Papers by Keyword: Expansive Agent

Abstract: The shrinkage of concrete is one of the main causes of cavity in concrete filled steel tube (CFST). In this paper, the effect of a kind of lab-made expansive agent on the deformation of CFST is studied. The laser displacement sensor combined with the round straight steel tube mould was adopted to test the deformation of concrete from the initial setting to 1 day after pouring, and the mechanical micrometer method was used to test deformation after 1 day in the laboratory. The full-scale model test was carried out to monitor the effect of expansive agent in CFST at the project site. The results show that, the addition of expansive agent produced significant expansion in the first 3 days, with the expansion basically compensating the autogenous shrinkage of the concrete; influenced by thermal deformation, the constraint action of steel tube to the core concrete in temperature rise stage was far greater than that in cooling stage. Compared with expansion produced in temperature rise stage, it is more important to produce expansion in cooling stage to solve the cavity problem of CFST.

Abstract: Pre-cast concrete products are sometimes manufactured in two cycles daily. It is ensured that they have the required strength at an early stage of demolding by increasing the steam curing temperature to reduce cost or increase productivity. However, the reduction in durability because of cracking due to thermal stress is a topic of concern. On the other hand, it has been known that fine blast-furnace-slag powder and expansive agents show high temperature dependence. Although they are used even in precast concrete products, the effect of steam curing on these materials is not known. Thus, in this study, the expression of compressive strength and expansion that are critical in improving the quality and productivity of precast concrete products was investigated, using high early-strength cement and two types of expansive agents, namely, ettringite-based and lime-based agents. Consequently, the strength expression was accelerated by the addition of expansive agent to ordinary cement and high early-strength cement. The extent of restrained expansion is greater for the lime-based expansive agent than for the ettringite-based expansive agent, and when an expansive agent is added to high early-strength cement, the extent of restrained expansion falls to a level lower than that in the case of ordinary cement.

Abstract: UltraHigh Performance Fiber Reinforced Concrete (UHPFRC) exhibits remarkable mechanical performance, which can allow to reduce the cross-section of structural members. However,a problem involving UHPFRC isthe likely tendency to crack at early age, due to autogenous and plastic shrinkages, caused by the very low water-to-binder ratio adopted. Therefore, this experimental work intends to detect the effectiveness of a possible solution for reducing the risk of shrinkage cracks in UHPFRC, by adding to the mixture a suitable combination of expansive and shrinkage reducing agents.Compressionand bending testswere carried out up to28 days of curing. Free drying shrinkage strains were evaluated up to 56 days of exposure to 50% relative humidity. The experimental results obtained by using expansive and shrinkage reducing agents were extremely encouraging in termsof free dryingshrinkage reduction, and even surprising in terms of flexural behaviour.

Abstract: The compensation effect of expansive agent on shrinkage of self-compacting concrete (SCC) are studied. The deformation of Concrete was tested by concrete comparator, and the qualitative and quantitative analysis of hydration products were done by X-ray diffractometer (XRD). The results show that the dosages of mineral admixture and curing conditions affect the application results of expansive agents markedly. SCC samples containing expansive agent cured in the water are expanding within the test time. In the case of watering condition and natural curing condition, the samples are always shrinkage within test time, but mineral admixtures have a good effect to resist the shrinkage of SCC. Microstructure analysis show that ettringite content is reduced slightly due to the reduction of the alkalinity of SCC resulted by more mineral admixture. However, when at water curing condition, humidity inside SCC is much higher, which makes ettringite content larger even though with more mineral admixture. So, curing is the most important factors for the micro-expansion of SCC containing expansive agent.

