Beton basınç dayanımının hızlandırılmış deneyle saptanması

Abstract

ÖZET Bu çalışmada; beton sertleşmesini hız land ırmakta kullanı lan hızlandırılmış kür yöntemlerinden, sıcak su yöntemi kulla nılarak, 2 0 C-2 8 günlük basınç dayanımının kısa sürede belir lenmesi amaçlandı. Araştırmada Erzurum- Aşkale çimento Fabrikasının ürettiği 3 KPÇ 325 çimentosu kullanıldı. 300, 350 ve 400 kg/m lük sabit üç çimento dozajı kullanılarak, su-çimento (E/C) oranı 0,40 ila 0,62 arasında değişen 40 seri üretim yapıldı. Üretilen be ton numunelerinin, 20 C-28 günlük basınç dayanımı değerleri 125 ila 313 kgf/cm2 arasında değişti. Hızlandırılmış basınç 2 dayanımı değerleri ise 29 ile 104 kgf/çm arasında değişti. Agrega olarak, Trabzon ve çevresinde yaygın kullanıma sahip olan kırmataş ve dişli kum kullanıldı. Standartlara uygunluk açısından 150x300 mm'lik silindir numuneler kullanıldı. Araştırma sonucu, hızlandırılmış deney sonucu ölçülen ba sınç dayanımları ile 20 C-28 günlük basınç dayanımları arasın da yüksek korelasyon katsayılı ilişkilerin varlığı belirlendi. Korelasyon katsayısı lineer ilişki için 0,965 üstel veya hiper bolik ilişki için 0,971 oldu. Hızlandırılmış dayanımların var yasyon katsayısı %3,4, 28 günlüklerin ise %5,3 bulunmuştur. Sonuç olarak; sıcak su yöntemiyle elde edilen basınç daya nım değerlerinden 28 günlük standart basınç dayanımına güveni lir bir biçimde geçilebileceği ve bu şekilde gerek üretimde sürekli kalite kontrolünün sağlanması, gerekse beton basınç dayanımının kısa sürede belirlenmesi amaçlarıyla hızlandırıl mış deneylerin katkılı portland çimentosu ile üretilen beton larda başarıyla uygulanabileceği kanısına varıldı. ııı

SUMMARY Rapid construction practices have led to the assessment of the quality of concrete at an age earlier then the customary 2 8 days after placement. Accelerated testing methods have been developed and even standardized in some countries. In 197 9 the Turkish Standards Institute adopted the ASTM C 684-74 procedures as the Turkish Standard (TSE 3323,1979). Use of mineral admixtures such as fly ash, slag, and natural pozzolans in the cement and concrete industry is practiced progressively in most countries. In Turkey these admixtures are usually added during the cement production stage. The production and domestic consumption of blended cements has been increasing since 1973, and in 1985 the share of the blended cements in domestic consumption exceeded 90 percent. Today, with a production rate of faurteen million metric tons per year, modified port land cement has an important place among the blended cements. It is known that the type of cement is one of the important parameters that affect the accelerated strength test result (Wills, M.H., 1975). In this study accelerated strength test results with modified Portland cement concretes were examined. The accelerated curing procedure applied was the warm water method. EXPERIMENTAL WORK Experimental work was carried aut at the Materials and Structures Laboratory in the Civil Engineering Department of Black Sea University. Fourty concrete mixes were tried and fallowing parameters were investigated: Chemical, physical and mechanical properties of cement are given in Table 2.1 Crushed aggregate was used. Crushed aggregate was produced also with the material obtained from the river bed. Specific gravities of aggregates ranged between 2,59 and 2,68. The maximum size of coarse aggregate was chosen as 25,4 mm (Square opening sieve). Concrete mix proportions are summarized in Table 2.6. For each concrete mix a 40 L batch was prepared in a 60 L. Capacity tilted drum mixer. An initial dry mixing for 2 minutes was followed by wet mixing for five minutes. From each batch 6 specimens were cast in 150x300 mm cyindrical reusable steel IVmolds with steel caps. Consolidation of specimens was achieved by external vibration on a vibrating table of 2 800 rpm Accelerated cure was applied to three specimens, while standard 2 8-day cure was applied to the remaining three. Three specimens reserved for the warm water method were placed in water at 35 C (95 F) immediately after casting. After curing for 2 3 1/2 hr the cylinders were removed from the water tank, demolded, capped, and tested at 24 hr. Accelerated curing tank and molds conformed to ASTM C 684-74 Specifications. Production, accelerated and standard curing, capping, and testing of specimens were carried out in accordance with the relevant Turkish Standards. A 60 ton capacity universal testing machine and a 2,5 kgf/cm2 sec loading rate were used for compression testing. Slump of concrete mixes ranged between 0-130 mm, accelerated compressive strengths ranged between 29 to 104 kgf/cm2 while 28-day compressive strengths ranged between 125 to 313 kgf/cm2. Strength results are reported in table 2.6. The average strangths were 63 kgf/cm2 for accelerated test and 217 kgf/cm2 for the 2 8-day test. The avarge compressive strength obtained atthe end of the accelerated curing was 29 percent of the 28-day strength. The average within- batch standard deviations computed by analysis of variance were found as 2,2 kgf/cm2 for accelerated strength and 11,6 kgf/cm^ for 2 8-day strength. Then the within-batch coefficient of variation was 3,4 percent for accelerated strength and 5,3 percent for 28 days strength. DISCUSSION OF RESULTS In Fig. 3.1 28-day compressive strengths are plotted versus accelerated strengths. Regression analysis shows that the relationship between 2 8-day and accelerated strengths can be expressed by either a linear or a power type equation. y=70, 857+2, 322x r=0,965 y=13,52x0/673 r=0,971 Where X is the accelerated strength, y is the 2 8-day strength, and r is the coefficient of correlation. Although several para meters (cement delivery, cement content, water-cement ratio, aggregate type and grading) were investigated through wide ranges, a high coefficient of correlation was obtained. There fore, it can be said that within the limits of this investigation, these parameters did not noticeably affect the relationship between the accelerated and 28-day strengths. When the linear relationship is used to estimate the 28-day strength from the accelerated strength, standard error of the estimate is computed to be 13,7 kgf/cm^ and 95 percent confiden ce limits of the estimate to be + 27 kgf/cm2 as indicated in Fig. 3.2. The high coefficient of correlation and narrow confi dence limits obtained here demanstrate the warm water method can be applied successfully also to modified port land cement concretes. In Fig. 3.3, regression curve obtained is compared to those given by other investigators who used the warm water method.Relationship 1 and 2 are from the Corps of Engineers' accelerated testing program, respectively for Type I or II portland cements and Type II portland cement + pozzolan. Relationship 3 and 4 represent the results of ASTM'S Cooperative Test program, respectively for Type I and Type III portland cements. Relation ship 5 represent the results of öztekin, for portland - pozzolan cements. A better agreement is observed between the modified Portland cement and Type II + pozzolan cements* regression lines. CONCLUSION Warm water accelerated testing method can be used for modified portland cement concretes as successfully as it is for Portland cement concretes. Although the parameters related to the compostion of concrete are varied extensively a relationship with a high coefficient of corelation (r=0,971) is obtained between the accelerated and 2 8-day strengths. A power-type equation represents best the ralatlonship between the accelerated and the 2 8-day strengths. However, a linear relationship gives an equally good coefficient of corre lation and it may be preferred for its simplicity and suita bility to statistical analysis. VI