CIVL 2026 Concrete Exam

Concrete is made up from which components?

- Mainly Fine/Course aggegates

- cement, water, SCM, -chemical admixtures

What do concrete mixing times depend on?

The type of mixer, speed of rotation, size of the batch, type of materials in the batch

What are the fresh properties of Normal Concrete

Temp -> greater than 5 degrees

Workability -> 0-150mm

Density -> 2100-2500 kg/m3

Air content -> 0-8%

What is Rheology

Science that deals with the flow of matter

Define Workability in Concrete

Property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished.

What factors affect workability and why?

Water content: Most important factor because water particles fill spaces between particles and lubricate the mix

Aggregate Quality: The less fines will produce a harsh mix which lacks consistency.

High temperatures and time reduce workability because of water evaporation

What is bleeding and how do you prevent it?

Bleeding is the upward movement of water. Excessive water movement and water pocket formation leads to a lack of durability.

Solution: Increase cement fineness or add admixtures to increase hydration rate

What are the different kinds of Slumps?

True slump, zero slump,

collapsed slump, shear slump

Pros and cons of slump test

Simple apparatus, easy and quick

Restricted to normal concrete. No measure of cohesion

What is the difference between entrained air, and entrapped air?

Entrained Air: Tiny micro bubbles which are not interconnected. This is desirable and added to concrete using admixtures

Entrapped air: All shapes and sizes which are interconnected. Undesirable property caused by improper compaction

Properties of "setting time"?

- Gives an indication of change in fluidity

- Setting precedes strength maturity

-2 types: Initial and final set.

Fresh properties need to be evaluated well before the initial set. Concrete starts hardening after the final set

Two types of mixed concrete?

Central mixed concrete is completely mixed at the plant, then discharged into a truck mixer

Truck mixed concrete is completely mixed in the truck

Name multiple ways of placing concrete

Buckets, wheelbarrows, conveyor belts, crane and bucket, trailer pump, boom pump

What occurs from improper placing? How do you place properly?

Improper placing can effect the air content

Adjusting the pump pressure can mitigate the magnitude of air loss in the pump

What is consolidation?

Consolidation allows for proper compaction and make finishing easier. This is done by working the concrete after placement to eliminate voids

What can cause honeycombing?

Honeycombing occurs when concrete hardens with some entrapped air. This is avoided through consolidation

Name different ways of finishing

Screeding, floating, trowelling, jointing/edging, broom finish

Name the concrete placing timeline

Batching, Transportation, placing, consolidation, screeding, floating, trowelling, edging, curing. Initial set @150min,

Final set @ 270 min

What is Curing?

Curing ensures continued hydration of the cement, strength gain of concrete, and avoidance of early age shrinkage.

The goal is to prevent moisture loss from fresh concrete.

How do slump flow and slump tests differ? What are they used for?

Slump flow is used for SCC mixes, whereas Slump is for normal concrete mixes.

Both are used to test workability and consistency of a mix

Define the following terms, and name which order they are used in.

Floating, Trowelling, Screeding, edging, jointing

Order: Screed, float, trowel, edging/jointing

Screeding: leveling surface

Floating: smoothening surface

Troweling: compacting top layer

Edging: makes neat edges

Jointing: creates narrow grooves

What problems arise during hot weather concreting, and how do you compensate for it?

Hot weather = increased evaporation and hydration reaction rates. This decreases set times and workability.

This can be fixed by using set retarders, crushed ice, cool aggregates

Why is it recommended that concrete is placed no more than 120 min after being batched

Placing after 120 minutes can occur in slump loss, honeycombing, improper compaction, and has an overall lower working life.

What causes honeycombing

insifficient fine material, or poor grading and mixing.

How do you fix honeycombing?

First remove the honeycombed area until only suitable concrete is exposed. Then fill voids with non shrink grout

What are C3A and C4AF roles in cement hydration?

They both don't have a huge role in cement hydration. They are generally used as fluxing agents

What is a fluxing agent?

Reduces melting temperatures in materials

What is Gypsum used for?

Gypsum is added to portland cement mix to avoid the formation of hydrogarnet.

Gypsum also helps slow down the chemical reaction with water to strengthen the mix

Prevents Hash

What is the difference between CSH and CH

CSH has high strength and durability. It takes up about 50-70% of the volume

CH has low durability and is a bi product. Takes up about 20-30% of volume

What is fly ash? What are it's characteristics?

by product of thermal powerplants

Increases workability, lowers pore size distribution, increases late strength and sustainability

What is granulated blast furnace slag characteristics?

enhances workability, durability, strength

used to control alkali silica reactivity

Late strength, sustainability

What are silica fume characteristics?

