Material used for filling the space or gap between the base or base plate of a column, trestle, machine etc. and the top surface of it’s foundation is called grout and the process of filling grout is called grouting. The main objectives of grouting is to maintain alignment of the erected structure or equipment etc. accurately as well as to render the combination of it and it’s foundation monolithic. In case of structures like column, trestle and suchlike, this monolithic nature helps uniform transfer of load and in case of a machine foundation it also imparts dynamic stability to the combination. Various other kinds of grouts are also available for various other purposes.
Grouts are integral elements of virtually all construction projects, especially industrial projects. Two of the most widely used grouts in such projects are: a) grouts prepared in-situ with Ordinary Portland Cement (OPC) and b) Ready to use or pre-mixed or pre-packed non-shrink grouts. Grout prepared in-situ is usually a simple mix of OPC & good quality, well graded sand. Sometimes, small sized stone chips are also added as described later in this post. The ratio of cement to sand normally varies from 1:1 to 1:2 as per requirements. These grouts are usually used in ordinary structures. For large machine/equipment foundations, tall or heavy structures etc., ready-to-use grouts, which come in many varieties with diverse characteristics meant for diverse purposes, could be a much better choice from strength, durability or even workability point of views. Few quite useful points about these two types of grouts are briefly discussed here.
Grouts prepared in-situ with OPC:
Water to cement (w/c) ratio for these grouts should be kept as low as possible while ensuring the required workability too. In other words, w/c ratio should be just enough to render the mix flowable which could be ascertained from trial mixes. Restricting this ratio is of utmost importance from both strength & durability point of views.
For gaps of thickness of 1″ and above, especially for large or heavy structures, it is appropriate to use good quality stone chips of size 6mm & downgraded for better performance. The ratio of cement : sand : chips may be kept at 1:1:2.
Unlike non-shrink ready-to-use grouts, these ones tend to shrink in volume after hardening. Since, the extent of shrinkage is a function of water content of the mix, it is quite necessary to maintain a low w/c ratio.
W/c ratio usually needs to be increased as the gap thickness increases. As a simple rule of thumb for these grouts, when the gap thickness is more than 1″ thick, the cement to sand ratio may be kept at 1:2 & max. w/c ratio restricted to 0.52. When the same is less than 1″ thick, the ratio may be adopted at 1:1 with the max. w/c ratio restricted to 0.45.
Contd. in Part-II….