Further to the CRUD TEST 1, where Indian cement and other materials were used with the US meshes sent by Tom, a second test was planned in order to understand the crud behaviour better. The crud test 2 was carried out in the same laboratory in Delhi as earlier, this time with American cement instead of the Indian one.
4 mortar mixes were used with ratios of (Cement : Fly Ash : Sand ) as follows –
• Mix 1 = ( 1 : 0 : 2) ................... No Fly Ash
• Mix 2 = ( 0.75 : 0.25 : 2) ..........25% Fly Ash replacement
• Mix 3 = ( 0.5 : 0.5 : 2) ..............50% Fly Ash replacement
• Mix 4 = ( 1 : 0 : 2) + Cr2O3 mixed in water (2 ppm)
2 types of GI meshes were used with each mix -
• a WWM sent from USA by Tom,
• a square wowen mesh from India - well passivated for
over 10 yrs.
making a total of 4x2 = 8 samples.
Like earlier, the samples were cast all on the same day
using these 4 mixes on 2 meshes
MIX 1 Mix2 Mix 3 Mix 4
1:0:2 0.75:0.25:2 0.5:0.5:2 1:0:2+Cr
US ww mesh A C E G
IND wowen mesh B D F H
|SAMPLES with varying ratio (US Cement : Indian Fly Ash : Sand)|
|CRUD TEST 2 Feb.'08|
• After a week of curing under 2 layers of damp Burlap( wetted every morning), the samples were turned upside down and observed closely for weak crud. There did not appear to be any.
• On light scrubbing with a nylon brush the mortar in the immediate vicinity of the wires did not prove to be any weaker than the rest of it.
• The samples were then left for another week to dry. Patches were then chipped with an improvised tool to bare the wires which were nearly 0.5 mm inside the surface. Again the chipped patches did not show any specific weak areas. Due to the absence of any reinforcement on the other face, the samples broke easily during chipping.
• Further investigation was carried out by applying a brush liberally soaked in water on samples A and G , waiting for 5 minutes and then observing. As per Tom's hypothesis, A was expected to have the maximum crud while G should have had no crud owing to the Chromium trioxide.
• Looking at the wet sample WET-A one might suspect some weak, spongy zones along the wires which seem to have soaked up extra water. However the WET-G sample also has similar appearance - darker zones along the wires, as if holding water
|CLICK sample names and PICTURES TO SEE ENLARGED VERSIONS|
|On giving the same wet brush treatment to all the samples, one observes a much decreased presence of 'dark wet zones' in samples C & D (25% fly ash) while a near absence of the same in samples E & F ( 50% fly ash ), indicating a better, dense mortar in samples incorporating fly ash. Also note the distinct difference in the mortar colour itself.|
1. In the two test programmes carried out in our Delhi laboratory on the GI meshes supplied by TOM from the US , ( the first one with an Indian Cement, the second one with a US cement ) there was no "CRUD" found. The light brushing with a nylon bristled brush did not indicate the presence of specially weak zones next to the GI wires other than some minor vacant pockets due to placement procedure.
2. In both these set of tests, the criterion for the judgement was limited to light brushing accompanied with visual observations only. A more in-depth test ( microscopic / chemical ) on the mortar in the immediate vicinity of the wires compared to the mortar elsewhere may throw a greater light on the characteristics of the two regions. However, from a practical, engineer / mason's point of view there was no problem worthy of mention found in the ferrocement made with the said materials.
3. A possible reason that could cause weak / loose / vacant pockets next to GI meshes placed in mortar is the 'springiness of the GI mesh'.The mortar is usually applied with a pressing action which exerts a push on the mesh deforming it slightly. When the trowel is moved away, the springy action of the mesh may cause it return to its 'HOME' position leaving a void underneath. This void may or may not get filled by sound mortar by more working around the area. The void may even attract bleed water to accumulate resulting in a very weak zone - easily removable by a toothbrush. ( This is just a hypothesis trying to explain an observed phenomenon ). The above description applies to a situation where the mesh movement due the springiness is small - perhaps less than 1 mm . When the movement is large, and the mesh aperture small enough, the whole layer of mortar may delaminate. The mesh while coming back to its 'HOME' position brings the layer of mortar in front along with it thus seperating it from its sister mortar behind the mesh. A sticky mortar such as one having a large amount of cementitious material ( cement + Fly ash) may have a lesser tendency to delaminate. However the culprit in such a case is the freedom of the mesh to move so much - regardless of whether it is a welded/woven mesh or a diamond lath. The solution lies in tying the mesh 'well enough' so as not to show an appreciable movement on pressing with the palm of your hand. I think the FC primer gives a similar guideline. See pic1 and pic2 of DELAMINATION during my initial trials 4 years ago with a square woven mesh with an aperture of 6 to 7mm.
In the opinion of the author:
1. Although there has not been any "CRUD" formation in our tests , an extra safeguard - (that of adding Chromium trioxide - if easily available) against a suspected phenomenon may always be welcome. Passivating the GI mesh is also reccomended.
2. Fly ash is a beneficial admixture in cementitious formulations - be it concrete or Ferrocement. Due to the pozzolanic reaction, it helps in improving the pore structure of the cementitious matrix. The more, the better !
3. There is a tremendous Power of thoughts in moulding and influencing the physical world around us. For the philosophical thinker or the genuine seeker willing to explore this aspect here is the link Power_Of_Thoughts
But watch it ! Read it with reverence. The words are from an enlightened soul - far above the mundane illusions of the physical world.
|The human being is far from perfect and has limitations. Every new discovery negates some earlier ones.
Only HE - The creator is PERFECT. Aye - you egoistic embodiment of Imperfection - BOW TO THEE !