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Latex modified concrete for the repair and rehabilitation of bridges
The Internat~ona/Joumat of Cement Composites and Lightweight Concrete, Volume 7, Number4
Latex modified concrete for the repair and rehabilitation of bridgest L. A. K u h l m a n n *
* The Dow Chemical Company, USA t Paper presented at the Seminar Resins in Construction 20+ Years Experience, London, Oct. 1984 and published by kind permission of FERFA (~) Construction Press 1985 0262-5075/85/07440241/$02.00
November 1985
SYNOPSIS Portland cement modified with styrene butadiene latex has been in use for over 25 years. This paper presents data on the physical properties of latex modified concrete and case histories are cited to show its application and usefulness. Latex modified cement systems are applicable wherever adhesion, durability, and compatibility with the base concrete are required. KEYWORDS Polymer concrete, latex (plastic), bridges (structures), repairs, durability, styrene butadiene resins, physical properties, shrinkage, elastic modulus, maintenance, bonding, concrete construction, adhesion tests. INTRODUCTION Portland cement modified with styrene butadiene latex has been in use for over 25 years. This paper will present data and case histories to fully explain its application and usefulness. Wherever adhesion, durability, and compatibility with the base concrete are required, latex modified cement systems are applicable. The bulk of the information presented here is from work done in the United States on styrene butadiene latex produced by Dew Chemical Co.; but, of course, the data are applicable worldwide. BACKGROUND During the past two decades, rigid concrete overlays have been replacing membranes and asphalt for the protection of bridge deck surfaces. In 1976, the Federal Highway Administration (FHWA) published the third volume of a three-part research study, 'Time-toCorrosion of Reinforcing Steel in Concrete Slabs' [1]. One of their conclusions was that a minimum of 25 mm of styrene butadiene latex modified concrete would provide sufficient protection for bridge decks exposed to deicing salts. These laboratory findings confirmed what had been experienced in the field during the prior decade, ie., styrene butadiene latex modified concrete was a practical system that worked. initial research on latex modified portland cement, started by Dow in 1956, was directed towards the development of a system for thin protective overlays for repair of bridges and pavements. A cooperative effort between The Dow Chemical Company and the Michigan Highway Department resulted in field trials in 1958 on the approach spans of the Bascute Bridge on US 23 in Cheyboygan, Michigan. Using the tools and equipment available at the time, unsophisticated by today's standards, basic construction techniques were developed which are still applicable today. As the years passed, equipment and mix design were improved to meet the needs of the industry. The original mix was a mortar, 3.25 parts sand to 1.0 part cement, applied 12.5 to 20 mm thick. Since then, more efficient mixes of concrete (1.0 part cement to 2.5 parts sand, 2.0 parts stone) have been developed and are the norm, typically applied 30 to 40 mm thick. Latex content,
241
Latex modified concrete for the repair and rehabilitation of bridges
0.15 parts styrene butadiene latex solids per 1.0 part cement, has remained constant. Surface preparation, originally done by small scarifiers and scabblers, is now often done in a single pass across a full lane width and the refuse automatically loaded into a waiting truck. Mixing was formerly done in one and three bag mixers or an occasional transit mixer; continuous feed mobile mixers are now the norm. Hand and mechanical screeds have given way to roller finisher machines. The development of improved techniques and equipment have resulted in durable and efficiently placed overlays. Continuation of the FHWA research resulted in the 1978 publicatinn of a report on prequalification and certification of styrene butadiene latices for bridge deck overlays [2J, which provided the criteria for latex manufacturers who were interested in entering the bridge overlay market. No products can be used in this application without first meeting the requirements contained in the report. The result of all these developments is that Dow styrene butadiene latex modified concrete has become a standard material of construction in the United States and Canada, and one that has an extensive history of high performance on bridge decks [3]. It is used not only for repair, but also to protect decks in new construction. The primary properties of latex modified concrete - - bond, impermeability, and freeze/thaw durability, as well as its compatibility with conventional concrete, make it an ideal material to repair old concrete and protect new Research and field evaluations continue to be conducted on this material by highway departments as well as Dow.
PROPERTIES OF LATEX Latex is a collodial dispersion of styrene butadiene in water. A stabiliser is added to improve freeze/thaw resistance and prevent coagulation when mixed with portland cement. Latex is milky white in appearance and contains approximately 48% solids. When dried in air it forms a semiclear rubbery compound having great adhesive power. When latex is added to portland cement, aggregate and water, a concrete with the colour, consistency, and workability of ordinary concrete results, but containing 20% to 35% less water. When cured, the concrete consists of hydrated cement and aggregate interconnected by a 'film' of latex particles, It is this 'film' of latex particles which imparts the physical and chemical properties that make latex modified concrete a superior product. It is an excellent bridge deck overlay and protective surface for both new construction and rehabilitation. Details of the chemistry and the mechanism of curing have been researched and reported by Eash and Sharer [4]. As normal portland cement concrete dries, the cement hydrates, and the gel shrinks. This creates stresses within the gel and eventually results m the development of microcracks (Figure 1). The propagation of these cracks lowers the tensile capacity of the concrete and increases its permeability to chlorides or
242
Kuhfmann
other iniurious chemicals. In latex modified concrete (Figure 2), the latex particles have coalesced to form the plastic film which surrounds the aggregate and coats the gel. Due to its elasticity and high bonding strength, the latex bridges the microcracks in the gel, and restrains their propagation. In this manner, the latex polymer increases the tensile capacity of the concrete and greatly reduces its permeability.
