Retempering a Stiff Ready-Mix Concrete Load
A ready-mix truck arrives with C30/37 concrete for a suspended slab. The batch ticket shows 340 kg/m³ of cement and 170 kg/m³ of free water, giving w/c = 0.50. The specified slump class is S3, 100-150 mm. At 75 minutes after batching the slump is only 60 mm. The driver proposes adding 25 L/m³ of water and remixing before discharge. The site has an approved superplasticiser, and the specification refers to EN 206, requiring site water addition to be prohibited unless permitted by the concrete producer. Cube results from an earlier pour on the same project gave a mean of 34 MPa and an individual result of 27.5 MPa. The slab is exposed to occasional wetting.
1. Calculate the w/c before and after the proposed water addition, and quantify the change.
Read the worked answer
Initial free water is given as 170 kg/m³ and cement as 340 kg/m³, so w/c = 170/340 = 0.500. Adding 25 L/m³ is adding 25 kg/m³ of water, so the new water content is 195 kg/m³ and the new w/c = 195/340 = 0.574. The water content rises by 25/170 = 14.7%, and the w/c rises by 0.074, or about 15%. That is a different mix from the tested one and exceeds the original 0.50 ratio.
2. Decide whether the engineer should permit the addition. Explain the effect on strength and durability, using the cube data as supporting evidence.
Read the worked answer
The addition should not be permitted. At constant cement content, extra water increases porosity, lowers strength and raises permeability. A rise from 0.50 to 0.574 is likely to reduce strength by roughly 10-15%, and it will increase carbonation and wetting-related deterioration in an exposed slab. The earlier individual cube result of 27.5 MPa already shows little margin against a C30/37 specification; adding water would make conformity less likely, not more. A 60 mm slump at 75 minutes is slump loss, not a reason to add water. The correct response is to reject the load or have the producer authorise a controlled admixture adjustment, not to retemper with water.
3. Set out the compliant actions and records if the slump must be corrected on site.
Read the worked answer
First, contact the producer and obtain written confirmation of what is permitted. If a superplasticiser is allowed, add it at the producer's dosage, mix it thoroughly in the drum and retest the slump; do not exceed the maximum w/c. Record the batch ticket number, truck number, time of arrival and discharge, initial slump, admixture type and quantity, final slump and the person authorising the change. If the producer does not authorise an adjustment, reject the load and record the rejection. Never allow the driver to add water informally. Keep the cube results from the earlier pour under review: a mean of 34 MPa and an individual of 27.5 MPa do not demonstrate C30/37 conformity and should trigger identity testing and a review of the supplier's control, not acceptance by assumption.