Bonded Concrete Overlays - Ardiansyah Kusuma … · Bonded Concrete Overlays 23-3 Sprinkel and Moen, 1999; Sprinkel and Ozyildirim, 1999b). The latex admixture is typically a 48%

23-1

23Bonded Concrete

Overlays

Michael M. Sprinkel, P.E.*

23.1 Introduction ......................................................................23-123.2 Key Issues for Successful Bonded HCC Overlays ...........23-2

Contractor Performance • Overlay Material Properties • Overlay Bond Strength • Overlay Thickness • Overlay Surface Characteristics • Overlay Protection Characteristics

23.3 Other Issues .....................................................................23-15Rapid Construction of Overlays • Cost • HCC Pavement Overlays • Service Life of HCC Overlays

23.4 Summary..........................................................................23-16References ...................................................................................23-16

23.1 Introduction

Overlays are usually placed on bridge decks to reduce the infiltration of water and chloride ions and toimprove the skid resistance, ride quality, drainage, and appearance of the surface. The protection providedby the overlay can extend the life of the deck. Three types are typically used: asphalt overlays onmembranes, polymer concrete overlays, and hydraulic cement concrete (HCC) overlays. Asphalt overlayson membranes and polymer overlays have been successfully used on decks that are in good conditionbut require a skid-resistant surface and protection against chloride intrusion (AASHTO, 1995b; NCHRP,1995). HCC overlays have been used successfully on similar decks, as well as on decks that require asignificant amount of concrete removal and rehabilitation. Overlays are placed on pavements to increasethe stiffness and to improve the skid resistance, ride quality, drainage, and appearance of the surface.Both asphalt and concrete overlays are used on both asphalt and concrete pavements. HCC pavementoverlays that are thicker than 4 in. are typically unbonded, and HCC overlays that are ≤4 in. thick aretypically bonded. The overlays can extend the life of pavements. This chapter deals only with bondedHCC overlays placed on concrete bridge decks and concrete pavements; however, the designs, specifica-tions, materials, and construction techniques should be applicable to concrete decks in buildings, parkinggarages, and similar concrete structures.

* Associate Director, Virginia Transportation Research Council, Charlottesville, Virginia, and Section Head, Trans-portation Research Board; expert in materials and construction, particularly public works.

© 2008 by Taylor & Francis Group, LLC

23-2 Concrete Construction Engineering Handbook

23.2 Key Issues for Successful Bonded HCC Overlays

Properly constructed, HCC overlays can last 30 years or more. Although effective long-lasting overlayshave been constructed, some overlays have cracked and delaminated and have had to be replaced beforethe bridge was opened to traffic. Key issues for long-lasting overlays are the contractor performance,material properties, bond strength, thickness, surface characteristics, and protection characteristics ofthe overlay. Many failures have been caused by placing too much emphasis on material compressivestrength and protection properties and too little emphasis on the other, typically more important prop-erties. Factors that often contribute to premature delamination of the overlay include poor surfacepreparation, use of mixture proportions with high shrinkage, and early shrinkage cracking in the overlay.Some other factors include the use of construction joints, use of thick overlays, and creep and shrinkageof newly constructed superstructures. Design, specification, material, and construction requirements andconstruction procedures that can be used to provide long-lasting HCC overlays follow.

23.2.1 Contractor PerformanceOverlays are difficult to construct. Obtaining and maintaining the high bond strengths for long-lastingoverlays requires that appropriate construction decisions be made with respect to the selection and useof concrete removal and surface preparation equipment and procedures, mixture proportions, andplacement and curing procedures. A properly equipped and experienced overlay contractor is more likelyto perform well and less likely to have problems with the construction. Use of performance-based end-result specifications in which the contractor is rewarded for the quality of the final product rather thanbeing compensated for following a prescription specification encourages good performance.

23.2.2 Overlay Material Properties23.2.2.1 Overlay Mixture Proportions

Materials that have been successfully used for bridge deck overlays include Type I/II Portland cementwith 15% styrene–butadiene latex by weight of cement, referred to as latex-modified concrete (LMC); lowwater/cement ratio hydraulic cement concrete mixtures; and Portland cement mixtures in which silicafume (SF), fly ash, or slag is substituted for a portion of the Portland cement (Sprinkel, 1984, 1992b;Sprinkel and Ozyildirim, 1999b; Tyson and Sprinkel, 1975).

For situations in which traffic must be placed on the overlay after a curing time of 24 hours or less,successful overlays include LMC constructed with Type III Portland cement (LMC-HE), Type II Portlandcement and 7% silica fume, and LMC overlays constructed with calcium sulfoaluminate and dicalciumsilicate cement, referred to as LMC very early strength (LMC-VE) overlays. Traffic can be placed on theLMC-VE overlays after only 3 hours of curing at concrete curing temperatures of 50°F and above(Sprinkel, 1998b, 1999). At temperatures of 80°F and above, a citric acid admixture must be added toretard the mixture. Typical mixture proportions are shown in Table 23.1 (Sprinkel, 1988, 1998b, 2001;

TABLE 23.1 Typical Mixture Proportions

Mixture LMC Silica Fume Fly Ash Slag LMC-HE LMC-VE

Type I/II cement (lb/yd3) 658 658 526 395 815 658Type cement I/II I/II I/II I/II III Rapid-setSilica fume (lb/yd3) — 46 33 33 — —Fly ash (lb/yd3) — — 99 — — —Slag (lb/yd3) — — — 230 — —Fine aggregates (lb/yd3) 1552 1269 1351 1369 1402 1552Coarse aggregates (lb/yd3) 1187 1516 1510 1510 1142 1187Water (lb/yd3) 146 282 254 254 164 146Air (%) 5 7 7 7 5 5 Admixtures Latex HRWR HRWR HRWR Latex Latex