MIX DESIGN The inclusion of latex in mortar or concrete really changes only one component of the mix - - l e s s water is required. A workable slump 100 to t50 mm can be achieved at a water-cement ratio of 0.40 or less. This includes the water in latex, typically 52%, by weight. For mortar, a typical mix design would be: Cement Sand Dow Latex Water
50 kg 160 kg 15.5 iitres i 10 litres
For concrete, a representative m~x design could be: Cement Sand Stone Dow Latex Water
300 780 520 93 72
kc] kg kg titres !itres, maximum
Unlike conventional concrete, a~r is not required for freeze/thaw durability. The latex itself apparently provides this protection. However, some air is entrained by the latex during the mixing process so it is common for a specification to include a maximum air content, say 6.5% , but not a minimum.
Figure t Conventional concrete
L¢3tex modified concrete for the repa~rand rehabi//tatlon of bridges
Kuhlmann
COMPRESSIVE STRENGTH
#_
40 -
~
7
5 mm x 150 mm cylindersiiii~iiiiiii!i]ii!~]iiiii'";iiiiiiiiii!ii';iiii!
30
c~ 20 (9
B_10 E O
O
0
I 0
7
I 14 Cure Time, days
I 21
28
FLEXURAL STRENGTH
! 1o ~
Figure2 Latex modified concrete n :s
P
t-
PROPERTIES
The properties of the latex, combined with the low water-cement ratio, produce a concrete that has improved flexural, tensile, and bond strength, lower modulus of elasticity, increased freeze/thaw durability and reduced permeability characteristics compared to conventional concrete of similar mix design. Shrinkage is slightly lower while compressive strength will typically be unchanged. Some of these properties are shown in Figures 3 and 4. It should be noted that most of these data are ranges of values, typical of those reported by government agencies and research reports. An exception to this is the modulus of elasticity. Although little work has been reported on this property, current data indicate that latex modified concrete will yield a modulus that is approximately 85% of conventional concrete made of the same materials. The shear bond data are based on a test developed by Dow. It consists of a conventional concrete cylinder, cured fora minimum of 28 days, belt sanded on one end, then capped with the latex modified mix. After curing, the assembly is mounted as shown in Figure 5, and the latex cap sheared off. The load at failure, divided by the cross-sectional area, is reported as the shear bond stress. All of these results are based on a latex modified concrete curing cycle of one day at 100% RH, and subsequent time in dry air, typically 50% RH. It is during this drying period that latex modified concrete achieves its ultimate properties. This is clearly shown in Figure 6. Compressive cylinders were weighed prior to testing, and their weight loss and strength gain plotted. The curve shows that strength increased as the cylinders continued to lose weight with time. Most recently, an electrical test has been developed by the FHWA to rapidly determine the chloride permeabiiity of hardened concrete [5]. The test requires only
,-
...... ......-.-.:.-.S;.::::-~.:.S_'~'S.:.~:.;..ili~i
03 t-
2 o9
5:
0 mm bars V
center point loaded
x Ll_
0 0
I
t
I
7
14
21
28
21
28
Cure Time, days SHEAR BOND STRENGTH IJ_
d0 3 t'-
O9 "G cO
(1) t-"
09
0
7
14 Cure Time, days
Figure 3 Typical physical properties of Dow latex modified concrete 6 hours, rather than the 90 days presently required bythe currently accepted ponding test [6] It is an electrical resistance test which has shown that increasing permeability is proportional to increase in measured current flow. Concretes of different water-cement ratios (0.60 and 0.40), as well as the Iowa Low Slump and latex concretes have been tested by the FHWA. The data from this study, highlighted in Figure 7, correlate well with the AASHTO 90 day ponding test. The durability of latex modified concrete has been measured by both freeze/thaw testing and resistance to
243
Latex modified concrete for the repair and rehabilitation of bridges
Kuhlmann
MODULUS OF ELASTICITY
50
c.. '" 40 ~
(Ii
en
E = 2.5 x 10" MPa
~
U5 30 OJ
> 'iii en OJ
~
o
20
()
10
o"-
-L.
o
.001
-'---
---L.,--
.002 Strain, mm/mm
.L-
.004
SHRINKAGE
Surface preparation A clean, sound surface is the key to any material being applied for adhesion. In the case of a concrete surface this is accomplished by first removing all unsound concrete on the surface. Scarifiers, shotblasters, or scabblers are used for this. Hand chipping follows if there are deep pockets of deteriorated concrete, or if concrete below reinforcing steel needs to be removed. The entire area is then blasted to clean off surface laitance from the concrete, and rust from the rebar. Sand blasting has been most common and efficient method, although water blasting has some merit in areas concerned with dust. In either case, the dust and debris must be removed, so that a clean surface is provided.
