Лако-красочные материалы — производство

Unless a portion of the original concrete is removed and replaced with a higher strength material, then a concrete overlay will usually result in an increase in member self weight. Applications at ground level are therefore more common than in suspended beams or slabs, although the use of polymer modified concrete overlays has found some favour in the refurbishment of bridge decks in North America(9).

Adhesives are often employed to achieve the necessary longitudinal shear connection for full composite action between the fresh and the older hardened concrete. This is because the inherent bond between fresh and hardened concrete cannot be relied on for a truly structural connection. The uncertainty of the purely cementitious bond has been related to the early occurrence of shrinkage stresses and the relatively slow rate of bond strength development. There are two prerequisites of a strong bond. The first is the careful preparation of the old concrete surface; removal of laitance and surface contaminants by grinding or gritblasting is imperative. The second is the selection of an appropriate adhesive.

Epoxies are preferable to other resin systems in that they can be formulated to bond to moist hardened concrete and yet be able to cure within the wet environment of the fresh concrete. Polyesters do not generally bond reliably under wet conditions. The system must also be selected with due regard to the temperature of application. Resins claimed to have good bonding performance at 20 °С may give rather poor bond strengths at the lower cure temperatures prevalent on UK construction sites during much of the year.

There are several records of tests carried out to demonstrate that epoxy adhesives can achieve good bond strengths with failures occurring in the substrate concrete rather than within the glue — line(10, 11). The main danger to the long-term integrity of the bond is the presence of water and thus water penetration to the glue-line should be prevented as far as possible.

In recent years the use of polymer latex slurries as bonding coats between new concrete and old has been the subject of continued debate. It is now universally recognised that PVA systems are unsuitable in damp conditions. SBR bonding agents have been utilised since the late 1960s on many UK construction sites whereas acrylic based systems are a more recent addition. The arguments have largely centred around the method of test and the effect of multiple coats. The slant shear test is recommended in BS 6319 (12) but it must be recognised that in this test the bond line is subject to compressive as well as shear stresses. Further, the result is sensitive to the method of preparation of the hardened concrete surface (see Chapter 4).

With SBR slurries the use of more than one coat can reduce bond strengths assessed using the slant shear test(13). It is now accepted that these materials form a skin if allowed to dry which can impair adhesion of subsequent layers.

Under normal conditions on repair sites it seems likely that a more reliable bond can be achieved by using a bond coat rather than by relying on any natural bond between fresh and hardened Portland cement concretes(2). However, bonding agents for use on site must be tolerant of site conditions, be reliable and cannot be selected solely on the basis of slant shear results. The bonding performance of carefully applied SBR and acrylic latex/cement slurry coats appears to be similar to that of epoxy resin bonding aids(14). However, other factors such as open time, barrier coat effects and cost may affect the final choice.