Why has my concrete not gone hard?
Six reasons a pour stays soft, how to tell which one you have, and the honest answer about whether any of them can be rescued.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
The usual causes are cold, dead cement, too much water, too little cement in the gauge, a mix that froze, or simply not enough time in low temperatures. Cold and time are recoverable — wait, and protect it. Wrong gauge, dead cement and frost damage are not, and the pour has to come out.
A pour that is still soft when you expected it to be hard is alarming and often not as bad as it looks, because cold slows the reaction dramatically and a slab in January can take several times as long as the same slab in June. The diagnosis matters, though, because two of the causes mean the concrete will never harden properly and leaving it in place stores up a failure.
Step by step
- Check the temperature record first.Look at what the overnight lows have actually been since the pour. Below a few degrees, concrete sets very slowly; near freezing it effectively pauses. This explains most cases and requires nothing but patience and cover.
- Check whether it froze.Frost damage usually shows as a surface that set and then went soft, crumbly and friable, often with a slightly scaly appearance. This is not recoverable and it does not improve.
- Look at how uniform the softness is.Uniformly soft across the whole pour points to the cement or the gauge. Soft in one area points to a bad batch. Soft only at the surface points to water or finishing.
- Check the cement you used.Squeeze what is left in the bag. Hard lumps mean it had already partly hydrated before it went in the mixer, in which case the pour will never reach strength.
- Protect and wait before doing anything drastic.If cold is the likely cause, cover with polythene and insulation and give it several more days. Do not walk on it, load it, or start breaking it out while it is still going.
- Break out anything that will not recover.Wrong gauge, dead cement or frost damage all mean the concrete has to go. It is a miserable job and it is far cheaper now, while it is weak, than after it has been built on.
- Get the next pour right.Fresh cement, a strict bucket gauge, water added slowly and judged by consistency, and protection from frost from the moment it is placed.
Tips
- Cold is by far the commonest answer and the least serious. Before assuming the worst, check what the nights have actually been doing.
- A slab that is soft only in the top few millimetres is a finishing problem, not a setting problem, and the concrete underneath is usually fine.
- If a contractor poured it, do not break anything out before they have seen it. Photograph the state of it and the dates.
If it doesn't work
Soft after a day or two, but the weather has been cold
Cause: Low temperature slowing hydration — near freezing the reaction very nearly stops — Fix: Wait, and insulate it. Cover with polythene and a layer of insulation or old carpet to hold what heat the reaction generates. It will get there, but much more slowly, and it is vulnerable to frost the whole time.
Set at the edges, soft in the middle, or vice versa
Cause: Uneven mixing between batches, or one batch gauged wrongly — Fix: Break out and replace the soft area. A localised soft patch does not recover and it becomes a hole in the slab.
Surface soft and sloppy, underneath firm
Cause: Too much water, or water and cement worked to the surface during finishing — Fix: The underlying concrete is probably sound. Remove the weak surface layer mechanically and treat as a dusting problem.
Whole pour is soft, crumbly and pale after several days in mild weather
Cause: Cement that had already reacted in the bag, or far too little cement in the gauge — Fix: It will not recover. Break it out and re-pour with fresh cement and a bucket-gauged mix.
Set, then went crumbly and soft after a frost
Cause: The mix froze before it had gained enough strength — Fix: Permanent damage — the internal structure was disrupted by ice. It has to come out. There is no additive or treatment that restores it.
Postcrete or a fast-set mix still soft after an hour
Cause: Too little water, water added before the powder, or old stock — Fix: Fast-set products need the water in first and the powder poured into it. If it is old stock, it may be partly hydrated. Dig it out while it is still soft and start again with fresh.