Concrete overlays

The advantages of conductive paints are that they are easy to apply and a concrete overlay is not required. They can be applied to complex shapes and are not a problem where weight restrictions are imposed. 

ACI (1993) Guide for polymer concrete overlays. ACI Material Journal, 90(5), 499-522. Brieton, P.R. (1994) Sealant materials properties and performance - an overview, in European Adhesive and Sealant Year Book 1994, FMJ International, Redhill, Surrey, pp. 7-8. 

Owing to this rapid cure, a compressive strength of 40 N/mm is possible within 2 h at temperatures down to 4°C (Shaw, 1993a). The properties of polyester binder depend primarily upon the chemical composition of the polyester resin component. Typical physical properties of cured polyester binders for concrete overlays are given in Table 9.3 (ACI, 1993). 

ACI Committee 548 (1993) Guide for Polymer Concrete Overlays, ACI Materials Journal (Sept.-Oct.), 504. 

Pull-out tests were also conducted to measure the bond strength between the polymer concrete overlay and the Portland cement concrete substrate before and after the thermal compatibility test. The results are shown in Table 9. 

Depuy, G. Polymer Concrete Overlays for the Repair and Protection of Concrete, ICPIC 2001. The Tenth International Congress in Polymeric Concrete, Honolulu, HI, 2001. 

Tables 3.7 and 3.8 give the ACI (American Concrete Institute) suggested guidelines for the mix proportions of latex-modified concretes for bridge deck applications and patching work respectively.l J Also, Standard Specification ACI 548.4.1 1 provides a guideline for the mix proportions of SBR-modifled concrete overlays, for new construction as well as repair and rehabilitation, of highway bridge decks as shown in Table 3.9.

Sprinkel, M. M., Twenty-Year Performance of Latex-Modified Concrete Overlays, Polymer-Modified Hydraulic-Cement Mixtures, STP-1176,pp. 141-154, American Society for Testing and Materials, Philadelphia (1993)... 

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. 

Conductive polymer mound anode system used platinized-niobium copper wire anode with the conductive polymer mounded on wire anode and rigid concrete overlay on top. Estimated cost is 137/m with a service life of 20 years. 

Activated titanium mesh secured to the concrete and covered with either conventional concrete or latex-modified concrete overlay at a cost of 137/m and service life of 35 years. 

Perforated zinc sheets fastened on the deck with a bed of mortar, then covered with concrete overlay. 

Performance of Stainless Steel Rebar in Concrete Corrosion protection of steel rebar can be achieved by (i) selection of corrosion-resistant steel (ii) use of coatings (iii) addition of corrosion inhibitors such as calcium nitrite to concrete mix (iv) addition of concrete sealers (iv) use of membranes (v) use of thicker concrete overlay (vi) cathodic protection. 

After milling, local, deeper removal is needed where cracking and spalling has occurred. The deck is then patched in the delaminated areas and a dense cementitious overlay is put back on. The use of concrete overlays is discussed in a later section in this chapter. 

Concrete overlays or encasement may absorb some of the chlorides, reducing the level at the concrete surface. They will certainly reduce the high chloride gradient that drives chloride further into the concrete if they are coupled with removal of some or all of the old cover concrete. This has been measured on bridge decks. 

Anodes have been developed in the form of conductive coatings, metals embedded in concrete overlays, conductive concrete overlays and probes drilled into the concrete. Anodes continue to be developed, applied in new configurations and to new structures. In the next section we will discuss the major components of the cathodic protection system, and particularly the anode systems that are available as these are the most prominent part of the cathodic protection system. Judicious choice of cathodic protection anode can maximize the cost effectiveness of the system. 

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. 

One of the first commercial, proprietary anodes for decks was a flexible cable with a conductive plastic round a copper condnctor. The cable was woven across the deck and then a concrete overlay applied. It was also used on substructures with a sprayed concrete overlay. This system was very popular in the 1980s but unfortunately the condnctive plastic started to fail after about five year s of service. This led to attack of the copper and, in some cases, expansion of the plastic which delaminated and spalled the concrete overlay. This system is no longer available. 

Coated titanium expanded mesh in a concrete overlay... 

Electrical connections may be made by self tapping screws, welding or brazing. Extra bars may need to be welded in if continuity between bars is inadequate. Connections are usually protected with epoxy glue applied over them. Cabling may be run through the concrete overlay, over the anode or from the back of the protected surface. Cables are run into junction boxes for splicing and connections. There must be at least two connections per zones for redundancy. There may be separate connections for the reference electrodes. 

Concrete Overlays (used on North American highway bridges according to specifications developed by highway agencies). 

Some waterproofing membranes on car parks do not require asphalt concrete overlays. The extra wear on these membranes, especially from scuffing of tyres as cars manoeuvre in and out of parking spaces, means that they have shorter lifetimes and need more regular monitoring and maintenance if they are to keep the deicing salts out of the concrete. 

In North America the market for impressed current cathodic protection of bridge decks has diminished (Russell, 2004). High-performance concrete overlays, full and partial deck replacement are preferred due to the cost of maintenance. 

Generally, when the rigid pavement shows extensive deep spalling together with surface deformation or other cracking, strengthening the asphalt or concrete overlay is the only effective treatment. 

Figure IS.31 Asphalt concrete overlay thickness required to reduce pavement deflections from a measured to a design deflection value. (From Asphalt Institute MS-17 3rd Edition, Asphak Overlays for Highway and Street Rehabilitation, Manual Series No.l7 [MS-17], Lexington, US Asphalt...

CONCRETE OVERLAY OVER RIGID OR FLEXIBLE PAVEMENT... 

Concrete overlay on rigid or flexible pavement is not a common practice as asphalt overlay. However, when concrete overlay is used, it is distinguished into thin overlay (50 to 100 mm) and thick overlay (>150 mm) construction. 

Thin concrete overlay over either flexible or rigid pavement presupposes intact pavement surface and ensures good bond between the old surface and the concrete overlay. 

Thick concrete overlay is also applied over damaged surfaces and is most commonly applied over existing rigid pavements. 

In the case of thick concrete overlay, the new slab needs to be free to move horizontally to deter the appearance of reflection cracks. The above can be achieved with the provision of an interlayer, which can be a polythene membrane or an asphalt interlayer. The use of an asphalt interlayer is preferred if the underlying concrete pavement has a significant remaining integrity (Thom 2008). 

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. 

In thin concrete overlay over jointed concrete pavement, the joints in the overlay should be in exactly the same position as the existing joints. In thick concrete overlay over an existing concrete pavement, such a requirement does not exist. 

When thick concrete overlay is placed over an existing concrete surface, the concrete material and in particular the aggregates must be compatible with the aggregates used in the existing concrete. This is to avoid shrinkage cracking (see Section 15.11.6 and Figure 15.21). 

As for the design of the thickness of concrete overlay, the thickness of the thin concrete overlay over intact surface seems to rely on proven past experience and in any case ranges from 50 to 100 mm. 

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. 

Additional information regarding bonded and unbonded concrete overlay over rigid pavements according to UK standards can be found in DMRB HD 32/94 (Highways Agency 1994). 

It covers asphalt or concrete overlays of flexible or rigid pavements as well as asphalt overlays of rigid pavements with an existing asphalt overlay and concrete overlay over a flexible pavement. In particular, it covers the following ... 

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