Coarse Aggregate Type Research Articles

MIX DESIGN FOR PFA CONCRETE.

A mix design procedure is proposed which uses the grading modulus of a pulverized fuel ash (PFA) as an important parameter in assessing its potential for use in concrete. An earlier mix design method facilitated the economic utilization of any type, grading and maximum size of coarse aggregate and any type of cement, with or without air entrainment, to achieve a required strength and workability in terms of the Vebe or modified Vebe times. The earlier rational method of mix design has been the basis for the present extension to include the use of various PFAs. In all, 126 concretes have been designed, cast and tested to derive an adjustment chart for various proportions and grading moduli of PFA and to check the entire design procedure for various PFAs, cements and aggregate combinations. (A)

In service deterioration of bituminous highway wearing courses due to moisture-susceptible aggregates

In the United Kingdom, cases of premature deterioration of bituminous macadam wearing courses have occurred with materials known to have been within specification and correctly laid. To determine the mechanism of deterioration, a detailed programme of site monitoring and laboratory studies was undertaken. Observations at sites in southwest England demonstrated progressive disintegration and removal of certain types of coarse aggregate with the greatest deterioration occurring during periods of maximum rainfall. Representative samples of the various types and qualities of the rocks composing these coarse aggregates were subjected to a series of artificial weathering tests. Particularly in the tests involving wetting—drying cycles existing cracks in the rock samples became extended and new cracks appeared to develop. Analyses of samples demonstrated the presence of platey and moisture-susceptible minerals. Expansion of these moisture-susceptible minerals is proposed as a mechanism for the disintegration of these rocks when used as wearing course aggregates. A concentration of such moisturesusceptible coarse aggregate particles in bituminous macadams may lead to significant deterioration when the wearing course is subjected to weathering and trafficking processes.

Properties of concrete made with three types of artificial PFA coarse aggregates

Data are presented on a comparative research on three artificial aggregates made from pulverised fuel ash by different processes. The properties of concrete produced from these artificial aggregates and river sand having an average 28 day compressive strength of about 30 MPa are also reported. The features of the artificial aggregates are compared with those of the common Dutch coarse aggregate river gravel.

Effect of specimen and crack sizes, water/cement ratio and coarse aggregate texture upon fracture toughness of concrete

Synopsis Tests were performed on cement mortar and concrete beams, in two stages, with a view to studying the influence of several specimen and mix variables upon the fracture behaviour of concrete. On the basis of the results from the first stage of tests, in which a single water/cement ratio and type of coarse aggregate were used, a simple formula was established to estimate the fracture toughness of concrete in terms of specimen dimensions, maximum aggregate size and notch depth, together with the mix compressive strength and modulus of elasticity (determined from separate standard cylinder tests). It was found to predict, with sufficient accuracy, the results from the second stage of tests in which, besides variation of the type of coarse aggregate and water/cement ratio, some of the specimen sizes were outside the range used in the first series.

Aggregate and concrete microfracture

Thin sections cut from cast concrete cylinders have been examined in transmitted light to investigate the significance of coarse aggregate type in “primary” microfracturing. Concretes manufactured with crushed-rock aggregates and gravel aggregates were studied and, in every case, the dominant fracture type was a parting of the aggregate-matrix bond. Bond cracking was least severe with the marble aggregate where epitaxial calcite over-growth was indicated. The ability of bond cracks to maintain continuity by bridging surface irregularities, via mortar cracks, reduced the inhibiting influence of rough-surfaced aggregate on bond-crack development.

The point-load strength test for concrete cores

Synopsis The point-load test, commonly used in geotechnical practice, is evaluated for application in concrete testing. The method uses portable equipment to test concrete cores which require no further preparation after removal from the structure. This paper examines the results of a laboratory investigation in which over 450 cores were cut and tested. The influence of specimen and aggregate size, core-length/diameter ratio, and aggregate type upon point-load strengths is examined, and minimum values for the length/diameter and diameter/aggregate-size ratios are proposed. For concrete strengths greater than 20 N/mm2 a linear relationship between core point-load index and cube compressive strength was observed, and the variability of the point-load strengths compared favourably with values reported for compressive testing of cores. The advantages of both the point-load test and the uniaxial compression test for cores are discussed, and it is argued that, provided a satisfactory method can be found for estimating the effect of the coarse aggregate type upon the point-load strength index, this simple test offers a practical alternative to the compressive testing of cores.

