Construction Materials and Lab Exam 1

ASTM

American Society for Testing and Materials

AASHTO

American Association of State Highway and Transportation Officials

Typical Cylinder Standards

6 x 12 Cylinders, 4 x 8 Cylinders.

Coarse Aggregate

Retained on a #4 sieve with 4.75mm openings

Fine Aggregate

Passing a #4 sieve with 4.75 mm openings

Specific Gravity

Ratio of material density to reference density (usually water)—the mass of a material divided by the mass of water whose volume is equal to the volume of the material at a specific temperature

High Density Gradation ( Well Graded)

has a good mix of all particle sizes which means the aggregates use most of the volume (less air) and less cement or asphalt is needed

One-size gradation (Uniform)

all same size = nearly vertical curve

Gap-Graded

missing some sizes = nearly horizontal section of curve

Open-Graded

-missing small aggregates which fill in holes between larger ones

-lower part of curve is skewed toward large sizes

Fineness Modulus

-Measure of the particle-size distribution of fine aggregate

-Only #100, 50, 30, 16, 8, 4, and 3/8" sieves are used

Absorption

The moisture content of an aggregate that is in the SSD condition

ASTM C33 fineness modulus of fine aggregate in concrete should fall within this range

2.3 - 3.1

If Fly Ash is used to replace some of the cement in a concrete you could expect it to?

Lead to a slower strength gain

Adding a water reducer without altering the other quantities of the ingredients in a concrete mix will?

Increase the slump of the mix

At Cool temperatures, concrete will...

Hydrate slower than if temperatures were high

Cement Paste

Portland Cement + Water

Mortar

Portland Cement Paste + Sand

Concrete

Cement + Water + Sand + Aggregates + Admixtures

Main Components of Cement

tricalcium silicate (C3S) , dicalcium silicate (C2S) , tricalcium aluminate (C3A) , tetracalcium aluminoferrite (C4AF)

Specific Gravity of Cement

~3.15

What is produced from the hydration of C3S & C2S (in cement)?

C-S-H (Calcim-silicate-hydrate)

During Hydration, what is C3S good for?

Early Strength Gain

During Hydration, What is C2S good for?

Later Strength Gain

What are the Stages of Hydration?

• Initial (ions released)

• Dormancy (plastic state, working period)

• Precipitation of Hydrates ( Acceleration period into crystallization)

Voids in Hydrated Cement

interlayer hydration space, capillary voids, trapped voids, entrained air

Stiffening

change from plastic to solid (initial and final)

-not the same as hardening, which is strength gain

-handling, placing, & vibrating must be completed before initial set

-finishing between initial and final

-curing after final set

False Set

premature stiffening within a few minutes due to humidity in cement during storage

• remix without adding water

Quick set and Flash set

occurs quickly 10-45 minutes due to imbalance of aluminates and sulfates( this is why gypsum is added)

• cannot be fixed

water-cement ratio

the ratio of water to cement in a concrete mix, which is the main factor which determines concrete strength

-water need for hydration, w/c=0.22 - 0.25 but typically use > 0.40

Effect of more water in water-cement ratio?

-Decreases Strength

-Decreases Durability

-Decreases bond between successive layers

-Decreases bond between concrete and rebar

-Increases Permeability

-More workability but causes voids

Admixtures for Concrete

Commonly used to improve properties of fresh and hardened concrete

Types of Admixtures for Concrete

1. Air Entrainer

2. Water Reducer

3. Retarder

4. Hydration Controller

5. Accelerator

6. Specialty Admixtures

Air Entrainers

-produce tiny air bubbles in hardened concrete to provide space for water to expand upon freezing

-Improve workability, resistance to de-icing chemicals, sulfates, & alkalis

-Decreases strength but this can be compensated with lower w/c ratio

Entrapped Air

Harmful Air bubbles in concrete that have been entrapped during the mixing process. 1% - 3% in normal concrete.

Water Reducers

Increase mobility of cement particles.

Improves workability - measured with slump test

Increase strength using lower w/c ratio

Types of Water Reducers

Standard, Mid-Range, High-Range ( Superplasticizers), increases workability temporarily, reduces cement cost

Retarders

• slows hydration process and initial set time

• long haul time

• may reduce early strength but usually doesn't reduce final set time much

Hydration Controllers

-Have the ability to stop and reactivate the hydration process of concrete. Consists of Stabilizer and Activator.

• Stabilizer: stops hydration up to 72hrs

• Activator: restarts hydration

Accelerators

• Used to speed up setting time and reduces the length of time for curing and protection

• Increase rate of strength gain

Most commonly used Accelerator

Calcium Chloride (CaCl2)

Specialty Admixtures

-workability agents

-corrosion inhibitors

-damp-proofing agents

-permeability reducing

-fungicidal, germicidal & insecticidal admixture

-concrete pumping aids

-bonding agents

-grouting agents

-coloring agents

Supplementary Cementitious Materials

Not cement but work "like cement" in a secondary reaction

• Fly ash

• Slag cement

• silica fumes

Fly Ash

• By-product of the coal industry, getting harder to obtain

• Increases workability and extends the hydration process

Slag Cement

-made from iron blast furnace slag

-used as a cementitious material in concrete

Silica Fume

• gold standard

• byproduct of the production of silicon production

• increases strength and durability

• reduces concrete corrosion induced by salts

Natural Pozzolans

not cementitious, but react with calcium hydroxide to form compounds possessing cementitious properties

Concrete = ?

