Material properties and workability characteristics of plaster mortar | ||||||||||||||||
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IntroductionThe decisive criteria in the assessment of the workability characteristics of plaster mortar are: good staying power during spraying, easy screeding during leveling, plasticity and no pilling during finish-ing. There is no standardised instruction for the description or measurement of these characteristics. Therefore, workability characteristics and consequently also material properties are determined by spraying the plaster mortar on test walls, treating it individually and finally assessing the fresh plaster from experience. In addition to such subjective assessment of plaster mortar, agitating tests are also carried out by which a speed-depending torque is related to specific properties of the product. Lastly, the assessment of the plaster mortar by means of the tests described in combination and inter-action with the respective mortar-technological tests acc. to DIN-EN forms the basis for making-up the mortar mix designs. In order to assess material properties and workability characteristics of plaster mortars more objec-tively than to date, a procedure and the required equipment have been developed which will be re-ported on in the present paper. | ||||||||||||||||
Introduction into the approachWith regard to character and behaviour plaster mortar is no fluid in the classical sense, but it is also no bulk material. Neither can this mortar be put into the group of suspensions, as the bulk properties predominate. Its properties and its behaviour under exterior forces are determined by both the fluid and the bulk phase. With regard to its structure plaster mortar may be defined as a fine-grained bulk material whose particles are wetted with a fluid. From the rheological point of view, the treatment of fresh mortar on the wall, e.g. by screeding, takes place in a range of very small rates of shearing and its appearance resembles a solid rather than a fluid. Therefore, contrary to the common characterisation of mortars by flow curves other well-known rheological experiments are used for the description of the properties of plaster mortar. These are the creep test, shear strain test and residual deformation test. The characteristic features of these tests can be seen in the following figures.
In some cases of material testing a coupling (serial arrangement) of at least two tests of this kind seems to be possible. Because of the irreversible structure of the plaster mortars care must be taken that the critical ranges of the angle of distortion are not too close to each other (e.g. jI,1 << jB ) . | ||||||||||||||||
Test procedureThe aim of the tests was to find out whether information on the workability characteristics of plaster mortars can be obtained by means of rheological investigations. On the one hand, according to present practice, the usual technological small-scale tests were carried out, which were, however, supple-mented by rheometrical measurements, on the other hand, separate tests on plaster mortar were made on laboratory-scale according to an adapted test procedure. A. Technological small-scale tests combined with laboratory tests Walls (4 x 1,75 2) made of vertically perforated bricks (500 x 240 x 110 mm3) served as plaster base. The plaster mortars were applied by means of a plastering machine of the type solomix by mtec. The ambient temperature in the test facility was about 20° C. Three plasterers independently applied and assessed the plaster mortar. Parallel to every operation of the plasterers measurements were made in the laboratory. After about half of the mortar mass had been used up, while the plaster mortar was being applied, 2-liter samples were sprayed into a vessel and the flow diameter was determined. After the plaster mortar had been applied a brick was removed from the upper part of the wall and simultaneously subjected to rheological investigations while the plaster mortar was subjected to further treatment by the plaster-ers. B. Laboratory tests Independently of the small-scale tests laboratory tests of a basic character were carried out. 2-kg samples acc. to DIN 18555 were mixed in a Hobart mixer and by means of a Hägermann flow table the flow diameter a was determined. The empirical value of 165 < a < 175 mm was fixed as the required range. Deviating samples were discarded. In order to include the absorbing capacity of the plaster base in the investigations the glass plates of the flow table (see DIN 1060) were either covered with a double layer of filter sheets or replaced by split tiles. For the rheological investigations a rheometer was available that had been developed after a draft by R.VOGEL Research Laboratory together with RHEOTEST Medingen GmbH. Details of the devices are dealt with elsewhere.