Abstract: Measuring techniques for vertical deformation and horizontal deformation of concrete were introduced in the paper. Based on these techniques, concrete deformation about reference concrete and shrinkage-compensating concrete were studied. The results show that workability of concrete was almost not affected by expansive agent. Usually deformation testing began when the mold has been removed 24 hours after casting according to present standard, but the reaserch shows that the time of starting measurement has a little influence on deformation of reference concrete, while its vastly affected on those of shrinkage-compensating concrete with expansive agent. Thus, its essential to judge time-zero accurately for shrinkage-compensating concrete. Before final set, concrete is on the plastic stage when the concrete has no enough stiffness to store expansive energy, so the measurement results should be zeroed to the time of final set especially for concrete with expansive to evaluate expansive validity better.

Abstract: The expansion of cement mortar mixing expansive agent under standard curing and membrane curing conditions and compressive strength in constrained conditions are studied in this paper. Results show that the early expansion effect of calcium sulphoaluminate hydrate-calcium hydroxide expansive agent is obvious, mid-to late effect of magnesia expansive agent increase clearly. Compare with standard curing, the shrinkage of the basic cement mortar increase and expansion effect of expansive agent reduce under membrane curing. The expansion effect of expansive agent B is good and persistent under membrane curing, its applicable to used in the closed environment, such as concrete filled steel tubes. The compressive strength of mortar with expansion agent is a little more than the basic mortar under the condition of limit.

Abstract: norganic polymer concrete of a new environment-friendly material has been the hot issue in engineering research so far. For this new material, the main job of the paper includes: we design mixture ratios of inorganic polymer concrete by ourselves, testing the concrete in the age periods of 3, 7, 28 d. The results indicate that this kind of material strength develops mainly in the first 3 d, the strength grows slowly in the later stage; the dry shrinkage of the configured concrete properties are measured, the curve of dry shrinkage shows that the dry shrinkage occurs mainly in the first 14 d and develops slowly in the late; And measure the expansion performance of the concrete member mixed the different categories expansive agent, the results show that the volume growth of mortar specimens to join HCSA expansion agent are obvious; Test results provide a certain basis for the inorganic polymer concrete of micro expansion.

Abstract: Portland cement, silica fume, superplasticizer and fly ash cemamsite were used to obtain a high strength lightweight aggregate concrete with lower water to binder ratio. An enhancement effect on the strength was gained by adding steel fiber. When the expansive agent added with the steel fiber a synergistic reinforcing effect on the compressive strength, tensile strength and bending strength was obtained because of more energy consumed to pull the fibers out under the different loads.

Abstract: Concrete deformation measured under a certain condition in the experiment are very different from those in practical engineering structures, therefore, its necessary to monitor concrete properties in structures. In the paper, by real-time continuous monitoring of reinforcement concrete slab prepared with expansive agent, time-dependent temperature variations and strain variations are obtained and analyzed, and indoor test of the mechanical properties and deformation of field sampling from the slab are presented, so that this completes a transition from the specimen in experiment to the structure in engineering. The result shows that: there was no early-age crack in the four month after casting; the temperature had a certain increase due to heat generation of cement, which can lead to shrinkage deformation in cooling period; the temperature fluctuate with ambient temperature and there are obvious peak temperature rise about 15°C; There was a certain expansion at heating period, which can be considered as compensating shrinkage caused at late age in a certain extent so that minimum strain value is just about-120με which are not beyond the ultimate tensile strain. In addition, the measured experimental data can provide helpful reference for early crack control and durability of concrete structure, and they can also be use to verify and improve the accuracy of the numerical results for slab, which is available in the future for basis to similar projects and research.

Abstract: Blended expansive agents consisting of MgO and CaO were prepared by calcining the mixtures of dolomite and magnesite. The mineral phases and microstructures of expansive agent were examined by XRD and SEM. The expansion properties of cement pastes containing 5% and 6% of expansive agent as well as the microstructure of hydrated expansive agent in cement paste were investigated. Results indicated that the contents of MgO and CaO in the blended expansive agent could be adjusted by changing the mix proportion of dolomite and magnesite. All the cement pastes containing expansive agent produced rapid expansion. At the same addition dosage, irrespective of curing temperature, expansive agent containing higher content of MgO produced greater expansion in cement pastes, particularly at late age, which probably ascribes to the relatively slow hydration of MgO.