CVG 5150 – ADVANCED CONCRETE TECHNOLOGY

Quiz II
Dr. Leandro F.M. Sanchez Date: September 22nd, 2023

1. Describe in your own words the production of aggregates for usage in concrete.
➢ The production of the aggregates used in concrete is done using the following steps:
• Extraction of crushed stone from the source
• Crushing
• Sieving
• Cleaning.

Now, let us talk about the production process in detail.

• Extraction of crushed stone from the source

o There are two sources for aggregate manufacturing which are Quarries and
Natural deposits of coarse-grained sand/soil. Figure 1.1 is a picture of a quarry
university and figure 1.2 is a natural deposit picture.

Figure 1.1 – Photo of a quarry taken from Laval University

 Figure 1.2 – Photo of natural deposits taken from Laval university

o Quarry manufacturing is influenced by a lot of factors which includes the nature

of the rocks, thickness of layers available, different types of materials presents,
location, etc.
o The crushed stones are obtained by blasting/drilling, where in most of the cases
blasting is preferred as it is more efficient.
o Manufacturing is done according to the performance requirement. Nowadays
modern aggregate plants have all the necessary equipment to carry out operations
involving crushing, cleaning, size separation and combining two or more factors
to meet the customers specifications.
• Crushing
o There are 3 types of crushers used to manufacture aggregates. They are Jaw
crusher, Gyratory crusher and Impact and hammer crusher.
o All 3 have different properties where jaw crusher is generally used to break down
the initial big size rocks as it reduces the size to 4 to 5 times ready to be put in the
secondary crusher. As a negative effect- it generates a lot of flat and elongated
particles which are not suitable for usage in concrete.
o Gyratory crusher is the most effective crusher, and it can be used as either of the
primary, secondary of tertiary crusher. It prevents aggregates from becoming
elongated and flaky. So, it produces aggregates required for concrete.
 o Impact and hammer crushers reduce the size to a significant 30 times but
produces a lot of fine particles.
• Sieving and Cleaning
o The piles are formed according to the sizes which are 0/5mm, 5/10mm and
10/20mm. Sieving is done after every crushing stage. It is shown in figure 1.3.
o Since soil deposits contains varying quantities of silt and clay which adversely
affect the properties of both fresh and hardened concrete, so it needs to be
removed by dry screening or washing. Generally washing is more efficient as it
removes more residues.

Figure 1.3 – Crushing stages and sieving.

 2. Why do we need to test for impurity and harmful substances in aggregates used for
concrete?
➢ We need to prevent impure and harmful substances in the aggregates as it may affect the
strength and properties of concrete in fresh as well as hardened state. They are as follows:
• Material finer than 80Microns in aggregate – The materials passing the 75 Micron sieve,
such material will appreciably increase the water requirements for workable concrete and
reduce the abrasion resistance of hardened concrete if present in large amounts. So, the
maximum percentage passing allowance is given in A23.2-5A for fine and coarse
aggregate.
• Variety of unsound particles can occur in small quantities in aggregates, soft particles
such as clay lumps, wood, and coal will cause pitting and scaling at the surface. Coal may
also swell in the presence of moisture or release undesirable organic compounds that
interfere with setting and hardening.
• Week, friable particles of low density, such as many shales and pumice, should also be
avoided if a good wearing surface is needed. Reactive materials such as sulfides, gypsum,
and cherts can also lead to problems.
• Particles bearing iron oxide and Chloride can cause corrosion of the reinforced steel in
concrete, so it needs to be treated to have a good performing concrete.
• Below are some images for your reference.

