Non-Shrink Grout: Grades, Flowable vs Pourable, and How to Pour It Under a Base Plate

Key points

• Non-shrink grout is used under base plates of machines, pumps and steel columns, under bridge bearings and in anchor bolt pockets.
• ASTM C1107 names three consistencies: plastic, flowable and fluid. “Pourable” is a maker’s word, not a class in the standard.
• Fosroc’s Conbextra GP2 sheet asks for 4.50 litres of water per 25 kg bag for flowable grout and 4.125 litres for pourable.
• IS 2974 (Part 1) asks for the concrete under the plate to be kept wet for at least 24 hours and for grout to be poured from one side only.

What is non-shrink grout?

Non-shrink grout (also written non shrink grout) is a ready-mixed powder of cement, graded fine filler and chemical additives that you mix with water and pour into a gap, where it hardens without pulling away from the steel above it. Ordinary cement grout loses a little volume as it sets and dries. Under a base plate that small loss is enough to leave the plate sitting on a few high points instead of a full bed, and then the bolts take the vibration.

The additives in a non-shrink grout make it expand slightly in the first hours, which makes up for that loss. Fosroc’s data sheet for Conbextra GP2, a common Indian product, describes a gas-producing expansion system that starts about 20 minutes after mixing and finishes at about 120 minutes, and an unrestrained expansion of 0 to 4 percent while the grout is still plastic.

Indian Standard IS 2974 (Part 1):1982, the code of practice for machine foundations, still describes the old site-mixed grout: one part Portland cement to two parts clean sharp sand, mixed just wet enough to be worked under every seating, with no quick-setting cement. It allows additives that give “non-shrink properties”, and then lets the maker’s instructions set the mix. That is the step from a site mix to a bagged grout.

The three ASTM C1107 grades

Bagged grouts sold in India, Conbextra GP2 among them, quote ASTM C1107, the American standard for packaged non-shrink cement grout. The standard splits these grouts by how they control volume:

• Grade A expands before it hardens (pre-hardening volume adjusting).
• Grade B expands after it hardens (post-hardening volume adjusting).
• Grade C does both.

The grade is printed on a data sheet only when the maker claims it. If the sheet only says “conforms to ASTM C1107”, ask which grade, because the grade tells you when the expansion happens.

What is the difference between flowable and pourable grout?

The difference is the amount of water, and with it how far the grout runs on its own. ASTM C1107 does not use the word “pourable” at all. It defines three consistencies by two lab tests: a flow table (the mortar is spread by 5 drops of a table in 3 seconds and the spread is measured) and a flow cone (the time a set volume takes to run out of a funnel).

Table 1: Consistency classes of non-shrink grout in ASTM C1107

Makers then use their own names. On the Conbextra GP2 sheet, “pourable” is the stiffer mix with less water (a water to powder ratio of 0.165), and “flowable” carries a little more water (0.17 to 0.18). So the word on the bag is not a reliable guide. Go by the litres of water per bag that the sheet gives for each consistency, and measure that water every time.

Table 2: Water, strength and yield of one non-shrink grout by consistency (Conbextra GP2 data sheet)

One N/mm² is the same as one MPa. Two things stand out. Extra water costs strength at every age, which is the same rule the water cement ratio guide explains for concrete. And a good bagged grout clears the standard’s minimums by a wide margin, so a site that adds “a little more water to help it run” is spending that margin on the wrong thing. ASTM C1107 also limits how much the grout may expand: no more than 4.0 percent in its early hours, and between 0.0 and 0.3 percent once hardened.

Where is non-shrink grout used on a site?

Anywhere a steel plate has to sit on concrete with full contact, or a bolt has to be fixed into a hole in hardened concrete. The Conbextra GP2 sheet lists base plates of turbines, compressors and boiler feed pumps, bridge bearing pedestals, and anchoring masts, anchor bolts and fence posts. On construction and quarry sites, the same job comes up again and again:

• Base frames of jaw and cone crushers and their motors, where the vibration is constant. The crushing stages are explained in the jaw crusher vs cone crusher guide.
• Legs, silos and mixer frames of RMC and batching plants, set on anchor bolts in a concrete plinth.
• Base plates of steel columns in sheds and factory buildings.
• Pump and generator skids, and crane rails where the rail sits on a sole plate.

The plinth the grout sits on is ordinary concrete, usually with a lean PCC bed and an RCC block above it. For pouring that block, compare concrete mixers and concrete pumps and talk to a dealer.

Cementitious grout or epoxy grout: which one should you use?

Use a cement-based (cementitious) non-shrink grout for most base plates and bolt pockets. Move to an epoxy grout when the gap is very thin, when the load is heavy and pounding, or when oil and chemicals will reach the grout. Epoxy grout is a resin, a liquid hardener and a graded filler, supplied as a set and mixed as a whole pack, with no water added.

Table 3: Cementitious vs epoxy non-shrink grout

Creep is the slow sinking of a material under a load that never goes away. It matters under a heavy machine that has to stay aligned for years, and it is why the epoxy sheets make a point of low creep. Neither type is better everywhere. The machine maker’s installation manual, where there is one, decides which grout goes under that machine.

How do you pour non-shrink grout under a base plate?

The method below follows IS 2974 (Part 1) clauses 6.3 and 6.4 and the application instructions on the Conbextra GP2 sheet. Where the two differ, both are given.

