P. F. Dux

Tensile Properties of Early-Age Concrete

In this paper, the tensile properties of concrete at very early ages and their measurements are reviewed, and the need for further study is clearly highlighted, A newly developed apparatus and procedures for uniaxial direct tensile testing of concrete specimens at ages of 1.5 hours or more after mixing, together with the experimental results obtained, are reported. An improved knowledge of various important early-age properties is presented. The tensile strength, Youngs modulus, and fracture energy of early-age concrete are all found to increase very slowly during the first 3 hours or so, but significantly increase thereafter. Strong correlations are shown to exist between these three parameters, especially between tensile strength and fracture energy. The high values of fracture energy obtained strongly suggest that early-age cracking of concrete involves a significant zone of plastic straining or microcracking in the vicinity of the crack tip. Early-age concrete is also shown to be more ductile than mature concrete.

Validation of Mualem’s Conductivity Model and Prediction of Saturated Permeability from Sorptivity

Advanced modeling of unsaturated water and contaminant transport in concrete requires knowledge of the unsaturated conductivity (permeability) function and the water retention curve. An assumption of simple Fickian diffusion is not sufficient due to the nonlinear water and contaminant fluxes. The Van Genuchten-Mualem conductivity model, widely used in describing hydraulic properties of soils, is validated by predicting experimental moisture content profiles obtained by nuclear magnetic resonance during simple absorption experiments with concrete cylinders. A new tortuosity parameter is suggested. Analytical methods are extended based on the Van Genuchten-Mualem model, to provide a means of estimating the saturated permeability from the much simpler sorptivity experiment. The predicted permeability is similar to the long-term experimental permeability. This suggests that only the very long-term saturated permeability is suitable for describing unsaturated moisture flux.

INELASTIC BEAM BUCKLING EXPERIMENTS

The paper describes a series of experiments on the buckling of simply supported laterally continuous I-beams in the inelastic range. Nine beams were tested in three groups of three, each group having a different predominant moment gradient. Points of load application were prevented from moving laterally and twisting. Measurements of geometrical and material imperfections are included in this report. Results of a number of subsidiary experiments such as plastic moment and stub column tests are also presented. The results add to the small amount of test data presently available on such beams and provide a comparison with theoretical predictions. In particular, support is shown for the theoretical curves of Nethercot and Trahair and for the current trend to multiple design curves based on moment gradient.

Performance of Permeability-Reducing Admixtures in Marine Concrete Structures

The use of permeability-reducing admixtures is a potential preventative of the chloride-induced corrosion of steel reinforcement, which is the main cause of the deterioration of concrete structures exposed to coastal environments. This paper presents an experimental investigation into the effectiveness of two typical commercially available permeability-reducing admixtures: one characterized by crystallization activity and the other by hydrophobic and pore-blocking effects. Concrete specimens were exposed to simulated coastal environments, and chloride concentration profiles at 28-, 365-, and 730-day exposures were determined by X-ray fluorescence spectrometry. The results suggested that the incorporation of the admixture, characterized by hydrophobic and pore-blocking effects, appeared to considerably enhance the concrete durability with respect to chloride-induced corrosion. The inclusion of the admixture characterized by crystallization activity however, seemed to have almost no detectable effect. This implies the necessity of exercising a degree of caution during specification. Copyright

Cracking of Plastic Concrete

Abstract The ACI recommendations for the prevention of the cracking of plastic concrete by limiting the evaporation rate from exposed surfaces are reviewed, and are shown to overestimate the evaporation from the surface of desiccated concrete. Pore moisture effects in desiccated concrete and the stresses arising from these effects are described. The stresses due to desiccation of early age concrete and its strength are compared, and crack initiation and propagation associated with these stresses are discussed. It is shown that ambient conditions conducive to cracking in early age concrete are encountered everywhere, and that, under favourable conditions, shallow cracks in desiccated concrete can propagate almost instantaneously to considerable depths.

Measurement of concrete expansive strains due to alkali-silica reaction in Australian concrete structures

The results of measuring the concrete expansive strains due to Alkali-Silica Reaction (ASR) are presented for four high strength concrete bridge structures. It is concluded that some of the structures are expanding at significant rates at an age of 10 years even though they are already severely cracked due to ASR. Other structures have stabilised at an age of 10 years and subsequent expansion is expected to be minimal. The collection of this field data indicated that the concrete structures affected by ASR exhibited a maximum strain in the vicinity of 5000 microstrain after 12 years. This is important information to input into an accelerated testing programme for ASR and forms the basis of another paper in this series.

Suctions, Fracture Energy and Plastic Cracking of Cement Mortar and Concrete

Plastic cracking of cement mortar and concrete is primarily attributable to desiccation by evaporation from unprotected surfaces. This causes high suctions (negative pressures) to develop in the pore water adjacent to these surfaces. Dissolved salts in the pore water can also contribute significantly to suctions. Quantitative expressions are available for all of the components of the total suction. The development of suctions over time is illustrated by the results of desiccation tests conducted on cement mortars, supplemented by data from the literature. It is shown that ambient conditions conducive to plastic cracking can arise almost anywhere, but that the extremely high suctions that develop in mature cement mortar and concrete do not imply that compression failures should occur A high value of fracture energy is derived from data from the desiccation tests that implies that plastic cracking is characterized by a significant zone of plastic straining or microcracking.

Development of an accelerated test on concrete prisms to determine their potential for Alkali-Silica reaction

An accelerated laboratory test on concrete prisms is being developed which shows good correlation with the observed field performance of concrete structures damaged due to Alkali-Silica Reaction (ASR). The procedure for the test was determined by collecting a substantial database on actual structures affected by ASR. The main factors common to the affected structures were the use of high cement contents in the range 450 to 500 kg/m3 in association with the use of initial steam curing.

Crack Depths in Desiccating Plastic Concrete

The depths of cracks in desiccating plastic concrete are estimated by considering the effects of the suction (negative pore pressure) associated with desiccation and applying five failure models derived from fracture, theories combined with theories drawn from geotechnical engineering under the assumption that plastic concrete is a frictional particulate material. The estimated crack depths vary with the depth of desiccation, the suction profile, and a small number of material parameters that depend on the model adopted and are comparatively easy to estimate accurately. Four of the models predict excessively large crack depths. The fifth, however, predicts shallower crack depths that increase with the age of the concrete and are consistent with those of analogous desiccation cracks in coal mine tailings. It thus offers a relatively robust method of estimating the depth of desiccation cracks. Confirmation of this with data for plastic concrete is clearly desirable but not possible at present.

Plastic Shrinkage Cracking of Concrete

Summary Plastic shrinkage cracking, which occurs during the first several hours after the casting of concrete, is of considerable economic significance in the concrete construction industry. In this paper, the mechanism of plastic shrinkage cracking and its related phenomena of evaporation and bleeding are first reviewed. Then, together with tensile strength of early-age concrete, the concept of matric suction, its effect on plastic shrinkage and an empirical relationship for its estimation are discussed. Practical implications of the discussion are also highlighted.