800
Cure Time, days
Figure 4 Typical physical properties of Dow latex modified concrete
deicing chemicals. A recent Dow study [7] of freeze/thaw resistance, using the ASTM C 666 test method produced excellent results (Table 1). No air entraining agent was used. Another measure of a concrete's resistance to freeze/thaw cycling is the ASTM Test C-672 , 'Scaling Resistance of Concrete Surface Exposed to Deicing Chemicals'. The Indiana State Highway Commission conducted such a test [8J, on both latex modified and conventional concretes. The results, summarised in Table 2 again demonstrate latex modified concrete's durability. (A durability factor of zero indicates no scaling and is the best rating possible)
INSTALLING LMC The proper use of latex modified portland cement systems has been documented in several publications [9, 10, 11] Briefly the procedure is as follows:
244
Figure 5 Shear bond test
Mixing Accurate proportioning and thorough mixing are key requirements of a mixer for latex modified concrete. Most drum type mixers will do a good job in this respect. Table 1 ASTM C666 FreezelThaw tests on LMC
Cure
No Cycles
Durability Factor
1 day wet 27 days dry
323
91
5.8
1 day wet
323
98
5.2
27 days dry 1 day wet
323
91
Air Content
51
27 days dry
Table 2 ASTM C-672 Scaling resistance
Latex Conventional
Air Content
No. Cycles
4.1% 4.9%
50 50
Rating
o 2
Late.~"r~,~od,',Oedconcrete for the repa/r and reha@//tat/on of bndges
Kuh/mann
Figure 6 Compressive strength 84 weight loss
1.50
2_ :Z:" © Z Lu }.-. u3 u.J
O9 O3 O _d
40 1.00 30
LU
20
Z UJ O
0,50
nW CL
O ©
0
I
I
1
I
I
3
7
14
21
28
I .....
0
35
C U R E TIME, D A Y S _~
However, for projects where significant quantities of quality concrete are distributed over large flat areas, the self-contained, mobile, continuous mixer is most common in the United States and Canada. Machines capable of carrying sufficient unmixed materials (sand, stone, cement, latex, and water) for at least six cubic yards of concrete are most often used (Figure 8). These machines are calibrated frequently to assure an accurate mix design. They also minimise waste and clean up time since the auger is the only part that contains mixed concrete
Placement There are two alternatives at this point. Latex modified concrete can be placed into the deep repair areas simultaneous with overlay, or a quality conventional concrete can be poured first to fill deep holes, brought to grade, cured, and then overlaid with latex modified concrete. If the latter is selected, the conventional concrete should be sandblasted prior to placement of the latex modified concrete. In either case, the placement of latex modified concrete is preceded by wetting the substrate concrete. This is normally done an hour before and is particularly useful in hot weather to cool the deck. Standing water and puddles are removed by compressed air. To enhance bond, the latex modified concrete is normally broomed into the surface of the concrete to enhance contact between the former and the concrete surface. Any excess stones that accumulate are discarded. An alternate to this is the use of a latex mortar grout prepared in a separate mixer and applied just ahead of the concrete overlay. This method has also worked well. Finishing Self-propelled roller finishers (Figure 9) have proven to be the most popular method of screeding and finishing latex modified concrete on bridge decks. The auger, rollers, and vibrating pan combine to provide the proper thickness of overlay. Prior to the pour, the finisher is calibrated
22°C, 50°RH
3o0
, =.
20o ~-
8 100 W/C=40 75 Low Slump 50 LMC 25
/
l
I
I
I
I
I
1
2
3
4
5
6
Time. hr
Figure7
Chloride permeability
with shims to assure the contractor and owner that the proper thickness will be applied to the deck. In locations where a drag or broom finish is desired, this is accomplished by an attachment on the machine. In either case, the finishing operation should be completed before the surface of the latex modified concrete overlay begins to dry.
Curing Soon after finishing, wet burlap is applied, followed by white or clear polyethylene film. The intent is to keep the surface damp for approximately 24 hours. This means the burlap is not to be dripping wet, and the polyethylene film is to be held down at the edges with lumber or suitable weights to prevent it from being blown off. After this initial damp cure, the film and burlap are removed to
245
Latex modified concrete for the repair and rehabilitation of bridges
&~h/rnanr',
modified concrete, or followed ACi 306-66 'Recommended Practice for Cold Weather Concreting'
CASE HISTORIES A N D CONCLUSIONS There have been thousands of new and old bridges repaired and overlaid with concrete containing Dow styrene butadiene latex. Most have been in the northern climate, where deicing salts are prevalent, although recently there have even been projects in Texas, Louisiana, Mississippi, and North Carolina. Some of the more notable projects were [12-18]:
Figure 8 Continuous mixer
l
t
. . . . . . . . .
