IS 7320 : 1974Specification for concrete slump test apparatus

IS 7320:1974 is the specification for the concrete slump test apparatus — the slump cone, tamping rod and base plate used to run the workability test on every concrete pour. The *test method* lives in IS 1199 Part 2; IS 7320 fixes the equipment so that a slump measured on one site is comparable to a slump measured anywhere else.

It sits in the fresh-concrete control stack:

• IS 1199 Part 2 — the slump test procedure (the method that uses this apparatus)
• IS 456 — workability requirements and acceptance for concrete · IS 10262 — mix design (target slump is a design input)

A slump number only means something if every cone is the same cone. IS 7320 fixes:

• Frustum mould: bottom internal diameter 200 mm, top internal diameter 100 mm, vertical height 300 mm, smooth metal not thinner than ~1.6 mm, with foot pieces and handles so it can be held down and lifted vertically without disturbing the concrete
• Tamping rod: 16 mm diameter, ~600 mm long, one end rounded — fixes the compaction energy (25 strokes per layer, 3 layers)
• Base plate / non-absorbent surface so water isn't drawn out of the sample

The engineering point: slump is a *comparative* index, not a fundamental property. Its only value is repeatability — a worn, dented or odd-sized cone, or a rod of the wrong diameter, makes the day-to-day slump trend (your real early-warning signal for water or aggregate moisture changes) meaningless.

Scenario: acceptance slump on M30 delivered to site, target 100 ± 25 mm.

Step 1 — check the kit: cone the correct 200/100/300 mm geometry, undented, clean and damp; 16 mm rod; flat non-absorbent base.

Step 2 — fill: place on the base, hold the foot pieces, fill in three layers (~1/3 volume each), 25 strokes of the rod per layer distributed evenly, the bottom layer rodded full-depth, upper layers just into the layer below.

Step 3 — strike off & lift: strike level, clean spillage from the base, then lift the cone vertically, slowly, in 2–5 s without twist.

Step 4 — measure: invert the cone beside the slumped concrete, lay the rod across, measure the drop of the highest point to the nearest 5 mm. A shear/collapse slump → re-test; if it recurs the mix is harsh, not the test.

Get 105 mm → within 100 ± 25 → accept on workability. The discipline (right cone, 25 strokes ×3, vertical lift) is exactly what makes 105 today comparable to 95 yesterday.

1. Dented / non-standard cone. Any departure from 200/100/300 mm changes the slump — the trend that warns you of extra water becomes noise.

2. Wrong rod. A rod that isn't 16 mm dia / rounded changes the compaction energy and the result.

3. Tilted or twisted lift. Lifting the cone with a twist or off-vertical gives a false low/high slump; lift straight up, steadily.

4. Absorbent base. Testing on dry timber/earth draws water out and under-reads slump.

5. Treating slump as a strength test. Slump checks consistency/water content batch-to-batch — it is not strength and not durability; its job is to catch *change*, which only works if the apparatus is standard.

• IS 1199 Part 2 — slump test procedure (uses this apparatus)
• IS 456 — workability requirements & concrete acceptance
• IS 10262 — concrete mix design (target slump is an input)
• IS 5515 — compacting factor apparatus · IS 1199 (other parts) — other fresh-concrete tests

IS 7320 is old (1974) and humble, but the slump cone is the single most-used QC tool on any concrete site, and its entire value rests on the apparatus being standard. Disputes over 'why did the slump jump' almost never trace to the cone — they trace to aggregate moisture or added water — *provided* the cone is the right cone. A site that runs a battered cone, a stick instead of the 16 mm rod, and a casual twist-lift has thrown away its cheapest early-warning instrument: it can't tell a genuine mix change from test scatter. Keep one undamaged IS 7320 cone and rod per pour gang, run IS 1199 Part 2 the same way every time, and plot the slump — the trend, made meaningful only by standard apparatus, is what actually protects the IS 456 concrete.