Lec 7 Flashcards
Why?
• Concrete is brittle in tension but relatively ductile in compression?
• The permeability of concrete containing very dense aggregates will be even higher by an order of magnitude than the permeability of cement paste (same w/c)?
• Mortar is always stronger than concrete (same cement content, w/c and et al.)
Interfacial Transition Zone
What is interfacial transition zone
The structure of cement past t the boundary with aggregate particles differs from that bulk cement oaste
What role does ITZ play?
stiffness, strength, and permeability of cementitious materials.
In general, this zone has lower density and decreased strength, compared to that provided by bulk cement paste.
ITZ can be referred to as the “weak link” in concrete.
https://ibb.co/YhYGBVH
- Transition zone
- Bulk cement paste
- Nonreactive aggregate
- Interfacial region
- Bulk paste
Microstructure of ITZ
- There is a water layer on the surface of coarse aggregate, and the local w/c near the CA is higher than the value of hydrated cement paste.
- Mainly ettringite and CH from hydration of C3S, C2S and C3A
- Due to the high w/c, large crystals are forming a porous structure. CH is plate-like.
- C-S-H, CH and ettringite continue to fill the poor structure.
What structure of ITZ differ it from bulk paste
⚫ There is less unhydrated cement.
⚫ There is higher porosity (and a lower density). The pores are
generally larger than those in the bulk paste.
⚫ There is less C-S-H.
⚫ There are large, oriented crystals of calcium hydroxide.
⚫ There is generally a greater concentration of ettringite.
How much ITZ in typical concrete?
Up 20-40% of the total volume
https://ibb.co/GkbDptd
Which image is the failure stress?
Sort the number in the graph from the top!
The 4th
4,3,2,1
https://ibb.co/37RmbyW
- Aggregate
- Hardened cement paste
- Transition zone
- Crack path and direction of propagatiom
3 general behaviors
- Fluidity/consistency: It must be capable of being handled and of flowing into the formwork and around any reinforcement, with the assistance
- Compatibility: of the air entrapped during mixing and handling should be capable of being removed by the compacting system being used
- Stability/cohesiveness: The concrete should remain as a homogeneous uniform mass. For example, the mortar should not be so fluid that it segregates from the coarse aggregate.
What property of general behaviors when combined together then become workability
Fluidity and compatibility
What does workability represent
The ability of concrete to be mixed, transported, and poured under conditions of minimal loss of uniformity
Workability is expressed in terms of three separate:
Consistency, mobility, compatibility
What factors affecting workability of concrete
Physical properties of cement and aggregate,
mixing ratio,
equipment for mixing,
transporting and pouring,
size and spacing of steel ,
size and shape of structures
What are the characteristics of good workability
Adequate amount of cement, appropriate content of fine particles and water, low content of coarse particles
For the matrix to have good plasticity, what should we do.
The fines content needs to be higher
What is the result of insufficient fines
Will produce rough concrete, easy to segregate, causing difficulty in mixing and pouring
- Smooth natural gravel
- More edges and slender crushed sandstone
Which one has better workability
1
What happen when the temperature is high?
the concrete setting time may be shortened and the hardening will be accelerated. At this time, additional water is required to maintain a certain workability.
What does consistency meaning in workability
indicator of the mobility of concrete, which depends on the proportion and constituent materials
Measured by slump test, the result is called slump
What is the general slump of concrete
5 to 18 cm
How to increase the slump up to 20-25 cm
by adding high- efficiency water reducing agent or strong plasticizer, and it would become a flowable concrete.
How slump loss happened
When concrete is standing still after mixing, the slump will decrease with time due to hydration and evaporation of water
Once the mixing water and the aggregate gradation change
when cement dosage, fineness and brand or room temperature change
Can slump represent the quality or workability of concrete?
The slump value cannot fully represent the quality or workability of the concrete, nor is it suitable for comparing the quality of different batches of concrete.
