Structural Capacity of Cracked Concrete Repaired

with a Low-Viscosity Injected Epoxy Resin

University of British Columbia

Department of Civil Engineering

Undergraduate Report: Civil 322

i

Compressive Strength of Cracked Concrete Repaired with

a Low-Viscosity Injected Epoxy Resin

Leader: Jeff Brackett

Members: Lindsay Burk, Adam Floor, Mike Hanson, Mary Jane Howie, Madeline

McKee, Dave Mori, Matt Ridley, Navratna Sharma

Faculty of Engineering, University of British Columbia

Abstract

In the repair of concrete compression structures, low-viscosity epoxy is often used as a

repair material. Although the epoxy is assumed to act as a sealant against water

intrusion, the structural capacity of the repaired concrete is unknown. The objective of

this study is to investigate the compressive strength of the repaired concrete, relative to

both undamaged and unrepaired damaged concrete structures. The repaired concrete

was not observed to regain significant compressive strength.

ii

Table of Contents

Abstract ............................................................................................................................ i

Table of Contents .............................................................................................................ii

List of Figures ..................................................................................................................iv

List of Tables ...................................................................................................................iv

1. Introduction ............................................................................................................... 5

1.1. Purpose .............................................................................................................. 5

1.2. Scope ................................................................................................................. 5

2. Literature Review ...................................................................................................... 6

2.1. International Research ....................................................................................... 7

2.2. Previous UBC Research .................................................................................... 7

3. Experimental Procedure ........................................................................................... 7

3.1. Sequence ........................................................................................................... 7

3.2. Mix Design ....................................................................................................... 10

4. Results .................................................................................................................... 11

4.1. Analysis of Results ........................................................................................... 14

5. Conclusion .............................................................................................................. 14

5.1. Recommendations ........................................................................................... 15

6. Acknowledgements ................................................................................................. 16

7. References ............................................................................................................. 16

8. Appendices ............................................................................................................. 17

8.1. Appendix A: Data ............................................................................................. 18

8.2. Appendix B: Mix Design ................................................................................... 21

8.3. Appendix C: Sample Calculations .................................................................... 22

8.4. Appendix D: “Recommendations” as per Sudip Talukdar ................................ 25

iii

8.5. Appendix E: Testing Procedure Photographs .................................................. 26

8.6. Appendix F: Sikadur 52 Product Data Sheet .................................................... 30

iv

List of Figures

Figure 1: Sketch of Sample Cylinder ............................................................................... 8

Figure 2: Average compressive strengths at 14 days.................................................... 12

Figure 3: Average compressive strengths at 14 days.................................................... 12

Figure 4: Percent change in compressive strength at 14 days. ..................................... 13

Figure 5: Percent change in compressive strength at 14 days. ..................................... 13

List of Tables

Table 1: Requirements for Test Cylinders ....................................................................... 9

Table 2: Compilation of average compressive strengths ............................................... 11

5

1. Introduction

1.1. Purpose

For the CIVL 322 project based learning course in civil engineering materials, our group

was assigned to research the effectiveness of concrete crack repair with epoxy. All

concrete batching and compressive strength testing was undertaken in the UBC CEME

Materials Laboratory.

Concrete as a construction material is particularly sensitive to cracking. Although

cracks may be an early indication of impending failure, they can also be the result of

initial errors such as inadequate mix design or curing, or from environmental effects

such as water intrusion or chemical exposure.

Low-viscosity epoxy is often used as a crack sealant. Although both epoxy and

polyurethane injection products are used as a concrete repair material, epoxy is

generally considered the better product for application requiring structural repair as well

as sealing. This is due to the higher average compressive strength.

The product selected for this laboratory research, Sikadur 52, has a 28 day

compressive strength of 61 MPa. Although both the compressive strength of the epoxy

and the concrete are generally known, the structural capacity of the repaired concrete is

unknown. We have investigated the compressive strength of the repaired concrete,

relative to the strength prior to the crack and the strength of the cracked concrete.

1.2. Scope

The scope of this study is limited to the Sikadur 52 epoxy resin product, although

general results may be drawn between similar products. The strength correlations

apply to compression members loaded axially. Results apply to cracks which are

parallel to the loading axis.

6

2. Literature Review

The presence of a crack in a concrete structure can dramatically decrease its structural

capacity. The strength of the structure will decrease progressively with the increase of

the crack’s size. The most common methods of crack repair in concrete are epoxy

injection or gravity filling. These repair methods are minimally disruptive and do not

require the replacement of existing concrete. These methods have been tested for use

in the repair of reinforced concrete in both industrial and commercial applications,

however not in residential applications. Proper preparation of concrete surfaces is

crucial to ensuring a proper bond of epoxy to concrete.

