Polymer-modified Concrete with Improved Flexural Toughness and Mechanism Analysis

By selecting different types of polymer mixing into concrete, the toughness of concrete is investigated, and results indicate polymer has obvious effect to improve the toughness of concrete. Microstructure of polymer-modified concrete were studied through environment scanning electron microscope and digital micro-hardness tester, results show that polymer acts as a flexible filler and reinforcement in concrete, and alters the microstructure at mortar and ITZ. By crack path prediction and energy consumption analysis, the crack path of polymer-modified concrete is more tortuous and consumes more energy than that of ordinary concrete.

Mechanics Behavior of Ultra High Toughness Cementitious Composites after Freezing and Thawing

Mechanical behaviors of UHTCC after freezing and thawing were investigated,and compared with those of steel fiber reinforced concrete（SFRC）,air-entrained concrete（AEC） and ordinary concrete（OC）.Four point bending tests had been applied after different freezing-thawing cycles（0,50,100,150,200 and 300 cycles,respectively）.The results showed that residual flexural strength of UHTCC after 300 freezing-thawing cycles was 10.62 MPa（70% of no freezing thawing ones）,while 1.58 MPa（17% of no freezing thawing ones） for SFRC.Flexural toughness of UHTCC decreased by 17%,while 70% for SFRC comparatively.It has been demonstrated experimentally that UHTCC without any air-entraining agent could resist freezing-thawing and retain its high toughness characteristic in cold environment.Consequently,UHTCC could be put into practice for new-built or retrofit of infrastructures in cold regions.

Spalling and Mechanical Properties of Fiber Reinforced High-performance Concrete Subjected to Fire

Spalling and mechanical properties of FRHPC subjected to fire were tested on notched beams. The results confirm that the internal vapor pressure is the leading reason for spalling of high-performance concrete （HPC）. At the same time, the temperature-increasing velocity and constrained conditions of concrete element also play significant roles in spalling. Steel fibers cannot reduce the risk of spalling, although they have obvious beneficial effects on the mechanical properties of concrete before and after exposure to fire. Polypropylene （PP） fibers are very useful in preventing HPC from spalling, however, they have negative effects on the strengths. By using hybrid fibers （steel fibers＋PP fibers）, both good anti-spalling performance and improved mechanical properties come true, which may provide necessary safe guarantee for the rescue work and structure repair after fire disaster.

Mechanical Properties of Layered Hybrid Fiber Reinforced Concrete

To improve the mechanical properties of concrete,Layered Hybrid Fiber Reinforced Concrete (LHFRC) was developed in this paper.Through comparative tests,the effects of layered hybrid fibers on a series of mechanical properties of concrete were discussed.The mechanical properties include compressive strength,tensile strength,flexural strength,compressive stress-strain relationship,flexural toughness and cracking resistance of concrete.The testing results and analysis demonstrate that layered hybrid fibers can significantly improve the flexural strength,toughness and cracking resistance of concrete while the cost of concrete increases slightly.

Effects of fibers on mechanical properties of high-performance concrete subjected to elevated temperatures

The compressive strength and ilexural toughness as well as fracture energy of fiber reinforced highperformance concrete （FRHPC） subjected to different high temperatures were studied. The results showed that after exposure at 300,600 and 900℃, the concrete mixes retained 88.1% , 41.3% and 10.2% of the original compressive strength on average, respectively. Steel fiber and polypropylene （PP） fiber were both effective in minimizing the damage effect of high temperatures on the compressive strength. The HPC reinforced with steel fibers showed higher flexural toughness and fracture energy before and after the high-temperature exposures. In comparison, PP fibers had minor beneficial effects on the flexural toughness and fracture energy. The mechanical properties of HPC reinforced with hybrid fibers （steel fiber ＋ PP fiber） were equivalent to or better than those of HPC reinforced with steel fibers alone. In addition, the failure pattern of FRHPC beams changed from pull-out of steel fibers at lower temperatures （20, 300 and 600℃） to tensile failure of steel fibers at higher temperature （900 ℃）.

Effect of composition on flexural behavior of engineered cementitious composites

In this paper, four point bending tests were carried out to investigate the flexural properties of PVA fiber reinforced engineered cementitious composites (ECC) with different mix proportions. Based on the test results, the flexural toughness was evaluated with the methods of JSCE and post crack strength method (PCSm), respectively. Several parameters such as amount of water reducer, amount of sand, and fiber volume fraction were investigated to study their effects on the flexural toughness of ECC beams. According to the test results, superfluous water reducing additions can cause adverse effect on strength of the matrix and interfacial bond between fibers and the matrix, resulting in decreased bending strength and flexural toughness of ECC beams. Increase of the fiber volume fraction can result in increased flexural strength and toughness due to enhanced bridging effect between fibers and cementitious matrix. High amount of sand can reduce ductility and strain hardening behavior of ECC material, and better flexural toughness can be achieved when the amount of sand by weight is set to 0.2 for current water/cement ratio.