TESTING AND ASSESSMENT OF EPOXY INJECTION CRACK REPAIR FOR RESIDENTIAL CONCRETE STEM WALLS AND SLAB-ON-GRADE

Transcription

1 CUREE Publication No. EDA-01 TESTING AND ASSESSMENT OF EPOXY INJECTION CRACK REPAIR FOR RESIDENTIAL CONCRETE STEM WALLS AND SLAB-ON-GRADE NAHB Research Center, Inc. Upper Marlboro, MD July 2002 CUREE Earthquake Damage Assessment and Repair Project Consortium of Universities for Research in Earthquake Engineering

2 CUREE, the Consortium of Universities for Research in Earthquake Engineering, is a non-profit organization incorporated in 1988 whose purpose is the advancement of earthquake engineering research, education, and implementation. There are 28 University Members of CUREE located in 18 states and approximately 340 individual professor members. As its name states, CUREE is focused on research, earthquakes, and engineering. A basic criterion for all CUREE projects is the objectivity of the methodological phases of work as well as objectivity in the dissemination or implementation of the project results. CUREE s Website Integrity Policy provides a succinct statement of this principle: CUREE values its reputation as an objective source of information on earthquake engineering research and is also obligated to reflect the high standards of the universities that constitute CUREE s institutional membership. The following Website Integrity Policy is designed to assure those who use the CUREE website that we adhere to criteria appropriate to our non-profit purpose, rather than conforming to minimal prevailing commercial standards. CUREE provides a means to organize and conduct a large research project that mobilizes the capabilities of numerous universities, consulting engineering firms, and other sources of expertise. Examples of such projects include: Organization of the large, multidisciplinary conferences on the Northridge Earthquake for the National Earthquake Hazard Reduction Program federal agencies to bring together researchers and users of research; Participation in the SAC Joint Venture (CUREE being the C ), which conducted a $12 million project for the Federal Emergency Management Agency to resolve the vulnerabilities of welded steel frame earthquake-resistant buildings that surfaced in the 1994 Northridge Earthquake; Management of the CUREE-Caltech Woodframe Project, a $7 million project funded by a grant administered by the California Office of Emergency Services, which included testing and analysis at over a dozen universities, compilation of earthquake damage statistics, development of building code recommendations, economic analyses of costs and benefits, and education and outreach to professionals and the general public; Establishment for the National Science Foundation of the consortium that will manage the Network for Earthquake Engineering Simulation; Conducting research investigations in the USA jointly with Kajima Corporation researchers in Japan since the 1980s; Conducting the Assessment and Repair of Earthquake Damage Project, aimed at defining objective standards for application to buildings inspected in the post-earthquake context; Participation as a sub-awardee to the Southern California Earthquake Center in the Electronic Encyclopedia of Earthquakes project funded by the National Science Foundation. CUREE Published by Consortium of Universities for Research in Earthquake Engineering 1301 S. 46 th Street Richmond, CA (CUREE Worldwide Website)

3 Disclaimer The Information in this publication is presented as a public service by the Consortium of Universities for Research in Earthquake Engineering (CUREE). No liability for the accuracy or adequacy of this information is assumed by them, their sponsors, or their contractors. CUREE Publishing First Printing: July 2002 The goal of the Assessment and Repair of Earthquake Damage Project is to develop guidelines that provide a sound technical basis for use by engineers, contractors, owners, the insurance industry, building officials, and others in the post-earthquake context. Based on experimental and analytical research and a broad discussion of the issues involved, the guidelines produced by the project will reduce disparities in the evaluation of building damage and the associated need for repairs.

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5 TESTING AND ASSESSMENT OF EPOXY INJECTION CRACK REPAIR FOR RESIDENTIAL CONCRETE STEM WALLS AND SLABS-ON-GRADE July 2002 Prepared for Consortium of Universities for Research in Earthquake Engineering Richmond, CA by NAHB Research Center, Inc. Upper Marlboro, MD

6 ABOUT THE NAHB RESEARCH CENTER, INC. The NAHB Research Center is a not-for-profit subsidiary of the National Association of Home Builders (NAHB). The NAHB has 200,000 members, including 50,000 builders who build more than 80 percent of new American homes. The NAHB Research Center conducts research, analysis, and demonstration programs in all areas relating to home building and carries out extensive programs of information dissemination and interchange among members of the industry and between the industry and the public. Research Center staff contributing to this project include: Jay H. Crandell, P.E. (technical reviewer); Shawn P. McKee, P.E. (principle investigator); Chad Garner (construction specialist); Bryan Adgate (technician). ii

7 ACKNOWLEDGEMENTS This research was done under subcontract to the Consortium of Universities for Research in Earthquake Engineering (CUREE) as part of the project Assessment and Repair of Earthquake Damage in Residential Buildings funded by the California Earthquake Authority. The aim of that project is to produce Guidelines for the Assessment and Repair of Earthquake Damage, and CUREE's Project Manager is Dr. John Osteraas. Technical oversight for this research project and manuscript review was provided by the Foundation Advisory Subcommittee, membership of which consists of: David Bonowitz, S.E. Rutherford & Chekene San Francisco, CA Professor Andre Filiatrault Department of Structural Engineering University of California, San Diego Robert Gaul, Vice President ChemCo Systems, Inc. Redwood City, CA Professor John F. Hall California Institute of Technology Pasadena, CA Ed Kavazanjian, G.E. GeoSyntec Consultants Huntington Beach, CA John Osteraas, Ph.D., P.E. Exponent Failure Analysis Associates, Inc. Menlo Park, CA Robert Reitherman, Executive Director Consortium of Universities for Research in Earthquake Engineering Richmond, CA Professor Jonathan Stewart University of California, Los Angeles Dimitri Vergun, S.E. Structural Engineer Santa Monica, CA iii

