Experimental study on fiber-reinforced concrete panels subjected to shear , Test results - Tesi

P,I I

Crack width from [mm];

LM:N

∆ I

I Reading of [mm];

Stress [MPa]; LM:N

I

Secant modulus of elasticity as calculated using the readings from

[MPa];

LR SHℎ LM:N

I

LM:N

[ Gauge length of [mm].

The elastic portion of the deformation was removed from the overall LVDT reading and

the rest of the deformation was taken as the crack width since there was no multiple

190

5. TEST RESULTS

crack observed. If result of the equation (5.11) was negative, the crack width was taken

as zero. Test observations show that in all cases the crack formed simultaneously on all

faces, but some occasional out-of-plane bending was observed, causing some

“compressive readings” on the LVDT: this did not mean that the face of the specimen

was surely in compression, though, as the LVDTs were located on a plane offset from

the face of the dog-bone. Thus, it was reasonable take the crack width as zero until the

equation (5.11) yielded a positive value; once that every LVDT has his procedure

completed, the values of crack width were averaged to obtain the tensile stress versus

crack width relationship.

Ending, linear interpolation was used to determine the tensile stress at regular intervals

of crack width (or tensile strain) for each specimen, so the average response of the

specimen tested in each set could be found.

5.3.4 Results of Data Analysis

The results of data analysis are summarized in the following Table 5.6 and Table 5.7,

which includes the tangent modulus of elasticity , the first-cracking tensile strength

,

, the strain corresponding to the first-cracking tensile strength the maximum

tensile strength and the strain corresponding to the maximum tensile strength

\

:

\ C

@ ] ^

A^ ^

<

Specimen Fibers

] F ⁄ ⁄

C @

2] 2]

(MPa) (MPa) (xDE ) A A^ A

^

ID Type

(%) [CV] [CV] [CV]

38,600 4.35 0.148

DC-P1 - - 0.514 4560

[9.31] [4.52] [9.70]

29,000 3.90 0.184

DC-P2 1.0 Steel 0.495 3680

[15.2] [11.5] [26.0]

35,700 4.49 0.132

DC-P3 2.0 MSN 0.629 5000

[20.8] [4.59] [9.2]

Table 5.6: Dog-bone Pre-cracked Tests Results Summarized. 191

5. TEST RESULTS

Specimens in this experimental program did non exhibit strain hardening behavior, as

the fiber volume fraction was too low to present such behavior (Bentur, 2007).

The effect of fiber addition on the properties of the concrete prior to cracking is

viewable in the reduction of the Elastic Modulus of Elasticity in tension in the previous

table: this modulus of elasticity was reasonably similar to that suggested in the

Canadian Concrete Design Handbook, which states that the secant modulus of elasticity

for concrete may be taken as: 2

4500

= (5.12)

Moreover, the cracking stresses for all concretes were reasonably similar, ranging from

0.5 0.63

2 2

and . =

AD

` ` at

Aa Aa

< = = `

at at

= ^_ ^b Aa

= =

Specimen Fibers of

^_ ^b

ID Type (MPa) (MPa) 3mm

(%) (mm) (mm) (MPa)

DC-P2 1.0 Steel 3.20 0.14 3.52 0.65 1.60

DC-P3 2.0 MSN 1.58 0.47 1.78 2.51 1.73

Table 5.7: Dog-bone Post-cracked Tests Results Summarized.

The cracking strain was similarly unaffected; after cracking, however, the results were

different than for plain concrete: the plain concrete exhibited a brittle failure, whereas

for FRC specimens a ductile and more gradual reduction in load was observed.

5.3.5 Influence of fiber content, fiber type, loading protocol and concrete matrix

strength

Results obtained by Susetyo shows that the influence of fiber content changes the

behavior of the all specimen: by increasing the amount of fibers in the concrete matrix,

improves the tensile behavior of the concrete. When 0.5% fiber by volume was added to

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5. TEST RESULTS

the matrix, although a strength reduction was observed immediately after cracking, the

concrete still retained a small amount of residual strength (Susetyo, 2009). Increasing

fibers up to 1.0% and 1.5% did not alter the pre-cracking response of the concrete than

by 0.5% of fiber by volume, but the post-cracking behavior was improved remarkably.

The influence of fiber type is viewable by the main differences obtained: shorter steel

fibers were more effective than the longer fibers, at volume fraction of 1.0%, maybe due

to the fact that there is a larger number of individual fibers in the mix for shorter steel

fibers, at a given volume fraction.

At large crack width the SFRC with shorter fibers began to lose the load-carrying

capacity rapidly and the residual stress dropped below that of the longer MSN fibers.

The response of MSNFRC specimens was different: the drop after cracking was larger

than SFRC regardless to the volume fraction; moreover, a large crack width was

required before the macro-synthetic fibers began engaged.

However, despite to this initial drop, the MSNFRC response regained some strength: in

most cases the maximum residual tensile stress occurred at a much greater crack width

than with the SFRC specimen, as much as 150% of the stress at engagement.

