International Concrete Abstracts Portal

The objectives of the studies presented in this paper were to investigate the effects caused by steel reinforcing bars on determining the depth of cracks which appear on the surface of a concrete structure. Numerical studies were performed to investigate the interaction of stress waves with reinforcing bars. A full scale reinforced concrete beam was constructed as an experimental specimen. The specimen was loaded until cracks occurred. To measure the depth of a crack penetrating into the specimens, two receivers were used and located on the opposite sides of the crack. The first receiver on the impact side is used to obtain the time of impact initiation. The second receiver located on the other side of the crack is used to trace the arrival of the P-wave diffracted from the bottom edge of the crack. Subsequently, the depth of the crack can be determined. Experimental results show that if tests are performed on the regions without reinforcing bars, the crack depth can be obtained easily because the second receiver initially responds to the arrival of the diffracted P-wave. In the presence of reinforcing bars, the initial disturbance at the second receiver is caused by the arrival of P-wave propagating through reinforcing bars but its amplitude is much smaller than that associated with the following arrival of the P-wave diffracted from the bottom edge of the crack. It is concluded that the presence of reinforcing bars does not make it difficult to measure the crack depth.

Strengthening of existing reinforced concrete beams is commonly carried-out by “jacketing”. This paper describes an experimental investigation into the behaviour of reinforced concrete beams strengthened by jacketing. Static and cyclic load tests to failure were carried-out on 12 reinforced concrete T-beams. The main variables examined were the degree of roughening of the surface of the original concrete, and the anchorage of the additional transverse reinforcement. The additional concrete was preplaced aggregate concrete. The strength of the bond between preplaced aggregate concrete and plain concrete was assessed by 39 slant shear tests, and a Mohr-Coulomb type failure envelope was derived. Static failure of the beam specimens was related to this failure envelope. The experimental results allowed conclusions to be reached on: the use of the slant shear test to predict the behaviour of full scale beams; the use of the Compressive Force Path Concept to enable reliable and economical strengthening of reinforced concrete beams; and the effect of cyclic loading on the ultimate load carrying capacity of jacketed beams.

Fifteen high-strength concrete beams were tested under concentrated load at two-point to investigate their ductile behavior including flexural strength with various compressive stress distribution. And six high strength concrete beams were tested to investigate the minimum reinforcement ratios requirements for high strength concrete. Thus major test variables were the compressive strengthes of concrete and tensile steel ratios. The test results were compared with flexural strength predicted by ACI 318-95. It was found that high strength concrete beams with maximum reinforcement ratio of ACI 318-95 accompany brittle failure and that compressive stress of high strength concrete beams is distributed linearly than that of normal strength concrete beams. New maximum reinforcement ratio is proposed for the ductile behavior of high strength concrete beams.

Applications such as soil, rock, and oil-well grouting all require enormous amounts of cement and are therefore good examples of areas where high volumes of ground slag or pozzolans such as fly ash could partially replace cement to produce low-cost, environmentally safe and durable grouts. This paper presents the results of a detailed study of the grain size distribution, three rheological properties, and five mechanical properties of high-volume fly-ash grout (containing over 55% fly ash) with and without high-range water reducing admixtures (HRWRA) and/or anti-washout agents (AWA). The rheological properties are reported for eight water-solids ratios (W/S). The effects of HRWRA and AWA on the flow time of low W/S grouts and the stability of high W/S grouts are investigated for grouting. The results indicate that the addition of fly ash in cement grouts reduces the flow time, improves stability, reduces drying-shrinkage, attains similar or closer compressive and bond strengths as pure cement grouts at later ages. Moreover, when HRWRA is used for low W/S grouts, the latter destabilize (produces three layers of unknown composition) and in those cases AWA should be used.

In this paper, a technic of both adding ultra-fine fly ash(UFFA) and silica fume(SF) for preparing high performance concrete(HPC) of C150~200 is presented. Under the condition of replacing 15% cement with UFFA or SF or their composite respectively, the strength characters of HPC with the same proportion of both UFFA and SF were systematically studied by comparing adding-both UFFA and SF sample(HPFASFC--- high performance’ fly ash and silica fume concrete) with adding only UFFA sample(HPFAC---high performence fly ash concrete) and adding only SF sample(HPSFC---high performence silica fume concrete). The experimental results showed that the strength of HPFASFC may be higher than that of HPSFC or HPFAC because of existence of effects of promoting and making up each other in strengthening of mineral admixture. The results of micro tests and analysis for the mechanism of composite effects proved the conclusion. It follows that there is a possibility for preparing high performance concrete with UFFA.