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9 TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS... iii INTRODUCTION...1 TEST PLAN...1 TEST PROCEDURES...4 TEST RESULTS...10 CONCLUSIONS AND RECOMMENDATIONS...22 REFERENCES...24 v

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11 INTRODUCTION This research report addresses the performance of epoxy injection crack repair in plain (i.e., unreinforced or lightly reinforced) concrete stem walls and slab-on-grade floors found in typical single-family residential construction. While the research is intended to address the efficacy of repairs in the context of possible seismic damage, the issue of cracked foundations and floor slabs has a much broader association with housing performance problems. In fact, it is ranked among the most common home warranty claims and home builder concerns [1] [2]. Unfortunately, there is little data regarding the effectiveness or appropriateness of various repair methodologies for plain, residential concrete foundations and floor slabs. Crack control methodologies are not generally employed in the construction of residential foundations and floor slabs. Thus, observable cracking in stem walls and slab-on-grade floors is common and is primarily attributable to concrete shrinkage and differential settlement. Historically, such cracks are typically ignored unless they lead to other problems. Obviously, it is desirable to repair cracks that compromise the structural performance of the foundation, although the circumstances under which that performance is compromised are not well defined. Sealing of cracks may also be desired to prevent moisture, pest, and/or potential radon infiltration into the residence. One of the most common and popular repair methods employed to repair cracks in concrete is epoxy injection. This repair method is minimally disruptive and cost-effective because it does not require the replacement of existing concrete. While extensively tested and widely used for repair of reinforced concrete in commercial and industrial applications, the performance of such repairs in typical residential applications is not well documented [3]. The testing program described herein addresses the effectiveness of repair for a range of typical crack widths and access limitations encountered in typical residential concrete. In particular, access for repair is limited to one side of the specimen. TEST PLAN A total of 18 stem wall and 18 slab test specimens were constructed to evaluate the performance of the epoxy injection repair process. An overview of the stem wall and slab specimens constructed is given in Table 1. TABLE 1 STEM WALL AND SLAB TEST SPECIMENS SPECIMEN TYPE Stem wall Slab DESCRIPTION 5.5 in thick x 18 in depth x 8 ft length 3.5 in thick x 3.0 in width x 4 ft length Page 1 of 24

12 Table 2 illustrates the test matrix used in this study. Two repetitions were performed for each specimen type, loading condition, and crack condition. Twenty-four flexural tests were performed on uncracked specimens to estimate the original modulus of rupture (MOR) and to crack the specimens for the purpose of subsequent repair and retesting. Twelve flexural and 12 shear tests were then performed on repaired specimens to determine the performance of the repaired specimens. Four uncracked slab specimens were also tested using the shear test setup intended to subject repaired cracks to a high transverse shear stress. In particular, the loading point was very close to the bearing reaction in the stem wall specimens which created arching of the load to the bearing surface through compressive stress in the concrete. Thus, the shear tests should not be interpreted as indications of crack repair performance relative to concrete shear capacity because the test configuration is not designed to produce a concrete shear failure (i.e., diagonal tension crack) and may overstate actual shear capacity of concrete under more normal loading conditions. The shear tests, however, do represent loading conditions on a repaired crack that could produce a high transverse shear stress at the plane of the crack (i.e., crack adjacent to a point load or linearly distributed uniform load). TABLE 2 TEST MATRIX SPECIMEN CRACK WIDTH LOADING REPS COMMENTS Stem wall Slab Flexure (3-point) 1 Crack initiation of all stem wall 12 Uncracked specimens; baseline original MOR Shear 2 Baseline for original transverse shear capacity <1/16 inch Repaired Flexure (4-point) 2 ( in) Shear 2 1/8-inch Repaired Flexure (4-point) 2 Evaluate effect of crack width on flexural Shear 2 and shear performance of repaired stem wall specimens 1/4-inch Repaired Flexure (4-point) 2 Shear 2 Flexure (3-point) 1 Crack initiation of all slab specimens; 12 Uncracked baseline original MOR Shear 2 Baseline for original transverse shear capacity <1/16-inch Repaired Flexure (4-point) 2 ( in) Shear 2 Flexure (4-point) 2 Evaluate effect of crack width on flexural 1/8-inch Repaired and shear performance of repaired slab Shear 2 specimens Flexure (4-point) 2 1/4-inch Repaired Shear 2 Notes: 1. The flexural 3-point loading was used to initiate all cracks that were set for repair. The specimens were cast in lumber forms. Each specimen contained 6x6 welded wire reinforcement in the outer third of the span. The stem walls contained four 0.5 inch diameter L -bolts in the top of the specimen to facilitate movement of the specimens. All stem wall and slab specimens contained two 0.5 inch diameter metal conduits. The conduits were greased prior to placing the concrete to reduce adhesion. Once the Page 2 of 24

