Лако-красочные материалы — производство

For the surface screeds and coatings applied for purely cosmetic purposes the repair will have little influence on the overall structural performance of the repaired member. However, with maintenance programmes now being formulated to provide the longest possible life before major renovation becomes necessary it is likely that patch repairs when they are carried out will be of a scale where structural integrity becomes of importance. Work in the USA(6) using cementitious repair mortars has shown the importance of taking the patch well behind the reinforcement steel to assist in ensuring the overall integrity of a reinforced concrete beam. Current UK guidelines(2) confirm that, for the most durable patch repairs, concrete should be cut away right around the affected reinforcement.

Possible concerns arise over the potential property mismatch between the repair material and the substrate concrete. Short-term problems may arise because the repair material contracts on curing relative to the surrounding concrete. With resin based mortars the

prime cause is the cooling which occurs following the exothermic reaction whereas with water-based cementious formulations drying shrinkage may occur. Fig. 6.3 shows the free shrinkage strain measured on 40 mm square by 160 mm long prisms of four different generic forms of concrete repair mortars, measurements starting when the materials had attained an age of 24 hours. Of particular note is the relatively low shrinkage on cure observed with the epoxy mortar as compared with some cementitious systems. Also of interest is the expansion observed with a magnesium phosphate system. This is deliberately induced by the incorporation of an expansive admixture(7) in the formulation. The volume decrease which occurs with polyester based systems on hardening has been referred to earlier in this chapter and must be considered in addition to the curing shrinkage. The manifestation of curing contraction is either initial tensile strains induced in the repair or cracking at the repair/substrate interface, both of which may reduce longer term structural capacity.

During service, incompatibilities in the form of differing elastic moduli and differential thermal movements between repair and substrate may create problems. The strengths, elastic moduli and coefficients of thermal expansion for nine repair systems are summarised in Table 6.1(8). The results show that all systems provide adequate compressive strength for the vast majority of applications, the values generally being well in excess of those expected in the substrate concrete. The resin-based materials have greater tensile and flexural strengths than the polymer modified cementitious systems which in turn have greater values than unmodified ordinary Portland cement materials and also of the substrate concrete. Inspection of the modulus of elasticity results, however, shows a reverse trend, the resin based systems generally having lower moduli than cementitious systems. Indeed, the moduli of some cementitious systems are in excess of those expected in the substrate concrete.

To illustrate the effect of modulus mismatch on structural performance consider the symmetrically repaired reinforced concrete prism shown in Fig. 6.4 and loaded axially in tension. The concrete has a modulus of elasticity in tension of 25 kN/mm2. For material C (Et = 14 kN/mm2) the elastic stress induced in the concrete at mid-height of the prism will be 2.5 times that in the repair material at the same position. Conversely for material D (£t = 43 kN/mm2) the elastic stress carried by the repair material is now 1.5 times that

carried by the concrete. This enhanced stress must be transmitted across the interfaces at the end of the repaired zone thus increasing the demands placed upon the repair/substrate adhesion. In addition, the elastic property mismatch at the end interfaces of the repaired zone can induce zones of stress concentration. For a low modulus repair material these occur on the surface of the stiffer concrete immediately adjacent to, but not at, the interface (Fig. 6.5). If the peak tensile stresses exceed the tensile capacity of the concrete then cracking will occur and the possibility of subsequent reinforcement corrosion being initiated outside the repaired area must be con­sidered. This point highlights the need for careful consideration of where patch repairs are curtailed with respect to the overall stress state in a structural member.

It can also be seen from Table 6.1 that the coefficients of thermal expansion/contraction of resin-based repair systems may be two or three times that of the substrate concrete. If the repair is carried out at, say, 10 °С then a rise in ambient temperature to 25 °С will potentially induce a

supporting a bridge deck, is where temporary trestles are erected to support the dead load of the deck but deflections induced by live loads apply a cyclic load to the columns during the repair operation.

The effect of a sustained compressive stress of 10 N/mm2 on the creep strains induced by 160 mm long by 40 mm square prisms of four different generic forms of repair material is shown in Fig. 6.6. The range of ultimate creep strains varies from over 4500 microstrain with one resin-based system down to less than 400 microstrain with some modified cementitious systems. These strains include the effect of any curing shrinkage/expansion which may also be occurring during the period under load.

Consider, a simple scenario involving the 600 mm circular reinforced concrete column shown in Fig. 6.7. Corrosion of reinforce­ment necessitates the removal of surface concrete to a depth of 25 mm behind the reinforcement. The column is subject to design axial loads of 2000 kN dead and 1200 kN imposed, both of which are fully relieved by propping whilst the patch repair is effected in order to prevent overstressing the remaining concrete core. A repair with the relatively low compression modulus modified cementitious mortar ‘E’ (Table 6.1) causes a 24% increase in concrete stress to 12.5 N/mm2 whereas with the high modulus flowing concrete T there is a 13% decrease in stress. The corresponding stresses in the repair materials are 7.7 and 11.5 N/mm2, respectively.

Taking creep into account, a repair with material ‘E’ loaded at an age of 1 month will cause the dead load stresses in the concrete

to increase from 7.9 to 10.8 N/mm2 with time. The latter value increases to 15.5 N/mm2 when live load is applied which may cause some concern in relation to the ultimate strength of the concrete. Meanwhile, the dead load stresses in the repair have reduced from 4.8 to 2.2 N/mm2.

It is thus apparent that care must be taken when selecting materials for the deep patch repair of reinforced concrete members to ensure that the material is structurally compatible with the substrate.