What is concrete finishing of flatwork?
Finishing concrete is the operation of
consolidating, leveling, and creating a concrete surface of
desired texture and hardness. The finish should be strictly
functional or decorative. Finishing makes the concrete
attractive and serviceable. The final texture, hardness, and
joint pattern depends on the concrete's end use.
When should concrete be cured?
As soon as possible after finishing the concrete.
When should finishing begin?
You should wait until all bleed water has
evaporated and concrete is firm enough to leave only 1/4" deep
footprint. Early finishing can cause crazing, dusting,
blistering and scaling. Outside concrete should not be steel
troweled. Bull float and broom finish is recommended. Use air
entrained concrete (5-8%) for freeze-thaw protection. Never
sprinkle water or cement on concrete while finishing it. This
may cause dusting or scaling. Never use a jitterbug. Fog
spraying is also effective during hot weather and windy weather,
but impractical in most cases. Evaporation Retardant is an
effective alternative. Saw cut joints as soon as practical,
especially during hot weather.
What is a control joint?
Control joints create weakened sections to
control crack locations and accommodate movements, caused by
temperature changes and drying shrinkage, and prevent vertical
but permit slight horizontal movement. NOTE: Strength and
durability factors. Adding 1 gallon of water to a cubic yard of
concrete will increase the SLUMP about 1". Decrease COMPRESSIVE
STRENGTH ABOUT 200 to 300 PSI. Increase SHRINKAGE potential
about 10%. Waste AS MUCH AS 1/4 bag of cement and proportionally
affects DURABILITY.
What are the common causes of surface
scaling?
Excessive slump, overworking of wet concrete,
premature finishing operations, inadequate or improper curing.
Little or not entrained air due to: failure to use Air
Entrainment Admixture agent, air worked out by overworking over
wet concrete or premature finishing.
What is the solution to surface
scaling?
Always use air entrained concrete for outside
work and indoor surfaces exposed to vehicular traffic; 5-8% is
right for most mixes. Use Low Slump concrete (4" max) and w/c
ratio no greater then 0.50
What are the two basic causes of cracks
in concrete?
(1.) Stress due to drying shrinkage or
temperature changes. These types of cracks are an inherent,
unavoidable property of concrete. Though unavoidable, they can
be controlled.
(2) Stress cracks due to applied loads. These are settlement
cracks and can be avoided by having a proper sub-grade, slab
design and proper concrete mix designed for the load.
What tests should you know about for
proper concrete placement?
Slump test (ASTM C-1431) Air Content (ASTM C231)
Temperature test (ASTM 1064). Call or write for a booklet of 40
pages concerning Concrete Placement Procedures.
