Лако-красочные материалы — производство

Applications of resins as true adhesives in new construction have been relatively scarce but increasing use is being made of epoxies in the joints between units in segmental precast, prestressed bridge construction (see Fig. 7.9). Using this technique long span bridges can be constructed by stressing precast concrete segments together to form a monolithic structure. The advantages of segmental

construction are that it speeds erection, can accommodate changes in horizontal and vertical curvature, and the relatively small segments are relatively easy to handle and transport. Cost savings of the order of 20% have been claimed over alternative methods of construction(l).

Traditionally the joint zones had been formed of widths in excess of 250 mm, which were filled with cast in situ concrete, or as relatively narrow joints, 20-40 mm thick, filled with dry packed sand/cement mortar. Concrete joints need time to cure and as a result it was sometimes necessary to temporarily support several segments before stressing could take place. In 1963, on a bridge over the Seine at Choisy-le-Roi outside Paris the precast segments were counter cast for the first time. This involves casting each segment against the previous one whose mating surface has been precoated with a release agent. The objective is to produce a very thin accurate joint of width 1-2 mm which can subsequently be filled with an epoxy resin. Although the resins are much more expensive than concrete the use of the thinner joint offsets this cost. Further, the waiting time before the segments can be stressed together is considerably reduced and hence leads to a faster speed of erection. The adhesive also assists in erection by acting as a lubricant during final alignment. More recent examples of use of the technique in the UK include the Trent Bridge near Scunthorpe on the M 180 motorway(ll) and the East Moors Viaduct, Cardiff(12).

The resin in the joint is not used in a truly structural sense since it is only designed as a gap filler to transmit compressive stresses. However, it does serve to more evenly distribute these stresses. The adhesives have not normally been used to resist the vertical shear which develops between adjacent units since keys are formed for this purpose, but there is no reason why they should not be designed to do so. The reluctance stems from the 120-year design life requirement for highway bridges in the UK and the lack of sufficient data on the long-term durability of adhesives. It is ironic, therefore, that one of the major claimed advantages of the technique is the relatively impermeable joint which is obtained compared with conventional cement mortars. The collapse of the Ynysgwas bridge in South Wales in 1985(13) bears testimony to the lack of resistance to moisture penetration of some wide sand/cement mortar joints.

A draft standard for acceptance tests and verification of epoxy bonding agents for segmental construction was published in 1978(14). The need arose because of the perceived difficulties of using epoxies for segmental work under conditions of varying humidity and temperature. The functions of the bonding agent are described as:

(1) To join the surfaces of the precast concrete segments in such a way that compression and shear, and in some cases also tensile stresses, are transmitted between the segments.

(2) To develop such strength, at a rate sufficient to allow continuous erection of the segments.

(3) To lubricate the surfaces of the joints to facilitate proper positioning of the segments during erection.

(4) To provide a moisture seal across the joints to protect the prestressing tendons against corrosion and to prevent leakage at joints during tendon grouting.

To fulfil these requirements the proposal suggests separate programmes of testing for (a) choosing an appropriate bonding agent and (b) site testing. The properties required and the proposed test methods are summarised in Tables 7.2 and 7.3.

Epoxy adhesives were used in 1978 during the construction of the U. S.’s first segmentally constructed cable-stayed bridge across the Columbia River(15). Here the deck is made monolithic by a combination of post-tensioning and compressive forces from the horizontal components of the stay cable forces; the epoxy mortar is called upon to provide both shear and tensile resistance across the joint.

Three adhesive formulations were chosen to cover the anticipated temperature range at the time of application, i. e. 4 °C-38 °С. Control of correct mixing and materials was achieved by measuring hardness of small specimens after curing for 20 minutes in a small oven. Subsequently slant shear and compressive strength tests were performed.

Construction of the M 180 bridge near Scunthorpe (Fig. 7.10) is particularly interesting since an aliphatic amine cured epoxy was used during severe UK winter conditions. A technical account of the site-testing carried out describes the procedures used to examine the curing characteristics of the adhesive in cold weather(16). Cubes bonded together to form a beam and subsequently tested for flexural strength, deflection and creep revealed little difference from monolithic concrete provided the concrete surface to be joined had been properly prepared. A minimum compressive stress of 0.3 N/mm2 is also recommended during the curing period. Single lap joints were used to monitor the effect of curing temperatures on subsequent strength under both site and controlled laboratory conditions. The results shown in Fig. 7.11 for site joints relate to the maximum temperature of the range experienced. They suggest that for the particular adhesive, the lowest reliable limit for full curing must be

• Adhesive fully cured — no creep

О Adhesive set but creeping under load

x No cure — test pieces separated on loading О Cured in refrigerator

Time (hours)

7 14 21

Time (days)

on cure rate of an epoxy (Ref. 16).

at least 3 days at temperatures in excess of 5 °С. Below this temperature site heating must be employed.

Creep of the epoxy resin system under sustained load has always been of some concern. However, with a thin glue-line and at the relatively low stress levels encountered in segmental construction, compressive creep should not be a problem with most epoxies(2).