At a crack width of 2.4 mm to 2.8 mm, steel fibers began to lose bond strength due to

the straightening of the end-hook; the macro-synthetic fibers performed more favorably

at this level of cracking (Carnovale, 2013).

The flexibility of the fiber is a significant properties that affect the engagement of the

macro-synthetic fibers. At first cracking, some fibers were oriented in non-orthogonal

directions to the crack. These fibers had to become bent around the matrix entrance

points at both sides of the crack and become aligned with the direction of the load,

before becoming effective: this does not happen instantly and some crack opening is

required to allow this alignment to occur. This explain the requirement of a relatively

large crack opening before that these fibers become engaged.

Therefore, at a small crack widths, only fibers perfectly aligned perpendicular to the

crack can transmit significant tensile stress across the crack.

Influence on loading protocol in response of MSNFRC and SFRC exhibited different

engagement characteristics: cyclic dogbones have similar engagement characteristics

than those by monotonic load, whereas for the MSNFRC have a different response, with

193

5. TEST RESULTS

little to no difference in residual load-carrying capacity when compared to the

monotonically loaded.

Ending, the concrete matrix strength has an important rules in the behavior of the

specimens: a high concrete matrix strength was indicated by stress at which the concrete

first cracked, and this means that a higher concrete strength would result in a higher

first-cracking stress (Susetyo, 2009).

Watching the post-cracking behavior of the specimens, they seem to don’t be

significantly influenced by the strength of the concrete matrix: the only difference is the

interfacial shear strength between the fibers and the concrete matrix (higher in high

strength concrete matrix than in the normal one) so at the onset of first cracking, the

fibers in high strength concrete specimens were subjected to a higher tensile stress than

those in normal strength concrete specimens.

Lastly, the increased interfacial shear stress at the onset of cracking may have resulted

in an amount of bond slip available before the fibers were fully de-bonded similar to

that in normal strength concrete, despite the higher interfacial shear strength of high

concrete strength (Susetyo, 2009).

5.4 Prisms - Modulus of Rupture Tests

In this section, results of the Modulus of Rupture (MORs) are summarized and

discussed in accordance with the ASTM C1609/C169M (2010).

For each batch two specimens were constructed, and tested at various age from 48 to 99

days, based on the availability of laboratory machinery.

5.4.1 Previous Tests

Previous tests done by Susetyo show that fiber addition increase the flexural strength of

the concrete. Improvements ranging from 30% to 300% were obtained in previous

experimental test, depending on the fiber content and the type of fibers used.

Improvements were also more pronounced in test sets made with normal strength

concrete than those made with high strength concrete; higher flexural strength

194

5. TEST RESULTS

improvements were obtained also by adding fiber to the normal strength concrete than

to the high strength concrete.

Even with a small amount of fiber, the flexural behavior of the concrete was much more

ductile than that of the plain concrete: the plain concrete fails immediately after

cracking, whereas the fiber reinforced specimens deform significantly before the failure

improving the energy absorption capacity of the element.

5.4.2 Test Observation

This kind of test show a variability in results: the unforeseen location of the crack had a

big effect on the cracking load and on the post-cracked peak load. The closer the crack

was to the mid-span of the specimen, the higher the cracking and peak loads in the four-

point bending condition.

Test set DC-P1 exhibited brittle behavior, since no fibers were present to transfer the

load across the crack, and failed immediately after the prism cracked.

Like dog-bones, by adding fibers in the matrix, a control in the abrupt opening was

obtained and in all cases, the FRC specimens exhibited an ability to carry residual

flexural loads after first cracking and the load-carrying capacity decreased steadily and

gradually as the crack continued to open.

For FRC specimens, a strain hardening behavior in flexure was observed.

A slight drop in load was observed after cracking, accompanied by a slight change in

stiffness as the load continued to increase: multiple cracks or crack extensions formed

until the peak load was reached.

It was observed that further deformations were localized at one of the cracks and the

load-carrying capacity began to drop in a rapid yet controlled fashion; a problem with

the DC-P2 prisms was found after testing both modulus: the midspan LVDTs were not

switched to collect data, and no results are so available.

Following pictures shows the modulus of rupture tested in this experimental program:

195

5. TEST RESULTS

Figure 5.23: DC-P1 modulus of rupture test crack pattern.

Figure 5.24: DC-P2 modulus of rupture test crack pattern.

Steel fibers and macro-synthetic fibers show different behavior for the modulus of

rupture tests: steel fibers specimen show a slight drop in load after cracking,

accompanied by a slight change in stiffness as the load continued to increase.

The macro-synthetic specimen shows a sudden drop in load at the onset of cracking

similar that of the MSNFRC dogbones and this drop continued to the point at which the

fibers became sufficiently engaged; from there the response softened, yet load increased

until the post-cracked peak was attained. After reaching this secondary peak, the load

carrying capacity dropped gradually as the crack mouth continued to open. 196

5. TEST RESULTS

Figure 5.25:

5. D