Most commonly the strain in reinforced concrete elements is measured by attaching sensors to reinforcement. However, the degree of reinforcement in massive concrete elements is relatively low and reinforcement is mostly concentrated in near-surface areas.

The suggestion made by Elena seems very good but I would also like to recommend you some deeper insight into wireless monitoring of thermal-moisture changes in concrete elements (I assume that strains in concrete due to early-age effects are of interest here). Please refer to the following paper:

Construction and Building Materials 40 (2013), pp. 1156-1166

N Barroca, L M Borges, F J Velez, F Monteiro, M Górski, J Castro-Gomes: Wireless sensor networks for temperature and humidity monitoring within concrete structures

The piece of metal could locally modify the strength and you may not acquire the actual strain. I used FOSs (fiber optic sensors) for acquiring radial strain in concrete cylindrical specimens (you can find some results on my profile). You can embed FOSs in your wall when you build it.

For details on FOSs, you can take a look to this paper:

Smart Materials and Structures Volume 5 Number 2

C I Merzbacher et al 1996 Smart Mater. Struct. 5 196 doi:10.1088/0964-1726/5/2/008

Fiber optic sensors in concrete structures: a review

C I Merzbacher, A D Kersey and E J Friebele

Most commonly the strain in reinforced concrete elements is measured by attaching sensors to reinforcement. However, the degree of reinforcement in massive concrete elements is relatively low and reinforcement is mostly concentrated in near-surface areas.

The suggestion made by Elena seems very good but I would also like to recommend you some deeper insight into wireless monitoring of thermal-moisture changes in concrete elements (I assume that strains in concrete due to early-age effects are of interest here). Please refer to the following paper:

Construction and Building Materials 40 (2013), pp. 1156-1166

N Barroca, L M Borges, F J Velez, F Monteiro, M Górski, J Castro-Gomes: Wireless sensor networks for temperature and humidity monitoring within concrete structures

The answer differs depending on the fact that the structure does already exist or you have to design the measurement device for a structure to be build. In this latter case you can apply fiber optics as well as electrical strain gauges inside the wall linked to reinforcements or enbedded into concrete mass if you use fiber optic sensors, both Brillouin or Bragg's. The sensor will measure the strain. you can calculate the stress by elastic modulus of the concrete since the strain should be approximatively the same for both materials. Obviously you can measure the strain along the sensing direction and along the reinforcement direction. The application of the sensor along the radial direction of the reinforcement bar is meaningless. The stress like the strain is a six component symmetric second order tensor but vertical and horizontal stresses can be reasonably considered uniaxial under certain hypotesis about structural symmetry. If you insert a piece of metal into the concrete, you can measure the strain if the metal is well embedded and made perfectly glued to the concrete; this will be achieved by using long pieces enough to transfer shear stress (cylindrical pieces long about 80 - 100 times the diameter should be good) and the measure have to be done at the middle of the piece, it is true that it modify the strength but does not affect the strain considerably except that for a small local effect.

If the structure already exists, in my opinion, stress at internal points can be measured only by cutting a small portion of the wall and inserting flat hydraulic jack but you will get only average vertical stress with a great uncertainity this test has practitional validity but I do not think it is good for research.

I used Brillouin FOS in concrete but only for measures of strain on the surfaces, you can find some references in "Bernini, R., Minardo, A., Ciaramella, S., Minutolo, V., Zeni, L. Distributed strain measurement along a concrete beam via stimulated brillouin scattering in optical fibers (2011) International Journal of Geophysics, 2011, art. no. 710941," some experiments with embedded FOS have not been pubblished yet.

Are you concerned by the strain which will be developed due to the heat of hydration or due to loading. We used FOS embedded in concrete beams and cylinders and did show a great response and sensitivity to loading due to temperature change and loading as well

Strain gauges and vibrating chord devices can be used. Please contact me personally as i did the monitoring of a 4mX40mx60m foundation of a skyscraper and I can suggest u some tricks.