圧裂引張試験におけるコンクリートの破壊機構に関連するパッドの影響

It is well known that the pads used in splitting tests have a great influence on the splitting tensile strength of concrete. This study was aimed at clarifying the effect of the padding materials on the splitting tensile strength of concrete in relation to the fracture mechanism of concrete by carrying out the experiments and the FEM analyses on both the actual and model concrete. The experimental results of the actual concrete showed that the influence of the padding materials on the splitting tensile strength depended on the types of fine and coarse aggregate, the volume fraction of coarse aggregate in concrete and the friction between pad and specimen. The FEM analyses showed that the stress distribution within the model concrete was not so much affected by the padding materials before cracking, while, after cracking, the padding materials influenced the initiation and development of cracks as well as the stress distribution of the model concrete. These facts suggest that the effects of the padding materials on the splitting tensile strength of concrete are closely related to the complicated structure of concrete.

Fifty Year Properties of Concrete

Compressive strength, unit weight, and volume changes of concrete for ages up to 50 years have been determined. Variables included are types of cement and coarse aggregate, mix proportions, consistency, methods of placement, and storage conditions. Over 2500 concrete cylinders, 132 concrete expansion prisms, and several thousand mortar cylinders and briquets were made in 1910, 1923, and 1937 for this program. Earlier reports on these tests were published in 1931, 1943, and 1961. Under outdoor storage in Madison the compressive strength of concrete made with coarsely ground cements having high C2S contents generally increased as the log cf the age out to 50 years, but concrete made with finer cements having relatively low C2S contents appeared to reach maximum strength at ages between 10 to 25 years and showed some retrogression thereafter.

Relationships between strength, deformation and age of autogenously healed concretes : Dhir, R Sangha, CM Munday, JG 7F, 7T, 14R. Quarry Management and Prods. V1, N3, Aug. 1974, P93–99

The authors describe an investigation on the autogenous healing of mortars subjected to strength failure. The effects of water/cement and aggregate/cement ratios on the healing process were studied. Six concretes, made with 3 types of crushed igneous coarse aggregate, and cured in humid conditions, formed the cylindrical test specimens. 367 tests were conducted using a constant strain rate of 1 X 1/100000/s to give controlled loading up to the maximum stress of 50 n/square mm. The influence of mix design and age was considered. Test ages were 7, 28, 60, 120 and 240 days. The authors concluded that (1) full recovery of concrete strength after fracturing to maximum stress is possible for all ages investigated and is unrelated to aggregate type and mix proportions; (2) full recovery of the 50% tangent modulus was possible only after initial testing at 7 days; (3) healed concrete exhibits brittle behaviour with a reduced ultimate strain at failure. /TRRL/

The workability of concrete subjected to vibration

An extensive investigation has been carried out to determine the Vebe time for a very wide range of mix proportions, including various combinations of maximum size, grading and type of coarse aggregate with different gradings and types of fine aggregate and different types of cement. Each variable has been related to a clearly defined physical test on the aggregate or cement. Comprehensive but simple to apply charts are derived which accurately predict the effect of any of the foregoing variables upon the Vebe time. An examination of the charts provides rational explanations, in quantitative terms, for many of the practical observations of the effects of each variable on concrete workability which have formerly had to receive a qualitative description only.