What is Bingham model
Related to behavior of fresh paste, mortar, and concrete
Flow starts when the applied shear stress reaches a yield stress (τy) sufficient to overcome the interparticle interference effects, and at higher stresses the shear rate varies approximately linearly with shear stress, the slope defining the plastic viscosity (μ).
https://ibb.co/ftNZrDV
- Conventional concrete
- Segregation zone
- SCC
Test of measurement of workability
- Slump test
- Compacting factor test
- Vebe test
- Slump flow test (mostly for mortar or self-consolidating concrete)
- Penetration test
Explain slump test
- The most common test
- For particles size smaller than 381 mm
- Has truncated cone (d=100mm, h= 300mm), semi-spherical dome bar (d= 16mm, l= 600mm)
The process of slump test
Page 22
1. The cone is filled with concrete in 3 equal layers, and each layer is compacted with 25 tamps of the tamping road
2. The cone is slowly raised and the concrete isnallowed to slump under its own weight
3. The slump is measured to nearest 5 or 10 mm using the upturned cone and slump rod as a guide
In traditional concrete, what is Self-Compacting Concrete (SCC)
Does not require tamping, it has the advantages of automatic pouring and reducing construction noise. It can flow through narrow reinforcement gaps or sections to achieve a sufficient filling formwork. (Invented by JP)
Types of slumps result
https://ibb.co/3WPYD4X
- Near-zero slump: <2.5 cm, too low water content, poor workability
- Trye slump: good workability
- Shear slump: insufficient strength of concrete cones, lack of plasticity and adhesion. Poor workability; less suitable for construction
- Collapse slump: 18-25 cm. Poorly mixed or too much water. Coarse stay in the middle; placement difficulty
- True, 2. Shear 3. Collapse
Explain compacting factor test
https://ibb.co/BjYrY3s
- Concrete is loaded into the upper hopper
- The trap door is opened, and the concrete falls into the lower hopper
- The trap door is opened, and the concrete falls into the cylinder
- The concrete is struck off level with the top of the cylinder
- The cylinder + concrete is weighted, to give partially compacted weight of concrete
- The concrete is fully compacted, and extra concrete added to fill the cylinder
- The cylinder + concrete is weighed, to give the fully compacted weight of concrete
Compacting factor= (weight of partially compact)/(weight of fully compact)
What is compacting factor test in short
Test for compaction at standard energy
• Test procedure
– Fill the top hopper with concrete first, then drop it into the middle hopper, and then drop it into the bottom test mold.
– Divided the test density by the fully compacted density to obtain the “compaction factor “
Explain vebe test
https://ibb.co/tcZy4Jr
https://ibb.co/pyVMh67
IMPORTANT
Process:
1. Slump test is performed in containers
2. A clear perspex disc, free to move vertically, is lowered ontol the concrete surface
3. Vibration at a standard rate is applied
Vebe degrees in seconds to complete covering the underside of the disc with concrete
Vebe test in general
Mostly used in Europe
By re-molding
Suitable with plastic properties
For particles small than 40 mm
Explain slump flow test
https://ibb.co/jTkMX1n
METHOD WAKTU LAB ENG MATERIAL
Flow = final diameter of the concrete
Measure the flow capacity under vibration
Test method—fill the concrete, then lift the cone, raise it 12.7 mm on the vibration table 25 times in 15 seconds, and then drop it to make the concrete specimen spread-out, and finally measure its flow diffusion value.
• Slump is caused by the self-weight of concrete without vibration
• The slump flow test uses vibration to force the concrete to deform.
Explain penetration test
https://ibb.co/XkHgqhw
Use The Kelly Ball Apparatus
For fresh concrete (quick checking)
• Penetrate the concrete with a penetrator and measure its penetration to assess the consistency of the concrete
• Suitable for construction site, the test process is very fast and can be tested on the trolley hopper or on the board of formwork.
