Autogenous deformation and fracture properties of cementitious materials: Effect of size and type of aggregates

Çimentolu malzemelerin otojen deformasyonu ve kırılma parametreleri: Agrega tür ve boyutunun etkisi

Abstract

Since high strength/high performance concretes (HSC/HPCs) are produced using rich mixes with high cement contents and low water/cement ratios, they are denser and more homogeneous than normal strength concretes. Low porosity and non-connected capillary pore structures are the most important properties of these concretes. There is an insufficient amount of water in concrete to complete the hydration process, and also the water income is prevented because of the impermeable character of concrete. Concurrent with the production of hydrates, after de-moulding (also before) time dependent volume changes of cement paste occur. Development of the empty pores due to chemical shrinkage is the main reason for diminishing the radii of menisci. This decrease causes self-desiccation by increasing capillary stress in the pore water. The absorption of free water into pores by the hydrates and formation of fine pores is driven by the chemical shrinkage. At later ages, however, self desiccation becomes the major drive mechanism. The development of empty pores continues during self-desiccation and it affects the hydration kinetics. This effect limits the ultimate degree of hydration and consequently the strength of cement paste. Since the tensile strain capacity of concrete at early ages is not sufficient enough to resist stresses caused by the external and internal restraints, autogenous deformation leads to the formation of cracks. These early age cracks may induce durability problems. Conventional curing techniques are not effective in mitigating this decrease of relative humidity (RH) in the hardened cement paste. Using pre-soaked fine lightweight aggregate (LWA) for preventing the decrease of RH in the hardened cement paste matrix was first recommended by Philleo (1991). It is known that the bond between lightweight aggregate and matrix is stronger than that of normal aggregate. This enhanced interfacial zone prevents the length changes developed under RH and/or thermal conditions. In application, concrete is usually designed on the basis of compressive strength theory, in which the brittleness of concrete is ignored. For this reason, fracture parameters of concrete have to be defined along with its brittleness. Because the LWAs produce more microcracks, the concrete containing these aggregates behaves more ductile (i.e., less brittle) than normal cementitious materials. On the other hand, the use of LWA instead of dense aggregate is expected to result in a lower strength. It is obvious that the amount of LWA for internal curing must be optimized according to the mechanical and fracture properties of cementitious materials. This study presents the results of experimental studies conducted for determining the restraining effect of both LWAs and normal weight aggregates (NWAs) on autogenous deformation and fracture properties of composites containing 70% paste and 30% aggregate by volume. It has been shown that the size of NWAs has no significant effect on autogenous deformation at 28 days, while the autogenous deformation of cement paste with 0-2 mm size fraction of NWA has been found higher than that with 2-4 mm and 4-8 mm at the age of 360 days. At early ages, the finer NWAs (0-2 mm) restrained the autogenous deformation of cement paste, because the modulus of elasticity of cement paste was not too high. On the other hand, addition of coarser size NWAs easily restrained the shrinkage of cement paste at later ages. The fine size LWAs, as expected, has been found to be more effective in mitigating the autogenous deformation of cement paste than coarse LWAs. The addition of LWAs to paste phases increased the degree of hydration, especially at early ages. The use of fine LWAs causes the water reservoirs to be distributed in proximity and results in a better internal curing, hence the degree of hydration of cement paste with finer LWAs is greater compared to that with coarser LWAs. While the compressive strength of composites slightly increases with the increasing size of NWA, the size of LWAs has no significant effect on compressive strength. Also, the moduli of elasticity along with the bending and splitting tensile strengths were not affected significantly by the size of aggregates (both NWA and LWA). Addition of fine sand aggregate at the size fraction of 0-2 mm increased the characteristic length significantly compared to the cement paste. Although the fracture energies of cement pastes with LWAs were found smaller than that of reference cement paste the addition of LWAs caused an increase in the characteristic length of cement paste.

Özet

Yüksek performanslı çimentolu malzemelerde erken yaşlarda oluşan otojen deformasyonu önlemek için önceden suya doyurulmuş hafif agregalar su rezervuarı olarak kullanılabilir. Sabit bir düşük su/çimento oranına sahip silis dumanı katkılı çimento hamurunun otojen deformasyonunu önlemek için, hacimce %30’u normal veya hafif agrega ile yer değiştirildi. Kullanılan agrega cins ve tane boyutunun oluşturulan kompozitlerin kırılma enerjisi, karakteristik boy ve kiriş açıklık ortası en son sehime olan etkileri incelendi. Sonuçlar, normal agregaların rijitliğinin çimento hamuruna göre yüksek olması sayesinde otojen deformasyonu bir miktar kısıtlarken, hafif agregaların çimento hamuruna su sürükleyerek otojen deformasyonu önlediğini gösterdi. Çimento hamuruna hafif agrega ilave edilmesiyle hidratasyon derecesi özellikle erken yaşlarda arttı. İnce hafif agrega ilavesiyle su rezervuarları hamur fazı içerisinde daha yakın dağılarak iyi bir içsel kürleme sağladı. Böylece, ince hafif agrega ile çimento hamurunun hidratasyon derecesi iri hafif agrega ilavesine göre daha yüksek bulunmuştur. Aynı zamanda içsel kürlemenin çimento hamurunun hidratasyon derecesini de artırdığı bulundu. Ayrıca, ince hafif agrega kullanımının iri hafif agregaya göre belirgin şekilde çimento hamurunun otojen deformasyonunu azalttığı belirlendi. İnce hafif agregalı kompozitlerin kırılma enerjisi, kirişin açıklık ortasındaki son sehim değeri, yarılmada çekme dayanımı ve basınç dayanımının iri hafif agregalı kompozitlere kıyasla daha yüksek olduğu görüldü. Normal agrega kullanımı ile kompozitlerin kırılma ve mekanik özelikleri artarken hafif agrega kullanımı ile azalmalar görüldü. Diğer yandan hafif agregalı kompozitlerin sünekliği çimento hamuruna göre daha yüksek bulundu.