The patented measuring technique and the measuring sensor are shown in Fig. 4. The sensor is im-mersed in the mortar pat which is lying on the plate of the flow table or has been applied to / sprayed on another base. The mortar pat must not be subjected to any further disturbance. Then the behaviour of the plaster mortar as to creep, shear strain, and residual deformation is measured. The required precise measurement concerns in the first line the three magnitudes time t, torque T and angle of distorsion j in variable arrangement to each other. In certain cases an additional measurement of the difference in height is required. | ||||||||||||||||
ResultsFive different plaster mortars were investigated, see Table 1. For the small-scale tests sacked material was used. The laboratory tests were carried out with mixtures made after mix designs according to works standard and selected variations were tested. Table 1 Description of factory-made dry mortars used
Table 2 shows the most important results of the plastering tests. In lines 1, 8 and 9 the judgement of the plasteres is quoted. In the language of these experts 'easiness' refers to the strength required in the operation of screeding, 'stickiness' means that the mortar adheres to the screeding board, and 'plasticity' refers to the behaviour of the plaster mortar in the second stage, the finishing. The flow diameters in line 3 may directly be related to these statements. Since the mortar was tested under the same prerequisites the assessments given by the plasterers are comparable to the rheometer measuring results in lines 2 and 10 to 13. |
Table 2
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Considering the data in lines 1, 2 and 3 it may be stated:
As a magnitude typical of the material the moment of fracture can also be determined for plaster mor-tar under defined laboratory conditions. These measuring results can be found in lines 4 to 7 of table 2. The data in line 5 were determined in a test arrangement acc. to Fig. 4, i.e. mortar pat on dense plaster base. The effect of a variation of the plaster base, e.g. for KZP, is shown in lines 6 and 7.
As far as the plasterers' statements on stickiness and plasticity (see lines 8 and 9) are concerened the rheological investigations still leave some questions unanswered. The reason is that, on the one hand, deviations from the desired condition 'not sticky' and 'good plasticity' occur exclusively and si-multaneously with KGP 2 and that, on the other hand, a classification under the well-known physical terms is uncertain. Considering in this context the results of the creep tests in lines 10 to 13 we find an indication in line 11. The values given here for the irreversible deformation jII,120 / jI,1 for KGP 2 deviate noticeably from those for other plasters. It is, however, too soon to draw conclusions from this observation as this requires information on the exact composition of the material, which is unfor-tunately not available. The data in lines 10 to 13 let us assume that then a number of conclusions might be drawn from the creep tests.
Since almost no quantitative data of the material composition of the mortars investigated are available the results of the creep test are of little value. Fig. 9 shows how much valuable information can therefore not be gained and shall give an incentive to carry on research work. In Fig. 9 the creep tests for the two mortars KGP and KZLP are compared. On the left side the absolute values are repre-sented and on the right a section of a non-dimensional plotting of the angle of distorsion in the form of j / jI,1 can be seen. |
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With equal flow diameter a = 171 mm and equal loading of the samples TV = TI..II = 15 mNm the differences in the materials are clearly seen. If we use for the assessment of the range of recovery an exponential function of the form or in non-dimensional notation , we find that the creep test yields altogether 7 data, namely the direct measured values jI,1; jII,1; jII,120 resp. jII,¥ and the indirect measured values a ; b ; c ; tanb. The representation of the absolute measured values in Fig. 9 reveals the difference in the order of magnitude of the first group, the non-dimensional representation shows the differences in the second group of values.
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Summary and conclusionsThe paper reports on tests carried out on plaster mortars in a facility for small-scale tests and in the laboratory. Conventional plasterers' assessments of the workability of the plaster mortars are com-pared to rheological measurements. The properties of fresh mortar determined by a newly developed equipment correspond to practical experience in the application. The comparison between the rheological measurements and the judgements given by the plasterers shows that there are no princi-pal discrepancies. Besides, the rheological parameters give useful advice regarding quality control and the development of new plaster mortars. In order to achieve more telling results of rheometer measurements, particularly regarding the creep test, it is necessary to concentrate on finding out material parameters by means of model mixtures. | |||
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