Pompa betonlarında işlenebilirliğin harç fazının reolojisine dayanarak belirlenmesi

Abstract

Pompa betonları iri katı tanecikler içeren viskoz sıvıdırlar, yüksek basınç altında boru içinde iletilirler. Normal taşman betonların özelliklerine ek özelliklere sahiptirler. Pompa betonları boru içinde minimum enerjiyle, kesintisiz ve niteliklerini yitirmeden iletilebilmelidirler. Yani beton homojenliğini korumalı, ayrışmamak, segregasyonu önlemek amacıyla yüksek kohezyona sahip olmalıdır. Aynı zamanda boru içindeki basınç kayıplarını önlemek için yüksek düzeyde akış kabiliyeti olmalıdır. İletimin kolaylığı açısından minimum sürtünme değerine, betonun optimum terleme düzeyinde ulaşılır. Bununla beraber boru içindeki beton, tıkanmaya sebep olacak aşın terleme yapmamalıdır. Böylece istenilen pompa gücü daha düşük seviyelerde kalacaktır. Bu niteliklerin gerçekleşmesinde iri agrega fazının miktarı ve yüzey özellikleri, iri agrega-harç fazının niceliksel dengesi yanında harç fazının kendine özgü nitelikleri de rol oynarlar. Sıvı harç fazının nitelikleri; kayma eşiği (x0) ve plastik viskozite (rjp!), olarak adlandırdığımız reoîojik sabitler olarak tanımlanır. Çalışmada, harç fazının kayma eşiği ve plastik viskozitesi gibi reoîojik parametrelerinin betonun kohezyonu ve pompanın itme gücüne etkileri incelenmiştir. Kohezyon ve güç, pompalanabilirliğin iki ana kriteri varsayılmıştır. Yuvarlak ve kırmataşlı agregalarla, akıcı ve yüksek mukavemetli betonlar üretilmiş, su ve harç içerikleri sabit tutulmuştur. Nominal çimento içeriği kontrol örneklerinde 300 ve 400 kg/mJ'tür. Bu çimento içerikleri harç deney serilerinde C sınıfı uçucu küller kullanılarak çimento ile kısmî yer değiştirme yöntemi ile azaltılmıştır. Beton serilerinde ise olumlu sonuçlar veren Orhaneli uçucu külü ile çalışmalar sürdürülmüştür. İstenilen işlenebilirliği elde etmek için sülfone naftalen formaldehit kondense süperakışkanlaştırıcı kullanılmıştır. Bundan dolayı harç fazlarının kalitesi, yani harç fazının akış özellikleri akma değeri ve plastik viskozite değerleri değişmiştir. Harç örneklerinde, reoîojik parametreler koaksiyal bir viskozimetreyle yardımıyla, taze betonun pompalanabilirlik deneyinde ise betonu, boru içinde harekete geçirebilmek için gereken basınç değeri (itme kuvveti) ile boru ucundan kopmadan çıkabilen beton kütlesinin ağırlığı (taze betonun kohezyonu varsayılan) tayin edilmiştir. Betonun kohezyonu ve harcın reoîojik parametreleri arasında (kayma eşiği,To ve plastik viskozite rjpı ) lineer bir ilişki kurulmuştur. Bu reoîojik parametreler ve itme kuvveti arasında basit bir ilişki elde edilememiştir. Çimentolu malzemelerin içeriği ve kompozisyonu, kaba agreganın pürüzlülüğü, harç fazının ve betonun ilişkisini etkilemiştir. Çalışmadan Uçucu kül kısmî ikâmesinin % 15-20 mertebesinde tutulması, çimento nominal dozajının 400 kg/m altına düşürülmemesi, süperakışkanlaştırıcı katkı miktarının bağlayıcı madde içeriğine oram %2.0-2.5'i oranında kalması ile pompalanabilme, mukavemet, durabilite ve ekonomi koşullan optimum düzeyde sağlanabilir.