Project Name Bascule Bridge on U S 23 Columbia River Bridge Marquham Street Bridge Wiscasset Bridge Denny Creek Bridge Clark's Summit Bridge
Year 1958 1982 1983 1983 1980 1979
Type Repair New Repair New New Repair
Sandusky Bay Bridge O'Hare Departure Ramp Q'Hare Parking Garage Soldier Field Stadium
1980 1978 1983 1978
Repair Repair Repair Repair
Locat!on Cheyboygan, Michigan Portland,Oregon Portland,Oregon Wiscasset,Maine Snoqualmie Falls,Washington Northeast Extension Pennsylvania Turnpike Sandusky, Ohio Chicago, Illinois Chicago,illinois Chicago.Illinois
At Chicago International Terminal, the elevated road was repaired with latex modified concrete in t978, and is performing well. The ramps of the airport parking garage were also repaired with latex modified concrete. The football stadium in downtown Chicago, Soldier Field, was also structurally restored in 1978 with latex modified concrete. The data presented in this paper show that latex modified concrete is a versatile and useful material that can have wide applications in concrete construction:
Figure 9 Double roller finisher
allow air drying. It is during the air cure period that latex modified concrete gains its physical properties. WEATHER LIMITATIONS Latex modified concrete cures at about the same rate as conventional concrete. It will, however, form a crust (a relatively dry layer) on the surface if exposed to dry air for prolonged periods, even though the concrete underneath is still quite plastic. This is caused by drying of the latex itself and, if not controlled, can result in tearing during the finishing operation. This condition is aggravated by hot, dry, windy conditions, and can be minimised by following ACI's Recommended Practice for Hot Weather Concreting, 305-72. A maximum evaporation rate of 0.75 to 1.00 kg/m2/hr is recommended. For cold weather construction, latex modified concrete is less sensitive than conventional concrete. This is because low temperatures usually result in air with low humidity, a desirable condition for the drying portion of latex modified concrete's cure cycle. Although there are research data which indicate that in four days at 5°C latex modified concrete will gain the same compressive strength as at 22°C, most United States specifications have either adopted at 7°C minimum for placing latex
246
REFERENCES 1. Clear, K. C. 'Time-to-Corrosion of Reinforcing Steel in Concrete Slabs, Volume 3: Performance After 830 Daily Salting Applications', Report No. FHWARD-76-70, Federal Highway Administration, Washington D.C., 1976, pp. 59. 2. Clear, K. C. and Choltar, B. H. 'Styrene Bu[adiene Latex Modifiers for Bridge Deck Overlay Concrete', Report No. FHWA-RD-78-35 Federal Highway Administration, April 1978, pp. 117. 3. Kuhimann, L. A. 'Performance History of Latex Modified Concrete Overlays', ACI Publication, SP-69, Applications of Polymer Concrete, American Concrete Institute, Detroit, !981, pp. 123--44. 4. Shafer, H. H. and Eash, R. D. 'Reactions of Polymer Latexes with Portland Cement Concrete', Trans. portation Research Record 542; 1975, pp. 1 -S. 5. Whiting, D. 'Rapid Determination of the Chloride Permeability of Concrete', Technical ReporL FHWA-RD-81/119, August 198t 6. AASHTO T259-78 'Resistance of Concrete to Chloride Ion Penetration' 7. Dow Letter Report to the Maryland State Highway Department. 8. Smutzer, R. K. and Hockett, R. B. 'Latex Modified Portland Cement Concrete .....A laboratory Investi-
Latex mod/hed concrete for the repair and rehabilitatlon of bridges
gation of Plastic and Hardened Properties of Concrete Mixtures Containing Three Formulations used in Bridge Deck Overlays', Indiana State Highway Commission, February 1981. 9. Kuhlmann, L. A. 'Latex Modified Concrete for Deck Repair and Rehabilitation', American Society of Civil Fngineers Specialty Conference on New Materials and Process for Street, Highway, and Airport, March 1983. ZO. 'Guide for Repair of Concrete Bridge Superstructures', Report No. ACi 546.1R-80, ACI Committee 546, American Concrete Institute, Detroit. 71. 'Guide for the Use of Polymers in Concrete', ACI Committee 548, American Concrete Institute, Detroit. 12. Pfeifer, D. W. and Perenchio, W. F. 'Coatings, Penetrants, and Specialty Concrete Overlays for Concrete Surfaces', Wiss, Janney, Etstner, and Assoc., Inc., September 1982. 13. Isenburg, J. E., Rapp, D. E., Sutton, C. J. and Vanderhoff, J. W. 'Microstructure and Strength of
Kuhlmann
Bond Between Concrete and Styrene-Butadiene Latex-Modified Mortar', Dow Chemical Report. 14. Bishara, R. F. and Keeran, P. F. 'Bond-Shear Strength of Latex Modified Concrete', Ohio Department of Transportation, April 1981. 15. Pfeifer, D. W. 'Utilization of Latex-Modified Concrete for Restoration of Seats in Soldier Field for Chicago Park District', Wiss, Janney, Elstner and Assoc., Inc., September 1978. 16. Kuhlmann, L. A. and Foor, N. C. 'Chloride Permeability Versus Air Content of Latex Modified Concrete and Aggregates', Cement, Concrete and Aggregates, Summer 1984 17. Bishara, A. G. 'Latex Modified Concrete Bridge Deck Overlays Field Performance Analysis', Ohio Department of Transportation, Report No. FHWA/ OH/79/004, October 1979, pp. 96. 18. Smutzer, R. K. 'A Field Evaluation of the Use of Epoxy Penetrating Sealers on Latex Modified Concrete Bridge Deck Overlays - - Report Two', Indiana Department of Highway, March 1983.