Bonded Concrete Overlays 23-3

Sprinkel and Moen, 1999; Sprinkel and Ozyildirim, 1999b). The latex admixture is typically a 48% solidssolution that is added to the mixture at the dosage of 3.5 gallons per bag of cement. High-range water-reducing (HRWR) admixtures (ASTM, 2008) are typically added to mixtures with silica fume to helpdisperse the silica fume and to provide the desired workability. Air-entraining admixtures are used withall but the LMC mixtures. The latex tends to provide air voids in LMC mixtures, and an air-detrainingadmixture has to be added when the air content is too high.

Aggregates used in overlays typically meet the requirements of ASTM C 33 (ASTM, 2007) and thecoarse aggregate is typically a No. 7 or No. 8. The LMC is batched and mixed at the point of dischargeusing calibrated mobile concrete mix trucks. The trucks have compartments in which the ingredientsare stored separately until conveyed to a mixer on the back of the truck. Other mixtures are typicallybatched at a ready-mix plant and delivered to the discharge point in ready-mix trucks. Although theoverlay mixtures in Table 23.1 can be used on pavements, typically the less costly mixtures are used.These include Portland cement concrete with fly ash or slag (Sprinkel and Ozyildirim, 1999a). Even theless costly mixtures are more expensive than asphalt overlays on pavements; however, in some situations,because of rutting of available asphalt mixtures, structural requirements for the composite pavement,and the availability of suitable asphalt and concrete paving materials and paving contractors, bondedconcrete overlays can be economical on a life-cycle cost basis.

23.2.2.2 Compressive Strength

Overlay mixtures are typically specified to meet a minimum compressive strength (ASTM, 2005). A typicalminimum compressive strength for opening an overlay to traffic is 3000 psi. Other values have been rangingfrom 2500 to 5000 psi depending on the requirements for the deck and the overlay mixture (VDOT, 2002).The minimum compressive strength provides an indication that acceptable concrete was provided. Moreimportantly, it provides an indication that the overlay can have acceptable bond strength when opened totraffic. Bond strength is discussed in more detail later. Compressive strength data for typical overlaymixtures described in Table 23.1 are shown in Table 23.2 (Sprinkel, 1988, 1998b; Sprinkel and Moen, 1999;Sprinkel and Ozyildirim, 1999b). The 4-in.-diameter by 8-in.-long cylinders were typically moist curedprior to testing, with the exception that LMC specimens were typically air cured in the lab after 2 days ofmoist curing. The numbers are average values obtained from several deck overlay projects.

23.2.2.3 Permeability

In recent years overlay mixtures have been specified to meet a maximum permeability (AASHTO, 1995b).The maximum permeability provides an indication that the overlay will be resistant to the penetrationof chloride ions and therefore can protect the reinforcing steel in the deck from corrosion for the life ofthe overlay. The maximum permeability is typically 1500 coulombs (C) at 28 days. Other values havebeen specified, ranging from 1000 to 2000 C at 28 days. Because the permeability typically decreases withage and because the early age permeability is affected by the ingredients and curing conditions, thepermeability at 28 days of age does not typically provide a good indication of the permeability at laterage. A procedure that accelerates the curing of the specimens by storing them at 100°F is being used toprovide a better indication of later age permeability using 28-day test results (VDOT, 2006). Permeabilityis discussed in more detail in the section on protection.

TABLE 23.2 Average Compressive Strength vs. Age

Age LMC (psi) Silica Fume (psi) Fly Ash (psi) Slag (psi) LMC-HE (psi) LMC-VE (psi)

3 hours — — — — — 3510 4 hours — — — — — 3810 5 hours — — — — — 407024 hours 1810 2960 3100 3400 3750 54407 days 3360 5140 5050 5330 5280 629028 days 4630 6980 6820 7180 6340 6710



Permeability data for typical overlay mixtures described in Table 23.1 are shown in Table 23.3(Sprinkel, 1988, 1998b, 2000; Sprinkel and Moen, 1999; Sprinkel and Ozyildirim, 1999b, 2000b). Thedata are typically based on tests of 4-in.-diameter by 2-in.-thick slices cut from the tops of cores takenfrom the overlays. The numbers are maximum and minimum values obtained from several deck overlayprojects. The range is often large because the overlay thickness and overlay ingredients varied fromproject to project. Some projects had 2-in.-thick overlays, and others had 1.5- to 2-in.-thick overlays,so as much as 0.5 in. of the bottom of the slice could be deck concrete with a higher permeability. Apermeability of <100 C is negligible, 100 to <1000 C is very low, and 1000 to 2000 C is low (AASHTO,1995). Whereas the permeability at 28 days is typically low for the mixtures, over time the permeabilitygenerally drops to the very low range. The permeability of the LMC-VE mixture drops to the negligiblerange over time.

