Reinforced Concrete Exam 1

What is a reinforced concrete?

A composite material that uses high compressive strength of concrete and high tensile strength of steel reincorcement

How does concrete react to forces?

Weak in tension (will have brittle failure), strong in compression

Tensile strength is 8-15% of its compressive strength

Where are the stresses in concrete when load is applied on top of it?

Compression at top, tension at the bottom

Where do we add reinforcing?

In places that have tensile stresses

How does reinforcing steel react to forces?

Strong in tension (to make up for concrete’s weakness), so-so in compression (slender members buckle, which is a brittle failure)

What is the key design assumption for reinforced concrete?

The bond between the concrete and steel is perfect, and they deform together

How does concrete form?

Through a chemical reaction

What are the material properties of concrete?

It is neither homogenous (different materials) nor linear (not fully Hooke’s Law). The properties vary depending on many factors, like quality, curing, admixtures, etc

What are design parameters for concrete based on?

Considerable research and testing efforts, and many design relationships are empirical

After mixing the concrete, what is important to consider?

Concrete generates hear during curing from the hydration process, and can cause chemical burns. It also cracks, which is why we cure and reinforce it.

What are advantages to reinforced concrete?

Economical short and long term (cheap and available), form into any shape, fire resistance, low maintenance.W

What are disadvantages to reinforced concrete?

Low tensile strength of concrete, forms and shoring (wasted material, labor intensive, time consuming), low strength for its weight, time-dependent volume changes (creep and fatigue), consistency of mixes and their strength

What are some types of reinforced concrete structures?

Normal/Mildly/Non-prestressed Reinforced Concrete, and Prestressed Concrete (Pretensioned b4 cast, Post-tensioned after cast)

How does normal reinforced concretes deform?

Internal stresses caused by external loads, steel passively provides strength

How does prestressed reinforced concrete deform?

Internal stresses introduced by compressing steel so that tension from service loads can be counteracted

What are some ways to form concrete?

Cast-in-place, precast, tilt-up

What are the main ingredients of concrete?

Portland cement, large aggregate (gravel), fine aggregate (sand), water, air, admixtures

What is Portland Cement

Mostly limestone blended and heated to 2600-3000F in a kiln, which releases CO2. The resulting clinker is ground into a “dust-like” consistency

How much of global carbon dioxide emissions is concrete?

8%

What are the 5 types or portland cement?

1. Normal

2. Moderate sulfate resistance - less cracks

3. High early strength - gets hotter faster

4. Low heat of hydration - slower strength gain and lower temperature rise

5. High sulfate resistance

They all reach the same strength

What affects aggregate strength?

Rock type (strong = felsite, traprock, quartzite; med = limestone, granite; weak = soft rocks), shape (angular best), grading (well-graded = less voids = stronger

How does water affect concrete?

NO SALTWATER bc corrode, high w/c ratio = low strength

What is air entrained concrete?

Concrete that has 5-8% volume of air to let water expand during freeze-thaw

what happens when cement materials are mixed?

Water and cement causes a chemical reaction (hydration), and the mixture hardens, heat is produced, and strength increases

Is making concrete easy?

No, it is highly dependent on the handling process and requires great quality control

How do you test compressive strength of concrete?

Take a cylinder, moist cure it for 28 days, test in UTM, divide highest load by area of cylinder, get between 3000-5000 psi if as-designed

How many tests goes it take to get a normally distributed curve?

More than 30 because it is highly variable

What happens if 15 to 30 tests are taken?

Must use equations from 30+ tests and adjust the answers

What happens if less than 15 tests are take?

New equations

What are the major factors affecting concrete strength?

w/c ratio, type of portland cement, aggregates, placement and curing conditions, age

What are the two ways concrete can fail?

1. When concrete is loaded above critical stress and left in place, the concrete will creep to failure

2. When concrete is cyclically loaded to above the critical stress, the concrete will fail from fatigue

What tests determine concrete’s tensile strength?

Flexural test (modulus of rupture) and split cylinder (splitting tensile strength)

What is creep?

An increase in concrete strain under constant load over time (reinforced steel CANNOT creep so it stops the cement from it)

What is shrinkage?

A decrease in volume during curing

• Drying shrinkage = loss of absorbed water

• Carbonation shrinkage = reaction between cementitious materials and air

What affects shrinkage?

• Water content (more = more)

• Cement content (more = more, aggregates restrain shrinkage)

• Cement fineness (finer = more SA = more)

• Member shape (large volume + small SA = less)

• Relative humidity (Largest for RH < 40% - partially recoverable)

• Duration of moist cure

What are the two types of reinforced steel bars?

Smooth bars = chemical bond

Deformed bars = mechanical and chemical bonds

What did they use in ancient Egypt?

Gypsum mortar heated to 130C to dry, used as plaster, very soluble

What did the ancient Greeks use?

Lime mortar as plaster that was heated to 1000C, very soluble in water

What did the Romans use?

Lime mortar with volcanic ask, stronger and more water-resistent because it didn’t dissolve

What the The Pantheon use?

Formed concrete into a 144’ diamater dome

What happened in the Middle Ages?

We forgot how to make concrete

When did concrete reappear?

1300s

What is the Eddystone lighthouse?

A lighthouse made with burned limestone with clay and ash, was set under water (hydraulic cement)

Who was John Smeaton?

The first “Civil Engineer”, considered the first “expert witness”

Who was Joseph Aspdin?

Refined cement process (burned in kiln more than before), ground up the clinker, patented Portland Cement

Why is it called Portland Cement?

The Cenotaph in England is made from Portland stone, a stone from Portland island, and Aspdin’s cement looked like it, so he named it Portland Cement

Who was Joseph Monier?

Credited with first use of reinforced concrete to make a flower pot with wire and bars