School Name: Las
Vegas Homeschool Team (Middle School)
Area of Science: Engineering
Title: Storming the Industry: The
Effect of Fun Additives to Prevent Concrete Damage due to Freezing and Thawing
Question: What is stronger than concrete and
steel?
Answer: Ice J
Introduction
Winter is a time when concrete is
damaged. Water from melted snow and ice is
absorbed into the concrete. When the temperature drops below freezing, the
water turns into ice and expands with force.
That force is powerful enough to exceed the “yield point” of the
concrete and crack it. This problem is very
well known in the industry.
Industry
When it is freezing outside, one
of the steps Mr. Hank Hern, the owner of HHH Construction, does is carefully cover
his freshly poured cement with plastic sheets.
He does this so that it won’t harden too soon or develop weak areas inside. Once it’s set, however,
Mr. Hank’s work is as vulnerable to the freeze-thaw cycle as any other
building, driveway or sidewalk. Billingsly
Engineering in Las Vegas is spending a lot of
time and money trying to figure out how to make New Mexico’s bridges and highways
safer. This is because moisture that soaks
into concrete, and then expands as ice with the same
force, can weaken a bridge and may cause it to crack under pressure, which
would be very dangerous. The engineers
try to prevent “spalling-“ the breaking off of concrete that exposes rebar, and
“delamination-“ longitudinal cracks along the underside of a bridge; by testing
concrete samples for durability. They do
this by following different recipes for making concrete and then comparing how durable
they are. These tests are very intense
and interesting to watch, and that’s where I got my idea from. I wanted to study whether I could improve
concrete strength by adding new materials that would help form a stronger bond. I picked things that we throw away as trash
and that we have a lot of in New
Mexico; plastic bags, dryer lint, and pinion shells.
Moisture
How does the water reach concrete
surfaces in the first place? Well, the
answer is, this happens naturally. In
the winter, water can freeze in many different forms just depending on the temperature,
the amount of humidity is in the atmosphere, and how hard the wind is blowing. Here are few of the examples I found in my
book.
Frost
occurs when water vapor in the air comes in contact with concrete driveways or
sidewalks that are already frozen. Frost
looks like a coating of fuzzy-appearing crystals (p.572).
Glaze
happens when supercooled water lands on a surface that is at freezing
temperature or below freezing. When this
happens, a smooth clear coating is formed.
Even though it’s thin, glaze can freeze and thaw in layers and become dangerously
slick (p.572).
Hail
is a type of precipitation that falls like rain but in the form of balls or
lumps of ice. Balls are formed before
falling to earth. This happens when
moisture moves up and down in the clouds and that causes the balls to grow in size. According to my book, hailstones can be as
small as peas to the size as grapefruits (p.572).
Sleet
is tiny pellets of frozen water that looks like granular snow. It falls like rain to create a coating that
makes surfaces like sidewalks and bridges dangerous for walking and driving on (p.572).
There are many different ways in which
moisture naturally reaches concrete, these are just a few. However it happens, freezing water can
negatively affect concrete durability. This
is because concrete is permeable. When it’s
drenched with water and then cooled below 00C, concrete cracks internally. Ongoing freezing and thawing causes the cracks
to grow and may eventually lead to “macroscopic degradation,” the scientific
name for “ice damage” (Schulson, Erland M).
Problem Definition: Macroscopic
Degradation
Why are some concrete things more
affected by macroscopic degradation than others? The basketball court across the street from
my house, for example, has a snake-like crack about 15 feet long. We can still play but have to be
careful. The concrete parking lot at my sister’s
school, however, has a round pothole about eight inches deep. Parents picking up their kids or dropping
them off have to drive all the way around it; or smash up their car. Potholes and cracks are examples of “differential
weathering” and it has to do with the difference in resistance of various rock
types (p. 587). In construction, workers
add water, sand and gravel (usually different sizes & shapes of rock) to the
cement. These ingredients bind together
to form concrete. When chalk and clay
has been heated and crushed, Portland cement is made. When water is added, it crystallizes into a
mass that binds sand and gravel together (p.102). Sometimes special additives are added. As the concrete hardens, sand and gravel are
joined together by the cement and form a rocklike mass with amazing “compressive
strengths” (p.102). Compressive strength
refers how much weight or pressure it can support. The workers at Billingsly Engineering test
compressive strength using a hydraulic ram.