The transport of the fresh concrete by pumping is an important process and widely used in the actual concrete technology. This process has been applied since the beginning of the twentieth century. Developments of the pumping equipment and new findings in mineral and chemical admixtures in recent decades forced the concrete technologists to undertake more sophisticated researches on pumping concretes. It is obvious that pumped concretes in hardened state should have sufficient mechanical strength and good durability as the normal concretes. They differ from them only in the fresh state and this difference affects certainly the mix composition and mixing procedure. There are many recommendations for the composition of the pumped concretes. They are not elaborated in standard specifications, but are very useful in the applications. Besides in this research for determining the gradation of the aggregates and finding the mix proportions the recommendations of the American Concrete Institute ACI-304.2R were used, the maximum size, volume and type of the aggregates, the absolute volume of the mortar phase were determined according to these recommendations. The workability of the pumpable concrete is usually determined by standard workability tests such as slump. But, we believe that, For the workability of the pumpable concrete which needs more complex properties the classic slump test is inadequate. We may list these complex properties, as follows: the cohesion of the concrete should be high enough to avoid the segregation, but at the same time the concrete should also have flowing consistency to prevent head loss along the pipe, the bleeding of the concrete should have an optimum value, too little bleeding increases the friction between concrete and pipe; too large bleeding causes blockage of the pipe. It is evident that slump test can not give sufficient information about these specific properties. However, the "two point workability test apparatus" which helps to obtain the yield value and the plastic viscosity of the concrete may give more detailed and necessary information on the workability of pumped concrete. Unfortunately this apparatus does not exist in The Construction Materials Laboratory of Istanbul Technical University. Nevertheless the rheological constans (yield value and plastic viscosity) of the mortar can be determined with a co-axial viscosimeter, and this process is aplied in this study. If the efficiencies of the mortar phase on the cohesion, fluidity, bleeding and the friction with the pipe of the concrete are taken into consideration this approach can be admitted beneficial and helpful. Theoretical investigation of the hydro-dynamic behavior of the pumped concrete in pipe is very difficult, even imposibie. The concrete mass which is gaining velocity under pressure gradient and transporting is a viscous suspension containing coarse and non uniform size solid particles. The motion of this suspension in the pipe should be uniform and laminary and should not exceed the limit of turbulence, this is necessary for preventing the segregation. On the other hand the transport must be achieved with Xil low pressure and energy. The factor increasing the pressure and energy is the friction between concrete and pipe wall; if the pressure of the pump is increased to overcome this friction another danger may appear; attaining the limit of segregation pressure the mortar can separate from the mass and the aggregate particles block the pipe. The role of the mortar in all these phenomena is primordial. The' gradation and the fineness of the sand, cement and admixtures contents and the water/cementitious material ratio of the mortar are the composition parameters of the mortar which affect the behavior of the concrete in the pipe. The mortar provides cohesiness of the concrete, but should be fluid enough to bulge out of the concrete and form a gliding layer on pipe wall. The mortar possessing these properties has a low yield value and moderately high plastic viscosity. The cohesion of the mortar may be increased by adding some fine mineral additive such as fly ash and the fluidity may be improved by adding a plasticizer or superplasticizer agents. It is supposed that the plasticizers decrease the yield value without lowering significantly the plastic viscosity. Therefore the combined use of a superplasticizer and fly ash seems as an optimum solution. This method is applied in this work. In this study the pumpability of concretes produced with mortars having different rheological constants was investigated. We may give a clear, short and concise definition for the pumpability as follows: The concrete which moves in a pipe without loosing its cohesion and homogeneity, necessitating low pressure and energy and causing no blocking is defined as pumpable concrete. In this investigation two properties are taken into account, such as concrete cohesion and the necessary pushing force of the pump. In a more advanced research the friction between concrete and pipe wail, energy loss along the pipe