13 specimens were initially cracked, inch diameter threaded rods were inserted into each conduit and tightened to prevent further degradation of the specimens. Figure 1 illustrates a typical slab form prior to concrete placement. Figure 1 Typical slab specimen with welded wire reinforcement and conduit. A conventional concrete mix with a design value of f c = 2,500 psi was used for the stem wall and slab specimen construction and was supplied by a local ready-mix company. Quantities of materials incorporated per cubic yard of concrete are shown in Table 3. Adjustments were made to optimize workability, which had a measured on-site slump of 6 inches according to ASTM C143 [4]. TABLE 3 CONCRETE MIX DATA PER CUBIC YARD CONCRETE STEM WALLS CONCRETE SLABS Mix Ingredient Quantity Mix Ingredient Quantity Cement Type I 435 lb Cement Type I 450 lb Concrete Sand 1,553 lb Cement Class F 67 lb #67 Washed Gravel 1,600 lb Concrete Sand 1,254 lb Daravair oz #67 Washed Gravel 1,800 lb WRDA with HYCOL oz Daravair oz Water 267 lb Water 267 lb For SI: 1 lb = 4.45 N, 1 oz = 0.28 N, 1 gal = 3.79 l Notes: 1 Daravair 1000 is an air-entraining admixture and is formulated to comply with Specification for Air-Entraining Admixtures for Concrete, ASTM Designation C WRDA with HYCOL is a water-reducing admixture and is formulated to comply with Specification for Chemical Admixtures for concrete, ASTM Designation C 194. Page 3 of 24

14 Concrete was placed in the forms via the chute on the concrete truck and consolidated using an electric vibrator. Cylindrical concrete specimens were also cast following ASTM C39 [5], ASTM C31 [6], and ASTM C192 [7]. The 6 inch x 12 inch cylinders were cast to represent the concrete incorporated in the test specimens. After 48 hours the cylinders were split into two batches. The first batch was moist cured, while the second batch was field cured. Cylinders were tested throughout the testing program to determine the representative compressive strength of the concrete. Table 4 summarizes the results. Test Specimens STEM WALL SPECIMENS 53 days (psi) TABLE 4 CONCRETE COMPRESSION TESTS 140 days (psi) Test Specimens SLAB SPECIMENS 65 days (psi) 145 days (psi) 3,121 3,168 2,751 2,761 Moist-Cured Moist-Cured 3,102 3,298 2,775 2,874 AVERAGE 3,112 3,233 AVERAGE 2,763 2,817 2,540 2,582 2,565 2,650 Field-Cured Field-Cured 2,282 2,656 2,644 2,768 AVERAGE 2,411 2,619 AVERAGE 2,605 2,709 TEST PROCEDURES Tests on the specimens commenced at a concrete age of 46 days and continued over a duration of 145 days from the time the specimens were cast. The specimens were tested in the Universal Test Machine (NIST Traceable) at the NAHB Research Center, Upper Marlboro, MD. A 200,000 lb load cell was used to enable load recordings and electronic deflection gauges were used to enable deflection recordings. Cracks were initiated in the specimens by applying a single load at mid-span. All specimens were tested under a displacement controlled protocol at a rate of 0.01 in/minute for the stem wall specimens and at a rate of in/min for the slab specimens. Once the cracks were initiated, inch diameter threaded rods were inserted into the conduits and tightened to prevent further degradation of the specimens. The stem wall specimens were set in a vertical position with a loose sand backfill and a 2 inch x 6 inch nominal size wood sill plate attached to the top of the wall using L-bolts to simulate actual conditions. The slab specimens were placed horizontally on a 3-inch thick loose sand layer. It is noted that loose sand represents a difficult boundary condition for epoxy injection, as it provides very little resistance to the flow of epoxy out of the crack. A loose sand layer is sometimes found beneath concrete slab-on-grade floors, thus the test conditions are representative of conditions likely to be encountered in the field. Footings and below grade portions of stem walls are typically cast directly against firm soil, thus the test conditions were more challenging than conditions likely to be encountered in the field. Page 4 of 24

15 Each crack was characterized as a <1/16-inch crack following initial testing with crack widths ranging from inch to inch. The 1/8-inch and 1/4-inch cracks were obtained by loosening the inch diameter threaded rod and rocking the specimens about mid-span until the desired crack width was obtained. All cracks were repaired using an experienced local contractor, Structa-Bond of Maryland, Ltd., and a structural concrete bonding process. The process consists of injection of two component epoxy resins, of various viscosities, into cracks in concrete. ChemCo Systems, Inc., the producer of one of the systems of epoxy injection used in this study, has developed a guideline specification for repairs [8]. The International Concrete Repair Institute (ICRI) has also developed a non-proprietary Guide for Verifying Field Performance of Epoxy Injection of Concrete [9]. These guidelines are generally applicable to crack repairs were the crack is accessible from all sides and sealed to prevent seepage of epoxy during the injection and curing process. Some guidance on conducting one-sided repairs is found in Epoxy Injection in Construction [10]. The quality assurance (QA) methods recommended in the Chemco and ICRI guidelines to verify complete crack filling, bond, and epoxy curing were specifically excluded from this study, reflecting the fact that QA methods are not commonly used in residential repair work. The following types of epoxy were used in the repairs: Low Viscosity ( Grade 1 ) Kemco 038 Reg IR (viscosity = 350 cp) Medium Viscosity ( Grade 2 ) MasterBuilder s CONCRESIVE Liquid LPL (viscosity = 90 cp at 77 F) High Viscosity ( Grade 3 ) MasterBuilder s CONCRESIVE Paste LPL For external sealing of the cracks and sealing of ports, MasterBuilder s CONCRESIVE 1422 epoxy was used. Concrete Repair The concrete surface was prepared by removing all loose aggregate, cement-based materials, and soil inside of and adjacent to the cracked area. Wire brushing by hand and compressed air were the methods used to prepare the concrete surface and crack. Once the surface was prepared, entry ports were placed along the crack (Figures 2 and 3) and an epoxy surface seal was applied to the exposed surface of the crack. Different diameter ports were used depending on the width of the crack. Figures 4 and 5 show the surface seal and ports along cracks in slab and stem wall specimens, respectively. Rubber grommets (ports) with a small inside diameter were used for the <1/16-inch cracks that were always injected with a low viscosity (Grade 1) epoxy repair mix using the automatic dispenser (see Figure 6). Drywall nails were used as plugs in the ports. For the 1/4-inch cracks, a rigid plastic tubing (port) with a larger inside diameter was used for manual injections using a caulk gun dispenser and the Grade 2 and Grade 3 epoxy repair mixes (see Figure 7). Page 5 of 24

