6. FULL-SCALE ARCH TESTS

6.1 Objectives

In undertaking comparisons with full-scale tests of arches the key objective is to demonstrate the accuracy of analytical solutions and the appropriateness of simplifying assumptions that it is always necessary to make. In all cases vertical displacement at load intervals have been used for result comparisons.

Full-scale tests have been selected where boundaries and loading are two-dimensional so that the validity of comparing their results with two-dimensional analyses has not been compromised by three-dimensional behaviour. Skew arch barrels and spandrel walls are examples of bridge features that generally give rise to three-dimensional structural behaviour.

6.2 TRL Laboratory Tests

The Transport Research Laboratory (TRL) ran a LINK funded programme in the nineties aimed at quantifying the benefits and limitations of various repair and strengthening methods used on masonry arch bridges. The programme was entirely experimental and consisted of a series of tests using identical brick arch arrangements. Comparisons with predicted results have been made with the two unstrengthened arches tested before the LINK programme got underway and on which the LINK arches were styled. These tests are described in papers by S K Sumon(11) and N Ricketts(12) and summarised in the following paragraphs.

6.2.1 Arrangement

The arch bridge has a 5m span, a rise of 1.25m at the crown and was 2m wide. The bridge including principal dimensions is shown in Figure 6.1. Each arch barrel comprised three brick rings each with stretcher bonding. The barrel was founded at its springings on two reinforced concrete abutments held rigidly to the Laboratory floor. A 2m wide steel box was constructed around the arch to retain the fill. The 2m wide loading, supports and retaining box ensure that the bridge behaviour is two-dimensional. All ultimate failure tests were carried out with line loads at the span quarter point.

Figure 6.1 5m Span TRL Laboratory Arch – Mortared and Separated Rings

The two unstrengthened tests selected for comparison with predicted results were similar except for the circumferential joints between the rings.

In one case, and for the remainder of the strengthened arch test series, these joints consisted of dry sand in order to reproduce the effects of partial ring separation (no gap but with eroded cohesion, full separation would need to include a physical gap and would be weaker still).

In the other case the circumferential joints were fully mortared.

6.2.2 Masonry

The arch barrel was assembled from non-engineering bricks, lime mortar and built to be representative of many bridges constructed before 1900. Hand made bricks with a compressive strength of 18.4 N/mm2 were laid with a 1:3:12 cement:lime:sand mortar with a compressive strength in the range 1.3 to 2.3 N/mm2. Brickwork prisms were tested which gave a mean compressive strength of 5.3 N/mm2 which is fairly consistent with Figure 4.2 in BD 21(5).

6.2.3 Fill

Type 2 road base material was used for the fill which was compacted in layers using a hand operated vibrating plate.

6.3 Bolton Laboratory Tests

6.3.1 General

C Melbourne has lead a number of studies where full-scale brick arches have been built and tested. These have included ultimate tests of multi-ring brickwork arches and multispan brick arch bridges in various conditions under laboratory conditions.

The following lists the tests that have been selected for comparisons with predicted results. All of these arches were constructed with similar materials and had detached spandrel walls.

6.3.2 Single 3m arch with two rings of brick

This is one of four tests (original test identification number 3-1) carried out to investigate the behaviour of multi-ring brickwork arch bridges by C Melbourne and M Gilbert(13). The test arrangement showing principal dimensions is shown in Figure 6.2. Both rings were laid using stretcher bonding and mortar used to bond between the rings. A knife edge load was applied across the full width using a hydraulic system and reaction rig at ¼ span(14). The width of the barrel, fill and distance between the separated spandrel walls was 2.88m.

Figure 6.2 3m Span Bolton Arch – Mortared Rings
6.3.3 Single 5m arch with four rings of brick

These are two of a series of three tests (original test identification numbers 5-2 and 5-3) again carried out to investigate multi-ring behaviour(13,14). The test arrangement showing principal dimensions is shown in Figure 6.3. All four rings were laid using stretcher bonding and either mortar or sand used to bond between the rings. Hence two conditions were considered; fully bonded rings and partially ring separated (no gap but with eroded cohesion). Here load was applied at the ¼ span using ties passing through the arch and a prestressing system. The width of the barrel, fill and distance between the separated spandrel walls was slightly greater than the 3m arches at 3.01m

Figure 6.3 5m Span Bolton Arch – Mortared and Separated Rings
6.3.4 3 x 3m multi-span arch bridge

This is one of three tests (original test identification number 2) carried out to investigate the behaviour of multi-span masonry arch bridges by C Melbourne, M Gilbert and M Wagstaff(15,16). Each span is based on the single span arrangement described in 6.3.1.

The test arrangement showing principal dimensions is shown in Figure 6.4. For this test a knife edge load was applied across the full width using a hydraulic system and reaction rig at ¼ span position over span 2. The width of the barrel, fill and distance between the separated spandrel wall was 2.88m.

Figure 6.4 3 x 3m Span Bolton Arch – Mortared Rings

6.3.5 Masonry

All of the tests described in section 6.3 were built with similar masonry using solid class ‘A’ engineering bricks and laid with a 1:2:9 cement:lime:sand mortar (BS 5628 mortar designation (iv)). The bricks had very high compressive strengths tested between 115 and 154 N/mm2 and the mortar a compressive strength in the range 1.9 to 3.2 N/mm2. Tested brickwork prisms gave an overall mean compressive strength of 25.8 N/mm2 which is significantly outside the bounds of strength properties given in BD 21(5) and, interestingly, according to BS 5628(17) is similar to masonry with a much stronger mortar such as 1:0:3 cement:lime:sand designation (i).