Effect of Iron oxide bearing particles Effect of Chloride bearing particle

3. How to test the suitability of aggregates for use in concrete in Canada against freeze-
thaw cycles?
➢ The suitability of aggregates for use in concrete in Canada against freeze-thaw cycles is
done by the following 2 methods:
• Performance against Freezing and Thawing (FT) in concrete as per ASTMC 666 –
This test technique includes two separate methods: Method A, Rapid Freezing and
Thawing in Water, and Method B, Rapid Freezing in Air and Thawing in Water, to
calculate the resistance of concrete specimens to fast successive cycles of freezing and
thawing in the lab. Both approaches are meant to be used in figuring out how different
concrete qualities affect how resistant concrete is against the freezing-and-thawing cycles
described in the procedure. Neither approach is meant to offer a precise estimation of the
lifespan that might be anticipated from a particular kind of concrete.
• Resistance of unconfined coarse aggregate to FT as per CSA A23.2-24A
According to CSA A23.2-24A, aggregate samples are put in separate plastic containers
that are filled with a solution of sodium chloride (NaCl) that is 3 percent by mass. The
aggregate samples are frozen at -0.4°F for 16 hours after immersion for 24 hours at room
temperature. They are then thawed for a total of eight hours at room temperature. The
aggregate samples are rinsed with water and oven-dried to achieve a consistent mass after
a total of five cycles of freezing and thawing. The effectiveness of freezing-thawing is
assessed by calculating the amount of loss of mass of each aggregate sample because of
freezing-thawing cycles.
4. Explain with your own words the petrographic analysis procedure and its
importance in assessing aggregates for concrete.
➢ The Petrographic Analysis is done to find out the constituents with their relative amount
and to determine the physical and chemical property of the aggregate and based on the
results the performance of the aggregates in the field can be predicted.
➢ Petrographic Number is calculated through the analysis which describes the overall
performance of the coarse aggregate. It ranges between 100 and 600. The lower the
number the better overall performance of the coarse aggregate.
➢ Different types of microscopes can be used to perform Petrographic analysis depending
on the properties required. They are: Optical microscopy, Stereo Microscopy and other
procedures include XRD, SEM, etc. It is a very subjective procedure as it depends on the
petrographers accuracy.
➢ The minerals present in the sample are segregated by using weight and standardised to
100%. After that, each rock type is rated based on hardness and durability performance
on a scale of 1 to 6, where 1 is excellent performance and 6 is poor performance.
➢ Now let me explain to you the calculation of the Petrographic Number which is the result
of our analysis.
For example:
We have 25% shale (grade 5.0) and 75% limestone (grade 1.0), then the PN (Petrographic
Number will be as follows:
PN = 25x5.0 + 75x1.0 = 125 + 75 = 200.
➢ So, our PN was 200 it falls close to 100 so it is a good quality aggregate.
5. What’s alkali-aggregate reaction (AAR)? To prevent AAR in concrete, we can either
test or use the risk assessment evaluation as per CSA.A23.2-27A. Please explain in
your own words both approaches.
➢ Alkali-aggregate reaction is the chemical reaction between Na+(sodium), K+(Potassium),
and OH-(hydroxide) ions form the pores of the concrete with the siliceous minerals
present in the aggregate because of which Silica gel is produced. In Canada AAR is the
biggest problem where deterioration time is around 5 years. So, it is very crucial to take
necessary measure to prevent it. There are two methods to prevent AAR in concrete:
• Test Method/Mitigation
o Use of Supplementary Cementing Material (SCMs) in correct dosage is very
important. Examples: Fly ash, Silica fume, blast furnace slag, etc.
o Tests are carried out in conventional concrete versus concrete with different
percentage of Slag and Silica fume where expansion is observed with respect to
time up to 24 months.
o It is found from the results of the above test that we require 50% slag and on the
other hand we only require 10% Silica fume to keep the expansion under 0.05%
over 24 months. The graphs are given below.

0.25
Control
0.30

0.20 Control
Expansion (%)

25% Slag
Expansion (%)

0.15 0.20
7.5% SF
35% Slag

0.10
50% Slag 10% SF
0.10
0.05
65% Slag 12.5% SF
0.00 0.00
0 6 12 18 24 0 6 12 18 24
Age (Months) Age (Months)

o Lithium can also be added in the concrete, and it turns out to be very effective, but
it is super expensive, so it is not practical to use it to prevent AAR.
• Risk Assessment Evaluation as per CSA.A23.2-27A
o The purpose of this assessment is to prevent any material to be present in concrete
which actively reacts with alkali.
o So, the aggregate is analysed and is passed through various tests like petrographic
analysis, Chemical method for ACR Accelerated Mortar Bar Test, Concrete Prism
test, etc. and a flow chart has been formed by the Canadian Standard where steps
are given if it falls under which category whether the aggregate can be used
directly without any admixtures or does it require any admixtures to prevent
AAR.
o To prevent AAR a performance based mix design has been formed in Canada
through which we determine the alkali-silica reactivity of the aggregates on the
basis of which we determine the level of risk of ASR(alkali-silica reaction).
o After that we decide the lifespan of the structure required. Accordingly, we can
take the preventive measures.
o Preventive measure include addition of SCMs according to the prevention level
required.
o Below is a given flowchart for the selection of aggregate.