The use of traditional Portland Cement based repair methods has been used for many

decades, however, less than satisfactory reports have been reported in technical

literature. In choosing a repair material one must consider why the repair is needed,

corrosion or cracking, along with the use of the structure being repaired. The

compatibility of the repair material as well as the substrate of concrete must be

considered as well in selecting a repair method and material. Dimensional compatibility,

the ability of the repair area to withstand volumetric changes without the loss of bond

and delamination, have been frustrating issues in concrete repair.

The bonding of a repair and the concrete substrate is essential in evaluating the

success of any repair method. The roughness of the repair area plays an important role

in the bonding of a repair material and the concrete substrate. Exposed coarse

aggregate will have a more durable bond in comparison to a smooth surface area. The

moisture content of both the repair material and the concrete substrate play a role in the

bonding properties as well. Likewise the shrinkage of the repair material needs to be

considered, Sikadur 52 has shrinkage free hardening, so shrinkage should play a small

role in bonding problems.

7

2.1. International Research

At the Lebanese American University, researchers performed compression testing on

concrete cube blocks to evaluate the effectiveness of crack repair with epoxy. Cracks

were formed using steel plate inserts instead of the above mentioned plastic inserts.

The research concluded that higher stress concentrations are induced by longer cracks.

The tests found that a shallower crack reduced the strength by 32.7% and deeper

cracks reduced strength by 40%. When the cracks are repaired with the epoxy, it was

found to reduce the amount of strength lost to 8 and 11% respectively. These results

clearly show the potential benefits in using the epoxy repair method.

2.2. Previous UBC Research

In 2010, a similar project was undertaken by a group of students in Civil 322 with the

intention of studying the effects of epoxy repair on concrete strength. The students used

a similar, but smaller plastic insert to create the simulated crack. They found that the

strength reduction between the control and the cracked cylinders was insignificant due

to the small depth of crack formed.

Upon review of the above experiment, it was suggested that the size and depth of the

crack was not sufficient enough to affect the compressive strength of the concrete. To

address this issue in our project, the depth of the insert was increased during this year’s

experiment, which will extend approximately three quarters of the length of the cylinder.

3. Experimental Procedure

Photographs of the experimental procedure can be found in Appendix E.

3.1. Sequence

Three Concrete mix designs of water cement ratios; 0.55, 0.45 and 0.35

Slump test, air test and cylinders batched for each mix design. Testing of fresh

concrete has been conducted in accordance with CSA specifications. This

8

testing includes slump (CSA A23.2-5C), air content (CSA A23.2-4C), and

cylinders casted in accordance with CSA A23.2-3C.

The cylinders used in this experiment are 75mm by 150mm, as seen in Figure 1,

and lubricated with form release oil. In order to simulate the crack in the

“damaged” and “epoxy repaired,” a rectangular insert approximately 5mm thick,

75mm wide, and 100mm high was inserted vertically into the top of the cylinder.

The insert was removed after 24 hours. The cylinders were cured in a water tank

with a basic pH.

Figure 1: Sketch of Sample Cylinder

30 cylinders casted per batch, 10 undamaged, 10 damaged with a crack and 10

damages with same crack and repaired with epoxy inserted between walls of the

crack.

9

W/C Ratio Undamaged Damaged Epoxy Repaired Total

14 days 28 days 14 days 28 days 14 days 28 days

0.35 5 5 5 5 5 5 30

0.45 5 5 5 5 5 5 30

0.55 5 5 5 5 5 5 30

Table 1: Requirements for Test Cylinders

Cylinders were casted in 2 separate lifts of concrete. A vibration table was used

to provide consolidation and removing entrapped air.

Sikadur 52 Epoxy Resin injected into repaired specimens as per Sikadur 52

Epoxy Resin representatives instructions. The product data sheet for Sikadur 52

can be found in Appendix F.

o Sikadur Anchorfix was used as a fast curing, more viscous epoxy sealant

for the crack boundaries. This contained the low viscosity Sikadur 52

Resin epoxy, otherwise it would free flow through the crack.

o Cylinders were repaired using the gravity feed method, and conforming to

product recommendations provided in the manufacturer’s product data

sheet (Appendix A).

