relatively cost effective, easy to produce, uses mostly available natural materials, easy to shape, easy to handle |
| input for the design of PCC mixes is either... | strength required by the design engineer, or modulus of elasticity (both are designated by the structural engineer) |
| responsibility of the material engineer for fresh concrete | workability, finishing characteristics, handling, placing, transporting, production |
| responsibility of the material engineer for hardened concrete | strength (f'c or compressive strength), durability, porosity |
| Portland Cement Association (PCA) requirements for the design of PCC mixes | workability/fresh concrete durability, strength, and appearance economy |
| Proportioning PCC mix design steps 1-5 | 1) Evaluate strength requirements 2) Determine water/cement ratio required 3) Evaluate coarse aggregate requirements a. Maximum aggregate requirements b. Quantity of the coarse aggregate 4) Determine air entrainment requirements 5) Evaluate workab |
| Proportioning PCC mix design steps 6-11 | 6) Evaluate the water content requirement 7) Determine cementing materials content and type needed 8) Evaluate the need and application rate of admixtures 9) Evaluate fine aggregate requirement 10) Determine moisture corrections 11) Make and test tri |
| Evaluating strength requirements of PCC | material engineers usually design the PCC mix higher than the designated strength recommended by th structural engineer to take into account the variation in: batching, mixing, material |
| to calculate the strength requirement for concrete mix, the following must be known | specified compressive strength (f'c) standard deviation of concrete (s) Risk measurement |
| risk assessment is specified be ACI as being | 10% a chance concrete will be below specified strength, or f'cr = f'c + 1.34s For larger values of standard deviation, ACI recommends using the following equation: f'cr = f'c +2.33s - 3.45 Note: The larger value from these equations are used in the P |
| Number of samples considered in the calculation of standard deviation should be a minimum of | 30 tests for the use of the f'cr = f'c + 1.34s and f'cr = f'c +2.33s - 3.45 equations to be used. If the number of tests is between 15-30 use the following adjustment factors: Number of Tests Modification Factor, F 15 1.16 20 1.08 25 1.03 30 or |
| If the number of tests is below 15, what do you do? | use equations from the book that are too long to go into detail about here... |
| W/C ratio determined by | compressive strength, exposure conditions, sulfate exposure Table 7.2 in the book is used for small projects, do NOT use for trial mixes as it give conservative values for W/C |
| Coarse aggregate requirements based on | maximum aggregate requirements, quantity of the coarse aggregate Large dense graded will provide most economical mix Maximum allowable aggregate size is limited by the dimensions of the structure and capabilities of the construction equipment |
| Largest maximum aggregate size for form dimensions | 1/5 of minimum clear distance |
| Largest maximum aggregate size for clear space between reinforcement or prestressing tendons | 3/4 of minimum clear space |
| Largest maximum aggregate size for clear space between reinforcement and form | 3/4 of minimum clear space |
| Largest maximum aggregate size for unreinforced slab | 1/3 of thickness |
| PERCENTAGE of coarse aggregate required in the mix is determined by... | nominal max aggregate size, fineness modulus of fine aggregate (lower values indicate finer mix) |
| Air entrainment requirements determined by... | exposure to freeze-thaw conditions *mild, moderate, or severe Deicing materials, aggregate size, workability requirements |
| workability of plastic concrete | ease of placing, consolidating, and finishing of freshly mixed concrete |
| lower workability | lower density |
| proper workability | dense concrete |
| higher workability | segregation, bleeding |
| slump test | ASTM C143 used to measure workability |
| water content depends on | slump value, maximum aggregate size, shape of aggregate |
| Aggregate shape sub-angular | reduction in water content by 12 kg/m3 or 20 lb/yd |
| Aggregate shape gravel with crushed particles | reduction in water content by 21 kg/m3 or 35 lb/yd |
| Aggregate shape round | reduction in water content by 27 kg/m3 or 45 lb/yd |
| cement content and type needed | use w/c ratio, water needed is already determined Note: minimum cement content for concrete exposed to severe freeze-thaw, deicers, and sulfate exposure = 334 kg/m3 minimum cement content for concrete placed under water = 385 kg/m3 |
| admixture need and application rate | in most cases, the need for admixture is stated as a part of the mix proportion and considered in the design amount needed per unit weight or volume are given by the manufacturer. when using more than one type, be careful, the final outcome may be unpred |
| fine aggregate determined by | the weights of all the other ingredients are already known, take total weight, subtract all knowns, what's left is fine aggregate weight |
| moisture corrections | based on the condition of water content of the fine and coarse aggregates, an adjustment of water and aggregate weights are determined (SSD condition is the reference value in this case) |
| trial mixes | based on the proportions found, verification of mix criteria are done through laboratory trial mixes to measure: compressive strength (three samples minimum), slump, air content if results are not verified, adjustments are to be made and tested again |
| mixing PCC | hand mixing, batch or continuous mixture mixers vary in size from 1 to 9 cubic meters |
| mixing time | (# of revolutions) varies depending on size and type of mixer. Over mixing causes segregation and early hydration. Under mixing causes non-uniform concrete |
| where is concrete mixed? | in central plants and delivered to the site by trucks in most cases concrete should be delivered to the site within 90 minutes of mixing |
| Methods of mixing | central mixed PCC (2-6 rpm) shrink mixed PCC (4-16 rpm) truck mixed PCC (4-16 rpm) |
| Depositing concrete | concrete should be deposited continuously as close as possible to the final postition |
| pumped concrete | slumps should be between 40 to 100 mm |
| consolidation of concrete is used to | reduce entrapped air (vibrators) |
| finishing placed concrete | straight edge, power float |
| curing | the process of maintaining satisfactory moisture content and temperature in the concrete for defined period of time |
| methods of curing | pond or immerse, spraying or fogging, wet covering, plastic sheets, membrane compounds, forms left in place, steam curing, blankets and covers, electrical, hot oil and infrared curing |
| minimum curing period depends on | type of cement, mixture proportions, required strength, ambient weather, size of element, shape of element (exposure conditions), method of curing Note: concrete should be cured at a minimum temperature of 5 C for a period of minimum 7 days or concrete a |
| What causes early volume change of PCC? | plastic shrinkage: loss of water from cement paste while concrete is in its plastic stage (about 1%) drying shrinkage: shrinkage after setting time Note: shrinkage usually causes cracks on the surface |
| shrinkage increases with | lack of curing, high w/c ratio, high cement content, low coarse aggregate content, lack of reinforcement (rebar's), aging |
| creep | gradual increase in strain with time under sustained load (long term process) |
| PCC permeability affects | concrete durability through the penetration of water and chemicals into concrete causing deterioration, disintegration, and corrosion of reinforcing steel |
| stress strain relationship of PCC | higher w/c ratio, lower strength |
| cube compaction | look it up |
| tensile strength | considered as close to zero in concrete. tested by applying load to a cylinder placed on its side (split tension test) |
| flexural strength | like a 3 point bending test |
| rebound hammer test | non destructive testing of hardened concrete (very inaccurate) |
| other tests | penetration test, ultrasonic plus velocity test, maturity test |
Created by:
mbreeden
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