Mechanics Solids 1 - Lectures (1-8)

Common civil engineering materials are…

steel, mineral aggregates, concrete, masonry, wood and soil  

High performance materials do….

• Higher in strength

• Lower Cost

• Improve durability

Classification of materials

• Metals

• Polymers

• Ceramics

• Composites

Material Selection Consideration

• Economics

• Mechanical Properties

• Non-mechanical Properties

• Production

• Aesthetics

• Sustainability

Yield stress rules of thumb are…

• 0.2 % offset 

• 0.5 % extension

Engineering Stress considers…

• Original Length 

• Original Area

True stress considers…

Calculus to find the area and length at specific points in time (instantaneous)

Toughness is…

Material’s ability to absorb amount of energy before failure or collapse

Modulus of Resilience is…

• The are under the elastic region

• Maximum energy an object can absorb before permanent deformation

Poisson’s Ratio is…

The ratio between the lateral strain over the longitudinal strain

Visco-elastic Materials are…

Mainly affected by temperature (Plastics or Polymers)

Creep is…

deformation caused in an object due to some external load that has been placed and not removed for a period of time

Fatigue is…

Deformation caused in an object due to constant cycling and un-cycling force

Fatigue life is…

Amount of cycling and un-cycling an object can sustain before reaching failure

Endurance limit is…

Stress below which fatigue failure will never occurred

Sieve Size

• Coarse Aggregates > 4 Sieve

• Fine Aggregates < 4 Sieve

• Sieve 4 is 3.75

Aggregates are…

• Mass of crushed lime stone

• Gravel

• Sand

Concrete is…

Coarse aggregate, filled with cement to close all the inner gaps

Why add aggregates…

• Reduce Cost

• Improve Stability

• Improve Durability

• Improve Strength

Natural Aggregates are…

• Sand

• Gravel

• River rocks

Manufactured Aggregates are…

• Steel slag

• Crushed waste

• Manufactured sand

Aggregates by density…

• Heavyweight = 2500-4000 kg/m³

• Normal weight = 2150-2550 kg/m³

• Light weight structural =

  • Semi low = 1850 - 2150 kg/m³

  • Low = < 1850 kg/m³

• Light weight non-structural= 250-1450 kg/m³

Properties of Aggregates are…

• Gradation

• Strength and E

• Moisture Absorption

• Soundness

• Abrasion Resistance

Gradation 

Describes particle size distribution for aggregates

• Open graded - Poor

• Gap graded - Intermediate size omitted

• Dense graded - Good

Fineness Modulus

• Nominal 2.3 - 3.1

• Higher FM = Coarser Aggregate

• Lower FM = Finer Aggregate

Strength and E affected by…

• Higher moduli = good quality aggregate

• Porosity dictates the strength of aggregates

Moisture Absorption in aggregates affects..

strength of mix and has a huge role on proportioning mix 

Aggregates shapes…

• Round - Less strength but improves workability

• Sharp - More strength, harder workability, needs more water

Aggregates Soundness…

• Ability to withstand weathering

• Ex.

  • Freeze-thaw in cold climates

Aggregates Abrasion Resistance

Ability to resist damage caused by external loads

Cement Paste is… 

Cement + Water

Mortar is…

Cement paste + sand

Concrete is…

mortar + gravel

Concrete contains…

• Portland cement

• Aggregates

• Water

• Air Voids

• Admixtures

Portland Cement is made out of two main materials…

• Calcareous - Lime bearing

• Argillaceous - Clay materials (Silica, Alumina and Iron) 

Portland Cement production process is…

1. Collect aggregates from quarry, crush and blend them together.

2. Crushed aggregates are placed on a kiln at 1650C where a chemical reaction occurs, separating limestone and carbon dioxide.

3. After, the kiln generates marble size objects called “clinkers”

4. Clinkers get taken out and 5% gypsum is added

5. They are crushed and blend together to form Portland Cement.

Clinkers are made out of…

• Alites (C3S)

• Belites (C2S)

• Aluminate (C3Al)

• Ferrite (C4AlF)

Hydration is…

A chemical reaction that hardens the cement paste

C-S-H gel is…

Calcium-Silicate-Hydrate, a strong paste that acts as binder for cement paste. Made out of alites and belites.

Hydration Setting and Hardening process…

1. Cement paste is fluid

2. Hydration reaction

3. Increase hydration, decrease in water-filled space

4. Solid matrix becomes more dense and rigid

Hydration initial and final set

• Initial - 1 mm needle is inserted 25mm down the cement paste for 30 seconds until it penetrates less than 25mm

• Final - Needle does not penetrate at all

Alite (C3S) is…

• The most important cementing compound

• Gives early strength

Belite (C2S) is…

• Much slower reaction

• Long term strength

Ferrite (C4AlF) is…

• Little strength

• Reduces temperature to produce alites

Aluminate (C3Al)

• Not liked by engineers since is susceptible for sulfate attacks.

