concrete barriers maybe wouldn’t be super fair cause it would take construction vehicles to mix the concrete and then the setting time 
, but i like the idea of upgrading the sandbag so its more durable, like maybe the 1st upgrade could drive wooden stakes through the tops of the sandbags into the ground and the 2nd upgrade could add barbed wire to the top of the bags maybe 
each upgrade could require more building materials to complete so it would realistically deplete the engineers material reserves to make a more durable structure
the wooden stake upgrade could make it 3x as durable to close range explosions so it would take multiple explosives to destroy it
and the barbed wire would deter anyone from just rushing in and jumping over the sandbag
i still think the sandbags should gradually reduce in size as they receive damage but in that respect i also think the engineer should be able to repair a damaged sandbag for a portion of building supplies