23.2.2.4 Shrinkage

In the years ahead, it is anticipated that overlay materials will be required to meet a maximum shrinkagerequirement to minimize or help eliminate cracks in overlays. Cracks are typically caused by a combi-nation of factors that include plastic, autogenous, and drying shrinkage. To minimize cracks in overlayscaused by autogenous shrinkage and drying shrinkage, a maximum length change after 28 days of dryingof less than 0.04% is a desirable goal. To account for autogenous shrinkage, the test procedure must be

TABLE 23.3 Permeability (AASHTO T277)

Age LMC (C) Silica Fume (C) Fly Ash (C) Slag (C) LMC-HE (C) LMC-VE (C)

28 days 1500–2560 950–2330 1000–1160 1040–1390 1320–2850 300–14001 year 200–2060 590–1280 290–300 570–820 320–1280 0–103 year 300–710 520–1460 300–360 500–590 — —5 year 450–500 780–910 — — 510 —9 year 100–400 — — — — 0–60

FIGURE 23.1 Length change of four LMC mixtures. (From Sprinkel, M.M., Latex-Modified Concrete Overlay Contain-ing Type K Cement, VTRC 05-R26, Virginia Transportation Research Council, Charlottesville, 2005.)



modified to include an initial measurement at the time the concrete sets rather than at 24 hours of ageas specified by ASTM C 157 (ASTM, 2006). Figure 23.1 shows the length change of four LMC mixtures(Sprinkel, 2005): LMC, LMC-HE, LMC-VE, and LMC mixtures made with Type K expansive cement(ASTM, 2004b). Six specimens were made from each of the four mixtures. Three specimens from eachmixture were moist cured for 28 days (wet cure), and three were wet cured for the typical 2-day periodand allowed to dry in the air in the laboratory after 2 days (dry cure). Each curve in Figure 23.1 is basedon the average of the measurements on three specimens, 3 × 3-in. by 11 in. long. The curves indicatethat the LMC and LMC-HE mixtures have a 28-day drying length change of approximately 0.04%, andthe long-term length change is approximately 0.08%. Some overlays made with these mixtures have fewcracks, but these mixtures are prone to plastic shrinkage cracking, and drying shrinkage causes the cracksto be wider. The curves indicate that the LMC-VE and LMC Type K cement mixtures have less lengthchange than the LMC and LMC-HE mixtures and should be less prone to cracking. Specimens from theLMC Type K cement mixture that were moist cured for 28 days showed a significant increase in length.This increase could cause the overlay to delaminate from expansion rather than from shrinkage; however,because LMC overlays are typically moist cured for 2 days, excessive expansion typically should not bea problem. Shrinkage is discussed in more detail in the section on protection.

23.2.3 Overlay Bond Strength

The service life of an overlay is usually controlled by a failure in the vicinity of the bond interface betweenthe overlay and the deck. The failure may be at the bond interface, in the base concrete just below thebond interface, in the overlay just above the bond interface, or at a combination of these failure locations.For convenience, unless otherwise indicated, a failure in the vicinity of the bond interface will be referredto as a bond failure. Factors that contribute to high bond strength include the following:

• The deck concrete is in good condition.• Surface damage from concrete removal from the deck surface is minimal.• Deck preparation provides a sound, clean, textured and damp surface.• Overlay concrete is properly consolidated and cured.

Bond strength is a function of the strength of the deck concrete, surface preparation, placement of theoverlay concrete, curing of the overlay concrete, and the strength of the overlay concrete. Overlay bondstrength cannot be higher than the strength of the concrete in the deck and overlay.

23.2.3.1 Deck Concrete Is in Good Condition

Factors that provide an indication that the deck concrete is in good condition include the following:

• The concrete has adequate strength and few cracks.• The reinforcement is not corroding.• The concrete is properly air entrained and not deteriorating from alkali–silica reaction, freeze–thaw

action, or other progressing material distress.

Properly prepared 4000-lb/in2 compressive strength concrete surfaces can provide tensile bond strengthsof approximately 280 lb/in.2 (Sprinkel, 2003b). Most overlays will have a compressive strength that exceeds4000 lb/in.2 which is more than adequate for most situations. Cracks in the deck will typically reflectthrough the overlay and may cause a reduction in bond strength in the vicinity of the crack. The exceptioncan be dormant shrinkage cracks that are not moving and may not reflect. Corroding reinforcement cancause cracks in the concrete around the reinforcement and will eventually cause a reduction in bondstrength. Concrete that is salt contaminated or failing because of distress from freezing and thawing oralkali–silica reaction should be removed prior to placing an overlay. Overlays should not be used to coverconcrete that should be removed.



