Making & Placing Concrete Flashcards
What are the six steps of concrete construction?
- mixture design to meet specification
- batching of ingredients and mixing of concrete
- handling and placement of concrete
- consolidation procedures
- finishing procedures
- curing of concrete; hydration and development of the microstructure
How is concrete ordered and priced? (3)
- concrete is typically measured, ordered, and priced by the cubic yard
- the typical transit-mix truck can mix and deliver up to 10 cubic yards
- wet concrete weighs approximately 150 lbs per cubic foot or 4,050 lbs per cubic yard
What determines concrete strength? What is the strength of normal and high strength concrete? (3)
- concrete strength varies with the design mix and ingredients
- normal strength concrete: 2,000 to 6,000 pounds per square inch (psi)
- high strength concrete: >6,000 psi to roughly 20,000 psi
What is required to produce high strength concrete, and what advantages does it possess? (5)
admixtures and supplementary cementitious materials are required to reach higher strengths
- reduces column sizes in tall buildings
- achieves higher earlier strength, allowing construction to proceed quicker
- satisfies more stringent structural requirements
How does the age of the concrete relate to its strength? (5)
- one day: 16% strength
- three days: 40% strength
- seven days: 65% strength
- 14 days: 90% strength
- 28 days: 99% strength
How is concrete normally mixed? (3)
- most concrete is prepared at batch plants and delivered to the construction site in transit-mix trucks
- the concrete ingredients are mixed in the rotating drum of the truck so that the concrete is ready to be placed upon arrival at the construction site
- smaller batches of concrete may be prepared on site by hand or with the aid or portable power mixers
Which part of the ASTM refers to slump testing, and how are these tests performed? (4)
- ASTM C143
- used to determine workability/consistency of fresh concrete
- concrete with too low of a slump may be stiff and difficult to place
- specified maximum slump is usually in the range of 3-5 inches
Which part of the ASTM refers to compression testing, and how are these tests performed? (4)
- ASTM C39
- samples gathered from each batch of concrete delivered to the site
- concrete is placed in a cylindrical mold and consolidated to eliminate voids
- cylinders are returned to laboratory, properly cured, and tested at appropriate times to verify the compression strength of the concrete
What are the most important factors in placing concrete, and what methods are used to do so? (4)
- avoid delays; place ASAP
- depending on conditions, concrete can be placed up to 90 minutes after mixing commences
- may be placed on site directly from the discharge chute of a transit-mix truck
- by combination of wheelbarrows, power buggies, crane-lifted buckets and pump trucks
What is aggregate segregation, and how can it be avoided? (4)
separation of large aggregate from the finer portions of the concrete mix
- place concrete as close to final position as possible
- do not push or drag concrete over large distances
- avoid dropping concrete from high heights
Why is concrete consolidated, and why should over-consolidation be avoided? (3)
- consolidation eliminates voids and air pockets within the concrete through hand rodding/tamping, screeding, and vibration
- especially critical with stiff concrete mixes
- over-consolidation can lead to segregation as larger aggregate descends and smaller aggregate rises towards the surface
Why is concrete finished? How is this accomplished, and when does it occur? (4)
- finishing is required for exposed surfaces
- various surface textures can be specified to add surface traction/friction or for architectural conditions
- expansion/construction joints are made to minimize cracking
- typically finishing steps start after surface bleeding and bleed water starts to evaporate
What is curing, and why is it important? When does curing occur, and how is it accomplished? (6)
- concrete hardens through chemical bonding of water and cement
- if concrete dries out prematurely, the hydration process stops and maximum strength is not achieved
- concrete strength is normally specified at 28 days
- however, concrete strength and durability can continue increasing at diminishing rates for years
- exposed surfaces of newly-poured concrete must be protected from water evaporation
- concrete may be regularly misted, covered with moisture-retaining materials, or treated with a chemical surface sealer
What are some tips for manufacturing and placing concrete in cold weather? (4)
- concrete must be protected from freezing
- pre-heating ingredients
- provide cover and temporary heat
- use admixtures to adjust the cure rate
What are some tips for manufacturing and placing concrete in hot weather? (6)
- start early
- add water as ice
- use admixtures to adjust the cure rate
- provide temporary shade or wind breaks
- provide extra labor
- bubble liquid nitrogen through the mix at the batch plant