247
Latex modified concrete for the repair and rehabilitation of bridgest L. A. K u h l m a n n *
* The Dow Chemical Company, USA t Paper presented at the Seminar Resins in Construction 20+ Years Experience, London, Oct. 1984 and published by kind permission of FERFA (~) Construction Press 1985 0262-5075/85/07440241/$02.00
November 1985
SYNOPSIS Portland cement modified with styrene butadiene latex has been in use for over 25 years. This paper presents data on the physical properties of latex modified concrete and case histories are cited to show its application and usefulness. Latex modified cement systems are applicable wherever adhesion, durability, and compatibility with the base concrete are required. KEYWORDS Polymer concrete, latex (plastic), bridges (structures), repairs, durability, styrene butadiene resins, physical properties, shrinkage, elastic modulus, maintenance, bonding, concrete construction, adhesion tests. INTRODUCTION Portland cement modified with styrene butadiene latex has been in use for over 25 years. This paper will present data and case histories to fully explain its application and usefulness. Wherever adhesion, durability, and compatibility with the base concrete are required, latex modified cement systems are applicable. The bulk of the information presented here is from work done in the United States on styrene butadiene latex produced by Dew Chemical Co.; but, of course, the data are applicable worldwide. BACKGROUND During the past two decades, rigid concrete overlays have been replacing membranes and asphalt for the protection of bridge deck surfaces. In 1976, the Federal Highway Administration (FHWA) published the third volume of a three-part research study, 'Time-toCorrosion of Reinforcing Steel in Concrete Slabs' [1]. One of their conclusions was that a minimum of 25 mm of styrene butadiene latex modified concrete would provide sufficient protection for bridge decks exposed to deicing salts. These laboratory findings confirmed what had been experienced in the field during the prior decade, ie., styrene butadiene latex modified concrete was a practical system that worked. initial research on latex modified portland cement, started by Dow in 1956, was directed towards the development of a system for thin protective overlays for repair of bridges and pavements. A cooperative effort between The Dow Chemical Company and the Michigan Highway Department resulted in field trials in 1958 on the approach spans of the Bascute Bridge on US 23 in Cheyboygan, Michigan. Using the tools and equipment available at the time, unsophisticated by today's standards, basic construction techniques were developed which are still applicable today. As the years passed, equipment and mix design were improved to meet the needs of the industry. The original mix was a mortar, 3.25 parts sand to 1.0 part cement, applied 12.5 to 20 mm thick. Since then, more efficient mixes of concrete (1.0 part cement to 2.5 parts sand, 2.0 parts stone) have been developed and are the norm, typically applied 30 to 40 mm thick. Latex content,
241
Latex modified concrete for the repair and rehabilitation of bridges
0.15 parts styrene butadiene latex solids per 1.0 part cement, has remained constant. Surface preparation, originally done by small scarifiers and scabblers, is now often done in a single pass across a full lane width and the refuse automatically loaded into a waiting truck. Mixing was formerly done in one and three bag mixers or an occasional transit mixer; continuous feed mobile mixers are now the norm. Hand and mechanical screeds have given way to roller finisher machines. The development of improved techniques and equipment have resulted in durable and efficiently placed overlays. Continuation of the FHWA research resulted in the 1978 publicatinn of a report on prequalification and certification of styrene butadiene latices for bridge deck overlays [2J, which provided the criteria for latex manufacturers who were interested in entering the bridge overlay market. No products can be used in this application without first meeting the requirements contained in the report. The result of all these developments is that Dow styrene butadiene latex modified concrete has become a standard material of construction in the United States and Canada, and one that has an extensive history of high performance on bridge decks [3]. It is used not only for repair, but also to protect decks in new construction. The primary properties of latex modified concrete - - bond, impermeability, and freeze/thaw durability, as well as its compatibility with conventional concrete, make it an ideal material to repair old concrete and protect new Research and field evaluations continue to be conducted on this material by highway departments as well as Dow.
PROPERTIES OF LATEX Latex is a collodial dispersion of styrene butadiene in water. A stabiliser is added to improve freeze/thaw resistance and prevent coagulation when mixed with portland cement. Latex is milky white in appearance and contains approximately 48% solids. When dried in air it forms a semiclear rubbery compound having great adhesive power. When latex is added to portland cement, aggregate and water, a concrete with the colour, consistency, and workability of ordinary concrete results, but containing 20% to 35% less water. When cured, the concrete consists of hydrated cement and aggregate interconnected by a 'film' of latex particles, It is this 'film' of latex particles which imparts the physical and chemical properties that make latex modified concrete a superior product. It is an excellent bridge deck overlay and protective surface for both new construction and rehabilitation. Details of the chemistry and the mechanism of curing have been researched and reported by Eash and Sharer [4]. As normal portland cement concrete dries, the cement hydrates, and the gel shrinks. This creates stresses within the gel and eventually results m the development of microcracks (Figure 1). The propagation of these cracks lowers the tensile capacity of the concrete and increases its permeability to chlorides or
242
Kuhfmann
other iniurious chemicals. In latex modified concrete (Figure 2), the latex particles have coalesced to form the plastic film which surrounds the aggregate and coats the gel. Due to its elasticity and high bonding strength, the latex bridges the microcracks in the gel, and restrains their propagation. In this manner, the latex polymer increases the tensile capacity of the concrete and greatly reduces its permeability.