23.2.3.2 Minimal Damage from Concrete Removal

Overlay construction typically begins with the removal of chloride-contaminated concrete and otherdeteriorated concrete from the deck. The depth of removal depends on the depth of deterioration andtypically varies across the deck. Removal may range from as little as surface texturing to as much as full-depth removal. Deep areas should be patched prior to placing the overlay, although on occasion smallareas are sometimes patched as the overlay is placed. In some situations, particularly new decks, it is notnecessary to remove a considerable depth of concrete, as simple surface cleaning can provide adequatebond strengths. Concrete can be removed to greater depths to improve the grade or surface profile priorto placing the overlay or to allow for a thicker overlay to be placed. On older bridge decks, concrete maybe deteriorated to the point where major concrete removal is required. Concrete removal options andtypical removal depths are shown in Table 23.4. In situations in which surface cleaning and texturing areall that is required, the preferred methods are grit blast, shot blast (Figure 23.2), and hydro demolition.When concrete must be removed to greater depths, the grit blast and shot blast methods are not practical.Scarification works well to remove concrete to just above the rebar but care must be exercised not to hitthe rebar. Also, follow-up removal by grit blast, shot blast, or hydro demolition is recommended to removefractured concrete. Fine milling heads, as shown in Figure 23.3, are being used to minimize damage tothe milled surface. Hydro demolition can be used to remove concrete around reinforcing steel as shownin Figure 23.4. When reinforcement is exposed, concrete removal should continue to a depth that is atleast 1 in. below the reinforcement so the bond interface is not located in the reinforcement. Scarificationfollowed by hydro demolition works well for removing the top half of the deck. The hydro demolitionequipment abrades the deck surface with water at nozzle pressures that are typically about 26,000 lb/in.2.The cuttings and water are usually collected and vacuumed from the surface. Large self-contained unitsare used for applications ranging from surface texturing to full depth removal of concrete. Large pneu-matic hammers are practical for full-depth removal. A variety of methods may be used to removedeteriorated concrete. Impact methods such as hammers and scarification may fracture the concrete leftin place, whereas hydro demolition does not. When impact methods are used, additional removal withother methods should be done to remove damaged and fractured concrete. Following concrete removaland patching the surface must be cleaned and textured to get good bond strengths (Sprinkel, 1997).

23.2.3.3 Surface Preparation Provides a Sound, Clean, Textured, and Damp Surface

A specification that, when enforced, should provide a properly prepared surface states: “Clean surface byshot blasting and other approved cleaning practices to remove asphalt, oils, dirt, rubber, curing com-pounds, paint, carbonation, laitance, weak surface mortar, and other detrimental materials that mayinterfere with the bonding or curing of the overlay” (VDOT, 2001). Techniques that can be used typicallyinclude a combination of the following: grit blast, shot blast, hydro demolition, power washing, and airblast. The quality of a grit blast depends on the skill of the person doing the blasting and the time devotedto the effort. Shot blasting, as shown in Figure 23.2, is a more mechanized way to prepare concretesurfaces to achieve high bond strengths. The shot blaster abrades the deck surface with shot and vacuumsup the shot and concrete cuttings. The shot does not leave fractures in the prepared concrete surface. Bymonitoring the speed and number of passes of the shot blaster, proper surface preparation can be

TABLE 23.4 Concrete Removal Options and Practical Removal Depths

Concrete Removal Option Practical Removal Depth

Grit blast <2 mmShot blast <6 mmDiamond grind <Rebar depthScarification (milling) <Rebar depthHydro demolition 1 mm to half depthPneumatic hammers 12 mm to full depth



FIGURE 23.2 Shot blast provides for good bond strength by cleaning and texturing the concrete surface.

FIGURE 23.3 Fine milling heads spaced ≤8 mm apart as shown are being used to minimize damage to the milledsurface.

FIGURE 23.4 Hydro demolition can be used to remove concrete around reinforcing steel.



achieved. The shot blaster typically removes up to 1/8 in. of the surface, and larger shot blasters canremove up to 1/4 in. of the surface. Hydro demolition equipment can also be used to prepare concretesurfaces to achieve high bond strengths. A low-pressure water blast is used for the final cleaning of thesurface prior to placing the concrete overlay. The saturated concrete deck that results from the hydroblasting provides for a good cure of the overlay concrete.

When it may not be clear what is required for acceptable surface preparation, the test method prescribedin ACI 503R (ACI Committee 503, 1993) or ASTM C 1583 (ASTM, 2004a) can be used to determinethe cleaning and texturing practice necessary to provide a tensile bond strength in test patches or a tensilesurface strength in prepared surfaces greater than or equal to some minimum value (AASHTO, 1995b;ACI Committee 503, 1993; ASTM, 2004a; Sprinkel, 1997). A minimum value of 250 psi has been used(VDOT, 2001). A lower quality limit of 150 psi was used in a performance specification in which thecontractor was paid for the percent of tests that were within the specification limits (Sprinkel, 2004b).A bond strength test result is usually based on the average of three individual tests. A failure at a depthof 0.25 in. or more into the base concrete is preferred (AASHTO, 1995b; VDOT, 2001).

As shown in Figure 23.5, metal disks or plates can be bonded to the prepared surface and pulled intension to provide an indication of the tensile strength of the prepared surface. A rapid-setting acrylicadhesive can be used to bond the test discs or plates to the surface. The advantage of testing the preparedsurface is that results are obtained in approximately an hour, and surface preparation activities can beadjusted as needed with negligible delay to the project. The disadvantage is that the bond strength of theHCC overlay is likely to be less than that of the acrylic adhesive used to bond the disks or plates. Also,bond problems resulting from the overlay mixture and placement are not identified ahead of time(Sprinkel, 2003b).

As shown in Figure 23.6, on larger jobs it may be practical to construct HCC overlay test patches withone or more overlay mixtures and different surface preparation procedures. Overlay mixtures and surfacepreparation procedures that give the desired results can be specified. Although days or weeks may berequired for the overlay concrete to cure sufficiently to get tensile bond strength test results, any problemsassociated with the deck concrete, surface preparation, and placement and curing of the overlay can beidentified ahead of time and appropriate adjustments made prior to placing the overlay. On very largejobs and where time permits, a full-width bridge deck section can be placed and tested with the concretemixture and procedures that gave good results for the test patch.