and blockage problems should be investigated. But ail these studies may be carried out on prototypes. A research on pumped concrete in laboratory can only be accomplished on models. The two properties cited above were studied in this work in laboratory scale. A model consisting of a rectified steel pipe having 11.5 cm internal diameter and 150 cm length was manufactured. The concrete filled in the pipe was pushed with an Enerpac actuator and the pressure was measured by a manometer. The necessary force to initiate the movement of the concrete mass was taken as a criterion for the pushing energy. The pipe had an inclination of 1 5 degrees. The piece of concrete falling down from the open end of the pipe was weighed and the weight was considered as a criterion of the tensile strength of the fresh concrete, i.e. its cohesion. The standard workability tests such as slump, slump-spread and K-slump and the 28- day compressive cube strength tests on the concrete sample were also carried out. Rheological tests on mortars were carried out using a Mettler RM 1 80 Rheomat co axial viscosimeter. The mortars were placed in viscosimeter tube following 8, 18 and 28 minutes from the beginning of their mixing with water. A program of 8 steps was performed, and the rotation rates and torques were measured in each step. Based on this data the angular deformation rate (y) and shearing stresses (x) were computed, and the y-x diagrams were drawn. All the mortars showed a tixotropic behavior conformable to Binghamien model. The yield value (x0) and the plastic viscosity (r\p\) of the mortars were obtained by linear regression. The sand, cement and fly ash were first mixed in dry state and then during 2 minutes were mixed with water by a 800 Mil rpm mixer. The superplasticizers were added after 2 minutes and the mortar was mixed for 5 minutes. Some principles were proposed and applied on the choice of materials and the computation of ingredients:. Concretes manufactured would be high performance concretes, therefore the concretes mixes were designed for 28-day compressive strength of 40 MPa and the water/cementitious material ratios were less than 0.42.. The concretes were of flowing consistency; their standard slump values were greater than 1 5 cm. » The natural stock of the ASTM type F fly ash is limited in Turkey and, therefore using ASTM class C fly ash in the research would be more realistic for the Turkish concrete industry. To realize this objective three kinds of fly ash commercially available in Marmara region were examined from the point of view of water and superplasticizer requirements to obtain high strength concretes and the most convenient one, Orhaneli fly ash, was used in the main research.. The fly ash content was to be determined by applying the partial replacement method. Thus a preliminary investigation was undertaken to find out the efficiency factors of the fly-ashes and these factors were used to estimate the fly ash content. » The ACI-304.2R recommendations was adopted for the aggregate gradation and mix desing. Therefore the maximum size of the coarseaggregate was 20 mm and the aggregate content by absolute volume was 0.55 m /m. The volume of the mortar phase was approximately 0.45 m /nı in all the series. The maximum size of the sand was 2.38 mm. Coarse aggregates were round, but to see the effect of the aggregate angularity and surface texture, crushed stone aggregates were also used in some series. PC-42.5 portland cement corresponding to ASTM type III cement and napthalene sulfonate formaldehyde condensate superplasticizer were used. Based on the preliminary test results on the fly ashes, the most suitable one, Orhaneli fly ash was chosen and used in the main research. Seyit Ömer fly ash was too fine (Blaine 402 m /kg) and Çayırhan fly ash contained less particles between 10 and 40 u.m size (28.3 %). Seyit Ömer fly ash concrete required high quantity of water and superplasticizer and the compressive strengths of the Çayırhan fly ash concretes were much scattered. The main principle in tests was to keep constant the volume of the mortar phase and to change its quality. This change was realized by choosing two different initial cement content, replacing cement by different amounts of fly ash and adjusting the XIV superplasticizer content accordingly. The initial cement contents in control specimens 3 3 were fixed as 300 kg/m and 400 kg/m. After replacement of fly ash the cement contents changed from 300 kg/ra to 190 kg/m, from 400 kg/m to 220 kg/m and the fly ash contents changed from 0 kg/m to 350 kg/m and from 0 kg/nr* to 490 kg/mJ, respectively. The water contents were kept constant but the superplasticizer additions increased significantly. It will be more convenient to summarize the conclusions of this research in three different categories: modifications in the rheological behavior of the mortar phase as effected by the composition parameter the corresponding changes observed in the properties of the concretes and the influences of the rheological constants of the mortar on