Determination of setting times by shear wave velocity evolution in fresh mortar using bender element method

This study aims at using modified bender element (BE) method to monitor the shear wave velocity (Vs) of mortar at early-age and determine initial and final setting times with the Vs method. Modifications of the traditional BE method have been made to overcome the aggressive cementitious environment and eliminate electromagnetic coupling. BE has been successfully employed in mortar samples with and without admixtures to evaluate the evolution of Vs during the early age (first 24 hrs) of hydration. An energy approach method of determining the first arrival time of S-wave has been proposed. Pulse wave velocity test, penetration resistance test and calorimetry test were carried out to comprehensively evaluate the setting process. The evolutions of Young’s modulus E, shear modulus G, bulk modulus K, and Poisson’s ratio ν with time were then determined based on Vs, Vp and r measurements. Log-normal distribution and soil-water characteristic curve (SWCC) were used to fit the Vs-time relationship. It was found that the time corresponding to the largest slope (i.e. maximum increasing rate) of the Vs curve corresponds to the final setting time. The initial setting time shows a linear relationship with the first inflection point and parameter a in SWCC equation. This proposed method of determining setting times with Vs had a high accuracy (R2=0.98). With the non-destructive nature and reliable results, bender element method of obtaining Vs and determining set times has a potential application in the cement industry

A Dynamic Principal Agent Problem with One-sided Commitment

In this paper we consider a principal agent problem with moral hazard, where the principal is committed to the contract but the agent can quit at a stopping time up to his choice. When the current agent quits the job, both the principal and the agent will incur a cost, and the principal will hire a new agent, possibly with a different type. Being aware of the agent's noncommitment, the principal's problem is to find the optimal contract satisfying the agent's (and all possible future agents') individual rationality. We characterize the principal's dynamic value function, which could be discontinuous on the boundary, as the (unique) minimal solution of an infinite dimensional system of HJB equations, parametrized by the agent's type. In particular, this dynamic problem is time consistent in certain sense.Some interesting findings are worth mentioning. First, self-enforcing contracts are typically suboptimal. The principal would rather let the agent quit and hire a new one. Next, the standard contract for committed agent may also be suboptimal. This is due to the presence of different types of agents in our model. In some markets, the principal may prefer no commitment from the agent so she can hire a cheaper agent from the market by designing the contract to induce the agent to quit. Moreover, due to the cost incurring to the agent, the principal will see only finitely many quittings.Comment: 48 page

The Implications of Diverse Applications and Scalable Data Sets in Benchmarking Big Data Systems

Now we live in an era of big data, and big data applications are becoming more and more pervasive. How to benchmark data center computer systems running big data applications (in short big data systems) is a hot topic. In this paper, we focus on measuring the performance impacts of diverse applications and scalable volumes of data sets on big data systems. For four typical data analysis applications---an important class of big data applications, we find two major results through experiments: first, the data scale has a significant impact on the performance of big data systems, so we must provide scalable volumes of data sets in big data benchmarks. Second, for the four applications, even all of them use the simple algorithms, the performance trends are different with increasing data scales, and hence we must consider not only variety of data sets but also variety of applications in benchmarking big data systems.Comment: 16 pages, 3 figure