F. 11. Read the text and speak on Cemsave Concrete and the tests

The control mix for this test series contained no granulate and consisted of an agg / cement ratio of 6.0 /1, a coarse agg / fine agg ratio of 1.6 / 1, and a free water / cement ratio of 0.5 / 1. In subsequent mixes the quantities of aggregates and free water were held constant and part of the OPC was progressively replaced by the granulate.

Table 1 shows the replacement percentages used and gives the workability of each resulting mix as measured by the slump test. From each batch of concrete twenty-four 100 mm cubes and twelve 100 mm diameter x 200 mm long cylinders were manufactured. These specimens were demoulded 24hr later and stored under water at a temperature of 20°C ±1°C until required for testing. Specimens were weighed, measured and tested for strength at ages of 2, 7, 16, 28, 84, and 220 days. At each age four cubes were tested in compression and two cylinders were subjected to the cylinder splitting test. The mean failure stresses for the compression and the indirect tensile tests were calculated and are shown in Table 2 and 3 respectively. In these tables the figures indicate the percentage of the 28-day strength result using 100 per cent OPC as the cementing agent.

Although the external faces of specimens containing Cemsave appeared white in colour, an examination of the failed specimens revealed that the colour of the interior of the specimens was blue. The intensity of this blueness was found to increase with increasing Cemsave content, but on prolonged exposure of the failed surface to air the blueness reduced considerably in intensity and in some cases virtually disappeared.

Fig. 1 shows the relationship between workability and the percentage by weight of the granulate used. It is to be noted that, when the quantity of the free-water used in the mix is held constant, an increased granulate content is associated with an increase in the mix workability. If the workability of a mix is to be kept constant however, the addition of granulate must be compensated for by a reduction in free-water content of the mix. This will reduce the effective water-cement ratio of the mix which will lead to an increase in strength properties of the resulting concrete. Work is continuing to investigate further the workability properties of concretes containing Cemsave.

Fig 2 shows the gain in compressive strength for mixes containing 0 per cent, 30 per cent and 50 percent Cemsave. From this graph it can be seen that concretes containing the granulate develop strength more slowly than mixes containing 100 per cent OPC, but the difference in strength decreases with increasing age. For the mixes used in this investigation no case was observed where concretes containing Cemsave displayed a strength in excess of the corresponding value of concrete containing 100 per cent OPC. Taking the 28-day compressive strength of the mix as a reference strength, mixes containing less than 40 per cent of the granulate developed the reference strength within three month but for mixes containing 50 per cent and 70 per cent a one-year period of time might well be required. The gain in tensile strength of the mixes used is indicated in Fig 3. The immediate observations are similar to those outlined above for the compressive strength results. Using the tabulated values of stress in Tables 2 and 3, Table 4 has been compiled which indicates the ratio of the indirect tensile stress of the concrete at the particular age under consideration.

Formwork striking time is often an important consideration on site and Fig. 4 relates the compressive failure stress of the concrete at an age of seven days to the Cemsave content. It can be seen that at this age, the concretes containing 40 per cent granulate developed only 60 per cent of the corresponding strength of concrete containing none at all.

F. 12. Read the text On Mixing and Moulding Equipment and answer the questions:

1. What machines are used for the mixing jobs?

2. Where is moulding equipment used?

Mixers are the most important item of the precast concrete products industries. Although both batch and continuous mixers are used, the former are in far more general use. A given charge of materials is placed in the batch mixer, mixed and discharged before the next lot of materials is added. Batch mixers are manufactured in a wide range of sizes, and with various combinations of mechanical features. The desirability of extremely strong construction is a good point to keep in mind when selecting a mixer, for a light machine, even though it may be adequate for the mixing job, may prove to be highly vulnerable when some foreign object is by chance introduced. Replaceability of parts subject to wear is another vitally important factor to be considered.

In continuous mixers, the proportioned materials are introduced at intervals, and the mixing goes on continuously during the passage of the materials through the machine. Maintenance of strict uniformity with continuous mixers is somewhat more difficult than with batch machines, and they are for this reason considered to be better suited to very large operations.

Moulding of precast plain and reinforced-concrete units is usually accomplished with the aid of vibration. Precast reinforced-concrete pipes are manufactured either by vibrating or by centrifugal process. When reinforced-concrete pipes are to be used in pressure lines the requisite impermeability, density and strength are best obtained by centrifugal casting. Pipes from 12 to 78 inches in diameter and from 8 to 12 feet long are made by this process. When the required amount of concrete has been placed into the mould, the latter is rotated at high speed to remove all excess mixing water.

F. 13. Read the text On Curing Equipment and answer the question: What is the most advanced form of concrete curing?

Concrete sets in the presence of moisture and heat. Chemical reactions stop almost completely when the temperature drops below 40 degrees F., and the lack of moisture during the setting period results in incomplete hydration of the available cement. Under either of these conditions precast reinforced concrete units fail to develop more than a fraction of the strength potentially available in the material. Obviously, then, the maintenance of favourable curing conditions is of prime importance. Although 28-day air curing and drying under cover seems to offer a fair solution to the problem, it creates, especially in high-production plants, some almost equally serious problems, by tying up large amounts of capital in equipment, and because of relatively large storage areas required. That is why high-pressure steam curing is very widely used now.

Although the equipment is more expensive, curing with high-pressure steam is perhaps the most advanced practical form of concrete curing. It reduces the curing period from days to hours, allowing the precast reinforced concrete elements to be delivered to the construction job as early as 15 or 16 hr after they are moulded.

Cylindrical steel autoclaves are employed in this process. The freshly-made units are wheeled into the autoclaves, tightly closed there, and steam at 120 lb per sq. in. gage pressure or more is turned on, the temperature inside reaching 350 to 360 deg. F in about 3 hr. After 7 to 8 hr. of curing the steam is turned off and the pressure is rapidly lowered during a 1/2 hr. period to atmospheric pressure after which the units are removed. In general units cured in this manner will have strength at least equivalent to that, obtained by 28 days of continuous moist curing at 70-80 deg. F.

Curing. Concrete becomes hard by the chemical combination of cement and water, during which process it is necessary to prevent as far as possible evaporation of the water from the surface of the concrete; this is called " curing", and is accomplished by covering the concrete, as soon as it can be done without damaging the surface, with damp cloths, wet straw, wet sand, etc., kept wet by sprinkling, or by immersing in water.

Hardening. The strength of concrete under favourable curing conditions increases with age. Hardening is very rapid in the early stages, but continues more slowly for an indefinite period amounting to years.