Abstract

As an emerging technology, slurry infiltrated mat concrete (SIMCON) will be used in the

near future to repair and retrofit this nation’s aging infrastructure. Having both a high strength in

compression and in tension, this new material is versatile in application.

Various researchers have tried to quantify how much improvement from retrofitting

flexural beams with SIMCON is possible with various kinds of retrofit layouts. Their results

outline the advantages and disadvantages of each retrofit layout. For example, adding SIMCON

on the compression region of a beam would improves the beam’s moment capacity and ductility

but still allowing the tension side to have flexural cracks where exposure to the environment can

cause corrosion.

Another experiment dealt with the effect of sizes of the beam by testing beams with six

different reference dimensions. The results demonstrate how the ratio of moment capacities

relates to the ratio of moments of inertia.

Furthermore, in the same experiment, broken

reference beams were repaired with SIMCON and

then retested.

To model the retrofit layouts that have not

been experimented, a MATLAB program was

written to simulate 11 different retrofitted layouts.

The results of the simulation further the

understanding of how SIMCON can improve a

beam depending on how SIMCON is added.

Introduction

A Type of High-Performance Fiber Reinforced Concrete

As American infrastructures continue to age, the construction industry continually

searches for newer ways to repair and retrofit cheaply and efficiently.  One of the widespread

types of necessary repairs is flexural retrofitting. Such repair becomes crucial when a beam

becomes unserviceable due to, for example, wide cracks exposed to the environment or fatigue.

Slurry Infiltrated Mat Concrete is a solution to this situation.

1

Fig. 1, SIMCON: continuous fiber mat.7

Slurry Infiltrated Mat Concrete (SIMCON) is one of the two commercially available

types of High-Performance Fiber Reinforced Concrete (HPFRC); the other type being Slurry

Infiltrated Fiber Concrete (SIFCON).  HPFRC differs from ordinary fiber reinforced concrete

(FRC) mainly because of its higher fiber volume fraction (Vf), which is more effective in

improving moment capacity and durability when used in retrofit.

While SIFCON has discontinuous hook fibers and a very high fiber volume fraction,

SIMCON consists of very long fibers fabricated as a mesh, Fig. 1, where the orientation of the

fibers can be accurately controlled.  Thus, SIMCON can have a high tensile strength but with a

low fiber volume fraction (commonly about 5%), as oppose to SIFCON, which requires a high

fiber volume fraction (4-14% for 1-inch-fibers) to achieve the same tensile strength.4

Cost Efficiency

Although both types of HPFRCs are effective in improving ductility, strength, energy

absorption, crack width, and eliminating the need for stirrups, SIMCON has advantages over

SIFCON in economy; the manufacturing and labor costs deter the popularity of SIFCON.

Existing machines can manufacture SIMCON.  Furthermore, SIMCON are shipped out in large

rolls, which can easily unravel to cover the subject beam or column, simplifying the construction

work.  The construction procedures for repair work with SIMCON are very similar to that of

regular concrete, thus working with SIMCON requires minimal retraining.4

Uses of SIMCON

The experiment by Dogon, Krstulovic-Opara, Uang, and Haghayeghi investigates the

effects of different placements of SIMCON on retrofitted beams.  It involved testing two

reference reinforced concrete beams with no SIMCON addition and six retrofitted beams with

three types of layout: having SIMCON on the top only, on the bottom only, and on three sides

(two sides and bottom of the beam).  The six variable beams behaved differently, suggesting that

the different layouts do not improve the performance of the beam in the same way.

The later experiment by Haghayeghi and Oluokun aimed to quantify how much the

performance of a retrofitted beam improves depending on the size of the subject beam. Lastly,

untested retrofit layouts are simulated to further the understanding of the layouts’ effects.

Slurry Mix

While the steel mat in SIMCON provides the tensile strength, the cement-based slurry

2

mix provides most of the compressive strength.  An example of SIMCON slurry mix is

1/0.31/0.6/0.3/0.045 by weight of Type I Portland cement, water, Ottawa silica sand #250,

microsilica, and superplasticizer, respectively.4  The mix does not have any coarse aggregates

because bigger rocks cannot fit through the small spaces between the steel fibers.  Only fine

aggregates can effectively bond with the mesh of thin fibers.  Microsilica sand is added to further

fill in the microscopic pores between the fine aggregates, therefore enhancing the compressive

strength of SIMCON.  Ultimately, the superplasticizer facilitates pumping this slurry mix into

the formwork by allowing the aggregates and cement to flow more thoroughly into the entire

steel mat.