Brickwork in the barrels was laid in stretcher bond in separate rings either fully bonded with mortar to adjacent rings or partially separated with sand.

6.3.6 Fill

50mm Graded crushed Limestone was used for the fill which was compacted in layers using a hand operated vibrating plate. Using a large shear box the fill material was confirmed cohesionless and had a measured angle of internal friction of 60°.

6.3.7 Tests not used for comparisons

Further tests at the Bolton Institute were carried out at the same time and considered other conditions including header bonding in barrels and spandrel walls. Header bonding, used to mechanically connect adjacent brick rings, has not been investigated here since allowance for brick fracturing would be necessary to predict ultimate failure. Modelling the necessary localised failure mechanisms would add further indeterminacy and analytical complexity to numerical models. Similarly, predicting tests including spandrel walls have also been avoided as more complex models with three dimensional representations and accompanying computational overheads would be necessary.

6.4 TRRL Field Tests

6.4.1 General

The Transport and Road Research Laboratory (TRRL) undertook a research programme to re-examine the MEXE method of assessing the traffic load carrying capacity of brick and stone arch bridges. The programme of research comprised the development of analytical models, a series of load tests to destruction of eight redundant bridges and a series of model tests.

Strathmashie Bridge(18,19), selected from the tests of redundant bridges, was not skewed and had a longitudinal crack in the barrel parallel to the back face of the south most spandrel wall. Hence, at least on one side of the bridge the spandrel wall was separated and would not have significantly influenced the test. The segmental arch barrel of the bridge had a span and rise of 9.43m and 2.99m respectively and was constructed from random rubble masonry. The bridge was dimensionally in good condition although the state of pointing was poor. Figure 6.5 shows the bridge principal dimensions.

Most of the other tests cannot be easily used for comparison with two dimensional analyses for any of the following reasons:

1. the arch barrel is significantly skewed;
    
2. the spandrel walls were fully attached to the barrel; and
    
3. the spandrel walls as well as the fill were directly loaded by the loading beam during the test

Figure 6.5 Strathmashie Bridge – 9.43m Random Rubble Arch

6.4.2 Masonry

No quantitative tests were undertaken on the masonry. The individual stones were typically 100mm with quit thin joints near the edges whilst being wider and more variable towards the centre. The mortar type was not noted but as the bridge was built around 1830 was probably lime based.

6.4.3 Fill

No quantitative tests were undertaken on the fill but it was described as cobbles graded down to sand.

6.5 Archtec Strengthened Tests

6.5.1 General

In order to test the practical implementation of Archtec, to validate the method of structural analysis, to help quantify key strength parameters and to illustrate the degree of strengthening that could be archived two full-scale tests of Archtec strengthening were carried out at TRL. Both tests were based on the arch arrangement developed for the LINK programme, as described in Section 6.2, so that unstrengthened test comparisons could easily be made. Both tests also used the partially ring separated form of the arch; Figure 6.1 shows the principal dimensions. The anchor arrangements were configured for the stationary test and, therefore, were arranged asymmetrically with respect to the span. It was recognised that in practice, with moving axle loads, anchor arrangements would have to be symmetric to reflect critical loading positions on both sides of the span. The first test was carried out in January 1998 and the second in June 2001.

This work was conducted outside of the LINK programme and was privately funded.

6.5.2 Archtec Test 1

The arrangement of the first Archtec(20) test is shown in Figure 6.6 and used the Standard Cintec anchor shown in Figure 6.7. The Standard Cintec anchor arrangement used for the test comprises a 25mm diameter stainless steel ribbed reinforcement bar grouted in a 65mm diamond cored hole. Anchors were arranged in three rows, A, B and C with the anchors in row B (total length of 10.8m) providing most of the added strength. Anchors A and B were drilled through the inner most ring of bricks with a minimum cover of approximately 20mm near the quarter-span position at the intrados. Anchors in row C were drilled through the middle ring of bricks on the opposite side of the span to the load.

Plan View

Section View

Figure 6.6 Archtec Test Number 1
Based on TRL 5m arch with separated rings

Test number 1 – Standard	Test number 2 – Multibar
Figure 6.7 Cintec Anchors used in Arctec Tests

6.5.3 Archtec Test 2

A second Archtec(21) test was carried out to provide further confirmation of strength parameters and test the Multibar form of the anchoring system. The arrangement is shown in Figure 6.8 and used the Multibar Cintec anchor illustrated in Figure 6.7. The Multibar anchor used in the test comprises of six 10mm diameter stainless steel ribbed reinforcement bars arranged in a ring and grouted in a 55mm diamond cored hole. Again anchors A and B were drilled through the inner most ring of bricks with minimum cover of approximately 20mm near the quarter-span position at the intrados. Anchors in row C were also drilled through the innermost ring of bricks in recognition that the anchors work well in compression as well as in tension.

The area of steel is approximately equivalent to the Standard 25mm case. Multibar anchors whilst not providing any significant advantages over the Standard arrangement with regard to strength do however have several practical advantages during the installation process.

As with the first test, anchors were arranged in three rows A, B and C with the anchors in row B (total length of 15m) providing most of the added strength. However, more anchors were used with the overall arrangement expected to represent the strongest array of anchors that could be practically used in an arch of this size. It was predicted that any further increases in strength by anchoring would diminish as the influence of masonry strength would become increasing important in determining the arch ultimate strength; analogous to an over reinforced concrete beam.

In the second test two anchors from row B had electrical resistance strain gauges attached so that predicted and measured strains could be compared and, therefore, help verify predicted anchor stresses (axial reinforcement, grout to masonry bond).

Plan View

Section View

Figure 6.8 Arctec Test Number 2
Based on TRL 5m arch with separated rings