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Figure 5. Mean selected water to binder mass ratio (W:B) for mortar compositions with different binder (B) types.

(EN 459-2:2001). Each batch is tested on all three brick types: both types of perforated bricks are used to add three bricks to a single-leaf wallet, and the facing brick is used to produce small columns of eight layers of three bricks in alternating bond. The masons are continuously interviewed and filmed during the procedure.

Reference compositions for lab experiments with the quarry sand are calculated by taking into account the average of the W:B chosen by the masons (Fig. 5). They are tested for flow (prEN 1015-3:1998), plunger penetration (prEN 1015-4:1998), bulk density and air content (EN 459-2:2001). The mixing procedure is not standardised, but attempts to approximate the procedure from the masons' programme (see above), using a table model Hobart mixer: 1.5 min mixing, 0.5 min scraping and homogenising, 2 min mixing. All mixing is done at low speed.

In the second section, with standardised sand, W:B and B:A are free to choose for the mason for the composition of small batches (1.2 liter). Lab experiments with average values for W:B and B:A from these tests sometimes yield visibly unworkable mortars. Hence it was decided to discard them from the lab experimental programme.

For description of the scientific laboratory tests to define workability aspects of mortar we refer to the other contribution (Hendrickx 2008).

3.2 Experimental results and discussion

In the first section of the programme (fixed B:A), the selected W:B values for mortars with each binder have a coefficient of variation of 3% to 10% between the different masons for one specific binder (Fig. 5). A systematic difference between some of the masons is observed. A correlation between W:B and the specific surface area of the binder is found. Air entrained

Figure 6. Mean selected water to binder mass ratio (W:B) as a function of specific surface area (SSA) of the binder in the mortar.

Figure 6. Mean selected water to binder mass ratio (W:B) as a function of specific surface area (SSA) of the binder in the mortar.

mortars have lower values than non air entrained mortars (Fig. 6).

The same trend in W:B is found in the second section of the programme, although with larger variation due to the varying B:A for each composition. The average selected B:A are between 0.15 and 0.2 for binders 1, 2 and 3; between 0.2 and 0.3 for binders 4, 5, 7a and 7b, and 0.4 for binder 6. The latter value is very high: it appears that the test persons added a high quantity of the ordinary Portland cement because a normal dosage gives an impression of being poor in binder.

The lab experiments on reference mortars revealed important differences in flow, yield stress, water retention, etc., although all mixes were brought to optimum water content. Different performance can be attributed to different behaviour of the binder particles on micro-scale.

What is important, is that the qualitative judgement of the different mortars is similar for all the masons, and this to a remarkable extent. The small variation in ideal W:B ratio between the different masons for each of the mortars is remarkable. It shows how craftsmanship, knowledge and experience seem to be a very good discriminator for mix optimisation and for quality assessment.

The applicability and use of mortar for different types of bricks and masonry was also taken into account by the masons. For example the masons revealed that Binder 2 had an important problem with the air entraining agent, which causes swelling and subsequent shrinkage of the mortar during mixing and in the first 10 minutes after mixing. The same problem seemed present in binder 3 to a lesser extent. The comments from the masons also depend on the type of brick used. For the small and large perforated bricks, it is judged important that the mortar adheres well to the (higher) vertical sides, that it does not fall into the perforations and that it allows a large unit to be manipulated in the mortar bed without too much force. For the small facing bricks, staining and floating of bricks are more important criteria. Floating must be understood as instability due to lack of stiffening of the mortar, which should occur upon dewatering in contact with the porous brick.

An inquiry about the relative importance that the masons of the test panel attribute to different properties of mortar (in fresh and hardened state), reveals three major concerns: workable time, adhesion to brick and workability. Mechanical resistance and yield are also considered important. Cost, thermal insulation, frost resistance and environmental nuisance have low appreciation.

The results of the scientific assessment of workability are given in (Hendrickx 2008).

3.3 Synthesis and conclusions

The variance in chosen W:B between the different masons in practical tests is small. This indicates that a marked transition in the mortar's behaviour takes place around a well-defined water content: from a granular material to a liquid material, from a frictional regime to a viscous regime. Small variations of water content around this transition point, give a very different mortar for the user. It seems more easy to identify that transition point with masons than using laboratory experiments, not to say with standardized tests.

Laboratory research (Hendrickx 2008) indicates that a combination of tests may be able to approach the masons judgement of workability. A combination is proposed of yield stress measurement with a vane apparatus, water retention tests with vacuum suction or filter plates, and density and air content tests to characterise the workability of a mortar in detail.

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