On the Semantics of Snapshot Isolation

Snapshot isolation (SI) is a standard transactional consistency model used in databases, distributed systems and software transactional memory (STM).It is formally defined both declaratively as an acyclicity axiom, and operationally as a concurrent algorithm with memory bearing timestamps.In this paper, we develop two simpler equivalent operational definitions of SI as lock-based reference implementations that do not use timestamps.Our first locking implementation is prescient in that requires a priori knowledge of the data accessed by a transaction and carries out transactional writes eagerly (in-place).Our second implementation is non-prescient and performs transactional writes lazily by recording them in a local log and propagating them to memory at commit time.Whilst our first implementation is simpler and may be better suited for developing a program logic for SI transactions, our second implementation is more practical due to its non-prescience.We show that both implementations are sound and complete against the declarative SI specification and thus yield equivalent operational definitions for SI.We further consider, for the first time formally, the use of SI in a context with racy non-transactional accesses, as can arise in STM implementations of SI.We introduce robust snapshot isolation (RSI), an adaptation of SI with similar semantics and guarantees in this mixed setting.We present a declarative specification of RSI as an acyclicity axiom and analogously develop two operational models as lock-based reference implementations (one eager, one lazy). We show that these operational models are both sound and complete against the declarative RSI model.

On the Shape Simulation of Aggregate and Cement Particles in a DEM System

Aggregate occupies at least three-quarters of the volume of concrete, so its impact on concrete’s properties is significant. Both size and shape of aggregate influence workability, mechanical properties, and durability of concrete. On the other hand, the shape of cement particles plays also an important role in the hydration process due to surface dissolution in the hardening process. Additionally, grain dispersion, shape, and size govern the pore percolation process that is of crucial importance for concrete durability. Discrete element modeling (DEM) is commonly employed for simulation of concrete structure. To be able to do so, the assessed grain shape should be implemented. The approaches for aggregate and cement structure simulation by a concurrent algorithm-based DEM system are discussed in this paper. Both aggregate and cement grains were experimentally analyzed by X-ray tomography method recently. The results provide a real experimental database, for example, surface area versus volume distribution, for simulation of particles in concrete technology. Optimum solutions are obtained by different simplified shapes proposed for aggregate and cement, respectively. In this way, more reliable concepts for aggregate structure and fresh cement paste can be simulated by a DEM system.

POROSIMETRY BY RANDOM NODE STRUCTURING IN VIRTUAL CONCRETE

Two different porosimetry methods are presented in two successive papers. Inspiration for the development came from the rapidly-exploring random tree (RRT) approach used in robotics. The novel methods are applied to virtual cementitious materials produced by a modern concurrent algorithm-based discrete element modeling system, HADES. This would render possible realistically simulating all aspects of particulate matter that influence structure-sensitive features of the pore network structure in maturing concrete, namely size, shape and dispersion of the aggregate and cement particles. Pore space is a complex tortuous entity. Practical methods conventionally applied for assessment of pore size distribution may fail or present biased information. Among them, mercury intrusion porosimetry and 2D quantitative image analysis are popular. The mathematical morphology operator “opening” can be applied to sections and even provide 3D information on pore size distribution, provided isotropy is guaranteed. However, aggregate grain surfaces lead to anisotropy in porosity. The presented methods allow exploration of pore space in the virtual material, after which pore size distribution is derived from star volume measurements. In addition to size of pores their continuity is of crucial importance for durability estimation. Double-random multiple tree structuring (DRaMuTS), introduced earlier in IA&S (Stroeven et al., 2011b) and random node structuring (RaNoS) provide such information.

Strength and Durability Evaluation by DEM Approach of Green Concrete Based on Gap-Graded Cement Blending

This paper discusses the particle packing background of cementitious materials. On micro-level the Portland cement and eventually the mineral admixture grains can be considered packed in the watery environment. Particularly for (super) high performance materials, the packing density can be quite significant. An economic and due to fast computer developments reliable way to study packing of the binder, is by modern discrete element modeling (DEM) approach. In this paper use is made of a concurrent algorithm-based dynamic system, HADES. Hydration is simulated based on spherical grains. Thereupon strength can be studied on the basis of packing density. For durability issues, the complex and tortuous 3D pore structure has to be investigated. This paper uses for the assessment of pore characteristics the robotics-inspired DraMuTS system. Hydrated Portland cement is compared with gap-graded rice husk ash-(RHA)-blended (green) Portland cement. Experiments on gap-graded RHA-blended PC concrete are used as reference. Packing density is shown improved by gap-graded packing. What is more spectacular are the effects of gap-grading with RHA on the pore characteristics obtained on the DEM-produced virtual materials. This paper discusses the expected positive effects on transport-based durability issues due to gap-graded packing-induced changes in the pore system