1) Define the term concrete (2 marks)!Concrete is a mix of cement, water, aggregates, cementitious materials, as well as admixtures that hardens (due to hydration of cement and water) and is used in building structures such as towers, houses, roads, highways.!!2) 3 reasons why concrete is the most widely used engineering material? (6 marks)!- Concrete is relatively cheap compared steel (or other options) therefore it is

economical to use concrete!- Concrete is durable, and structures can have a life of 100 years or so.!- Concrete is flexible, as in, it can be moulded into any shape we desire therefore it

is better in a design perspective and also in convenience.!!3) How does (i) adequate workability and (ii) admixture influence or improve the properties of concrete?!(i)!(5 marks)!- Easier to place the concrete!- Easier to consolidate the concrete!- Easier to finish the concrete!- Consolidation removes air voids and water, increasing its density and increasing

the strength and durability of the concrete!- Can reach congested areas easily therefore there is guarantee that concrete will

have filled all the unreachable areas too?!(5 marks)!(ii)!- Can slow down hydration when in hot weather!- Can increase hydration when in cold weather!- Can help in making space for expansion (air entrainment)!- Can be used as an escape boundary for capillary water (air entrainment)!- Can increase the strength and durability of concrete (fly ash and silica fumes)!- Can increase workability without sacrificing strength !!4) How does the shape of aggregate particles influence the properties of concrete mix? (4 marks)!- Rounded shape : gives less friction therefore high workability, also has less

surface area, therefore does not need more water, therefore high workability. but do not bond well with cement paste therefore the resulting concrete is not strong.!

- Rough edges: Friction - low workability, needs more water to lubricate - low workability, Bonds well, high surface area, higher strength.!

5) Can wash water from the concrete mixer be used as mix water? (3 marks)!Water used to wash the concrete mixer is allowed to uses as mix water, as long as the mixer has been used to mix the same type of cement and admixtures as the current concrete mix. This too allowing that the water is fit to drink when it was used to wash the mixer (impurities can affect the mix)!!6) State the cause of sulphate attack in concrete and indicate two ways to control this problem (8 marks)!Sulphate attack!- gypSum and Attringite..!- C3A!!!Sulphates from the environment (K,Na,Ca,Mg) enter into !permeable concrete and !in the presence of moisture, !attack the C3A in the concrete and !form gypsum and enttringite. !This is larger than the reactants, therefore it increases the volume and causes cracking. !Use SRPC (less C3A),!Use proper curing to lessen the chance of sulphates entering!!7) Discuss the effect of compaction and curing on the durability of concrete (5 marks)!Compaction:!- Removes air voids in the concrete, makes it more denser, less porous, reduces

the segregation of the concrete. Denser concrete is stronger, thus more durable. Less porous means less chance of permeability vulnerability attacks such as sulfate, carbonate, attacks. Also means higher density. No segregation, stronger more durable because the mix constituents are not separated and bond well. !

Curing:!- Basically reduces evaporation from capillary pores and provide water if missing.

Well cured concrete will have blocked pores (or segmented) therefore is impermeable. Provides enough water, to hydrate all the cement: all hydrated = great strength therefore great durability. !

!8)What is meant by transition zone?!It is the zone between aggregate and the hydrated cement paste, .!!As cement particles and aggregate are considerably different is size, it is difficult for the cement to actually reach unto the walls of the aggregate, this is because of the wall effect. And also, the shearing stress put upon by the aggregate, also decrease the number of cement particles near the aggregate. Therefore the TZ is a zone with less cement and more water, i.e more porous and a weak link. !!!9) Explain (i) Carbonation (ii) Corrosion of steel!Carbonation:!- CO2 enter the concrete!- In the presence of water forms carbonic acid!- React with the Ca(OH)2 in the concrete to form CaCO3, which forms the

passive layer of protection around the steel!- This protection is removed, but concrete does get denser and stronger!- But it opens the door to corrosion!!Corrosion:!- When the passive layer is removed, Oxygen and water can corrode the steel!- It becomes a cell.. The anode and cathode have reactions..!- Cathode: Fe2+!- Anode: 2e-!- Fe(OH)2!- Fe(OH)3!- This goes on to turn to rust!- Rusting increases the volume of the steel, expansion and cracking. !!10) With the help of suitable curves, show how !(i) the effect of concrete temperature on the slump and the water

requirement to change the slump.!!(ii) The effect of ambient temperature on the water requirement of concrete.!!!!

