SITE TESTING OF MICROFINE CEMENT GROUTS

As name suggests, microfine and ultrafine cement injection grouts are much finer than normal cements. Average particle size is at-least three to four times smaller than that of normal cements. As fineness of such materials is more than 8000 cm2/gm, Blaine fineness may not be a correct measure. Particle size distribution (PSD) measured by laser diffraction methods is an appropriate method. PSD analyser confirming to ISO 13320 based on low-angle laser light scattering (LALLS) techniques using the Mie theory of light scattering should be used. Another method to know the fineness is B E T method (Nitrogen adsorption) described in IS: 15388 – 2003. Both these tests can only be carried out in sophisticated lab. At sites (which often are at remote places) following tests can be conducted to evaluate suitability of microfine cements.

Viscosity: Less viscous the grout, easier it is to inject in the strata with minimal pressure. Viscosity can be measured as per IS 14343: 1996 Annexure A at site with the help of Marsh cone. It gives time required to pass 1 ltr of grout through the orifice of 5 mm dia. The Marsh Funnel Test is a good indicator of true viscosity of the grout slurry; it is also an important index property for quality control of field mixes. Marsh cone flow time of 30 to 40 seconds is desirable for a fluid grout. (For water, Marsh cone flow time is 27 seconds).

Penetrability of the grout can be analysed by using “Sand Column” test. As per Finnish standard SFS EN 1771, fluid grout is passed through glass tube filled with fine sand. Speed of travel of grout and quantity of grout collected after passing through the tube indicates the suitability of material for injection process. Fluid grout qualifying the Marsh cone flow test shall be used for the sand column test. Grout is injected from the bottom of the tube at uniform pressure 0.75 bar. Particle Size Distribution (PSD) is an important contributor for superior groutability / penetrability of any microfine cement grout. Before finalising the materials, PSD (d, d, d) shall be evaluated carefully. Grouts with PSD on coarser side will not be able to pass through sand column.

Filtration stability of the grout: Many a times, despite selecting finest of the grout materials, grout may not get injected in the porous strata or fine fissures. Filtration of the grout is a result of a plug building at fracture constrictions which reduces the penetrability of the grout. Highly fluid grouts may bleed under pressure. Separated particles clog the crack entrance or superficial porous strata. The European Standard Association SFS EN 14497 explains the filtration stability test of highly fluid grouts. This is easy, reliable and standalone test which can be easily conducted at site.  The device is composed of a 1000 ml metal tube with a 25.6 mm diameter, rubber pistons and woven metal wire mesh with apertures of 125, 100, 75 microns. A fluid grout is sucked through metal tube fitted with steel wire mesh at one end. Grout is then emptied into the measuring cylinder and the volume is recorded. Procedure is repeated for next fines wire mesh. Efficiency of grout mixing machines do influence the result of this test.

Bleeding test: Bleeding of grout refers to the separation of solid particles from the fluid. It is defined as the height of clear water (H1) in percentage of the total height (H). For this test, one liter of grout is poured into a cylinder with a diameter of 60mm and after certain time, the height of clear water is measured. Highly fluid particulate suspension grouts are tend to bleed. Due to particle shape, size and granulometry, water gets entrapped in the inter-particle voids. This water gets squeezed out due to pressure or settlement of particles and is termed as bleed water. Post bleed phenomenon, hardened grout shrinks which defeats the very purpose of injecting these materials in the cracks or voids. Microfine & ultrafine cement grouts are less prone to bleeding (at W/C = 1) due to its compact & finer particle size.

Fresh wet density by mud balance apparatus or density cup, setting time, etc. are also some of the tests that can be carried out at the site.