Retrofit Layouts

The decision on which sides of a beam to retrofit with SIMCON affects which properties

of the beam would improve.  Adding SIMCON to every face of the beam would enhance nearly

every property of the beam, although at a high cost.  If a budget can only allow adding SIMCON

to one side of the beam, the engineer has to consider which properties of the beam are worth

improving.

In an experiment by Krstulovic-Opara, Dogon, Uang, and Haghayeghi,4 six beams with

three distinct layouts of SIMCON retrofit were tested against two reference beams, as illustrated

in Fig 2.  The three layouts included placement of 1-inch-thick SIMCON on the top, on the

bottom, and on the two sides and bottom (“U-Jacket”) of the beam.  The reference beams were 6

inches by 6 inches beams with steel reinforcement on the bottom and on the top of the beam.

Stirrups were placed near the supports to resist shear.  The six variable beams with SIMCON

retrofit were simply reference beams with SIMCON additions.

The beams were then loaded at 0.0336 inches per minute until flexural failure.  Every

3

Fig. 2, Reference beam and retrofitted beam layouts; units are in centimeters.4

beam followed ACI 318-89 in order to produce a ductile flexural failure.4 The average tensile

strength of SIMCON was 23 ksi at 1.1% strain.4

Each layout of the SIMCON retrofitted beam did not behave in the same fashion.  The

beams with SIMCON on the top only and on the bottom only behaved similarly until the first

steel yield point; both had the same number of major cracks and the same pattern of flexural

crack pattern.  Nearing the ultimate moment capacity, the top-only beam had flexural cracks that

propagated very close to the top surface, meaning that the SIMCON layer on the top significantly

improved the compressive strength of the beam.  The bottom-only beam, however, did not have

such prominent flexural crack propagations—the bottom SIMCON layer effectively slows down

the initialization of the flexural cracks.  Except for the delamination of the SIMCON layer and

the critical crack, the bottom-only beam had only very fine, discontinuous flexural cracks.

The U-jacket layout outperformed the top-only and bottom-only layout in terms of

maximum moment and adherence; the SIMCON layer of U-jacket layout beams did not

delaminate like the other two retrofitted beam layouts.4 The U-jacket doubled the maximum

moment of the reference beam, significantly higher than the other layouts, as shown in Table 1

and Fig. 3.

Beam type Moment, kip-in. Curvature, e-5 in.^-1 Moment, kip-in. Curvature, e-5 in.^-1Reference beams 25 4.47 143 254

Bottom-only layout 90 19.8 183 76.2Top-only layout 54 5.69 196.4 279"U-jacket" layout 107 16.3 312 317

First crack Maximum moment

The experiments and modeling by Krstulovic-Opara et al.4 demonstrate the following

relationships between SIMCON layout and effects on the beam:

Lever arm: all three layouts lengthened the lever arms and therefore the maximum

moment capacity.

Delamination: although the top-only and bottom-only layouts exhibited detached

SIMCON layers before or after the maximum moment, this problem can be avoided by

installing shear studs or shear keys between reinforced concrete and SIMCON.

4

Table 1, Experimental values of moment and curvature.4

Crack initialization: SIMCON exhibits multiple cracks mechanism in flexural load,

which means, except for the large critical crack, it would have only microscopic,

discontinuous cracks. Consequently, the bottom-only layout and the U-jacket layout had

very few and very small flexural cracks (which also propagated upward very slowly),

compared to the top-

only layout, as shown

in Fig. 4.

Crack propagation: if

a beam’s tension

region is retrofitted

with SIMCON, the

upward propagation

of flexural cracks is

impeded with the

SIMCON’s high

elastic modulus,

compared to the steel

reinforcement, which

is related to its

multiple cracks mechanism. Such retrofit layout would actually have a lower ductility

factor compared to the top-only beam. When a beam’s compression region is retrofitted

instead, flexural cracks are allowed to travel very far up the beam’s height, almost to the

very top, due to the SIMCON strengthening the compressive side, which is also shown in

Fig 4.

