Lecture #5 - Fresh Concrete Flashcards
Name the three ways to order concrete.
common, prescription, performance
What does the owner specify in common ordering?
type of cementing materials, class of exposure the concrete must withstand, compressive strength needed for structural requirements, maximum size of coarse aggregate, air content, slump at discharge point, admixtures and any other properties
What does the owner do in common ordering?
certify the plant, equipment and materials to meet the standardds, assure the mix proportions will produce the specified the required concrete, assure the concrete strength will meet the required standards
What must the owner specify for prescription ordering?
everything from common ordering, amount of cementing material, propertion of fine and coardse aggregate by mass and total water content (all for one cubic meter of concrete)
What must the supplier do for prescription ordering?
certify the plant, equipment and materials to meet the standards
What must the owner specify for performance ordering?
exposure class of conrete, architectural, structural and durability criteria
What must the supplier do for prescription ordering?
certify the plant, equipment, and materials to meet the standards, ensure the performance criteria are measured and recorded, ensure the concrete complies with the performance criteria, certify the slump and workability measures
Where is most concrete prepared and how does it get to the construction site?
batch plants, in ready mix trucks
Define shrink-mixed conrete.
concrete mixed partially in a stationary mixer and completed in a truck mixer
Define truck-mixed concrete.
concrete mixed completely in a truck mixer
What is a concrete mixer for?
small batching
How can concrete be placed?
with a chute, bucket or hopper, pump, wheelbarrow
What is workability?
the ease of placing, consolidating and finishing freshly mixed concrete and the degree to which it resists segregation
What are bleeding and settlement?
the development of a layer of water at the top or surface of freshly placed concrete caused by the settlement of solid particles (bleeding is a form of segregation)
What is consistence?
the wetness of concrete, segregation and excess bleeding can occur if a mix is too wet, a mix that is too dry will be difficult to place and compact
What consumes wasted energy (workability)?
vibrating the form, vibrating the compacted concrete
What are the voids in concrete?
bubbles of entrapped air due to the grading of the mix or wetter mix, spaces after the excess water has been removed
What factors affect workability?
water content, aggregate type and grading, aggregate/cement ratio, admixtures, fineness of cement, time, temperature, air-entrainment, water reducer (plasticizer), superplasticizer
What tests are used for fresh concrete?
slump test, air test, temperature, samples cast in the field
What is the slump test?
a rough measure of the workability of wet concrete as it arrives on site, concrete is placed into a conical cylinder; the cylinder is removed and the loss in height of the concrete mass is measured, low slump=difficult to place, high slump=too much water
How can you control segregation?
place concrete as close to the final position as possible because if it moves too much it is likely to segregate
Why must concrete be consolidated in the forms?
to eliminate trapped air and to completely fill the space around the rebars and in all corners of the formwork
How do you get concrete to inaccessible places in the forms?
pump it through hoses
What can you use to consolidate concrete?
repeatedly thrusting a rod, spade, or immersion type vibrator into the concrete at closely spaced intervals throughout the formwork
What does poor consolidation result in?
porous, weak concrete with poor durability
