Effect of aggregate gradation on size
effect in split-tension cubes
Journal Name:
- Dicle Üniversitesi Mühendislik Fakültesi Mühendislik Dergisi
Keywords (Original Language):
| Author Name | University of Author |
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Abstract (2. Language):
In general, if geometrically similar specimens do not
behave similarly for different sizes, this is called a
size effect. It is known to structures become more
brittle as their size increases, but the classical
procedure uses working stresses which are the same
in design. Size effects occur in concrete in any
loading conditions like tensile or shear loading
Kani was one of the first to demonstrate the size
effect in concrete structures. He observed the shear
strength of similar concrete beams decreases with
increasing beam depth. Due to the fracture in a
structural element being driven by stored elastic
energy released from the whole structure, this size
effect can be well interpreted by fracture mechanics.
The fact that the strength of brittle materials is
affected by the presence of imperfections is first
suggested by Griffith, who is the founder of linear
elastic fracture mechanics (LEFM). Due to his
conclusion, it can be expected that the value of the
ultimate strength will depend upon the size of
specimens. As specimen size increases the strength
is expected to be decreased since the probability of
presence of weak links is increased. Traditionally,
the size effect in fracture of concrete structural
elements has been explained as Weibull’s theory. He
showed that if tensile tests are performed on two
geometrically similar specimens with different
volumes, the corresponding ultimate strengths are
different. Weibull’s approach has been widely used
for estimating safety factor of materials. In the early
1980s, it is realized that neither LEFM nor
Weibull’s approach were adequate for predicting
size effect in cementitious materials.
The experimental investigations on fracture
mechanics of cement-based materials until 1970s
indicated that classical linear elastic fracture
mechanics (LEFM) is invalid for quasi-brittle
materials such as concrete. This inapplicability of
LEFM is due to existence of an inelastic zone with
large scale and full cracks in front of the main crack
tip in concrete. This so-called fracture process zone
(FPZ) is ignored by LEFM. Consequently, several
investigators have developed non-linear fracture
mechanics approaches to describe failure of
concrete/reinforced concrete structures. eterministic
size effect laws among these non-linear approaches,
for instance size effect law (SEL) by Bazant (1984),
suggest that size effect on strength is primarily
related to a relatively large FPZ in concrete. One of
the main requirements in this law is the need to test
samples, which are geometrically similar and made
of the same material, and which must provide a
minimum size range=1:4.
The split-tension test has been used to indirectly test
the tensile strength of quasi-brittle materials such as
concrete and rock. Recently, concrete splitting
specimens have been commonly used in concrete
fracture because they have certain advantages, such
as compactness and lightness, compared to beams.
Additionally, cubical and cylindrical test specimens
have the following advantages:
a) These specimens are easy to handle, and there is
no risk of breaking them during handling.
b) The same molds can be used to cast specimens for
both fracture and strength tests.
c) In determining the fracture parameters of cementbased
materials, the contribution of the weight of the
specimen can be ignored, unlike notched beams.
In experimental studies, it was determined that
maximum aggregate diameter and aggregate type
are effective in fracture behavior of concrete. In the
samples without notch subjected to splitting tensile
test, it has seen that as the sample diameter
increases, the nominal strengths decreases and there
is a very strong size effect. In addition, how it will
behave in the actual structure size still remains
uncertain theoretically. In size effect studies, β =
(t/d) is the ratio of the distributed load width to
specimen depth not taken into consideration up to
now in splitting tensile test, although its importance
has been emphasized several times in scientific
studies in the literature. was taken as 0.10 and
0.16 in this study. When the test results of the cube
sample without notch produced by gap-graded
aggregate granulometry and analysis results
produced by continuous aggregate granulometry
taken from the literature were compared, it has been
observed in this study that the mechanical behaviors
of the mixtures prepared by gap-graded aggregate
granulometry are better than continuous aggregate
granulometry particularly in terms of ductility.
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Abstract (Original Language):
Beton ve betonarme yapılarda göçme analizi yapılırken birçok lineer ve lineer olmayan yaklaşımlar
kullanılmaktadır. Betonarme bir yapıyı kırılma mekaniğine göre analiz edebilmek için ilk önce, kullanılan
malzemenin kırılma parametrelerinin belirlenmesi gerekir. Betondaki neme ve zamana bağlı olarak mekanik
sabitlerin değişmesi ve boyut etkisinden dolayı, Lineer Elastik Kırılma Mekaniği (LEKM), kırılma
parametrelerinin tespitinde yetersiz kalmıştır. Bunun nedeni çatlağın ucunda yer alan diğer malzemelere
göre daha büyük bir yer işgal eden kırılma süreci bölgesinin (KSB) var olmasıdır. Betonun kırılma
parametrelerini belirlemek için şartnameler ve araştırmacılar tarafından birçok lineer olmayan kırılma
mekaniği modelleri önerilmektedir. Bu lineer olmayan yaklaşımlardan Bazant (1984) boyutun dayanım
üzerindeki etkisini inceleyerek betonda daha büyük bir KSB olduğunu öne sürmüş ve ‘Boyut Etkisi Kanunu’
geliştirmiştir. Betonun kırılma mekaniğinde kiriş numuneler yaygın olarak kullanılmakla birlikte,
taşınabilirlik ve hafiflik açısından bazı avantajlara sahip olduğundan küp ve silindir numuneler son
zamanlarda kullanılmaya başlanmıştır. Bununla birlikte karşılıklı basınç kuvveti uygulanan küp yarma testi
ile ilgili deneysel ve teorik çalışmalar oldukça sınırlıdır. Sunulan bu çalışmada, su/çimento oranı 0.6 olan,
maksimum agrega çapı 8 mm'lik kesikli ve sürekli granülometrili beton karışımları hazırlanmıştır. Aynı
geometriye sahip farklı boyutlu (4:2:1) küp numuneler üzerinde boyut etkisi incelenmiştir. 50x50x50 mm3,
100x100x100 mm3, 200x200x200 mm3’lük küp numunelerin 28 günlük yarmada-çekme dayanımları
belirlenmiş, elde edilen deney sonuçları Bazant’ın “Boyut Etkisi Kuralı”na göre analiz edilmiştir. Sonuç
olarak kesikli granülometriye sahip betonların daha sünek davrandığı tespit edilmiştir.
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