Durability: ACI specifies that continuous cracks must be below 0.0078 in. to be safe from

the environment. Fortunately, SIMCON flexural cracks, aside from the critical crack, are

at most 0.0039 inch. (These cracks are also discontinuous, whereas the ACI code refers

to continuous cracks.) Furthermore, due to SIMCON’s low water content, SIMCON has

a higher percentage of unhydrated cement particles than regular concrete. Such condition

allows the SIMCON’s permeability to decrease gradually due to autogenous healing (a

process where moisture converts cement’s calcium hydroxide into limestone, sealing the

5

Fig. 3, Moment-curvature behavior of tested beams; solid lines represent the experimental results; dashed lines represent the analytical models.4

cracks 8). Therefore, if corrosion is an important factor, the engineer must retrofit the

beam’s tensile region with SIMCON.

Beam Size Effect

The magnitude of improvement from a retrofit is correlated with the size of the subject

beam. In the experiment conducted by Krstulovic-Opara et al. in 1997,4 the two reference beams

had the same dimensions, while every layout of SIMCON had a thickness of 1 inch. Two years

later, Oluokun and Haghayeghi6 conducted another experiment with SIMCON-retrofitted beams

with a scope that included beams of different sizes.

The specimens included 12 reference reinforced concrete beams with six different sizes

and 12 corresponding retrofitted beams, as illustrated in Fig. 5. Every retrofitted beam had a U-

jacket layout of 1-inch thick layer of SIMCON on the bottom and on the sides. Beam 1, 2, and 3

were 6-inches wide, while beam 4, 5, and 6 were 8-inches wide. The reference beams were

designed to have ductile flexural failure, ACI 318-95.6

6

Fig 4, Top: the top-only layout allowed the flexural cracks to propagate up very close to the top surface due to the SIMCON layer strengthening the compression region. Bottom: the bottom SIMCON slowed the flexural crack initialization and the upward propagation of cracks; notice that the only major crack on the SIMCON is the critical fracture.4

The slurry mix and concrete were

designed to have compressive strength of 10

ksi and 6 ksi, respectively. The fiber

volume fraction was 5.39%.6

The beams were loaded at two

points, approximately at every third of the

beam length on a Material Testing System

machine. Three linear voltage differentials

transducers (LVDT) were placed at the

midspan to measure displacement and

curvature.

Every retrofitted beam displayed a “pseudoductile” response: after the SIMCON

fractured, the load capacity dropped because the bottom steel reinforcements were then carrying

substantially more load. Every retrofitted beam also failed in the same manner: after the

SIMCON fractured, the concrete at the top was crushed as the critical fracture from the SIMCON

propagated upward.6 The moment-curvature plot shows significant improvement in moment

capacities but not for ductility, as shown in Fig. 6.

The differences in improvement from retrofitting between the beam sizes were not large.

However, they demonstrated a strong correlation between capacities and dimensions; the ratio of

moment capacity of retrofitted beam to that of reference beams is almost linearly related to the

7

Fig. 5, Top: cross-sections of reference beams. Bottom: cross-sections of retrofitted beams.6

Fig. 6, Moment-curvature response of reference and retrofitted beam 1.6

ratio of moment of inertia of the retrofitted beam to that of the reference beam,6 as seen in Table

2. The following equation represents this relationship:

(1)

A higher ratio of the

area of SIMCON and the area

of reference beams strongly

correlated with a higher

increase in moment capacity.

For example, retrofitted beam 1 consisted of 33% SIMCON compared to retrofitted beam 4,

which is 26% SIMCON. However, if the SIMCON layer thickens by 1 inch to 2 inches thick,

beam 1’s moment capacity increased by 69% and beam 4’s capacity increased only by 20%

compared to the original 1-inch-thick SIMCON retrofitted beams.6 Reference beam 1 and beam

4 had the same height – only their widths were different, by 2 inches. Therefore, to achieve the

substantial magnitude of moment capacity improvement, beams with larger areas need thicker

layers of SIMCON on the tensile side.

