Concrete Flashcards
What are Clinkers?
Cement raw materials finely ground and carefully proportioned before heated to temperatures from 2,000 to 3,000 degrees (F) to from hard pellets
What is the composition of concrete
Mixture of aggregates and often controlled amounts of entrained air and held together by a hardened paste made from cement and water
What is air entrainment
Air entrainment is the intentional creation of tiny air bubbles in concrete.
What are the 3 components of concrete
Water
Aggregate: Sand/gravel
(Portland) Cement
What is the most commonly used cement in the military
Portland Cement
What is the chemical reaction called when water and cement mix?
Hydration
What is the optimal temperature for cement hardening
73 degrees
What 2 functions does water play in concrete
- Effect hydration
* Improve workability
What determines the strength of concrete
ratio of cement to water
Does more water or less water strengthen concrete
less water
Aggregates make up what total volume of finished cement
60-80%
What is the primary purpose of aggregate in cement
To gain economy
What are the most common contaminating materials in concrete
dirt, clay, salt
What are the rules of thumb for the maximum size aggregates should be in concrete
- The maximum size should not exceed 1/5 the minimum diameter of a wall or similar structure
– Not to exceed 1/3 the slab thickness
– Not to exceed 3⁄4 of the clear space between reinforcing bars
What are the 5 types of Portland cement
– Type I (Normal Portland) – Type II (Modified) – Type III (High early) – Type IV (Low heat) – Type V (Sulfate resistant)
What are 3 other cements
• White Portland • Air Entrained (Type I, II, III) • Oil Well Cements • Waterproofed Portland cements • Plastic cement
What does a bag of Portland Cement weigh
– Bags-94 lbs (contains one cu ft of loose
measurement)
How many and what are the types of CONCRETE
- Reinforced
- Prestressed
- Precast
- Lightweight
What are the benefits of precast concrete
- In dead-loads making savings in foundations and reinforcement - Improved thermal properties - Improved fire resistance - Savings in transporting and handling pre- cast units on site - Reduction in formwork and propping
What are the disadvantages of concrete
- Low tensile strength
- Expands and contracts
- Shrinks when dry
- Heavy
What do construction joints do?
join concrete that is paved at different times
Do construction joints allow for movement of concrete members
No
What do contraction joints do
control natural cracking from stresses caused by concrete shrinkage, thermal contraction, and moisture.
At what angel are transverse contraction joints cut
At a right angle to the pavement centerline and edges.
What do isolation joints do
separate the pavement from objects or structures, and allow independent movement of the pavement, object or structure without any connection that could cause damage.
Where are isolation joints used
where a pavement joins sidewalks and buildings, and intersects other pavements or bridges.
What do expansion joints do
Permit volume change movement of a concrete structure or member.
What is the minimum width of an expansion joint
1/4′′ wide
1 person can manually mix approximately how much concrete per hour
1 cubic yard
Describe the hand mixing sequence of concrete
Three part gravel, then Two part sand (mix dry 3x), then One part Portland cement, and add water
Project Volume (PV) = ?
length x width x height x number of pours
Loss Factor = ?
if PV 5400 cubic ft (or 200 cubic yards), add 5% to project
Loose Volume (LV) = ?
LF x 1.5 (and round up)
Equation for calculating cement from Loose Volume (LV)
1/6 x LV = number of bags of cement (rounded up)
Equation for calculating sand from Loose Volume (LV)
2/6 x LV = cubic feet (yards) of cement
OR
LV/3 = cubic feet (yards) of cement**
**This may be “funny” math
Equation for calculating gravel from Loose Volume (LV)
3/6 x LV = cubic feet (yards) of gravel
OR
LV/2 = cubic feet (yards) of gravel
Equation for calculating gallons of water from number of bags of cement
Water (in gals) = number of bags of cement x 8
Forms must be what in order to hold concrete properly until it sets
Tight, rigid, and strong
What materials are forms usually made of
Wood, metal, earth, fiber
What are the elements of wooden forms
Sheathing, batten, yoke, and bracing
What does the yoke do in a wooden form
ties the top of sheathing together in column form
What is the difference between a vertical keyway and a horizontal keyway
one is vertical and the other is horizontal
What is the maximum rate of filling concrete to avoid excessive pressure on form
4 ft per hour
How deep does the land under a slab need to be moistened (if plastic isn’t being used as a barrier between the slab and the ground)
6 inches
Slabs which will support load bearing walls must be what?
Reinforced with rebar
True or False: Rebar in slabs must sit above the frost line
False. Must be below frost line
