An essential step toward sustainability is evaluating how a given product interfaces with adjacent products and what affect this has on the performance of the combined material. In this case how does the interface between ESCS lightweight aggregate and the hydrating cementitious mortar matrix affect the performance of structural concrete?
Internal Curing: With lightweight concrete the cementitious hydration is enhanced due to the process of internal curing. Time-dependent improvement in the quality of concrete containing lightweight aggregate is greater than that with normalweight aggregate. This is due to better hydration of the cementitious fraction provided by moisture available from the slowly released reservoir of water absorbed within the pores of the lightweight aggregate. This process of internal curing is made possible when the moisture content of the aggregate, at the time of mixing, is in excess of that achieved in 1-day submersion. Campbell and Tobin first documented this fact in 1967. Their tests confirmed that availability of absorbed moisture within the lightweight aggregate produced a more forgiving concrete that was less sensitive to poor field-curing conditions.
High cementitious concrete is vulnerable to self-desiccation and benefits significantly from the added internal moisture. This application is especially helpful for concrete containing high volumes of pozzolans that are sensitive to curing procedures. While improvements in long-term strength gain have been observed, the principal contribution of internal curing rests in the reduction of permeability that develops from a significant extension in the time of curing. In 1959 Powers et al., showed that extending the time of curing increased the volume of cementitious products formed, which caused the capillaries to become segmented and discontinuous.
For additional information on Internal Curing see ESCSI publication 4362.1, Internal Curing: Helping Concrete Realize its Maximum Potential.
Cracking, Contact Zone, Elastic Compatibility, Permeability: Concrete failing prematurely should not be tolerated. Whether by micro-cracks or macro-cracks, a major source of failure is initiated at cracks. Therefore, mitigating cracking becomes an essential element in sustainability. Adding lightweight aggregate to concrete mitigates crack formation, as demonstrated in the following narrative. Core samples taken from hulls of 80-year-old lightweight concrete ships as well as 40 to 50-year-old lightweight concrete bridges reveal that the concrete has a dense contact zone at the lightweight aggregate/cement matrix interface. This zone has very low levels of microcracking throughout the cement mortar matrix (Sturm et al. 1999).
Explanation for this high resistance to weathering and corrosion involves several physical and chemical mechanisms including superior resistance to microcracking. This excellent performance is developed by the significantly higher aggregate/matrix adhesion (contact zone) and the reduction of internal stresses due to elastic matching of the lightweight aggregate and cementitious mortar matrix phases (Holm, Bremner, and Newman 1984). This elastic matching is present regardless of the lightweight aggregate size.
High ultimate strain capacity is also provided by lightweight concrete as it has a high strength/modulus ratio. The strain at which the disruptive dilation of concrete starts is higher for lightweight concrete than for equal-strength normalweight concrete. A well-dispersed pore system provided by the surface of the lightweight fine aggregates may also assist the air-entrainment system and serve an absorption function by reducing concentration levels of deleterious materials in the matrix phase.
Permeability investigations conducted on lightweight and normalweight concrete exposed to the same testing criteria has been reported by numerous researchers Khokrin (1973), Nishi et al. (1980), Keeton (1970), Bamforth (1987), Bremner et al. (1992). It is of interest that, in every case, despite wide variations in concrete strengths, testing media (water, gas, and oil), and testing techniques (specimen size, media pressure, and equipment), lightweight concrete had equal or significantly lower permeability than its normalweight counterpart. Khokrin (1973) further reported that the lower permeability of lightweight concrete was attributed to the elastic compatibility of the constituents and the enhanced bond (improved contact zone) between the lightweight aggregate and the mortar matrix.
One principal difference between lightweight concrete and normalweight concrete is the development and positive influence of the contact zone. The contact zone in lightweight concrete is the interface between two porous media: the lightweight aggregate particle and the hydrating cementitious binder and has been demonstrated to be significantly superior to that of normalweight concrete. This improvement in the quality, integrity, and microstructure stems from a number of characteristics that are unique to the interface between lightweight aggregate and surrounding cementitious mortar matrix. These characteristics include but are not limited to the following:
The alumina and silicate rich pozzolanic surface of the fired ceramic ESCS aggregate combines with the Ca(OH2) liberated by hydration of the portland cement.
Reduced microcracking at the matrix lightweight aggregate interface because of the elastic similarity of the aggregate and the surrounding cementitious matrix.
The modulus of elasticity of concrete depends on the relative amounts of paste and aggregate and the modulus of each constituent. In normalweight concrete there is an elastic incompatibility between the higher moduli of sand, stone, and gravel and the surrounding cementitious matrix. In contrast the moduli of the lightweight aggregate particles is more closely matched to that of the matrix.
Essentially, a lower modulus of elasticity (Ec) value for lightweight concrete results in a reduced stiffness, as defined by the product of modulus of elasticity and moment of inertia (EI). Reduced stiffness can be beneficial at times in cases requiring improved flexural response, such as bridges, structures where differential settlement may occur, etc.
Hygrol equilibrium between the two porous phases: lightweight aggregate and a porous cementitious matrix is fundamentally different than the usual condition with dense aggregates, where bleed-water lenses form around the non-absorbent coarse natural aggregates that have a w/cm ratio significantly higher than the matrix. The accumulated water at the interface is subsequently lost during drying leaving voids and a weak low-quality aggregate/matrix interface (ACI 213R-03).
When pozzolans are added, the high-quality microstructure of the contact zone of concrete containing lightweight aggregate is moderately enhanced. In contrast, when high-quality pozzolans are used in concretes containing normalweight aggregates, this zone of weakness is significantly improved.