Priz geciktirici katkıların uzun süre karıştırılmış beton özelliklerine etkisi

Abstract

Beton üretiminde, betonun işlenebilme süresiyle ilgili problemlerle sıkça karşılaşılır. İşlenebilme süresinin kısalığından kaynaklanan problemler priz geciktirici katkı kullanarak çözülmeye çalışılır. Bu çalışmanın amacı priz geciktirici katkıların taze ve sertleşmiş beton özellikleri üzerindeki etkisini incelemektir. Bu amaçla değişik katkılarla ve farklı katkı yüzdeleriyle aynı çimento ve aynı beton bileşimine sahip 8 ayrı üretim yapılmıştır. Üretim sırasında bir transmikser sürekli betonu karıştırmış ve işlenebilme çok küçük değerlere düşene kadar belirli zaman aralıklarında taze beton deneyleri (çökme, taze birim ağırlık, hava miktarı, terleme, penetrasyon direnci) yapılmış ve ileri günlerde gerçekleştirilmek üzere sertleşmiş beton deneyleri (basınç, eğilme, hidrolik rötrepermeabilite) için numuneler alınmıştır. Betonun işlenebilme değeri düşük seviyelere indiğinde yeniden su verilerek işlenebilme baştaki değerine getirilmiş ve tekrar numuneler alınmıştır. Deneyler sonunda kullanılan katkının ve miktarının beton özelliklerini değiştirmede etkin olduğu gözlenmiştir. Priz geciktiriciler, su indirgeyici kullanarak üretilen betonlara eklendiğinde etkilerinin daha da belirginleştiği görülmüştür. Yeniden su verme işleminin beton özelliklerini olumsuz yönde etkilediği gözlenmiştir.

In many cases long workability time of concrete is required. In ready mixed concrete transportation, extended mix time is a problem especially due to the heavy traffic, breaking down of the ready mixed truck or trouble in placing equipment at the site. Also hot weather concreting, prevention of the harmless heat of hydration, prevention of the construction cold joints need longer workability time. Increasing problems with the supply of long workability time may be solved through using retarding admixtures. The purpose of this study is to examine the effects of retarding admixtures on the properties of long time mixed concrete. Retarding admixtures are products which retard the setting of concrete. Many organic and inorganic compounds, including those derived from industries as by-products, can be used as retarding admixtures. Unrefined Na, Ca, NH4 salts of lignosulfonic acids (containing sugars such as glucose, mannose, fructose, xylose etc.) and modifications and derivatives of this group not only act as retarding admixtures but also reduce water requirements. The other well known class of retarders is hydroxycarboxylic acids, their salts (generally Na, Ca or triethanolamine salts of adipic, gluconic, tartaric, succinic, citric, malic or heptonic acids) and derivatives. Carbohydrates including sugars are also good retarders. Many inorganic compounds based on phosphates, fluorates, oxides (Pb or Zn), borax and magnesium salts are known to act as retarders. Retarders are generally used as neutral or slightly alkaline aqueous solutions. They are usually supplied in plastic containers (3-25 liters), steel drums (210 liters), and portable (1000 liters) or bulk storage tanks (2000-3000 liters) commonly constructed of mild steel or heavy polypropylene with a steel frame. Retarders are added to mix with fully automatic or semi automatic dispensers in the amounts of 0.2 to 5 % by weight of the cement. Addition procedure could be obtained by trial batches. Storage and shelf life is very long (at least 10 years) if they are protected from severe frost. Retarding admixtures are neither combustible nor flammable. Because retarders are in general mildly alkaline, contact with skin and eyes must be avoided. In this study some combinations of two retarding and one water reducing admixtures are used. The mix proportions and the cement type of the concrete mixes remained the same except the reference one. In the reference mix to obtain the workability and water/cement ratio the same as admixture added concrete, both the water and cement amounts were increased 10 % while the increased absolute volume was reduced from aggregate. The concrete mixes prepared with and without admixture are as follows: IX a) Series A: Addition of a commercial retarder (A ) (gluconate based) 1 % by weight of cement. b) Series B: Addition of the same retarder (A) 2 % by weight of cement. c) Series C: Addition of retarder (A) 2 % by weight of cement 1.5 hour after the initial mixing of water and cement, (without admixture at the beginning). d) Series D: Addition of retarder (A) 2 % by weight of cement 1.5 hour after the initial mixing of water and cement with