From a material viewpoint, modern concrete's frequent propensity to plastic shrinkage cracking can be attributable to a combination of low water-binder ratio use and ever-changing properties of binding materials. To obtain a better understanding of this phenomenon, this paper explores the effects of cement fineness and alkali content on the plastic shrinkage cracking of concrete manufactured with two water-binder ratios. Results indicate that within the range 275-385m 2 /kg, cement specific surface area is approximately directly and inversely proportional to hydration rate and evaporation rate respectively; a trend generally leading to higher plastic shrinkage and resulting areas of plastic cracking. Similar effects were observed for alkali contents which resulted in increased levels of plastic shrinkage. Furthermore, while decreasing crack tendency was noted as alkali content increased from 0.4 to 0.8% by mass of cement, further increases in alkali content caused significant decreases of compressive strength and slump; thereby lowering overall concrete performance. It is also found that plastic shrinkage cracking is closely related to the kinetics of plastic shrinkage. In summary, the experimental programme confirmed that cement with relatively low surface area (less than 340 m 2 /kg) and low alkali content (less than 0.8%) is preferred for modern concretes with minimal plastic cracking problems.
Abstract-This paper studies a novel user cooperation method in a wireless powered communication network (WPCN), where a pair of distributed terminal users first harvest wireless energy broadcasted by one energy node (EN) and then use the harvested energy to transmit information cooperatively to a destination node (DN). In particular, the two cooperating users exchange their independent information with each other to form a virtual antenna array and transmit jointly to the DN. By allowing each user to allocate part of its harvested energy to transmit the other's information, the proposed cooperation can effectively mitigate the user unfairness problem in WPCNs, where a user may suffer from very low data rate due to the poor energy harvesting performance and high data transmission consumptions. We derive the maximum common throughput achieved by the cooperation scheme through optimizing the time allocation on wireless energy transfer, user message exchange, and joint information transmissions. Through comparing with some representative benchmark schemes, our results demonstrate the effectiveness of the proposed user cooperation in enhancing the throughput performance under different setups.
Abstract. In this paper, we consider a wireless powered communication network (WPCN) consisting of a multi-antenna hybrid access point (HAP) that transfers wireless energy in the downlink to wireless devices (WDs), and receives the information transmissions from the WDs in the uplink. To enhance the throughput performance of some far-away WDs, we allow one of the WDs acting as the cluster head that helps relay the information transmitted by the other cluster members. This cluster-based cooperation can effectively enhance the throughput performance of some far-away WDs. However, its performance is also limited by the high energy consumption of the cluster head. To tackle this energy imbalance issue, we propose in this paper to exploit the capability of multi-antenna energy beamforming technique at the HAP, which focuses more transferred power to the cluster head to balance the energy consumption of all the WDs. Specifically, we derive the throughput performance of the proposed scheme and demonstrate through simulations that the proposed multi-antenna enabled cluster-based cooperation can effectively improve the throughput fairness performance of the WPCNs.
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