The 12 reference beams were tested 4 weeks after casting. However, the 12 retrofitted

beams were tested 9 weeks after casting and 30 days after the addition of SIMCON.6 The long

time span between the tests of the reference beams and the retrofitted beam might have

influenced the conclusions. However, such procedures were necessary because four of the

broken reference beams needed to be repaired and then tested at the same time as the retrofitted

beams. Casting more reference beams and retrofitted beams for comparison with the repaired

beams may have posed a budget and time issue.

Repairing with SIMCON

8

Table 2, Average increase in moment capacity of retrofitted beams with respect to the moment capacity of reference beams.6

After the reference beam 2 and 5 were tested, Oluokun and Haghayeghi6 repaired those

four broken beams with the same processes used in retrofitting beams (after all of the crushed

concrete was thoroughly removed.) The original steel reinforcements were not replaced. All of

the crushed portions of concrete were replaced with the slurry mix when the mix was pumped

into the SIMCON wooden formwork. Oluokun and Hagayeghi6 believed that the “slurry mix

infiltrated most of the cracks,” which was highly plausible due to the slurry mix’s high content of

fine aggregates and superplasticizers.

When tested, the four repaired beams failed in the same manner as the retrofitted beams:

SIMCON fractured, allowing the critical crack to propagate upward until the top concrete

crushed. The improvements in moment capacity and ductility factor were comparable to the

improvements observed in the retrofitted beams,6 as shown in Fig. 7. In fact, the ductility factor

for the repaired beam 5 was even higher than that of the retrofitted beam 5, as shown in Table 3.

Whether or not the repaired beams’ superior ductility factor is consistent with other beam

repaired with SIMCON, more experiments need to be performed on beams repaired with

SIMCON. Nonetheless, the performance of the repaired beams proved that SIMCON is a

reliable material for repairing concrete beams.

9

Fig. 7, Moment-curvature response of the repaired beam 5 compared with its reference beams.6

Analytical Modeling: Moment Curvature

The experiments by Krstulovic-Opara et al.4 and Oluokun and Haghayeghi6 covered the

scope of the different retrofit layouts, the effects of beam size, and the performance of repaired

beams. There are still some retrofit layouts that

were not addressed. With limited time and

resource, the best course of action is to model

the other layouts.

General Procedures

Since reinforced concrete and SIMCON are

nonlinear materials, a numerical nonlinear

analysis model is well suited for simulating a

reinforced-concrete-SIMCON composite beam.

Haghayeghi and Oluokun2 used a numerical

procedure similar to the nonlinear analysis of

concrete beams and columns, as laid out in Fig

8., which involves discretizing the cross section

area of the beam into slices and then calculating

the stresses in each layer for the corresponding

materials by assuming linear strain distribution

along the height of the beam.

10

Table 3, Comparison between the ductility factors of reference, retrofitted, and repaired beams.6

The first step is to input section geometry and properties.2 They include the width,

height, thickness of SIMCON and concrete and the location,

amount, and bar size of longitudinal steel reinforcements and

stirrups. Furthermore, the program also requires the maximum

strength of concrete and the grade of steel.

The second step is the discretization of the cross section area into thin layers.2 The

recommended number of layers is 100. However, for an extremely large section, it is important

to make sure that a layer is smaller than roughly a third of the longitudinal steel reinforcements’

diameter. Each layer is then divided into areas of every material in the beam.

The next step begins the iterative loop to find points on the moment-curvature plot. The

procedures described here do not always conform to the cited literature because the procedures

have been modified for accuracy and speed. The bisection method is used to find the correct

bottom strain, εbot, corresponding to a given top strain.1 The program begins with setting the

topmost strain, εtop to a small value, e.g. dε = 10-5. The bisection method procedure then begins

to find the correct εbot by first guessing that εbot1 is zero and εbot2 is the steel fracture strain, εsm.

(Assuming that there is no axial force at the neutral axis, the error of guessing the first εtop being

less than εaxial is not possible.) The average of εbot1 and εbot2 is the estimated bottom strain, εbot,est.

For every εbot,est, the material model provides the stress each material in each layer.