a water reducer (lignosulfonate based) 0.4 % by weight of cement at the beginning. e) Series E: Addition of another commercial retarder (B) (dextrin based) 0.35 % by weight of cement. f) Series F: Addition of retarder (A) 2 % by weight of cement 1.5 hour after the initial mixing of water and cement with a water reducer 0.4 % by weight of cement at the beginning. g) Series G: Reference mixture with no admixture. H) Series H: Addition of retarder (A) 2 % by weight of cement. The first 4 series (Series A, B, C, D) were produced in the autumn of 1993 and others were produced in the summer of 1994 to see the effects of the weather conditions on the performance of retarders. Most of the tests were carried out at a commercial ready-mixed concrete plant. The concrete used is taken to a ready mixed truck from the batching plant in the morning. The drum of the truck continued its rotation at 1.5 rev/min rate, approximately. First group of specimens were taken in the first 10 minutes after initial mixing and following groups of specimens were taken at the intervals of 1.5 hour, 2.5 hour and 1 hour until the workability drops down to low values. The tests were repeated for each group as the following hours: time (hour) »^ V V V V v t=0 t=1.5 t=4 t=5 t=6 At the end, the concrete was brought to initial workability by retempering, and specimens were taken for further tests. The following test procedures were followed for the experimental program: Fresh concrete tests: The fresh concrete was tested for slump, bleeding and setting times following ASTM test procedures CI 43, C232 and C403, respectively. Also unit weight and air percent of the fresh concrete were measured. Hardened concrete tests: Compressive strength: Compressive strength tests were conducted at 7 and 28 day ages on 150x150x150 mm cubes. The concrete cube strength was calculated as the average of three specimens which were cured in water at 20±2 °C until the time of testing. Flexural strength: Flexural strength was determined at 28 day ages on 100x100x500 mm prisms. Flexural strength was calculated as the average of two specimens which were cured in water until the time of testing. Drying shrinkage: Length and weight changes were measured on 100x100x500 mm prisms. The results are the average of two specimens of each group which were cured in the air at 23±2 °C and 50 % relative humidity. Permeability: After 28 days 70x70x70 mm specimens were dried in the oven. They were put in the water such that they could absorb the water from one face only. Their weights were measured at specific times and the permeability coefficients were calculated as the average of three specimens. Freezing and thawing resistance: Tests were conducted following ASTM C 666 on 100x100x300 mm specimens. The specimens were kept for 2 hours at -18±2°C in the air and 1 hour at 5±2°C in water for 100 cycles. At the end of every 20 cycles their weights, ultrasound pulse velocities and resonant frequencies were measured. Reinforcement bonding strength: Steel bars were insert in 200x300x300 mm concretes at the same distances from the corners. They were pulled out by a hydraulic jack and the failure loads for bonding were measured with a load cell. The effect of retarders on workability was determined by studying the slump loss of the concrete. Although they have the same initial slump the rate of slump loss is different. For example; at the end of 4 hours the slump decreases to 3.5 cm for dextrin based retarder in series E while it is only 10.5 cm with gluconate based retarder in series A. The lowest rate of slump loss, in other words the longest workability time is achieved in series F which contained a modified lignosulfonate based water reducer 0.4 % by weight of cement added at the beginning and a gluconate based retarder 2 % by weight of cement 1.5 hours after the initial mixing of water and cement. In series the rate of bleeding and the total amount of bleeding water decreases with the mixing time. In series F (0.4 % water reducer + 2 % gluconate based xi retarder) it is observed that gluconate based retarders extends the time of bleeding while slowing down its initial rate. These retarders cause the bleeding occur in a longer time which may help to prevent plastic drying crackings. Although the mixing times of retempered groups are longer (up to 6 hours in some series), they started setting earlier than the initial groups. Extended mixing and retempering decreases the effect of retarder. This behaviour may be explained as follows: mixing continuously affects the accumulation of admixtures negatively around cement particles and retempering water removes the retarder around cement particles. Although there is more than 500 % differences in initial setting times, the elapsed time between initial and final setting is more or less the same for all series. The longest setting time among the series is obtained in series F (0.4 % water reducer + 2% gluconate based retarder). Showing a kind of synergistic effect. Unit weights and air contents showed a decrease with retempering. 