When the acceptable cleaning and texturing practice has been determined, it is necessary to ensurethat the practice is carried out over the entire deck surface. This is accomplished by determining themacro texture of the prepared surfaces upon which the acceptable bond tests were done (the macro

FIGURE 23.5 Metal disks or plates are bonded to the surface and pulled in tension.



texture is usually determined prior to construction of the test patches or test sections) and monitoringthe macro texture of the prepared deck surface to ensure that the surface macro texture is the same orheavier than that on which the bond tests were performed (Sprinkel, 1997).

Two methods can be used to monitor the macro texture of the prepared deck surface (Sprinkel,1997, 1998a). As shown in Figure 23.7, a quantitative macro texture result for the prepared surfacecan be determined by measuring the macro texture using the procedure specified in ASTM E 965(ASTM, 1996). A qualitative indication of the macro texture of the prepared surface can be determinedby using the International Concrete Repair Institute (ICRI) molded standards shown in Figure 23.8(ICRI, 1997). The texture of the prepared surface is compared to the texture of the ICRI moldedstandard determined to best represent the prepared surface upon which acceptable bond tests weredetermined. A result for the macro texture measurement is based on the average of ten sand patchtests. Use of the ICRI nine molded standards requires less effort, but the owner and contractor mustbe able to agree on the qualitative assessment. Recent research indicates a strong correlation betweenlaser surface macro texture measurements and the ASTM E 965 procedure (Stroup-Gardiner andBrown, 2000). Laser measurements to monitor surface preparation are a third option that will likelysee greater use in the future.

Minimum macro texture values based on ASTM E 965 that have been specified in contracts include0.06 in. and 0.08 in. (Sprinkel, 1997; Sprinkel and Ozyildirim, 1999b). Whereas 0.06 in. is often anacceptable minimum value, more or less cleaning may be required to obtain high bond strengths.Typically more cleaning and therefore texturing are necessary for old concrete surfaces than for new

FIGURE 23.6 An overlay test patch is tested for bond strength.

FIGURE 23.7 Macrotexture depth measurement to ensure surface preparation (ASTM E 965).



ones. Also, the type and size of the aggregates in the deck can affect the optimum minimum texturevalue. For the same blasting effort, decks with lightweight aggregates will typically have a higher texturethan decks with hard, dense aggregates because the aggregates abrade away in the lightweight decks andthe mortar abrades away in the decks with hard aggregate. The minimum texture that provides goodbond strength will depend on the materials in the deck, the condition of the deck, and the cleaningtechnique used.

As the surface is prepared, the screed is set to the proper elevation. A trial run of the screed is madewith a spacer block attached to the bottom of the screed to ensure that proper cover between the screedand top reinforcement and the required minimum overlay thickness are obtained. After the screed is setand cleaning and texturing of the surface are complete, the prepared surface is wetted with potable waterand covered with a polyethylene sheet to protect the prepared surface until the sheet is removed minutesprior to placing the overlay concrete. Any contamination of the prepared surface prior to placing theoverlay must be removed, and dry surfaces should be rewetted so the surface is damp but has no standingwater where the overlay concrete is being placed.

23.2.3.4 Overlay Concrete Is Properly Placed and Consolidated

Figure 23.9 shows the typical concrete placement procedure which is as follows:

• Deposit the overlay concrete onto the prepared surface.• Broom mortar from the overlay concrete onto the prepared surface, and discard the excess coarse

aggregate from the mortar.• Consolidate and strike-off the concrete using a screed that vibrates.• Finish the surface using a float that is typically attached to the back of the screed.

Some hand-finishing is typically required along the edges of the placement. Proper consolidation of theconcrete at the bond interface is required for high bond strength. The vibrating pan on the front of ascreed, vibrating rollers, and a vibrating strike-off bar can consolidate overlays <2.5 in. thick. Thefrequency of vibration can be adjusted, and care should be taken to ensure that the screed is providingadequate consolidation. Surface vibration decreases with the distance from the surface and typically failsto properly consolidate the concrete at depths greater than approximately 2.5 in. Internal vibrators areused to consolidate areas that are deeper than 2.5 in.

FIGURE 23.8 ICRI molded standards ensure proper surface preparation. (From ICRI, Selecting and SpecifyingConcrete Surface Preparation for Sealers, Coatings, and Polymer Overlays, Guideline No. 03732, International ConcreteRepair Institute, Sterling, VA, 1997.)



Some recent experience indicates that, with proper consolidation of the overlay at the bond interface,the use of brooms to apply mortar from the overlay or from separate containers does not contribute tobond strength; therefore, this step can be eliminated. Moreover, it is a step that, when done incorrectly,can cause lower bond strengths. Incorrect applications include the mortar drying before the overlayconcrete is placed, retempering of a separate container of bonding mortar that results in a low strengthlayer of mortar, or placement of bonding mortar on a prepared surface that has a texture so heavy thata thin and uniform layer of bonding mortar cannot be placed.

23.2.3.5 Overlay Concrete Is Properly Cured

The overlay concrete must be properly cured to achieve high bond strengths. The bond strength cannotbe higher than the strength of the overlay concrete, and the strength of the overlay concrete is a functionof the quality and completeness of the curing. As can be seen in Figure 23.10, from the time the overlayconcrete is deposited on the prepared surface until the curing materials are applied, water evaporatesfrom the surface of the overlay concrete. The evaporation rate can be estimated using a nomograph that

FIGURE 23.9 Screed strikes off and consolidates the overlay concrete.