16 Figure 2 Insertion of entry ports for a <1/16-inch crack slab specimen. Figure 3 Insertion of entry ports for a typical 1/4-inch crack stem wall specimen. Page 6 of 24

17 Figure 4 Surface seal paste and ports installed along the crack plane of a typical slab specimen. Figure 5 Surface seal paste and ports installed along the crack plane of a typical stem wall specimen. Page 7 of 24

18 Figure 6 Automatic metering, mixing, and dispensing device used in a <1/16-inch crack slab specimen. Figure 7 Caulk gun used to manually dispense the epoxy mixture in a 1/4-inch crack slab specimen. Page 8 of 24

19 Based on an inspection of the 1/8-inch cracks, including use of a magnifying glass to assess roughness of the concrete surfaces inside the crack (not all cracks had the same interior surface roughness or uniform width), the contractor selected an approach deemed suitable to effectively fill the crack with epoxy. For some, the low viscosity (Grade 1) epoxy was selected which required the use of the smaller ports as described above for the <1/16-inch cracks. For others, a medium viscosity Grade 2 or blended viscosity (Grades 2 and 3 mixed together) epoxy was used based on contractor experience or trial and error. It should be noted that the medium (Grade 2) and high (Grade 3) viscosity epoxies were mixed by hand, whereas the low (Grade 1) viscosity epoxy was automatically mixed and dispensed. Hand mixing was done in batches of quantity sufficient to perform repairs on approximately two specimens at a time. The spacing of the ports was generally equal to the depth of the wall or slab 4 inches for the slabs and 5-1/2 inches for the stem walls but in some cases, a spacing as wide as three times the slab thickness was used. All repairs were done with access to only one side of the specimen to represent field repair conditions. With all crack widths, the epoxy was injected until it flowed out of the adjacent port. The port being injected was then plugged with a nail. The ports were generally injected sequentially across the crack on the slabs and from bottom to top on the stem walls. However, on some stem walls, the injection was initiated at a more centrally located port. The contractor stated that, given that the ports are spaced at a distance equal to the depth of the crack, the epoxy should reach the next port and the back of the crack simultaneously. After the last port was filled in each specimen, the contractor re-opened the first port and injected additional epoxy if deemed necessary by judgment to ensure that the crack was completely filled. A clear indication of complete filling, such as refusal of further injection or an increase of injection pressure, was not experienced due to the nature of the one-sided repair resulting in only partial surface sealing of the crack. When the crack was properly filled, epoxy ran out of the last port. Otherwise, the entire injection process was repeated until the crack was considered full. It should be noted that during the repair process, the contractor stated that every one has pumped different so far, indicating the variation in real and perceived conditions experienced in making the repairs and selecting the appropriate epoxy mix. Climate conditions were generally warm and humid with periods of thunderstorms and rain that disrupted repairs. The delays due to weather did not appear to have an impact on the effectiveness of repairs. Table 5 summarizes the epoxy selection and repair procedure used for the three crack width categories and various stem wall and slab specimens. Page 9 of 24

20 TABLE 5 SUMMARY OF CRACK REPAIR METHODS CRACK WIDTH SPECIMEN SAMPLE NUMBER SELECTED EPOXY AND APPLICATION <1/16-inch All stem wall and slab 1/8-inch After crack preparation, Grade 1 low viscosity epoxy was applied with automatic mixer/injector. specimens SLABS #7 Bending First tried Grade 2 medium viscosity epoxy, but too thin. Then, parts A and B of the high viscosity epoxy (Grade 3) were mixed with the Grade 2 by hand and manually injected using a caulk gun to fill the crack. Ports were spaced at 10 inches on center. #8 Bending After crack preparation, Grade 1 low viscosity epoxy was injected #9 Shear Same as #8 above #10 Shear Same as #8 above STEM WALLS #5 Bending After crack preparation, Grade 1 low viscosity epoxy was first injected in the lowest port to create an epoxysand plug at the base. Then, the intermediate viscosity epoxy (Grades 2 and 3) was mixed by hand and manually injected using a caulk gun. #7 Bending Same as #5 above, except Grade 2 medium viscosity epoxy was used instead of blended Grades 2 and 3. Injection was done from the middle port only. #8 Shear Same as #7 above. #13 Shear After crack preparation, Grade 1 low viscosity epoxy was applied with automatic mixer/injector for a long time. 1/4-inch All stem wall and slab specimens After crack preparation, Grade 1 low viscosity epoxy was first injected in the bottom port to create an epoxysand plug on the inaccessible base of stem walls only. For all slabs and stem walls, a Grade 3 high viscosity epoxy was mixed by hand and manually injected using a caulk gun to fill the crack. For slab #4, a Grade 2 medium viscosity was attempted first, then followed with Grade 3 high viscosity epoxy. TEST RESULTS Initial Cracking A three-point loading configuration (Figures 8 and 9) was used to ensure cracking near mid-span of the specimens while still allowing a natural crack to occur and propagate through the specimens. However, because of the non-homogeneous material property characteristics of concrete, crack locations varied within a few inches of the center span of the specimens. Figure 10 depicts a typical initial cracking failure of a stem wall specimen. The crack formation resulted in a sudden decrease in applied load. The specimens were subject to this initial testing primarily to form a crack suitable for repair and subsequent re-testing. However, the data were also used to determine an index of the original modulus of rupture (MOR) of the concrete. Tables 6 and 7 summarize the initial cracking test data. Due to their short length, the specimens were essentially free of uniform axial strain that would develop in typical residential applications due to restrained shrinkage. Therefore, the flexural MOR determined in this test program Page 10 of 24