MIX DESIGN The inclusion of latex in mortar or concrete really changes only one component of the mix - - l e s s water is required. A workable slump 100 to t50 mm can be achieved at a water-cement ratio of 0.40 or less. This includes the water in latex, typically 52%, by weight. For mortar, a typical mix design would be: Cement Sand Dow Latex Water
50 kg 160 kg 15.5 iitres i 10 litres
For concrete, a representative m~x design could be: Cement Sand Stone Dow Latex Water
300 780 520 93 72
kc] kg kg titres !itres, maximum
Unlike conventional concrete, a~r is not required for freeze/thaw durability. The latex itself apparently provides this protection. However, some air is entrained by the latex during the mixing process so it is common for a specification to include a maximum air content, say 6.5% , but not a minimum.
Figure t Conventional concrete
L¢3tex modified concrete for the repa~rand rehabi//tatlon of bridges
Kuhlmann
COMPRESSIVE STRENGTH
#_
40 -
~
7
5 mm x 150 mm cylindersiiii~iiiiiii!i]ii!~]iiiii'";iiiiiiiiii!ii';iiii!
30
c~ 20 (9
B_10 E O
O
0
I 0
7
I 14 Cure Time, days
I 21
28
FLEXURAL STRENGTH
! 1o ~
Figure2 Latex modified concrete n :s
P
t-
PROPERTIES
The properties of the latex, combined with the low water-cement ratio, produce a concrete that has improved flexural, tensile, and bond strength, lower modulus of elasticity, increased freeze/thaw durability and reduced permeability characteristics compared to conventional concrete of similar mix design. Shrinkage is slightly lower while compressive strength will typically be unchanged. Some of these properties are shown in Figures 3 and 4. It should be noted that most of these data are ranges of values, typical of those reported by government agencies and research reports. An exception to this is the modulus of elasticity. Although little work has been reported on this property, current data indicate that latex modified concrete will yield a modulus that is approximately 85% of conventional concrete made of the same materials. The shear bond data are based on a test developed by Dow. It consists of a conventional concrete cylinder, cured fora minimum of 28 days, belt sanded on one end, then capped with the latex modified mix. After curing, the assembly is mounted as shown in Figure 5, and the latex cap sheared off. The load at failure, divided by the cross-sectional area, is reported as the shear bond stress. All of these results are based on a latex modified concrete curing cycle of one day at 100% RH, and subsequent time in dry air, typically 50% RH. It is during this drying period that latex modified concrete achieves its ultimate properties. This is clearly shown in Figure 6. Compressive cylinders were weighed prior to testing, and their weight loss and strength gain plotted. The curve shows that strength increased as the cylinders continued to lose weight with time. Most recently, an electrical test has been developed by the FHWA to rapidly determine the chloride permeabiiity of hardened concrete [5]. The test requires only
,-
...... ......-.-.:.-.S;.::::-~.:.S_'~'S.:.~:.;..ili~i
03 t-
2 o9
5:
0 mm bars V
center point loaded
x Ll_
0 0
I
t
I
7
14
21
28
21
28
Cure Time, days SHEAR BOND STRENGTH IJ_
d0 3 t'-
O9 "G cO
(1) t-"
09
0
7
14 Cure Time, days
Figure 3 Typical physical properties of Dow latex modified concrete 6 hours, rather than the 90 days presently required bythe currently accepted ponding test [6] It is an electrical resistance test which has shown that increasing permeability is proportional to increase in measured current flow. Concretes of different water-cement ratios (0.60 and 0.40), as well as the Iowa Low Slump and latex concretes have been tested by the FHWA. The data from this study, highlighted in Figure 7, correlate well with the AASHTO 90 day ponding test. The durability of latex modified concrete has been measured by both freeze/thaw testing and resistance to
243
Latex modified concrete for the repair and rehabilitation of bridges
Kuhlmann
MODULUS OF ELASTICITY
50
c.. '" 40 ~
(Ii
en
E = 2.5 x 10" MPa
~
U5 30 OJ
> 'iii en OJ
~
o
20
()
10
o"-
-L.
o
.001
-'---
---L.,--
.002 Strain, mm/mm
.L-
.004
SHRINKAGE
Surface preparation A clean, sound surface is the key to any material being applied for adhesion. In the case of a concrete surface this is accomplished by first removing all unsound concrete on the surface. Scarifiers, shotblasters, or scabblers are used for this. Hand chipping follows if there are deep pockets of deteriorated concrete, or if concrete below reinforcing steel needs to be removed. The entire area is then blasted to clean off surface laitance from the concrete, and rust from the rebar. Sand blasting has been most common and efficient method, although water blasting has some merit in areas concerned with dust. In either case, the dust and debris must be removed, so that a clean surface is provided.
