Wk 2 Cement Mortar and Concrete

What concrete comprised of?

• cement

• water

• aggregates

• optional admixtures

What do admixtures do?

they tailor workability, setting and durability (adjusts properties) - usually in liquid or powder form

What are the good qualities of concrete?

• mouldability

• rapid early gain (quick gain in strength after being placed)

• versatility

• high long term compressive capacity

What is mortar?

a mix of cement water and sand (with no course aggregate)

What is the use of cement?/ what is cement?

a powder substance used for bonding parts to form a single whole

What are the two types of cement?

• Hydraulic cement - consists of inorganic, non-metalic compounds that react with water to set

• Portland cement - a type of hydraulic cement that contains Calcium Oxide and silicon dioxide

What are aggregates?

the materials mixed with cement and water to provide bulk and modify the physical properties of the concrete

What type of reaction occurs between cement and water?

hydration

starts of by remaining plastic until it sets and begins to gain strength (hardening)

What is gel porosity?

degree of hydration that governs strength stiffness and permeability

What is essential for continued hydration and densification?

adequate moisture and time

What is fine aggregate and its use?

e.g. sand

• improves cohesion

• workability

• finish

What is the use of coarse aggregate?

• provides bulk

• reduces shrinkage

What is grading?

the distribution of particles of aggregates amongst various sizes - usually expressed in terms of cumulative percentages of larger or smaller than each other

What is proper grading?

The way we use different sized aggregates in combination to minimise voids and reduces the demand for paste (cement and water mix)

How are aggregates graded?

Using a tower of sieves with successively reducing sizes

• aggregate placed inside and vibrated for a specific period

• aggregates in each sieve are then weighed

• weights are used to provide a profile of the distribution of different particle sizes

• can create the aggregate grading curve

What are the properties aggregates should have?

• hard

• strong

• chemically inert

• non-porous

• durable

• good adhesion

• good shape (sharp vs rounded)

recycled materials can be used as aggregates

What happens when w/c is so low that it is below critical?

too little water can lead to

• incomplete hydration

• harsh mixes

• poor consolidation (not properly mixed - nonuniform)

What happens when w/c ratio is too high?

• increases porosity

• reduces strength

What will low w/c cause in general?

• increased strength

• increased durability

but it still needs to have sufficient water for workability

examples of admixtures and their uses?

• air-entrainers (freeze thaw protection)

• plasticizers - reduce water demand (makes it more flexible)

• accelerators/retarders - speed up or slow down the setting process

What is workability?

the ability to place concrete without segregation of its components

• influenced by w/c, aggregates, admixtures

• poor workability causes voids, weak bonds to the reinforcements

• workable concrete defined as suitable for placing and compacting under site conditions

What is the slump test?

• fresh concrete placed in a mould (trapezium shaped)

• in 3 layers

• when each layer is added, the concrete is compacted using a tamping rod 25 times (penetrating the layers below without touching the bottom layer)

• the mould is lifted and the concrete settles

• the way it settles will measure the consistency (used as quality control)

What is the vebe test?

Measures time required for a concrete volume, shaped as a conical frustum, to spread into a cylindrical form under vibration. After the cone is lifted, vibration causes the concrete to spread, and a stopwatch records the time until it fully contacts the glass disc

• time known as the vebe degree (VB) measured in seconds

• time indicates consistency of concrete

Hand mixing vs machine mixing

Mixing: the combining of the materials to form a uniform and workable mass, ensuring uniform coating of aggregate with paste

Hand mixing

• suitable for small scale works

• requires thorough mixing

• poor mixing can cause segregation and weak zones/inconsistent properties

Machine mixing

• used in most construction projects

• more efficient and ensures better uniformity

What must we do with transport to ensure that the mixed materials don’t segregate?

• transport promptly (can use a pump, bucket, wheelbarrow)

• minimise delays

• maintain temperature

• avoid contamination

What must we do with placing to avoid segregation of materials?

• place as close to the final position as possible to ensure bonding and compaction

• place in layers

• avoid drops of more than 1.5 meters - avoids cold joints (a visible seam that forms when fresh concrete is poured onto hardened concrete due to a delay between pours caused by mixing times)

Purpose of compaction

remove entrapped air

to make the concrete more dense and strong/durable

What happens if the concrete is insufficiently compact?

• voids

• honey combing

• poor bonding to the reinforcement

• higher permeability

• less strong

• air becomes trapped - significantly reduces strength

Methods of compaction?

Manual - rodding tamping or light hammering

Mechanical - interal vibrators, table vibrators, surface vibrators

What can over vibration cause?

• bleeding

• segregation

• ensure the correct vibration frequency, amplitude and time

What is curing?

the maintenance of optimal conditions of temperature (greater than 10 degrees celsius) and humidity to promote the hydration of cement

• It plays an important role on strength development and durability of concrete

• Methods : water curing (sprinkling/ponding), membrane curing compounds, plastic sheeting and steam curing (precast)

• Proper curing reduces permeability, cracking and enhances long-term strength

Strength development of concrete?

• most of the strength is developed in the first 3 days (40-50%)

• increases to 60-70% after a week

• curing beyond 28 days will increase strength and durability

What is durability?

ability to resist environmental/service actions (chlorides, sulfates, freeze– thaw, carbonation) over service life.

How can we achieve durability?

low permeability - low w/c, good compaction, good curing

• account for exposure environment (de-icing salts, marine, industrial)

• quality control, placing and curing

What can cause durability issues in concrete?

• reinforcement corrosion (the steel around it deteriorating/rusting)

• freeze thaw damage (we can minimise by air entraining the mixes)

• sulfate attack (reduce permeability)

• carbonation

• alkali-silica reaction

What are the desirable properties of hardened concrete?

• strength

• water tightness (low permeability)

• durability

• resistance to cracking

• abrasion resistance