International Concrete Abstracts Portal

Industrial and mining wastes, often extremely fine-grained, are being used as fillers and binders in structural concrete and consolidation by compaction, as opposed to high-frequency vibration. Impact and static compaction can be industrially used for the automatic production of such building elements as bricks and blocks, whereas roller-compacted concrete is more suitable for field applications. Compressive strength characteristics under air-dry, sealed, and soaked conditions of portland cement mixtures containing a complete range of combinations of dihydrate phosphogypsum (by-product of the phosphate fertilizer industry) and fine aggregate (crushed lime rock) are presented. Cylindrical specimens were prepared according to the Modified Proctor procedure (impact compaction). Based on these results, strength comparisons are made for selected constituent proportions in the cases of: 1) consolidation by static compaction; 2) consolidation by high-frequency vibration; 3) site consolidation by vibratory roller compactor; and 4) substitution of the dihydrate phosphogypsum with the hemihydrate form (other available by-product). It is shown that consolidation by compaction is advantageous because of the contribution of phosphogypsum to strength development. Laboratory-compacted samples of the by-product alone indicate that strengths of over 1000 psi (6.89 MPa) can be achieved. In addition, low-cement (7.5 percent) mixtures using hemihydrate gypsum waste exceed the 4000 psi (27.56 MPa) mark.

Concretes were prepared at degrees of consolidation varying from 100 to 85 percent. Mixtures were typical of those used for pavement applications with cement factors ranging from 520 to 610 pounds per cubic yard (308 to 360 kg/m3) and air contents ranging from 5 to 9 percent. Additional concretes were intentionally overvibrated to the point of incipient segregation. Test specimens were cast for determination of compressive strength, bond of reinforcing steel to concrete, permeability of concrete to chloride ions, and resistance of concrete to freezing and thawing in water. Results show that compressive strength is reduced by about 30 percent for each 5 percent decrease in degree of consolidation. Bond stress is reduced even more dramatically, suffering a loss of approximately 50 percent for 5 percent reduction in degree of consolidation. Overconsolidation has little apparent effect on compressive strength, and may increase bond strength by virtue of displacement of air in these air-entrained concretes.

Fiber reinforced concrete is reinforced by a small amount of short fibers randomly dispersed into cement concrete, and this composite material is superior to normal plain concrete, with respect to flexural strength, impact resistance, and ductility. But the consistency of fiber reinforced concrete is extremely decreased by adding fiber so that, in some cases, it becomes difficult to place and mold by the ordinary method. These phenomena are observed irrespective of kinds of fibers used. The effects of compaction methods on the strength of fiber reinforced concrete with poor consistency and containing a fairly large amount of fibers were investigated. Conventional steel fiber and alkali-resistant glass fiber were used. Test specimens (10 x 10 x 60 cm) of plain and fiber reinforced concretes were compacted by external vibration with temporary or continuous compressive loading, and were tested in flexure. The mechanism of compaction effects was discussed. Test results indicate that the compaction with compressive loading increases the flexural strength of both types of fiber reinforced concretes and also does extensibility of glass fiber reinforced concrete, although the improvement is made within a certain limit of compaction loading.

The effect of mixing methods on compaction properties is described. A new mixing method presented divides the mixing water into two portions and adds them at different times. The concrete produced by this mixing method is called "SEC concrete" and shows low bleeding and excellent workability. Concrete employed for the experiment was lean concrete whose compactability is most important. Compactability was evaluated by means of angle of internal friction, cohesion, acceleration propagation, density, strength and modulus of elasticity. From the experimental results, the following conclusions were obtained: a) the mixing method of concrete had great effect on the compactability of produced concrete subjected to vibrating compaction; and b) double-mixed concrete showed better compactability than conventional concrete.

Current consolidation practices and a recently completed laboratory investigation to determine the effects of coarse aggregate factor, maximum aggregate size, vibrator spacing, and the method of vibrator mounting on the achieved consolidation of CRCP is reviewed. The theory and principles of consolidation by internal vibration are also reviewed. Vibrator spacing, concrete mix design, and acceleration in the concrete are parameters found to be important. A vibratory spacing of 24 in. (610 mm) is required to produce adequate consolidation. The concrete mix should contain coarse aggregate with a top size of 1 « in. (40 mm) and a coarse aggregate factor determined by the Fineness Modulus Method of mix design. The superplasticizers with or without retarders have to be used with great caution due to slump loss, making it difficult to consolidate the stiff concrete mixes used in slipformed paving construction.