Surcharge for additional reinforcement of the base plate

Hello dear forum,

I have a question and hope someone can shed some light on it.

Yesterday we unexpectedly received mail from our general contractor in the form of an additional offer for the extra reinforcement in the base slab. We are building with a basement, with a waterproof concrete tank due to groundwater. The construction service description only mentions an assumed ground pressure of 250 kN/m2 and that, in case of unfavorable soil conditions, increased loads or groundwater, the reinforcement proportion may increase and the costs for this will be determined additionally. Total thickness 25 cm, C25/30 reinforced concrete, two-layer up to Q335A reinforcing mesh.

The costs for the waterproof concrete tank have already been billed with a hefty surcharge, so I assumed that the additional effort due to the groundwater is already covered.

I find it hard to imagine increased loads with our construction method, 10 by 10 m, two full floors in solid construction.

From the soil report, I can’t quite tell if we have unfavorable (apart from groundwater) soil conditions. The foundation is supposed to be on an elastically supported base slab in the existing medium dense to firm SU and SE soil. The bedding modulus is specified with ks = 11.5 MN/m3 and the allowable bearing stress should be limited to 150 kN/m2.

I have already consulted Google extensively but found nothing helpful, so here are my questions:

1. Can I somehow deduce from the information in the soil report whether the required ground pressure of 250 kN/m2 is met? I can’t quite put the data in relation.

2. Then, the kilogram of additional steel is billed at a flat rate of €2.30, based on the gross quantity and regardless of whether it is mats or round steel of the supplementary reinforcement. Since it concerns almost 2 tons of additional steel overall (in the result almost three times the original amount), the price is accordingly. Is this method of billing common and is the price reasonable?

Thank you in advance for your help.

Best regards Carsten

Hello construction expert,

Thank you for the feedback. I am aware that due to the contractual relationship we are practically a hostage of our general contractor and our scope for action is very limited. Nevertheless, it is important for me to know whether the surcharge is within a reasonable range or not, in order to possibly limit the extent of my teeth grinding.

Therefore, my question again is whether there are similar experiences or if the €2.30 per kg is excessive.

Best regards
Carsten

So once again my question, whether there are similar experiences or if the €2.30 per kg is exaggerated.

We paid €1.41 per kg of steel (including delivery + installation). At least, that’s how it was stated. Whether the steel was cheap and other items were more expensive? No idea. But you do have a comparison value

Hello Wastl,

thank you very much. With about €0.90 difference per kg and nearly 2 tons of steel, I consider our "offer" rather expensive.

Best regards, Carsten

We had around 3800 additional costs for our foundation slab (approx. 10.64 x 10.94). This was calculated by the structural engineer after the soil investigation report. We found this very steep, yet you have to place a lot of trust in the calculations. To our knowledge, we are also the only ones in our building area who have an additional reinforcement or upgraded foundation slab. Many have waived the report and relied on the report that the municipality made about 5 years ago. However, they already have large settlement cracks... But we built without a basement.