All Concrete mixes designed to meet the following criteria:

o Target slump of 150mm (+/- 25mm)

o Air content of 7% (+/- 1%) with use of air entraining agent (Darax 2)

o Maximum nominal coarse aggregate of 10mm

o Fineness modulus of 2.8

o No supplementary cementing materials (fly ash, silica fume)

10

The 0.55 water cement ratio exceeded the specified slump. This has no negative

effect on comparison of the strength results as the strengths will be compared

within each mix design. Mix with low water cement ratio required the addition of

plasticizer to meet specifications.

Compressive testing has been conducted in accordance with CSA A23.2-9C.

The cylinders have been ground before the compression testing in order to

ensure planeness as per CSA A23.2-9C clause 4.1.3.

3.2. Mix Design

This experiment required three mix designs; water cement ratios of 0.55, 0.45, and

0.35. The initial mix design was based off the example provided in Design and Control

of Concrete Mixtures (DCCM). This is the absolute volume method. All components

were designed and proportioned for one cubic meter of concrete. The bulk density of

the coarse aggregate required for our design was provided by Sudip, Appendix D.

Proportioning was then calculated by keeping the mass of the coarse aggregate and

water constant, while adjusting the mass of the cement and fine aggregate. The

required water cement ratios were met using these adjustments. The final mix designs

for all three batches are provided in Appendix B.

On batch day, air entrainment and the modified batch proportions were added. Based

on previous experience, Sudip recommended the amount of 10mL of Darex II air

entrainment which we added to the mix on site. The modification equations, provided

by Sudip, for the water content and fine aggregate are presented below.

𝑀𝑎𝑠𝑠 𝐻2𝑂 𝑎𝑑𝑑𝑒𝑑 = 𝑀𝑎𝑠𝑠 𝐻2𝑂 𝑚𝑖𝑥 − 𝑀𝑎𝑠𝑠 𝐻2𝑂 𝑚𝑖𝑥 (𝐴𝑏𝑠 − 𝑀𝐶)

𝑀𝑎𝑠𝑠 𝐹𝑖𝑛𝑒 𝐴𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 𝑎𝑑𝑑𝑒𝑑 = 𝑀𝑎𝑠𝑠 𝐹𝑖𝑛𝑒 𝑎𝑔𝑔. 1 + 𝐴𝑏𝑠 − 𝑀𝐶

𝐴𝑏𝑠 − 𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛𝑠 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑡𝑕𝑒 𝑓𝑖𝑛𝑒 𝑎𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 𝑖𝑛 𝑑𝑒𝑐𝑖𝑚𝑎𝑙

𝑀𝐶 − 𝑀𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑜𝑓 𝑡𝑕𝑒 𝑓𝑖𝑛𝑒 𝑎𝑔𝑔𝑟𝑒𝑔𝑎𝑡𝑒 𝑖𝑛 𝑑𝑒𝑐𝑖𝑚𝑎𝑛𝑙

11

4. Results

In order to determine how using Sikadur 52 Epoxy Resin to repair a crack affects the

strength of concrete, the compressive strengths of undamaged, damaged and repaired

cylinders was tested and recorded. A compilation of all data collected for this

experiment can be found in Appendix A. The average compressive strengths and the

standard deviations of the results were calculated for each water cement ratio and have

been compiled in Table 2 below.

W/C Age Undamaged Damaged Repaired

Mean Std.Dev Mean Std.Dev Mean Std. Dev 0.35 14 days 57.51 1.81 52.18 2.10 52.15 2.17

28 days 63.84 0.92 54.95 1.31 57.11 0.88 0.45 14 days 39.74 1.41 35.87 1.09 36.58 1.23

28 days 44.08 0.18 41.45 0.90 37.66 5.39 0.55 14 days 34.56 1.53 31.18 1.17 - -

35 days 40.76 0.32 - - 37.70 1.52 Table 2: Compilation of average compressive strengths

A graphical analysis of the results seen in Table 2 indicates that the Sikadur 52 epoxy

does not significantly increase the compressive strength of the concrete. Figure 2

graphs the 14 day average compressive strengths and Figure 3 graphs the 28 day

average compressive strengths for the undamaged, damaged and repaired cylinders.

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0.35 0.45 0.55

Stre

ngt

h (

MP

a)

W/C Ratio

14 Day Cylinder Strength (Average)

Undamaged Damaged Repaired

12

Figure 2: Average compressive strengths at 14 days.

Figure 3: Average compressive strengths at 14 days.

Unfortunately, due to problems with the epoxy on the 0.55 W/C ratio cylinders, repaired

strength could not be determined for 14 days. The Sikadur 52 epoxy was applied to the

cracked cylinders after 7 days, but did not set properly, and was still tacky after 14 days.

