Reinforced concrete overlays

Conductive overlay systems are not practical propositions on vertical surfaces or surfaces where weight restrictions are important. However, they are proven cathodic protection systems, and should be considered in conjunction with other reinforced concrete cathodic protection system anodes. 

Latex-modified mortars and concretes have become promising materials for preventing chloride-induced corrosion and for repairing damaged reinforced concrete structures. In Japan and the USA, latex-modified mortar is widely used as a construction material in bridge deck overlays and patching compounds, and for finishing and repairs [99]. Polymer-cement hydrate-... 

Rehabilitation is achieved by overlays such as latex-modified concrete, low-slump concrete, high-density concrete, and polymer concrete. They are commonly used for the rehabihtation of bridge decks. This procedure extends the life of a bridge deck by about 15 years. Impressed-current CP systems on bridge decks are now a routine rehabihtation teehnique because of the cooperative research with industry and states in the development of durable anodes, monitoring devices, and installation techniques. Titanium mesh anode, used in conjunction with a concrete overlay to distribute protective current, serves as a durable anode for use in impressed-current CP of reinforced concrete bridge deeks and widely accepted by state and other transportation agencies. 

One important requirement is that the interfacial resistance between the anode and the concrete. However, the electrical resistance of the anode system should be proportionately lower than the combination of the interfacial resistance, the concrete cover resistance, and the steel to concrete resistance, otherwise the current will not distribute evenly to the steel. For atmospherically exposed reinforced concrete structures the anode is usually a distributed anode system , such as a paint coating on the surface, an expanded metal mesh across the surface encased in a concrete overlay, strips of anode in slots across the surface or a series of small point or discrete anodes embedded in the concrete cover or among the rebars. 

The CRCPs with asphalt surfacing layer may be further distinguished into CRCPs with thin asphalt overlay (minimum thickness of 30 mm) or CRCPs with an asphalt overlay of 100 mm. The latter is known as rigid pavement with continuously reinforced concrete base (CRCB) and asphalt overlay. 

Figure 14.1 Characteristic types of rigid pavements, (a) Unreinforced concrete pavement (URCP) (b) jointed reinforced concrete pavement (JRCP) (c.i) continuously reinforced concrete pavement (CRCP), without asphalt overlay (c.ii) CRCP, with thin asphalt overlay and (d) rigid pavement with continuously reinforced concrete base (CRCB) and asphalt overlay. (Adapted from Highways Agency, The Manual of Contract Documents for Highway Works [MCDHW], Volume 3 Highway Construction Details, Department for Transport. London Highways Agency, 2006c.)...

In thin-bonded concrete overlay over asphalt surface, transverse joints should be provided at very close distance (no more than approximately 1 m Thom 2008) and the slab should be either fibre reinforced or steel reinforced, if slab thickness allows it. 

Thickness determination of the thick concrete overlay may be carried out by considering the failed concrete pavement as a sub-base layer. Usually, the minimum thickness considered for an unreinforced concrete slab is 150 mm and that for a reinforced slab is 200 mm. 

Similar predictive equations for smoothness degradation have also been developed for HMA overlays of rigid pavements, jointed reinforced concrete pavements (JRCPs) and continuously reinforced concrete pavements (CRCPs), which can be found in the MEPDG manual (AASHTO 2008). 

A relatively inexpensive bonding agent that may be used to ensure a positive bond of a new concrete overlay is a latex-reinforced cement slurry grout. Portland cement and sand (in a ratio of 1 3 by volume) are combined with a gauging liquid that is a mixture of equal volumes of water and an acrylic or polyvinyl acetate latex emulsion, as described in the table. This emulsion must be nonre-emulsifiable. The gauging hquid, based on the latex blend, is added to the cement-sand powder until a creamy paste is developed. This paste is scrubbed into the surface of the base slab, covering the surface thoroughly. New concrete is then placed on this bond hue, which is approximately M6-in thick. 

J2 Corrosion of steel reinforcing ban (reban) in a concrete bridge head, (a) The corrosion cell which arises due to water penetration and chloride pick-up from the concrete or from deicing salts, (b) The expansive corrosion products may cause cracking of the concrete. Tn severe cases, delamination results, (c) Impressed current, cathodic protection may be applied, (d) A flexible anode mesh may be installed (which is then covered with a concrete overlay)i Power usage is < 10 Wm" of the concrete surface. (Courtesy Raychem Ltd.)... 