21 probably represents an upper bound on the expected strength of a typical stem wall or slab in situ Figure 8 Stem wall crack initiation test set-up using a three-point loading applied at specimen end bearing points and at mid-span Figure 9 Slab crack initiation test set-up using a three-point loading applied at specimen end bearing points and at mid-span. Page 11 of 24

22 Figure 10 Typical initial failure crack of a stem wall specimen subjected to three-point loading. TABLE 6 SUMMARY OF THE INITIAL CRACKING TEST DATA FOR THE STEM WALL SPECIMENS 1 SAMPLE NUMBER ULTIMATE LOAD (lb) LOADING SPAN (in) M max 2 (in-lb) MOR 3 σ max (psi) 9 5, , , , , , , , , , , , , , , , , , , , , , , , Average = COV = 13.1% Note: 1 Specimens were tested from 46 to 52 days after casting. Field cured compressive strength is estimated as 2,411 psi based on data in Table 4. 2 Calculation of M max does not include effect of specimen weight of approximately 100 lb/ft. 3 MOR calculated based on moment, M max at mid-span, not at the location of crack initiation. Page 12 of 24

23 TABLE 7 SUMMARY OF THE INITIAL CRACKING TEST DATA FOR THE SLAB SPECIMENS 1 SAMPLE NUMBER ULTIMATE LOAD (lb) LOADING SPAN (in) M max 2 (in-lb) MOR 3 σ max (psi) 14 1, , , , , , , , , , , , , , , , , , , , , , , , Average COV 13.8% Note: 1 Specimens were tested from 57 to 65 days after casting. Field cured compressive strength is estimated at 2,605 psi based on data in Table 4. 2 Calculation of M max does not include effect of specimen weight of approximately 106 lb/ft. 3 MOR calculated based on moment, M max at mid-span, not at the location of crack initiation. While the initial estimated MOR is used as an index to compare with the MOR determined for the same specimens after repair (see ratios reported later in Tables 8 and 10), it should not be considered as an actual representation of a particular repair s effectiveness relative to the original strength of the concrete. As used in this report, the ratio is not corrected for actual crack location relative to theoretical moment in the initial three-point load tests reported values overstate the actual MOR by roughly 2.5 percent for every inch off center that the crack initiation occurs. It is also not corrected for effects in differences in cure time between initial and repaired specimen testing (minor impact). It also does not take into consideration possible effects of cumulative damage where initial testing may have influenced performance of the concrete in repaired specimen testing, particularly in the near mid-span regions of the specimens that were highly stressed in the initial testing. However, this latter concern may be representative of conditions that are present in the field when cracks are caused by external forces imposed on the concrete stem wall or slab. As a final concern, differences in volume effect related to the amount of concrete exposed to high stress is not considered. For example, a greater volume of concrete was subjected to higher bending stress in the four-point load test than in the three-point load test. In effect, the normal variation in concrete strength along the length (or volume) of a specimen would tend to decrease MOR values in the four-point loading (repaired specimen flexure tests) relative to the three-point loading used for initial cracking. Therefore, the ratio of initial cracking MOR to repaired specimen MOR is used in this report purely for the purpose of providing a relative comparison or index between specimens. For the purpose of determining effectiveness of concrete repair, the more reliable basis is considered to be Page 13 of 24

24 the location of cracking experienced in post-repair testing relative to the location of the repaired crack. An index using code-nominal MOR is also provided for a relative comparison to design practice using ACI 318 [11]. Strength reduction factors are not applied to the code nominal values as required by ACI 318 for design purposes. Using data in Tables 4, 6, and 7, the average MOR estimated from the initial cracking tests is related to the concrete compressive stress as follows: MOR = 9.9 sqrt(f c ) for the stem wall specimens and MOR = 6.7 sqrt (f c) for the slab specimens. These values are within the range expected for normal weight concrete and agree reasonably well with the value of 7.5 sqrt (f c) recommended in ACI-318 Section for determination of the effective moment of inertia of concrete beams. It is noted that ACI-318 Section 22.5 requires the use of 5 sqrt(f c) for flexural strength design of unreinforced concrete beams [11]. Stem Walls After Repair The stem wall specimens were repaired as described previously and tested according to two different procedures. A four-point loading procedure was used to induce a region of maximum moment and zero shear in the middle third of the repaired specimen to evaluate the performance of the repaired specimen in pure flexure (Figure 11). Figure 12 depicts the setup used to develop increased transverse shear forces in the vicinity of the repaired crack to evaluate the transverse shear performance of the repaired specimen. Two test specimens of each crack width were tested using the two procedures. All repaired cracks were within the central third span of each specimen. Performance attributes of interest were primarily location of crack relative to initial repaired crack and, secondarily, MOR of the repaired specimen Repaired Crack 5.5 Figure 11 Stem wall flexural test set-up using a four-point loading applied at end bearing points and at one-third points along span. Page 14 of 24