11) What are the effects of adding fibers to concrete?!- Increases the tensile strength by delaying growth of cracks!- Increases the flexural strength by transmitting stress!- The fibers are used to reduce shrinkage cracking. These fibers are used in slabs

and pavements that have large exposed surface leading to high shrinkage crack. !- Glass, Steel, Cellulose, Kevlar!!12) 2 Advantages and Disadvantages of precast concrete!- Can continue work even in bad weather!- High quality, strong, factory controlled concrete!- Needs skilled labor to put together the building!- The joints? are sophisticated?? difficult to bring changes..?? It is difficult to

bring changes (remodelling) to the structure once it has been put together!!13) The need to test concrete is due to occurrence of conditions. List three conditions??!!!14) What is meant by accelerator and retarder? (6 marks)!Accelerator:!- Increases the rate of hydration of cement!- Less setting and hardening time!- CaCl2!- Used in cold weather!!Retarder:!- Increases the setting time of concrete!- Sugar is very effective!- Hot weather, long distance transport!!15) Why is gypsum added to OPC? (2 marks)!Gypsum delays the flash setting of cement (due to reaction of C3A) by reacting with some of the C3A and forming ettringite. !!16) Identify the root causes of and how to minimise (10 marks)!(i) sulphate attack!(ii) ASR!(i) How to minimise?!

- Use SRPC (less amount of C3A)!- Use proper curing procedures so the concrete is not permeable!- Use air entraining to provide space for expansion!(ii) Root cause?!- Reactive Alkali in the cement, Reactive silica in the aggregate, they react to form

a gel. This gel expands by osmotic pressure (semi permeable membrane) or absorption swelling and causes cracking..!

- We can use cement with less reactive alkali!- We can use alkali that have less reactive silica!- We can provide air entrainment to make up for expansion!!!!17) Why is it important to consider the free water content of the mix, and exposure of the structure?!Free water content: !- Is the free water in the concrete!- It will be added to the mixing water during mixing!- Therefore, the w/c ratio of the mix will be higher than calculated!- This will lead to a higher slump, lower strength, bleeding, porosity and

permeability.!- Therefore the free water content amount of the agg needs to be subtracted from

the total water amount that was going to be used, to compensate for the increase in water when mixed. Or else it will end up in lower quality concrete than we expected.!!

Exposure of the structure:!- A lot of factors of the concrete depends on type of environment the structure will

be exposed on. Such as nominal cover for concrete, which will decide how thick the cover should be around the reinforcement, to ensure that concrete cracking/ permeability cannot reach the reinforcement quickly!

- A maximum w/c ratio is also needed to be placed, as more water means more porosity, permeability and less durability, therefore it will be easier for concrete in extreme exposures to be attacked!

- A minimum grade is also needed to ensure strength and durability against these exposures as some structures may be in very harsh conditions such as places where waves are hitting it successively and also maybe swamps / sewage.!!

!18) State 5 solutions for plastic shrinkage of concrete (5 marks)!- Proper curing (reduce the evaporation)!- Dampen the sub grade and formwork, ensuring that any excess water is

removed prior to placing concrete!- in hot weather, lower the temperature of the fresh concrete by using chilled

mixing water or replacing some of this water with crushed ice!- Protect concrete surfaces from drying out!- The use of sufficient proportions of synthetic or steel fibres!!19) What is meant by curing of concrete? What would happen if not cured? Discuss three methods!!Curing of concrete is the process where evaporation of water is stopped so full hydration can occur, and strength of the concrete develops!!If not cured, the concrete will not gain its full strenght, will undergo plastic shrinkage due to evaporation, the capillary pores will not have segmented, it will be permeable. So less durability and compressive strength!!!1. Impermeable membrane: which helps in stopping evaporation and provides

ingress of water!2. Wet curing: spraying water or curing spray, wet sand, on the concrete surface!3. Steam curing: concrete is passed through tunnels with steam (which is wet so

its moist curing) used in precast concrete making!!20) Describe the classes/categories of lightweight concrete (10 marks)!- Aerated: Gas bubbles are introduced that leave voids in the concrete, Foam:

foaming material is added while mixing, which forms little gas bubbles!- Lightweight aggregate: only low density, porous aggregate is used!- No fines: only coarse aggregate is used, this leaves pores in the concrete

structure!- Autoclave Aerated concrete: precast concrete made with gas bubbles. !!21) What are the methods in making ferrocement?!- Chicken wire mesh is used as reinforcing? + water + cement + sand / fine

aggregates (4 marks)!- Making the reinforcement into the wanted shape, (chicken wire mesh and small

diameter bars as skeletal steel)!

- Preparing the mortar with water + cement + sand in correct proportions!- Plastering the mortar on the surface of the mesh!- Curing and coating it!!Can be used to make low cost houses, sculptures, boats!!22) What is the difference between cement and concrete?!