Portland Cement Concrete (PCC)

Order of Operations for Concrete

I Mix Design

II Trial Mixes & Testing

III Batching

---------------------------Start the clock

IV Mixing

V Transporting

VI Pouring (Placing)

VII Vibrating (Consolidation)

----------------------------Initial Set Here

IX Finishing

----------------------------Final Set Here

IX Curing

X Maintenance

Slump Test

A test for mixed concrete to determine consistency and workability.

-Fill a cone in 3 layers, 25 rods each layer

-Life cone off (3-7 seconds) and measure distance it slumps from original height

Air Content Test for Fresh Concrete

~Measures total air content (Capillary and entrained)

~Only entrained is good but we can't tell the difference from this test

1. Pressure Method (common)

2. Volumetric Method (for lightweight aggregate)

3. Gravimetric Method

4. Chase Air Indicator

Screeding

striking off excess concrete to create a smooth and level surface, done after consolidation

Finishing Concrete

smoothing and imprinting the surface of the concrete with the desired texture

Curing Concrete

Maintain moisture & temperature in the concrete to promote continued hydration and strength gain

Seal surface

impervious paper or plastic sheets

membrane forming compounds

leave forms in place

Properties of Hardened Concrete

1. Early Volume Change

2. Creep

3. Permeability

4. Stress-Strain Relationship

Early Volume Change

Results in cracking, slight swelling, or curling from non-uniform drying

Creep

• long term, gradual, deformation under sustained load

• Concrete appears to be less stiff over time. Effective modulus decreases

• Generally takes place in cement paste, not in aggregate

Permeability

-As w/c = 0.3 to 0.7; coefficient of permeability increases by a factor of 1000

-caused by voids: poor consolidation & excess water

-Allows water & chemicals to penetrate

-Reduces durability & resistance to frost, alkali reactivity, and other chemical attacks

stress-strain relationship

• Increasing w/c decreases both strength and stiffness

• almost linear at small strains

Bond and paste strength are improved by...

low w/c ratio

use of pozzolans, smaller and fractured aggregate

Purpose of ASTM

Common language, quality assurance

ASTM Standard Types

Material Specs, Standard Tests, Standard Practices

ACI

American Concrete Institute, responsible for building codes and revisions, concrete inspection

6×12 Cylinder testing

3 layers rodded 25 times (5/8” rod)

4×8 cylinder testing

2 layers rodded 25 times (3/8” rod)

Procedures for cylinder testing

• Sample from center of concrete truck

• store in 100% humidity for 28 days (2 cylinders)

• compression and strength tests

• tests to be made once a day every 15 trucks (150 yd³)

Strong concrete

Brittle failure

Weak concrete

Ductile failure

Aggregate makeup in concrete

80-85% weight, 60-75% volume

Improved freeze-thaw resistance

Weaker concrete, more air entrapment

Increased workability

Weaker concrete, higher w/c ratio

Angular aggregate

Best for asphalt

Rounded aggregate

Best for concrete

Specific gravity is affected by

Voids

Loss

Aggregate is put in salt solution and loss is measured to analyze soundness and durability

Oven Dry

no water, MC=0

Air Dry

Voids have some water in them

Saturated Surface Dry

Voids are completely full, surface has no free water

Wet/Damp

voids full+free water on surface

Gradation measurement in coarse aggregate

1-9 (coarse to fine)

How to minimize silica reaction in portland cement

Fly ash pozzolans

Typical coarse SSD specific gravity value

2.65

Stage 1 of cement production

crushing and grinding of raw materials into slurry

Stage 2 of cement production

heat and melt in kiln (2500-3000F) to make cement clinker

Stage 3 of cement production

Add gypsum to delay set time, pulverize to fine powder

Role of fineness

Controls rate of hydration and growth

Role of hydration in cement

Continues hardening and strengthening process, curing

Purpose of C-S-H (Calcim-silicate-hydrate)

Makes paste strong

Capillary Voids

Comes from surface, decreases strength, increases permiability

Typical w/c ratio

0.22-0.25 minimum, usually >0.40

Increasing w/c ratio increases workability but decreases

strength, durability, bonding between layers, bonding to rebar

Type I Cement

Normal

Type II Cement

Moderate sulfate resistance

Type III Cement

High early strength (speeds up early production)

Type IV Cement

Rarely used, low heat of hydration

Type V Cement

High Sulfate Resistance

Purpose of gypsum

delays set time, prevents quick set/flash set

Curing Process

Maintain water presence, seal surface so water can’t escape, spraying, misting

When to cure

immediately after set, minimum 7 days