Some factors affecting the compressive strength of concrete

Summary Abrams's law of compressive strength versus water/cement ratio is well known and many investigators have studied the possible effects of other factors, such as cement content and sand concentration, upon concrete strength. In particular, the British and American literature is often at variance concerning these secondary effects so that a primary aim of the present paper is to reconcile these conflicting views. Any considerations of secondary effects require a very clear definition of what is meant by ‘compressive strength’ and ‘water/cement ratio’. Furthermore, different aggregates can give significantly different results. These considerations are illustrated by presenting data for curves relating compressive strength and water/cement ratio for three distinct criteria for compressive strength and two types of coarse aggregate. The paper then re-examines the effects of varying both the cement content and the sand concentration, for a given water/cement ratio, upon the compressive strength. It is shown that the effects of these secondary factors are in general fairly small and that rational explanations can be given for the often conflicting conclusions obtained by earlier investigators.

The Effect of Qualities of Artificial Lightweight-Aggregate upon Compressive Strength of the Concrete

Qualities of lightweight aggregate such as its specific gravity, shape of particles and grading can be controled in its producing process, and these factors will have important effect on the quality of structural lightweight concrete. To find sufficient quality of the aggregate to be used in structural lightweight concrete, the relation between these factors and compressive strength of concrete were investigated.Four types of coarse aggregates were used; the coated-type expanded shale, the peletized-type expanded shale, and the river gravel and crushed stone. River sand was used in all concrete mixes. The particles of expanded shales were previously divided into 7 groups of specific gravity ranging from 0.9 to 1.6, and a certain class from them in which all particles had almost uniform specific gravity was used in a concrete mix. The concrete mix was determined by trial mixes so as to obtain the highest possible volume fraction of coarse aggregate using 1:1 mortar (W/C=30%).The compressive tests were carried out on φ10×20cm cylinders, and the main results obtained are as follows.(1) The relation between the specific gravity of expanded shale aggregate and the compressive strength of concrete appeared differently in accordance with the strength level of concrete. In the case of high strength concrete, the strength was limited by inherent strength of aggregate, and its inherent strength of aggregate was proportional to the specific gravity of the aggregate.(2) If we evaluate the desirable quality of lightweight concrete in the form of specific strength, the ratio of strength to the specific gravity, there is the most suitable specific gravity of the aggregate which brings the maximum specific strength of concrete. From the results of this experiment, the most suitable value of specific gravity was estimated as follows for each strength-levels of concrete:The required concrete strength (kg/cm2) Suitable s.g. (Saturated Surface-dry)200∼300 1.3about 400 1.4about 550 1.5about 700 1.5∼1.6(3) The shape of the particles does not make any difference to the specific strength of the concrete, whereas the grading is one of the factors that affect the strength of concrete.

The effect of coarse aggregate on the mode of failure of concrete in compression and flexure

Summary Experiments have been made to determine the flexural and compressive strengths of concretes containing thirteen different coarse aggregates. In the compression tests, observations were made by an ultrasonic pulse technique to detect the onset of cracking prior to failure. The main results were as follows. Concretes containing smooth gravels began to crack at lower compressive stresses than did concretes containing coarser-textured aggregate. The compressive strengths did not show such appreciable differences. Concretes which best resisted pre-cracking in compression also gave highest flexural strengths. The relation between the flexural strength of the concrete and the stress at which cracks first occurred in compression was independent of the type of coarse aggregate; no such unique relation existed between flexural strength and ultimate compressive strength. Tentative conclusions are drawn on the mode of failure of the concrete as follows. Pre-cracking in a cube when tested in compression probably arises from local breakdown in the adhesion between the coarse aggregate and the cement. The average compressive stress at which the first cracks occur is determined by those properties of the aggregate which influence the aggregate-cement bond. Pre-cracking in compression does not lead to the immediate fracture of the concrete specimen. A contribution to the ultimate strength is provided by the mechanical interlocking of the coarse aggregate after pre-cracking has occurred. Thus the compressive strength of concrete is usually greater than the compressive strength of its comparable mortar. In flexure, pre-cracking causes fracture of the concrete specimen and the only properties of the coarse aggregate which influence the flexural strength are those which appear to determine the aggregate-cement bond. In general the flexural strength of concrete is lower than the flexural strength of the corresponding mortar.