Summary Report on Work package 3.2 Test Methods for
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Growth Contract No. G6RD-CT-2001-00580 Summary Report on Work package 3.2 Test Methods for Passing Ability Swedish Cement and Concrete Research Institute, CBI Universität Stuttgart, IWB Peter Billberg (CBI) Örjan Petersson (CBI) Mikael Westerholm (CBI) Timo Wüstholz (IWB) Hans-Wolf Reinhardt (IWB) Draft version 2004-10-14 Test methods for passing ability -Summary- Page 2 Introduction The European project, TESTING-SCC, investigates suitable methods for assessing the three key properties of fresh self-compacting concrete, i.e. passing ability, filling ability and resistance to segregation. Work package 3.2 deals with methods to measure SCC passing ability by using the L-box and the J-Ring. The work in this investigation is shared by Swedish Cement and Concrete Research Institute, CBI, and University of Stuttgart, IWB. The work at IWB has mainly been focused on the J-Ring while the work at CBI has focused on the L-box. For a concrete to be considered having self-compacting properties it should fulfil all the three above mentioned functional requirements; filling ability, passing ability and segregation resistance. An insufficient passing ability can indeed be enhanced by poor filling ability as well as poor segregation resistance. But even if these requirements are fulfilled, blocking can occur due to the mix design itself. This is the reason why reliable test methods able to assess passing ability are so important. Blocking mechanisms The mechanism of blocking can be explained by observing the flow of concrete through an opening. Due to the change of the flowing path of the aggregate particles around the opening it is possible that some aggregate particles form a stable arch, which blocks the rest of the flow of the concrete. The arching develops easier when the size of aggregate is large relative to the size of the opening, if the total content of the aggregate is high and also if the shape of the particles deviates from spherical, i.e., crushed aggregate. It is also possible that the surface friction between the flowing concrete and the surface of the obstacle (e.g., the reinforcement bars and the texture of the apparatus material used in the J-Ring and L-box tests) influences the blocking behaviour by supporting or preventing the building of an aggregate arch. Scope of performed investigations A large number of different SCC mixes were tested at CBI and IWB in order to evaluate the limitations and reliability of the L-box and J-Ring respectively. All mixes used are based on the mixes developed in WP 2 and the original as well as local materials were used. For the L-box, the following parameters were evaluated: - Arrangement of reinforcement bars - Material of the L-box - Pre-treatment with form releasing oil (or not) - Relation between empirical slump-flow and L-box results - Relation between fundamental rheology parameters and L-box results For the J-Ring, the following parameters were evaluated: - Two different ways to evaluate blocking: ∆s, difference in spread with or without the J-Ring and stJ, step of blocking - Worktop conditions (material and moisture) - Type and number of reinforcement bars - Cone orientation and lifting height Test methods for passing ability -Summary- Page 3 Results and Conclusions The following conclusions from the evaluation of the two methods L-box and J-Ring can be drawn from the work within WP 3.2: L-box: Blocking was assessed using L-box frames with different types of bars, i.e. deformed and smooth bars. Any possible influence of friction due to form material was tested using L-boxes made of plywood and plexiglass, respectively. In addition, tests were done with L-boxes pre- treated with form oil and without form oil. The results of these tests did not show any significant differences that can be linked to the different materials, type of bars or pre- treatment of the L-box. The conclusion regarding the gaps in the different frames for the L-Box is that it should have the same spacing between the bars and also between bars and wall. The original frame, frame 1, has 34 mm spacing between the bars and 48 mm between bars and wall. Video sequences from the testing using frame 1 show that the concrete flow is higher between bars and wall compared to between bars. The 3rd tested type of frame, frame 3, has an even spacing of 41 mm, which performs better and gives a more uniform flow of concrete through bars and wall and between bars. The rheology of a series of SCC mixes were measured with a ConTec 4 viscometer in order to establish relations between the fundamental parameters yield stress (τ0, Pa) and plastic viscosity (µpl, Pa·s) and the results from empirical test parameters of slump-flow and L-box. The results showed good correlation between yield stress and the slump-flow value and between plastic