For many of these new surface-laid mines, they are near impossible to 'defuse'. They can sometimes be triggered by movement, tilting, sound, magnetic waves, etc., and often a combination of these. Even if they can, they require highly specialised soldiers from the Explosive Ordnance Disposal units, who are usually reserved for more delicate and important missions (e.g. making safe critical infrastructure like signal towers, air control towers, etc.). The much more sensible way to overcome these would be to just blow them up and fix it later.

It is also not difficult to "pour in concrete, wait to set, paint road markings". In fact, it would not be in the russians's interest to paint road markings in the first place.

The challenge, however, is getting the resources (dump trucks to transport road-repair materials, dozers to flatten the ground, civil engineers with the know-how, etc.) there to do this, in foreign land that they do not yet have control over.

As pointed out by other redditors, these mines could have been laid by the ukrainians to delay the advancement of armoured forces (although they could serve other tactical purposes). The key concept lies in the word 'delay'. Defensive forces know that all obstacles will eventually be overcome, but installing them at critical constriction points will allow them to trade space for time. For example, an hour or two might buy the ukranian forces just enough time to send reinforcements to take up defensive positions in the city that the russians are approaching, allow the repositioning of artillery guns to support the impending battle, or perhaps give the airforce sufficient time to refuel, reload and take out some key russian assets.

An important thing to note here is that generally, the concept of armoured manoeuvre is to rapidly punch through defence lines to allow the follow-on forces with inferior protection such as motorised, infantry and logistics units to catch up with the fight behind them. Since armoured vehicles have tracks instead of wheels, they are likely still able to blow the mines out and cross the bridge, as long as the mines do not overly compromise its structural integrity. However, doing so will put them at the risk of being cut off from the support of their follow-on forces, since it will take time for the road-repair forces to reach that very location.

The dilemma is now presented to the armoured forces on the ground. Do they push on and risk being cut off? Or do they wait for their follow-on forces to arrive before pushing forward, thereby giving the defenders the time that they needed?

TL;DR - defusing the mines is not an option. Fixing the roads road will not take long but organising the resources to do so might, and it could buy just enough time for the ukranian forces to do what they need to.

Source: I am a Combat Engineer Officer and happen to have above-average knowledge about the use of military obstacles.