25 9 18 Span SAMPLE NUMBER Figure 12 Stem wall shear test set-up (see Table 9). Performance attributes of the repaired specimens tested in pure flexure using the test set-up shown in Figure 11 are presented in Table 8. As discussed in the following paragraphs, there was a distinct difference in the failure mode of the <1/16-inch and 1/4-inch crack specimens versus the 1/8-inch crack specimens. CRACK WIDTH (in) TABLE 8 SUMMARY OF POST-REPAIR STEM WALL FLEXURAL TEST DATA 1 ULTIMATE LOAD (lb) LOADING SPAN (in) M max 2 (in-lb) MOR σ max (psi) RATIO OF REPAIRED MOR TO INITIAL MOR RATIO OF REPAIRED MOR TO CODE- NOMINAL VALUE OF 250 PSI 9 <1/16 7, , <1/16 7, , /8 6, , /8 5, , /4 7, , /4 6, , FAILURE MODE 3 Bending, new crack in concrete Bending, new crack in concrete Bending, failure in repaired crack 4 Bending, failure in repaired crack 5 Bending, new crack in concrete Bending, new crack in concrete Notes: 1 Specimens were tested within 105 days to 109 days after casting. 2 Calculation of M max does not include the specimen weight of approximately 100 lb/ft. 3 Each specimen was dismantled after testing to determine if the failure plane intersected the repaired crack. 4 Crack was completely filled with Grade 3 high viscosity epoxy, but epoxy appeared not to completely harden. Epoxy bond on surfaces of cracked concrete failed during test. 5 Top two-thirds of crack was void of Grade 2 medium viscosity epoxy. Facing surfaces of the crack did have a thin remnant coating of epoxy that appears to have seeped out of the crack prior to curing. Page 15 of 24

26 Most significantly, the failure mode of the <1/16-inch and 1/4-inch crack specimens was a new flexural crack that propagated through the parent concrete, not through the repaired crack, indicating that both the bond strength and the material strength of the epoxy exceeded the strength of the concrete adjacent to the repaired crack. The two specimens with lowest initial strength (Specimens 1 and 9) as shown in Table 6, had repaired strengths (MOR) essentially equal to their original strengths. The two specimens with the highest initial strengths (Specimens 2 and 11) had repaired failure strengths of less than their original value, but greater than the original strengths of the two weaker samples (Specimens 1 and 9). Thus, the repairs to these specimens provided strengths comparable to the initial concrete strength without any loss of strength associated with the epoxy repair itself. The repaired strength of all four specimens also exceeded the code-nominal flexural strength of 250 psi allowed by ACI- 318 for unreinforced concrete with a nominal design compressive strength, f c, of 2,500 psi. Figure 13 Typical failure of stem wall specimens outside of repaired crack. Figure 14 Dismantled stem wall specimen #1 (1/4-inch crack) with the parent concrete evident along the failure plane. The repair strategies used with the 1/8-inch crack specimens did not perform as well, resulting in failures in the repaired crack and ratios of initial to repaired specimen MOR of much less than one (i.e., 0.6 and 0.5 in Table 8). In specimen #5, there was an apparent mixing problem of the epoxy which resulted in a relatively soft epoxy after curing. The epoxy coverage in the joint was near 100 percent indicating that an appropriate viscosity was used. However, the failure occurred along the epoxy-concrete bond even though the bond was sufficient to cause a residue of concrete to remain adhered to the epoxy surface. The apparent mixing problem may be attributed to a possible measuring error in mixing the two part epoxy, a consequence of varying Page 16 of 24

27 conditions during the overall repair process, or a procedural problem in blending the Grade 2 and Grade 3 epoxy mixes. For specimen #7, the lower performance can be attributed to selection of an epoxy grade with a viscosity that was too low (i.e., Grade 2). As a result, a thin layer of epoxy coated each side of the crack plane, but with roughly two-thirds of the repair having voids (i.e., the top two-thirds of the crack was not filled because of apparent seepage of the epoxy from the crack by gravity prior to cure). The bottom one-third of the crack was bonded which gave some capacity for the direction of loading as tested. If this specimen had been tested in the opposite direction (with the voids on the tension side of the specimen during testing), the performance would have been substantially lower. Figures 15 and 16 show failure along the repaired crack of specimen #7. Figure 15 Typical failure along repaired crack of a 1/8- inch crack stem wall specimen. Figure 16 Dismantled stem wall specimen #7 (1/8-inch crack) with epoxy coating only the crack surface in the top two-thirds (above line in figure). The parent concrete in the bottom third indicates that the crack was filled with epoxy in this region (below line in figure). Table 9 provides a summary of the shear test data for the stem walls. Although the shear set-up in Figure 12 was used to increase the shear stresses in the vicinity of the repaired crack, the failure mode was characterized as a bending failure by formation of a new crack in the concrete near mid-span well away from the location of the repaired crack in all but one case (specimen #8), consistent with commentary paragraph R of ACI 318 [11]. Specimen #8 used a repair strategy identical to that of specimen #7 discussed above for the bending tests. That is, the selected epoxy viscosity was too low, resulting in about the top two-thirds of the repaired crack being void of epoxy (except for a coating remaining on the crack surfaces). With the exception of specimen #8, the repaired cracks were subjected to a minimum transverse shear stress of 150 psi Page 17 of 24

