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
What is a Poker Vibrator? A poker vibrator consist of an electric weight encased in a housing and may be driven by an electric or pneumatic motor or a petrol or diesel engine. Poker diameters range from 25 to 75 mm. How To Use Poker Vibrator? A poker vibrator should be used as follows Insert … Read more
Objective To determine quantity of silt in fine aggregates in the field Theory There are two types of harmful substances preset in fine aggregates i.e. organic matter produced by decay of vegetable matter and/or clay and silt, which form coating thus preventing a good bond between cement and the aggregates. If present in large quantities, … Read more
What is Contraction Joints ? Contraction joints are planes, usually vertical separating concrete in a structure or a pavement placed at a designed location. Contraction joint is placed in such a way that it interferes least with the performance of the structure as well as prevents formation of objectionable shrinkage cracks elsewhere in concrete. Why … Read more
The aggregates should be stored near the mixer A clean dry and hard surface should be selected for the storage. Aggregates should not be dumped on loam or grass. In the absence of clean and hard surface a platform constructed of wooden planks, old iron sheets, bricks or lean cement concrete may be used. During … Read more
Methods Used For Curing of Concrete There are various methods of curing. The adoption of a particular method will depend upon the nature of work and the climatic conditions. The following methods of curing of concrete are generally adopted. Shading concrete work Covering concrete surfaces with hessian or gunny bags Sprinkling of water Ponding method … Read more
When concrete is compacted with vibrators the following precautions may be taken to produce a concrete of good quality. The formwork should be strong and tight so that there is no leakage of mortar. The leakage of mortar results in honey combing. A lining of plywood may be provided to prevent the leakage if the … Read more