the concrete pumpabilities. As mentioned above for a good pumpability the yield value (x0) should have a low value and the plastic viscosity (rjpi) should not decrease too much. Due to the addition of superplasticizer to the mortar, the decrease in x0 was 78% and that in rjpi was only 35%. This result is in conformity with the aim proposed. The addition of a fly ash to a superplasticized mortar increased the values of To and rjpi in the same manner, though not so significant. It is interesting to note that this increase in To takes place at all the fly ash replacement ratios, up to 40-50% of the cement content (15-25% in partial replacement) beyond which rjpi begins to decrease. Based on the experimental results and economic necessity it can be suggested that the selection of the superplasticizer/cementitious material ratio between 2 and 2.5 % (by total material), cement content 400 kg/m, addition of fly ash between 25-35% may yield optimum results. In that case x0 has a value between 38 and 48 Pa, and rjpi between 0.8-1.0 Pa. sec. The addition of a superplasticizer to the concrete mixes increased the consistencies as expected. But in the case of crushed stone aggregate or fly ash addition into the mix the rate of increase in slump values diminished significantly. If a higher initial cement content ( 400 kg/m ) is choosen and a quantity of fly ash less than 1 5 % replaces partially the cement, the quantity of superplasticizer required to obtain concretes having slumps exceeding 170 mm also increases, non the less the solution is sufficiently economic. Slump-spreading tests and K- slump tests were more sensitive and reliable than the classic slump test in determining or quantifying the workabilities of the pumped concretes which are indeed flowing concretes. On the other hand it is possible to put forward the same opinion on the friction between concrete and pipe wall by analysing the K-siump test values. In this series of tests two different values namely K and W are measured, the first indicates the seepage ability of the mortar phase from the concrete mass, and the second the bleeding of the mortar phase. K should be large and K-W low for providing a continuous sliding layer on the pipe wall. These two conditions were realized for concretes having rounded aggregate, low cement content partially replaced by fly ash at a ratio of 1 5 %. XV The pushing force required in concretes soleiy superplasticized was greater than that required in concretes superplasticized and containing fly ash. This difference observed was significant in the concretes made with crushed stone aggregates. The cohesiveness of the concretes was significantly influenced by the ingredients. Fly ash replacements exceeding 1 5% resulted in a decrease in compressive srengths. This result indicates that the efficiency factors of the fly ashes are not constant but are functions of the cement contents. This result was expected, because the pozzolanic activity is also affected by the quantity of the lime produced during cement hydration. The evaluation of the effects of the rheological properties of the mortar phase on the pumpabiiity constitute the third part of the conclusions, in fact the determination of these relations were the main purpose of the work. The influences of the To and rspj of the mortar on the pushing force required could not be expressed with a simple analytical relation. The amount of the cementitious material per cubic metre of concrete was another important parameter affecting this relation. The required pushing force for a rich concrete is lower, and the addition of a superplasticizer causes significant decrease in the value of this force. The presence of a crushed stone coarse aggregate is a second factor effecting an increase in the value of the pushing force. It was possible to establish linear relation between the cohesion of the concrete and the rheolojical constants of the mortar phase. In general the increase in x0 and r|pi values induce the increase in the cohesiveness of concrete. In the case of lean concretes containing a high proportion of fly ash the rate of increase in cohesion with increasing x0 is significantly high. As a final conclusion, it can be said that the determination and improvement of the rheological constants of the mortar phase are necessary and sufficient for obtaining a pumpable concrete providing that a coarse aggregate of sufficient quality is choosen. From economy, strength and durability points of view the most convenient solution to obtain a good pumpable concrete is to keep the nominal cement content at the level of 400 kg/m, to replace 15-20% of this cement content by fly ash and to add sufficient quantity of superplasticizer for the fluidity required. The following topics may be proposed for further research in this area: measurement of the radial pressure of the concrete on the wall of the prototype pipes during transport, determination of the bleeding rate and capacity of the mortar phase under pressure and the segregation pressure of the concrete mix in the pipe.