28 (assuming uniform stress across the beam section containing the crack) without failure of the repaired crack. Specimen #8 failed at a transverse shear stress of 66 psi. SAMPLE NUMBER TABLE 9 SUMMARY OF POST-REPAIR STEM WALL SHEAR TEST DATA CRACK WIDTH (in) ULTIMATE LOAD (lb) LOADING SPAN (in) V max 1 (lb) 15 Uncracked 27, ,802 Bending 16 Uncracked 23, ,521 Bending FAILURE MODE 12 <1/16 18, ,813 Bending, new crack in concrete 10 <1/16 19, ,801 Bending, new crack in concrete 8 1/8 7, ,531 Shear, failure in repaired crack /8 19, ,125 Shear/Bending, new crack in concrete 3 3 1/4 20, ,967 Bending, new crack in concrete 4 1/4 21, ,776 Bending, new crack in concrete Notes: 1 Calculation of V max does not include the specimen weight of approximately 100 lb/ft. 2 Grade 2 medium viscosity epoxy was used. Crack was mostly void of epoxy except for a remnant coating on facing surfaces of the crack. 3 Grade 1 low viscosity epoxy was used. Interestingly, specimen #13 (also with a 1/8-inch repaired crack) did not experience a transverse shear failure indicating that the repair strategy used was potentially effective. The low viscosity (Grade 1) epoxy was used. Based on stem wall specimen #7, one would expect that using the low viscosity epoxy should have resulted in notably poor performance due to increased voids. Because the ultimate failure of shear test specimen #13 was characterized as a combined shear (diagonal tension) and bending failure of the concrete, it is difficult to assess the actual performance of the repair in this specimen, except to state that the transverse shear capacity along the plane of the repaired crack was sufficient to cause a different failure mode to occur which is not associated with the transverse shear capacity of the repair itself. It should be noted that the crack that formed during the shear test of specimen #13 did occur in the high shear region and intersect the repaired crack, following along the repaired crack for about one-fourth of the stem wall depth (height) and then angling outward from the repaired crack. Slabs After Repair The slab specimens were repaired as described previously and were tested following two different test procedures. A traditional four-point loading procedure was used to induce a region of maximum moment and zero shear in the middle third of the specimen to evaluate the flexural capacity of the specimen after repair (Figure 17). Figure 18 depicts the setup used to develop increased transverse shear stresses in the vicinity of the crack to evaluate the transverse shear performance of the specimen after repair. A shear span of 10 inches was used to accommodate the angling of cracks across the surface of the specimens. The specimens were loaded such that the top of each slab specimen was subject to tension. Page 18 of 24

29 Repaired Crack Figure 17 Slab flexural test set-up using a four-point loading applied at end bearing points and at one-third points along span (see Table 10) Span Repaired Crack 30 Figure 18 Slab shear test set-up (see Table 11). Table 10 summarizes the flexural test results for slab specimens tested using the set-up shown in Figure 17. As with the stem wall tests, all of the repaired slab specimens with <1/16-inch and 1/4-inch cracks failed by development of a new crack in the specimen (see examples in Figures 19 and 20). Thus, these specimens indicate that an effective epoxy viscosity and repair strategy was selected and executed. This outcome is further evidenced by ratios of repaired to initial MOR that generally exceeded a value of 1. As an additional point of comparison, the MORs of the repaired specimens are compared to the code-nominal flexural strength of unreinforced concrete as similarly done in Table 8 for the stem wall specimens. All specimens, except specimen #8, exceed the code nominal flexural strength by a margin of about 30 percent or more. Page 19 of 24

30 SAMPLE NUMBER CRACK WIDTH (in) ULTIMATE LOAD (lb) TABLE 10 SUMMARY OF SLAB FLEXURAL TEST DATA LOADING SPAN (in) M max 1 (in-lb) MOR σ max (psi) RATIO OF REPAIRED MOR TO INITIAL MOR RATIO OF REPAIRED MOR TO CODE- NOMINAL VALUE OF 250 PSI 14 <1/16 3, , <1/16 3, , /8 3, , /8 1, , /4 3, , /4 2, , Notes: 1. Calculation of M max does not include weight of the specimen of approximately 106 lb/ft. FAILURE MODE Bending, new crack in concrete Bending, new crack in concrete Bending, new crack in concrete Bending, failure in repaired crack Bending, new crack in concrete Bending, new crack in concrete Figure 19 Typical failure of slab specimens with repaired <1/16-inch crack width (new crack adjacent to but outside of repaired crack) Figure 20 Typical failure of slab specimens with repaired 1/4-inch crack width (new crack outside of repaired crack) Page 20 of 24

31 Results for the 1/8-inch crack width varied depending on epoxy viscosity selection. For example, the low viscosity (Grade 1) epoxy injection used for specimen #8 was ineffective (see MOR ratio of 0.7 in Table 10 and Figure 21), resulting in failure of the repaired crack. On the other hand, the blended viscosity (Grades 2 and 3) epoxy injection was effective for specimen #7 (see MOR ratio of 1.2 in Table 10) and the failure resulted in a new crack in the concrete adjacent to the repaired crack. Table 11 provides a summary of the shear test data for the slab specimens tested using the set-up shown in Figure 18. The failure mode was characterized by a bending failure in all but one case (Specimen 10). The low transverse shear load achieved is attributed to the shear test configuration which resulted in bending failures of the concrete preceding transverse shear failures by a large margin. This test configuration condition was necessitated to place the repaired cracks entirely within the high shear region of the test set-up (i.e., the span between the applied load and the nearest bearing support on the opposite side of the crack was longer than desired due to the angling of cracks across the surface of the slab specimens). The results do indicate that the cracking behavior of unreinforced slabs will be controlled by flexure for likely loading conditions, consistent with ACI 318 commentary paragraph R [11]. However, in sample #10 a transverse shear failure (not diagonal tension failure as associated with concrete) did occur in the repaired crack as shown in Figure 22. Both specimens #9 and #10 where repaired with low viscosity (Grade 1) epoxy injection, but specimen #9 failed in flexure at a location away from the repaired crack. It should be noted that use of the low viscosity (Grade 1) epoxy in specimen #8, 9 and 10 resulted in only partial filling of the 1/8-inch cracks. SAMPLE NUMBER TABLE 11 SUMMARY OF SLAB SHEAR TEST DATA CRACK WIDTH (in) ULTIMATE LOAD (lb) LOADING SPAN (in) V max 1 (in-lb) 17 Uncracked 3, ,434 Bending 18 Uncracked 4, ,132 Bending FAILURE MODE 11 <1/16 3, Bending, new crack in 2,481 concrete 12 <1/16 4, Bending, new crack in 2,768 concrete 10 1/8 1, Shear, failure in repaired crack 2 9 1/8 3, Bending, new crack in 1,931 concrete 3 1 1/4 4, Bending, new crack in 2,614 concrete 2 1/4 4, Bending, new crack in 2,572 concrete Notes: 1 Calculation of V max does not include specimen weight of approximately 106 lb/ft. 2 Crack was only partially filled with Grade 1 low viscosity epoxy 3 Though the repaired crack did not fail, it was only partially filled by visual inspection of the crack surface. Page 21 of 24