800
Cure Time, days
Figure 4 Typical physical properties of Dow latex modified concrete
deicing chemicals. A recent Dow study [7] of freeze/thaw resistance, using the ASTM C 666 test method produced excellent results (Table 1). No air entraining agent was used. Another measure of a concrete's resistance to freeze/thaw cycling is the ASTM Test C-672 , 'Scaling Resistance of Concrete Surface Exposed to Deicing Chemicals'. The Indiana State Highway Commission conducted such a test [8J, on both latex modified and conventional concretes. The results, summarised in Table 2 again demonstrate latex modified concrete's durability. (A durability factor of zero indicates no scaling and is the best rating possible)
INSTALLING LMC The proper use of latex modified portland cement systems has been documented in several publications [9, 10, 11] Briefly the procedure is as follows:
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Figure 5 Shear bond test
Mixing Accurate proportioning and thorough mixing are key requirements of a mixer for latex modified concrete. Most drum type mixers will do a good job in this respect. Table 1 ASTM C666 FreezelThaw tests on LMC
Cure
No Cycles
Durability Factor
1 day wet 27 days dry
323
91
5.8
1 day wet
323
98
5.2
27 days dry 1 day wet
323
91
Air Content
51
27 days dry
Table 2 ASTM C-672 Scaling resistance
Latex Conventional
Air Content
No. Cycles
4.1% 4.9%
50 50
Rating
o 2
Late.~"r~,~od,',Oedconcrete for the repa/r and reha@//tat/on of bndges
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Figure 6 Compressive strength 84 weight loss
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However, for projects where significant quantities of quality concrete are distributed over large flat areas, the self-contained, mobile, continuous mixer is most common in the United States and Canada. Machines capable of carrying sufficient unmixed materials (sand, stone, cement, latex, and water) for at least six cubic yards of concrete are most often used (Figure 8). These machines are calibrated frequently to assure an accurate mix design. They also minimise waste and clean up time since the auger is the only part that contains mixed concrete
Placement There are two alternatives at this point. Latex modified concrete can be placed into the deep repair areas simultaneous with overlay, or a quality conventional concrete can be poured first to fill deep holes, brought to grade, cured, and then overlaid with latex modified concrete. If the latter is selected, the conventional concrete should be sandblasted prior to placement of the latex modified concrete. In either case, the placement of latex modified concrete is preceded by wetting the substrate concrete. This is normally done an hour before and is particularly useful in hot weather to cool the deck. Standing water and puddles are removed by compressed air. To enhance bond, the latex modified concrete is normally broomed into the surface of the concrete to enhance contact between the former and the concrete surface. Any excess stones that accumulate are discarded. An alternate to this is the use of a latex mortar grout prepared in a separate mixer and applied just ahead of the concrete overlay. This method has also worked well. Finishing Self-propelled roller finishers (Figure 9) have proven to be the most popular method of screeding and finishing latex modified concrete on bridge decks. The auger, rollers, and vibrating pan combine to provide the proper thickness of overlay. Prior to the pour, the finisher is calibrated
22°C, 50°RH
3o0
, =.
20o ~-
8 100 W/C=40 75 Low Slump 50 LMC 25
/
l
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2
3
4
5
6
Time. hr
Figure7
Chloride permeability
with shims to assure the contractor and owner that the proper thickness will be applied to the deck. In locations where a drag or broom finish is desired, this is accomplished by an attachment on the machine. In either case, the finishing operation should be completed before the surface of the latex modified concrete overlay begins to dry.
Curing Soon after finishing, wet burlap is applied, followed by white or clear polyethylene film. The intent is to keep the surface damp for approximately 24 hours. This means the burlap is not to be dripping wet, and the polyethylene film is to be held down at the edges with lumber or suitable weights to prevent it from being blown off. After this initial damp cure, the film and burlap are removed to
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Latex modified concrete for the repair and rehabilitation of bridges
&~h/rnanr',
modified concrete, or followed ACi 306-66 'Recommended Practice for Cold Weather Concreting'
CASE HISTORIES A N D CONCLUSIONS There have been thousands of new and old bridges repaired and overlaid with concrete containing Dow styrene butadiene latex. Most have been in the northern climate, where deicing salts are prevalent, although recently there have even been projects in Texas, Louisiana, Mississippi, and North Carolina. Some of the more notable projects were [12-18]:
Figure 8 Continuous mixer
l
t
. . . . . . . . .