1. Prepare the concrete. Clean off oil, grease, loose bits and laitance (the weak, dusty skin on top of concrete), and roughen the surface. IS 2974 asks for it to be soaked and kept wet for at least 24 hours. Just before grouting, remove all standing water and blow out the bolt holes.
2. Prepare the plate. It must be free of oil, grease and scale. Drill small air relief holes where a high spot could trap air. Grease any levelling shims that are to be pulled out later.
3. Grout the bolt pockets first, before the gap under the plate.
4. Build leak-proof formwork with a foam strip or sealant under it. IS 2974 asks for forms high enough to give about 150 mm of grout head on the pouring side. The GP2 sheet keeps the gap between form and plate edge to no more than 150 mm on the pouring side and 50 mm on the opposite side, so that little grout surface is left open to dry.
5. Mix enough for one continuous pour. Put the measured water in first, add the whole bag slowly and mix for 5 minutes. A slow drill at 400 to 500 rpm with a paddle does up to 50 kg; more needs a grout mixer.
6. Pour quickly and from one side. At 30°C the GP2 sheet asks for placing within 20 minutes of mixing, so the expansion happens under the plate and not in the bucket. IS 2974 sets one hour from mixing as the outer limit for cement grout. Pour across the shortest distance, keep the head topped up, and do not stop until grout comes out on the far side.
7. Cure it. Keep the exposed grout wet with hessian and water, or use a curing compound. IS 2974 then asks for two coats of an oil and alkali resistant coating on exposed grout and concrete, and a final alignment check of the machine.

Weather changes the method. Below 10°C, the GP2 sheet asks for warm water at 30 to 40°C and formwork left in place for at least 36 hours. Above 40°C, it asks for mixing water below 20°C.

How far grout travels under a plate

The head of grout pushes it under the plate, and a narrow gap slows it down. The GP2 sheet gives the maximum flow distance for its flowable mix, poured straight after mixing at 30°C onto a soaked base:

Table 4: Maximum flow distance of flowable non-shrink grout (Conbextra GP2 data sheet)

Read it this way: a 1.2 m wide plate on a 30 mm gap needs more than a 100 mm head, or the grout may stop short of the far side. Raise the head box, or choose a runnier grade the maker rates for that distance.

Why does non-shrink grout crack or leave a gap under the plate?

When a grouted plate rings hollow when tapped, or the grout shows cracks and a dusty face, the cause is almost always in the method, not the bag. The usual culprits:

• Too much water. The grout bleeds and separates, the solids settle, and a weak watery layer collects under the plate. The GP2 specification clause says the grout “must not bleed or segregate”.
• Placed too late. Grout that sat in the bucket past its placing time has spent its expansion before it reached the plate.
• Poured from two sides. Two grout fronts meet and trap a pocket of air between them. IS 2974 and the GP2 sheet both say pour from one side.
• A stop in the middle. The first batch starts to stiffen and the next one cannot push it along. Have all the bags, water and labour ready before the first bag is opened.
• A dry base. Dry concrete sucks water out of the grout. A base left with standing water does the opposite and dilutes it. Hence the soak, then the blow-out.
• Too thick a pour without aggregate. Neat cement grout heats up in deep sections. Beyond 100 mm the GP2 sheet adds 50 to 100 percent of clean 10 mm aggregate by weight of the powder.
• No curing. The exposed shoulder dries out and cracks at the surface.

A hollow patch under a working machine is not a cosmetic defect. It lets the plate rock, which loosens bolts and can crack the plinth. Cracks and voids in the concrete itself are a separate problem, covered in the guide to honeycombing in concrete.

How much non-shrink grout do you need?

Work out the volume inside the formwork, then divide by the yield of one bag. The GP2 sheet gives the yield of a 25 kg bag as 12.5 litres when pourable and 13.3 litres when flowable, which is about 75 to 80 bags for one cubic metre.

A worked example: a crusher motor base plate is 600 mm by 400 mm on a 40 mm gap. The formwork sits 150 mm off the pouring side and 50 mm off the other three sides, so the area inside it is 800 mm by 500 mm. At 40 mm deep that is 0.8 × 0.5 × 0.04 = 0.016 m³, or 16 litres. Two 25 kg bags mixed flowable give 26.6 litres, which covers it with grout to spare for the head box and the bolt pockets. The sheet tells you to allow for wastage but does not fix a figure, so round up and open the second bag before you start.

How is non-shrink grout different from other site chemicals?

Grout fills a gap under something; a repair mortar rebuilds missing concrete; an admixture changes a whole concrete mix. Mixing them up is a common and costly mistake. The types of construction chemicals guide sorts each family by the stage of work it belongs to, and the concrete admixture guide covers what goes into the mix itself. For checking the concrete under the plate, the cube test guide explains how its strength is tested.

More material topics are collected in the building materials guides.

The grouting clauses on this page are from IS 2974 (Part 1):1982, Code of Practice for Design and Construction of Machine Foundations, clauses 6.3 and 6.4. Water, strength, yield, flow and placing figures are from the Fosroc Conbextra GP2 India data sheet; epoxy gap ranges are from Fosroc’s Conbextra EP10 and EP65 product pages. ASTM C1107 grade, consistency, strength and height-change requirements are as summarised by the standard’s published scope and by MC-Bauchemie’s comparison of grout specifications. Brand names are used only to identify the data source. All were read in October 2026.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. Grout properties and placing limits vary between products; follow the data sheet of the grout you buy and the machine maker’s installation instructions.