FIGURE 23.10 Wet burlap is placed on the overlay surface as soon as possible to prevent plastic shrinkage cracks.



takes as input data the air temperature, relative humidity, concrete temperature, and wind speed. Whenevaporation rates are high, the contractor can take steps to reduce the rate. Special precautions aretypically followed when evaporation rates exceed 0.1 lb/ft2/hr. These include fogging the air over thesurface of the overlay to increase the relative humidity, batching the concrete at a lower temperature, andinstalling wind breaks to reduce the wind speed over the surface of the overlay.

Regardless of the evaporation rate, the wet burlap curing material should be placed on the finishedconcrete surface as soon as possible to stop further evaporation and to prevent plastic shrinkage cracking.As shown in Figure 23.11, polyethylene is placed on the wet burlap as soon as practical to help keep theburlap wet. Soaker hoses can be placed under the polyethylene to maintain the burlap in a wet conditionduring the curing period. The burlap should not be allowed to dry, as dry burlap can remove water fromthe overlay that is necessary for curing. LMC is typically moist cured for 2 days and other HCC overlaysfor 3 to 7 days. Longer curing periods are beneficial for the quality of the concrete but often not practicalbecause of the need to accelerate construction and to open the overlay to traffic. LMC-VE overlays arecured for 3 hours or until opened to traffic.

23.2.3.6 Minimal Full-Depth Cracks

Plastic shrinkage cracks can occur when water evaporates from the overlay concrete because there is nowater on the surface to protect the concrete from evaporation. These cracks are often fine and shallowbut typically become wider, as shown in Figure 23.12, because of drying shrinkage. The cracks canpenetrate the full thickness of the overlay either initially or at a later age because of drying shrinkage.Shrinkage of the overlay can stress the bond interface because the overlay is trying to shorten relative tothe deck. These stresses are the highest around the perimeter of the overlay, along construction joints,and along full-depth cracks in the overlay. These stresses can cause delamination of the overlay, and thearea of delamination can increase with time. Although cracks compromise the protection provided bythe overlay, cracks that are full depth can increase the risk of delamination of the overlay along the cracks.The overlay concrete must also be properly cured to help prevent full-depth cracks.

23.2.3.7 Joints

To accommodate traffic, overlays are often constructed one lane at a time with traffic in the adjacentlane which results in longitudinal construction joints. These joints are like full-depth cracks in the overlayand should be avoided because they can cause premature delamination of the overlay. When possible,overlays should be placed over the entire width and length of the deck span and without any joints. Jointsoften control the time to repair or replace properly constructed overlays. Bridge decks with more than

FIGURE 23.11 Polyethylene sheeting covers wet burlap for the curing period.



one span may have expansion joints to allow for movement between the spans. The overlay should notbe placed over the joints because the overlay will crack and delaminate along the joints. Joints must beconstructed in the overlay directly above the joints in the deck. Typically, the joints in the overlay aresimilar to and replace the joints in the deck. In some situations, the construction of an overlay providesthe opportunity to replace leaking joints with joints that will perform better. Either new joints or formsfor new joints are placed directly above the joints in the deck, and the overlay concrete is placed againstthe joints or forms. When forms are used, they should be constructed with a compressible layer of materialto minimize compression stresses in the overlay and shear stresses at the bond interface that may becaused by expansion of the adjacent deck spans. Also, the forms should be removed shortly after theconcrete sets so the joint in the overlay can allow for movement between the adjacent deck spans (Sprinkeland Ozyildirim, 1999b). The open joint must be filled with a suitable joint material in accordance withrecommended practice.

23.2.4 Overlay Thickness

Bonded concrete overlays for bridge decks are typically designed to have a nominal 1.5-in. thickness. Theminimum thickness is 1.25 in., and the installed thickness typically ranges from 1.25 to 2 in. A mixturewith No. 7 or No 8 coarse aggregate works well for these overlays. In situations in which the minimumthickness of the overlay is greater than 2 in., the typical overlay mixture should be redesigned to reduceshrinkage by increasing the maximum size of the coarse aggregate and if practical to reduce the cementmaterials content. The optimum concrete mixture has a maximum coarse aggregate size that is one thirdthe minimum thickness; for example, No. 67 aggregate should be used in 2.25- to 3-in.-thick overlays,and No 57 aggregate should be used in 3- to 4-in.-thick overlays. Bonded concrete overlays are typicallynot designed to be thicker than 4 in., but thicker overlays are sometimes constructed to provide a suitablegrade and profile for the surface. The thickness of an overlay is often varied over the deck surface toimprove the drainage, surface profile or ride quality and to fill areas not patched prior to placing theoverlay; however, the overlay is typically designed for the thinnest areas. When practical, the mixtureshould be changed to accommodate the thicker areas.

The thickness of an overlay is a factor in performance. Mortar rather than concrete is used for overlaysthat are thinner than 1.25 in. The mortar typically has higher shrinkage and therefore is more prone tocracking than concrete. The cracking can cause delamination of the overlay. Shrinkage of the overlay canstress the bond interface because the overlay is trying to shorten relative to the deck. The stress increasesas the thickness of the overlay increases; therefore, overlays thicker than 2 in. are more prone to delam-ination. The stress can be reduced by changing the mixture to reduce shrinkage. Overlays have delami-nated before being opened to traffic when a concrete mixture designed for a 1.5-in.-thick overlay wasplaced 4 in. thick. The optimum nominal thickness for a bonded concrete overlay is 1.5 in.