Bi product of silicon metal

significantly increases cement strength and durability

reduces cement corrosion

Reduces workability, size, increases water demand

Difference between silica fume and fly ash

Silica fume is more expensive.

Fly ash is a bi product of thermal powerplants

Silica fume is a bi product of ferrosilicon electronics (silicon metal)

Benefits of using Superplasticizer?

Increases workability, flow, reduces amount of water needed

Benefits of Set accelerator?

increases rate of hydration

Accelerates development of early strength

Benefits of Set Retarders?

Offsets the effect of hot weather

Good for long hauling distances

Provides time for special finishes

Benefits of air entraining admixture

produces tiny air bubbles in concrete to provide space for water to expand upon freezing

can increase durability of concrete without entrainers

Freeze thaw resistance

Ingredients in portland cement?

Limestone, sand, shale, iron ore, gypsum

What is interlayer hydration space?

pores inbetween the layers of CSH.

Can contribute up to 28% of porosity of paste

Do interlayer hydration spaces effect cement strength?

nahhh nah nah

How are capillary voids created?

result of hydrated cement paste having a lower bulk specific gravity than cement particles

What is the relationship with strength and the water/cement ratio?

The lower the water/cement ratio, the higher compressive strength in concrete

What admixture do you use if there is a lot of rainfall on site and work must stop for the time being?

Set Accelerator to increase rate of hydration

What admixture to you use if the construction crew needs more time to finish concrete than expected?

Set Retarder to slow rate of hydration

What admixture would you use in below freezing environments?

Air entrainers to increase durability under freezing

What admixture do you use for concrete around tightly spaced reinforced steel?

Superplasticizer.

Preferably a mix with smaller aggregate sizes and higher volumes of sand

Which admixture do you use if the mix is going for a long haul

Set retard er

Which admixture would you use if high early strength is needed, but not a high ultimate strength

Set Accelerator

Why is it necessary to measure air content of concrete on site rather than at the plant?

Air content can significantly change during the handling and hauling process

What is the typical range for compressive strength of concrete

20 - 40 MPa

What are the main forms of curing?

ponding immersion

membrane curing

spray fogging

What is the effect of high temperature on curing of concrete

At high temperatures, the compressive strength increases very quickly, but it also plateaus earlier than it normally would

A few days after casting slabs of concrete, you notice non connected cracks on the surface. What are they? How could have they been prevented?

Plastic shrinkage. Caused by allowing water/deleterious agents to enter concrete

Could be avoided by reducing evaporation. Hide concrete from sun with plastic sheet overtop

What is the cause of drying shrinkage?

Evaporation of surface water

What can alleviate shrinkage?

Curing

reducing water/cement ratio

reduce volume of paste, increase volume of aggregates

Crack control joints

Why is the compressive stress-strain graph of concrete non-linear?

When the ITZ starts to crack, the non linearity begins

What is a rebound hammer used for?

Measuring hardness

What is electrical resistivity used for?

measures voltage difference, water content, porosity

What is ultrasonic pulse velocity used for?

Finding cracks and discontinuities

Would it be necessary to use an air entrainer in a tropical climate?

No, they are typcially used in cold environments to increase pore space

What is the term "slab on grade"?

Concrete placed on ground/earth surface

What are potential problems with using CaCl2 as an admixture?

It contains chloride which accelerates corrosion of rebar.

What is freeze thaw damage in concrete?

During freezing, the capillary pores expand. Then when the concrete thaws the pores stay and the concretes durability is reduced

After about a year in construction, you notice whitish powdery deposits on some of the blocks, what could this be?

Could be efflorence. This is caused by alkali sulphate leaching out of the concrete.

This occurs when calcium hydroxide comes in contact with air

What is a SCM?

Supplementary Cementitious Material

How are self consolidating concrete properties achieved?

High use of SCM

High use of plasticizers

lower sizes of course grades

increased volume of fine aggregetes

How are self consolidating concrete properties measured?

Passing ability

filling ability

segregation resistance

Why would high performance concrete be preferred over conventional concrete?

higher workability and strength.

It is specifically engineered to accustom the specific structure it is used for

How do fibers effect:

workability

strength

ductility and energy absorption

reduces workability

does not increase compressive or tensile strength

increases ductility and energy absorption