7 and 28 day compressive strengths gave the maximum value for series G (reference mixture). This is due to high cement dosage in series G (10 % higher than that of other series). At the beginning series H which contains 2 % gluconate based retarder gives the highest compressive strength. The compressive strength of series F reaches series H (2% gluconate based retarder) after 2 % of gluconate based retarder added into mix which contained initially 0.4 % of water reducer. Compressive strength of series E which contained 0.35 % of dextrin based retarder were about 20 % lower than these series. Although the mixes were agitated from 1.5 hours to 6.5 hours, the difference in their compressive strengths at the end of mixing were not greater than 10 % of their initial strengths. All the series were retempered at the end of their mixing times and due to increase in water/cement ratio, the compressive strengths were decreased. This decrease was proportional the workability of concrete before retempering. For example, in series E (0.35 % dextrin based retarder) the slump decreased to 3.5 cm at the end of mixing time and its compressive strength decreased 25 % when it was retempered to reach the initial workability while this number was 10 % for series H (2 % gluconate based retarder) which had a 10 cm slump before retempering. Flexural strength changes were not more than 10 % up to 6.5 hours mixing time. Flexural strengths decreased as compressive strengths after retempering. Slopes of drying shrinkage and weight loss versus time curves of all series are reducing by time and becoming almost horizontal after 160 days. Retempered groups of each series gave higher values for drying shrinkage and weight loss. Series G (reference mixture) has the highest permeability coefficient which means it is the most permeable of all series due to the high amount of mixing water. After series G (reference mixture) was mixed for 1.5 hours its permeability coefficient decreased 27 %. In all other series containing admixtures permeability coefficients were low at the beginning and increased by the mixing time. Due to the increase in water/cement ratio, permeability coefficients decrease in all series after retempering. xn Freezing and thawing tests which were done for series G (reference mixture) and series H (2 % gluconate based retarder) showed that gluconate based retarders extremely enhance the freezing - thawing resistance. However in series G (reference mixture) after 80 cycles the specimen began to fall into pieces, although there was no change in the mechanical properties of series H (2 % gluconate based retarder) after 1 00 cycles. In literature retarding admixtures are mentioned to improve the reinforcement bonding strength. In this study bonding test gave higher results for reference mixture (series G) than series H containing 2 % of gluconate based retarder by weight of cement. This is again due to high amount of cement and high compression strength of the reference mixture. General conclusions about this study can be summarized as follows: -Gluconate and dextrin based retarders used in this study increases the slump of fresh concrete. -Gluconate based retarders extend the workability time, and give less slump loss than dextrin based retarders and reference mixture. -Gluconate based retarders extend the initial setting time but it has no effect on the time difference between initial and final setting. -Gluconate based retarders control the bleeding which could prevent plastic shrinkage crackings. -Gluconate and dextrin based retarders have no negative effect on compression and flexural strengths. -Gluconate and dextrin based retarders decrease the permeability. -Freezing and thawing resistance of gluconate based retarders are much better than control mixture. -Dextrin based retarder, although designated as ASTM C494 Type D admixture, it showed no retarding effect at all. At the moment more research is being done for the microstructure and distribution of pores caused by retarding admixtures.