viscosity and the T50- value. Also the T40- values from the L-box are in good agreement with the plastic viscosity when using the frame 4B with only two bars. Another conclusion is that the major factor influencing the calculated blocking ratio, H2/H1, is the flowability, either expressed as slump-flow value or yield stress. A decrease of flowability, i.e., an increased yield stress results in a steeper slope of concrete in the L-box and thus, the blocking ratio decreases. The basic mechanism of blocking depends, however, mainly on the paste volume of the concrete together with the characteristics of coarse aggregate (size, shape and amount) relative to the gap. The viscosity expressed as either T50- value or plastic viscosity does not influence the blocking behaviour of SCC. J-ring: Two methods of evaluating the blocking behaviour were compared. The method which uses the difference between the spread diameters from the slump-flow test and the J-Ring test could not be recommended as a knock out criterion due to the large scatter despite of the fact that the difference between the measured spread diameters decreases with an increasing filling ability. The relatively large scatter could be traced back to the fact that two subsequently performed measurements have to be compared. It could be shown by a simple consideration that the blocked concrete volume can be expressed with a linear relationship by the so-called step of blocking stJ, which can easily be measured with the J-Ring test. Test methods for passing ability -Summary- Page 4 Tests with different surface conditions on the worktops were performed and it could be shown that mainly the filling ability is influenced. But also the passing ability seemed to be influenced by the surface friction. Also tests with 16 and 22 smooth steel rods in the J-Ring test were performed and the effect on the blocked concrete volume – expressed by the step stJ – was checked. An interrelationship between these results was found, i.e., larger steps due to smaller opening when the 22 bars was used. Blocking behaviour was investigated by use of conventional rebars with a diameter of 16 mm instead of smooth rods with a diameter of 18 mm. No difference in blocking could be observed. Also tests with different orientations of the Abrams’ cone were performed. Under common testing conditions only an influence on the flow time T50 respective T50J was found. An inverted cone orientation seems therefore also to be possible. L-box vs. J-Ring: The step of blocking, stJ, was measured with the J-Ring (16 bars with spacing 40.5 mm) simultaneously with the blocking ratio in the L-box (three bars with even spacing of 41 mm). Thus, the distance between the bars in the JRING and L-box was approximately the same. The test results are shown in Fig S1. The blocking criteria H2/H1≥ 0.8 for the L-Box seems to correspond to a step of blocking stJ< 10 mm for the J-Ring. 0.9 0.8 0.7 L-box, blocking H2/H1 [-] 0.6 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 J-Ring, step of blocking [mm] Fig S1. Simultaneously performed blocking tests with L-box vs. J-Ring. Test methods for passing ability -Summary- Page 5 Recommendations L-box: In the case of dense reinforced constructions, e.g. civil engineering structures, the L-box equipped with three bars should be used with a criterion of 0.8 as blocking ratio. For less dense reinforced structures the L-box with only two bars, should be used. The blocking ratio criterion should also in this case be 0.8. The L-box should have the same spacing between the bars and between bars and wall in both cases. Since both the filling ability and the segregation resistance influence the measured blocking ratio in the L-box, it is important that these requirements are fulfilled in order to find out if blocking is due to undesired mix design. Thus, the L-box can not be used to assess all three functional requirements simultaneously. J-ring: The method which uses the difference between the spread diameters from the slump-flow test and the J-Ring test could not be recommended as a knock out criterion due to the large scatter despite the fact that the difference between the measured spread diameters decrease with an increasing filling ability. Thus, based on general considerations linked with the test results, only the measurement of the step stJ can be recommended as a quantitative criterion for assessing the blocking behaviour with the J-Ring. Recommendation of numerical values for a quantification of the blocking behaviour should build up a base for practical users to decide if a SCC mix blocks and if so, how much it blocks. These values are necessary for the quality control as well as for concrete producers and for customers, e.g., for acceptance tests on site. All tested SCC mixtures having a step stJ < 10 mm showed apparently no blocking. This value could be recommended as a knock out criterion.