To determine whether εbot,est is correct, we find the resulting force by taking the sum of the

multiplication of every layer area and its corresponding stress to determine whether the beam is

in equilibrium. If not, we produce a new εbot,est by setting the new εbot1 or εbot2 (new εbot1 if there is

too much tension) as the previous εbot,est and calculate for the stresses and the resulting force

again.1 Repeat until the resulting force is within a set acceptable range, i.e., less than 0.01.

After equilibrium is achieved, we calculate for the moment and curvature, and record any

other important properties, such as the correct εbot and the neutral axis. This step would produce

a point on the moment-curvature plot. To calculate for more points, we continually increase εtop

by dε increments and then calculate the corresponding moment and curvature for each increment.

The final εtop should be reasonably past the concrete crushing strain in order to observe the

starting point of the moment-curvature plot’s descending branch.

11

Fig. 8, Flow chart of computation procedure.2

The compressive and tensile models of SIMCON were derived from known fiber

reinforced concrete models by modifying the coefficients, which were determined

experimentally from tests done on both SIFCON and SIMCON.2

Modeling SIMCON

Both compressive and tensile behaviors of SIMCON are divided into two parts: the

ascending branch and the descending branch.3 The ascending branch is when the stress increases

with strain. The descending branch is the stress after the ultimate stress, σcu, is already reached.

When SIMCON is undergoing compression or tension, the ultimate stress and the

corresponding strain are calculated with the following equation5:

(2)

(3)

where σvmu and εvmu are the virtual compressive strength and the corresponding strain when Vf =

0, α and β are an experimentally determined coefficients. These four components and σcu and εcu

are tabulated5, 3 in Tables 4 and 5. These values in compression are not the same as those in

tension. For compression, some specific values are dependent on how the slurry mix was

applied.5

12

Table 4, Specific values for modeling SIMCON in compression.5

Table 5, Specific values for modeling SIMCON in tension.3

In compression and tension, the behavior of SIMCON follows the stress-strain relationship until

the stress reaches the ultimate strength – this is the ascending branch.5, 3 The stress-strain

relationship is as follows,5, 3

for (4)

where , (5)

and Ec is the elastic modulus.

Beyond the respective ultimate stress, in the descending branch, SIMCON in

compression is still correlated to the strain, but with a new relationship as follows5

for (6)

where σasy is the asymptotic stress on the stress-strain relation curve, which was determined

experimentally (and tabulated). The m and b are coefficients related to the stress and strain at the

point of inflection on the stress-strain curve,5 see the Appendix for their codes.

The descending branch of SIMCON in tension is related to the opening of cracks, which

is as follows,3

for (7)

where , (8)

Φ is an experimentally determined coefficient and δ is the width of the crack’s opening.

According to Krstulovic-Opara and Malak,3 δ can be approximated to be 0.5*d (d = width of a

fiber strand) in order to produce good prediction modeling; L/d is a commonly used quantity in

FRC.4

SIMCON Models in Simulation

To build upon the experiments Krstulovic-Opara et al. and Oluokun et al., I have used

MATLAB to analytically model retrofitted beams of different cross-section layouts. The

reference beam, as shown in Fig. 9, is modeled after the beam 1 in the experiment of Oluokun et

13

al. The 11 retrofitted beams layouts are shown in

Fig. 10. The MATLAB code is located in the

Appendix.

Results

After forming the moment-curvature plot for

the 12 beams, the plot of beams with similar layouts

are shown together in one graph in order to compare

how slight changes in the placement of SIMCON

can change the properties of the beam.

The first comparisons involves the top-only

and bottom-only retrofitted beams and how

thickening the SIMCON layer from 1 inch to 2

inches affects and properties. Both top-only layouts doubled the reference beam’s ductility ratio

and increased moment capacity by about 25% and 50%, as illustrated in Fig. 11. However,

retrofitting on the tension side only does not improve the ductility factor or the moment capacity

significantly (only by about 10-20%), as shown in Fig. 12. Therefore, for this particular

reference beam, if retrofitting is limited to only one side, retrofitting the compression would

improve the moment capacity and ductility factor more substantially.

14

Fig. 9, Cross-section of the reference beam.

For the beam retrofitted on the top and bottom, ductility factor improved by about 300%

or more, as shown in Fig 13. The most outstanding out of the three beams in this category is

beam 4.2, which has about a ductility factor of about five times greater than that of the reference

beam and a moment capacity that is about twice as much—the biggest improvement in moment

capacity out of all the beams.