This was later found to be due to improper settings on the application gun, which should

have been set to a 2:1 ratio. As a result sufficient data could not be collected to draw

accurate comparisons from this batch of cylinders.

When testing the 28 day strength of the 0.45 water cement ratio, there were anomalous

results for a series of cylinders, where compressive strength was only half of what it

should have been. This occurred immediately following a high strength test had been

performed on the machine. We halted testing and resumed after the machine had been

calibrated; the following tests met the expected strength. Though this anomalous data

was not used for our final results, it can still be found in Appendix A.

Using the average damaged strength for each age and water cement ratio the percent

change of compressive strength of the repaired cylinders compared to the damaged

was calculated. Figure 4 illustrates the data found for the 14 day strengths. The percent

change in strength of the repaired cylinders ranged from a decrease in strength of 6% to

an increase in strength of 5%.

0.00

20.00

40.00

60.00

80.00

0.35 0.45 0.55

Stre

ngt

h (

Mp

a)

W/C Ratio

28 Day Cylinder Strength (Average)

Undamaged Damaged Repaired

13

Figure 4: Percent change in compressive strength at 14 days.

The percent change of compressive strengths at 28 days can be seen in Figure 5. The

percent change in strength of the repaired cylinders ranged from a decrease in strength

of 24% to an increase in strength of 5%. The decrease of 24% seems to be an extreme

value and could be due to experimental error.

Figure 5: Percent change in compressive strength at 14 days.

Given the above data, it appears as though the Sikadur 52 Epoxy Resin does not have

a significant effect on the compressive strength of the cracked cylinders.

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

Pe

rce

nt

Ch

ange

(%

)

Percent Change 14 Days

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

1 2 3 4 5 6 7 8

Percent Change 28 Days

14

4.1. Analysis of Results

The results presented in Table 2 are indicative of the quality of the test specimens used

in this experiment. They show a low standard deviation for most test results, with the

exception of an outlier in 0.45 W/C ratio at 28 days, indicating that the fabrication of test

samples and application of repair materials was conducted in a careful and consistent

manner. The results obtained using these concrete specimens should be accurate

reproducible, provided that the specified testing and repair procedures are followed.

With respect to the actual test results, we found that the actual difference in strength

between undamaged and damaged cylinders was relatively small given the apparent

severity of the crack, with values between 6 and 14 percent strength loss. This gives a

smaller range over which the potential strength of repaired concrete samples can be

evaluated.

The results for the repaired cylinders show only a marginal difference in strength, with

the exception of the 0.45 W/C ration at 28 days, which actually showed a 9% decrease

in strength. This decrease is strongly influenced by an outlier in the break results from

that set. Overall, we did not observe any significant gains in compressive strength by

repairing cylinders. The average increase in compressive strength across sample

cylinders was 0.8%.

5. Conclusion

As shown in the analysis of results, the Sikadur 52 is not effective in increasing the

compressive strength of cracked concrete. The average gain in compressive strength

was found to be only 0.8%

During the experimental process, 5mm cracks were created, larger than most cracks

found. Furthermore, the concrete surface that bonds with the epoxy resin is smooth,

due to the rectangular insert used when casting, to simulate a crack. Due to the reduced

surface area of the concrete in comparison to the volume of the crack, less of the epoxy

resin bonded directly with the concrete.

15

Due to error in application of epoxy resin, the data for repaired strength of 0.55 W/C

ratios, 14 day cylinders cannot be used. Additionally, machine calibration errors cause

inaccurate measurements of the strength of the 28 day, 0.55 W/C damaged cylinder.

Due to these errors, the 0.55 W/C cylinders are not used in the analysis of this project.

However, when comparing the effectiveness of the epoxy resin with respect to the 0.35

and 0.45 W/C cylinders, there does not seem to be a trend. From the data, it can be

concluded that differing water-cement ratio does not change the effectiveness of epoxy

repaired concrete compressive strength.

5.1. Recommendations

When repairing cracks for the purpose of structural integrity, the epoxy resin is not

enough to restore the structure to pre-crack strength. More traditional methods of

concrete repair should be used if compressive strength is to be restored. That said, the

epoxy can still be used as a sealant to prevent freeze thawing, chemical attacks, and

other forms of corrosion that would affect the long term strength of the concrete.

To further test of compressive properties of epoxy repaired concrete, it is recommended

that more realistic cracked concrete conditions be applied. The 5 mm cracks are larger

than usual crack damage, and the crack surface is smooth. Furthermore, we

recommend testing be done on more crack thickness variations. The epoxy resin could

be more effective when applied to cracks that are less than or more than 5mm under

the same cylinder diameter. It is also recommended that the machine use to record the

compressive strength of cylinder be tested and calibrated before use with the real

cylinders.

