Figure 1. A Demec strain gauge. Figure 2. Demec strain gauge point and nailin fixing. circumstances, the order of accuracy obtainable is not justified. It often demands a high level of skill from the operator. IS THAT CRACK GETTING WIDER? It seems to be a basic fact of life that cracks appear to get wider the longer they are studied. This, as suggested earlier, may be due to the crack getting dirtier and therefore more noticeable, it may be psychological in that its existence isworrying or it may actually be a fact that a crack is widening due to movement. The only sure way of discovering the true nature and speed of the apparent movement is to monitor the fracture accurately. The simplest way of checking for movement of a fracture is to use a ruler to carefully draw pencil lines across the line of the fracture. Any movement will be shown by misalignment of the pencil lines. This method, whilst quick and of no cost whatever, is far from precise and no record of temperature, seasonal or other movements is possible. The traditional way of monitoring cracks is by the use of glass telltales but this cannot be recommended at all.These are susceptible to breaking due to frost or vandalism, and are difficult to fix properly. They often become detached at one end thus suggesting, by still being intact, that no movement has taken place. No record of progressive or climatic movement can be kept and they are unsightly, an important consideration in historic building work. Mortar pats were also sometimes used but they suffer the same problems as glass telltales. Modem plastics telltales using two overlapping plastics plates, one marked with cross hairs and the other with a grid pattern, are accurate only to l ·0 mm, an accuracy which may well not be good enough to detect thermal and climatic movement. These suffer the same somewhat unsightly characteristics as glass telltales. Although their cost is small per unit, it is better and cheaper to invest in a vernier gauge rather than a dozen or so plastics telltales. These plastics telltales CONTEXT49 Horizontal \ Figure 3. Typical setting out of crack monitoring location points. do have the advantage that owners of buildings can inspect them for their own peace of mind between the visits of a building professional. They are also useful for attaching at high points were access is difficult and once it is fixed it may be possible to read the telltale through binoculars. Movements of cracks in structures can be monitored very satisfactorily using a demountable strain gauge (the Demec) together with suitable locating points fitted to the structure adjacent to the cracks. The 200 mm long gauge is shown in Figure 1. Gauges of a wide variety of other lengths are also available from the manufacturers but it is generally felt that the 200 mm gauge is best in most circumstances. The locating points used for the Demec gauge are 6 mm-diameter stainless steel discs that have a small hole drilled at the centre for accurate positioning of the conical points of the gauge. These steel discs can be fixed to the structure with either sealing wax or glue. An alternative method of fixing locating points is to drill small holes into the structure and to insert a hammer-in fixing consisting of a flanged expansion sleeve and a nail. The nail is driven flush with the surface and drilled with a BS1 centre drill in order to receive the conical gauge point. Similarly a brass cross headscrew in a plastics wall plug can be used but care needs to be take to ensure that the points of the gauge sit firmly and do not shift about. Care must be taken with drilled-in fixings to avoid the base material being damaged by the wedge action caused by tightening them up. These locating points are shown in Figure 3. For monitoring movement in timber structures brass screws can be inserted at suitable locations. The type of locating point used is dependent upon the material to which the fixing is being made, the degree of exposure to the weather and the location in relation to possible vandalism. A soft wall material can make it difficult to stick the discs on successfully and therefore the drilled-in fixing is advisable, but conversely it can be difficult to drill into a hard material and the sticking on of discs is easier and quite satisfactory. At low levels the stick-on points can be vulnerable to vandalism. Drilling into mortar joints is to be avoided as the fixings will eventually work loose and the readings will become unreliable. Figure 3 shows the typical layout of locating points in order that the crack can be monitored for both horizontal and vertical movements. Using the Demec gauge, movements of 0-025 mm are easily traceable and, with care, more accuracy can be achieved. The only visible signs of this form of monitoring are three small discs or screw heads at each monitoring point. No particular skills are necessary for setting up and using the Demec gauge although interpretation of the results must be done with care and thought. At about £400 in 1994, the Demec strain gauge,althougheasytouseandrobust, is rather expensive for occasional use. Alternatively, a good vernier gauge (purchased from a suppliers of engineering tools) can be used to measure accurately (to at least 0· l mm) between the shanks of brass screws set in plastics plugs around a crack in the pattern shown in Figure 3. Cheap verniers can be obtained from DIY shops but forthe sake of a few pounds the better quality engineers' verniers are much more reliable. A good vernier usually incorporates a depth gauge which is a useful facility for monitoring fractures in comers of walls. Care is needed in the reading of a vernier gauge as these can be rather confusing. Modem verniers with digital readout facilities are now available at relatively modest cost. It is often a good idea to set up a dummy reading point such as two locating points on the same piece of stone (not across a fracture) which can be used to check for damage to the gauge or for problems with misreading of the instruments. The cost of brass screws and plastics plugs for use with the vernier is minimal. On a 'consumer guide' basis the best value for money is the vernier gauge and brass-screw system although the Demec is to be recommended if a lot of accurate monitoring is foreseen. Ian Hume is Chief Engineer, Conservation Engineering Branch, English Heritage. In the next issue of Context we will look at ways of checking for settlement movements and the tilt which this can cause. 19
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