The U-jacket and full-jacket layouts do not perform as well as the top-and-bottom layouts

given the same or greater amount of SIMCON, as shown in Fig. 14. However, beam 6.0

improves the ductility factor by the most out of every retrofitted beam. These beams did not

improve the moment capacity as much as the augmented top-and-bottom layouts because their

SIMCON are not localized at the top or bottom, thus not effectively lengthening the lever arm.

15

Fig. 10, Modeled retrofit layouts.

16

Fig. 11, Moment-curvature response of the top-only retrofitted beam layouts. The reference beam’s curve is the bottommost curve.

Fig. 12, Moment-curvature response of the bottom-only and sides-only retrofitted beam layouts. The reference beam’s curve is the bottommost curve.

17

Fig. 13, Moment-curvature response of the top-and-bottom retrofitted beam layouts. The reference beam’s curve is the bottommost curve.

Fig. 14, Moment-curvature response of the U-jacket and full-jacket retrofitted beam layouts. The reference beam’s curve is the bottommost curvet.

Conclusions

The conducted experiments and simulations demonstrate that slurry infiltrated mat

concrete has a lot of potential as a material for retrofitting. The experiment by Krstulovic-Opara

et al. not only demonstrated how SIMCON improves moment capacity and ductility factor, but it

also showed the advantages of retrofitting with SIMCON, such as corrosion prevention.

The simulation shows that there are many effective ways to retrofit with SIMCON,

whether it be enlarging the lever arm or improving energy absorption. A material that has the

compressive behavior similar to concrete and yet can bear more than 10 ksi in tension has a

versatility similar to kind that brought structural steel into widespread popularity. Perhaps in the

near future SIMCON would used extensively in construction as well. Beam made out of HPFRC

like SIMCON and SIFCON can be smaller in dimensions compared to the traditional reinforced

concrete beams with the same capacity.4

With a better understanding of how reinforced-concrete-SIMCON composite beams

work, engineers will be able to make smarter decisions.

18

Fig. 15, Moment-curvature response of every modeled beam.

References

1. Chai, Rob Y. H. “Confined Concrete.” ECI 232: Advanced Topics in Concrete Structures. University of California, Davis. 26-31 Oct. 2011. Lecture.

2. Haghayeghi, Abdol R., and Ajiboye F. Oluokun. “Prediction of Flexural Strength of Concrete Beams Retrofitted with Slurry Infiltrated Mat Concrete (SIMCON).” American Concrete Institute: Structural Journal 95-S50 (1998): 558-569.

3. Krstulovic-Opara, Nevin, and Sary Malak. “Tensile Behavior of Slurry Infiltrated Mat Concrete (SIMCON).” American Concrete Institute: Materials Journal 94-M5 (1997): 39-46.

4. Krstulovic-Opara, Neven, Erdem Dogon, Chia-Ming Uang, and Abdol R. Haghayeghi. “Flexual Behavior of Composite R.C.-Slurry Infiltrated Mat Concrete (SIMCON) Members.” American Concrete Institute: Structural Journal 94-S46 (1997): 502-512.

5. Krstulovic-Opara, Neven, and Mohammad Jamal Al-Shannag. “Compressive Behavior of Slurry Infiltrated Mat Concrete.” American Concrete Institute: Materials Journal 96-M46 (1999): 367-377.

6. Oluokun, Ajiboye F., and Abdol R. Haghayeghi. “Flexural Behavior of Concrete Beams Retrofitted or Repaired with Slurry Infiltrated Mat Concrete (SIMCON).” American Concrete Institute: Structural Journal 95-S59 (1998): 654-664.

7. “SIMCON: Continuous fiber-mat High-Performance Fiber Reinforced Cementitious Composites.” Photo. Emerging Construction Technologies. 2011. 22 Nov. 2011 <http://rebar.ecn.purdue.edu/ect/links/technologies/civil/simcon.aspx>.

8. “What is Autogenous Healing.” Cement Lining Corporation International. 2011. 22 Nov. 2011 <http://www.cementlining.com/clci/faqs.htm#faq4>.

19

Appendices

20