In conclusion, while the Sikadur 52 is reported to be effective at sealing cracked

concrete, it does not significantly restore concrete compressive strength.

16

6. Acknowledgements

Sudip Talukdar, UBC CIVL322 T.A.

Daryl McColl Sika Representative, Sika Canada Inc.

7. References

Conversation with Sudip Talukdar on January 13, 2011 during introductory meeting at

CEME Materials Laboratory.

Issa, Camille A. and Debs, Pauls. “Experimental Study of Epoxy Repairing of Cracks in

Concrete.” Nov. 2004. Department of Civil Engineering, Lebanese American University,

Lebanon.

Morgan, D.R. “Compatibility of Concrete Repair Materials and Systems”. July 1995.

AGRA Earth & Environmental, Burnaby, BC

CUREE. “Testing and Assessment of Epoxy Injection Crack Repair for Residential

Concrete Stem Walls and Slab-On-Grade. 2002.

Sika. “Sikadur 52 Super-Low Viscosity Injection Resin – Product Data Sheet.” Edition

06.2010/v1. 2010.

Sika Canada Inc., Sikadur 52 Product Data Sheet, Edition 07.2007, CSC Master Format

03 64 23. Pointe Claire, QC., (2007), pp1-2.

17

8. Appendices

18

8.1. Appendix A: Data

19

20

21

8.2. Appendix B: Mix Design

EPOXY REPAIR MIX DESIGNS

PROJECT 12 - ST3

20-Jan11

0.55 W/C Ratio 1 m3 .05 m3 .075 m3 0.1 m3

Volume

(m3)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Coarse Agg. 0.266 713 35.65 53.475 71.3

Water 0.216 216 10.8 16.2 21.6

Air 0.07 0.196 0.0098 0.0147 0.0196

Cement 0.125 393 19.65 29.475 39.3

Fine Agg. 0.323 853 42.65 63.975 85.3

0.45 W/C Ratio 1 m3 .05 m3 .075 m3 0.1 m3

Volume

(m3)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Coarse Agg. 0.266 713 35.65 53.475 71.3

Water 0.216 216 10.8 16.2 21.6

Air 0.07

0 0 0

Cement 0.152 480 24 36 48

Fine Agg. 0.296 781 39.05 58.575 78.1

0.35 W/C Ratio 1 m3 .05 m3 .075 m3 0.1 m3

Volume

(m3)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Mass

(kg)

Coarse Agg. 0.266 713 35.65 53.475 71.3

Water 0.216 216 10.8 16.2 21.6

Air 0.07

0 0 0

Cement 0.196 617 30.85 46.275 61.7

Fine Agg. 0.252 665 33.25 49.875 66.5

22

8.3. Appendix C: Sample Calculations

8.3.1 Elastic Modulus

Young’s Modulus of Elasticity is defined as the slope of the tangent of the Stress Strain

curve at 40% of the failure load in uniaxial compression.

The average 40% failure load for the two 0.35 W/C cylinders tested is 183 kN,

translating to an applied stress of approximately 23 MPa. At this applied stress, the

strain is approximately 0.78. These values give a stress/strain value of 29.5 GPa.

23

y = 60587x - 2.132R² = 0.959

0

5

10

15

20

25

30

0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004 0.00045 0.0005

Stre

ss [

MP

a]

Strain (mm/mm)

Combined Stress Strain Curve for 0.35 W/C

y = -2E+07x2 + 52495x + 0.295R² = 0.969

0

2

4

6

8

10

12

14

16

18

20

0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004 0.00045

Stre

ss [

MP

a]

Strain (mm/mm)

Stress Strain Curve for Cylinder 0.45 W/C

24

y = 60045x + 0.731R² = 0.86

0

2

4

6

8

10

12

14

16

18

20

0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004

Stre

ss (

Mp

a)

Strain (mm/mm)

Stress Strain Curve for 0.55 W/C Cylinder

25

8.4. Appendix D: “Recommendations” as per Sudip Talukdar

26

8.5. Appendix E: Testing Procedure Photographs

Concrete drum mixer Application of Sika Anchorfix

Grinding cylinders prior to compression testing

27

Undamaged, repaired, damaged cylinders prior to compression testing (side)

Undamaged, repaired, damaged cylinders prior to compression testing (top)

28

Compression testing

Undamaged, damaged, repaired cylinder after compression testing (side)

29

Elastic modulus testing

30

8.6. Appendix F: Sikadur 52 Product Data Sheet

31

32