CV301 Lab 4 Flexural Strength of Concrete 09-08-20

8/10/2019 CV301 Lab 4 Flexural Strength of Concrete 09-08-20

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Revised 09-06-23, WKS Datasheet No. 7.6a & 7.6b

MOHAWK COLLEGE OF APPLIED ARTS AND TECHNOLOGY

BUILDING AND CONSTRUCTION SCIENCES DEPARTMENT

Flexural Strength of Concrete (The Modulus of Rupture Test)

INTRODUCTION

Concrete pavement carries load as a simple, plain, (non-reinforced) concrete beam. The

strength of the concrete in flexure is the most important requirement. In previous labs then

primary interest has been the compressive strength of concrete. This strength is used in the

structural design of reinforced concrete, where tension, in which concrete is very weak, isassumed to be taken entirely by the reinforcing steel.

The rebars in concrete pavement are not important from the point of view of bending

stresses. There are dowel bars at joints and possibly shrinkage control bars. As the correlation

between it and compressive strength is only approximate, it is usually measured directly and

many specifications for concrete pavement specify only the flexural strength of the concrete,

and not the compressive. The flexural strength is referred to as the modulus of rupture of the

concrete.

Fibre reinforcement of concrete mixes has been in use for over 20 years mainly to

provide added resistance to crack propagation and improve its resistance to failure in tension.

In this lab, half the class will cast and test concrete beams in in flexure while the other half

does the same, only with polypropylene fibres added to the mix. This will enable the class to

determine whether or not the addition of fibre reinforcement had a significant effect on the

flexural strength of the mix.

PROCEDURE

A. Mixing and Casting

1. Each group will cast one 152.4 mm x 152.4 mm (6" x 6") x 914.4 mm (36") beam and two

101.6 mm x 203.2 mm mini cylinders.


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2. The mix proportions are shown in Table 1. Calculate the quantity required for the beams

and cylinders. An extra 15% should be allowed for waste and also as a margin for possible

error in the assumed density of the concrete. The volume of the air test container is

7.07910-3m3. If a density of 2350 kg/m3is assumed, this calculation can be done in

advance to save time in the laboratory.3. After making the moisture determinations on the aggregates and correcting for them,

each group will mix a batch. Groups 1 to 4 will add 25 grams of polypropylene fibres to

their batch. The laboratory mixers being used must first be dampened, spraying the

insides of the mixer with water from the hose and making sure that all sides of the

paddles are moistened then finally turning the mixer to the drain position for at least

three minutes.

4. The slump, fresh density and air content of the concrete should be measured and then

the beam and cylinders should be cast by rodding or vibrating depending on the slump.

5. The procedure for casting the beams is as follows:

Vibration(Slump less than 80 mm): One layer is used and the mold is over-filled so that

after consolidation the top of the concrete will be slightly above the edge of the mold.

Vibrate the concrete along the centreline of the mold at not less than 150 mm intervals.

Be careful not to touch the sides or bottom of the mold with the vibrator. Remove the

vibrator slowly after each insertion to eliminate voids. After vibrating, tap the sides of

the mold gently to dislodge any air bubbles.

Rodding(Slump greater than 80 mm): Two equal layers will be used. The number of

roddings per layer is specified as one for each 14 cm2

of surface area of the beam. Afterrodding each layer spade around the sides of the beam with a trowel.

Table 1 Mix Design Proportions

Design SSD

Masses (kg)

SSD Mass

for Required

1 m3(kg)

SSD Mass for

Required

Volume (kg)

Cement 20.676

Water 9.924

CA 30.686

FA 38.714

Total 100.000 2350


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6. The tops of the cylinders should be finished in the normal manner. The beams are to be

struck off level, finished with a sponge float and covered with plastic.

7. The following working day each group must strip and tag their beams and cylinders and

place them in the curing tanks, unless otherwise instructed.

B. Flexural Testing of the Beams(After one week of curing)

1. The beam will be tested on its side relative to the position in which it was cast.

2. The span should be 457.2 mm (3 times the depth). The load should be applied to the

specimen at the third points as illustrated in Figure 1 (152.4 mm from each support).

3. The specimen should not be removed from the curing tank until just before testing.

Even a small amount of drying can adversely affect the results. Two tests will be made on

each beam. Therefore, for the first test, position the beam with one end about 30 mmfrom the support.

4. The points of support and loading should be marked on the beam.

5. The test should be carried out at a rate of loading indicated by the instructor.

6. After the load test, the average depth and width of the specimen at the failure section

must be measured to the nearest mm.

Figure 1 Third-Point Loading Proportion Requirements for PCC Flexural Strength Test


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C: Testing of Cylinders

1. After determining the hardened density of the cylinders, they should be capped and

tested in compression in the usual manner.

CALCULATIONS

1. The modulus of rupture is calculated as follows:

CASE I where fracture occurs within the middle third of the span:

2db

LP=R

where R = modulus of rupture in kPa

P = maximum load in kN

L = span length (457.2 mm) in metres

b = average width in metres

d = average depth in metres

CASE II where fracture occurs outside the middle third of the span as measured along

the beam bottom by no more than 5% of the span length (grace zone):

2dbaP3=R

where

a = distance in metres of the fracture from the nearest support measured

along the bottom centre line of the beamif left support is closest,

record as positive, if right support is closest, record as negative

CASE III where fracture occurs more than 5% outside the middle third, the results of

the test are discarded (i.e. the test is indeterminate).

2. The constant k, which is sometimes used in converting compressive strength to modulus

of rupture is calculated as follows:

cf1000

R

k

2


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where

f'c = compressive strength in MPa

R = modulus of rupture in kPa

REPORT

1. Visit the website for the Ontario Provincial Standards:

http://www.ops.on.ca/home.asp

and select Online Standards or click on the OPS link on the instructors homepage. Using

the standards indicated on the Report Forms, fill in the required information for the report

The National Research Council website should also be examined as it relates to fibre

reinforcement in concrete mixes (http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd223-print_e.html).

2. The diagram in Datasheet 7.6b must be dimensioned and the fracture lines accurately drawn

thereon and their distance to the nearest support dimensioned. The appearance of the

specimens after stripping should also be reported, indicating whether there was any

honeycombing, large air bubbles, etc.

3.

The average Modulus of Rupture value obtained by your group should be evaluated in the

report against the OPS criterion.

4.

A tabular summary of the class results (posted on the instructors website) should also be

presented in the report with the non-reinforced data grouped separately from the fibre

reinforced data.

5.

Finally, a non-paired statistical comparison should be made (instructions and examples

posted on the instructors website) on the Modulus of Rupture data to determine whether

or not the fibre reinforcement had a statistically significant effect.

CV301 Lab 4 Flexural Strength of Concrete 09-08-20

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