Project Name Bascule Bridge on U S 23 Columbia River Bridge Marquham Street Bridge Wiscasset Bridge Denny Creek Bridge Clark's Summit Bridge
Year 1958 1982 1983 1983 1980 1979
Type Repair New Repair New New Repair
Sandusky Bay Bridge O'Hare Departure Ramp Q'Hare Parking Garage Soldier Field Stadium
1980 1978 1983 1978
Repair Repair Repair Repair
Locat!on Cheyboygan, Michigan Portland,Oregon Portland,Oregon Wiscasset,Maine Snoqualmie Falls,Washington Northeast Extension Pennsylvania Turnpike Sandusky, Ohio Chicago, Illinois Chicago,illinois Chicago.Illinois
At Chicago International Terminal, the elevated road was repaired with latex modified concrete in t978, and is performing well. The ramps of the airport parking garage were also repaired with latex modified concrete. The football stadium in downtown Chicago, Soldier Field, was also structurally restored in 1978 with latex modified concrete. The data presented in this paper show that latex modified concrete is a versatile and useful material that can have wide applications in concrete construction:
Figure 9 Double roller finisher
allow air drying. It is during the air cure period that latex modified concrete gains its physical properties. WEATHER LIMITATIONS Latex modified concrete cures at about the same rate as conventional concrete. It will, however, form a crust (a relatively dry layer) on the surface if exposed to dry air for prolonged periods, even though the concrete underneath is still quite plastic. This is caused by drying of the latex itself and, if not controlled, can result in tearing during the finishing operation. This condition is aggravated by hot, dry, windy conditions, and can be minimised by following ACI's Recommended Practice for Hot Weather Concreting, 305-72. A maximum evaporation rate of 0.75 to 1.00 kg/m2/hr is recommended. For cold weather construction, latex modified concrete is less sensitive than conventional concrete. This is because low temperatures usually result in air with low humidity, a desirable condition for the drying portion of latex modified concrete's cure cycle. Although there are research data which indicate that in four days at 5°C latex modified concrete will gain the same compressive strength as at 22°C, most United States specifications have either adopted at 7°C minimum for placing latex
246
REFERENCES 1. Clear, K. C. 'Time-to-Corrosion of Reinforcing Steel in Concrete Slabs, Volume 3: Performance After 830 Daily Salting Applications', Report No. FHWARD-76-70, Federal Highway Administration, Washington D.C., 1976, pp. 59. 2. Clear, K. C. and Choltar, B. H. 'Styrene Bu[adiene Latex Modifiers for Bridge Deck Overlay Concrete', Report No. FHWA-RD-78-35 Federal Highway Administration, April 1978, pp. 117. 3. Kuhimann, L. A. 'Performance History of Latex Modified Concrete Overlays', ACI Publication, SP-69, Applications of Polymer Concrete, American Concrete Institute, Detroit, !981, pp. 123--44. 4. Shafer, H. H. and Eash, R. D. 'Reactions of Polymer Latexes with Portland Cement Concrete', Trans. portation Research Record 542; 1975, pp. 1 -S. 5. Whiting, D. 'Rapid Determination of the Chloride Permeability of Concrete', Technical ReporL FHWA-RD-81/119, August 198t 6. AASHTO T259-78 'Resistance of Concrete to Chloride Ion Penetration' 7. Dow Letter Report to the Maryland State Highway Department. 8. Smutzer, R. K. and Hockett, R. B. 'Latex Modified Portland Cement Concrete .....A laboratory Investi-
Latex mod/hed concrete for the repair and rehabilitatlon of bridges
gation of Plastic and Hardened Properties of Concrete Mixtures Containing Three Formulations used in Bridge Deck Overlays', Indiana State Highway Commission, February 1981. 9. Kuhlmann, L. A. 'Latex Modified Concrete for Deck Repair and Rehabilitation', American Society of Civil Fngineers Specialty Conference on New Materials and Process for Street, Highway, and Airport, March 1983. ZO. 'Guide for Repair of Concrete Bridge Superstructures', Report No. ACi 546.1R-80, ACI Committee 546, American Concrete Institute, Detroit. 71. 'Guide for the Use of Polymers in Concrete', ACI Committee 548, American Concrete Institute, Detroit. 12. Pfeifer, D. W. and Perenchio, W. F. 'Coatings, Penetrants, and Specialty Concrete Overlays for Concrete Surfaces', Wiss, Janney, Etstner, and Assoc., Inc., September 1982. 13. Isenburg, J. E., Rapp, D. E., Sutton, C. J. and Vanderhoff, J. W. 'Microstructure and Strength of
Kuhlmann
Bond Between Concrete and Styrene-Butadiene Latex-Modified Mortar', Dow Chemical Report. 14. Bishara, R. F. and Keeran, P. F. 'Bond-Shear Strength of Latex Modified Concrete', Ohio Department of Transportation, April 1981. 15. Pfeifer, D. W. 'Utilization of Latex-Modified Concrete for Restoration of Seats in Soldier Field for Chicago Park District', Wiss, Janney, Elstner and Assoc., Inc., September 1978. 16. Kuhlmann, L. A. and Foor, N. C. 'Chloride Permeability Versus Air Content of Latex Modified Concrete and Aggregates', Cement, Concrete and Aggregates, Summer 1984 17. Bishara, A. G. 'Latex Modified Concrete Bridge Deck Overlays Field Performance Analysis', Ohio Department of Transportation, Report No. FHWA/ OH/79/004, October 1979, pp. 96. 18. Smutzer, R. K. 'A Field Evaluation of the Use of Epoxy Penetrating Sealers on Latex Modified Concrete Bridge Deck Overlays - - Report Two', Indiana Department of Highway, March 1983.
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