The hydraulic ram puts pressure on a concrete samples until they break. Compressive strength is measured in
megapascals (MPa) or pounds per square inch (psi). This test takes place after
the concrete has had a chance to harden.
Normally, that’s twenty-eight days.
Twenty eight days is a long wait, especially if there’s an urgent
need. Sometimes three-day and seven-day
strengths are used to predict 28-day compressive strengths.
Road Construction
Roadway pavement is either
flexible or rigid (p.71). Flexible
pavement is made up of sand and gravel mixed with a tarry material called asphalt. The base of Flexible pavement consists of a thin
layer of asphalt that fills in the cracks.
Like a lasagna, roads consist of several layers
of harder asphalt, and topped with gravel or stone chips. Another method uses a layer of gravel about 1
to 3 inches thick placed directly on the base.
Heavy rollers then smash down the stone layer and then heated tar is
sprayed on to hold everything in place.
Rigid
pavements are the other type of roadways in New Mexico.
Rigid pavement is made of concrete that is 6 to 12 inches thick that is
on top of a layer of compacted gravel. Another
problem is that concrete shrinks when it is cold outside and expands when hot. This too can cause cracks.
Use of Additives
The workers at Billingsly
Engineering test special ingredients in their lab hoping to decrease cold
weather damage out in the field. When
water freezes and turns into ice, its volume expands by about nine percent. If
there is not enough room for this expansion, the ice will break the needles of
the interlocking cement paste crystals.
Additives are poured in the cement mixer that create
small air bubbles inside the concrete when it dries. These air bubbles provide a
place for expanding ice crystals to relieve it’s
pressure.
Research Objective
There are many things that we
normally just throw away that might offer improved workability and resistance
to spalling, delamination, and macroscopic degradation. The primary purpose of this study is to
determine whether adding additional items improves the durability of hardened
concrete.
Methodology
Permission will be requested to
remove enough cement from the mixer to fill 6 forms. The following ingredients will be added:
Form
1: Cement + shredded Walmart bags
Form
2: Cement + dryer lint
Form
3: Cement + pinion shells
The other three forms will be
identical to the ones above but will be sprayed with melted Walmart bags before
being put in freezer.
Form
4: Cement + shredded Walmart bags,
sprayed with
melted Walmart bags
Form
5: Cement + dryer lint, sprayed with
melted
Walmart
bags
Form
6: Cement + pinion shells, sprayed with
melted
Walmart
bags
Once hardened, these six blocks
will be put into the freezer with the other samples from the original
batch. To imitate the freeze-thaw cycle
that occurs in the field, they will be frozen then thawed 300 times. (Actually, that’s probably more intense than
what happens outside). Finally, all the
blocks, including my six experimental ones, will be tested for durability.
Data Analysis
How can computers help us understand?
We will be using a program called
“Mathematica” to add, subtract, divide and compare the data that we
receive. The numbers will tell us which
of the blocks are most durable.
Progress to Date
Our methodology has been approved
by the owner of Billingsly Engineering.
We’re not sure how were going to melt the plastic bags but will figure
that out by Thursday, February 22.
That’s the day we’ll set our forms.
We’re currently practicing with Mathematic on “fake data” to try to
learn the program.
Team Members: Moriah Scott (age
11) & Elena Martinez (age 12)
Sponsoring Teacher: Rey Martinez
Consultants: Mrs. Leanne
Billingsly, Engineer; Dr. Tahani Hossein, Computer and Mathematical Sciences
Department, New Mexico
Highlands University.