32 Figure 21 Failure along repaired 1/8-inch crack of slab specimen #8 (new crack within repaired crack). Figure 22 Failure along repaired 1/8-inch crack in slab specimen #10 (new crack within repaired crack). Summary of Results It is apparent from the overall test data that use of the low viscosity (Grade 1) epoxy is effective in the <1/16 in crack specimens with a one-sided repair approach. However, for the loose sand boundary condition and the repair procedures employed in this study, the low viscosity (Grade 1) is unreliable in the 1/8-inch crack application with a onesided repair. The medium viscosity (Grade 2) performed well in a 1/8-inch crack slab specimen (only used in one specimen), but did not perform well in the 1/8-inch crack stem wall specimen (only used in one specimen). The high viscosity (Grade 3) epoxy effectively filled the crack in a 1/8-inch crack stem wall specimen, but did not perform well due to possible incomplete hardening of the epoxy. Therefore, the test results do not clearly distinguish an appropriate epoxy viscosity for the 1/8-inch crack specimens. However, based on the limited testing, it appears that the Grade 2 medium viscosity epoxy is suited for slab applications where the crack width is about 1/8 inch and the depth does not exceed about 4 inches. The Grade 3 high viscosity epoxy appears best suited for the stem wall applications where the crack width is about 1/8 inch, and the height of the crack is more than several inches. The high viscosity (Grade 3) consistently performed well in all slab and stem walls specimens with 1/4-inch crack widths. CONCLUSIONS AND RECOMMENDATIONS The purpose of this research program was to evaluate the performance of epoxy injection crack repair of unreinforced concrete stem walls and slabs-on-grade for differing loading conditions, crack widths, and epoxy repair strategies (e.g., crack preparation, epoxy mix viscosity, and injection method) with access limited to one side Page 22 of 24

33 of the specimen. The geometries, material properties, and accessibilities of the specimens were considered to be representative of residential construction practice, although no attempt was made to model restrained shrinkage stresses present in actual construction. Two repetitions were performed for each specimen type, loading condition, and crack condition (52 tests). An experienced local contractor repaired the specimens from the accessible side of the specimen (i.e., one-sided repair) in an environment that represented difficult in-situ conditions. The following list summarizes major conclusions and recommendations supported by the test program: 1. With appropriate selection and preparation of epoxy viscosity (Grade), proper mixing of components, and proper execution of the injection, crack repairs made from one-side only of the concrete element were effective in creating a repair that was comparable to the uncracked strength of concrete specimens that were free of restrained shrinkage stress and that exceeded the unfactored code-nominal strength of concrete assumed in design. Therefore, access to all sides of an element is not required to achieve a fully effective crack repair. 2. Crack repairs were completely effective for <1/16-inch and 1/4-inch crack widths, in part because the epoxy viscosity selection appears straight-forward. For example, all <1/16-inch wide cracks used a low viscosity (Grade 1) epoxy with an automatic mixing/injecting system. For all 1/4-inch wide cracks, a high viscosity (Grade 3) epoxy was manually mixed and manually injected using a caulk gun. 3. A variety of viscosities and methods were used for repair of the 1/8-inch wide cracks, several of which were successful. Unsuccessful repairs were the result of epoxy seeping out of the cracks into the sand bedding. The variability of results for the 1/8 in wide crack repairs illustrates the importance of the contractor's judgment and the need for quality assurance (QA) under current practice for repair of cracks with limited access to fully seal the crack surface against seepage of injected epoxy. More specific guidance on epoxy selection for this type of repair should help to prevent problems as experienced in this study, while QA procedures should help to determine when an unsuitable procedure or epoxy selection has been used so that corrections can be made early in the repair process. 4. The technique of pre-injecting wider cracks (cracks 1/8-inch wide and wider) with a low viscosity (Grade 1) epoxy and allowing that material to jell in the adjacent soil is a recognized means of creating an "epoxy-sand" seal on the inaccessible sides of cracks. However, the volume of epoxy and number of pre-injections required is highly dependent upon the width of the crack, and the permeability and porosity of the underlying soil. Proper execution of this technique requires careful attention of an experienced contractor. However, more detailed guidance on selection of epoxy viscosity to minimize potential seepage in the presence or absence of a properly executed epoxy-sand seal warrants additional research to lessen the potential for contractor error. Page 23 of 24