• Responsible for initial set

Sulfate attacks are…

When sulfate enters concrete and reacts with aluminate causing expansion and cracking

Fineness Cement is…

Overall particle size distribution measured by surface are per unit mass

Max size of cement paste is…

0.09 mm

Portland Cement Types…

• Type 1:

  • No requirements of any sort

  • Susceptible to sulfate attacks

  • Used in pavements, sidewalks

• Type 2:

  • Moderate sulfate and heat hydration needed

  • Improved sulfate resistance by lowering aluminate

  • Used in mass concrete works

• Type 3:

  • Early strength is needed

  • Highest heat hydration and more alites added

  • Used in cold weathers and emergency repairs

• Type 4:

  • Long term strength is needed

  • Low heat hydration

  • Low aluminates

  • Used in hydro-dams

• Type 5:

  • Severe sulfate attacks

  • Used for underground water lines and pipes

Workability is…

Ease to mix, work and use the cement paste

Water Content (w/c)…

• In theory: 0.25

• In practice: 0.35 - 0.40

• Excessive water will create air-pockets weaking the compressive strength of concrete

Amount of water mix should account for…

• Hydration

• Curing and Evaporation

• Absorption by Aggregate

• Workability

Slump test…

1. Invert Cone

2. Fill cone with 3 layers of fresh concrete

3. For each layer of concrete, mix 25x

4. Scrape excess away

5. Lift off cone

6. Measure vertical height decrease

Non-potable water needs to…

Meet potable water ratios by 90% or above

Effects of using non-potable water…

• white stanning

• corrosion of steel

• volume instability

• reduce durability

Admixtures are…

anything other than aggregates, water and cement

Types of Admixtures… 

• Air-Entrainers

• Retarders

• Accelerators

• Water-Reducers

Why add admixtures…

• Lowers costs

• Maintains quality of cement paste through its process

• Achieves certain properties more efficiently

Air-Entrainers

• Improve freeze-thaw cycle durability

• Tiny air bubbles into concrete

• Entrainers (1mm<) Entrapped (1mm>)

Water-Reducers

• Improve workability without increasing the w/c ratio

Super-placizers…

• 3-4x more effective

• Particles are more workable

• Creates flowing for 30-60 min

Accelerators…

• Increase rate of heat hydration (strength)

• Reduces curing time

• Used in cold weather

Retarders…

• Offer extra-time between mixing and pouring

• Lower heat hydration

• Used in hot temperatures

Pozzolan are…

Siliceous materials that react with CaOH released by hydration to form C-S-H.

Proportioning is…

Determine the quantity of concrete ingredients

Order of Concrete Proportioning….

1. Mix Design

2. Trial Mix & Design

3. Batching

4. Mixing

5. Transportation

6. Pouring

7. Vibration

8. Finishing

9. Curing

10. Maintenance

Methods of Proportioning…

• Arbitrary Volumetric: Mix ingredients and water until desired outcome

• Absolute Volume: Using density values for ingredients to calculate the absolute volume each element will occupy per unit volume of concrete.

• Field data

• Trial mixes

Quantity that you are normally designing for in m³…

1m³

Design Tables are for…

Normal concrete mix

Steps for Concrete Design…

1. Select Slump

2. Required Strength

3. Select Coarse Aggregate

4. Estimate Water and Air Content

5. Estimate w/c ratio

6. Weight of Cement

7. Estimate Admixtures 

8. Estimate fine aggregates 

9. Adjust for aggregate moisture 

Curing is…

The process of maintaining satisfactory moisture content and temperature in the concrete for a period of time.

Curing time…

3 days minimum at 10C to reach 40% strength

Curing affects…

• Durability

• Strength

• Water-tightness

• Absorption Resistence

Curing temperature effects…

• High Temperature:

  • Heat hydration is faster, early strength

• Low Temperature:

  • Slow hydration, long term strength

• Below 0C:

  • No hydration, no strength 

Curing Methods…

• Constant presence of water:

  • Ponding, sprinkling

• Evaporation:

  • Plastic sheets cover concrete to keep evaporated moisture inside

• Supply heat:

  • Steaming concrete

Curing Properties…

• Shrinkage:

  • Deformation due to lack of water from structural pores

• Permeability:

  • Allows the chemicals and water to penetrate easily

Shrinkage Types…

• Plastic Shrinkage:

  • Water evaporates from exposed surface

• Dry Shrinkage:

  • Concrete shrinks due to water-filled gaps getting evaporated