FIGURE 23.12 Shrinkage cracks are apparent in an overlay placed on a bridge deck.



23.2.5 Overlay Surface Characteristics

Obtaining overlays with desirable surface characteristics, including good skid resistance, ride quality,drainage, and surface appearance, is typically not a problem. These factors are easily achieved withgood design and construction practices. The overlay must be designed and the screed must be set toprovide a final grade that provides for good ride quality and good drainage. The surface of the plasticconcrete must be struck off and finished to the final grade shown on the plans. Good skid resistanceis obtained by tining the plastic overlay concrete or by saw-cutting grooves into the hardened concretesurface. Tined valleys and saw-cut grooves are typically 1/8 in. wide, 1/8 in. deep and spaced 3/4 in.apart (VDOT, 2002).

23.2.6 Overlay Protection CharacteristicsThe protection provided by an overlay is a function of its thickness, permeability, diffusion constant,shrinkage, and cracks. Construction joints, like cracks, can affect the level of protection provided. Thethicker the overlay, the greater the cover over the reinforcement and therefore the greater the protectionprovided. As mentioned earlier, however, overlays thicker than 2 in. are more likely to delaminate. ManyHCC overlay mixtures can provide low permeability. Use of styrene–butadiene latex or supplementalcement materials, such as silica fume, fly ash, and slag, and good concreting practices easily provide foroverlay concretes with a low permeability (<1000 C) (AASHTO, 1995b). Permeability values for typicaloverlays are shown in Table 23.3 (Sprinkel, 1988, 1998b, 2000; Sprinkel and Moen, 1999; Sprinkel andOzyildirim, 1999b, 2000). The diffusion constant for the overlay concrete can be calculated using Fick’ssecond law of diffusion. It provides an indication of the rate at which chloride moves through the overlay.The lower the constant, the longer it takes for chloride to penetrate the overlay. Concretes with a lowpermeability typically have a low diffusion constant.

Cracks compromise the protection provided by the overlay by allowing chlorides and water to bypassthe low-permeability overlay concrete and travel directly to the reinforcement. Shrinkage can cause cracksin the overlay. The lower the shrinkage, the less prone the overlay is to cracking and delamination.Construction joints, like cracks, can allow for the penetration of chlorides. In addition to shrinkage,cracks can be caused by thermal contraction when overlay concrete is placed that has a much highertemperature than the deck. Cracks can be caused by cracks in the deck reflecting through the overlay, bysuperstructure creep and shrinkage when overlays are placed on newly constructed structures, and bytraffic when the traffic loading causes tensile stress in the surface of the overlay. Most cracks in overlaysoriginate as plastic shrinkage cracks and widen because of one or more of the other causes. Typically, arelatively crack-free overlay can be obtained when a low-shrinkage mixture is properly placed andprotected from plastic shrinkage cracking.

Gravity-fill polymers can be placed in shrinkage cracks to help seal them. Products that have beensuccessfully used include high-molecular-weight methacrylate, epoxy, and urethane (Sprinkel, 1992a,1995). Typically the overlay surface is flooded with the monomer, and brooms are used to work the liquidinto the cracks. Cracks that are not close enough to justify flooding the surface can be treated individually.Silane can also be used to seal cracks. The silane is easy to apply and makes the surface of the crackhydrophobic so it repels water. Although an overlay with sealed cracks cannot be expected to performas well as an overlay with no cracks, application of the gravity-fill polymer or silane sealer is the onlypractical way to reduce the penetration of chlorides and water through the cracks.

Because the cracks will likely widen with age, it is best to wait as long as practical to seal them. Thebest time is a function of the shrinkage of the concrete and the use of the structure. Most of the shrinkagehas occurred in approximately 3 months for a typical HCC overlay. The shrinkage may occur faster inthe summer than in the winter. As much as approximately 25% additional shrinkage may occur after 3months, so waiting 6 months to a year or more before sealing the cracks is desirable but often notpractical. Another option is to seal the cracks once at an early age before opening the deck to traffic andagain after 2 years after most shrinkage has occurred. Very early strength materials may be effectivelysealed at an earlier age.



23.3 Other Issues

23.3.1 Rapid Construction of Overlays

In the past, most bridge decks and pavements were closed to traffic for the construction of an overlay.As traffic volumes have continued to increase, it has become difficult if not virtually impossible to closemany structures for overlay construction. Overlays are typically placed one lane at a time with traffic inadjacent lanes. In the most heavily congested situations, lanes are only closed for short periods of time,such as weekends or nights, and short segments of overlay are placed during each lane closure. Thesequence of construction steps for the construction of LMC-VE overlays follows:

Phase 1• Close lane at 9 p.m.• Mill deck surface.• Patch deck.• Cure patches.• Open lane at 5 a.m.

Phase 2• Close lane at 9 p.m.• Shot blast surface.• Wet surface.• Place overlay.• Cure overlay 3 hours.• Open lane at 5 a.m.

Although this sequence has been used successfully, weekend lane closures are preferred for the constructionof LMC-VE overlays and are required for the construction of SF and LMC-HE overlays (Sprinkel, 2006).

