7. DISCUSSION

7.1 Test Results

7.1.1 Crack Monitoring

Significant movement across the four pre-existing cracks was recorded. Movement across the cracks was significantly reduced following the installation of the strengthening anchors demonstrating their effectiveness in tying the masonry across such discontinuities.

Referring to Figure 6.1, the maximum absolute crack displacement for the unstrengthened case was 0.046mm compared with a corresponding value of 0.009mm for the strengthened case indicating a reduction of approximately 80%. At many locations strengthening virtually eliminated movement with readings close to the gauges resolutions.

The implications are that Archtec strengthening prevents significant movement of preexisting transverse cracks under live load. The main benefit of this behaviour would be the reduction in load cycle derived hysteretic damage; opening and closing of cracks under traversing traffic. Reducing this type of damage will almost certainly be beneficial to the bridge service life.

7.1.2 Intrados Strains

Measurable strains were recorded around the intrados of the both arches and the data appears sensible and logical. As expected, significant strains were detected across the pre-existing transverse cracks and measurable strains were also recorded elsewhere.

These macro strains resulting from the averaged effects of elastic and crack opening/closing behaviour following Archtec strengthening are significantly altered. Peak values, both compressive and tensile, are reduced. The representative results shown in Figure 6.2 illustrate this phenomenon; the shaded area marking where reduced strain has occurred. Strengthening appears to have influenced intrados strains in two ways as follows.

1. Strains measured across pre-existing cracks.

The largest reductions in strain occur in the vicinity of the cracks (the cracks were also monitored separately and discussed in section 7.1.1). Here the tendency is for strengthening to reduce compressive strains with the measured values being greatly influenced by crack movement. Again this supports the finding made by direct monitoring of the cracks, that by reducing crack movements Archtec strengthening helps limit further load cycle derived hysteretic damage.

2. Strains measured away from cracks.

Away from the cracks there has been a general reduction in tensile strains. For the two cases illustrated in Figure 6.2, this trend is most notable at the centre of the span between the quarter points. This is particularly important as by reducing tensile strains Archtec strengthening lessons any susceptibility to further cracking and loosening of bricks which is clearly beneficial to the bridge’s serviceability.

7.2 Comparison of Predicted and Test Results

7.2.1 Intrados Strains

Using the DE model and methodology described in section 6.2, intrados macro strains have been calculated and are compared against measured values in Figure 7.1. Again the representative load cases LC8 and LC9 acting on the south span have been used. The figure shows two graphs each with four curves;

• measured values (reproduced from Figure 6.2), labelled "Measured".
   
• measured values with the contribution attributed to movement across the significant pre-existing cracks removed, labelled "Ignore Crack".
   
• numerically predicted values, labelled "Predicted".
   
• numerical predicted values factored to make some allowance for 3D behaviour, labelled "Scaled Predicted". (As noted in Section 5, the 2D analysis is inherently conservative and, as anticipated, gives upperbound values with regard to strain, etc, since the 3D effects of transverse distribution, bridge skew and contribution of spandrel walls are not accounted for).

Macro strain results, positive values are tensile, are plotted against gauge numbers that mark the relative position around the intrados, see Figure 4.1.

Adjustment to the raw data both, measured and predicted is necessary to make valid comparisons between the two sets of results. As previously explained predicting the effects of closure of the pre-existing cracks is beyond the scope of the numerical analysis undertaken. In addition the 2D analysis was expected to be conservative and the need to factor the results to take account of 3D effects was anticipated. For the purposes of comparison, the predicted results have been reduced by a factor of 2 to illustrate correlation in the distribution of predicted and measured strains, even though the magnitudes have been conservatively over-predicted in the particular analysis undertaken.

The adopted factor of 2 is considered a reasonable allowance for the 3D effects, in this case. It should be noted that the 2D analysis gives upperbound conservative values, as is appropriate for assessment and design work. Whilst 3D DE analysis of masonry arches is now feasible it has not yet been fully developed. Correlation of 3D analysis against the results of Pop Bottle test is beyond the stated objectives of the test, although the data could be of subsequent use for such verification in the future.

Once the above adjustments are made, for both cases the measured (curve marked "Ignore Crack") and predicted (curve "Scaled Predicted") results compare very well. Even before any adjustment is made to the predicted results (curve "Predicted"), the distribution of strain compares closely with that measured.

Figure 7.2 shows similar results for Archtec strengthened test. Again once adjustment are made to the raw data for the effects attributable to initial crack widths and full 3D behaviour measured (curve marked "Ignore Crack") and predicted (curve "Scaled Predicted") results compare very well. As for the unstrengthened case before any adjustment is made (curve "Predicted") to the predicted results the distribution of strain compares closely with that measured.

These comparisons show that 2D DE simulations have been used to conservatively predict intrados strains in a defective arch barrel in its existing condition and after Archtec strengthening under working loads (serviceability limit state).

Gauge Number, Radial Position

Load case LC8 – A pair of 11.4 tonne axles at mid-span of the south span

Gauge Number, Radial Position

Load case LC9 - A Pair of 11.5 tonne axles at pier quarter-span of the south span

Figure 7.1 Unstrengthened Test – Predicted versa Test Intrados Macro Strains

Gauge Number, Radial Position

Load case LC8 – A pair of 11.4 tonne axles at mid-span of the south span

Gauge Number, Radial Position

Load case LC9 - A Pair of 11.5 tonne axles at pier quarter-span of the south span

Figure 7.2 Strengthened Test – Predicted versa Test Intrados Macro Strains

7.2.2 Anchor strains

Using the DE model including Archtec strengthening and the methodology described in section 6.2 anchor strains have been calculated and are compared against the measured values in Figure 7.3. Again the representative load cases LC8 and LC9 acting on the south span have been used. The figure shows two graphs each comparing the predicted and measured strains along the relative length of each anchor; measured values (reproduced from Figure 6.3) and numerically predicted values. Positive strains indicate tension with 50 µå indicated a bar stress of 10 N/mm2.

Bearing in mind the relatively small strains that are being compared both the distribution and magnitude of the results compare well. Some adjustment might be warranted to cater for finite crack widths and 3D behaviour but the approach to use is less clear than for intrados strain results. A pattern is discernable which suggests that measured strains do exhibit additional compression in the proximity of the cracks (close to 97 and 127 on the abscissa) compared with predicted values which do not allow for pre-existing open cracks.

These comparisons show that 2D DE simulations have been used to predict anchor strains in a defective arch barrel that has been strengthening using Archtec under working loads (serviceability limit state). Consequently simulations can be used to assess the level of stress in the bars.

Relative Position

Load case LC8 – A pair of 11.4 tonne axles at mid-span of the south span

Relative Position

Load case LC9 - A Pair of 11.5 tonne axles at pier quarter-span of the south span

Figure 7.3 Predicted and Test Anchor Strains