The grading of coarse aggregate may be varied through wider limits than that of fine aggregates, since it does not affect the workability, uniformity and finishing qualities of concrete mix. As per IS-383 the grading limit of coarse aggregate, both for single size as well as graded should be as per the table given below. … Read more
Aggregate Classification [Based on Shape] According to shape the aggregate is classified as Rounded aggregate Irregular or partly rounded aggregate Angular aggregate Flaky aggregate Elongated aggregate Flaky and elongated aggregate 1. Rounded Aggregate The aggregate with rounded shape has the minimum percentage of voids ranging from 32 to 33%. It gives minimum ratio of surface … Read more
Determination of setting time of concrete helps a site engineer in guiding the following areas. Importance of Setting Time of Concrete Whether or not to use any plasticizer To find out effectiveness of set controlling admixture Regulating maximum time of mixing transit, placing and compacting Whether or not to provide protection against adverse weather condition … Read more
The setting time of concrete depends upon following factors, such as Water-cement ratio Temperature condition Type of cement used Use of mineral admixture Use of chemical admixture (such as plasticizer) Other factors which may affect the setting time of concrete includes Absorption of mixing water by aggregates Grinding of aggregates in the mixer Of all … Read more
The various process involved in any concreting operation involves the following stages. Mixing Transporting Placing Consolidating As concrete passes from one stage to another, it starts to lose its plasticity characteristics. If after placing it is not disturbed for a longer time period, then it starts to become stiffening. As a result concrete changes its … Read more
Objective Determination of initial and final setting time of concrete as per IS-8142-1976. Apparatus • Non absorptive metal container having minimum lateral dimension of 150 mm and minimum depth of 150 mm. • Penetration resistance apparatus having a pressure gauge of capacity 700 to 900 N. Removable needles of 645, 323, 161, 65, 32 & … Read more
To get better efficiency, the sequence of charging the loading skip is as under 1. First place half the quantity of coarse aggregate in the skip. 2. Place half the quantity of fine aggregate over the previously placed coarse aggregate. 3. Then pour full quantity of cement (i.e. one bag of cement) over it. 4. … Read more
For preparation of special products like precast concrete membranes, some special compaction methods are used, such as • Vibropressing • Shock • Centrifugation • Air jets Vibropressing Compacting in this method is effected by applying external pressure on top and vibrating from bottom. Table vibrators are usually made use of in this process. This process … Read more
The following properties of aggregate affect the properties of hardened concrete. 1. Grading Aggregate grading has no significant effect on the properties of hardened concrete. 2. Bulk Density This has no direct effect but has an indirect effect on drying shrinkage. Bulk density is an indicator of packing capacity, which influences water requirement, which in … Read more
Grading In case of sands, it has been found that materials having a large deficiency or excess of any size fraction adversely affect the workability of concrete. The most important size fraction in a concrete sand are those passing the 0.30, 0.15 and 0.075 mm sieves. If these fractions are too low the concrete tends … Read more
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