23.3.2 CostThe cost of an overlay is a function of the cost of materials, surface preparation, labor, equipment, overhead,and traffic control. Cost data based on a review of bid tabulations obtained from the Virginia Departmentof Transportation bridge office for the period of 1994 and 1995 indicated the following average total costsper square yard for overlays: LMC and SF, $130; LMC-HE, $92; and LMC-VE, $96 (Sprinkel, 1999). Therapid overlays cost less than the conventional overlay. Traffic control costs are high for conventional overlaysbecause of the requirements for concrete barricades, removing and installing permanent and temporarypavement markings, and longer construction time. The rapid overlays are installed in a short time usingcones for delineation and without the need to replace pavement markings, so the traffic control costs areless. The LMC-VE costs more than the LMC-HE because of the higher cost of the cement.

23.3.3 HCC Pavement OverlaysBonded pavement overlays are similar to bonded bridge deck overlays, and the concrete mixtures andconstruction procedures are similar. One difference is that a slip-form concrete paving machine is usedfor pavement overlays and a bridge deck vibrating screed is used for bridge deck overlays. The slump ofthe overlay concrete used on pavements is typically 1 to 4 in., and the slump of the overlay concrete usedon decks is typically 4 to 7 in. Overlays on decks can increase the stiffness of the deck, but the increaseis typically not a significant factor in the performance of the deck. On the other hand, overlays are oftenplaced on pavements to increase the stiffness of the pavement which should extend the life of thepavement. Deflections were reduced by 33 and 36%, respectively, after 2- and 4-in.-thick overlays wereplaced on 8-in.-thick continuously reinforced pavements in Virginia (Mokarem et al., 2007; Sprinkel andOzyildirim, 1999a, 2000a). The increased stiffness has been maintained for 10 years, and it is anticipatedthat the overlays will perform well for 20 years or more.



23.3.4 Service Life of HCC Overlays

The service life of an overlay is usually controlled by a failure in the vicinity of the bond interface betweenthe overlay and the deck or pavement surface. The service life of a well-bonded overlay is usually reducedby cracks and construction joints. Joints and full-depth cracks increase the probability of delaminationof the overlay in the vicinity of joints and cracks. The delaminations often spread to other areas. Jointsand cracks may control the time to repair of properly constructed overlays. Other common causes offailure are due to the overlay being placed on deck or pavement concrete that is deteriorating or on deckconcrete that is salt contaminated and the reinforcement is corroding.

The service life of a well-bonded deck overlay with no full-depth cracks and joints placed on concretein good condition should be controlled by the time it takes for the overlay to allow chlorides to reach thereinforcement in the deck and cause corrosion-induced spalling; however, many years are required forchlorides to penetrate the low-permeability overlays, and this type of failure is unlikely (Sprinkel, 2003a,2004a). Also, loss of skid resistance or loss of ride quality rarely controls the life of a hydraulic cementconcrete overlay. The exception is when ride quality is affected by the presence of spalling caused by lowbond strength or corrosion or by reinforcement in situations where the reinforcement was corrodingwhen the overlay was placed. It is reasonable to expect that the service life of overlays will increase withan increase in bond strength, with a decrease in the incidence of cracking and construction joints, witha decrease in permeability, and with a decrease in chloride at the level of the deck reinforcement.

The probability that long-lasting HCC overlays will be constructed increases with the use of low-shrinkage concrete mixtures; the use of good surface preparation procedures; proper consolidation ofthe overlay; placement of overlays when evaporation rates are low; good curing of the overlay; theconstruction of 1.25- to 2.0-in.-thick overlays; use of no construction joints; and the construction ofoverlays on bridges that are rigid and subject to low creep or pavements that have adequate stiffness,good drainage, and good base materials.

23.4 Summary

Properly constructed, HCC overlays can last 30 years or more. The six key issues for long-lasting overlaysare contractor performance, material properties, bond strength, thickness, surface characteristics, andprotection characteristics of the overlay. HCC overlay properties that support long-lasting overlays includeadequate bond strength, low shrinkage, low permeability, and few cracks and construction joints. A soundand stable substrate and good surface preparation are required to obtain adequate bond strength. Qualitymixture proportions that provide low shrinkage, low permeability, and few cracks are required to protectthe deck from chlorides and moisture. Proper placement that includes adequate consolidation, strike off,and finishing is required to obtain good bond strength and low permeability. Good curing that minimizescracking and premature loading is required for long-lasting protection of the deck. Other factors includeconstruction of overlays with a thickness of 1.25 to 2 in. and using as few construction joints as possible.Finally, the contractor must construct an overlay that satisfies the requirements of a good specification.

References

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AASHTO. 1995b. Standard Method of Test for Electrical Indication of Concrete’s Ability to Resist ChlorideIon Penetration, AASHTO T 277. American Association of State Highway and TransportationOfficials, Washington, D.C.

ACI Committee 503. 1993. Use of Epoxy Compounds with Concrete, ACI 503R. American ConcreteInstitute, Farmington Hills, MI.

ASTM. 1996. Standard Test Method for Measuring the Macrotexture Depth Using a Volumetric Technique,ASTM E 965. American Society for Testing and Materials, West Conshohocken, PA.



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ASTM. 2004b. Standard Specification for Expansive Hydraulic Cement, ASTM C 845. American Societyfor Testing and Materials, West Conshohocken, PA.

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Department of Transportation, Richmond.


Nabeaure Tower in Yokohoma, Japan, with precast ECC coupling beams in the building core for seismic resistance;the 41-story building was completed in 2007.


Bonded Concrete Overlays - Ardiansyah Kusuma … · Bonded Concrete Overlays 23-3 Sprinkel and Moen, 1999; Sprinkel and Ozyildirim, 1999b). The latex admixture is typically a 48%

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