Portland cement concrete overlays for new reinforced concrete are applied as part of a two-stage construction process. The... 

Concrete overlays have been used as a protection against reinforcing steel corrosion in new bridges. In general, the overlays are susceptible to cracking, especially on continuous structures, though this is a characteristic of all rigid overlays. ... 

Figure 6.4 Dimensions of the substrate for measuring plastic shrinkage cracking using a bonded overlay, from N. Banthia.R. Gupta and S. Mindess, Development of Fiber Reinforced Concrete Repair Materials , Canadian J. of Civil Engineering, Vol. 33, No. 2,2006, pp. 126-1 33 with permssion from NRC Research Press.

N. Krstulovic-Opara, A.R. Haghayeghi, M. Haidar and P.D. Krauss, Use of conventional and high-performance steel-fiber reinforced concrete for bridge deck overlays . Ad Mater. J. 92,1995, 669-677. 

Anode system. The anode system, which consists of the anode material plus its overlay, must supply the required current for the anticipated service life and distribute it to the reinforcement that needs to be protected. Anode materials and current density aspects have been dealt with in a previous section. The general requirements of an anode system are it has to adhere to the concrete surface it should be suitable for appHcation to the surface needing protection (top, bottom, horizontal, vertical, flat, curved), it should be durable and have low installation cost it should produce acceptable weight addition and change of the appearance and dimensions of the structure. If an overlay is used, it should have durable bond to the substrate concrete, sufficient mechanical strength and electrical characteristics equal to those of base concrete (ionic conductivity). 

Execution. In the execution phase, the CP system is applied to the structure following the design. During the work, the complete concrete surface is checked for cracking, delaminations, cover depth, steel continuity and the presence of metal objects that might cause short circuits. If necessary, continuity is provided and metal objects are connected to the reinforcement. Subsequently the cracked and spalled areas are removed and repaired. The reference electrodes and other monitoring probes are embedded. Then the anode is applied, with overlay or top coat as designed. All electrical coimections are made and the power source is installed. 

Hot asphalt surfacing overlay restores micro- and macro-texture and is the most suitable treatment for restoration of skid resistance of continuously reinforced pavement surface. All types of hot asphalts used in restoration of skid resistance of flexible pavements can be used, namely asphalt concrete for very thin layers, SMA and porous asphalt. 

The second part of this book is devoted to current and potential applications of adhesive materials in construction. Chapter 6 deals with both the repair and the strengthening of concrete structures, covering applications ranging from non-structural patch repairs and resin overlays to externally bonded steel plate reinforcement. The theme of repair and strengthening is extended to applications involving steel, timber and masonry structures. A number of case histories are reviewed and discussed with reference to the successes and failures, and the results of allied research work are presented. In Chapter 7 a number of applications of adhesives in new construction are described, and specific examples are given. The final chapter. Chapter 8, examines the potential for future developments in adhesive usage. 

Zhang, J., Li, V. C. (2002) Monotonic and fatigue performance in bending of fiber-reinforced engineered cementitious composite in overlay system, Cement and Concrete Research, 32 415-23. 

The REZKLAD epoxy-based system consists of a Concrete Primer and Flooring Compound, plus an Expansion Joint Compound and Surface Sealer available for use where necessary. The CLADKOTE Flooring Compound is a modified polyester, resin-based monolithic overlay for concrete. The composite of resins and siliceous reinforcing material cures to a tough, chemical resistant topping. CLADKOTE C utilizes a 100% carbon filler specifically designed for service in hydrofluoric acid and fluoride salts. 

Adequate anode lifetime is obviously also an important factor related to the magnitude and imiformity of current flow. A variety of anode systems have evolved for cathodic protection of reinforcing steel, each with certain advantages and limitations. Continuous surface anodes have been based on conductive bituminous overlays and conductive surface coatings. The former are suited only to horizontal surfaces. In general, good current distribution is achievable with such systems. Discrete anodes have been used without overlays and with cementitious overlays. For horizontal surfaces, anodes without overlays can be recessed in the concrete surface. Nommiform current distribution is a... 

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