Concrete

1
Concrete

• AGEN 145
• Paul Crovella

2
Concrete Components

• Concrete is a mixture of
• Large Aggregate (big stones cheap)
• Small Aggregate (smaller stones a little more
  expensive)
• Sand
• Water
• Cement (the most expensive component)
• Air

3
Concrete mixes

• The components should be mixed to provide an
  adequate strength, at a reasonable cost.
• Strength is measured in compressive strength
  after 28 days. Fc typically ranges from 3000 to
  4000 psi.
• The strength depends on the final use, and
  whether it will be exposed to weather
• e.g. 3000 psi mix for footings (underground)
• 3500 psi for slabs on grade (loaded in
  conpression)
• 4000 psi for walls (loaded in bending)

4
Concrete strength

• Concrete is very strong in compression, very weak
  in tension-
• Concrete will typically be reinforced with fibers
  or welded wire fabric for slabs, and with rebar
  for walls or heavy duty floors
• All concrete must have a minimum amount of
  reinforcement to resist thermal cracking
• Large expanses of concrete which change temps
  will change size control joints are used to
  control where the crack occurs

5
Concrete curing

• Water and cement combine in an exothermic (heat
  producing) reaction known as hydration. The
  water must be present in order for the concrete
  to cure.
• More water than that needed for curing will make
  the concrete easier to work, but weaken the final
  product.
• Concrete workability measured in inches of slump

6
Concrete curing

• For concrete to cure correctly
• It must not be allowed to freeze
• It must not have the water evaporate out
  concrete can be covered with plastic, wet burlap,
  flooded with a hose, sealed with silicone, etc to
  keep the water in while initially curing (3-7
  days)
• Speed of the reaction depends on the concrete
  temperature dont place concrete when
  temperatures are high flash set will result.

7
Concrete estimating

• Concrete is estimated in cubic yards (referred to
  as yards). One cubic yard is equal to 27 cubic
  feet
• Convert all measurements to feet.
• Determine volume of concrete in cubic feet
• For slabs - Thickness x Width x Length
• For discs Pi x radius x radius x thickness
• Divide cubic feet by 27 to convert to cubic yards
• Normally add at least 10 extra for waste
  multiply by 1.10

8
Practice problems

• How much concrete is needed for a 68 by 320
  barn with a 4 slab?
• How much concrete is needed to fill 50 postholes
  18 diameter to a depth of 6?